Solar Cells: From Materials to Device Technology 9783030363536, 9783030363543

https://www.springer.com/gp/book/9783030363536 This book addresses the rapidly developing class of solar cell materials

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Solar Cells: From Materials to Device Technology
 9783030363536, 9783030363543

Table of contents :
Preface I......Page 5
Preface II......Page 7
Contents......Page 8
1.1 Introduction to Nanomaterials......Page 10
1.2 History of Nanomaterials......Page 11
1.3 Nanomaterials Are All Around Us......Page 12
2.1 Surface Area of Nanomaterials......Page 14
2.4 Chemical Properties of Nanomaterials......Page 15
2.5 Magnetic Properties of Nanomaterials......Page 16
2.6 Optical Properties of Nanomaterials......Page 17
3 Common Synthesis Techniques......Page 18
3.1 Sol–Gel Technique......Page 19
3.2 Co-precipitation Technique......Page 21
3.4 Microwaves Assisted Synthesis......Page 22
3.5 Plasma Synthesis Technique......Page 23
3.6 Simple Heating Method......Page 24
3.7 Solvent Evaporation Technique......Page 25
3.8 Mechanochemical Process......Page 26
3.9 Sonochemical Technique......Page 27
3.10 Chemical Vapor Deposition Technique......Page 28
4 Applications of Nanomaterials......Page 30
References......Page 31
Perspective of Nanomaterials in the Performance of Solar Cells......Page 33
1 Introduction......Page 34
2.1 Solar Cell Technologies......Page 35
2.2 Classifications of Third-Generations Solar Cells......Page 37
3 Nanomaterials- and Nanocomposites-Based Solar Cells......Page 39
5 Focal Points of Nanostructured Materials......Page 43
6.1 Stability......Page 48
6.2 Light Harvesting Efficiency......Page 49
6.3 Higher Surface Area......Page 50
6.4 Recombination Rate......Page 52
7 Challenges in Nanostructured Solar Cells......Page 53
8 Summary and Future Perspectives......Page 54
References......Page 55
1 Introduction......Page 63
2.1 Synthesis of Material-Based (TiO2, ZnO) Solar Cells......Page 66
3.1 Upconversion Rare Earth Materials for Photovoltaic Applications......Page 73
4 Dye-Sensitized Solar Cells......Page 74
5 Organic and Polymer Solar Cells......Page 77
References......Page 81
1 Introduction......Page 87
2 Basic Concepts to Solar Cells......Page 88
3 Inorganic Photovoltaic Devices......Page 90
3.1 Perovskite-Based Solar Cells......Page 92
3.2 Advanced Materials for Charges Transport in Solar Cells......Page 96
3.3 Application of SWCNTs in Perovskite-Based Solar Cells......Page 102
3.4 Stability Issues in Inorganic Solar Cells......Page 106
4 Organic Solar Cells......Page 110
4.1 Main Electron Donor/Acceptor Molecules Used in OSCs......Page 112
5 Tandem Solar Cells......Page 118
6 Conclusion......Page 121
References......Page 122
1 Introduction......Page 131
2 Metal Oxides......Page 132
3 Sulfides......Page 134
4 Case Studies......Page 135
4.1 Cu2S......Page 136
4.2 CuBiW2O8:S......Page 139
References......Page 142
1 Introduction......Page 147
2 Classical Review of Photovoltaic (PV) Cell......Page 150
3.1 Nanowires......Page 151
3.2 Nanotubes......Page 152
3.3 Nanopillars......Page 153
4 Inorganic-Based Nanomaterials for Solar Cells......Page 155
5 Organic-Based Nanomaterials for Solar Cells......Page 157
7 Nanomaterials for Perovskite Solar Cells......Page 158
8 Nanomaterials-Based Photovoltaic Cells Efficiency......Page 159
9 Solar Cell Characterization......Page 160
References......Page 162
1 Carbon-Based Nanoscience and Nanotechnology......Page 167
2 Structures and Properties of Carbon Nanotubes......Page 168
2.1 Types of Carbon Nanotubes......Page 170
2.2 Single-Wall Nanotubes......Page 172
2.3 Multiwall Nanotubes......Page 173
3.2 Reaction Mechanism......Page 174
4.1 Arc Discharge Method......Page 176
4.3 Chemical Vapor Deposition Method......Page 179
5 Applications and Functionalization of CNTs......Page 180
6 World Power Demand and CNT Solar Cells......Page 181
7 CNT-Based Perovskite Solar Cells......Page 182
8 Photoactive Side of Predominantly Semiconducting Nanotubes......Page 184
9 CNT Dispersion and Thin Film Formation......Page 187
10 Heterojunction and Environmental Stability of CNT-Based Solar Cells......Page 188
References......Page 189
Basic Concepts, Engineering, and Advances in Dye-Sensitized Solar Cells......Page 193
1 Introduction......Page 194
2 Basic Concept of DSSC......Page 195
3 Components of DSSC......Page 198
4 Device Construction of DSSC......Page 202
5 Basic Characterizations for DSSC......Page 204
6 Factors Influencing the Efficiency of DSSC......Page 206
7 Engineering and Advances in DSSC......Page 209
7.1 Role of Nanomaterial Properties in DSSC......Page 210
7.2 Dyes and Electrolytes Compatibility in DSSC......Page 213
7.3 Catalytic Ability of Different Counter Electrodes......Page 216
7.4 Role of Plasmonic Nanostructures in Improving the Efficiency of DSSC......Page 217
7.5 Tandem Dye-Sensitized Solar Cell......Page 222
8 Merits and Demerits of DSSCs......Page 226
9 Summary and Future Directions......Page 231
References......Page 232
1 Introduction......Page 242
2 Solar Cell and Generations of Solar Cell......Page 243
2.1 First-Generation Solar Cell......Page 244
2.2 Second-Generation Solar Cells......Page 245
2.3 Third-Generation Solar Cells......Page 246
3 Quantum Dots......Page 247
4 Quantum Dot Solar Cells......Page 249
4.1 Metal-Semiconductor Junction Solar Cell (Schottky Cell)......Page 250
4.2 Quantum Dot-Sensitized Solar Cells......Page 251
4.3 p-I-n Quantum Dot Solar Cell......Page 252
4.4 Polymer-Semiconductor Structure Configuration......Page 253
5.1 CdSe-Based Quantum Dot Solar Cell......Page 254
5.2 CdS-Based Quantum Dot Solar Cell......Page 255
5.3 PbS-Based Quantum Dot Solar Cell......Page 257
5.5 Graphene Quantum Dot-Based Solar Cell......Page 258
5.6 Other Quantum Dot Solar Cell......Page 260
6 Conclusion......Page 261
References......Page 262
1 Introduction......Page 266
2 Perovskite Material for Solar Cell Applications......Page 268
3.1 ‘A’ Site......Page 270
3.2 ‘M’ Site......Page 271
3.3 ‘X’ Site......Page 272
4 Structure, Phase Transformation and Electronic Characteristics of Perovskite Materials......Page 273
5 Band-gap Tuning of Perovskite Materials......Page 274
6.1 Preparation Methods for Perovskite Films......Page 277
6.2 Device Architecture and Working Principle......Page 279
7.1 J-V Hysteresis......Page 282
7.2 Long-Term Stability......Page 284
References......Page 285
1 Introduction......Page 289
2.1 Oxygen Quantity......Page 291
2.2 Annealing......Page 292
2.3 Fabrication Method......Page 294
2.4 Doping......Page 295
3.1 Structural Properties......Page 297
3.2 Wettability......Page 298
3.3 Optical Properties......Page 299
3.4 Electrical Properties......Page 301
4.1 Efficiency......Page 302
4.2 Hysteresis......Page 303
4.3 Stability......Page 305
5 Conclusion......Page 306
References......Page 307
1 Introduction......Page 312
1.1 Radiations in Space......Page 313
2 Radiation Environment and Devices in Space......Page 314
2.2 Ionization Damage Mechanisms......Page 316
3 Interaction with Matter......Page 318
3.1 Photoelectric Effect......Page 319
3.3 Compton Scatterings......Page 320
4.1 Effect on Solar Cells......Page 321
4.2 Effect on Photodiodes and LED’s......Page 323
5 Conclusion and Future Prospectus......Page 324
References......Page 325
Multi-junction (III–V) Solar Cells: From Basics to Advanced Materials Choices......Page 329
1 Introduction......Page 330
1.1 Limiting Factors Involved in Multi-junction Solar Cells......Page 331
2 Multi-junction Solar Cells Design......Page 332
2.1 GaAs/Si Tandem Solar Cell......Page 333
3 Multi-junction Solar Cell Performance......Page 334
3.1 Current Density–Voltage Curves......Page 335
3.2 Band Gap Conversion Efficiency......Page 337
3.3 Spectral Distribution Effects......Page 338
3.4 Anti-reflection Coating in Multi-junction Solar Cells......Page 340
4 Material Choice and Growth......Page 343
4.1 Metal Organic Chemical Vapor Deposition......Page 344
4.3 Gallium Indium Phosphide Solar Cells......Page 345
4.4 Germanium Solar Cells......Page 347
5 Future Consideration for Developing Advanced Multi-junction Solar Cells......Page 348
References......Page 350

Citation preview

S. K. Sharma Khuram Ali   Editors

Solar Cells From Materials to Device Technology

Solar Cells

S. K. Sharma Khuram Ali •

Editors

Solar Cells From Materials to Device Technology

123

Editors S. K. Sharma Department of Physics, Faculty of Science and Technology The University of the West Indies St. Augustine, Trinidad and Tobago

Khuram Ali Department of Physics University of Agriculture Faisalabad Faisalabad, Pakistan

ISBN 978-3-030-36353-6 ISBN 978-3-030-36354-3 https://doi.org/10.1007/978-3-030-36354-3

(eBook)

© Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface I

Solar cell is designed to convert sunlight into electrical power using photovoltaic properties. The advantage of a solar cell resides in a nearly permanent, uninterrupted power source at a minimal operating cost. Moreover, it has the ability to convert sunlight directly into electricity instead of some intermediate form of energy. A higher power to weight ratio is another advantage the solar cells have as compared to other power sources such as batteries. On the other hand, the low power to unit area of sunlight ratio (which necessitates using large area arrays), the relatively low efficiency, and the degradation that takes place in hostile high-energy radiation environments are some of their main disadvantages. The role played by solar cells in a wide range of resource investigation, meteorological, broadcast, communications, scientific, and space development research cannot be denied. It can be rightly commented that without the development of solar cells, we would not have the sophisticated weather, communications, military, and scientific satellite capabilities that we have today. Increased life, improved conversion efficiency, and reduced cost are the basic objectives for research and development of solar cell materials. Thin-film technology has been significantly improved since the last few years. For sufficient absorption of the solar spectrum, it is required that the wafer thickness should be >700 µm. It is not desirable for commercial or large-scale production of solar cells because it is a large thickness for a Si wafer in terms of cost and effective collection of photo-generated carriers. This type of solar cell is generally made by depositing one or more thin films, on a glass, plastic, or metal substrates. Initially, wafers of up to 200 µm thickness were used to fabricate silicon-based thin-film solar cells. This technique allows thin-film cells to be flexible and lower in weight. However, it is also mandatory to have a general understanding of the problems that appear when the thickness of a silicon wafer is decreased. Incorporating quantum dots is an updated approach to harness solar cells. Efficiency of thin-film-based solar cells has been remarkably increased with the addition of quantum dots. They have properties of band gap tunability, which makes them suitable for multijunction solar cells. Quantum dots have energy levels that are tunable by altering their size. These tunable energy levels, in turn, determine the band gap. Quantum dots in v

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semiconductors are nowadays a subject of great importance primarily due to their electronic structures and tunable optoelectronic characteristics. In addition, band gaps for single-junction solar cells using lead sulfide (PbS) colloidal quantum dots (CQD) can be fabricated which can operate on far infrared frequencies. Such configuration is difficult to achieve with conventional solar cells. Currently, quantum dot-based nanostructures have been primarily restricted to lasers, sensors, and LEDs, etc. However, the blue shift in the band gap energy and the size-dependent increase in oscillator strength are the characteristics that can be used for promoting better photovoltaic devices as compared to traditional photovoltaics. Solar cells with multiple stacked p–n junctions are made with multiple layered semiconducting materials from III–V compound semiconductors. These types of III–V cells have the highest conversion efficiency under concentrated sunlight. Since their evolution, the multijunctions solar cells have achieved great attention in the photovoltaic industry due to broad choice of materials with direct band gaps and high absorption coefficients. However, manufacturing price and process complexities are also involved with the high efficiencies gained in III–V multijunction solar cells. Recently, organic–inorganic hybrid solar cells have gained a rapid improvement in their performance. Their overall power conversion efficiency (PCE) has exceeded 20% because of high charge carrier mobility, efficient light harvesting, and long carrier lifetime. A perovskite absorption layer is sandwiched between a hole contact layer and an electron contact layer in a typical planar heterojunction perovskite solar cell. Irrespective of the compatibility of low-temperature sputtered NiOx films with flexible devices, their low PCE (below 10%) makes them unappealing to the research community. Therefore, for flexible perovskite solar cells, it is very purposeful to explore low-temperature processed NiOx films with effective hole extraction capabilities. It is observed that on an ITO glass substrate, a solution-derived NiOx hole contact layer-based inverted planar heterojunction perovskite solar cell can gain a PCE of as high as 16.47%. Before this technology is fully accepted in the market, many hurdles need to be overcome. At this time, organic photovoltaics (OPVs) are applicable only in limited markets that require lightweight, flexibility, stability, and variable angle performance. The continuous development of OPV with the improvement in their light harvesting ability and the management of active layer materials will occasion widely applicable technologies for electrical generation. St. Augustine, Trinidad and Tobago Faisalabad, Pakistan

S. K. Sharma Khuram Ali

Preface II

Reducing the cost and increasing the conversion efficiency are the crucial tasks in order to make solar cell energy competitive. Though important progress has been made in recent years, a complete auxiliary of traditional energy sources by solar cells still requires improvement in device performance. This book is devoted to the rapidly developing class of solar cell materials and designed to provide much needed information on the fundamental scientific principles of these materials, together with how these are employed in photovoltaic applications. Moreover, a special emphasize has been given for their space applications by thorough study of radiation-tolerant solar cells. This book will present comprehensive research outlining progress on the synthesis, fabrication, and application of solar cell materials from fundamental to device technology. St. Augustine, Trinidad and Tobago

S. K. Sharma

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Contents

Synthesis and Processing of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . Muhammad Yasin Naz, Shazia Shukrullah, Abdul Ghaffar, Khuram Ali and S. K. Sharma

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Perspective of Nanomaterials in the Performance of Solar Cells . . . . . . Hafiz Muhammad Asif Javed, Wenxiu Que, Muhammad Raza Ahmad, Khuram Ali, M. Irfan Ahmad, Anam ul Haq and S. K. Sharma

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Materials for Solar Cell Applications: An Overview of TiO2, ZnO, Upconverting Organic and Polymer-Based Solar Cells . . . . . . . . . . . . . . Navadeep Shrivastava, Helliomar Barbosa, Khuram Ali and S. K. Sharma Recent Advances in Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marcio A. P. Almeida

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Photovoltaic Materials Design by Computational Studies: Metal Sulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 Edan Bainglass, Sajib K. Barman and Muhammad N. Huda Photovoltaic-Based Nanomaterials: Synthesis and Characterization . . . . 139 Kanwal Akhtar, Naveed Akhtar Shad, M. Munir Sajid, Yasir Javed, Muhammad Asif, Khuram Ali, Hafeez Anwar, Yasir Jamil and S. K. Sharma Carbon Nanotubes: Synthesis and Application in Solar Cells . . . . . . . . 159 Shazia Shukrullah, Muhammad Yasin Naz, Khuram Ali and S. K. Sharma Basic Concepts, Engineering, and Advances in Dye-Sensitized Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Mohan Chandra Mathpal, Promod Kumar, F. H. Aragón, Maria A. G. Soler and H. C. Swart Quantum Dot Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 Hafeez Anwar, Iram Arif, Uswa Javeed, Huma Mushtaq, Khuram Ali and S. K. Sharma ix

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Organometal Halide Perovskite-Based Materials and Their Applications in Solar Cell Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Amna Bashir and Muhammad Sultan Effect of Oxygen Vacancies in Electron Transport Layer for Perovskite Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Mohamad Firdaus Mohamad Noh, Nurul Affiqah Arzaee and Mohd Asri Mat Teridi Solar Cells and Optoelectronic Devices in Space . . . . . . . . . . . . . . . . . . 307 Khuram Ali, Syedda Shaher Bano, Hasan M. Khan and S. K. Sharma Multi-junction (III–V) Solar Cells: From Basics to Advanced Materials Choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 Khuram Ali, Afifa Khalid, Muhammad Raza Ahmad, Hasan M. Khan, Irshad Ali and S. K. Sharma

Synthesis and Processing of Nanomaterials Muhammad Yasin Naz, Shazia Shukrullah, Abdul Ghaffar, Khuram Ali and S. K. Sharma

Abstract Nanomaterials have emerged as a distinct class of modern materials. These materials are of significant importance due to their unique optical, electrical, thermal, and magnetic properties. Due to their tunable physical and chemical characteristics, including melting point, electrical conductivity, wettability, heat conductivity, light absorption, catalytic activity, and scattering, these materials have also gained recognition in high-tech engineering applications. These characteristics reflect better performance and working efficiency of nanomaterials relative to their bulk counterparts. Although there are many naturally occurring nanomaterials, most nanomaterials are engineered in laboratories. Such materials are purposefully synthesized in accordance with the industrial requirements. This chapter deals with the fundamentals of nanomaterials, their history, properties, and industrial applications. Different methods of synthesis of nanomaterials, their merits, demerits, and scale-up potential are also discussed in this chapter. Keywords Nanomaterials · Properties of nanomaterials · Synthesis routes · Chemical vapor deposition

1 Fundamentals of Nanomaterials 1.1 Introduction to Nanomaterials Nanomaterials are representing a major area of science and technology with full growth in numerous application domains. These materials have gained fame in hightech advancements due to their tunable physical and chemical traits, including melting point, electrical conductivity, wettability, thermal conductivity, light absorption, M. Y. Naz (B) · S. Shukrullah · A. Ghaffar · K. Ali Department of Physics, University of Agriculture, Faisalabad 38040, Pakistan e-mail: [email protected] S. K. Sharma Department of Physics, Faculty of Science and Technology, The University of the West Indies, St. Augustine, Trinidad and Tobago © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_1

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catalytic activity, and scattering. These traits reflect the improved performance of nanomaterials as compared to their bulk counterparts. The International Organization for Standardization defines the nanomaterial as a “material having external or internal nanoscale dimensions or surface structure” [1, 2]. Presently, the following scientific terms are being used for the materials with nanoscale dimensions [1–3]. • Nanoscale: Roughly, the sizes in the range of 1–100 nm. • Nanoscience: The study of materials at nanoscale to understand their size and structural properties, to compare the advent of individual atoms and molecules, and to understand the bulk material related differences. • Nanotechnology: Practical use of the scientific knowledge to manipulate and control the matter on the nanoscale dimensions for different industrial applications. • Nanomaterial: A material with external or internal dimensions on the nanoscale. • Nano-object: A material possessing one or more peripheral dimensions at nanoscale. • Nanoparticle: An object with all three external dimensions grown at nanoscale is called nanoparticles. If the shortest and longest axes lengths of the object are different, then it is referred to as a nanorod or nanoplate instead of nanoparticles. • Nanofiber: A nanomaterial with two similar dimensions, extending at nanoscale, and a longer third dimension is called nanofiber. • Nanocomposite: A multiphase material having at least one phase on nanoscale. • Nanostructure: Arrangement of interconnected constituent parts in the nanoscale region. • Nanostructured material: A material with nanoscale structural dimensions. Nanomaterials are the keystones of nanoscience and technology. Nanotechnology is widely known as an interdisciplinary research area, which has been recognized worldwide during the past few years. Nanotechnology has shown substantial commercial impact, which will grow exponentially in the coming years [4]. Figure 1 depicts the evolution of science and technology of materials over time [1, 5, 6]. This chapter is aimed at a review of the progress to date in the science and technology of ultrafine particles, which form the building blocks of nanomaterials. An attempt is made to outline the progression of nanomaterials from fundamental research and commercial standpoint.

1.2 History of Nanomaterials The nanomaterials, driven from nanoparticles, have evolved as a distinct class of materials [7]. These materials are of special interest due to their unique optical, electrical, thermal, magnetic, and other properties [4]. Materials with dimensions in micrometers mostly reveal the physical characteristics same as the bulk materials. Contrarily, materials with dimensions in nanometers exhibit physical characteristics totally different from the bulk. In this size range, the materials exhibit some amazing properties since transitions from bulk form to atoms and/or molecules take place. For

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Fig. 1 Evolution of science and technology over time

example, nanosized crystals may have very low melting points and the difference can be up to 1000 °C. Also, lattice constant may decrease due to the appearance of surface atoms or ions as a significant fraction of a total number of atoms or ions. The surface energy, associated with these species, plays a substantial role in thermal stability of the material [5]. Very first scientific report on nanomaterials was published by Michael Faraday in 1857. He was the first who synthesized colloidal gold particles. Nanocatalysts have also remained under investigation for the last 70 years. In early the 1940s, the nanoparticles of precipitated and fumed silica were being sold in Germany and America as rubber reinforcement. The silica was being used as a substitute for ultrafine carbon black. The nanosized silica has found many applications in everyday consumer products, including nondairy coffee creamer, optical fibers, automobile tires, and catalyst supports. During 1960–70, nanopowders of metals were developed for magnetic recording tapes. Granqvist and Buhrman, 1976, reported the production of nanocrystals from an inert gas evaporation technique. In recent years, several structural and functional materials have been developed through nanophase engineering for manipulation of catalytic, mechanical, electrical, magnetic, electronic, and optical functions. These materials include both organic and inorganic structures [4, 8].

1.3 Nanomaterials Are All Around Us Although many nanomaterials exist naturally, majority of the nanomaterials are engineered. They are designed and synthesized according to the needs of commercial processes and products. Synthetic nanomaterials are being used in sunscreens, sporting goods, cosmetics, stain-resistant tires, clothing, electronics, and many other everyday

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items. In medicine, these materials are being used for imaging, diagnosis, and drug delivery. The unique properties of nanoscale are attributed to an increase in relative surface area and appearance of new quantum effects. The high surface area to volume ratio of nanomaterials improves their mechanical strength and chemical reactivity. Also, the quantum effects at nanoscale become more significant in determining the characteristics of the materials [4]. Nanoparticles can be treated as a common type of materials in many different environments. These particles pass by almost undetected unless somebody is looking for them. Nanoparticles are being produced through different physical and chemical processes. The volcanic ash, fine sand, dust, ocean spray, and even biological matter are the natural sources of nanoparticles [9]. The synthetic or anthropogenic nanoparticles are classified into incidental nanoparticles and engineered nanoparticles. The size and shape of the incidental nanoparticles are poorly controlled. These are the by-products of human activities and are made of a hodge-podge of the elements. The major sources of incidental nanoparticles are diesel engines, fire, and large-scale mining [9]. Figure 2 shows the common sources of incidental nanoparticles. Engineered nanoparticles are specifically designed and synthesized through different physical and chemical processes. It is possible to control their size, shape, and compositions by deliberately choosing the synthesis technique. Although engineered nanoparticles are becoming more sophisticated over time, the simple nanoparticles produced through relatively easy chemical reactions [9]. The nanophase ceramics, nanostructured semiconductors, nanosized metallic powders, single nanosized magnetic particles, nanostructured metal clusters, nanostructured metal-oxide thin films, and polymer-based composites are few examples of engineered nanomaterials.

Fig. 2 Few sources of incidental nanoparticles

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2 Properties of Nanomaterials The factors making the nanomaterials considerably different from their bulk counterparts are: I. Large surface area to volume ratio. II. Quantum confinement effect.

2.1 Surface Area of Nanomaterials Nanomaterials possess the large surface area to volume ratio. The ratio of the atoms at the surface to those within the body changes dramatically on a successive division of a macroscopic object into smaller ones [5]. The constituent particles exhibit a relatively larger surface area to volume ratio when compared to same mass of the material in bulk form. Figure 3 shows a cube of 1 m3 volume 6 m2 surface area. If divided into eight smaller cubes, the surface area would increase to 12 m2 . The surface area would increase further by cutting down the cube in a large number of smaller particles. In response, the surface atoms will increase by making the materials chemically more reactive.

Fig. 3 Illustration of surface area to volume ratio

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2.2 Quantum Confinement The bulk materials are known for continuous electronic energy levels. Contrarily, the nanomaterials do not have continuous energy levels due to confinement of electronic wave function in one, two, or three physical dimensions. The quantum confinement of nanomaterials can be classified as: I. 1-D confinement—thin film, quantum well. II. 2-D confinement—nanotubes, nanorod, and quantum wire. III. 3-D confinement—precipitates, colloids, and quantum dots.

2.3 Structural Properties of Nanomaterials The mechanical properties of nanomaterials are of quite basic interest since it is problematic to grow macroscopic objects of high density and grain size less than 100 nm. However, two materials have attracted the interest and achieved the industrial importance. These materials are plastic-deformed metals and the polymers composited with nanotubes or nanoparticles. However, earlier is not accepted as a nanomaterial due to its larger grain size. The experimental investigations on structural traits of bulk nanomaterials are impaired by the main experimental problems in synthesizing the specimens of fairly defined porosities and grain sizes. Therefore, molecular dynamics and model calculations are of key importance for understanding the structural properties of such materials [10]. The addition of nanoparticles, nanorods, or nanotubes into polymers significantly improves their structural properties. Possible changes in structural properties depend on the filler type and the method of conducting filling. The filling method is of great importance because the anticipated advantages filling may lose if the filler particles aggregate into larger particles. The polymers filled with nanofillers exhibit a wide range of failure strengths and strains, depending on the shape and particle size of the filler and the degree of agglomeration. The polymers reinforced with silicate platelets have shown the best structural properties of economic relevance. Although nanofibers or nanotubes added polymers are the best composites, sometimes these composites exhibit the least ductility. On the other hand, the composites filled with carbon nanotubes may exhibit high mechanical strength and strain at rupture [4, 11].

2.4 Chemical Properties of Nanomaterials The small atomic clusters exhibit high ionization energy as compared to the corresponding bulk material. Owing to different crystallographic structures and high surface area to volume ratio, the nanomaterials exhibit high radical alteration in chemical

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reactivity. The catalysis, utilizing nanoscale systems, can significantly increase the selectivity, rate, and efficiency of a chemical reaction. Porosity and surface area of nanomaterials are important physical properties influencing the utility, quality, and handling of the nanomaterials. Therefore, these materials should carefully be engineered to carry out specific functions. For example, surface chemistry of carbon nanotubes is driven by the surface defects and basal and edge carbon atoms. The structural defects of graphene layer or nanotubes act as active sites because of increased density of unpaired electrons. These active sites determine the rate and efficiency of the reactions [12, 13].

2.5 Magnetic Properties of Nanomaterials Magnetite nanoparticles are found in animal cells, which help them in cruising. The most fascinating characteristic of magnetic nanoparticles is the lessening of multidomains into a single domain with a reduction of particle size to some limit [14]. These materials have multifunction applications, such as color imaging, ferrofluids, bioprocessing, magnetic storage devices, and refrigeration. A high proportion of surface atoms in magnetic nanomaterials develops different magnetic coupling with neighboring atoms and consequently different magnetic properties than the bulk counterpart. As shown in Fig. 4, the bulk ferromagnetic materials have multiple magnetic domains. In contrast, magnetic nanoparticles often have a single magnetic domain exhibiting the superparamagnetism phenomenon [4, 15]. The gold and platinum are non-magnetic in bulk form and magnetic at nanoscale. In such materials, the surface atoms not only differ from bulk atoms but also exhibit different properties due to interaction with other species. The interaction of nanoparticles with other chemical species modifies their physical characteristics by capping them with molecules. Fig. 4 Illustration of superparamagnetism in nanomaterials

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The large spin–orbit coupling of these noble metals can yield to a large anisotropy and therefore exhibit high ordering temperatures. More surprisingly, permanent magnetism was observed up to room temperature for thiol-capped Au nanoparticles. For nanoparticles with sizes below 2 nm, the localized carriers are in the 5d band. Bulk Au has an extremely low density of states and becomes diamagnetic, as is also the case for bare Au nanoparticles. This observation suggested that modification of the d band structure by chemical bonding can induce ferromagnetic like character in metallic clusters [4].

2.6 Optical Properties of Nanomaterials The optical aspect of nanomaterials is also fascinating for many communication applications. The most important applications utilizing the optical properties of nanomaterials are the optical detectors, sensors, lasers, imaging devices, optical displays, phosphor, solar cell, photocatalysis, biomedicine, and photo-electrochemistry [4]. The optical absorption and emission in bulk materials depend on transitions between valence bands and conduction bands. The low dimensional metals and semiconductors exhibit large changes in their optical properties, such as color. For example, the colloidal solution of gold nanoparticles has typical deep red color when particle size is very small. However, the color of solution changes to yellow with an increase in particle size. This change in color with size is attributed to the resonance of surface plasmons, which normally occurs in low dimensional materials. The frequency and intensity of light, emitted from the nanoparticles, also depend on the size of the particle. The size of the nanoparticles of semiconductors in the form of quantum dots effects their nonlinear optical properties and gain for certain emission wavelengths. Other characteristics influenced by the small dimensions include photocatalysis, photoemission, photoconductivity, and electroluminescence. The spacing between energy levels increases with a decrease in particle size, which helps confining the system and enhancing the surface plasmon resonance. The resonance of surface plasmons is related to the coherent excitation of free electrons in the conduction bands, which results in in-phase oscillations. The free electrons polarize relative to cationic lattice due to electric field of incoming light. A net charge difference appears at the boundaries of nanoparticle, which acts as a restoring force. This force sets the electrons into dipolar oscillation at a certain frequency. The energy of surface plasmon resonance is governed by the dielectric medium around the nanoparticles and free electrons’ density. The resonance sharpens with increase of particle size. The larger particles enhance the scattering length. The resonance frequency of all the noble metals falls in the visible region of the light spectrum [5, 11].

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2.7 Electronic Properties of Nanomaterials How the particle size influences the electrical conductivity of nanomaterials is a complex phenomenon? The electrical conductivity is based on distinct mechanisms which are classified into grain boundary scattering, surface scattering, quantized conduction, ballistic conduction, widening and reduction of band gap, Coulomb charging and tunneling and a change in microstructures. An increased perfection, reduced impurity, minimum dislocation, and low structural defects also influence the electrical conductivity nanomaterials [5]. Only one electron wave mode is observed in electrically conducting carbon nanotubes. This mode transports the electrical current through the nanotubes. Since orientation and length of carbon nanotubes vary, these tubes would touch the mercury surface at different times by providing a set of information. This information is about the resistance of different nanotubes and the effect of tube length on resistance [4]. Changes in electronic properties of low dimensional materials are related to the wave-like properties of the electrons and scarcity of the scattering centers. When the system size is comparable to the de Broglie wavelength of electrons, the energy states exhibit a more pronounced discrete nature. In some cases, the conductors change to insulators when the size is well below the critical length scale. Conduction in extremely enclosed structures such as quantum dot is very susceptible to the presence of other charge carriers, and hence, the dot charge state is known as the Coulomb blockade effect. This results in single electrons conduction procedures that in turn require a minimum quantity of energy to run the switch, memory elements, or transistor [15].

3 Common Synthesis Techniques Nanomaterial synthesis techniques are split into two primary groups, namely topdown approaches and bottom-up approaches. These methods are used for assembling atoms together or disassembling or dissociating bulk materials into finer parts made up of very few atoms. The top-down approach begins with larger objects as shown in Fig. 5, which are reduced to lower sizes to achieve fine features and nanoscale materials. This strategy was originally adopted for the microelectronics sector, but with the development of nanopatterning and thin-film deposition methods, this strategy entered the nanofabrication regime. This regime includes physical vapor deposition, thin-film deposition, chemical vapor deposition, plasma-enhanced chemical vapor deposition, laser ablation, etc. [16]. Nanoscale materials are manufactured in a bottom-up strategy in the form of building blocks that combine to form bigger nanoparticles for various uses [16, 17]. Nanoparticles or nanospheres synthesis and assembling into macroscopic particles are some bottom-up approach material development techniques.

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Fig. 5 Top to bottom and bottom to top synthesis techniques for nanomaterials

3.1 Sol–Gel Technique This technique is used for condensation and hydrolysis of metal alkoxides or liquid precursors into solid nanomaterials [16–20]. As illustrated in Fig. 6, a sol–gel method includes the development of inorganic networks through sol formation (colloidal suspension) and sol gelation into gel [17], which is a continuous liquid phase network. Metals or metalloid components surrounded by reactive ligands are the prevalent precursors used to synthesize these colloids. In contact with water or diluted acids, the starting materials are processed to produce dispersible oxides and sol. The liquid is separated from the sol to produce gel and the size and shape of the nanomaterials are governed by this sol–gel transformation. On calcination, the gel shifts to oxide. The reactions for synthesis of nanomaterials through sol–gel technique by hydrolysis and condensation metal alkoxide are defined as: Hydrolysis: MOR + H2 O → MOH + ROH Condensation: MOH + ROM → M–O–M + ROH. The sol–gel process is performed by a sequence of separate steps such as: • Formation of stable metal alkoxide solutions or precursors of solved metal. • The formation of bridged networks of oxide or alcohol also known as gel. • Network gelation by a polycondensation reaction. This reaction increases the solution’s viscosity. • Gel aging to achieve solid mass. The polycondensation reactions continue unless the gel becomes solid. This stage involves the contraction of gel networks and the expulsion of solvent from gel pores.

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Fig. 6 Illustration of synthesis of nanomaterials through sol–gel technique [17]

• Ostwald ripening and phase transitions may occur at the same time as syneresis. Gel aging may take seven days. It is essential to avoid cracks in the cast gel. • Water removal and drying of volatile liquids from the gel network. The drying step involves constant rate period, critical point, falling rate period, and second falling rate period. • The gel densifies and decomposes at higher temperatures (T > 8000 °C). The pores in the gel collapse and other organic species volatilize. Sol–gel is an effective technique for synthesis of nonmetallic inorganic compounds at very low temperature. These materials include ceramics, glass, and glass ceramics. Sol–gel processing is preferred over high-temperature counterparts based

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on firing ceramics and melting of glass. The major problems associated with bottomup approach, however, are least control over particles’ growth and agglomeration of the particles. It is also essential to ensure that the growth reaction is complete and that the product does not contain any unwanted reactants. The production rate of nanomaterials is comparatively small through this technique [17]. The other problems associated with such a method are costly raw materials and long drying and sintering [18–25]. The sol–gel technique could not attain its complete industrial potential because of these constraints.

3.2 Co-precipitation Technique Co-precipitation is the most widely used technique of synthesizing nanomaterials among all known techniques [20–26]. Nanoparticles have been produced through co-precipitation of aqueous solutions of raw materials. This method involves the thermal decomposition of the solids and the formation of the precipitates. The complete process is broken into nucleation, growth, coarsening, and agglomeration of the particles. Nucleation and development steps should be well controlled for the synthesis of monodispersed nanoparticles. Both nucleation and synthesis steps can be controlled by altering the mixing ratio of reactants, adjusting the reaction parameters, adding surfactants, changing solvents, and changing the injection sequences. Relatively fast nucleation and steady growth of particles result in monodispersed nanoparticles. Nanoparticles’ molecules follow the Oswald ripening during slow growth mode, while fast-growth mode results in dispersed distribution of nanoparticles’ size and uneven morphology. Smaller particles disappear during the Oswald ripening phase while bigger particles continue to grow. This trend is ascribed to the least stable molecules on the particle surface. The molecules within the particles are already packed and well-ordered. The large particles have reduced volume-tosurface ratios, hence the low energy states. When the process attempts to reduce its energy, molecules at the surface of smaller particles tend to spread into the solution and attach to the bigger particles [27–30]. The method of co-precipitation is widely used in biomedical applications as it includes less harmful materials and processes. In this method, metallic nanoparticles are made from aqueous salt solutions in this method. In an inert atmosphere, a base is added to the solution at room temperature. The response to co-precipitation is merely expressed as: Fe2+ + 2Fe3+ + 8OH− → Fe (OH)2 + 2Fe(OH)3 → Fe3 O4 + 4H2 O The ferrous hydroxide suspensions are partly oxidized with separate oxidizing agents for the production of spherical nanoparticles in solution or the aging of stoichiometric mixtures of ferrous and ferric hydroxides takes place in aqueous media [31]. The size and shape of the particles depend on salt type, pH value, reaction temperature, ratio of ferric and ferrous ions, stirring rate, and dripping speed of the

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basic solution and ionic strength of the media. The particle size can also be controlled over one order of magnitude by changing pH and ionic strength of the medium of precipitation. Sometimes co-precipitation yields particles with wide a distribution of sizes, which often requires selection of secondary sizes [20, 32–34].

3.3 Green Synthesis Technique The method of green synthesis is preferred over physical and chemical methods. This method is cost-effective, simple to scale, and environmentally friendly for bulk synthesis. Additional green synthesis can be performed at comparatively low pressure, temperature, and energy without the use of toxic chemicals [21, 35]. However, biosynthesis of plant-based metal nanoparticles is in its infancy and needs further research efforts to be known as a fully developed process. The synthesis of nanoparticles from the plant extract, exudates, inactivated plant tissues, and other parts is a non-toxic and safer approach as compared to other well-known synthesis techniques. Other biological techniques for the production of nanoparticles include fungi, microorganisms, enzymes, and plant extracts [20]. The use of plants in biosynthesis is primarily due to the existence of reducing agents, which play a crucial part during the synthesis process [20]. The commonly known plant-based reducing agents are flavonoids, citric acid, ascorbic acids, extracellular electron shuttles, reductases, and dehydrogenases.

3.4 Microwaves Assisted Synthesis Microwaves are a form of electromagnetic energy in the electromagnetic spectral range of 300–300,000 MHz. Microwave heating is the best process whereby microwaves are directly coupled with the molecules of the reaction mixture. This process ensures fast heating, short reaction time, and clean chemistry of the reaction. Microwave chemistry is also referred to as green chemistry because it produces no hazardous material such as gas fumes or heat using internal energy sources. Microwaves utilize electromagnetic radiations that pass-through material and produce heat via molecular oscillation. Microwave heating generates the same amount of heat in the whole material at a high speed and at a high reaction rate at the same time. Synthesis aided by microwaves has become a tool of significant importance for fast organic synthesis. Some of the main benefits include a dramatic reduction in reaction time, enhanced conversion efficiency, clean product formation, and a wide scope of development of new reaction conditions [22]. Water is a good microwave absorber and a polar solvent. Figure 7 shows a simplified schematic illustration of heating mechanism of the water molecules through microwaves [23]. Polarized water molecules attempt to orientate during microwave

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Fig. 7 Water molecules under microwave irradiation in an alternating electric field

heating with the quickly changing alternating electric field. In response, heat produces due to the rotation, collision, and friction of the water molecules. When ionic salts are dissolved in water, they affect the structure of the water molecules. The relaxation time of the molecules at lower concentrations initially reduces and then rises. The presence of the ions is assumed to cause a breakage of the hydrogen bonding in water. Those water molecules that are aligned with the ions are relatively fixed, but those that are not coordinated do not experience such powerful intermolecular hydrogen-bonding impacts, and thus have reduced relaxation times in this free state. The impacts are reversed at higher concentrations and the relaxation time of water in concentrated salt solutions is higher than that of pure water, probably owing to a higher ordering of water molecules when there is a large amount of ions in the solution. The metallic nanomaterials of different morphologies, including nanospheres, nanopolygonal plates, nanosheets, nanorods, nanowires, nanotubes, etc. can be prepared within few minutes through microwave heating. The morphology and size of nanostructures can be regulated by changing multiple experimental parameters such as metal salt concentration, surfactant chain length, solvent, and temperature of the reaction.

3.5 Plasma Synthesis Technique Different kinds of plasma reactors that can be used to synthesize nanocrystals have a number of common characteristics to help understand suitable conditions for plasma formation and growth of nanocrystals. The main characteristics are: • Reactor vessel: It describes the plasma volume and separates the mixture of precursor gases (often diluted in an inert carrier gas) from the surrounding air.

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• Electrical power: An electric power supply is used to convert the gas mixture into plasma through direct current electric field or time-varying radio or microwave frequencies. • Plasma electrodes: These are the means of plasma coupling such as electrodes, resonant cavities, or antennas. • Gas controllers: Such devices are attached to the plasma reactors for controlling the supply of precursor gases or vapors and to create the desired gas pressure in the plasma reactor [18]. In low-pressure plasmas, two distinct types of synthesis strategies are adopted: batch reactors and flow-through reactors. The performance of batch type reactors depends on ability of the discharge plasmas to trap the negatively charged species produced during gas ionization. The charged species may be subjected to a constant flow of gases and allowed to expand to larger dimensions. Particles are gathered after switching off the plasma and enabling the particles to drop down on a substratum by gravity or through the gas flow to a filter. In a flow-through reactor, nanoparticles are transported through the reactor in a gas flow soon after they nucleate and develop. As the nanoparticles grow, the precursor will be depleted and particle growth will ultimately be limited in the gas stream by the available precursor. Particles are brought into the gas that flows out of the reactor and gathered on a substratum or using filters [21].

3.6 Simple Heating Method The easiest way to produce the nanoparticles is to heat the precursor material in a heat-resistant crucible. Figure 8 shows a schematic representation of the gas phase synthesis method of single-phase nanomaterials from a heated crucible [4]. This method is only suitable for materials with high vapor pressure at temperatures of up to 2000 °C. The energy is supplied to the precursor material through arc heating, Joule heating, or electron-beam heating. The atoms evaporate into an atmosphere that is either inert or reactive (so that a compound is formed). In order to perform reactive synthesis, materials with very low vapor pressure must be fed into the furnace in the form of an appropriate precursor such as organometallics, which decomposes into the furnace to produce a condensable material. The atoms of the evaporated material lose energy by collision with the cold gas atoms and undergo condensation through homogeneous nucleation into tiny clusters. The precursors react in the gas phase when a compound material is synthesized. The precursors form a compound with the material that is injected independently into the reaction chamber. If they remain in a supersaturated region, the clusters would continue to grow. To control their size, a carrier gas is used to remove them quickly from the supersaturated environment. Three parameters play a key role in controlling the cluster size and its distribution: i. Evaporation rate, which depends on energy input. ii. Condensation rate, which depends on energy removal.

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Fig. 8 Schematic representation of the gas phase synthesis method for single-phase nanomaterials from a heated crucible

iii. Gas flowrate, which for removal of clusters. This method can be extended from laboratory to industrial scale due to its inherent simplicity [4].

3.7 Solvent Evaporation Technique The first technique established for the preparing of nanoparticles was solvent evaporation. Using this technique, polymer solutions were formed in volatile solvents and emulsions were formulated with the use of dichloromethane and chloroform. Nowadays, ethyl acetate is preferred for the stated purpose due to its better toxicological profile for obtaining the polymeric particles with dimensions smaller than 500 nm. During the preparation stage, the emulsion is transformed into a nanoparticular suspension when the solvent is evaporated, after which the solution can be diffused through the continuous emulsion phase by performing some standard procedures. The procedures involve high-speed ultrasonication or homogenization and evaporation of the solvent through magnetic stirring at room temperature or under reduced pressure. The magnetic stirring results in solidified nanosized particles, which are collected through ultracentrifugation and washed to remove surfactants. Finally, the product is lyophilized [24] (Fig. 9). This technique comprises of the preparing of nanoemulsions, formulated in a volatile solvent solution with a polymer dissolved. Dichloromethane and chloroform are the commonly used solvents, but are often substituted by ethyl acetate, a less toxic solvent, and therefore much more suited to the synthesis of the controlled release systems in which the encapsulation of drugs is involved. This technique is performed under vacuum where suspension is developed by evaporating polymer solvent from

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Fig. 9 Illustration of a solvent evaporation technique

emulsion droplets, which can be diffused through the continuous phase. This slow method includes a rapid evaporation period in which at least 90% of the remaining solvent is removed, followed by a slow evaporation period in which the remaining solvent is extracted. Due to the elevated solvent loss, droplet size dramatically reduces during the first step to reach a minimum value. On the contrary, a substantial rise in droplet diameter due to coalescence characterizes the second step. In polymers with interfacial adsorption characteristics, this coalescence process can be accentuated, whereas in polymers with bad surface-active characteristics, coalescence is low. Furthermore, partly miscible solvents can be used in emulsion precipitation and evaporation conditions can be changed. In this case, the removal of volatile solvents can be done by distillation [25].

3.8 Mechanochemical Process Mechanical attrition is a technique established in 1970 to produce fresh alloys and phase mixtures from powder particles as an industrial process. The quantity limitations for nanocrystalline preparation can possible be controlled through this technique; therefore, nanocrystalline powders can be produced on large scale. It also provides many options in the preparation of various structures in nanostructured powders such as crystalline–crystalline or amorphous–crystalline and atomic bonding like metal–metal, metal–semiconductor, and metal–ceramic materials. The most significant benefit is that mechanical milling can be performed at lower temperatures, allowing the newly shaped grains to grow very slowly [6]. The high mechanical force leads to the destruction of materials and creates a distinct structure. Mechanochemical technique has been commonly used in advanced material synthesis, covering nearly all aspects of material science.

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Planetary ball mill is a typical mechanical process whereby the planetary ball mill is used for grinding of the materials by rotating the big plate continuously and simultaneously rotating the containers. The speed of the plate revolution and the speed of the container rotation can be adjusted independently [26]. Particles larger than 100 nm cannot, therefore, be excessively cold welded. As a consequence, the average grain size can be reduced by up to 2–20 nm. The mechanism of formation of microstructure is very distinct from other techniques of synthesis, but it is very comparable to the final microstructure. Due to mechanical attrition, a severe plastic deformation happens, resulting in a constant refinement of the inner composition of the powder particles to nanometer scale [27]. Mechanical milling is susceptible to contamination; it is possible to use atmospheric control for chemical reactions between atmosphere and milled powders. This has resulted in a novel and cost-effective technique for production of nanopowders. A conventional ball mill can be used as a chemical reactor at low temperatures for mechanochemical processing. Ball milling improves the reaction kinetics of the reactants due to intimate mixing and refinement of grain structure to nanometer scale. Since reaction takes place during actual milling, a suitable reactive gas (oxygen or nitrogen, atmospheric air) and a suitable precursor are used during milling stage. A variety of metals can be used in reactive milling where metal powders are transformed to nanocrystalline metal–ceramic composites [6]. Reducing the process cost and product industrialization can be achieved using a variety of precursors such as carbonates, fluorides, oxides, hydroxides, sulfates, chlorides, etc. For mechanical attrition, various ball mills such as tumbler mills, shaker mills, planetary mills, vibratory mills, etc. have been developed. The material powder is put in a sealed container with tungsten carbide or steel coated balls. Kinetic energy of balls depends on their mass and velocity. Since steel or tungsten is high-density materials, they are preferred as a milling material [6].

3.9 Sonochemical Technique Sonochemistry is a technique in which the application of strong ultrasound radiation causes molecules to undergo a chemical reaction. Acoustic cavitation, which involves the formation, growth, and fall of bubbles in an ultrasonically irradiated liquid, is the driving force responsible for the sonochemical process. Although in the construction of reactors cavitation is prevented, still acoustic cavitation is the key to sonochemical processing owing to its capacity to regulate and restrict its impacts to the reaction rather than the reactor. This technique has been used widely to create extraordinary characteristics of nanosized materials. Unique process conditions such as elevated temperature, elevated pressure, and elevated cooling rates make it easier to form smaller particles and different product sizes compared to other techniques. In conducting sonochemical experiments, the primary benefit is that it is very cheap [6].

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Fig. 10 Schematic of a chemical vapor deposition reactor

3.10 Chemical Vapor Deposition Technique Chemical vapor deposition (CVD) is referred to as activating chemical reactions between a substrate surface and a gaseous precursor in order to deposit a thin solid film onto the substrate. Surface activation and growth activity can be achieved either with increased temperatures (thermal CVD) or with plasma (plasma-enhanced CVD). A PECVD process is carried out at significantly lower temperatures compared to the thermal CVD process [6]. CVD is a widely used technique in material processing industry due to its low set-up cost, high production yield, and ease of scale-up. As shown in Fig. 10, CVD has been developed as a novel manufacturing process in many industrial sectors such as semiconductors and ceramic industry [28]. A typical CVD process involves the mass transportation of reactants delivered by the gaseous or liquid precursor to the substrate surface, adsorption of reactants at substrate surface, chemical reaction of the reactants at the substrate surface to form thin film, desorption of by-products of the reaction from substrate surface and removal of by-products and unreacted reactants. The deposition rate of a film in CVD process is mainly controlled by the slowest step in this serial process [16]. Since adsorption and desorption at the substrate surface usually occur at relatively faster rates, the deposition rate is anticipated from the competition between mass transportation of reactants in the first step, by-products in the last step, and the surface reaction rate in the third step. The whole process is illustrated in Fig. 11. Mass transportation is a function of gas flowrate and its partial pressure while surface reaction rate is an exponential function of process temperature [16].

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Fig. 11 Illustration of a typical CVD process involving five growth steps

The chemical response of the surface is dominant at elevated temperatures. As soon as it hits the surface, the precursor will react with the substrate and form a thin film. Therefore, mass transportation controls this process. By raising the supply of reactants such as gas or liquid precursors, mass transportation can be improved and consequently the rate of deposition. Usually, the deposition occurs at various nucleation sites and expands and coalesces together. Since deposition rate is governed by the mass transportation, if the surface has a pronounced topography, the film tends not to be conformal and the deposition rate is also usually difficult to control. On the other side, the surface reaction rate is slow for a low-temperature regime and becomes the dominant phase. In this case, the deposition rate will remain the same regardless of any change in the flowrate of reactants. The surface will have an adequate supply of reactants due to the slow reaction rate, and reactions generally occur concurrently in all surface fields, resulting in a very uniform and conformal deposition. However, if the temperature is too low for thermodynamic reasons, the reaction may not happen at all. It reveals that for a CVD method, selecting the right temperature is of significant importance. CVD reactors are classified into normal CVD reactors and PECVD reactors. In a normal CVD process, reactants can be either gases or liquids and there can be more than one reactant. The reactant generally has a thermal decomposition reaction at the surface of the substrate if there is only one reactant involved in a CVD process. If more than one reactant is involved, these would be the precursor and a reducing agent. The normal CVD reaction temperature varies from case to case; however, it should be high enough to initiate the decomposition of the precursors and activation of the substrate surface [16]. Like a PVD method, DC or RF plasma is also used in many modern thin-film deposition methods involving CVD reactors. The plasma helps in breaking down the precursors before they reach the surface of the substrate. Therefore, it considerably

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reduces the surface temperature needed for the chemical reactions. The plasmas are generated and sustained under very low pressures [16]. A typical CVD reactor includes gas delivery system, loading/unloading system, reaction chamber, vacuum system, energy system, process control system, and exhaust gas treatment system. The gases are fed into the reaction chamber at temperatures in the range of 500–1200°C. Other than the precursor gas, argon and nitrogen are used as carrier gases. When gases come into contact with the heated substrate in the reactor they react and form a strong layer on the surface of the substrate. The critical operating parameters of this method are the temperature and working pressure [6]. Finally, CVD is being widely used for synthesis of carbon nanotubes and is the most promising technique as compared to arc discharge and laser ablation techniques. Recently, a swirled floating catalyst CVD reactor is developed for up-scaling nanotubes’ production capacity. A vertical reactor was placed inside a furnace and cyclones for collecting the final product [6]. It is worth mentioning that although CVD is a flexible technique, it requires numerous experimental works for standardization of the appropriate growth parameters [6].

4 Applications of Nanomaterials Nanotechnology is being implemented through a range of applications from optical communications, electronics, biological systems, and automotive engineering to new materials. Many possible applications have been explored and a number of nanomaterials’-based devices and systems have been studied previously [29, 30, 36, 37]. These applications are based on (i) the peculiar physical properties of nanosized materials, for example, gold nanoparticles used as inorganic dye to introduce colors into glass and as low-temperature catalyst, (ii) the huge surface area, such as mesoporous titania for photo-electrochemical cells, and nanoparticles for various sensors, and (iii) the small size that offers extra possibilities for manipulation and room for accommodating multiple functionalities [5]. Future applications of nanomaterials include next era laptop chips, better insulation materials, kinetic power penetrators, lethal weapons, phosphors for high definition televisions, low cost flat panel display, cutting tools, pollution remediation, high-density materials, powerful magnets, high performance batteries, highly sensitive sensors, aerospace additives with superior performance, advanced weapon platforms, satellites, medical implants, machinable ceramics, drug delivery, smart buildings, etc.

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5 Conclusions While replication of natural phenomena is one of the most promising research areas of nanotechnology, researchers are still attempting to understand their amazing complexities. Nanotechnology is a rapidly expanding area of research where novel properties of materials manufactured on nanoscale can be used to benefit humanity. Nanomaterial always performs better than their bulk counterparts due to some tunable physical and chemical properties. By selecting suitable conditions and synthesis method, it is possible to tune the melting point, electrical conductivity, wettability, thermal conductivity, light absorption, catalytic activity, and scattering properties of nanomaterials. Conclusively, chemical vapor deposition provides good control over structural parameters during synthesis of nanomaterials. This method can be extended from laboratory to industrial scale due to its inherent simplicity and low process cost.

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Perspective of Nanomaterials in the Performance of Solar Cells Hafiz Muhammad Asif Javed, Wenxiu Que, Muhammad Raza Ahmad, Khuram Ali, M. Irfan Ahmad, Anam ul Haq and S. K. Sharma

Abstract Solar cells have a great promise to solve the world energy crises in a sustainable way. In recent years, numerous efforts have been devoted on different aspects and performance of solar cells with a common task of achieving higher efficiency to compete with the traditional energy resources. Cost-effectiveness and enhancing power conversion efficiency are the major tasks in the photovoltaic technology. New field of nanotechnology has developed promising possibilities to improve the quality, stability, and performance of solar cells. Nanomaterials can contribute to solar cell design in different ways, which play an important role in their performances. Developments of nanomaterials-based solar cells could reduce the cost and stability for bulk power generation as well as enhance the power conversion efficiency. This book chapter reviews the performances of traditional solar cells and focuses on different contribution of advanced nanomaterials in solar cell advancement. Keywords Generation and classification of solar cells · Nanotechnology · Performance parameter · Stability · Light harvesting efficiency

H. M. A. Javed · K. Ali · M. Irfan Ahmad · A. Haq Department of Physics, University of Agriculture Faisalabad, Faisalabad 38000, Pakistan H. M. A. Javed (B) · W. Que (B) Electronic Materials Research Laboratory, International Center for Dielectric Research, Key Laboratory of the Ministry of Education, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China e-mail: [email protected] W. Que e-mail: [email protected] M. R. Ahmad Center for Advanced Studies in Physics (CASP), GC University Lahore, Lahore, Pakistan S. K. Sharma Department of Physics, Faculty of Science and Technology, The University of the West Indies, St. Augustine, Trinidad and Tobago © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_2

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1 Introduction The demand for energy is swelling year to year in all over the world and the handling of this demand will be the biggest challenge in the near future. Particularly, energy demand has become a serious issue in third world countries. These countries are focusing to find the proper solution of this issue because it is considered very important for their development. Scientists and researcher are more interested in renewable energy sources than the non-renewable energy sources. Out of these renewable energy sources, solar energy is the most interested area of the researcher. The use of solar energy is always considered as the most beneficial, infinite, and environment-friendly source [1]. Researchers are trying to find an inexpensive and clean energy method to use the solar energy [2]. The most common effort for fulfilling the demand of energy is the use of solar cells which convert the sunlight into electrical energy [1, 2]. The traditional solar cell is consisting of silicon wafers which traps the fallen sunlight on it and converts it into electricity. Researchers are trying to synthesize the new materials that are inexpensive and eco-friendly. Generally, these materials have different physical and chemical characteristics which depend upon their size and shape. When their size is less than 100 nanometers (100 nm), they are considered as nanoscale or nanotechnology. So, at nanoscale, their magnetic, chemical, optical, and electronic properties along with reactivity response are entirely different from their bulk scale. Due to these qualities, the concept of nanotechnology or nanoparticles (NPs) is used in the manufacture of solar cells as it reduces the manufacturing costs as a result of a low-temperature processing similar to printing instead of the high-temperature vacuum deposition process typically used to produce conventional cells made with crystalline semiconductor material. These NPs have also reduced the installation cost of solar cells because at nanoscale they can be rolled like a sheet which is not possible with conventional crystalline panels. This rolling characteristic has been developed in solar cells due to semiconductor thin films. Currently, available nanotechnology solar cells are not as efficient as traditional ones. Researchers have improved the efficiency level using quantum dots [3]. The conventional solar cells depend upon the silicon wafers are known as firstgeneration solar cells and have the power conversion efficiency of about 25.6%. The second-generation solar cells were fabricated with thin films and CdTe-based solar cells have the efficiency of about 19.6% [4]. Third-generation solar cells are also called nanomaterials-based solar cells, e.g., dye-sensitized solar cells (DSSCs), quantum dot solar cells, hybrid solar cells, organic solar cells, and perovskite solar cells (PSCs). DSSCs and PSCs are unique due to their performance in the research field of photovoltaic and can approach the theoretical efficiency limit [5].

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2 The Generations and Classifications of Solar Cells 2.1 Solar Cell Technologies 2.1.1

First-Generation Solar Cell Technologies

First silicon-based solar cell was manufactured in Bell laboratories 1954 with 6% efficiency. Solar cells based on silicon materials are the mostly used according to the single-cell PV device, more sufficient element on earth is silicon. Silicon is a semiconductor material and it is appropriate for photovoltaic applications with 1.1 eV energy bandgap. Crystallites silicon solar cells are divided into three main types based on arrangement of Si wafers. Different types of silicon are used to make the different solar cells with different efficiency, specifically: • Polycrystalline (poly c-Si) • Monocrystalline (mono c-Si) • Amorphous silicon cells. The earliest technology of solar cell is still very important and valuable for solar cells. Solar cells are manufactured from thin silicon wafers. These solar cells are called monocrystalline solar cells. Early in 1963, c-Si modules were produced commercially, when Sharp Company of Japan started the manufacturing photovoltaic modules commercially and produced a 242 W PV [5]. This solar cell has more efficiency (up to 26%) which means more electric energy from the specific area of panels. Czochralski process is used to single crystal wafers of silicon; it covers about 30% of the PV market. Amorphous and polycrystalline silicon are less pure than that of a single layer of crystalline silicon, less expensive, and most favorable. Monocrystalline solar cells are much expensive as compared to the polycrystalline materials because this is made by a single layer of crystal. Polycrystalline silicon solar cells have the highest efficiency of about 21% [6]. Silicon solar panel typically contains two layers: a p-type positive layer and n-type negative layer. The pure crystalline silicon is a poor conductor of electricity because none of its electrons are free to move about. Other atoms are purposefully mixed in with the silicon atoms, such as phosphorous that have five electrons in its outer shell. It bonds with its silicon neighbor atoms, but in a sense, the phosphorous has one electron that does not have anyone to hold hands with. It does not form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place [7]. Impure silicon with phosphorous atoms takes less energy to knock loose one of our “extra” phosphorous electrons because they are not tied up in a bond. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called N-type because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon. The other part of a typical solar cell is doped with the element boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type has free openings and carries the opposite (positive) charge [8].

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Second-Generation Solar Cell Technologies

The second generation of solar cell is also called thin film solar cells. In this generation, solar cells are manufactured from very thin layers in few micrometers. Thin film solar cells could actually deliver electricity with lower cost than c-Si wafer-based solar cells. Thin films can be arranged into lightweight and flexible structures, which easily combined into structural component building-integrated PV (BIPV) [9]. There are three basic types of thin-film solar cells that have been commercialized: • Amorphous silicon • Cadmium telluride • Copper indium gallium selenide (CIGS). Numerous tiny electronic devices and calculators can be operated by amorphous silicon solar cells. A biggest disadvantage of amorphous silicon-based panels is that those are less efficient per unit area. There are limited major differences between second-generation solar cells and first-generation solar cells. The most prominent difference is the direct bandgap and the indirect bandgap types of the semiconductor materials, but both types depend on the p-n junction model. Copper indium gallium selenide (CIGS) solar cells, CdTe solar cells, and amorphous solar cells have highest efficiency of 21%, 21.4%, and 11.8%, respectively [8–13].

2.1.3

Third-Generation Solar Cell Technologies

The third-generation solar panels are totally changed from other first and second generations as their efficiency do not depend on the design of p-n junction. The third generation of solar panels is being fabricated by different types of nanomaterials, polymers, and organic dyes. First-generation solar cells are very costly and toxic. On the other hand, in the second-generation solar cell, a very limited number of materials are available for second-generation solar cell. Recently, scientists are working on third-generation solar cells to improve the power conversion efficiency by developing new nanomaterials and techniques. In 1991, Gratzel introduced a new kind of solar cells, called Gratzel cells. Later on, they are called dye-sensitized solar cells. The main parts of dye-sensitized solar cells contain a nanostructured semiconductor oxide, an organic dye sensitizer, a redox electrolyte, and a counter electrode. Nowadays, studies on the features of dye-sensitive solar cells are very extensive to overcome the production cost and to get very high performance. Performance of solar cell depends on many parameters, for example, size of particle, bilayer TiO2 photo-anode, nature of organic dye, and liquid electrolyte. However, due to the liquid electrolytes in dye-sensitized solar cell, many difficulties are observed, for example, short-term stability due to evaporation of organic solvent, leakage, electrode oxidization, and limited inorganic salts solubility [14–20]. The most advanced type of third-generation family is perovskite solar cell. Perovskite solar cells demonstrate more advantages than other solar devices, due to their transparency, simple manufacturing process, cost-effective, high efficiency, environmental friendly, and flexibility.

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Perovskite solar cells perform very well in terms of factors such as lifetime, efficiency, and recombination rate under laboratory conditions as compared to other solar cells, so the price could be controlled for commercial applications [21–30].

2.2 Classifications of Third-Generations Solar Cells 2.2.1

Organic Solar Cells

The organic solar cells practice organic nanomaterials that manage semiconductors, organic polymers, and organic elements, which deliver power and lightweight. It is also known as flexible solar cells that use electrical phenomenon impact to generate power. Organic solar cells are actually manufactured from organic chemical compound materials [31–34]. Organic solar cells have attained importance due to their unique parameters, such as less weight, flexibility, and affordable. Researchers analyzed and mentioned the 3D organic photovoltaic cell structures with rear connection electrical diversions that have a complete extended capability [35–40]. Scientists introduced an advanced model of organic solar cell with graphene, and the graphene is synthesized by different strategies, e.g., a biochemical vapor method, a surface oxidization treatment at the earlier step of the graphene advancement. After that, the graphene was turned to polythene terephthalate substrate. In addition, researchers introduced an advanced structure of organic solar cells by which effectiveness has extended from 5.8 to 7.11% in integrated Au/Ag bimetallic nanocomposite which is highly sensitive plasmonic material [41–45]. The Au/Ag bimetallic nanocomposites distinctly feature converging dual plasmon resonance peaks to a single plasmon resonance peak, strongly depending on the packing density and the unit size. It is expected to achieve high open-circuit voltages in organic solar cells. The best result of voltage attained after several attempts is 0.44 V, and the efficiency improves from 3.1 to 4.4% [44–50].

2.2.2

Dye-Sensitized Solar Cells

Dye-sensitized solar cell (DSSC) was invented by Michel Gratzel and his colleagues in 1991 [44]. The DSSC is still in evolutionary process. Researchers are improving the electrical properties and performance of DSSC by incorporating the suitable materials into TiO2 film. Recently, Fe3 O4 has earned good name in the field of biomedical and as a photocatalyst because of its high specific surface area, biocompatibility, and good dispersion. Now, this material (Fe3 O4 ) has attained attention of researchers to modify the optoelectronic devices. There are many reports of enhancing the photovoltaic properties in DSSC [45–50]. So, this new DSSC has produced power conversion efficiency of 3.54% [51]. Some researchers used pigment of TiO2 nanoparticles and multiwall carbon nanotubes in DSSCs. The photo-anode carbon-based TiO2 has also

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been reported for DSSCs [52, 53] by achieving the capability of 5.45% that is 61% more as compared to untreated photo-anode. Multi-walled carbon nanotubes (MWCNTs) are hollow, cylindrical-shaped allotropes of carbon that have a high aspect ratio (length to diameter ratio). MWNTs consist of multiple rolled layers of concentric nanotubes of graphene inside other nanotubes. Carbon nanotubes can also be effective semiconductors with the right arrangement of atoms. Researchers have created discrimination in the photocurrent –voltage characteristics of DSSCs by using chemical treatment of multi-walled carbon nanotubes (MWCNTs) on TiO2 films [54–59].

2.2.3

Nanocrystals Solar Cells

Nanocrystal solar cells (NSCs) are fabricated on a substrate with a coating of nanocrystals. The substrates are generally silicon or various organic conductors. In spite of considerable approach, the nanocrystals solar cells were not suitable for bulk scale production due to lower efficiency, high cost and effectiveness above Si based Solar cells [60, 61]. Researchers introduced a nanostructured semiconducting material on exterior of crystalline Si(C-Si) by metal-helped biochemical engraving (MCE) method [62]. Structure is most fitting to stop the ultraviolet light in lightweight mode. Nanocomposite is enclosed by zinc oxide with traditional dye [Laali, Zobo] and designed dye, dye employed nanocomposite exhibits a lot of vital activity.

2.2.4

Polymer Solar Cells

Polymers-based solar cells are also the part of third generation and also named as plastic solar cell. The recorded highest energy conversion efficiency currently is 10% [63]. In order to further improve the performance, new polymers with various molecular structures and their applications in photovoltaic devices are under intensive investigations. These cells are fabricated which have greater skillfulness with ease of fabrication. Regardless, it needs further advancements to extent efficiency since it is so much lingering behind the normal solar cells [63–68]. The researchers define a unique poly (3-hexylthiophene)(P3HT)/C-70 photoactive composite by heptane/o-dichlorobenzene and modified effectiveness of 24% was achieved [69]. Nickel/Titania nanocomposite with poly(3-hexylthiophene)/[6,6]phenyl-C70-butyric has been achieved, which could be used for chemical optoelectronic devices. Execution of polymer chemical compound-based solar cell is much better by presenting gap conditions among dynamic coated metal sheets.

2.2.5

Perovskite Solar Cells (PSCs)

Perovskite is unique in all photovoltaic devices to build up an advanced PV structure. Perovskites are class of blends pictured by the structure ABX3 , where X addresses

Perspective of Nanomaterials in the Performance of Solar Cells

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halide group as anion, for example, I− , Br− , Cl− , A and B are cations of novel estimations [70]. The distinctive resources are employed to fabricate the device and focused on the importance of perovskite solar cells [71] by continuing progressions in perovskites solar cells and thus lead-free salt perovskites [72]. Perovskite solar cell was organized by comprising Si p-i-n nanowire cluster loaded up with alkyl radical ammonium ion lead halides. The efficiency of 13.3% was obtained as a result of their additional features and lightweight [73].

3 Nanomaterials- and Nanocomposites-Based Solar Cells Nanoscale devices and objects have provided a revolutionized opening in the field of solar energy. Now, the conversion of solar energy has become highly efficient and low cost through nanostructured devices. New challenging tasks associated to demonstrate high level of efficiency and stability are now being addressed in the research community. Nanostructured materials have recognized themselves as an inexpensive energy absorbing option and have overridden the traditional resources. These materials have ability to manipulate light and control energy flow at nearly the atomic level. Nanostructured solar cells are a type of third- or next-generation solar cell and include those that are based on nanostructures and/or nanostructured interfaces such as nanowire, mesoscopic, and quantum dot solar cells as shown in Fig. 1a–c. They hold great promise toward new approaches for converting solar energy into either electricity (in photovoltaic devices) or chemical fuels [74]. The conventional solar cells are fabricated from bulk semiconductors. The semiconductor absorbs the light as a result of electrons and holes are generated. After creation of electrons and holes, they apart from each other and move toward different contacts to produce voltage V and current I and thus power (P = I × V ). These materials have large dielectric constants due to this property the electrons and holes are speedily partitioned from one another and do not interact. Nanostructuring eliminates the need of high-dielectric-constant semiconductors and opens the new paths for synthesizing new types of materials and for designing the new designs. For example, two dissimilar materials, where one is n-type (conducts electrons) and other is p-type (conducts holes), can be intermixed with nanoscale morphology. It is understood that absorption of light causes an excited state (exciton) that undergoes rapid charge transfer producing electrons and holes in separate phases, making their interaction less probable. Based on this mechanism, two types of solar cell are designed: organic photovoltaic devices (OPD) and dye-sensitized solar cells (DTSCs). Another next-generation approach for photovoltaics is based on semiconductor nanocrystals. The most important properties for photovoltaic applications are the strong size-dependence of the bandgap, and the large modification of the relaxation dynamics of photo-excited charge carriers that are created by the absorption of photons within energies larger than the band gap. The bandgap of the absorber layer could be optimized to improve the absorption of photons and to limits the output voltage of the solar cells. Because only the radiation with higher energy than the bandgap is

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Fig. 1 Potential benefits as well as of the scientific challenges that need to be overcome in nanostructured solar cells are a quantum dot solar cell, b nanowire solar cell; holes (h+ ) are extracted from the outer layer (red) and electrons (e– ) flow through the core of the nanowire (blue), and c mesoscopic solar cell

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absorbed, narrower bandgap materials absorb more solar photons, resulting in higher photocurrents. However, the output voltage is linearly proportional to the bandgap, and thus wider bandgap materials allow higher voltages. In the Shockley–Queisser analysis, there is an optimal bandgap that achieves the highest efficiencies and ranges between 1.2 and 1.4 eV. Semiconductors with bandgaps lower than 1 eV are generally not employed in single-layer solar cells. Quantum confinement effects in quantum dots (QDs) can increase the bandgap by more than 1 eV compared with the bulk value, expanding the range of semiconductor materials viable for photovoltaics. A prototypical example is PbS. Bulk PbS has a bandgap of 0.4 eV but PbS QDs can have bandgaps from ~0.6 to ~2 eV depending on their size. Quantum dot solar cells (QDSCs) (Fig. 1a) are treated from colloidal suspensions at ambient air temperatures and relatively have low fabricating costs. One approach to relax such requirements has been explored by using radial p-n junction Si nanowires. A core-annular p-n junction (Fig. 1b) is created along the length of the wire. When the incident light generates charge carriers at the junction, minority carriers only need to traverse the nanowire diameter in order to be collected. With less strict requirements on the minority carrier lifetimes, lower-grade silicon can be used. Two different substances, first is n-type and second is p-type, are combined at nanoscale structural morphology (Fig. 1c). Sweetening of NMs as energy resources has tough excellent headway, particularly within zone of top-notch energy gathering solar cells. For quick progression as for energy process and developments, distinctive methodologies are planned for the arrangement which has set a trend to use of nanocomposite materials widely. New methodologies are in use for upgrading the reaction rate in batteries anodes [75]. Nanomaterials will grasp reversible reaction, create a hopeful instrument for fresh energy, and ensure large storage cutoff points. NMs have benefits of wellbeing ecofriendship with supportability attitude and have potential for money saving [76–84]. Different materials are tried as photo-anode in DSSC. Titanium dioxide is the most abundantly used photo-anode in DSSC due to its unmatched characteristics. The band structure of Titania describes the valence band composed of O 2p states and the conduction band mostly consists of Ti 3d states. At the same time, the density of states shows other less important contributions, notably from Ti 3d states in the valence band [85–88]. Titanium dioxide is most fitting for DSSC because of its greater physical phenomenon band edge, exterior region, dyes filling, and lepton disposition. Titanium dioxide nanoparticles based solar cells generally show the efficiency of about 13% that was the most amazing among the other materials [88]. Zinc oxide is another wide bandgap semiconductor large lepton flexibility that is most correct for photo-electrode in dye-sensitized solar cell [86]. Nb2 O5 -based cells indicate high electric circuit potential and effectiveness on account of the nice physical phenomenon band edge [88]. Among the most Nb2 O5 based cells, a proficiency of sixth has been practiced to this point utilizing nanorods [89]. Different nanostructured metal oxide, for example, Al2 O3 , SnO2 , V2 O5 , ZrO2 , CeO2, and Fe2 O3, are analyzed as photoanodes in DSSC’s [90–93]. Dopedsemiconductor materials exhibit an improvements in I-V characteristics because of different band edges.

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Many researchers reported that one of the planner selective layers of mesoporous electrons is used for simplification of perovskite solar cells for constructing the solar panels art large scale. The conversion efficiency power can be enhanced in perovskite solar cell by doping the titanium dioxide with electron selective layer (ESL). In this way, titanium dioxide limits the conduction band. It can be done with different techniques. The simplification of perovskite solar cells (PSCs), by replacing the mesoporous electron selective layer (ESL) with a planar one, is advantageous for large-scale manufacturing. PSCs with a planar TiO2 ESL have been demonstrated, but these exhibit unstabilized power conversion efficiencies (PCEs). The planar PSCs using TiO2 are naturally limited due to conduction band misalignment. It has demonstrated with a variety of characterization techniques. Latest studies showed the potential of SnO2 -based ESLs. These devices have not shown high efficiency without hysteretic behavior. For the first time, SnO2 achieves a barrier-free energetic configuration, obtaining almost hysteresis-free PCEs of over 18% with voltages of up to 1.19 V [94]. Many researchers have reported to prepare lithium doped titanium dioxide nanoparticles by hydrothermal method with assist of ionic liquids. For organic oxidation, potential is consumed as photocatalysts. For the good assistance of ionic liquids, Cr6+ , a toxic element, is converted into Cr3+ , a non-toxic element. The microstructural studies of Li doped titanium dioxide nanoparticles are investigated through Fouriertransform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The optical properties are observed with UV-Visible spectroscopy. The synthesized Li doped titanium dioxide has anatase phase. The photocatalytic activity of Li-TiO2 nanoparticles results showed better degradation as compared to the TiO2 nanoparticles [95]. It is reported that three-dimensional (3D) perovskite structures are not much stable than that of two-dimensional (2D) perovskite structure due to organic legends which acts as a counterpart. A method in which 3D perovskite structure is chemical reacted with n-butylamine (BA) to produce the rational structure of 2D and 3D structure. Different forms of 2D perovskite layers are produced by different mechanism with the reaction of n-butylammonium iodide (BAI). (BA)2PbI4 was produced by the chemical reaction of n-butylamine (BA) and MAPbI. The 2D perovskite mixture has less protection than the (BA)2PbI4 due to the number of organic ligands. BA treatment has much smoother two-dimensional perovskite layer on three-dimensional layer that results good exposure. The stacking structure of two-dimensional and threedimensional structure shows the better photovoltaic stability as compared to threedimensional counter parts to bear the heat stress [96–107]. The inorganic solar cells provide some central focuses over regular daylight-based cells regarding soundness, assimilation properties, lifetime, and capability with less reliance on social gathering adapt [109–113].

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4 Central Features of Nanomaterials Nanomaterials can be categorized into three types according to their source: natural, incidental, and engineered. The engineered nanomaterials (NMs) are specially designed for many commercial goods and processes. These NMs have many applications, such as stain-resistant clothing, cosmetics, sunscreens, electronics, sporting goods, tires, as well as many of real-life daily items, and are used in medicine for purposes of diagnosis, imaging, and drug delivery. Nanophase engineering has flourished rapidly and is producing variation in the structure of materials. Nano-engineering has capability to manipulate the catalytic, mechanical, optical, electrical, magnetic, and electronic functions in organic and inorganic materials. The final productive outcome is the designing of advanced nanomaterials, which is mostly based on the functional and efficient nanostructures [115–118] (Table 1). Important applications of nanomaterials have been tabulated in Tables 2 and 3 along with practical illustrations. Ideally, these postulates show the convenient application, notably in several cells as well as battery-controlled metal cells, skinny film cells (e.g., adaptable, printable, or perhaps clean cells), fluid powered cells, reaction stream cells, customary cells, super-capacitors, and sun-based/imperativeness sections [119–125]. In any case, the running with subjects still ought to be essentially surveyed, notably regarding the usage of ordinary materials, such that common place standard and inorganic materials for imperativeness applications [126–136]: • Failure to change inanimate assertion frameworks to trademark constituents because of complexities related with configuration incited or seeded progression dealing with methodology. • Influence of electric arrangement and charge carriage properties on atomic introduction and requesting at the trademark material boundary. • Restricted stimulating spread sizes owed to vague or semicrystalline pattern of standard materials. • Trouble with respect to monitoring sub-atomic introduction, pressing, and crystallization at the substrate boundary owed to imperfect working of typical and inorganic constituents.

5 Focal Points of Nanostructured Materials Nanotechnology is an approving advancement that provides larger opportunities for selecting the linked matters. The creation parts and machines are less than 100 nm providing the fine styles to manage an imperative and safe trade. It can help to overcome current performance barriers and substantially improve the collection and conversion of solar energy. A number of nanoscale physical phenomena have been identified that can improve the collection and conversion of solar energy. Nanoparticles and nanostructures have enhanced the absorption of light, increased the conversion of

Nanostructure structures

Nanospheres

Nanowires

Nanotubes or nano-arrays

Porous 3D nanostructured networks

Nanofilms, nanosheets, or nameplates

S. No.

1

2

3

4

5

Polyaniline, graphene and its composites, MOFs, etc.

MnO2 core/PEDOT shell coaxial nanowires, Cu nanowires with Fe3 O4 , etc.

Si, SnO2 , TiSi2 , etc.

Sn, SnO2 , Sb, Fe3 O4 , LiFeO4 , etc.

Examples

Table 1 Primary highlights and focal points of nanostructured materials

High likelihood of rational applications in light of clear electron and molecule trade given by high essentialness densities

interrelated pores and nanothickness suggest high molecule change, little molecule scattering lengths, simple electron and molecule get to (in view of broad imperativeness thickness by methods for the high surface region), high mechanical characteristics with long cyclical times, and great rate capacities

High molecule movement through straightforward invasion of the electrolyte, which can garbs fluctuations in the material in the volume midst of cycling

Improved energy over steady electron pathways

Massive negated volumes and connective electron paths consider fast electron and molecule trade and moreover keeping up a vital separation from aggregate with the amazing pads related with volume changes

Role

[125, 127, 129]

[116, 117, 121, 125]

[75, 118, 120, 135]

[118, 120, 123, 124]

[115, 119, 120, 123]

References

36 H. M. A. Javed et al.

Nanostructure course

1-D

2-D

3-D

S. No.

1

2

3

Mesoporous and micro porous materials (i.e., metal regular structures, 3D graphene, nanohybrid materials, etc.

Graphene carbon-secured nameplates, polymers, etc.

Carbon nanotube, carbon/metal-based nanowires/nanotubes, etc.

Resources/example

Indistinguishable advantages from 1D materials yet without a significant number of their confinements (i.e., high inner opposition because of little size and totals and higher perspective proportions without interconnections)

High explicit capacitance Enhanced cyclability Not thoroughly researched as an electrochemical call Energy stockpiling material

Very much well-ordered development simple and productive automated exchange. High ionic transition can oblige fluctuations in the material volume for enhanced cyclability. No requirement for additional covers or conductive constituents. Negated volume among adjacent ID NMs takes into account simple entrance of the electrolyte

Welfares for energy resources

Table 2 Arrangement of nanostructured materials for energy applications dependent on their auxiliary measurements

[79, 119, 136]

[77, 136]

[79, 84]

References

Perspective of Nanomaterials in the Performance of Solar Cells 37

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Table 3 A relative study of several solar cell materials [160, 161] III Generation Amorphous semiconductor

Nanocrystal

Dye sensitized

Polymer

Perovskite

Efficiency (%)

4–8

7–8

~10

~3 to 10

31

Temperature dependency

Stable at high temperature

Stable at high temperature

Not stable at very high temperature

Not stable at very high temperature

Stable at high temperature

Energy bandgap (eV)

1.1–1.5

~3

~3

~2

~1

Merits

Merits riveting capability is high, efficient acknowledgements to low price of materials and producing method

Versatile, tough

Versatile, tough

Versatile, tough

Versatile, tough, more potency

Energy (TWy)

~0.1

~5

~0.01

Demerits

Reliability is a smaller amount once it’s use for outside requests

Demand of fixing period and area is high

Demand of fixing period and area is high

Demand of fixing period and area is high

Demand of fixing period and area is high

light to electricity, and provided better thermal storage and transport. The technologies fall short of potential performance because of poor control over feature size and placement, unpredictable micro/nanostructure, poor interface formation, and in many cases, short lifetimes of laboratory devices. Among various contraptions, the sun-based pro, the imperativeness part, photocatalysis, and sun-based photovoltaic have utilized the nanomaterials to build the ability [134–136]. It is discovered that by utilizing nanomaterials, the scene radiation can be stretched out by various events while the ampleness of the sunlight-based gatherer has 10% higher showed up distinctively in connection to that of a standard measurement plate sun-based master. From the past research, it has been shown that nanotechnology is a principal asset for an enormous social occasion of the adjacent planetary system in help of able, sensible imperativeness change, aggregating, and confirmation, to the degree [137]. • Adapting the correspondence of light with materials which empowers the insignificant effort of semiconductors into contraptions, for example, photovoltaic. • Making progressively incredible photostimuli for changing over daylight into built fills. • Developing new materials and layers for encapsulating the packages on situational requests.

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• Creating the variable powers into electrical essentialness (and the alternate way), imperativeness and power thickness in cells. • Improving the capacity in zones from highlights and strong state. Most of the future progress in clean energy will be relating to nanotechnology. Scientists have achieved fruitful outcomes in advanced clean energy materials in recent years. The positive roles of nanostructures in clean energy research are well accepted, and there are still some fundamental issues debated in real applications. For example, high surface area leads to self-discharge, poor cycling, and calendar life of batteries and their inferior packing of nanoparticles also lead to lower volumetric energy densities. Small particle size of photocatalysts results in their poor cycling performance due to their instability [137–139]. In solar cell research, carrier multiplication is the key phenomenon for the excitation of multiple electrons from valence band to conduction band. This phenomenon is different from the theory of a conventional solar cell. The carrier multiplication principle takes place in nanostructure material and enhances the work. The carrier multiplication effect in quantum dots (QDs) can be understood as creating multiple excitons and is named as multiple exciton generation (MEG). MEG increases the energy conversion efficiency of nanocrystal-based solar cells [140–143]. Quantum confinement (QC) improves the Coulombic interaction that drives MEG [144, 145]. Down-conversion (DC) intends to utilize the free energy of photons with energy higher than the bandgap of the solar cell, which is otherwise lost to thermal reaction. In DC process, a separate material from the solar cell is used to split photons with energy at twice the bandgap energy into two lower energy photons, which are better matched to the solar cell’s bandgap. Photons with a lower energy than the bandgap is lost in a normal solar cell. The principle of the up conversion technique is that two or more photons are converted into a photon with energy higher than the bandgap energy. High energy photons will lose the energy above the bandgap energy limit. Downconversion is a process where a high-energy photon is converted into several lowerenergy photons with energies above the bandgap. This process intends to increase the current of the solar cell by increasing the number of absorbed photons imposed upon the solar cell while retaining its voltage characteristics. This increase in current subsequently increases the overall efficiency of the system. The DC process can be considered as modifying the solar spectrum to better match the solar cell properties, as opposed to changing the solar cell itself, enabling the efficiency increase of the underlying solar cell beyond the Shockley–Quiesser limit [139–149].

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6 Performance Parameters of Nanomaterials-Based Solar Cells 6.1 Stability Solar cells are lightweight, flexible, and they boost light harvesting by absorbing lights. Although special attentions were given to the existing solar cells in terms of efficiency, stability, and cost, but still, there is great demand for novel technologies that can simultaneously improve both PCE and stability while using an inexpensive electrode material. An imaginative golden triangle is shown in Fig. 2 for a better understanding of the technical gauge for commercialization photovoltaic technology. Silicon photovoltaic solar cells are looking to capture the 90% of the total market because of their excellent efficiency of 21% with lifetime of 25 year more at reasonable cost. On other hand, nanomaterials-based solar cells have high efficiency more than 23% and low manufacturing cost, with considerable half life of that crystal structure. However, perovskite solar cell has problem of stability. So for one year is a longest life time of nanomaterials-based solar cell which is very short time as compared to 25 years. Many factors are involved in determining the lifetime of perovskite solar cell and can be divided in two groups. One is called extrinsic and other is called intrinsic. Extrinsically, environmental condonation such as oxygen and moisture can be removed or controlled by encapsulation but the most important critical issue is the large size of perovskite materials. For intrinsic, there is the movement of charge at interface between layer and perovskite. There are three main intrinsic factors, i.e., instability of perovskite hygroscopicity, ion migration, and thermal instability [150–189]. Fig. 2 Golden triangle of solar cell performance

Perspective of Nanomaterials in the Performance of Solar Cells

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6.2 Light Harvesting Efficiency Harvesting of light is a performance parameter of solar cells. The effective harvesting of sunlight has become a big challenge in the field of PV solar cell. The understanding to increase the path length of photon and for photoexcitation improvement in less absorbing material like that of silicon is helpful for knowing the optimum operation of this structure. The resulting bandgap of photonic in solar cell cannot achieve harvesting efficiencies. Light harvesting of solar cell is similar to the process of photosynthesis. The process of separation of charges is similar like photosynthesis process which is photochemical reaction. The ability to capture the solar energy cannot occur by recombination of separate charges. The light harvesting at semiconductor junction is the process at nanoscale at which light interacts with matter. The conversion of solar energy to electrical energy by using photovoltaic (PV) cells is one of the key elements of the future energy supply of mankind. By optimizing the optics, the number of photons which are converted by the semiconductor material can be maximized. First of all, optical approaches are necessary to avoid losses by reflection or unwanted absorption which does not generate charge carriers. The light has to be trapped efficiently within the cell. And the photon energy is far away from being optimized. Due to the large range of wavelengths in the solar spectrum almost all photons have the wrong energy because the photon energy may exceed the bandgap energy of the semiconductor or fall below that. In the first case, the photon generates an electron-hole pair and the excess energy is thermalized, and in the second case, the photon is transmitted or absorbed without generating an electron-hole pair, e.g., by free carriers. Only photons with exactly the energy of the bandgap of the semiconductor are generating electron-hole pairs with the maximum efficiency. Thus, the researchers are facing the need the spectral control of the incident photons. At all the basic levels, it is understood that more energy can be attained by separating into light capture charging in conversion photonic process. Different materials with different structures shape new technology which brings efficient and cheap solar cells in coming generation of solar cells [188–190] (Table 4).

6.2.1

X-Ray Diffraction (XRD)

X-ray diffraction (XRD) may be a standout among the foremost essential non-ruinous devices to study a large variety of issues ranging from liquids, to powders and precious stones. From analysis to creation and engineering, XRD is an irreplaceable technique for materials portrayal and internal control. XRD analysis is based on constructive interference of monochromatic X-rays and a crystalline sample: The Xrays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. Rigaku has engineered up the newest and most novel high-resolution X-ray diffractometer (XRD). Due to its versatile approach, it can be used for thin films, nanomaterials, powders, or liquids and allows mapping measurements within suitable samples [149].

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Table 4 A relative study of various photovoltaic cell materials [150–159] Parameters

I Generation

II Generation

Mono semiconductor

Poly semiconductor

CIGS

CdTe

Efficiency (%)

14–17.5

12–14

10–12

9–11

Temperature dependency

Not stable at very high temperature

Not stable at very high temperature

Smart at high and low temperature

Smart at high and low temperature

Energy bandgap (eV)

1.1

1.1

Eg > 1.2

1.5

Merits

Merits handiness of raw materials in masses, eco-friendly, decent in terms of consistency, steadiness,

Efficiency cheap related to mono-semiconducting material

Versatile, tough, high potency

Versatile, tough,

Energy (TWy)

~2.5

~2.5

0.02

0.02

Demerits

Interval intense and difficult producing procedure

Potency is a smaller

Amount riveting capability is less

Toxic to presence of Cd

Cost

Luxurious

Luxurious

Needs solely fiftieth of expense of traditional semiconducting material-based cells

Needs solely fiftieth of expenditure of customary semiconducting material-based cell

6.3 Higher Surface Area A typical silicon PV cell produced around 0.5–0.6 V DC current under open circuit and no load condition. The electronic circuit output of a PV cell depends upon the efficiency and size (surface area), and this is directly proportional to the intensity or glancing of the striking sunlight on the surface of cell. The surface area-to-volume ratio increases as the diameter of the nanoparticle decreases or vice versa. It means that when a given volume of material is made up of smaller particles, the surface area of the material increases. The working performance in a full sunny day, with commercially available PV cell with surface area of 160 cm2 will produce a power of approximately 2 W. But this can be changed if the sunlight decreases in a cloudy day

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during day time. Its intensity is 40% of peak value, and the cell would produce about 0.8 W of power. This can be taken in terms of length. Length is directly proportional to the resistance. Short distance creates lower resistance, so that smaller cells will waste less energy and be a little more efficient. That is why smaller size cell exceeds larger size cell because smaller size has greater efficiency [183–185]. Typically, silicon solar cells are about 5 or 6 inches square to match the size of the new silicon.

6.3.1

BET Analysis

Brunauer–Emmett–Teller (BET) theory explains the mechanism of physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of materials. In 1938, Stephen Brunauer, Paul Hugh Emmett, and Teller distributed the first article regarding the BET hypothesis within the Journal of the Yankee Chemical Society. The specific surface area of a powder is determined by physical adsorption of a gas on the surface of the solid and by calculating the amount of adsorbate gas corresponding to a monomolecular layer on the surface. Brunauer–Emmett–Teller (BET) theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of materials. Therefore, normal BET examination is usually crystal rectifier at the boiling temperature of N2 (77 K). The BET strategy is generally utilized in surface science for the computation of surface territories of solids by physical sorption of gas particles. The mixture surface territory express surface region in units of volume to boot the units of the monolayer volume of the adsorbate gas is Avogadro’s number [186].

6.3.2

SEM Analysis

Scanning electron microscope (SEM) pictures are useful for examining the fine structure of solar cells. Even in large area commercial devices, for example, an SEM photograph can show the depth of the rear surface aluminum alloyed layer. Many of the solar cell features are of the order of microns and so not possible to view with an optical microscope. An additional advantage of an electron microscope is its higher depth of field. With an electron microscope, it is possible to have the whole device in focus at once, whereas in an optical microscope at high magnification, only parts of the device will be in focus at any time [187]. SEM is also used as a diagnostic tool for analyzing the degradation of a polycrystalline photovoltaic cell. The SEM characterizes the surface morphology of hot spot regions (degraded) cells in photovoltaic solar cells. In recent years, production of hetero and multi-junction solar cells has experience tremendous growth as compared to conventional silicon (Si) solar cells. Thin film photovoltaic solar cells generally are more prone to exhibiting defects and associated degradation modes. To improve

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the lifetime of these cells and modules, it is imperative to fully understand the cause and effect of defects and degradation modes. Many researchers have used SEM to study poly-Si cells before, and after reverse biasing, the reverse biasing was done to evaluate the cells’ susceptibility to leakage currents and hotspots formation. After reverse biasing, some cells were found to exhibit hotspots as confirmed by infrared thermography. The surface morphology of hotspot regions was characterized using SEM. The preliminary result indicated that hotspots are formed in the regions of high inhomogeneity [188]. Before analyzing, the samples of solar cells were placed on the stubs using carbon double side tab. The JEOL JSM-6390LV (SEM) device is considered with an accelerated voltage of 12 keV. This uniform acceleration voltage helps to maintain uniform sample electron beam interaction across the sample morphology. The SEM micrographs revealed the defected region in the cell. Each region of interest is diagnosed with electron dispersive X-ray spectroscopy (EDX) for chemical composition analysis. The elemental composition at various regions plays a major part in PV cell defect formation. Though the elemental composition of the module might have change form, due to impact ionization, moisture and electrolytic process after a long exposure in the case of outdoor deployment. For ancient imaging within the SEM, examples should be electrically semiconductor, in any event at the surface, and electrically grounded to stay the aggregation of electric charge. SEM should be extra ordinary clean with metal articles and conductively mounting to an example stub. Non-conducting materials are usually lined with an ultrathin coating of electrically conducting material, unbroken on the instance either by low-vacuum sputter coating or by high-vacuum evaporation. Semiconductive materials in current use as an example coating incorporate gold, gold/palladium compound, platinum, iridium, tungsten, chromium, osmium, and carbon. Coating with substantial metals might expand signal/clamor proportion for tests of low nuclear range (Z) [188].

6.4 Recombination Rate Open-circuit voltage and short-circuit current are greatly affecting by surface recombination. At the top of the surface, there exists a very high recombination rate which is mostly creating unfavorable influence on current of short circuit. The upper surface is also considered to the highest region of carrier generation in the solar cell. Reduction in upper surface recombination is consistently passivating layer insult by reducing the number of dangling silicon bonds at the top surface. The electronics industry mostly use passivate layer by growing dioxide layer on silicon. Silicon nitrate are mostly used as a dielectric layer in commercial solar panels [189]. However, silicon solar cells are using the passivating layer as insulator. The surface is passivated as silicon dioxide layer which cannot use on Ohmic metal contact. Instead, under the upper interactions, the influence of the recombination of surface can be reduced by enhancing the doping. While high doping creates damages

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Fig. 3 Techniques for reducing the impact of surface recombination

the diffusion length. The interaction areas do not involve in carrier generation and hence the effect on carrier gathering becomes unimportant. In addition, junction is closed by very high recombination area; possible doping is increased to lower the recombination option [190]. Figure 3 is illustrating the surface recombination mechanism.

7 Challenges in Nanostructured Solar Cells Nanostructured solar cell systems exhibit different properties and have allowed new ways of approaching solar energy conversion for electricity generation or fuels. The large surface-to-volume ratio of nanomaterials provides various benefits. Though nanostructured solar cells have many advantages, some limitations are still tagged with them. In general, there are two main limitations for every solar cell. Firstly, at night, solar energy cannot harvest. Secondly, all time solar radiated energy is not same. A large amount of energy is necessary to photovoltaic device to produce electric energy. Moreover, the intensity of solar energy is fluctuated in whole day. Solar radiations have to bear a lot of hurdles for reaching the surface of earth. These are traveling time from Sun to surface of earth, weather conditions, and location during summer season as to the winter season. The radiation of sun is very less intense in winter season. Scientists and researchers are trying to overcome these limitations of solar technology. They are trying to develop high-efficiency solar cell with capability of energy storage for night usage [191, 192].

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Nanostructured solar cells have attractive features for commercial point of view but these devices have also additional challenges. For nanostructured devices, improper passivation of internal surfaces can hinder long-term stability. Device stability is obviously an important consideration for photovoltaic technologies as longer the photovoltaic system operates, the lower is the total cost. Stability issues are being appeared due to the chemistry [193–195] or due to device configuration [196]. The commercial viability of the nanostructured solar cell product is made possible by encapsulation route which helps to settle the stability issues. Further efforts are being made on the stability and low maintenance. The overall cost and efficiency are kept in a satisfactory range of the customer [197]. The characteristics of nanostructures for photovoltaics are based on two approaches: (i) significant reduction in material usage and/or associated final costs; (ii) photovoltaic devices with a higher limiting efficiency than that determined by the Shockley–Queisser analysis. Both approaches, individually or in combination, can lead to significantly lower costs per kWh. For household energy consumption, the energy is measured in rated cost per peak watt. This energy depends upon the power conservation efficiency which is calculated by the fraction of the energy converted by solar cell to total energy irradiance. The total consumption cost per peak watt is two times greater than the module (cost per peak watt) and it is inversely proportional to power conservation efficiency. For calculating the overall cost per rate, another factor is also included that is the brightness of sun which changes day by day. For numeric value, the value of cost rate is calculated by multiplying the cost per peak watt with 0.05. Currently, photovoltaic conversion cost is 1 and the mean value of this cost 1-0-10$/watt which is comparable with the grid energy which is called grid purity [195, 196].

8 Summary and Future Perspectives A device that produces electrical energy directly from sunlight by photocatalytic effect, physicochemical process, is called a solar cell. Many solar cells are connected with each other to form solar modules also called solar panels. In the 1960s, single-junction silicon photovoltaic cells can produce ~0.5 to 0.6 V, and then, it was improved gradually. Initially, it has high cast users that work for space investigations. The solar cells also depend upon semiconductor industry and after the invention of integrated circuits, their prices had been decreased substantially in late 1971, normally its cost was some $100 per watt. The efficiency of solar cells has been distinguished into thermodynamic efficiency, reflectance efficiency, conductive efficiency, and separations of charge carrier efficiency, and they are responsible for the overall efficiency of solar cells. The conversion of incident light power into electrical power is known as power conversion efficiency, and also it has some other parameters to check the efficiency limits such as temperature coefficients, shadow angles, and temperature dependent curve for efficiency. It is hard to measure directly such parameters, so some other parameters such as integrated quantum efficiency, quantum efficiency, fill factor and VOC ratio, and

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reflectance ratio are substituted into external quantum efficiency. The materials that have single layer of light absorbing material are known as single junction otherwise multi-junction. The separation and absorption mechanisms are different in single and multi-junction materials. Till now, the solar cell family is divided into three generations called first, second, and third generation. The first generation consists of waferbased solar cells and photovortices technologies. The second generation consists of thin film solar cells. The third generation is considered as the emerging photovortices which also include on the thin film solar cells technologies. The third-generation solar cells are in evolutionary development phase, in which organic, inorganic, and organometallic compounds are often tested. The third-generation solar cells have high efficiencies but also have stability issues. Beside these major disadvantages, it has bright future to achieve high efficiency for commercial applications. Among third-generation solar cells, perovskite solar cells and dye-synthesized solar cells have great future because of their increasing efficiency. These cells have increased their efficiency up to 20% till 2014 and it is predicted its efficiency will approach to 27.3% by the end of 2018. In conclusion, next-generation solar cells must meet demanding desires regarding the power conversion efficiency (PCE), price cost, and life stability of ten to fifteen years. We have confidence in that nanostructured solar cells (NSCs) have great potential capability to attain such mentioned objectives.

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Materials for Solar Cell Applications: An Overview of TiO2 , ZnO, Upconverting Organic and Polymer-Based Solar Cells Navadeep Shrivastava, Helliomar Barbosa, Khuram Ali and S. K. Sharma

Abstract The present chapter gives an overview of third-generation solar cells with special emphasize on important synthesis protocol for ZnO, TiO2 , and rare earthbased upconverting materials for their utilization in the field of solar cells. Moreover, we have discussed working principle and basic requirements for organic-based solar cells, which is in major focus of research worldwide. This is booming research field and has enormous scope to serve humankind to combat energy scarcity and futuristic application for harvesting the solar energy. Keywords Solar cells · Upconversion · TiO2 · ZnO · DSSC · Polymer solar cells

1 Introduction The whole world is suffering from the sustainable energy sources as large dependency is on petroleum, coal, and hydro energies. This is leading to CO2 emission and finally ending to global warming scenario. This also affects the social–economic– health issues. The fossil fuels are not able to meet the future need due to their reserved resources and rapid increasing prices. In the modern world, energy issues have become urgent problem facing by habitants as the energy requirements are growing ever faster. Hence, it is important to find clean, renewable, and sustainable energy sources and considered as one of humanity’s greatest challenges. The sun N. Shrivastava Institute of Physics, Federal University of Goias, Goiania, GO, Brazil H. Barbosa Institute of Chemistry, Development of Inorganic Materials with Rare Earths (DeMITeR), Laboratory of Photoluminescent Materials (LAMAF), Federal University of Uberlândia, Uberlândia, MG 38400-902, Brazil K. Ali Department of Physics, University of Agriculture, Faisalabad 38040, Pakistan S. K. Sharma (B) Department of Physics, Faculty of Science and Technology, The University of the West Indies, St. Augustine, Trinidad and Tobago e-mail: [email protected] © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_3

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emits an enormous and endless energy, and our planet receives this solar energy each year more than twice as energy of petroleum and nuclear resources. Hence, solar energy is great alternative compared to these traditional energy sources and has potential to meet our energy demands, with no exception. In general, solar energy can be harvested using two classes of technologies: photovoltaic (PV) and solar thermal (ST). PV converts solar photons absorbed by PV cell into charge which can be collected and used to generate electrical power, whereas ST technology utilizes heat energy from the sun to heat a fluid. Harvesting energy directly from sunlight using PV technology is considered as being one of the most important ways to address growing global energy needs using a renewable resource [1]. Solar PV technology is increasingly being deployed globally leading to approximately 500 GW installed capacity at the end of 2018, and this is expected to continue toward multi TW levels within a few decades. PV has been reported to be a net contributor to greenhouse gas emission reduction, while module efficiency is gradually increasing, getting closer and closer to the Shockley–Queisser limit, and the leveled cost of electricity is plummeting to much less than traditional sources in some regions. Solar thermal technology is not the major focus of the present chapter, and hence, it is not discussed here. PV can be found in a variety locations; from the PV cells on a calculator to vast multi-mega Watt generating power stations, there are numerous ways of implementing the technology such as building-integrated photovoltaic (BIPV), concentrator photovoltaics (CPV), and space photovoltaics (SPV), floating PV, and others (Fig. 1). (a)

(b) Global New investment in renewable energy , by region in 2017, $BN

Fig. 1 a Solar energy distribution and capture [2]. The AM0 and AM1.5 solar irradiation spectra show the solar energy distribution outside of the Earth’s atmosphere (white line) and at the Earth’s surface (black line), respectively. The colored bars show the absorption range of green plants, purple bacteria, red algae, and typical silicon PV (photovoltaic) panels, as these are the most prevalent light-harvesting systems. The absorption peak maxima (where available) are depicted by darker shading in each colored band. The energy contained in the 300–350 nm and 1000–4045 nm regions is only captured by black absorbers and corresponds to 1.37% and 26.31% of the total energy in each spectrum, respectively. b Global new investment in 2017, in $BN, by the major economies in the world [3]

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We can divide PV devices into three major groups [4]: (i) first-generation solar cell, (ii) second-generation solar cell, and (iii) third-generation solar cell often also termed as future generation solar cells. First-generation solar cells are composed of silicon (Si) wafers (monocrystalline and polycrystalline). The monocrystalline Si homojunction and heterojunction cells have 25.1% and 25.6% efficiencies, respectively, whereas the polycrystalline Si cells have 20.8% efficiency [5]. The maximum efficiency of Si-based single-junction solar cells (the absence of non-radiative recombination) is limited to 33.5% for AM1.5G spectrum at 25 °C according to Shockley–Queisser (SQ) limit [6]. At present, silicon solar cells (wafer technology) dominates and taking up ~93% of the global PV installation market with power conversion efficiency (PCE) of commercial modules of around 20% and lifetime of more than 20 years. The cost of silicon modules dropped down significantly from ~70$/WP in 1970s to ~0.36$/WP in 2017. However, the main disadvantage is that Si technology needs expensive materials with higher processing temperature, which means that the production is still expensive [5]. Second-generation solar cells are based on “thin-film” technology that consists of semiconductor materials such as copper indium gallium diselenide (CIGS—21.7%), single-crystalline gallium arsenide (GaAs—28.8%) and polycrystalline cadmium telluride (CdTe— 21.5%) showing promising efficiencies [5]. Multi-junction solar cells surpass SQ limit due to several absorber layers for harvesting light in different regions of the solar spectrum and have reached highest PCE of 38.8% under one sun condition with a five junction (GAInAs/GAInP/GaAS/AlGaInAs/AlGaInP) tandem geometry. However, a big challenge for the PV community with these solar cells is the high production cost [4, 5]. Researchers have pushed toward new kinds of solar cells and developed “third-generation” solar cells to overcome the limitations of the previous solar cells and tried to reduce high production cost [4]. Though the efficiencies of third-generation solar cells are relatively low as compared to Si/multi-junction solar cells, they have low production cost due to cheap fabrication processing techniques. This makes third-generation solar cells appealing to the PV community. In recent years, solution-based/processed PVs have attracted significant interest due to their high PCE, cost-effective fabrication, and added functionality such as flexibility, being aesthetic and lightweight. The first development was based on dye-sensitized solar cells (DSSCs) [7], which now demonstrate 14.3% efficiency [8, 9]. The most recent breakthrough in the field of third-generation solar cells is the development of organic–inorganic halide perovskite solar cells that now demonstrate efficiency about 22% [10]. Progress of perovskite solar cells (PSCs) has been remarkably impressive as these can be fabricated by solution processing at low temperatures, and the production requires less energy than Si solar cell. Another potential market advantage of the PSCs is the deployment of flexible PV technology. It is not only interesting due to the quest for low-cost manufacturing and high throughput (output/efficiency) but also by considering its properties of being lightweight, flexible and thin that would make it easy to integrate on any surface (e.g., building-integrated photovoltaics (BIPV), automotive integrated photovoltaics (AIPV) or structure (either curved, rigid, or flexible), and even in portable and indoor electronics [11].

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Despite the high-efficiency reports, the PSCs still suffer from issues such as longterm operational stability, toxicity (the most efficient device employ lead, Pb), and reproducibility. The reproducibility arises from their rapid crystallization, which largely depends on the processing conditions and also the substrate or the selective contacts underneath [11]. For a high-efficient device, the choice of selective contacts is crucial for efficient charge extractions as well as its stability as perovskite deposition on metal oxides such as ZnO and TiO2 has shown degradation due to a possible interfacial reaction [12, 13]. In the present chapter, we mainly deal with the fabrication and importance of new solid-state and organic solar cells based on the third generation.

2 Synthesis of Materials 2.1 Synthesis of Material-Based (TiO2 , ZnO) Solar Cells Rising world population and industrial growth have led to accelerated energy consumption, while the uncontrolled release of toxic and industrial wastes in the air and water has resulted in pollution associated diseases worldwide as well as abnormal climate changes. It is therefore imperative to develop sustainable clean energy-efficient and environmentally friendly technologies for the serious environmental problems that have become a major concern. Titanium dioxide (TiO2 ) that exhibits multifunctionality has attracted intense effort worldwide due to its potential applications in harvesting, storage and transfer of energy, removal of air pollutants, and applications in biomedicine [14, 15]. However, wide band gap (3.0–3.2 eV) of TiO2 restricts its optical activity only to the ultraviolet (UV) radiation, corresponding for only ~5% of the solar radiation and thus resulting in inefficient photocatalytic properties [16]. Nowadays, co-doping of TiO2 nanomaterials with different elements has received considerable attention to further improve their photocatalytic activity. The high ratio of the surface area caused by the small particle size brings many benefits in the photocatalytic field for many TiO2 -based devices, since it facilitates the reaction/interaction between the devices and the interactive media, which occurs mainly at the external interface of the particle and depends heavily on the surface area of the material.

2.1.1

Synthetic Methods for TiO2 Nanostructures

Sol–Gel Method: In a typical sol–gel process, a colloidal suspension is formed from the hydrolysis and polymerization reactions of the precursors which are composed of inorganic metal salts or organic metal compounds, such as metal alkoxides. The complete polymerization and the consequent solvent outlet lead to the transition of the liquid sol into a solid gel phase. TiO2 nanomaterials have been synthesized with

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the sol–gel from the hydrolysis of a titanium precursor [17]. This process normally proceeds via an acid-catalyzed hydrolysis step of titanium (IV) alkoxide followed by condensation. Anatase TiO2 nanoparticles with different sizes and shapes could be obtained with the polycondensation of titanium alkoxide in the presence of tetramethylammonium hydroxide [18]. In a typical preparing method, titanium alkoxide is added to the base at 2 °C in alcoholic solvents in a three-neck flask and is heated at 50–60 °C for 13 days or at 90–100 °C for 6 h. A secondary treatment involving autoclave heating at 175 and 200 °C is performed to improve the crystallinity of the TiO2 nanoparticles. A prolonged heating time below 100 °C for the as-prepared gel can be used to avoid the agglomeration of the TiO2 nanoparticles during the crystallization process, obeying the kinetics of the reaction [19]. By heating amorphous TiO2 in air, large quantities of single-phase anatase TiO2 nanoparticles with average particle sizes between 7 and 50 nm can be obtained. Micelle and Inverse Micelle Methods: Aggregates of surfactant molecules dispersed in a colloidal liquid are called micelles when the surfactant concentration exceeds the critical micelle concentration (CMC). CMC is the concentration of surfactants in free solution in equilibrium with surfactants in aggregate form [16]. In micelles, the carbonic chains with hydrophobic characteristics of the surfactant are oriented toward the interior of the micelle, while the hydrophilic groups are oriented toward the aqueous medium. The lipid concentration determines the self-organization of the surfactant/lipid molecules. The values of H2 O/surfactant, H2 O/titanium precursor, ammonia concentration, feed rate, and reaction temperature are significant parameters in controlling TiO2 nanoparticle size and shape. Amorphous TiO2 nanoparticles with diameters of 10–20 nm were synthesized and converted to the homogeneous anatase phase at 600 °C and to the more thermodynamically stable rutile phase at 900 °C. Li et al. prepared TiO2 nanoparticles with the chemical reactions between TiCl4 solution and ammonia in a reversed microemulsion system consisting of cyclohexane and polyphenols [20]. The produced amorphous TiO2 nanoparticles are converted into anatase when annealed at temperatures from 200 to 750 °C and into rutile at temperatures higher than 750 °C. The crystallinity of TiO2 nanoparticles initially (prepared by controlled hydrolysis of titanium alkoxide in reverse micelles in a hydrocarbon solvent) could be enhanced by annealing in the presence of the micelles at temperatures considerably lower than those required for the traditional calcination in resistive furnace. Hydrothermal Method: Hydrothermal synthesis is conducted in steel pressure vessel (autoclaves) under controlled temperature and pressure with reaction in aqueous solutions. The temperature can be elevated above the boiling point of water, increasing the pressure of vapor saturation. This method is widely used to prepare nanoparticles [21]. For instance, the nanoparticles can be prepared by adding a 0.5 M isopropanol solution of titanium butoxide into deionized water ([H2 O]/[Ti]: 150) and then peptized at 70 °C during 1 h in the presence of tetraalkylammonium hydroxides (peptizer). After filtration and heat treatment at 240 °C for 2 h, the as-prepared powders are washed with deionized water and absolute ethanol and then dried at 60 °C. TiO2 nanowires have also been successfully obtained with the hydrothermal method by various researchers [22]. Normally, TiO2 nanowires are obtained by treating TiO2

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white powders in a 10–15 M NaOH aqueous solution at 150–200 °C for 24–72 h without stirring in an autoclave. Solvothermal Method: This is almost identical to the hydrothermal method with only exception that the used solvent here is non-aqueous. Moreover, the temperature can be elevated much higher than that in hydrothermal method, since a variety of organic solvents with high boiling points can be chosen. For instance, TiO2 nanoparticle was prepared using titanium (IV) n-butoxide (purity 97%) as the starting material. Titanium n-butoxide is suspended in toluene and placed in autoclave, in a test tube. The same solvent was filled in the gap between the test tube and the autoclave wall. The autoclave was purged completely by nitrogen; after that, it was heated up to the desired temperature at 573 K with the rate of 2.5 K/min. The temperature of the autoclave was held constant at 573 K for 2 h and then cooled down to room temperature. The obtained TiO2 powder was washed by methanol for several times and finally dried in air [23]. Electrodeposition: Electrodeposition is applied to produce a homogeneous, usually metallic, coating on a surface by the reduction action on the cathode. The substrate to be coated is used as a cathode and immersed in a solution containing a salt of the metal to be deposited. The metallic ions (Ti3+ ; TiIV ) are attracted to the cathode and reduced electrolytically to the metallic form. With the use of the template of an anodic alumina membrane (AAM), TiO2 nanowires can be prepared by electrodeposition [24]. In a typical synthesis, the electrodeposition was carried out in 0.2 M TiCl3 solution with pH = 2 with a pulsed electrodeposition approach, and titanium and/or its compound are deposited into the pores of the AAM. By heating the abovedeposited template at 500 °C for 4 h and removing the template, pure anatase TiO2 nanowires can be obtained. Sonochemical Method: Ultrasound production is a phenomenon based on the process of creating, expanding, and imploding cavities of vapor and gases. The implosion of these gaseous cavities is called cavitation, promoting activation effects in chemical reactions [25]. The cavitation produces intense local heating (~5000 K) and high pressures (~1000 atmosphere). The origin of the cavitation is due to the fact that, during the expansion, the gases adsorbed in the liquid around the cavity or at the interface evaporate resulting in the expansion of the cavity [25]. The bubbles propagate the expansion according to the oscillation of the ultrasound pressure. Sometimes, the collapse of the bubble becomes very violent due to the converging spherical geometry and the inertia of the liquid moving into that collapsed bubble. Yu et al. have applied the sonochemical method in preparing highly photoactive TiO2 nanoparticle photocatalysts with anatase and brookite phases using the hydrolysis of titanium tetra isoproproxide in pure water or in a 1:1 EtOH-H2 O solution under ultrasonic radiation [26]. Microwave Method: Microwave irradiation is electromagnetic radiation in the frequency range 0.3–300 GHz, which corresponds to wavelengths of 1 mm to 1 m [27]. Due to the fact that the electromagnetic radiation produces an oscillating field, the dipoles or ions continuously attempt to realign themselves in the electric field. Depending on the timescales of the orientation and disorientation phenomena relative to the frequency of the irradiation, different amounts of heat are produced

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through molecular friction and dielectric loss. Thus, the electric dipoles of solid systems change their orientation with a lag time because of crystalline lattice or short-distance interactions. If the microwave radiation has a frequency of the same order of magnitude as this time of lag, the dipole realignment suffers only a small lag with the oscillating electric field, and the solid can absorb the radiation and convert it to heat [27]. Thus, in the use of microwave-assisted dielectric heating for solid-state synthesis, it is necessary that at least one species absorb microwave radiation [27]. However, in the frequency of domestic microwave apparatus and at room temperature, few refractory materials can generate heat since they are called low dielectric loss insulators. When the dielectric loss of a substance is high, absorption of microwave radiation occurs and decay of excited vibrational states, releasing the energy absorbed as heat [27, 28]. This method is also known as microwave-assisted solid-state (MASS) method. Thus, it is necessary to use a material with high dielectric loss, also called microwave susceptor (charcoal or Fe2 O3 ), to promote the spot heating of the sample. Once heated, the dielectric loss of the precursor material undergoes changes, and microwave radiation engages directly with the ceramic material, warming it up punctually. This makes it possible to reach very high temperatures (~1500 °C) in a very short time, reducing the average time of synthesis to a few minutes. Microwave radiation is applied to prepare various TiO2 nanomaterials [29]. Corradi et al. found that colloidal titania nanoparticle suspensions could be prepared within 5 min to 1 h with microwave radiation, while 1–32 h was needed for the conventional synthesis method of forced hydrolysis at 195 °C [29].

2.1.2

Synthetic Methods for ZnO Nanostructures

Zinc oxide (ZnO) nanoparticles have a wide range of multiple applications as the materials promising candidate due to its various applications like optoelectronic devices [30], cosmetics [31], gas sensors [32], biosensors [33], solar cells [34], superconductors [35], varistors [36], photodetectors [37], photocatalyst [38], etc. ZnO is distinctive electronic and photonic wurtzite n-type semiconductor with a wide direct band gap of 3.43 eV at 2 K and a high exciton binding energy (60 meV) and deep violet/borderline ultraviolet (UV) absorption of the solar spectrum when compared to TiO2 [39–42]. Moreover, ZnO nanoparticles can be prepared by simple low-temperature processes. For the preparing of ZnO, a variety of techniques have been developed (Table 1) such as sputtering [43], sol–gel [44–46], vapor–liquid–solid growth [47], physical vapor deposition [48] zinc–air (Zn–air) system [49], coprecipitation [50], microemulsion [51], thermal evaporation [52], microwave-assisted hydrothermal synthesis [53], metal organic chemical vapor deposition [54], molecular beam epitaxy [55], solvothermal [56], sonochemical [57], wet chemical [58, 59], and electrochemical deposition [60]. Among these methods, the wet chemical has a promising potential for device applications because it is very simple at ambient conditions, low temperature, no catalyst, low cost, and high yield. Additional advantages of ZnO are that it can be easily prepared by wet chemical synthesis, presenting excellent stability under high

Synthesis

Very simple, ambient conditions

Complicated, inert atmosphere

Complicated, ambient conditions

Simple, high pressure

Synthetic method

Coprecipitation

Thermal decomposition

Micro-emulsion

Hydrothermal synthesis

100–220

20–50

100–320

20–90

Reaction temperatures (°C)

Hours–days

Hours

Hours–days

Min–h

Reaction period

Table 1 Summary comparison of synthetic methods taken from [65]

Water–ethanol

Organic compound

Organic compound

Water

Solvent

Very narrow

Relatively narrow

Very narrow

Relatively narrow

Size distribution

Very good

Good

Very good

Not good

Shape control

Medium

Low

High/scalable

High/scalable

Yield

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energy radiation [61]. ZnO exhibits a tetrahedral configuration and large ionicity at the borderline between that of covalent and ionic semiconductors [62]. A ZnO crystal can exhibit three different forms: hexagonal wurtzite, cubic zinc blende, and rocksalt [63]. ZnO hexagonal wurtzite is the most thermodynamically stable structure. Cubic zincblende, however, can be stabilized by growing ZnO on cubic substrates. ZnO will exist in the rocksalt structure only at higher pressures [64]. ZnO is generally an ntype semiconductor with the presence of intrinsic or extrinsic defects such as oxygen vacancies (VO ), zinc interstitials (Zni ), and zinc vacancies VZn and affect its optical properties and electrical behavior [61]. The intrinsic point defects of ZnO drawing a fundamental role in the electrical behavior of this material. Regarding Zni , it is generally considered to act as a donor. On the other hand, VZn is usually considered to act as an acceptor. When ZnO is photo-induced by solar radiation with photonic energy (hv) equal to or higher than the excitation energy (Eg) electrons from the filled valence band (VB) are promoted to an empty conduction band (CB) [62]. This photo-induced process produces electron-hole (e- /h+ ) pairs. The electron-hole pairs can migrate to the ZnO surface and be involved in redox reactions. The mecanism can generate ecb - and hvb + where ecb - = electrons in the conduction band and hvb + = electron vacancy in the valence band, respectively. Both of these entities can migrate to the catalyst surface, where they can enter in a redox reaction with other species present on the surface. ZnO has been shown that to exhibit higher absorption efficiency across a larger fraction of solar spectrum compared to TiO2 . The photo-activity of a catalyst is governed by its ability to create photogenerated electron–hole pairs. The major constraint of ZnO as a photocatalyst, however, is the rapid recombination rate of photogenerated electron–hole pairs, which disturbs the photo-degradation reaction. Additionally, it has also been noted that the solar energy conversion performance of ZnO is affected by its optical absorption ability, which has been associated with its large band gap energy. Therefore, researches have been devoted to improving the optical properties of ZnO in order to minimize band gap energy and inhibit the recombination of photogenerated electron–hole pairs. A comparison of various synthesis methods to prepare ZnO-based nanomaterials is given in Table 1. Improvement of ZnO as photocatalyst: ZnO is usually an n-type semiconductor mainly due to the oxygen vacancies (VO ), which can provide more electron charge carriers. The major drawbacks in the fabrication of ZnO semiconductor are the difficulties in obtaining a stable and reproducible p-type ZnO. The high purity of p-type ZnO is optimal for various applications due to its high radiative stability. Doping has been a strategy adopted to improve the physical and chemical properties of ZnO, incorporating impurities such as metals or non-metals, to shift energy from the ZnO valence band upwards and reduce the band gap energy to the ultraviolet-visible region. Metal doping of ZnO can improve the photo-activity of catalysts by increasing the trapping site of the photo-induced charge carriers and thus decrease the recombination rate of photo-induced electron–hole pairs [66]. This phenomenon can occur without causing any large lattice distortion. Besides, energy band gap decreases, and the ZnO-doped material could be applied in dye degradation and solar cells. However,

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highly active photocatalysts can be obtained by coupling two semiconductors having different band gaps. Higher-efficient charge separation may be achieved due to photo-induced electrons that are transferred away from the photocatalyst. Therefore, the heterostructure of nanomaterials acts as an attractive alternative for enhancing the photo-activity of photocatalysts. For ZnO coupled with other semiconductors, TiO2 /ZnO, SnO2 /ZnO, SnO2 /ZnO/TiO2 , and Co3 O4 /ZnO are the most investigated materials for photocatalytic processes [62]. In the ZnO/TiO2−x Ny array, the ZnO electrons are transferred from the conduction band to the TiO2−x Ny conduction band, while conversely the photogenerated holes are transferred from the valence band of TiO2−x Ny to the valence band of ZnO [67]. The occurrence of such phenomena suppresses the recombination of the electron/hole (e− /h+ ) pairs, increasing the charge carriers. In this system, ZnO could increase concentration of free electrons in the CB of TiO2 , which implies that charge recombination is drastically reduced in the electron transport process. An improvement in the redox processes is expected in this combination of matrices, since the separation of charges increases the lifetime of the charge carriers. Solar cells: Reducing the use of fossil fuels and their consequent production of gaseous pollutants has been the main target for the search for clean and environmentally friendly sources of energy. Thus, ways to convert sunlight into electrical energy have become the focus of the global energy field. Since 1991, dye-sensitized solar cells (DSCs) have attracted attention due to its low cost and simple preparation. DSCs prepared with TiO2 nanocrystallites sensitized by ruthenium-based dye have achieved a conversion efficiency above 11% [68]. Zinc oxide has recently been explored as an alternative material in dye-sensitized solar cells with great potential. The main reasons for this increase in research surrounding ZnO material include: (i) ZnO having a band gap similar to that for TiO2 at 3.2 eV and (ii) ZnO having a much higher electron mobility ∼115–155 cm2 V−1 s−1 than that for anatase titania (TiO2 ), which is reported to be ∼10−5 cm2 V−1 s−1 [69]. There are reports in the literature that relate about the higher state of nanocrystallite aggregation induces a more effective photon capture in the visible region, as well as a strong dispersive effect of light. This phenomenon optimizes the absorption of incident light, weakening the transmittance of thin films, which may present strong interference in the solar conversion process [69]. ZnO nanocrystallites have been demonstrated as an effective approach to generate light scattering within the photo-electrode film of DSCs while retaining the desired specific surface area for dye molecule adsorption. The maximum energy conversion efficiency of 5.4% was achieved on photo-electrode films that consist of polydisperse ZnO aggregates of nanocrystallites. This efficiency is a more than 100% increase over the 2.4% achieved for films only including nanosized crystallites. The polydispersity in the size distribution of ZnO aggregates was indicated to be positive in causing light scattering in a broad wavelength region and therefore, in enhancing the light-harvesting capability of the photo-electrode film. The presence of nanometric material promotes enhanced light scattering for increased photon absorption.

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3 Rare Earth-Based Materials Rare earths (RE) include 17 elements of the periodic table, the “lanthanide series”— La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu (Z: 57–71) and the elements Sc (Z: 21) and Y (Z: 39). The term “rare” can be misinterpreted since La, Ce, and Nd are more abundant than the Pb, Ni, Mo, and As elements, while Tm and Lu, the least abundant, are found in greater quantity than Au, Ag, Pt, and I. Thus, the term “rare” refers to the difficulty of chemically separating RE elements from their minerals, since their chemical properties are very similar [70, 71]. The RE elements (except Sc and Y) exhibit general electronic configuration [Xe] 4fN 6 s2 or [Xe] 4fN−1 5d1 6 s2 (La, Ce, Gd, and Lu), where N represents the number of electrons in shell 4f, but due to lower values of the first ionization energies [72], the predominant oxidation state of these elements is trivalent (3+) [73–75]. However, some RE ions exhibit the divalent (e.g., Eu2+ and Yb2+ ) and tetravalent (e.g., CeIV and TbIV ) states that can be found with some stability, since these species have completely empty electronic level (CeIV ), partially filled 4f7 (Eu2+ and TbIV ) or fully filled 4f14 (Yb2+ ) [76]. Not by chance, these ions have higher ionization energies for the process RE2+ → RE3+ , in the case of Eu2+ and Yb2+ and lower to the process RE3+ → REIV for the Ce3+ and Tb3+ ions [72].

3.1 Upconversion Rare Earth Materials for Photovoltaic Applications There is plenty of literature available related to down-conversion (DC) phenomenonrelated solar cells. So we discuss a novel experimental results related to upconversion (UC), which is a phenomenon in which the absorption of two or more low-energy photons leads to the emission of a higher-energy photon [77]. Trivalent lanthanide ions, which have many energy-level structures, are good candidates for sensitization and activation. In this sense, Er3+ , Tm3+ , and Ho3+ ions are generally applied as activators to give rise to efficient visible emissions under low pump power densities. In order to enhance upconversion luminescence efficiency, the Yb3+ ion is usually co-doped as an excellent upconversion sensitizer due to its large absorption cross section in the 900–1100 nm NIR region, corresponding to the 2 F5/2 → 2 F7/2 (Yb3+ ) transition. In fact, the Er3+ –Yb3+ couple is by far the most studied upconversion system. For photovoltaic applications, the energy transfer mechanisms are based on some combination of mechanisms are based on some combination of excited state absorption (ESA) and energy transfer upconversion (ETU) processes. In this sense, many groups have pursued new luminescence-based upconversion materials with the aim to enhance their efficiency. Er3+ -doped upconversion luminescence materials are the most potential upconverters for c-Si solar cells due to the ground-state absorption (GSA) of Er3+ in the range of 1480–1580 nm (4 I15/2 → 4 I13/2 transition). Ho3+ ion has a relatively large

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absorption band in the 1150–1225 nm spectral range due to 5 I8 → 5 I6 transition [72]. The GSA centered at about 1540 nm results in upconversion via an ETU mechanism, giving rise to four emission bands: 4 I11/2 → 4 I15/2 , 980 nm; 4 I9/2 → 4 I15/2 , 810 nm; 4 F9/2 → 4 I15/2 , 660 nm; and 4 S3/2 → 4 I15/2 , 550 nm. These emission bands match well with the absorption of c-Si.

4 Dye-Sensitized Solar Cells Dye-sensitized solar cells (DSSCs) are next-generation photovoltaic cells that can be used to create low-cost, environmentally friendly, and flexible solar panels. Unlike conventional Si-based solar cells, DSSCs primarily consist of photosensitive dyes and other substances such as an electrolyte solution and metal oxide nanoparticles. In this sense, Shan and Demopoulos [78] reported in 2010 for the first time, the application of rare earth-doped upconversion luminescence materials for improving the NIR sunlight harvesting in DSSCs. In their studies, the Er3+ , Yb3+ co-doped LaF3 part of the nanocomposite helps capture near-infrared (NIR) light by converting it into visible light absorbable by the dye, hence opening the road for the development of DSSCs with higher conversion efficiency and photocurrent output. The green emission located at around 543 nm generated by the upconversion nanocomposite can be efficiently absorbed by the N719 dye, which indeed leads to photocurrent generation upon 980 nm laser excitation. Different upconversion materials consisting of NaYF4 :Er3+ /Yb3+ hexagonal nanoplatelets (particle size ~800 nm) were tested in DSSCs by Shan et al. [79]. The upconverting phosphors were directly placed on the rear side of a counter electrode. This design enables a dual-mode functionality that provides both light reflection and NIR light capture. Photocurrent was observed for the DSSC device when irradiated with a 980 nm laser, clearly demonstrating the upconverting function of the NaYF4 :Er3+ /Yb3+ nanomaterials. Several other groups also reported the enhanced NIR response of the DSSCs by using upconverting materials NaY(MoO4 )2 , doped with Ln3+ (Ln = Er and Ho), exhibiting efficient, visible, and near-infrared emitting [80]. These materials were prepared by a conventional solid-state reaction, and sensitization of Ln3+ (Ln = Er and Ho) from the host with different doping concentrations of Ln3+ was investigated. Under excitation at 310 nm, the phosphor shows intense and characteristic emission of Er3+ and Ho3+ in both the visible and NIR region due to sensitization by MoO4 2− group. In the excitation spectra, there is a broadband ranging from 250 to 380 nm which is associated with the O2− –Mo6+ charge transfer transition in the MoO4 2− group. An efficient energy transfer from MoO4 2− to rare earth ions was observed at 410, 531, 553, 658, and 1536 nm (Er3+ ) and emissions at 384, 440, 468, 483, 541, 660, 754, and 1195 nm (Ho3+ ). As a result, Er3+ or Ho3+ single-doped NaY(MoO4 )2 phosphor may have potential application in modern lasers and photonic technology. Yb–Er–F triply-doped TiO2 upconversion nanoparticles (YEF–TiO2 –UCNPs) composite structures were prepared as photoanodes of the flexible DSSCs by using

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hydrothermal and spin-coating approaches [81]. The as-synthesized YEF–TiO2 UCNPs which could enhance the absorption of N719 dyes by converting the nearinfrared light to visible light exhibited better upconversion luminescence properties than the Yb–Er co-doped TiO2 upconversion nanoparticles. Moreover, the appropriate YEF–TiO2 -UCNPs could enlarge light-harvesting range of the formed flexible photoanode and improved the photovoltaic performance of the DSSCs. Notably, this novel DSSC configuration greatly overcomes the drawback of charge recombination induced by the conducting upconversion layer when used internally. Phosphors and Glasses: The rare earth-doped luminescence materials exhibit promising applications which include radiation detection, sensors for structural damage, optical memory media, identification markers, medical diagnostics, optical probes for in vivo bioimaging, and solar cell sensitizers [82–85]. Photon management by different means opens the opportunity to create new applications in diverse fields. In particular, the combination of rare earth ions with wide band gap semiconductors has attracted a lot of interest and recognized a lot of success in the technologies of lasers, optical fibers, fluorescent lamps, light-emitting displays, bioscience and imaging [86]. In the field of solar energy, these combined materials can act as photon converter structures with functionalities such as down-shifting (DS), down-conversion (DC), or upconversion (UC), which might enhance the solar cell efficiency. The concept is that photons from the solar spectrum can be converted to photons that match better the absorption wavelength range of the solar cell. Table 2 summarizes the development of phosphors and glasses as down-shifting layers for a variety of photovoltaic devices. In these studies, the principal strategy Table 2 Phosphors and glasses used as down-shifting layers for photovoltaic applications Materials NaY(MoO4)2 :Er3+ ,Yb3+ TiO2

:Yb3+,

Er3+ ,F

Preparation method

Wavelength excitation (nm)

Wavelength emission (nm)

Solid-state reaction

980

384–754, 1195

Hydrothermal method

980

525–800

SrAl2 B2 O7 :Eu3+ , Gd3+ , Sm3+ , Nd3+

Glass melting

254, 365

400–700

Tm3+ /Tb3+ /Sm3+ co-doped borate glass

Melt quenching

358

452–645, 700, 800

La2 O3 :Eu3+

Sol–gel Pechini method

285

570–700

YVO4 :Bi3+ , Eu3+

Aqueous precipitation

250–400

570–700

YAG:Ce3+ , Cr3+

Solid-state reaction

400–500

688

YAG:Ce3+ , Nd3+

Sol–gel

400–500

850–950, 1062

Cr3+ –Yb3+ -doped

Glass melting

300–700

950–1100

Hydrothermal method

250–320

450–600

Co-precipitation

230–320

570–700

fluorosilicate glass LaVO4 :Dy3+ Y2 O3

:Eu3+

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has been to convert the ultraviolet-blue radiation into red-NIR (near-infrared) emission by utilizing Stokes-shifted photoluminescence. Among RE3+ ions, NIR-emitting Nd3+ and Yb3+ ions have attracted significant researches. Their typical NIR emission around 1000 nm is just above the band edge of c-Si where the solar cell exhibits the higher spectral response. With the aim to overcome the weak luminescence of Nd and Yb ions, several research groups are applying other luminescence species with higher absorption coefficients to sensitize the Nd3+ or Yb3+ . An example could be Eu, Gd, Sm, Nd co-doped in SrAl2 B2 O7 nanocrystal phosphors prepared by glassceramic technique [87]. Phosphorescence measurements indicated that the formed SrAl2 B2 O7 phase is a promising host phosphor for rare earth ions to give different emission colors used in different applications. These glasses based on alumina borates compositions doped with rare earth ions attracted much interest due to its special properties as high UV transparency, nonlinear properties, exceptional optical damage threshold, and their ability to withstand the harsh condition in vacuum discharge lamps or screens [88]. Optical absorption spectra exhibited a characteristic UV absorption line at 357 nm with no further absorption in visible or NIR regions. SrAl2 B2 O7 glass doped with Gd3+ also exhibits no visible or NIR absorption spectra while Eu3+ -, Nd3+ -, and Sm3+ -doped samples exhibited characteristic absorption bands in both visible and NIR regions. The multicolor phosphorescence was attributed to the alternation of rare earth in glasses. Tm3+ /Tb3+ /Sm3+ triply-doped transparent borate glasses (Na2 O–CaO–P2 O5 – B2 O3 –ZrO2 ) were successfully synthesized via melt-quenching technique [89]. These glasses may be a promising candidate for white light-emitting diodes, luminescent materials, and fluorescent display devices. Among the various RE3+ ions doped, Tm3+ /Tb3+ /Sm3+ triply-doped compounds can be employed as good activators because of their unique spectral characters and emission intensity in glasses. In this material, while the concentration of Sm3+ ions increasing, both of fluorescence decays of Tm3+ and Tb3+ become quicker. This confirms that energy transfer of Tm3+ → Sm3+ and Tb3+ → Sm3+ occurs [89]. Besides, there are typical spectral overlapping between the Tm3+ , Tb3+ emission upon excitation at 358 nm, and the Sm3+ absorption. The overlap of the terbium 5 D3 → 7 F4 and 5 D4 → 7 F6 emission bands with the Sm3+6 H5/2 → 4 M17/4 , 6 H5/2 → 4 I9/2 , and 6 H5/2 → 4 G7/2 absorption peaks, respectively. The emission bands (1 D4 → 3 F4 and 1 G4 → 3 H6 ) of Tm3+ overlap with the absorption bands (6 H5/2 → 4 G7/2 and 6 H5/2 → 4 I9/2 ) of Sm3+ [89]. Thus, potential energy transfer processes from Tm3+ to Sm3+ and Tb3+ to Sm3+ are confirmed in the Tm3+ /Tb3+ /Sm3+ co-doped glass samples upon excitation at 358 nm. With increasing the content of Sm3+ , the luminescent color can be tuned easily from blue to white. Thus, Tm3+ /Tb3+ /Sm3+ triply doped glass (Na2 O–CaO–P2 O5 –B2 O3 –ZrO2 ) is a promising candidate for the development of white light-emitting diodes, luminescent materials, and fluorescent display devices. In addition, host sensitization via energy transfer (ET) from an excited host lattice to rare earth ion (RE3+ ) also offers an effective way to improve the luminescence signal. The photoluminescence emission process occurs as a result of a radiative electron transition in which the electron decays from a more energetic state to a less energetic

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state [76]. First, the electron must be excited to a state of higher energy by UV or visible light, for example. After the excitation, the nuclei adjust their positions to the new excited situation, so that the inter-atomic distances are equal to the equilibrium distances belonging to the excited state. During the relaxation process, there is no emission. The system can return to the ground state spontaneously under the emission of radiation from the lowest energy level of the excited state. The emission occurs at energy lower than the absorption due to the relaxation process. The energy difference between the maxima of the absorption and emission bands of the same transition is called the Stokes shift. The luminescence emission spectrum represents the luminescence intensity obtained in the visible (or UV, IR) wavelength range with a fixed excitation wavelength. On the other hand, an excitation spectrum is an impression of the intensity of luminescence at a given emission wavelength over a range of excitation wavelengths. Thus, the excitation–emission spectra consist of a two-dimensional image which displays the intensities of the luminescence as a function of the excitation and emission wavelengths. Inorganic solid materials that give rise to luminescence are called phosphors or, recently, luminescent materials [74]. Luminescent materials generally require a host lattice crystalline structure that constitutes the bulk of the phosphors. The characteristic luminescence properties are obtained by doping the matrix material with relatively small amounts of foreign ions (RE ions). The luminescence of inorganic solids can be classified into two mechanisms: luminescence of localized centers or activators and luminescence of semiconductors through band-to-band excitation [90]. To improve the luminescence from an activator without efficient absorption for the available excitation energy, a sensitizer ion is often incorporated into the host material to transfer its excitation energy to the activator. The emission color can be readily adjusted by varying the dopant concentration or composition without changing the host lattice.

5 Organic and Polymer Solar Cells As mentioned earlier various types of solar cells have become an interesting topic in recent two decades. Most used solar cells are based on silicon but limited by power conversion efficiency (PCE) of 25% on laboratory scale and 11–16% in commercial arrays and lifetimes of about 20 years. But thick silicon layers make these cells relatively expensive. Hence, the needs for the alternative solar energy technologies and a lot of research efforts have been put into this area. Organic thin-film solar cell technologies, e.g., polymer solar cells (PSCs), have been shown to be a promising alternative to the silicon-based solar cells. The word polymer originates from the Greek words poly (many) and meros (parts). A polymer does indeed consist of many parts, called monomers, which are linked together in a long chain to form a polymer. Conjugated polymers consist of a backbone chain of alternating single and double bonds. Because of this, there will be an overlap of p-orbitals which in turn enables delocalization of electrons across the polymer backbone, making it a one-dimensional semiconductor. Normally, polymers are regarded as insulators,

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unable to conduct electricity or absorb sunlight. For their discovery and contribution to the development of conjugated polymers, Shirakawa, McDiarmid, and Heeger were awarded the Nobel Prize in chemistry in 2000. The main advantage of polymer solar cells (PSCs) is the potential of flexible and lightweight devices that can be solution processed by cheap and efficient methods such as roll-to-roll processing, inkjet printing, or spray coating. Nowadays, the efficiency of laboratory-scale devices is ~9 to 10%. However, in order to compete with silicon-based solar cells and other energy sources, relatively low stability and short lifetime of polymer solar cells (PSCs) have to be improved. The active layer of PSCs usually consists of a blend of a conjugated polymer as light-absorbing material and electron donor and a second material, often a fullerene derivative, as electron acceptor. The conversion of solar energy into electrical energy requires the conjugated polymer in a solar cell able to absorb a substantial amount of the available photons, in which photon energies that can be absorbed are defined by the band gap (Eg) and absorption coefficient of the semiconducting polymer. The absorption coefficient is described as the amount of photons a material absorbs at a given wavelength. Ideally, a material should absorb all available photons. For the potentially highest power output for a single layer cell, the band gap should be between 1–1.5 eV and 1250–830 nm, as described by Shockley–Queisser. An organic solar cell (OSC) is a photovoltaic device whose active layer comprises p-conjugated polymers and small molecules. Among the arguments for pursuing research on OSCs are that most—if not all—of the components can be deposited from solution in a roll-to-roll manner that the materials are in principal earth-abundant, that devices can be semi-transparent or aesthetically pleasing, that the devices are ultra-flexible and even stretchable, and that the materials and whole devices can be extraordinarily lightweight. Organic photovoltaic devices are thus unique not only in that they could have low costs per module, but that their thinness and extremely small mass could also reduce the costs associated with transportation and installation of modules (part of the balance-of-system costs, which are generally independent of the particular photovoltaic technology of the module). Organic photovoltaic cells with a single-component active layer sandwiched between two electrodes with different work functions only led to very low power conversion efficiency due to poor charge carrier generating and unbalanced charge transport. Functional principle of organic photovoltaic cells: Contemporary organic photovoltaic cells are often based on a heterojunction made of two materials: an electron donor (D) and an electron acceptor (A). In the molecular case, the basic energy-level diagram used to describe the operating principle of an OPV cell is shown in Fig. 3. The molecular energy levels involved in the light conversion into electricity are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. The ionization potential (IP) is the energy needed to remove an electron from a neutral molecule and is related to the HOMO energy level. The fundamental gap, Efund , or HOMO-LUMO energy gap is defined as the difference between IP and the electron affinity (EA). The EA is the amount of energy released when an electron is added to a neutral molecule and is used to estimate the

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Fig. 2 a Basic energy-level diagram and illustration of energy gap in molecular case of an organic semiconductor, b functional principle of a D/A solar cell

electron-withdrawing ability of a molecule to act as an electron acceptor in an OPV device and thus linked to the LUMO energy level relative to vacuum. In the material case, the molecular energy levels are broadened into electronic bands. The upper occupied band is referred to as the valence band, and the lower unoccupied band is referred to as the conduction band. The material band gap is defined as the energy difference between the top of the valence band and the bottom of the conduction band. As simply described in Fig. 2, the photovoltaic effect of converting sunlight into electricity in an organic photovoltaic cell can be summarized as the following four steps: (i) photon absorption and exciton generation; (ii) exciton diffusion to the D/A interface; (iii) exciton dissociation into free charge carriers (electrons and holes), and (iv) charge transport and charge collection at the electrodes, which produce current in the external circuit. A bilayer heterojunction configuration containing a p-type layer for hole transport and an n-type layer for electron transport can be implemented to improve the photocurrent of the solar cell device Generally, the synthesis of polymer solar cells is performed using green synthesis method following the common reaction as given below: Metal-mediated crosscoupling reactions: This group can be subdivided by two subgroups of reactions. Stille polymerization: The most common reaction methodology employed to generate alternating donor–acceptor, low-band gap polymers is the Stille-based condensation [91, 92] and polymerization of bromide-terminated and trimethylstannylterminated monomers and alternatives. Suzuki polymerization: The Suzuki reaction is in some aspects environmentally preferable to the Stille reaction, though it brings its own set of potential complications [93, 94]. The base, water, and phase transfer catalyst that are required for the typical Suzuki reaction to add significant complexity to the reaction setup, which will be exacerbated at large scale where multiphase reactions are sometimes slower and more difficult to mix. The stability of monomers is often troublesome, as boronic esters have a tendency to condense into cyclic trimmers [93]. For illustration, the following is the palladium-catalyzed cross-couplings in organic synthesis [46]. The Suzuki cross-coupling reaction involves the formation

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Fig. 3 Reaction mechanism of a a Suzuki cross-coupling reaction and b a Stille cross-coupling reaction

of sp2 -hybridized carbon atoms (Fig. 3a). The method employs the use of a palladium catalyst and a base to facilitate the reaction between a halide or a triflate and a boronic compound [95]. The first step in the mechanism is oxidative addition of the halide to the palladium (0), forming a palladium (II) complex. The reaction cycle continues with a transmetallation step with the boronic compound, catalyzed by a base. In the final step, the product is expelled by reductive elimination, regenerating the palladium (0) catalyst. The Stille cross-coupling reaction (Fig. 3b) involves the formation of sp2 hybridized carbon atoms. The method employs the use of a palladium catalyst to facilitate the reaction between a halide or a triflate and an organotin compound. Their action mechanism is similar to that of the Suzuki cross-coupling; the difference is the absence of a base in the Stille cross-coupling. This allows for polymerization of basesensitive monomers. A problem with the Stille reaction is the toxicity of the organotin compounds [96]. This is a disadvantage compared to Suzuki cross-coupling, both at laboratory scale and for future up-scaling of the process. Other well-known

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reactions for polymer solar cells (PSCs) are heterogeneously catalyzed polymerizations, direct heteroarylation polymerization, conjugated polymers comprising of biologically derived materials, and water-forming polycondensation reactions

6 Summary To summarize, we have discussed several important synthesis protocols of TiO2 and ZnO-based materials for their significance in solar cell applications. A novel phenomenon of upconversion solar cell, harvesting rare earth characteristics, has shown a huge impact to fight the issue-related solar energies. Dye-sensitized solar cells, incorporating rare earths and other dyes, have shown potential importance and hence require more attention in the solar cell applications.

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92. Li W, Furlan A, Roelofs WSC, Hendriks KH, van Pruissen GWP, Wienk MM, Janssen RAJ (2014) Wide band gap diketopyrrolopyrrole-based conjugated polymers incorporating biphenyl units applied in polymer solar cells. Chem Commun 50:679–681. https://doi.org/10.1039/ C3CC47868H 93. Molander GA, Ellis N (2007) Organotrifluoroborates: protected boronic acids that expand the versatility of the Suzuki coupling reaction. https://doi.org/10.1021/ar050199q 94. Suzuki A (1999) Recent advances in the cross-coupling reactions of organoboron derivatives with organic electrophiles, 1995–1998. J Organomet Chem 576:147–168. https://doi.org/10. 1016/S0022-328X(98)01055-9 95. Suzuki A, Miyaura N (1995) Palladium-catalyzed cross-coupling reactions of organoboron compounds. Chem Rev 95:2457–2483. https://doi.org/10.1021/cr00039a007 96. Boyer IJ (1989) Toxicity of dibutyltin, tributyltin and other organotin compounds to humans and to experimental animals. Toxicology 55:253–298. https://doi.org/10.1016/0300483X(89)90018-8

Recent Advances in Solar Cells Marcio A. P. Almeida

Abstract The last years the growth of the global population has resulted in high demand for electricity consumption. Photovoltaic devices have shown a big potential to obtain energy power from solar irradiation when compared with other sources. Currently, photovoltaic silicon-based technologies are the most used around the world, but its high cost is still a big problem for global consumption. A short approach to fundamental concepts to inorganic and organic solar cells will be described in this chapter. Moreover, it will be showing new models of solar cells as well as advances and challenges in the development of inorganic and organic solar cells with high efficiency and stability. Keywords Photovoltaic devices · Solar cells · Energy

1 Introduction The high growth of the world population, along with industrial and urban development, has resulted in high demand for energy consumption. The use of renewable energy sources such as biomass, wind, hydroelectric, geothermal, and photovoltaic (solar energy) is of fundamental importance for the sustainability of the planet. The photovoltaic systems are one of the fastest-growing renewable sources in the world, representing 20.5% of power generation. Only in 2017, new installation totaling more than 100 GW, with highlight to China with 48 GW, which reflects a total of 4426 TeraW/h (global), an increase of close 35 percent annual when compared with 2016 [1]. By definition, photovoltaic systems are technologies able to convert sunlight into electrical power (direct current, DC), from semiconductor materials, which is measured in watts. In historical terms, the first functional application of photovoltaic system was reported by Fritts in 1883 [2], but only in 1954, a researcher at Bell laboratory in the USA discovered the potential of pn junction diodes to generate voltage when M. A. P. Almeida (B) Curso de Ciência e Tecnologia, Universidade Federal do Maranhão, Campus Bacanga, São Luis, Maranhão 65085-580, Brazil e-mail: [email protected] © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_4

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the light was on, which within a year 6% efficient to Si pn junction solar cell was achieved [3], which was also made by Reynold et al. to film heterojunction solar cell based-Cu2 S/CdS [4]. The first application toward photovoltaic system has made to space solar cells (in the 1960s), which was used to power satellite applications from the USA and USSR, and only in the 1960s, its commercial production began, with highlight to Japan and Europe [5]. Currently, dominant technologies for the use of solar cells are supported by silicon (91%) [6]. However, silicon photovoltaic systems have dominant technology over the world, and in the long years, its cost still has been high, which results in this being unfeasible for domestic use. In the last years, research has reported other alternatives of materials to replace based-silicon solar cells [7]. Several reviews have presented new classes of inorganic, organic, and hybrid materials as a high potential to solar cells [8]. In some cases, the power efficiency has been similar to the photovoltaic based-silicon system, a topic which will be described in the last items.

2 Basic Concepts to Solar Cells Solar cells are recognized as pn junction. As illustrated in Fig. 1, a basic solar cell is composed of a junction of two (or more, e.g., tandem solar cells) materials, one p-type and other n-type, connected by two electrodes. When a solar cell is under sunlight, its electrons valence bands (VB) are excited to the conduction band (CB), generating a charge electron/hole pair. The energy between VB and CB is called bandgap, which due to range sunlight, generally to solar cell systems, low bandgap values are requested 1.1–2.5 eV [9]. This concept of the bandgap is not applied to perovskite-based solar cells, (PSC) since in these devices, the perovskite acts as a sensitizer, which has high bandgap values. In the ideal photovoltaic system, pn junction and the electrons are concentrated on the n-side, while the holes are all on the p-side. The electrons on VB in p-type semiconducting are driving to CB in p-type semiconducting which are collected by an external circuit, an electrode.

Fig. 1 a Illustration of solar cell. Reprinted with permission from Ref. [9]. Copyright 2017 Elsevier. b pn junction to solar cells

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The electron flow between the two electrodes is responsible for generating electric current. Therefore, over the world, researchers have concentrated many efforts in the development of new p-type and n-type materials with the potential to use in solar cells, a discussion which will be further approached in the next section. Currently, the photovoltaic technology (PV) can be classified into three large groups: (1) solar cells of the first generation, (2) solar cells of the second generation, and (3) solar cells of the third generation, Fig. 2. For solar cells of the first generation, the silicon-based technologies (single, multi-crystalline silicon) are predominant, with highlight to crystalline silicon, but that another category of the solar cells as hydrogenated amorphous silicon (a-Si–H) PV is also widely used. In PV of the second generation, the highlight concerns thin-film technologies. In this category, GaAsbased, CdTe (and analogous), and CuInSe (and analogous) are found. Finally, in the third generation (also of the thin film), new materials are found with the potential to overcome the technologies currently used, called advanced semiconductors. This group of the materials falls within the dye-sensitized solar cells (DSSCs), perovskites and organic-based solar cells, and quantum dot PV.

Fig. 2 Classification of solar cell technologies

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3 Inorganic Photovoltaic Devices The first solar cell inorganic-based material was developed at Bell laboratory in 1953 with an efficiency of 6% [3], and your first application was in the solar-powered Sputnik II (US-Soviet Union, USSR) and Vanguard 1 (USA) satellites [10]. Photovoltaic devices, from their discovery until their manufacturing toward commercial purposes, several other solar cells inorganic-based materials have been presented such as Cu2 S/CdS [4], GaAs [11], GaAlAs/GaAs [11b], and CdTe [12]. However, only in the 1980s companies in Japan, the USA, and Europe began manufacturing, but to silicon-based solar cell using the concept of the pn junction it is still predominant the use of silicon-based. In a global overview, crystalline silicon (c-Si) solar cells achieve about 25% of efficiency and dominating almost 90% of the market [13], whereas CdTe-based photovoltaic technologies cover only 5% (PCE 20%) [14a] and others such as CuInGaSe2 (GIGS, PCE 19.5%) and amorphous silicon (a-Si:H, PCE 10%) represent the remaining share. Although silicon photovoltaic technologies are dominant in the global market, much effort has gone to the development of new thin-film-based photovoltaic technologies with new materials, which has been reported in several reviews [13, 15]. Materials such as GaAs, InP, and their derivates have arisen as new alternatives to replace silicon in solar cells. These new materials have displayed highest conversion efficiencies between 18.4 and 37.9% (InGaP/GaAs/InGaAs), which their efficiency will depend on the method and composition used in the device building [14a]. For example, Moon et al. have shown that GaAs thin-film solar cell obtained by epitaxial lift-off (ELO) method can achieve a high efficiency of 22.08% [16] (Fig. 3), while Bauhuis et al. also to GaAs-based solar cell (thin film) presented an efficiency to 24.5% [17] and Kayes et al. to same material (GaAs) have found efficiency to 27.6% [18]. Although GaAs-based solar cell and its derivate have achieved great results of efficiency, high cost of producing device-quality epitaxial layers or substrates make it impracticable for industrial production. Fig. 3 J–V curve of the GaAs thin-film solar cell. a By Moon and co-works. Reprinted with permission from Ref. [16]

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Fig. 4 a Diagram of typical dye-sensitized solar cell using ruthenium (II) complex. Reprinted with permission from Ref. [19e]. Copyright 2010 American Chemical Society. b cyclometalated bis-tridentate Ru(II) complexe in DSSC. Reprinted with permission from Ref. [21]. Copyright 2011 American Chemical Society. c I–V curve and IPCE spectra for JK-91 and JK-92. Reprinted with permission from Ref. [22]. Copyright @ Elsevier

Besides the compounds mentioned above, (polypyridyl) ruthenium(II)-based complexes and its homologs in the last years have also exhibited applications for solar cells. Differently from photovoltaic devices described before, here the Ru(II) complex acts as a sensitizer into the device, as illustrated in Fig. 4a, b. These compounds, to their photovoltaic devices, an efficiency between 6 and 11% [19], have been displayed. These values of PCE are still little expressive when compared with other photovoltaic devices, for example, perovskite-based ones, which can achieve PCE of 22.7% [20]. However, this does not impair the studies of Ru(II) complexes toward the solar cell, since these compounds have a very versatile structure, which shows that better results to PCE can be achieved. The versatility of application of Ru(II) compounds at solar cells is related to the 2,2 -bipyridine molecules and their derivate coordinates on Ru(II) ion. Such molecules can optimize the bandgap values as well as its stability, which is attributed to the ability that bipyridine ligand has to make electronic transition metal-ligand type.

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3.1 Perovskite-Based Solar Cells Currently, among several inorganic materials used with the purpose of developing photovoltaic technologies, perovskites have been widely studied and reported in several reviews [23]. The perovskite-based photovoltaic device first arose in 2007 with a PCE of 2.2% [24]. Since the first reported PCE of 2.2% for solar cell based on perovskite until nowadays, many improvements were made in the perovskite structure resulting in a PCE of 23.7% [25], as illustrated in Fig. 5. In order to improve the efficiency of perovskite-based photovoltaic devices, researchers have focused their efforts on some strategy, like chemical composition/structure, morphological evolution [26], and methodology of synthesis. Among all factors described before, perhaps the chemical composition is the most expressive concerning efficiency. For example, Deepa et al. [27] have described the importance of doping of Cs+ into Csx MA1−x PbI3 . For Cs+ content, it was achieved a PCE of 17.1%, which was higher than without Ce+ doping. The insertion of the Cs+ into the perovskite structure increase V oc , J sc , and FF values, Fig. 6a. Cho et al. [28] reported the influence of the surface passivation in increase of both PCE and stability. For passivation of composition (FAPbI3 )0.85 (MAPbBr3 )0.15 , a PCE of 21.3% was achieved, as well as better stability was exhibited, Fig. 6b, c. In this work, the authors described that using the technique of passivation, it was possible to reduce the amount of the traps and defects which are located more at the surface of the perovskite film, factors responsible for the recombination of electron– hole pairs. This observation agrees with observations made by Wang et al. [29] in the paper reported. For the FA0.83 Cs0.17 Pb(I0.6 Br0.4 ), the composition C60 doped, besides a better PCE, an improvement to stability was also achieved, Fig. 6d. As described before, an improvement in the performance of the perovskite-based photovoltaic device depends on the chemical composition, which can be made Fig. 5 Efficiency evolution of perovskite solar cells from 2006 to 2017. Reprinted with permission from Ref. [8b]. Copyright @ Royal Society of Chemistry

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Fig. 6 a J–V curves of Csx MA1−x PbI3 with and without Cs+ addition. Reprinted with permission from Ref. [27]. Copyright @ Royal Society of Chemistry. b J–V curves of (FAPbI3 )0.85 (MAPbBr3 )0.15 . Reprinted with permission from Ref. [28]. Copyright @ Royal Society of Chemistry. c J–V curve hysteresis of the passivated cell for (FAPbI3 )0.85 (MAPbBr3 )0.15 . Reprinted with permission from Ref. [28]. Copyright @ Royal Society of Chemistry

through doping using different ion metals [30]. Another alternative to it is the employment of organic cations mix [31]. Zhang et al. [32] showed that to perovskite hybrid system, to the mix ionic composition of MA and FA cation, a performance over 20% can be achieved, Fig. 7a. For this structure, the pristine (MAPbI3 ) has presented a performance of 18.59%. When the guanidinium organic structure is added into perovskite, the PCE value was increased to 20.26%, which is justified by an increase of all photovoltaic parameters, with highlight to FF, which suggests a reduction of defects into the interface as well as less hysteresis. Jodlowski et al. [33] also analyzed the influence of organic cations into the perovskite structure films (MA1−x Guax PbI3 ) but not to the guanidinium (CH6 N3+ , Gua). For MA1−x Guax PbI3 with x = 0, the PCE observed was 18.00%, when that to x = 5, 10, 14, and 17, it was found 18.81, 19.09, 20.15, 19.29%, respectively. Here, it was observed that the performance increased only 5 and 10%, which are attributed to improvements in the photovoltaic parameters J sc , V oc , and FF. Here, it is worth highlighting to MA1−x Guax PbI3 perovskite with x = 25 that though the PCE values found were 18.3%, this device presented a better stability when compared with other

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Fig. 7 a J–V curve of FAx MA1−x PbI3 (x = 0, 10, 20, 30%). Reprinted with permission from Ref. [32]. Copyright 2017 American Chemical Society. b J–V curve of MA1−x Guax PbI3 perovskite solar cells. Reprinted with permission from Ref. [33]. Copyright 2017 Springer Nature. c Thermal stability test of MA1−x Guax PbI3 perovskite solar cells. Reprinted with permission from Ref. [33]. Copyright 2017 Springer Nature

devices, Fig. 7b and c, which can be attributed to the hindrance of moisture that the guanidinium cation proves into the perovskite structure, therefore avoiding the degradation of film. Another factor that influences the efficiency of perovskite-based solar cells is the morphology of the perovskite thin film [26]. The morphology has a direct relation with the amount of defects which can be present on the film surface, the bulk of the grain or boundary between neighboring grains. Thus, growth control in thin films has been attracted many efforts, since this when optimized, reduced defect density can be obtained and therefore fewer recombination sites, which contributes to the enhancing of PCE [34]. Wang et al. [35] showed that though different anti-solvents (toluene, chloroform, diethyl ether, and di-isopropyl ether) used in the deposition process for obtaining the perovskite-based solar cell, several PCE values have been found, Fig. 8g. The device obtained with di-isopropyl anti-solvent presented 17.19%, which is superior to the PCE of the device obtained with the anti-solvent most used in the manufacturing of perovskite thin films, chlorobenzene. Indeed, Fig. 8a–f shows clearly as the anti-solvent influences in the grain and therefore in PCE. The FF value to the device obtained with di-isopropyl ether was 0.79, which is higher than the FF value

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Fig. 8 Top view SEM images of perovskite films prepared by the anti-solvent deposition method with different solvent treatments: a without antisolvent. b Toluene. c Chlorobenzene. d Chloroform. e Diethyl ether. f di-isopropyl ether. h J–V curves of perovskite solar cells fabricated with different anti-solvent treatment. Reprinted with permission from Ref. [35]. Copyright 2017 Royal Society of Chemistry

to the device obtained with chlorobenzene, 0.62. For perovskite, a great FF value indicates fewer defects in the device and therefore a reduction in the recombination of electron/hole pair rate.

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3.2 Advanced Materials for Charges Transport in Solar Cells Both inorganic and organic solar cells, being great charge-transfer materials, play an efficient role in PV systems. These materials, when working efficiently, promote great charge separation and therefore optimize the PCE. Electron-transporting materials (ETMs) extract electrons in conduction band from sensitizing and driving to the photoelectrode (e.g., anode for perovskite solar cell normal) while hole-transporting materials (HTMs) is responsible for collecting holes of the valence band from sensitizing and transport to the external circuits, cathode, if we have the configuration glass/FTO/ETM/sensitizing/HTM/Au, Fig. 9. Currently, in the research, there are several options of inorganic electron transport materials, but TiO2 is the most used material, which is reported on several reviews [36]. For HTM, although there are many alternatives to inorganic materials, the organic molecules are most used, with highlight to spiro-OMeTAD.

3.2.1

Advances in the Development of ETM

It is known to all that three basic criteria are necessary for a great ETM: (1) good optical transmittance in the visible range, (2) energy level of ETM near the sensitizing, and (3) presenting a great electron mobility. The second item, the energy level, is very important because depending on ETM, the configuration of the device can be normal or an inverted type, for example, for perovskite, Fig. 9b–d. Currently, in the

Fig. 9 Schematic illustration of the most common device architectures of PSCs: a (n–i–p) mesoporous and planar structures. b Energy diagram of the different components of a conventional PSC. c Inverted (p–i–n) mesoporous and planar structures. d Energy diagram of the different components of an inverted PSC. Reprinted with permission from Ref. [37]. Copyright 2018 Royal Society of Chemistry

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research, TiO2 is the most widely used ETM in the world [38]. However, researchers all over the world have found new alternatives of ETM, because in order to obtain ETM (thin films) with great crystallinity high temperature (>450 °C) annealing is required, which results in the process becoming expensive. Other alternatives of ETM such as SnO2 [39], WO3 [40], ZnO [41], C60 [39c], and PCBM [39c] have to arise as options to replace TiO2 . Luo et al. [41d] showed in a huge review of the potential that ZnO has to act as ETM. They showed that the efficiency performance of photovoltaic devices with ZnO as HTM depends on the synthesis method of zinc oxide as well as the device structure, which can be mesoporous, mesoporous inverted, planar and inverted planar structure, Fig. 9a–c. For such configurations using ZnO as ETM, it is possible to achieve PCE values ranging from 2.56 to 17.17%. Son et al. [42] showed that PCE to ZnO-based photovoltaic devices might be enhanced through control of the growth using different concentrations of the precursor solution as well as immersion time. In the optimizing process, it was possible to achieve PCE of 11% when used the concentration 35 mM of solution precursor with time fixed at 180 min and 90 °C. In 2015, Xu et al. [43] presented the TiO2 /ZnO bilayer as new propose of ETM, Fig. 10. This new design brings as news, the reduction of interface recombination, as well as the increase of PCE from 13% (ZnO-based device) to 17.17%. Li et al. [44] have also made the same observation to ZnO-based as ETM, and to its device, a PCE of 17.3% was achieved. Also, in 2015, Chang et al. [45] showed a new approach in the design of solar cells with ZnO as ETM. Unlike Xu et al. (deposition by spin coating), it was used atomic layer deposition (ALD) method to deposit ZnO in the design of the inverted perovskite device, which was achieved a PCE of 16.50%. Comparing both methodologies used in the building of photovoltaic devices, the device made by the ALD method is better than the spin coating method since the device made by ALD has a great stability at the air in 45 days and low hysteresis. Similar to the ZnO, SnO2 has also attracted much attention of the researchers because of its application as ETM in solar cells. Thus, SnO2 structure has shown a high potential to replace TiO2 in photovoltaic devices, and such efficiency depends on the methodology used, data that was well written by Xiong et al. in their review [39c]. For an improvement of the efficiency in devices containing SnO2 as well as other materials, it is very important to understand band alignment engineering between ETM and perovskite. For example, Baena et al. [46] described the importance of energies level on the conduction band (CB) in ETMs to electrons extraction from perovskite. Following such criteria, Baena and co-collaborators obtained the device FTO/SnO2 /Perovskite/Spiro-MeOTAD/Au type, which presented PCE of 18.4%. Moreover, to this device containing SnO2 , it was observed a low hysteresis as well as stability at the under air by 30 days. Other authors, such as Song et al. [47], Chu et al. [48], Huang et al. [49], and Anaraki et al. [50], have reported SnO-based photovoltaic devices with PCE over 20%. These results are encouraging, since such efficiency conversion is near to the values found to TiO2 -based devices, and moreover, these devices presented a reduced hysteresis and good stability.

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Fig. 10 a Schematic diagrams of the device structure. b Energy levels. c Perovskite surface morphology SEM image. d Current–voltage curves of the planar perovskite solar cell. Reprinted with permission from Ref. [43]. Copyright 2019 Royal Society of Chemistry

3.2.2

Advances in the Development of HTM

As previously described, the 2,2,7,7-tetrakis-(N,N-p-dimethoxyphenylamino)-9-9spirobifluorene abbreviated as Spiro-OMeTAD, which was firstly presented in 1998 by Graetzel’s group [51] (Fig. 11a) is the most used HTM in the research on solar cells [52]. However, the synthesis of this HTM with high purity requires some care, which makes it very difficult. Furthermore, to its activity, and improve conductivity, some additives as lithium bis(trifluoromethane)sulfonimide (LiTFSI, doping) and 4-tertbutylpyridine (TBP) are necessary, which reduce the stability of the PSCs because of its hygroscopic properties [53]. Thus, the researchers have been concerned to find out new alternatives to HTM (inorganic or organic) of low cost and high stability [54], Fig. 11b.

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Fig. 11 a Scheme for the electron transfer processes (inj., injection; reg., regeneration; rec., recapture; hopping) occurring in the dye-sensitized heterojunction. Reprinted with permission from Ref. [51]. Copyright 1998 Springer Nature. b Energy level diagram of representative inorganic HTMs (CuI, CuSCN, Cu2 O, CuO, CuS, Cu2 ZnSnS4 , NiOx, MoO3 , V2 O5 , and CuGaO2 ), along with HOMO level of Spiro-MeOTAD. Reprinted with permission from Ref. [55]. Copyright 2018 American Chemical Society

Due to these many drawbacks, for the use of Spiro-OMeTAD, in 2016, Graetzel’s group showed again a new option for the HTM, the 2 ,7 -bis(bis(4methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b ]dithiophene-4,9-fluorene], FDT, which is cheaper (60US$g−1 ) than Spiro-OMeTAD (500US$g−1 , high purity, Merck). For perovskite-based photovoltaic devices with FDT, it was obtained a PCE of 20.2%, while that for devices using Spiro-OMeTAD, the PCE was 19.7% [56], Fig. 12a. Though the increase of efficiency using FDT proved to be slightly higher than the devices with Spiro-OMeTAD, this result is encouraging, since the obtaining cost for FDT is cheaper than Spiro-OMeTAD and, furthermore, the FDT is soluble in more solvent environmental friendly, as toluene instead of chlorobenzene. Similar to the Graetzel’s group, Xu et al. [57] showed that SFX-based 3D oligomers called X55 (Fig. 12b, c) have a great potential to replace the SpiroOMeTAD. For efficiency of the device with X55 HTM, it was found a PCE of 20.8, while that for the device using Spiro-OMeTAD, the PCE observed was 18.8%. Another important factor observed for these HTMs was its stability since, for the device with X55, after six months stored, the PCE exhibited was 19.3% while that to the device with Spiro-OMeTAD PCE was 16.9%. Wang et al. [58] recently have proposed two new alternatives of HTM for solar cells, organic molecule, which was called X61 and X62. For a photovoltaic device with X62 HTM, it was observed PCE of 15.9 while that to control device (with Spiro-OMeTAD), PCE value was 10%. In this work, the improvement of PCE is addressed to the shift of the valence band to a more negative value, which potentiates the hole transfer process from the perovskite layer to the HTM film.

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Fig. 12 a J–V curves of perovskite solar cells fabricated with FDT, and Spiro-OMeTAD. Reprinted with permission from Ref. [56]. Copyright 2016 Springer Nature. b J–V curves of perovskite solar cells fabricated with X55 and Spiro-OMeTAD. Reprinted with permission from Ref. [57]. Copyright 2017 Elsevier. c The chemical structure for X55. Reprinted with permission from Ref. [57]. Copyright 2017 Elsevier

Bi et al. [59] in its work also showed a new approach of HTM of easy obtained. In this work, the researcher showed that the N ,N ,N ,N -tetrakis(4methoxyphenyl)spiro[fluorene-9,9 -xanthene]-2,7-diamine HTM labeled of X59 exhibit efficiency (19.8%) similar to the photovoltaic device containing SpiroOMeTAD, PCE of 20.8%. Although the PCE value for the device with X59 is lower than the one with Spiro-OMeTAD, the synthesis of X59 HTM was classified as being a more interesting alternative, a significantly more positive factor in the designing of new HTMs. All HTMs described previously are organic molecules. In the last years, inorganic materials have also attracted much attention to applications as HTMs. These new materials have shown ability to improve PCE and stability in perovskite-based photovoltaic devices, as well as reduce the cost of perovskite solar cells. Such materials are mostly copper oxide-base, which are reported in literature [60]. For example, Rao et al. [61] obtained an inverted perovskite-based solar cell using CuOx as HTM, Fig. 13a, b. To this device, it was observed a PCE of 19.0%, as well as the absence of hysteresis, which can be attributed to fewer defects in the interface

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Fig. 13 a Schematic illustration of the energy level diagram to a device containing CuOx as an HTM. b J–V curve of the perovskite-based device containing CuOx as an HTM. Reprinted with permission from Ref. [61]. Copyright 2016 Elsevier

perovskite/HTM. This result (PCE) is closer to values found in the literature to PCE in systems using Spiro-OMeTAD as HTM. Zhang et al. [62] described the potential that CuGaO2 to the HTM. The synthesis of CuGaO2 is easier than Spiro-OMeTAD, and moreover, this oxide has better stability. Thus, when it was made a comparison of PCE to different HTM, the device with CuGaO2 presented PCE (18.51%) better than the device with Spiro-OMeTAD, 17.14%. The difference between PCE values is slightly small; however the device with CuGaO2 exhibits excellent stability when stored under humidity, which was not observed to the device with Spiro-OMeTAD. Liu et al. [63] also showed the versatility of copper oxides for HMT. It was made Cu2 O-based (modified) device, which was observed a PCE to 18.9%, while that to the device with Cu2 O (unmodified), the PCE was 11.9%. The device using Spiro-OMeTAD as HTM exhibits a PCE of 20.0%, which is slightly higher than Cu2 O-based device, but the stability studies to these devices show that both devices with Cu2 O display better stability than Spiro-OMeTAD. Though the results described previously look great, who presented the best proposal of new inorganic HTMs was Chen et al. [64] with Zn: CuGaO2 -based solar cell, Fig. 14a, b. In this solar cell, besides improving PCE (20.67%), thermal stability and resistance under a humid environment have been both observed. Herein, though it has been exhibited an inverted perovskite solar cell, which was used NOx instead of TiO2 , the improvement of PCE may be addressed to the insertion of Zn2+ ion into CuGaO2 because Zhang et al. [62] reported a PCE to CuGaO2 -based solar cells with lower PCE. The insertion of Zn2+ improves the alignment of the valence band of HTM with the perovskite, which facilitates the hole transport.

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Fig. 14 a Schematic illustration of the device structure and J–V curve between a device with Zn:CuGaO2 . b Comparison of J–V curve between a device with and without Zn:CuGaO2 . Reprinted with permission from Ref. [64]. Copyright 2019 Elsevier

3.3 Application of SWCNTs in Perovskite-Based Solar Cells 3.3.1

A Short Approach to Carbon Nanotube

CNT structures, due to their versatility, can become available as the most incredible electronic and optical applications. It depends on their chirality since CNTs can display different diameters, resulting in different semiconducting or metallic characteristics [65]. It takes place due to their molecular structure present different structure band which results in different bandgap, factors acquainted for their conducting properties. General reduction of diameter to CNT is followed by an increase of bandgap value, which makes it an easy tuning structure. For electronic applications, the chirality of SWCNT, (n, m) index has to satisfy n − m = 3j, wherein j is an integer to SWCNT metallic and n − m = 3j + 1 or 3j + 2 to SWCNT semiconducting [66]. This overview previously described shows us the potentiality to the most diverse type of applications that CNTs display nowadays. Generally, p-type CNTs are the most used to perovskite-based solar cell, but CNTs can also present n-type characteristic as well as it can be obtained from ptype semiconducting. This depends on surface functionalized, and synthesis methods used [67], which results in a different type of defects created in their structure. For example, photovoltaic applications of single-chirality SWCNTs have been shown better than SWCNT chiral mixtures and metallic semiconducting [68]. Here, the factor crystallinity is very important, since a mixture of CNTs has lower crystallinity and therefore more defects, which are responsible for photoluminescence. The increase of crystallinity (prove by Raman spectroscopy) in chiral CNTs reduces the amount of defects, leading to a lower e− /h+ recombination rate, making it ideal to photovoltaic applications such as solar cells. The development of new materials toward solar cells is supported in an improvement of p and n-type material, which can be achieved by functionalization technical. CNTs have been known either p- an n-type material, and it depends on the functionalized group. For example, SWCNTs exhibit p-type majority charge carriers due to

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oxygen impurity [69]. Therefore, SWCNTs (not functionalized) can act as electron transfer material (ETM) in solar cells system. Unlike SWCNTs, when functionalized with organic molecules nitrogencontaining electrons donors such as polyethyleneimine (PEI) [70] and phosphine [69c], they can convert SWCNT from p- to n-type. It happens because the doping molecule transfers partial electrons to conduct band SWCNT (LUMO) and block in part hole in the valence band [69c]. This versatility to CNTs shows us the potential to this class of the material in the most diverse applications, thus toward the development of electronic devices.

3.3.2

SWCNTs into Perovskite-Based Solar Cells

Perovskites are the most commonly studied materials for the application in solar cells, a fact which has resulted in a large amount of reviews reported [36c, 71]. Though perovskite structure has presented PCE over 20% [20, 72], three issues of fundamental importance have to be overcome concerning devices containing such structures: (1) low stability, (2) high hysteresis, and (3) charges recombination, electron/hole pair. The low stability in perovskite structure is a consequence of degradation of perovskite structure under humidity [73], heating, and/or on a long time under irradiation, while the hysteresis can be attributed to charge accumulation either in trap states of the perovskite/TiO2 interface or in the HTL, for example, Spiro-OMeTAD involving LiTFSI and TBP as p-dopants [74]. To high recombination rates, they are results of localized new states created into the structure during synthesis or deposition process, which are derived from structure defects [75]. Thus, to photovoltaic systems containing SWCNTs on perovskite working as a p-type semiconductor (Fig. 15a–c),

Fig. 15 a Schematic of a MaPbI3 solar cell, using an s-SWCNT interfacial layer to extract holes. b Fast PHT is observed from MAPbI3 to s-SWCNTs, while recombination is slow. c J–V curve of a representative MAPbI3 solar cell with and without an s-SWCNT hole extraction layer, only showing the fourth quadrant. Reprinted with permission from Ref. [76b]. Copyright 2017 American Chemical Society. s-SWCNT, semiconductor single wall carbon nanotube

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Fig. 16 SEM images of (a, d) CH3 NH3 PbI3 films with CNTs, (b, e) conventional CH3 NH3 PbI3 films, and (c, f) CH3 NH3 PbI3 films with s-CNTs. g XRD patterns of pristine CNTs, s-CNTs, conventional CH3 NH3 PbI3 film, CH3 NH3 PbI3 films with CNTs and s-CNTs. Reprinted with permission from Ref. [78]. Copyright 2016 Royal Society of Chemistry

it is possible to achieve an increase of J, V oc , and FF as well as reduction of the recombination rates and hysteresis, resulting in a gain of PCE [76]. Currently, solar cells containing structure perovskite/CNTs display PCE to values between 10 and 20% [72a, 77]. Zhang et al. [78] have obtained perovskite solar cells with functionalized SWCNTs (sulfonic group, s-SWCNT), which has presented great results. In this work, it was observed an increase (from 12.50 to 15.10%) of PCE to devices with s-CNTs. The incorporation of s-SWCNTs into perovskite increased the crystallinity and grain size of perovskite, which was proved by XRD pattern and SEM images, Fig. 16. The increase of crystallinity and grain size indicates that to such structure, there is a less amount of structural defects to perovskite as well as a reduction of the presence of grain boundaries, observations also reported at several works [34a, 79]. The grain size is directly related to optoelectronic properties of perovskites, since, films with little grain generally, much grain boundaries (harmful to PCE) have been found. Such factor takes place because grain boundaries, which are from defect states, forward to high recombination rate, which has been investigated by photoluminescent measurement and high-resolution confocal [79b, 80]. Zhang et al. [78] in your work have displayed the importance of synthesis method in the improvement of PCE in solar cells perovskite-based. In this work, all J–V data to the device with s-CNTs has been presented better, with highlight to FF which presented the value of 0.75, suggesting that to such device, there is less interface defect as well as structural defects in the perovskite, observations that are suggesting the existence of a smaller amount of hysteresis. Thus, the control of growth to perovskite single-crystal films is one of the key roles to improve the optoelectronic properties in photovoltaic devices [34a, 81]. The control of morphology in the preparation of the film is also important to improvement, which has been described by Zhang et al. [78]. Differently from Wang et al. [82], concerning building a FTO/TiO2 /CH3 NH3 /PVSK-SWCNTs/SWCNTsphotovoltaic system, it was described that to SWCNTs when added on the top of peroveskite a PCE of 15.73% (Fig. 17a, b) can be achieved, which is a better alternative compared to pristine. Here, similar to the Zang et al. it took place an increase of

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Fig. 17 a J–V curves of various devices. b J–V curves measured in the forward and reverse scanning directions. Reprinted with permission from Ref. [82]. Copyright 2019 American Chemical Society

J–V data indicating that the method used (CNT bridging) results in superior charge extraction and transferability. The hysteresis is another factor that has worried the researcher since its presence also contributes to the reducing of the PCE. The CNTs have also been able to reduce hysteresis, an observation made by Wang et al. [82]. The reduction of hysteresis— like observed here by the author indicating less structural defects and therefore little recombination rates, which is supported by photoluminescence impedance measurement. The CNT bridging method has shown effectiveness to an improved enhancement of exciton separation and carrier transport as well as the superior conductivity resulting in great stability during ninety days, results which can be considered very promising. As it has been previously described, the papers have shown that preparation method to perovskite-CNTs films is an important factor to a better PCE. We can support this affirmation when compare the Aitola’s works [77c] with Tiong’s works [77a]. Both works have SWCNTs on the top of perovskite. Aitola et al. have added SWCNTs via simple press transfer from method filter paper (dry method), whereas Tiong co-works has used spin coating method are quite different, since to Aitola’s work, a PCE reduction from 18.4% to 15% occurred, while to Tiong’s, an increase from 9.8 to 10.1% has been observed. The key role to better PCE here is the homogeneity of the films. In the dry method, it is possible to obtain a regular film while to spin coating method this is not possible, which strongly interferes in the separation of charge process, factor important to efficiency in solar cells. To also methods, the data to J–V curve is better when compared with control. It was also observed that to devices with s-CNTs, a smaller hysteresis has been observed as well as a better stability.

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3.4 Stability Issues in Inorganic Solar Cells The first application of perovskite structure as an absorbing material was made in 2009 by Kojima et al., which for PCE, 3.8% was achieved [83]. This discovery has been a milestone in the history of new materials for the development of photovoltaic devices. From Kojima and co-workers until nowadays, several contributions have been given, and currently, perovskite solar cells (PSC) have achieved an efficiency over 22%, [20, 23b]. Although perovskite-based solar cells have been achieved PCE to 22%, stability issues have been a big obstacle toward their commercialization as a manufactured product. The PSC is sensitive to moisture, thermal stress, molecular oxygen, and even long time under ultraviolet light. So, currently, great efforts have been made to improve the stability of PSC [84]. Among several factors, one that has been influencing the stability for perovskitebased solar cells is humidity, which makes them one of the biggest challenges to be overcome. The moisture is very dangerous because it causes degradation of perovskite as illustrated below, proposed mechanism by Walsh and co-workers to form halide acid and organic base [85]. Therefore, it creates new methodologies that to be able to obtain perovskite films with more stability, it becomes necessary. In order to improve the stability of perovskite solar cell, You et al. [86] have used a new strategy to build the perovskite-based photovoltaic device, Fig. 18a. In this new approach of the device, metal oxides were used to protect the perovskite thin film. To this new structure (ITO/NiOx /perovskite/ZnO/Al), which NiOx to work as p-type (HTML) and ZnO as n-type (ETML), an efficiency of 16.1% has been achieved and after a 60 day (1440 h) storage in air at room temperature, a loss of only 10% in PCE took place. In this new design, NiOx and ZnO beyond work as HTML and ETML also act to prevent the perovskite structure to moisture, which avoids its degradation, which are encouraging results. Other works reported [87] also forward to results similar described by You and colleagues. For example, Zang

Fig. 18 a J–V curves for ITO/NiOx (80 nm)/perovskite/ZnO(70 nm)/Al stored in ambient air for several days. b Stability of the devices in an ambient environment (30–50% humidity, T = 298.15 K). Reprinted with permission from Ref. [86]. Copyright 2015 Springer Nature

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et al. [88] have described a perovskite structured ZnO-based device. The results are too clear concerning the effect that the zinc oxide has in both, acting as ETM and protecting the perovskite structure, which after 90 (2160 h) days it is still possible to have 88.7% of PCE. Among metals oxides used to encapsulant perovskite structure, aluminum oxide also has stood out. Choi et al. [89] using the atomic deposition layer method, obtained a perovskita-based photovoltaic device encapsulated with Al2 O3 . To this device, excellent durability (Fig. 19a) to PCE was achieved, which is attributed to the capacity of Al2 O3 to prevent moisture in the perovskite structure. In this same approach, Wu et al. [90] have obtained a better result using a bismuth interlayer in their device. Besides, an improvement to PCE (18.02%) has been achieved and after 500 h stored at ambient air in the dark 95–97% of their initial PCE. As displayed in Fig. 19b–c, the bismuth oxide prevents the entrance of water and molecular oxygen in the perovskite structure resulting in the low rate of degradation of the device. Another strategy that has shown great results in stability of perovskite-based solar cells is used to doping of metals ions [91]. Depending on metal ion and oxidation state, the chemical bonds can be stronger enough and therefore to a more stable structure. For example, Chan et al. have called attention to the potential that the alkaline-earth metal cations (Ba2+ , Ca2+ , Sr2+ , and Ba2+ ) can offer in the replacement of lead in perovskite-based photovoltaic devices [91b]. In this work, it was obtained a perovskite structure doping with metal cations from two groups of the periodic with highlight to barium. To Ba2+ doped (3.0 mol%) perovskite device, it was observed an increase of PCE from 11.8 to 14.9% when compared with pristine perovskite, which results from better J–V curve parameter (J sc , V oc , and FF). The best of all is that besides the improvement, a better stability in control atmosphere (N2 with H2 O, 0.1 ppm) has been achieved to this device with 3% mol% Ba2+ , Fig. 20a. The Cs+ doping in the perovskite structure has been another great alternative to improve stability. Zhang et al. [91d] have observed that the replacement of organic cations by Cs+ (5%) improved the stability of the device, observations also made by Saliba et al. [92]. These results suggest in the perovskite structure to form a stronger bond justifying a better stability under humidity. Although it has been reported some inorganic materials with the potential to prevent moisture in the perovskite structure, organic materials are also related to such proposal, which are greatly described by Yang et al. [93]. Yang and co-works have described the importance that organic polymers can act to functionalization of perovskite thin films and thus prevent the moisture. With this new approach to build perovskite thin films to solar cells, they also have achieved successful functionalizing ammonium salts (tetra-methyl ammonium, hexadecyl trimethyl ammonium, tetra-ethyl ammonium, tetra-butyl ammonium, and tetra-hexyl ammonium) on top of perovskite film, which after stored in 55% relative humidity (in the dark) shown to be stable for 500 when compared with the unfunctionalized. This ammonium salts are low soluble in water, justifying its action to prevent water, with highlight to tetra-ethyl ammonium (Fig. 21).

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Fig. 19 a PCE’s of full mesoporous (FAPbI3 )0.85 (MAPbBr3 )0.15 /HTM/Au/50 nm Al2 O3 PSCs during storage at room temperature (dark) in 50% RH for 7500 h (~10 months). Reprinted with permission from Ref. [89]. Copyright 2018 Elsevier. b Stability tests to FTO/NiMgLiO/PVK/PCBM/BCP/Bi/Ag devices. Reprinted with permission from Ref. [90]. Copyright 2019 Springer Nature. c Schematic diagram to: the device structure type FTO/NiMgLiO/PVK/PCBM/BCP/Bi/Ag; energy level diagram; SEM cross-sectional image of the device (scale bar: 500 nm); J–V curves of a typical large-area. Reprinted with permission from Ref. [90]. Copyright 2019 Springer Nature

Using the same previous methodology, Meng et al. [94] also have obtained great results of the stability of perovskite-based devices. In this work, Meng and colleagues have shown the potential that the 1-dodecyl mercaptan (NDM) has to modify the interface between perovskite and hole transport layer (HTML) and prevent water into perovskite. For such device (with NDM), it was possible to achieve an improvement to PCE (15.04%) when compared with the pristine (without NDM, PCE 12.75%) and a better stability after stored in air at room temperature over 2400 h. Several other

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Fig. 20 a Stability of PSCs with the pristine and 3.0 mol% Ba2+ -doped perovskite films when stored the glovebox system with the integrated gas purification system. Reprinted with permission from Ref. [91b]. Copyright 2017 Royal Society Chemistry. b Long-term stability measurements of both solar cell without any encapsulation under ambient conditions where the relative humidity is 30% RH. Reprinted with permission from Ref. [91d]. Copyright 2017 Royal Society Chemistry

Fig. 21 a J–V curves with reverse and forward scans for typical PSC devices based on MA, TMA, and TEA films. b Evolution of the normalized photovoltaic parameters of unsealed devices which were stored in 55 5% relative humidity and dark conditions. Reprinted with permission from Ref. [93]. Copyright 2016 Springer Nature

works have also called attention to the potential that organic molecules have to prevent water when they are supported on top of perovskite films. Such assumption suggests that the size [93, 95] (Fig. 23) of the organic molecule, as well as low solubility in water, are pre-requisites for a great design of a structure moisture-tolerant and therefore provide an improvement of stability (Fig. 22).

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Fig. 22 a Schematic structure of a PSC, modified by NDM molecules on the interface of MAPbI3 and an HTL. Schematic structure for the surface of pristine MAPbI3 and MAPbI3 anchored with NDM. b Current-voltage curves of the best PSCs using pristine MAPbI3 (orange curve) and NDManchored MAPbI3 (blue curve). c Time evolution of the efficiency for the two different devices stored in air at room temperature out of the sun. Reprinted with permission from Ref. [94]. Copyright 2019 American Chemical Society

4 Organic Solar Cells It is well known that OSC is a junction between two-organic molecular structure, one electron donor and another electron acceptor (Fig. 24) so-called n–p heterojunction as illustrated in Fig. 25a–c. Though inorganic-based materials, photovoltaic devices have exhibited great results (PCE > 22% [25]), organic solar cells (OSCs) since their discovery in 1986 by Tang [96] with PCE of ~1%, great efforts [97] have been made by researchers in the last decades, and currently, a record PCE of 14% has been achieved by Zhang et al. [97a]. For organic molecules used in OSCs, depending on the molecular weight, they can be classified into conjugated polymers (high molecular weight compound with repeating units in the polymer chain), oligomer (low molecular weight compound with repeating units), and small molecules, which are considered low molecular compounds without repeating units. For the first proposal of OSC (by Tang), it was presented a donor–acceptor bilayer structure wherein the electron acceptor material was coated on donor material. In this photovoltaic device, donor material is responsible for absorbing the light and exciting the electrons from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) of the donor material, and such electrons

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Fig. 23 a Molecular structure of β-cyclodextrin. b Schematic diagram to show the interactions between the perovskite and β-cyclodextrin molecules. c Schematic diagram to show the mechanisms of the degradation process of the pristine perovskite. d The water-resistant property of the perovskite with β-CD. Reprinted with permission from Ref. [95a]. Copyright 2019 Royal Society Chemistry

follow LUMO of the accept material as illustrated in Fig. 25d. Moreover, there are other two configurations to OSC, one which the donor and acceptor materials are mixed to form a junction in a single layer, which is known as the bulk-heterojunction (BHJ) configuration, and another wherein the position of the electrodes are inverted, a concept this introduced by Glatthar et al. [99]. In this last configuration, there is a change in the position of the electrodes (e.g., ITO or the FTO), so these act as the cathode and the high work function metal acts as the anode, as well as the polarity of charge collection also is inverted.

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Fig. 24 a Chemical structures of the fullerenes. b Chemical structure of some conjugated polymers used in OSCs. Reprinted with permission from Ref. [98]

Fig. 25 a Conventional device structure to BHJ OSCs. b Inverted structure device to BHJ OSCs. Reprinted with permission from Ref. [100]. Copyright 2017 Elsevier. c Structure bilayer to OSCs proposes by Tag. d Principle of working to conventional OSCs. c BHJ OSCs

4.1 Main Electron Donor/Acceptor Molecules Used in OSCs Among OSCs, BHJ is one of the most studied configurations in the last years [101]. For this model, which consists of electron donor/acceptor blend system, π -conjugated polymers are used as electron donor materials, which can highlight poly(3-hexylthiophene), so-called P3HT, among others [102]. A typical structure scheme of the bilayer OSCs is P3HT (or analogous) as donor mater and [6,6]-phenylC61 -butyric acid methyl ester (PC70 MB) or analogous as the electron acceptor. For this structure of the OSCs, PCE over 5% [103] has been achieved, but many efforts aiming at improving the performance of BHJ solar cell devices have been made. For

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polymers act as an electron donor, it is necessary that it presents absorption in the visible range, since this ensures a better absorption of photons in the visible range. Currently, the challenge for improving in OSCs is to develop donor materials (polymers) with high absorption on the visible range, which implicates in obtaining low bandgap π -conjugated polymers with reasonable energy levels able to harvest more photons from solar light [104]. For these materials besides having a low bandgap (between 1.2 and 1.9 eV), it is also required that the energy level HOMO to be − 5.2 and −5.8 eV while for LUMO energy level −3.7 and −4.0 eV is required [105]. This last condition is directly related to the alignment between HOMO and LUMO of electron acceptor, which improves the carrier charges and so enhances the PCE value. Although P3HT is ordinarily used as donor material, other polymers like PCPDTBT [106], PCDTBT [106a, 107], PTB7 [108], and DPPTT [109], which present absorption in visible, have been presented as an alternative of electron donor exhibiting PCE similar or better than P3HT when combined with electron acceptor (fullerene or no-fullerene). Bin et al. [110] knowing the importance of absorption of visible spectral to organic molecules to the improvement of PCE in OSCs studied the influence of the replacements of thiophene conjugated side chain of the benzodithiophene (BDT) for different branched alkyl (Fig. 26) in the photovoltaic performance of OSCs. For simple replacement of the 2-iso-octyl (J52) by alkylthio ramification (J60), alkylthio linear (J61), and tri-N-proprylsilane (J71), it was observed a change in the profile in the absorption spectra, which resulted in a narrowing of the bandgap. As illustrated in Fig. 27, the absorption spectrum to J60 and J61 is similar, but a red-shift was exhibited when compared with J52. This suggests that the groups added are strong electron donors favoring the electronic transitions in low energy. Thus, the improvement of the absorption in the visible range to these molecules and its use as an electron donor in OSCs resulted in an increase of PCE (from 5.51 to 11.20%, Table 1). The work of the Bin and collaborators shows the following to the engineering of donor molecules able to obtain of OSCs of high performance: Here, it is important to highlight that although the absorption of the donor is improved, the HOMO and LUMO of donor/acceptor must have the max of alignment, a condition necessary to charge carrier. Xu et al. [111] use the concept of the composite to prepare new donor materials to solar cells. Herein, we see the importance of synergism as well as the visible light absorption of the composite to improve the performance of the OSC. With an adding of PTB7-Th weight ratios, it rises a new band at 750 nm. For device optimized (ratio 0.7), it was found a PCE of 12.27%, while that to content 0.0 and 1.0, the PCE was 9.39 and 10.10%. Fei et al. [112] found PCE value slowly higher than it was found by Xu et al., but to a different donor/acceptor system. In this system, it was used the PFBDB-T/C8-ITIC-based cell, which presented PCE of 13.2%. For this great performance, the authors attributed the replacement of hydrogen in PBDB-T by fluorine. This substitution is responsible for the shift of the absorption band to more wavelength, which results in a narrowing of the bandgap, and thus more absorption in the visible range.

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Fig. 26 Chemical structure of the copolymer. Reprinted with permission from [110a]. Copyright 2016 American Chemical Society Fig. 27 a Absorption spectra of the donor polymers in film states. Reprinted with permission from Ref. [110a]. Copyright 2016 American Chemical Society

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Table 1 Properties of the copolymers Copolymers

λmax (nm)

λedge (nm)

Eg (eV)

V oc (V)

J sc (mA/cm2 )

FF (%)

J52

538, 590

630

1.96

0.73

13.11

0.57

5.51

J60

550, 598

642

1.93

0.91

16.33

0.60

8.97

J61

552, 600

642

1.93

0.89

17.43

0.61

9.53

J71

528, 573

632

1.59

0.94

17.40

0.68

11.20

PCE (%)

Although the results presented above are good, recently, Zhang et al. [97a] have presented a new design of OSCs using PBDB-T-2Cl as a donor material. Unlike Xu and collaborators, the hydrogen was replaced by chloride instead of fluorine. For the device containing PBDB-T-2Cl, it was found a PCE of 14.4%, which was addressed to lower HOMO and LUMO levels of the chlorinated polymer when compared with a fluorinated polymer. Furthermore, when it was made a comparison of stability between two donor polymers, the device with PBDB-T-2Cl presented a better stability. So far, we have described only to electron donor, but it is known that the electron acceptor also has its importance in the process of manufacturing of OSCs. Although in order to be building an ideal organic donor material, the choice of the acceptor pair to the flow of the hole is important, and this is only possible if its HOMO and LUMO present a good alignment with donor material, since these conditions toward to increase the V oc and therefore improve PCE, Fig. 28a. Following this pathway, in the literature, there are a huge number of options to acceptor material in OSCs, which generally are classified into two big groups, fullerene and non-fullerene acceptor groups [98, 113]. Furthermore, non-fullerene acceptors can be divided into two categories: (1) small molecules [113d, 114] and (2) polymer acceptors [115]. In the last decades, fullerene-based acceptors have been widely used in OSCs, which PCE values found to these solar cells are from 1 to 10% [116]. For example, Kim et al. [117] obtained fullerene-based OSCs PPDT2FBT/PC71 BM (donor/acceptor), which presented PCE of 8.16%. However, concerning the same device, it was added another donor material (mix PPDT2FBT:IDT2BR), and the PCE value was increased to 9.02%. Herein, the improvement of PCE can be addressed to a better alignment of the HOMO/LUMO of donor material with HOMO/LUMO of the acceptor, PC71 BM. Zhao et al. [118] showed that to fullerene-based OSCs, the choice of solvent influences the improvement of PCE. In this work, the authors addressed the improvement of PCE (11.7%) to the synergistic effect of solvents 1,2,4trimethylbenzene (TMB) and 1-phenylnaphthalene (PN) in the solar cell, which acted directly in the morphology control. When making the comparison between the two devices without (TMB) and with a solvent mix (TMB-PIN), it was observed an increase to all photovoltaic parameters with highlight to V oc . The increase of V oc

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Fig. 28 a Simple energy diagram of a donor/acceptor interface showing the HOMO and LUMO energy offsets (HOMOs and LUMOs ). b Example of a device configuration, with a depiction of idealized morphology of the donor/acceptor active layer. c Most commonly used electron acceptors in solution processable bulk heterojunctions: [6,6]-phenyl-C61 -butyric acid methyl ester (PC61 BM) and [6,6]-phenyl-C71 -butyric acid methyl ester (PC71 BM). Reprinted with permission from Ref. [114a]. Copyright 2015 American Chemical Society

and J sc in the device indicates a better alignment between HOMO and LUMO of donor/acceptor blend and more absorption of the visible spectrum, resulting in great PCE. Currently, it is known that to fullerene-based OSCs, poor light absorption, limited chemical, energetic tunability, high cost, and instability morphologic are issues that hinder practical applications of fullerenes acceptors in OSCs. Thus, the researchers have been devoted to developing non-fullerene electron acceptors, which the PCE values are from 1 to 16% [113a, 113d, 114c, 116b, 119]. For example, in 2016, Zhao et al. [119a] showed that it is possible to obtain non-fullerene OSCs of high performance. For this device, it was used the 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2 ,3 -d ]-s-indaceno[1,2-b:5,6 b ] dithiophene) (ITIC) as acceptor, which presented a PCE of 11.21%. The results exhibited by Zhao and co-work agree with article reported by Li et al. [114c], which used an analogous acceptor (IT-M) in their device, resulting in a performance of 12.05%. Currently, the best results of non-fullerene-based OSCs were reported by Li et al. [120b] and Cui et al. [116b], which displayed PCE of 13.9 and 15.08%, respectively, Fig. 29. The new PCE values found are very positive in scientific communities, since such values were recently achieved only in inorganic-based solar cells.

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Fig. 29 a Chemical structure of the IT-4F acceptor. b J–V of the P2:IT-4F-based blend cell. Reprinted with permission from Ref. [119b]. Copyright 2018 American Chemical Society. c Chemical structure of the acceptor BTP-4Cl used in OSCs. d J–V curve of the PBDB-TF:BTP-4F blend cell. Reprinted with permission from Ref. [116b]. Copyright 2019 Springer Nature

Among the wide number of electron acceptors used in OSCs, the 2,2 [[6,6,12,12-Tetrakis(4-hexylphenyl)-6,12-dihydrodithieno [2,3-d:2 ,3 -d ]-s indaceno[1,2-b:5,6-b ]dithiophene-2,8-diyl]bis[methylidyne(3-oxo-1H-indene2,1(3H)diylidene)]] bis[propanedinitrile] (so-called ITIC) and its derivates have been widely used in organic-based photovoltaic blends [120]. Its potential as an acceptor to the improvement of PCE depends on the substituted groups in structure (Fig. 30), as it showed by Yang et al. [121]. With a great optimization in the ITIC structure, the charge carrier can be improved, which is observed by an increase of V oc and J sc resulting in high performance of the OSCs. For example, Zhao et al. [122] showed that to ITIC when four hydrogen atoms are substituted by fluorine atoms (IT-4F), the performance in the device increases from 11.05 to 13.1%. Comparing the photovoltaic parameters of two devices (PBDBT:ITIC vs. PBDB-T-SF:IT-4F), though after adding of fluorine in both, donor polymer and acceptor material, it has taken a reduction of V oc (from 0.90 to 0.88 eV), J sc increased from 17.02 to 20.88 eV, which means a better absorption of visible spectrum, data which is supported by the absorption spectra, since the PBDB-T-SF:IT-4F

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Fig. 30 a Nonfullerene structure acceptor materials. Reprinted with permission from Ref. [121]. Copyright 2019 Royal Society of Chemical. b Molecular structure of the fluorinated donor and acceptor. c J–V curve of PBDB-T-SF:IT-4F blend used at the photovoltaic device. Reprinted with permission from Ref. [122]. Copyright 2017 American Chemical Society

blend presents a shift-red. As shown above, IT-4F depending on substituents in both, donor and acceptor, an improvement of PCE can be achieved. Li et al. [119b] using different polymer (PDTB-EF-T) obtained PCE of 14.2%. However, though the results presented above are great, it was Cui et al. [116b] who has presented BJH OSCs with excellent performance to PBDB-TF:BTP-4Cl blend with PCE of 16.5%. Cui and co-worker showed that the replacing of the fluorine atoms by chlorine in the acceptor could increase photovoltaic parameters (V oc , J sc , and FF), resulting in higher PCE. Such replacement is responsible to exhibit an extended optical absorption and meanwhile displays a higher voltage when compared with PBDB-TF:BTP-4F.

5 Tandem Solar Cells Both inorganic and organic solar cells have achieved excellent PCE values, as previously described. The efficiency of a solar cell depends on the bandgap energy of the semiconductor since the spectrum of sunlight has a wide variety of wavelength (Fig. 31), from infrared (lower energy) to ultraviolet, higher energy. Thus, for singlejunction solar cells (a particular bandgap), only a little wavelength range of solar spectrum can be used, which limits the performance of the photovoltaic device. According to Shockley and Queisser, for inorganic single-junction solar cells, PCE value under non-concentrated air mass 1.5 global (AM1.5G) sunlight can reach a

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Fig. 31 Schematic structure of the tandem organic solar cell. Reprinted with permission from Ref. [123]. Copyright 2009 Royal Society of Chemistry

maximum value of ∼33% with the bandgap between 1.1 and 1.4 eV [124]. Shockley– Queisser limitation can be overcome when the concept of multi-junction solar cells are employed. Solar cells tandem are known as a photovoltaic systems composed by stacking of the solar cells with different bandgaps. This configurates a better absorption of energy in the visible spectrum Fig. 32 and therefore improves the performance. Theoretical calculus made by De Vos et al. [125] described that for

Fig. 32 Standard solar spectra for space and terrestrial use. Reprinted from Ref. [126]. Copyright @ PVeducation

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two inorganic subcells, the performance of the photovoltaic device could rise up to 42% with bandgap of 1.9 and 1.0 eV, respectively, while to three inorganic subcells having a bandgap of 2.3, 1.4, and 0.8 eV, a PCE of 49% can be achieved. The use of the concept of a tandem for the organic system requires solving two well-known problems in the designer of organic photovoltaic devices: (1) poor charge carrier mobility and (2) low light absorption in the visible spectrum, which can be overcome by using a combination of different materials, donor–acceptor composite, which can increase the absorption of visible range as described by Xue et al. [127] and Ameri et al. [123]. Thus, through the tandem system, the performance is improved because thermalization losses of high-energy photons are reduced by conversion in the subcell with a wide bandgap and transmission losses are lowered by absorption of the low energy photons in the subcell with a small bandgap [128]. Currently, silicon-based solar cells, which are widely used around the world, present PCE between 25 and 26% [129]. As already described, the best PCE values for inorganic and organic single-solar cells are reported to 14.3% [98a] and 23.7% [25, 130], respectively. For multi-junctions, both inorganic and organic, performance values have already been overcome. For example, Werner et al. [131] already obtained in 2016 a perovskite-based tandem solar cell with PCE of 21.2%. This PCE value is the same photovoltaic which has been overcome by Bush et al. [132] and Jost et al. [129], which achieved PCE values of 23.6% and 25.5%, respectively. By comparing single and multi-junction of perovskite-based solar cells, there was an increase in PCE of 1.8% when compared with the value found by Li and coworkers [25]. Although the increase is small, in this model of multi-junction, there is a stability gain about the other perovskite-based photovoltaic devices. Besides, when compared with silicon-based solar cells, it looks that perovskite-based solar cells will achieve the max of performance within a few years, and solving the issues of stability, such technologies (cheaper than silicon technologies) will be able to manufacture in large scale and therefore commercialized. For organic-based multi-junctions, Meng et al. [133] have exhibited a new model of multi-junction for a two-terminal monolithic solution-processed tandem. In this design of solar cell, a PCE value of 17.3% was achieved, which is currently a record value with 2.9% more than the PCE found by Zhang and co-workers [97a], 14.4%. Making a comparison of multi-junction made by Meng and co-workers with singlejunction made by Zang and co-workers took place a reduction of J sc (14.21 V) and FF (0.72%), but V oc was increased to 1.63 V, which means that took place an increase in charge carries, therefore improving the performance of device. The results above described to tandem solar cells are great, but there are better results to GaInP/GaAs//Si-based tandem solar cells with values from 27.9% (over silicon-based) to 35.9% reported by Yamaguchi et al. [15a]. For example, Cariou et al. [134] in 2017 related a triple-junction tandem solar cell with 30.2% under 1-sun AM1.5g. Although these results are promising, Essig et al. [135] have shown some improvement for monolithic two-terminal III–V//Si tandem solar cell, which reached an excellent PCE of 35.8%. As it previously showed, the PCE values for tandem solar cells are much higher than other photovoltaic systems, but due to issues of growth

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Fig. 33 a Design of the four-terminal GaInP/GaAs//Si triple-junction solar cell. b NREL-certified J–V characteristics of the GaInP/GaAs top cell and the Si bottom cell when illuminated under AM1.5g spectral conditions at 25 C. The bottom-cell J–V curve was measured while the top III–V tandem cell was biased at its maximum power point. Reprinted with permission from Ref. [135]. Copyright 2017 Springer Nature

techniques, such photovoltaic systems are more expensive than silicon-based solar cells and therefore they are valid for large-scale production (Fig. 33).

6 Conclusion In summary, here we showed that photovoltaic technologies, which already are used all over the world, have a big grown potential, which depends on research advance. The OSCs in a short time have presented an expressive increase in PCE. The OSC devices have attracted attention due to being lighter, more flexible, and present a low manufacturing cost. The limitation for OSCs is directly related to issues of poor charge mobility and low absorption in the visible range. Another option to reduce the cost of producing solar cells is that an option to replace silicon-based technologies is perovskite-based solar cells. The PCE for these solar cells is almost ready at the silicon technologies, but issues of stability (also for OSCs) are still challenging to be overcome and so make them available for large-scale production. Finally, tandem solar cells are currently photovoltaic technologies that present a better performance, but the high cost in the manufacturing process makes it more expensive than silicon technologies, which makes it currently impracticable for commercial applications. Thus, we have tried to show the last advances in inorganic and organic solar cells aiming to improve performance.

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Photovoltaic Materials Design by Computational Studies: Metal Sulfides Edan Bainglass, Sajib K. Barman and Muhammad N. Huda

Abstract Materials design for the next generation of solar cell technologies requires an efficient and cost-effective research approach to supplement experimental efforts. Computational research offers a theoretical guide by applying cutting edge methodologies to the study of electronic structures of newly predicted materials. In this chapter, we present our recent research efforts on sulfides. First, we will also provide a brief overview of oxide-based photovoltaic materials. We have conducted a density functional theory (DFT) study of two sulfide systems: acanthite Cu2 S and S-doped triclinic CuBiW2 O8 . With these two systems, we will demonstrate both the cation and anion doping mechanisms. In Cu2 S, we investigate the effects of various cation doping in Cu sites, namely Zn, Sn, Bi, Nb, and Ta and contrast their electronic structures with that of a previously studied Ag-doped Cu2 S system. A subsequent charge analysis provides a correlation between dopant charge states and detrimental mid-gap trap state concentrations. We then present our best dopant choice for Cu2 Sbased photovoltaic systems. Finally, for CuBiW2 O8 , a new experimentally verified DFT-predicted quaternary oxide, the effects of S-anion-doping in O sites are studied, and results indicate favorable photovoltaic properties. This highlights the potential of S-anion-doping as a mechanism for engineering suitable band gaps for solar cell applications. Keywords DFT · Doping · Alloying · Cu2 S · CuBiW2 O8

1 Introduction Highly efficient, yet cost-effective, materials suitable for solar-to-electric energy conversion applications remain both elusive and in great demand, as continued dependence on fossil fuel risks the health of both the environment and the global population [1–3], and hence, the energy independence becomes a necessity beyond the economic aspect. Photovoltaic technology is well-established as one of the most viable candidates for tackling future energy demands. However, the challenge of designing E. Bainglass · S. K. Barman · M. N. Huda (B) Department of Physics, The University of Texas at Arlington, Arlington, TX 76019, USA e-mail: [email protected] © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_5

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new materials that can meet the host of properties required for efficient solar power conversion is a formidable one. Extensive research has been conducted over the decades in this pursuit, and though advances have been made, photovoltaic technology contributes less than 5% of the electric grid [4]. In part, this is due to the balance required between high efficiency and cost-effectiveness. New materials must be stable and reliable, earth-abundant, environmentally safe, and inexpensively scalable to meet global demands. To achieve the cost requirement, device efficiencies must be improved beyond the current state. Power conversion efficiencies of commercially available Si-based solar cells, for example, range from 18 to 22% [5]. Devices that surpass this efficiency tend to rely either on novel designs [6, 7], rare components, isolated conditions, or all of the above, and exist outside of any commercial markets. Other designs utilize inexpensive and abundant components but suffer from efficiencies too low for global scalability. Hence, additional research must be conducted to obtain a better understanding of material properties in hopes of increasing the utilization of solar energy. Traditionally, scientific breakthroughs in material science and engineering have been achieved primarily through experimental efforts. However, as computational resources became increasingly available, a significant theoretical effort has pushed for deeper investigations of the physical and chemical inner workings of materials [8, 9]. Today, computational studies are a common and often essential component of new materials research. They offer a cost-effective and efficient method for large-scale materials design and study and have produced many new materials, as evident in crystallographic databases such as the Inorganic Crystal Structure Database (ICSD) or the Materials Project [10]. In the following sections, we briefly give an overview of oxides and sulfides, two promising material groups at the frontier of photovoltaic materials research. We highlight the inherent difficulties and challenges of metal oxides in solar energy conversion applications and introduce sulfides as a promising alternative. Lastly, we present a computational investigation of the electronic properties of Cu2 S, a pure metal sulfide, and a study of the effects of S-doping in CuBiW2 O8 , a recently discovered novel quaternary oxide.

2 Metal Oxides Many aspects of PV devices incorporate metal oxide compounds, from transparent conductive layers, to transport barrier layers and photoactive light-harvesting layers. Oxides have been utilized in solar cells as insulators, semiconductors, and conductors due to their large range of conductivities [11]. They are non-toxic and abundant in nature due to their ability to form stable chemical bonds with most elements [12]. Many structural flavors of oxides exist including non-crystalline amorphous forms. This overall flexibility is highlighted in the emerging all-oxide solar cell field [13]. Of the various oxide materials, transition metal oxides are especially suited for PV applications. Their partially filled d-orbital character yields properties both unique and complex. Many binary transition metal oxides exhibit band gap energies

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larger than that of silicon (1.12 eV) and have thus been considered for transparent conducting layers (TCO) [14–16]. However, the requirement of both a wide gap and high conductivity is often at odds due to the band gap property. Much research has gone into the effects of dopants on the properties of transition metal oxides in attempt to tailor suitable band gaps. Most doped oxides in solar cells are n-type materials, especially for TCOs [17], due to intrinsic oxygen vacancies, and exhibit high electron mobilities due to strong metal–oxygen orbital overlap [18]. In contrast, highly electronegative O 2p states mixed with localized 3d states at oxide valence band maxima (VBM) hinder shallow acceptor states required for p-type doping and tend to instead result in large hole effective masses [19–21]. In some cases, such as Cu2 O, this challenge is resolved due to the proximity of Cu 3d and O 2p energy levels and intrinsic Cu1+ vacancies. Additionally, the ns2 electronic configuration, e.g., Bi3+ , has shown the presence of anti-bonding states at the VBM, which, through hybridization with O 2p states, can lead to reduced effective masses and higher hole mobilities [22]. This is analogous to the chemical modulation technique [23] of O 2p—Cu 3d VBM hybridization, which has yielded dispersive valence bands in Cu(I) ternary oxides. The technique has since been extended to the chalcogen group, as Cu 3d orbitals hybridize better with S, Se, and Te p orbitals [24, 25]. Cu2 O has also been utilized as a light-absorbing layer due to its 1.4–2.2 eV band gap [26] resulting in power conversion efficiencies of ~8% [27]. According to the Shockley–Queisser detailed balance limit [28], the band gap of Cu2 O can, at best, reach a ~22% theoretical conversion efficiency. On the other hand, cuprous oxide (CuO) has a theoretical efficiency limit of ~31% owing to its 1.4 eV band gap [29]. However, CuO has yielded poor efficiency thus far, which has been attributed to high defect concentration or interface recombination [30, 31]. Some improvements to CuO efficiencies were found in the nanoscale, reaching an efficiency of 2.88% [32]. To enhance the efficiencies of Cu-based oxides, we must pay careful attention to the challenges inherent in the oxide group. Oxides tend to be wide-gap semiconductors due to the high electronegativity of oxygen. Furthermore, 3d transition metal oxides suffer from greater localization at the band edges which generally results in large carrier effective masses. Thus, a study of different anion species may be required. Several of these have been studied in the scope of solar fuel generation, including the nitride and sulfide groups, as well as Se (CIG(S, Se)) and Te (CdTe). Theoretically, the reduced electronegativity of these species with respect to oxygen is expected to upshift the VBM, thus providing a mechanism for band gap engineering. And, as previously mentioned, Cu 3d tends to hybridize more strongly with the chalcogen group, which may lead to lower effective masses at the band edges. These are desirable properties for efficient solar energy conversion.

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3 Sulfides Cu-chalcogenides are well-known energy harvesting materials, especially in photovoltaic applications. These materials are composed of Cu1+ cations and chalcogen anions such as S, Se, and Te. Chalcogenides may also include other cations along with Cu from groups IIB, IIIA, IVA, and VA. Such compounds include Cu (In, Ga) (S, Se)2 and Cu2 ZnSn(S, Se)4 or CIG (S, Se) and CZT(S, Se), respectively, both regarded as prominent solar cell absorbers. Apart from these complex multinary compounds, the binary Cu-chalcogenide Cu2 S has also been found to be a promising solar absorber due to its suitable band gap of 1.2 eV. Of these, a CIGS-based solar cell was found to be superior due to its high solar conversion efficiency of nearly 20%, which was first reported by NREL [33]. Several other materials have shown similar efficiencies, including Si, GaAs, CdTe, and the perovskite group. However, Ga, In, Cd, and Te are relatively scarce and very expensive. Even though Si-based technologies are widespread around the globe, pure grade Si is not cost-effective nowadays and thus unsuitable to meet the global solar energy demand. In contrast, CZTS- and Cu2 Sbased solar cells, with reported solar-to-light conversion efficiencies of 12.6% [34] and nearly 10% [35], respectively, can be cost-effective alternatives in this respect due to the abundance of Cu, Zn, and Sn in Earth’s crust and their eco-friendly, non-toxic nature. To meet future energy demands and replace fossil fuel with cost-effective alternatives, thin film-based solar cells are so far the best candidate since they require less material and provide a pathway to go beyond the established Si-based solar cell technology [36, 37]. However, both CZTS and Cu2 S suffer from some intrinsic drawbacks which require careful attention and proper study to develop the necessary solutions. Defect-free synthesis of CZTS is rather challenging. Due to the similar ionic radii and tetrahedral coordination of Cu and Zn, the cations may swap their lattice sites with relative ease. In addition, annealing multinary compounds at high temperatures during synthesis results in defects to the crystal and electronic structures, which would eventually impact device activity [38]. In an earlier study, we have shown from thermodynamic considerations that defects would destabilize single-phase stability of CZTS [39]. Although Ba-substitution in Zn sites, leading to the chalcogenide family BaCu2 SnSex S4−x (BCT (S, Se)), provides a mechanism for controlling the coordination environment, the resulting conversion efficiency is well below expectation [40]. As a counterpart to multinary complex chalcogens, Cu2 S suffers from intrinsic p-type defects due to spontaneous Cu vacancy formations in the crystal. Hence, the performance of Cu2 S-based thin-film solar cells degrades in air over time. Careful crystal stability and electronic structure studies, along with possible doping effects on this material, may shine light on the prospect of their photovoltaic uses in the near future.

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4 Case Studies The density functional theory (DFT) is the state-of-the art tool for calculating electronic structures of materials. DFT methodology shifts the quantity of interest in electronic calculations from the complex wave function to the electronic density scalar. It establishes a one-to-one correlation between the two quantities and defines an energy functional of the latter. The energy functional can then be variationally minimized under constraint leading to the ground-state density of the system. With this quantity in hand, and via the one-to-one correlation with the wave function, all ground-state properties of the system are now obtainable. The mathematical rigor of the theory is encapsulated in the Hohenberg–Kohn theorems [41]. Applications of it involve iterative solutions of the so-called Kohn–Sham equations [42], a set of oneelectron Schrödinger-like equations describing a fictitious non-interacting system that, by construction, leads to the same ground-state density as the real, interacting system of interest. The theory has seen tremendous growth over the past 50 years and has since become a staple in the study of electronic structures of materials [43–47]. One of its many successes is the reproducibility of experimental results [48]. In the following subsections, we discuss results from two separate DFT studies of Cu(I) systems with a focus on photovoltaic properties. First, we address the properties of sulfides by studying the effects of doping on the relatively new acanthite phase of Cu2 S. Then, we present a study of the effects of S-doping on the novel CuBiW2 O8 quaternary oxide, dubbed CBTO. In both studies, the generalized gradient approximation of Perdew–Burke–Ernzerhof (GGA-PBE) [49, 50] was used to model exchange-correlation effects. The Kohn–Sham equations were solved in plane wave form, as implements in the Vienna ab initio Simulation Package (VASP 5.4.4) [51, 52], under the pseudo potential frozen-core approximation of the projector augmented wave (PAW) [53, 54] method. Plane-wave kinetic energy cutoff values of 520 and 600 eV were used for Cu2 S and CBTO, respectively. Reasonable convergence criteria were used for ionic relaxations (≤0.01 eV/Å in ionic forces) and electronic densities (≤10−6 eV in total energy). Brillouin Zone (BZ) integrations were performed on a 7 × 3 × 3 -centered k-point mesh for Cu2 S and a 7 × 7 × 7 Monkhorst-Pack [55] mesh for CBTO. A smaller 5 × 5 × 5 mesh was deemed sufficient for S-doped CBTO via convergence testing. Due to the well-known underestimation of band gap energies by pure DFT calculations, further exacerbated in localized d-electron systems [56–58], the Hubbard U parameter was applied (DFT + U) on Cu 3d to enhance on-site Coulomb interactions following the methodology of Dudarev [59]. An effective U value of 7 eV was used in the case of Cu2 S, while a value of 6 eV was used for CBTO, both adopted from previous studies [60, 61]. The different values are due to the unique chemical environments of the two systems. We note that the Bader charge analysis [62–66] conducted on doped Cu2 S was derived from a pure GGA-DFT calculation.

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4.1 Cu2 S Of the many experimentally known phases of Cu2 S [67–71], only the monoclinic low chalcocite structure forms below 104 °C (Fig. 1a). Cu2 S-based thin-film solar cells have previously shown efficiencies near 10% [35, 72]. However, the tendency to spontaneously form Cu vacancies in all phases and the consequent excessive p-type doping in the crystal make Cu2 S behave as a degenerate semiconductor [73, 74]. Hence, the efficiency of Cu2 S-based solar cells degrades over relatively short time periods [75, 76]. Recently, a new phase of Cu2 S known as acanthite [77] (Fig. 1b) was computationally derived from a materials database search and found to be thermodynamically more favorable than its naturally occurring counterpart (low chalcocite). Despite the new phase not showing any indication that its tendency for Cu vacancy formation is any less than that of low chalcocite, a theoretical study did show that Ag alloying in the acanthite phase can reduce this tendency, as well as control the diffusion of Cu ions inside the crystal structure [60]. The study also found that the electronic band gap of the acanthite phase can be increased with Ag alloying without introducing any intermediate or defect states to its band structure. This suggests the potential for tailoring a suitable band gap for photovoltaic applications via Ag alloying in Cu2 S. Experimentally reported band gaps for Cux S (1 ≤ x ≤ 2) are all within 1.1–1.2 eV. A recent study of silver copper sulfides comprising both experimental and theoretical efforts demonstrated promising electronic structures for AgCuS (stromeyorite) and Ag3 CuS2 (jalpaite), both suitable for high-efficiency solar cells [78]. Hybrid density functional theory was implemented in band gap calculation yielding 1.27 and 1.05 eV, respectively, for AgCuS and Ag3 CuS2 . Hence, Ag doping of Cu sites, or Ag alloying in the crystal structure, offers an opportunity to tune the band gap of Cux S-based materials for photovoltaic applications. It was also reported in a separate study that favorable growth of Cu2 S thin-film interfaces with suitable metal oxide thin films such as TiO2 and Al2 O3 could help stabilize the intrinsic Cu vacancy formation and minimize the Cu diffusion in the

Fig. 1 Crystal structures of a low chalcocite and b acanthite phases of Cu2 S. The low chalcocite has 96 Cu and 48 S atoms. The acanthite phase is presented as a 3 × 2 × 2 supercell for comparison. The unit cell of acanthite has 8 Cu and 4 S atoms

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system [79]. As an earth-abundant, non-toxic, and relatively cost-effective alternative, this material may provide the best outcomes compared to other ternary and quaternary materials currently available. However, understanding the fundamental characteristics of the electronic structures of the different phases of Cux S and the effects of various doping schemes requires more attention. For example, it is known that the low chalcocite phase of Cu2 S has a very complex crystal structure consisting of 96 Cu and 48 S atoms per unit cell. The Cu atoms are distributed in 24 symmetrically inequivalent sites within the crystal structure. This renders computational studies highly expensive and inefficient. On the other hand, the acanthite phase has only two inequivalent Cu sites. Hence, defect-related studies in this phase are far easier and become computationally feasible. Our recent investigation [80] shows that doping Cu sites in the acanthite phase reproduces the effects of doping on the low chalcocite phase. Thus, the simpler acanthite phase presents a computationally feasible alternative for defect-related electronic structure studies. Furthermore, the established structural correspondence allows results to be extrapolated to the more complex low chalcocite phase. We next present band structures and corresponding density of states for Ag, Zn, Sn, Bi, Nb, and Ta doping of Cu sites in the acanthite phase of Cu2 S. Results show that Ag doping does not introduce any intermediate or defect midgap states, as was previously mentioned [60] (Fig. 2). The DOS plot shows that the compositions of both the valence band maximum (VBM) and the conduction band minimum (CBM) are similar to those found in pristine acanthite and low chalcocite phases (Fig. 2). Added Ag 4d states are found deep in the valence band and do not contribute to the band edges. Under Zn doping, the Fermi level shifts up in energy, as the higher charge state of Zn with respect to Cu causes Zn 4s electrons to partially occupy the CBM. This adds extra charge carriers to the system, making it an n-type semiconductor. For Sn doping, a defect-related or intermediate midgap state is observed about the Fermi level. The DOS plot shows that the band is primarily composed of Sn 5p and Cu 3d. This kind of band is known to affect the photo-absorption of the system. A similar effect occurs in Bi doping, where Bi 6p shows significant contributions to both band edges. These phenomena become more pronounced in Nb and Ta doping, which suggests that the higher the charge states of the dopant, the more defect bands it introduces to the system, causing the material to tend toward a metallic character. We note that these effects appear to be limited to the CBM, and apart from the defect bands, the overall shape of the Cu2 S VBM is retained in all doping scenarios. A Bader charge analysis of Cu2 S (Table 1) shows that Ag and Cu have near identical charge states which is expected as they are isovalent, while Sn is almost twice that value. It is now evident that the coordination of a dopant in the system correlating to its charge state will have a direct effect on the electronic structure of the system. Thus, due to the proximity of the charge states of Ag and Cu, we conclude that the defect-free band gap and favorable crystal structure of Ag-doped acanthite Cu2 S make it the best candidate to tailor for a suitable band gap and optical absorption.

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Fig. 2 GGA + U calculated band structures (top) and density of states plots (bottom) for Ag, Zn, Sn, Bi, Nb, and Ta doping (separately) of a single Cu site, denoted as M Cu , in 2 × 2 × 2 supercells (64 Cu and 32 S atoms) of Cu2 S acanthite. The Fermi level is set to 0 eV. The density of states (DOS) is given in arbitrary unit. Ag plots are included for comparison. AgCu and ZnCu introduce no intermediate or defect mid-gap states. However, the CBM in ZnCu shifts to a lower energy and intercepts the Fermi level. For SnCu , an intermediate band is introduced at the Fermi level, which is composed of both occupied and unoccupied states. Meanwhile, BiCu occupies a band from the CBM and moves it below the Fermi level. For NbCu and TaCu , the system shows near-metallic characteristics Table 1 Calculated average Bader charge states of different species in doped Cu2 S Average Bader charge states Cu

Ag

Zn

Sn

Bi

Nb

Ta

S

+0.37

+0.30

+0.71

+0.66

+0.52

+1.23

+1.28

−0.74

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4.2 CuBiW2 O8 :S A recent computational materials prediction study by Sarker et al. suggested a triclinic ground-state structure for the quaternary Cu(I)-oxide CuBiW2 O8 [61], or CBTO, for potential photovoltaic application. For this DFT-predicted structure, an indirect band gap was calculated at 1.43 eV, well suited for absorptions in the visible range. Calculated optical absorption spectra showed a slightly higher optical band gap of ~1.49 eV, which was attributed to O 2p—W 5d transitions with O 2p situated slightly below the VBM. This is because the Cu 3d (VBM)—W 5d (CBM) transitions are not so favored by quantum selection rules, and Bi 6p contributions at the CBM are relatively small. Analysis of the band structure of CBTO found no mid-gap Cu 3d states, as compared to those found in CuWO4 thought to cause its high resistivity. Furthermore, dispersive bands were found at the CBM indicating low electron effective mass and high mobility. However, the top of the valence band was quite flat, indicating less favorable hole mobilities. The ns2 md0 electronic configuration, exhibited by Bi–W– O systems, has been suggested as a mechanism for reducing hole effective masses via the introduction of dispersive anti-bonding s orbitals at the top of the valence band [81, 82]. The contrary result in CBTO may then be due to occupied and highly localized Cu 3d states at the VBM, which are slightly higher in energy than Bi 6 s states. CBTO was previously synthesized over 27 years ago [83] though no data regarding its structural, optical, and electronic properties was provided in that report. However, a recent collaborative study [84] of the quaternary compound successfully confirmed the computationally predicted triclinic structure via solid-state synthesis methods performed in a Cu-rich environment. Moreover, the study provided the first measurements of the optical, electrical, and photoelectrical properties of CBTO. Light absorption measurements yielded an indirect band gap of 1.46 eV, as compared to the 1.43 eV gap produced from photoluminescence spectroscopy—both in excellent agreement with the theoretical value. Hall effect measurements determined CBTO to be a p-type semiconductor, as expected for Cu(I) oxides, with hole majority carrier mobility of 0.32 cm2 V−1 s−1 and carrier lifetime of 2.12 ns. The diffusion length was calculated at ~42 nm, a value comparable to that of BiVO4 [85], an ns2 md0 material well known for its good light absorption and transport properties. To improve hole carrier mobility, as well as the band gap energy, we look toward S-doping in CBTO. These improved outcomes are expected from the lower electronegativity and ionization potential of sulfur with respect to oxygen. Thus, as a dopant, S is expected to upshift the VBM and narrow the band gap. Additionally, due to the proximity of their energy states, S 3p is expected to hybridize with Cu 3d at the VBM. Therefore, the localized Cu 3d character of the VBM should give way in part to the more dispersive S 3p states, consequently reducing hole effective mass. The electronic properties of CBTO: S was investigated within the framework of DFT under similar conditions considered in the original CBTO study, as outlined earlier. However, since the publication of the original computational work, a lower ground-state structure of CBTO was derived from CuBi2 O4 (see acknowledgement).

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Calculated XRD peaks for the new triclinic structure were found to be a better match with the experimental diffraction patterns. For this reason, the new CuBi2 O4 -derived CBTO structure was used in the current study (Fig. 3). The affinity of S to Cu becomes apparent at the single S-in-O site doping level, with S pulling Cu toward it, hybridizing its 3p orbitals with Cu 3d states at the VBM. This is due to the proximity of Cu 3d and S 3p electron energies, as mentioned above. Moreover, as S distorts the Bi octahedron, the now closer Cu 4s orbitals hybridize with Bi 6p and significantly decrease in density at the VBM. The linear O–Cu–O bond found in the pristine cell bends toward angles reminiscent of the S-Cu-S network of the low chalcocite Cu2 S phase, which leads to sharper, more localized Cu 3d and W 5d densities. The top of the valence flattens, with a calculated hole effective mass of 7.19 m0 , as compared to 2.16 m0 in pristine CBTO. Electron effective mass increases slightly from 0.68 m0 to 0.87 m0 . The band gap is reduced to 1.534 eV, compared to the 1.618 eV calculated for the new CBTO structure. The narrowing of the gap is not due to an increase in the VBM, as expected, but rather a decrease of both band edges. This may be due to asymmetric distortions caused by the 1:3 ratio of S-to-O coordination about Cu atoms. The gap remains direct about the high symmetry Z point. The CBM character exhibits similar localization, with a slight increase in Cu 4s and Bi 6s contributions, though far less than the primary contributions from empty Bi 6p, O 2p, and W 5d states. The localization effect is not limited to Cu and W orbitals and is observed throughout the electronic landscape. This effect intensifies with additional S-doping as the cell is transitioned to a 50/50 S-in-O doping/alloying level, where the lowest energy configuration is obtained with Cu fully bonded to the 4 S sites (Fig. 3). More pronounced effects are observed at this level of doping. All cation–cation distances are increased as the larger S pushes atoms away from previously O-occupied sites. This is true for all but neighboring Cu and W atoms, where the distance decreases with respect to the pristine structure. This is caused by the increased attraction of the Cu– S environment. With the now fully coordinated CuS4 cluster, the Cu–S–W network becomes the primary photoactive site. This is highlighted in the density of states, in which, and as expected, the VBM sees a primary S 3p contribution over that of Cu 3d. At the conduction band, Bi 6p and O 2p hybridize more strongly than at the single S-doping level. More importantly, as the S 3p density of states increases about the Cu–S–W network, the W–S bond length decreases by ~0.2 Å leading to greater W 5d-S 3p hybridization at the CBM. As a result, the CBM detaches from the rest of the conduction band and shifts down below 1 eV, reducing the band gap to 0.766 eV. Here, the narrow gap is indeed caused by a VBM upshift of 0.461 eV, as expected, in addition to a CBM downshift of 0.391 eV. Despite the increased localization, both electron and hole effective masses are reduced with respect to the pristine cell, with 1.99 m0 for holes and 0.43 m0 for electrons, suggesting an overall increase in carrier mobility for CBTO:S4 . Hence, S-anion-doping in CBTO may lead to improved electronic properties suitable for photovoltaic applications.

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Fig. 3 Unit cells, band structures, and density of states plots for pristine CBTO (top), single S-in-O doped CBTO (center), and 50% S-doped CBTO (bottom). Symmetry points were obtained from AFLOW [86]. Densities are plotted in arbitrary units. Pristine CBTO contains 1 Cu, 1 Bi, 2 W, and 8 O atoms. Doping was applied to the unit cell to model high S concentrations. Band gap energy decreases from 1.618 eV (pristine) to 0.766 eV (50% doping) due to increasing interactions along the Cu–S–W network. S 3p hybridization at both VBM and CBM reduces effective masses of both charge carriers

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5 Summary and Conclusions In this chapter, we have outlined the benefits and effectiveness of computational studies applied to new materials research for photovoltaic applications. An overview of oxides was provided including the success of the material group, as well as the difficulties it faces. As a viable alternative, sulfur-based solar absorbers were discussed in detail, with emphasis on their potential for suitable photovoltaic material properties. Two case studies were presented, discussing first the electronic properties of the pure sulfide compound Cu2 S and second the effects of S-doping in the novel newly predicted quaternary oxide CBTO. In Cu2 S, we showed that Zn doping leads to partial occupation of conduction band minima by 4s electrons, increasing charge carrier concentrations resulting in an n-type material, while the remaining dopants introduce intermediate defect states leading to metal-like characteristics. A correlation between dopant charge states and mid-gap defect state concentrations was suggested, with lower charge state dopants, which are isovalent to the host cation, deemed favorable for photovoltaic applications. By comparing these results to Ag, which has previously been shown to minimize intrinsic Cu vacancies and diffusion, as well as increase the band gap without introducing detrimental mid-gap trap states, we conclude that Ag alloying is the best candidate for band gap engineering in Cu2 S systems. For S-doped CBTO, S-in-O anion doping was shown to cause distortions in the pristine unit cell forming a photoactive Cu–S–W network through Cu 3d-S 3p and W 5d-S 3p hybridizations at the VBM and CBM, respectively. These interactions lead to widespread localization of electron densities in the material. Despite the localization, S 3p hybridizations reduce overall charge carrier effective masses and narrow the band gap below 1 eV. This suggests that anion doping may provide an additional mechanism for tailoring suitable photovoltaic material properties. Acknowledgements All computations were performed on Texas Advanced Computing Center (TACC) servers. We acknowledge Dr. Pranab Sarker for the discovery of the new triclinic ground state of CBTO. This work was partially funded by NSF grant# 1609811.

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Photovoltaic-Based Nanomaterials: Synthesis and Characterization Kanwal Akhtar, Naveed Akhtar Shad, M. Munir Sajid, Yasir Javed, Muhammad Asif, Khuram Ali, Hafeez Anwar, Yasir Jamil and S. K. Sharma

Abstract Improving the conversion efficiency and reducing cost are the major tasks to make more energy competitive-based photovoltaics and able to replace the traditional fossil energies. In organic/inorganic-based solar cell development, nanotechnology seems to be the most promising branch. Nanostructure materials with large band gap synthesized from III–V and II–VI elements are gaining more attention because of their potential use in emerging energy applications. Nanostructures with different morphologies including nanowires, nanosprings, nanobelts and nanocombs can be prepared. Variations in atomic arrangements to minimize the effect of electrostatic energies produces from different ionic charges on the polar surfaces are the major reason of diversified range of nanostructures. In this book chapter, we will focus on the contribution of different nanomaterials in the advancement of solar cell technology. Keywords Nanomaterials based solar cells · Photovoltaic devices · Inorganic nanomaterials · Photovoltaic cell efficiency · Characterization parameters

1 Introduction The fast-growing rate of world population demands new sources to sustain and improve the amenity of life. The depletion of fossil fuels and declining its reserves K. Akhtar · Y. Javed (B) · M. Asif · K. Ali · H. Anwar · Y. Jamil Department of Physics, University of Agriculture, Faisalabad, Pakistan M. Asif e-mail: [email protected] N. A. Shad · M. Munir Sajid Department of Physics, Government College University, Faisalabad, Pakistan S. K. Sharma Department of Physics, Faculty of Science and Technology, The University of the West Indies, St. Augustine, Trinidad and Tobago

© Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_6

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subsequently is influencing environmental degradation, health, global warming issues and creating alarming situation for humanity. The renewable future might be guaranteed through contraction of greenhouse gas emissions and engendering new resources to meet energy demands. So, clean and renewable energy sources are best alternate to meet the world sustainability energy demands with zero quantity of carbon emission. Solar, hydro, wind, biomass and geothermal are considered the clean energy sources in which solar energy is main and excessive source for renewable [1]. The sun is the main source of solar energy, excessively available and inexhaustible rather than other renewable energy resources. Moreover, the captured power on earth from sun is roughly equal to 1.8 × 1011 MW, which is generally larger energy rate than all the present sources. There are diverse solar-based technologies, which are involved in cooking, heating, drying, refrigeration, thermal power and photovoltaic [2, 3]. Photovoltaic (PV) is the finest source to fill the gap of world energy crises by direct conversion of sun light into electricity, based on simple design that necessitating low maintenance and high output. Therefore, PV has applications in numerous fields such as communication, refrigeration, street light, disaster relief, water pumping, charging vehicle batteries, satellites and space vehicles [4–6]. Solar cell based on photovoltaic effect was first presented by a French scientist Edmond Becquerel [7], who observed the deflection of electrical voltage as sun light fall on material. Few years later in 1883, a solar cell composed of selenium(Se) material was manufactured by Charles Fritts based on photovoltaic effect [8]. Afterward, it was discovered that silicon possessed more efficacy than selenium. Therefore, in 1953, first silicon-based solar cell was fabricated. First cell was silicon-based wafers which convert solar energy into electrical power. Advanced PV technology was established on electron-hole formation in two layers (p-type and n-type) semiconductor material when photon falls on p–n junction [9]. The photovoltaic technology broadly categorized into four generations based on their characteristics that are abridged in Table 1. Using solid-state physics, solar cells attained its maximum possible efficiency by employing different materials such as cadmium telluride (CdTe), silicon (Si) and copper indium diselenide (CuInSe2 ), whereas efficiency was further improved by triple-junction compounds based on indium gallium phosphide, indium gallium arsenide and gallium arsenide [15]. Later on, chemistry seems to be an alternate Table 1 Classification of different solar generations Photovoltaic cells

Characteristics based on efficiency

Wafer-based silicon cell (1-G)

Attaining high efficiency and cost [10]

Thin film-based cells (2-G)

Copper indium gallium selenide (CIGS),cadmium telluride and amorphous silicon [11, 12]

Organic cells (3-G)

Nano crystalline-based cells [13]

Inorganic in organic-based cells (4-G)

Polymers-based and most advanced cells [14]

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option that may boost photovoltaic cell conversion efficiency; therefore, these days polymer-based and hybrid solar cells become area of interest for researchers. Nanomaterials prepared through chemical routes offer enough chances in efficiency improvement by accumulating light trapping and photo-carrier collection. Nanomaterials have attained completely altered chemical and physical properties from bulk materials owing to its high surface-to-volume ratio. This factor is further diversified by architecture of the nanomaterials such as nanowires [15, 16], nanopillars [17–19], nanocones [20], quantum dots [21], nanotubes, nanofibres, nanorods, nanosheets and nanopores. It is also observed that light trapping is due to wide photon path in nanostructures [22–24] that boost the electron–hole pair formation possibility. Photovoltaic related properties of nanomaterials are (i) energy band-gap flexibility and inter-changeability which relies on size (ii) improved optical path owing to multiple reflections and (iii) substantial decrease in the probability of recombination of charge carriers [25]. Moreover, nanomaterials offer flexibility in photovoltaic assemblies having capability to alter solar energy conversion via novel approaches [26, 27]. Quantum dots (QDs), on the other hand, have size-dependent band gap [28–30] that is useful to capture maximum solar spectrum depicted in following Fig. 1.

Fig. 1 Size dependency of quantum dot on band gap. Band gap of semiconductor material increases with the decrease in size. Adapted from [31]

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2 Classical Review of Photovoltaic (PV) Cell PV is considered an electronic device, attaining semiconducting properties composed of P-N junction that made it a feasible device to convert solar energy into electricity based on photovoltaic principle [32]. When a semiconductor material exposes to light, photons of light are immersed by the semiconductor crystal and removes substantial free electrons in the crystal that cause the electricity. Not all materials have properties for solar cell except have capable to trap sun light and the visible spectrum conversion into electricity. This occurred thanks to the creation of e− -hole pairs during photons absorption with energies greater/equal to band gap [33]. The air mass (AM) is a level, where atmosphere diminishes the light to reach the Earth’s surface. Broadly it is categorized into two levels; one is AM0 and other AM1.5. AM0 is a level of light spectrum outside the atmosphere while AM1.5 for terrestrial solar cell is standard solar spectrum at solar zenith angle of 48.2°. Graph of sun light spectrum, elaborated that temperature of blackbody radiations increases from 1000 to 2000 K and its highest values can attained by using Wien’s displacement law [12]. Graph is showing blackbody maxima shifted toward greater wavelengths conferring to Wien’s displacement law with temperature (Fig. 2). It indicates that photovoltaic cell having greater-energy band gaps corresponding to greater radiator temperature. Estimated band gap value of silicon-based solar cells is hv = 1.12 eV (it usually goes up to 1.11 mm) as a result shows the matching with blackbody at temperature range of 2610 K [34, 35]. In the visible region, major portion of the solar spectrum varies from 0.38 to 0.76 mm.

Fig. 2 Comparison of blackbody spectra and solar spectra taken at earth-sun distance in semilogarithmic scale. Reprinted with permission from [34]

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So, first assortment standard of PV absorbent is energy band gap which is around 1 eV and several materials lies in this energy gap such as silicon, gallium arsenide (GaAs), copper indium diselenide (CIS), cadmium telluride (CdTe), Cu2 ZnSnS4 (CZTS) and other materials [36–38]. Secondly, the e− -hole creation by holding electric voltages and migration of electron from one terminal to other until equilibrium. Conversion efficiency (η) and fill factor (FF) are basic defining terms for the behavior of PV cell, where fill factor is defined as the ratio of maximum power (Pmax ) with respect to theoretical power and conversion efficiency is ratio of output power to input. The fill factor and conversion efficiency are described below [39]. FF =

IMP · VMP Pmax = PT Isc · Voc η=

η=

Pout Pin

Pmax FF × Voc × Isc = Pin Pin

Pout is maximum output power, Pin is input power, V oc is open circuit voltage and I sc short circuit current density. Each parameter has particular effect on the performance of solar cell. The typical FF value is 0.70 for commercial use. The classical photovoltaic-based cell demands thicker materials to show efficiency in optical absorption, but carrier collection efficacy of particles can diminish due to wider length [40, 41].

3 Architecture-Based Classification of Nanomaterial The light absorption and carrier collection properties of different architectures of nanomaterials can enhance the efficiency of solar cell. Therefore, different forms of nanomaterial are reviewed to explore their technological relevant properties. These architectures include nanowires, nanotubes, nanopillars, nanobelts, nanofiber etc. [42, 43].

3.1 Nanowires The geometry of nanowires offers potential benefits against wafer-based or thin filmbased technology and improves light trapping, band-gap, and defect tolerance, forexample, Si nano-wires improved path length of the incident solar radiation varies up to 73 cm−2 [44]. Within sheet resistance, silver nanowires mesh electrodes exhibited small transparency that is why its flexible substrates coincides with organic solar cell and 19% improved values of photocurrent are reported [45]. The deposition of silver

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Fig. 3 Schematic diagram of core-shell nanowires. a Core ZnO nanowires array first grown in ITO substrate by CVD, b ZnS growth on the ZnO nanowires by pulsed laser ablation, c PMMA layer coating by spin coater, d then final prepared cell for measurements. Reprinted with permission from [53]. Copyright (2015) American Chemical Society

nanowires is carried out via very low-cost protocol i.e., simple brush painting and attained efficiency of 3.231% [46]. Diverse deposition ways are used for nanowires, most simple is colloidal lithography. It takes less time and appropriate for large scale deposition [25]. Vapor liquid solid technique is also very efficient to fabricate core shell nanowires [26, 27]. Nanowires are fabricated via different protocol which can be top-down and bottom-up approaches. With different deposition tools, nanowires can directly grow on substrate e.g., chemical vapor deposition (CVD) [47, 48] and molecular beam epitaxy (MBE) [49–52]. Nanowires epitaxial growth is organized at moderate temperature by employing catalyst droplet. Following schematic is showing different steps involved in the synthesis of NW (Fig. 3). This can be clearer in SEM images of the grown nanowires (Fig. 4). Figure 5 provides cross-sectional view of the Si-based nanowires grown by metal assisted chemical etching.

3.2 Nanotubes Nanotubes are also considered important architecture of solar cell applications. Different material-based nanotubes have been fabricated for the applicability in solar cell. To develop Schottky junction of Carbon nanotubes conducting layers when

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Fig. 4 Structural characterization of the nanowires shown schematically in Fig. 3: a, b top view of the core-shell grown nanowires. c High resolution image of nanowires showing ZnO and ZnS structures. d Line profile of the nanowires by energy dispersive spectroscopy indicating presence of Zn, O and S materials present in the nanowires. Reprinted with permission from [53]. Copyright (2015) American Chemical Society

placed on n-type silicon, photovoltaic cell having efficiency up to 1.9% under exposure of AM1.5 boosted up to 8.6% by doping of trifluoramethanesulfanyl amide [55]. For the optimization of solar cell efficiency, graphene can be used as compared to the silicon (ITO/Si) under indium tin oxide (ITO)-based Schottky solar cells. Nanotubes based on titanium dioxide can be used tremendously for dye-sensitized solar cells as transparent photoanodes. Nanotubes of titanium oxide widely used in diversified fields due to their electronic properties such as greater surface-to-volume ratio, high electron mobility and low carrier recombination ability. For active charge transfer, direct pathways tremendously increase the electron transport capacity [56, 57].

3.3 Nanopillars Nanopillar photovoltaics possessed many promising features which can substitute classical photovoltaics because of cost effectiveness. These are listed below: 1. Increase the collection carrier efficiency 2. Without use of expensive methods, growth of crystallized nanomaterials

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Fig. 5 Cross-sectional view Si nanowires arrays taken by SEM: a conventional metal assisted chemical etching process using b nonconductive tape, c one-layer and d three-layer conductive carbon tapes. Sample configurations are presented in the insert figures [54]

3. Decrease in the optical losses. Nanopillars fabrication process is demonstrated in the Figs. 6 and 7 given below [58].

Fig. 6 In the fabrication process, four main steps have been involved: anodization, electropolishing, wet etching and anodization step. Modified with permission from [58]

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Fig. 7 Schematic figure for nanopillar solar cell. Large part of a-SiH-nanopillar structure describe the transport and photogeneration mechanism [59]

3.4 Nanorods ZnO acceptor and conjugated polymer donor have been used as hybrid polymerbased solar cells. ZnO nanorods in poly [1-methoxy-4-(2-ethylhexyloxy-2,5phenylenevinylene)] actively formed the heterojunction hybrid bulk-based solar cells. Up to 0.045% efficiency can be obtained. This exposes the decrease in high interior resistance of photovoltaic cells [60–62]. Figure 8 presenting the SEM micrographs for different zinc (Zn2+ ) concentrations. With the increase in the zinc concentration, bigger diameter of ZnO nanorods can be obtained. Electron mobility for zinc oxide nanoparticles and bulk zinc oxide has been found up to 1–1000 cm2 V−1 s−1 . Significant difference explains the high quality of ZnO nanoparticles in comparison with bulk ZnO. Polymer donors having energy difference up to 0.3 eV leads toward the effective separation of excitons to form free carriers [61].

4 Inorganic-Based Nanomaterials for Solar Cells Inorganic nanoparticles exhibit promising physical properties due to their size in nanometric dimensions. These particles possessed unique optical and physical properties of nanocrystalline quantum dots might lead toward their tremendous use in different biomedical and electrooptic applications. Core/shell inorganic nanoparticles with hierarchical multicomponent assemblies exhibit many functional and enhanced properties.

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Fig. 8 Scanning electron microscope top view micrographs of zinc oxide nanoparticles with different zinc concentrations a 0.0125M, b 0.025M, c 0.05M and d 0.1M. Reprinted with permission from [61]

CdSe: Cadmium selenide is considered as primarily ionic compound having covalent character. It is present in three crystalline forms but wurtzite hexagonal structure is most common among all. It is n-type semiconductor. Nanoparticles of cadmium selenide are useful photocatalysts. Core/shell structures having combination of CdTe/CdSe showed the emission of near infrared radiations which does not exist for the CdTe nanoparticles individually. Core/shell structure unswervingly dependent on the lowermost lattice mismatch among the used nanomaterials [63, 64]. Polymeric solar cells based on the CdTe quantum dots having single wall carbon nanotubes containing composites of poly (3-octylthiophene)-(P3OT) revealed carrier transport and good exciton dissociation. Current density of the short circuit and voltage of open circuit were estimated in the range of 0.16 mA/cm2 and 0.75 V, respectively [65, 66]. Silicon-based nanomaterials: High performance of silicon-based solar cell can be attained through silver screen printed contacts. Silver nanoparticles can be introduced in the paste to enhance the contact compactness, conversion factor and fill factor. Silicon-based nanowires tremendously used in solar cells. Diameters of silicon nanowires vary from 200 to 1.5 mm. Diffusion length of minority carriers has been found around 2 mm, minimum lifetime of the carrier is 15 ns, whereas the velocity of maximum ability of surface recombination is 1350 cm s−1 (Fig. 9) [67].

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Fig. 9 Micrograph taken through field emission scanning electron microscopy of the as-synthesized silver NPs prepared by using solvothermal method. Reprinted with permission from [67]

For the silicon bulk materials, minimum lifetime of carrier is around 30 ms, diffusion length is about 200 mm and surface recombination velocity is about 8600 cm/s. For improving the efficiency of solar cells, effect of solar cells seems more obvious when compared with the data available previously [68, 69]. Recombination velocity seems to be reduced with the different structures of nanomaterials, which revealed the increased photon collection [70].

5 Organic-Based Nanomaterials for Solar Cells Organic-based nanomaterials gained tremendous attention from the last few years. Key phenomenon in the manufacturing of photovoltaic cells was accordingly mastered such as light trapping, exciton generation effect and onedimensional (1D) nanomaterials for Schottky barrier arrays [71]. Organic solar cells/photoelectrochemical solar cells consist on counter electrode (metal/semiconductor) and photoactive(semiconductor) electrode. In an electrolyte, both are immersed which composed of redox couples. e− h pairs are formed when light having greater energy as compared to semiconducting material is absorbed [72]. Figure 10 is showing schematic of the organic-based solar cell.

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Fig. 10 Schematic diagram for FTO/PEDOT:PSS/P3HT:PCBM(PSCs-2) FTO/TiO2 /dye/PEDOT:PSS/P3HT:PCBM (PSCs-1) heterojunction solar cells [73]

and

Table 2 Photoelectric performances of the perovskite solar cell (PSC-I) [73] P3HT:PCBM (2:1)

J sc (mAcm−2 )

V oc (V)

FF

Z (%)

Atm. annealed

3.96

0.83

0.67

2.19

Barrier

3.63

0.82

0.68

2.04

Vacuum annealed

4.3

0.83

0.67

2.37

Table 3 Photoelectric performances of the perovskite solar cell (PSC-II) [73] Poly(3-hexylthiophene)(P3HT):[6]-phynyl-C61-butyric acid methyl ester(PCBM) (2:1)

J sc (mAcm−2 )

V oc (V)

FF

Z (%)

Atm. annealed

3.27

0.78

0.62

1.58

Barrier

2.83

0.80

0.65

1.48

Vacuum annealed

3.59

0.80

0.66

1.90

6 Nanomaterials for Dye-Sensitized Solar Cells Titanium oxide(TiO2 ): TiO2 nanomaterials can be synthesized by combining the chemical methods with the dealloying processes. Solar cell performances based on TiO2 nanoparticles are listed in the Tables 2 and 3.

7 Nanomaterials for Perovskite Solar Cells Achieving high short circuit current density and high open circuit voltage simultaneously is great challenge in the field of enhanced efficiency-based perovskite solar cells. It is because of complex nature of excitonic hybrid-based inorganic-organic semiconducting nanomaterials. Zhang et al., developed an effective and facile method

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to fabricate the high-efficiency-based plasmonic devices. They prepared the solar cells based on incorporation of gold nanoparticles into TiO2 mesoporous films and depositing passivating MgO films in the modified silver NPs with mesoporous titania through pyrolysis and wet spinning of magnesium salt. Plasmonic solar cells based on MgO and silver NPs result in high conversion power efficiency about 16.1%, high current density of short circuit of 21.76 mA cm−2 and high voltage of open circuit up to 1.09% [74]. Perovskites are excitonic-based photovoltaic materials like quantum dots and organic dyes. These materials also revealed the noticeable conductance for holes and electrons. Because of the broad spectrum of p–p direct optical transitions, these materials possessed greater optical absorption. Hence, perovskite layer is squeezed between electron conducting and hole conducting materials matched with current collecting electrodes in highly efficient perovskite solar cells. Zhang et al., used poly(3-hexylthiophene) and stable CH3 NH3 PbI2 Br as hole transporting layer and light absorbing layer to fabricate mesoscopic perovskite solar cells with high conversion power efficiency up to 6.64% was reported. With bromine, partial substitution of iodine in the perovskite results in prolonged lifetime of the charge carriers [75].

8 Nanomaterials-Based Photovoltaic Cells Efficiency Perovskite organometal halide-based solar cells have been considered as important photovoltaic technology because of their promising power conversion efficiency (PCE) and low-cost materials. In 2012, PCE for solid-state perovskite-based solar cell was 9.7% and it was reported high as 19.3% in 2014. In the Table 4, efficiency of photovoltaic cells has been listed. Table 4 Photovoltaic performance depending on cell configurations and material types of perovskite solar cell [76] Material (cell configuration)

V oc (V)

PCE

J sc (mA/cm2 )

FF

Reference

MAPbl3 (ZnO nanorod/MAPbl3 /spiro-MeOTAD)

1.02

8.9

16.98

0.51

[77]

MAPbl3 (MAPbl3 /PCBM) MAPbl3−x ClX (MAPbl3−x ClX /P3HT)

0.60

3.9

10.32

0.63

[77]

0.921

10.4

20.8

0.542

[78]

MAPbBr3 (mp-Al2 O3 /MAPbBr3 /PDI)

1.30

0.56

1.08

0.4

[79]

MASn0.5 Pb0.5 I3 (mp-TiO2 /MASnx Pb(1−x) I3 /P3HT

0.42

4.18

20.04

0.50

[80]

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9 Solar Cell Characterization In this section, we have discussed solar cell characterization considering the capabilities of electrical power generation mechanism. These characteristics are related to the working of cell structure including charge carrier life time. Key cell characterization includes current versus voltage (I–V ) curve (Fig. 11) performance under solar simulator and measurements of spectral irradiance. From I–V curves, open circuit voltage, short circuit current and maximum power output can be identified. Many additional solar cell parameters with different relationships are used for the characterization of solar cell efficiency [81, 82]. • Current Versus Voltage (I–V) Curves: Measurement of (I–V ) module of a solar cell provides information about solar cell parameters such as open circuit voltage (V OC ), short circuit current (I SC ), power efficiency, fill factor, maximum voltage (V max ) and maximum current (I max ) at maximum power (Pmax ). For single junction generic cell, parameters at specific solar illumination level are mentioned in the figure given below. Internal loss is represented by fill factor (FF) which is communicated by I–V curve in 4th quadrant of current voltage in rectangular shape [84, 85]. As shown in I–V curve obtained through photovoltaic cell, electrical generation depends on many factors including operating temperature, solar radiation spectrum, applied electrical load and orientation of cell related to solar beam input cells. To compare different cells, current and voltages are measured based on common operating conditions. Standard test conditions (STC) are the set of primary operating conditions which is also called standard reporting conditions (SRC) [86, 87]. For SRC, standard reference spectrum is 100 W/m2 total irradiance and 1.5 global air mass. This spectrum typically corresponds for mid latitudes at earth surface. Operating temperature condition for SRC in specified device is 25 °C. In outdoor setting, it is difficult to get SRC conditions according to practical point Fig. 11 Representative I–V curve of the solar cell. Reprinted with permission from [83]

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of view, whereas in indoor environment, electrical measurements usually performed in laboratories under simulated sunlight [88, 89]. • Solar Simulator Performance: Solar simulator is optical light source having broad band output comparable to sun over wide response range of solar cell technologies. For the characterization of solar cells, solar simulator is widely used for irradiance exposure to devices and materials. Solar simulator is widely used in pulsed mode or steady state mode [90, 91]. Light emitting diodes, xenon arc lamps, quartz tungsten halogen lamps and metal halide arc lamps are being widely used in solar simulators. Xenon-based solar lamps are most common among them. Currently, solar simulator is evaluated on three basic unique criteria: (1) nonuniform irradiance within test measurement plane, (2) over certain range of wavelength, spectral match with reference spectrum, (3) temporal irradiance instability during measurement. Irradiance shows power of radiations per unit area. It is expressed in watts/meter2 . Irradiance measurement is usually expressed as wavelength function called spectral irradiance in W/m3 [92, 93]. • Measurements of Spectral Irradiance: Spectral output of any light source or solar simulator is measured through spectroradiometer calibrations. These instruments are typically equipped spectrographs with silicon-based charge coupled devices or fast photodiode arrays which provides reliable and sensible information about intensity of light and spectrum [94]. Two or more types of detector such as InGaAs-and Si-based arrays are used. For spectroradiometer calibration, light source should be calibrated at 1000 W for correct irradiance measurements. Set of operational conditions are important for the calibration of reference lamp: (1) from bulb filament center, proper placement and alignment of spectroradiometer optics at some definitive distance (2) use of high accuracy and stable power supply with proper feedback mechanism for the precise control over lamp current [28]. So, 0.1% better output accuracy is required typically. Input current of the lamp maintained 0.01% better during calibration. For low uncertainty budget, periodic calibration of spectroradiometer is needed. On regular basis, irradiance characteristics of the simulator should be measured due to shape of spectral out and age of lamps [95, 96]. • I–V Curve Measurement: For I–V measurements, correct setup of solar cell is a network based on 4 wire connection which is also known as Kelvin configuration (Fig. 12). These connections are usually referred as sense leads and source leads. Current flow is measured through source leads while voltage is measured through sense leads [97]. Source current will stop flowing due to high input impedance. Hence, voltage can be measured only. To take I–V data, setup based on 2 wire connection is usually used. Voltage drop is observed with the flow of current through leads. In the cell, large photocurrent is produced under comparable illumination/SRC that is why this effect is significant for the solar cells with larger area [98, 99].

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Fig. 12 Circuit diagram for the I–V measurements. Reprinted with permission from [83]

10 Conclusion To make use of photovoltaic energy in our daily life, improvement in the performance of solar cell is considered as the major challenge. For low-cost fabrication and high performance of solar cells, processes based on nano-electrochemistry seem to be essential. These processes can be carried out at ambient temperature or low temperature ranges, which minimizes sensibly the energy bills related to the manufacturing of photovoltaic cells. Conversion efficiency is relatively high when obtained through conventional solar cells in comparison to the solar cells based on nanomaterials. But nanomaterials-based photovoltaic solar cells gained much attention because of their potential applications in everyday life and low manufacturing cost. To overcome the limitations encountered through solid-state physics leads the shift toward nano-electrochemistry. Great opportunities are available to enhance the photo-carrier collection and light trapping abilities. Enhanced efficiency of inorganic-based solar cells is observed as compared to organic-based solar cell. Most promising next generation solar cell is based on organic materials which can be much better than inorganic materials. Conversion efficiency of solar cells is small and difficult to implement on the large scale. More efforts are needed to make high conversion efficiency-based solar cells.

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Carbon Nanotubes: Synthesis and Application in Solar Cells Shazia Shukrullah, Muhammad Yasin Naz, Khuram Ali and S. K. Sharma

Abstract Unique structures and outstanding properties of carbon nanotubes (CNTs) have drawn significant attention of scientific community working in materials science and engineering. Researchers are taking interest in dealing with certain constraints of solar power systems and harnessing maximum energy from the sun. Construction, working life, manufacturing cost and efficiency of the solar cells are the key factors in defining their widespread use. Different strategies are being adopted to develop stable materials for manufacturing the low cost but highly efficient solar cells. Owing to high thermal stability, mechanical strength, surface area to volume ratio and electrical conductivity, CNTs can be a good choice as a solar cell material. CNT-based solar cells are fascinating the world due to their reduced manufacturing cost and high efficiency. Also, the future CNT-based hybrid solar cells would be much cheaper than the traditional energy source cells. This chapter discusses the carbon-based nanoscience and nanotechnology, structures and properties of CNTs, methods of synthesis of CNTs and use of CNTs in manufacturing the efficient solar cells. Keywords Carbon nanotube · Chemical vapor deposition (CVD) · Photovoltaicscells · Heterojunction solar cell

1 Carbon-Based Nanoscience and Nanotechnology Nanoscience and nanotechnology are the study of engineering of matter, manipulation of structures and particles of nanometer scale (about 1–100 nm), which are the forefront of modern research. The properties of nanometer-sized objects can be qualitatively different from those of ordinary matter because they result from a more direct expression of the laws of quantum mechanics [1]. In the last twenty years, carbon plays a leading role in materials science due to nanometer-sized systems and S. Shukrullah (B) · M. Y. Naz · K. Ali Department of Physics, University of Agriculture, Faisalabad 38040, Pakistan e-mail: [email protected] S. K. Sharma Department of Physics, Faculty of Science and Technology, The University of the West Indies, St. Augustine, Trinidad and Tobago © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_7

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Fig. 1 Different carbon nanostructures: single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), fullerenes (C60, C240 and C540) and a carbon nano-onion

unique ability to form long chains to create new materials, that’s why it is known to be versatile element on the earth. Diamond and graphite are considered as the classical allotropes of carbon, whereas discovery of fullerenes was referred to as the third allotropic. Soon afterward, new and intriguing forms of carbon were discovered, named as carbon nanotubes (CNTs). CNT is one of the allotropes of carbon which includes fullerenes, diamonds and graphite, as shown in Fig. 1. CNTs have extraordinary interest and promising applications in the field of nanoscience and nanotechnology due to their exceptional and highly desirable multiple chirality, fascinating electrical, thermal, magnetic and mechanical properties. Recently, CNTs have been used in many types of solar cells applications due to incredible structured arrangement of carbon atoms, large surface to volume ratio, multiple chirality nature and absorbing power [2].

2 Structures and Properties of Carbon Nanotubes The search was given new impetus when CNTs was first reported by Radushkevich and Lukyanovich in a Russian language journal and largely unknown to scientific world due to language barrier and heightened Cold War tension at that time [3]. One-dimensional material research started with the discovery of CNTs in 1991 by Japanese scientist “Sumio Iijima” [4]. Iijima synthesized nanotubes by using arc discharge technique and was first explored by using transmission electron microscopy

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(TEM). Subsequently, the floodgates have been opened to better understanding of this marvelous material. CNTs captivate the great attention in scientific field due to outstanding properties. The carbon atoms are strongly attached with each other by covalently bound, approximate distance between the carbon–carbon atoms is nearly about 0.14 nm. Whereas, the layers between them are in weak long-range of weakly van der Walls-type interaction and inter-layer distance is about 0.34 nm. The weak inter-layer coupling gives graphite the property of a seemingly very soft material, the property which allows using graphite in a pen to write with. The C–C bonding structure of nanotubes makes them more incomparable to diamond and graphene. The structure of CNT is graphitic sheet rolled in cylindrical shape of diameter in the order of a nanometer with both open ends. Hollow cylindrical structure is formed due to bonding mechanism of carbon atoms. Carbon atomic structure is 1s2 2s2 2p2 , two electrons fill in 1s, while the other electrons fill the sp and when carbon atoms combine to form graphite, then hybridization of sp2 occurred as shown in Fig. 2 [5]. In this hybrid sp2 -orbitals, sigma-bond (σ-bond) appeared at 120°, while π-bond (pi-bond) is found at 90°. The chemical bonding of CNTs is composed entirely of sp2 bonds, like those of graphite. This bonding structure, which is stronger than the sp3 bonds found in diamond, provides the molecules with their unique strength. CNTs naturally align themselves into ropes held together by van der Waals forces. Under high pressure, CNTs can merge together, trading some sp2 bonds for sp3 bonds, giving great possibility for producing strong, unlimited length wires through high pressure CNT linking [6]. Due to delocalization of π-bond, it acts as thermally and electrically conductive. This ability of nanotubes exhibit much more extraordinary properties than graphite in term of electrical, chemical, mechanical, thermal and biological due to rolling of graphite [7]. Rolling of graphite sheets in seamless manner by linear combination of base vectors a and b can be written as r = na + mb where n and m as integers that defines the structure of nanotube. Based on values of n and m, nanotubes can be divided into three categories. For n = m, resultant nanotube will be armchair, m = 0 results in zig-zag and for other non-zero values of n and m, result will be chiral nanotube (Fig. 3). Fig. 2 Hexagonal bonding structure for the graphite sheet [5]

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Fig. 3 Roll up vector as combinations of base vectors a and b [8]

Rolled axis of graphene sheet at corresponding θ of each structure of nanotubes such as zig-zag (10, 0), armchair (6, 6) and chiral (8, 4) nanotubes are shown in Fig. 4. The diameter d and chiral angle θ of the nanotubes can be given as,  d = 0.783 n 2 + nm + m 2 A0 √ 3m −1 θ = sin 2 2(n + nm + m 2 ) Theoretically nanotubes deliberated perfect structures where as in reality defectfree structures will almost seem impossible. Defects were such as discontinuity, bamboo like structure, capped, cone-shaped walls, bent tubes, toroidal, helical and branched. The extraordinary chemical, physical, mechanical and electrical properties of CNTs have made them useful in many areas and still new industrial applications are being reported frequently. For example, CNTs show a tensile strength of almost 50 times higher than steel, thermal conductivity greater than the diamond and one of the lowest resistivities among the nonmetals. A summary of most important properties of CNTs is given in Table 1. Based on such exceptional properties, vast applicability of CNTs has been demonstrated in optical materials and electronics [10], structural materials and polymeric additives [11] and biosensor [12] and energy-storage devices [13]. In terms of commercial applications, CNTs are being effectively used as paint additives and the electrical conductivity enhancers for composite materials [14].

2.1 Types of Carbon Nanotubes CNTs are considered as nearly one-dimensional structures (1D buckytube shape) according to their high length to diameter ratio. Most important structures are SWCNTs and MWCNTs. A SWCNT is considered as a cylinder with only one wrapped

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Fig. 4 Schematic illustration of SWCNT rolling vectors (n, m), and the corresponding armchair, zig-zag and chiral structures. Adopted from Zhang and Zhao [9] Table 1 Details of some common properties of CNTs Property

SWCNTs

MWCNTs

References

Tensile strength (GPa)

75

150

[15]

Young’s modulus (GPa)

900–1700

690–1870

[16]

Resistivity ( m)

10−6



[17]

Maximum current density (A m2 )

107 –109



[18]

Quantized conductance (k

)−1

6.5



[19]

Theoretical quantized conductance (k )−1

12.9



[20]

Thermal conductivity (W m−1 K−1 )

1750–5800

3000

[21]

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graphene sheet, while MWCNTs are like a collection of concentric SWCNTs. The length and diameter of these structures differ a lot from those of SWCNTs and of course, their properties are also very different. The bonding in CNTs is sp2 and consists of honeycomb lattices and is seamless structure, with each atom joined to three neighbors, as in graphite. The tubes can, therefore, be considered as rolled up graphene sheets. The type of CNT depends on how the graphene sheet is oriented on rolling. This can be specified by a vector (called chiral vector), which defines how the graphene sheet is rolled up. • At θ = 30◦ , m = 0 for all zig-zag tubes • At θ = 0◦ , n = m for all armchair tubes • At 0 < θ < 30◦ they are called chiral tube. The value of (n, m) determines the chirality of CNT and affects the optical, mechanical and electronic properties. CNTs with |n _ m| = 3i are metallic like as in (10, 10) tube, and those with |n _ m| = 3i ± 1 are semiconducting like as in (10, 0) tube, whereas i is an integer. The armchair and zig-zag tubes structures have high degree of symmetry. These terms refer to the arrangement of hexagons around the circumference [2].

2.2 Single-Wall Nanotubes A single-walled carbon nanotubes (SWCNTs) can be formed by the rolling of a single layer of graphite into a seamless cylinder. SWCNT consists of two separate regions with different physical and chemical properties. Generally, CNTs have a length to diameter ratio of about 1000, that’s why it was considered as nearly one-dimensional structure. Most SWCNTs have a diameter of 3000 °C

>3000 °C

50 nm), the efficiency was found to be increased due to constructive interference between transmitted and scattered waves from the Au nanoparticle while for smaller particles, the efficiency decreases due to destructive interference [24]. In a similar study, due to the localized surface plasmon resonance the Au nanoparticles embedded in the nanoparticulate-TiO2 film strongly absorb the light [162]. Gold nanoparticles of 100 nm in diameter were incorporated into TiO2 nanoparticles for dye-sensitized solar cells (DSSCs). At the optimum Au/TiO2 mass ratio of 0.05, the power conversion efficiency of the DSSC improved to 3.3% from a value of 2.7% (without Au) [162]. A clear physical difference between the SPP resonances and the dominant metasurface collective resonance has been studied by Li et al. [163] through the fabrication, characterization, numerical simulations, and theoretical analysis of plasmonic metasurface with subwavelength features of two-dimensional Au patch arrays on ITO-coated glass substrate. They observed the difference between these two resonant phenomena is because of their

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sensitivity to the incident angle [163]. The collective metasurface resonance is based on the plasmonic resonance of an individual metasurface element and only weakly affected by the array coupling, whereas the SPP-coupled grating modes are lattice effects that are very sensitive to the incidence angle. Thus, high tolerance of incident angles and polarization in transmission and reflection of the metasurface resonance is potentially suggested to be useful for plasmonic solar cell [163]. Chen et al. [164] have tuned the SPR by varying Au NP’s size in Au-embedded TiO2 nanotubes arraybased photoanode material. The overall PCE of 4.63% was recorded for Au/TiO2 NT with 19.0% enhancement in comparison with the bare TiO2 NT-based DSSC. The enhanced PCE was attributed to the coupling effect of SPR of small-sized particles (~10 nm) and scattering effect of large-sized particles (50–200 nm) [164]. Rho et al. [112] reported an enhancement in PCE of DSSC made with TiO2 nanoparticles (NPs)/nanotubes (NTs)–silver nanoparticle composites. PCE for only TiO2 NPs film is 8.04%; for TiO2 NPs/NTs is 8.78%; and for TiO2 NPs/NTs-Ag@TiO2 NP’s composites is 10.6%. The significant enhancements in PCE were recorded due to the fast electron transport through nanotubes and plasmonic enhancement of Ag NPs [112]. Many other common plasmonic structures that have been used so far for enhancing the efficiency of DSSC are TiO2 –Au plasmonic nanocomposite (PCE 6%) [165]; Au–TiO2, and Ag–TiO2 plasmonic hybrid nanocomposites [166]; Ag nanoparticle/Nb2 O5 composite as plasmonic surface passivation layer on SnO2 photoanodes with an enhancement of 32% in PCE (total PCE 6.5%) [167]; designing of Au plasmon-based SiO2 @Au@TiO2 microspheres embedded with Au nanoparticles as photoanode (PCE 7.75%) [168]; post-annealing of Ag plasmonic compact layer with anatase TiO2 (PCE 9.45%) [169]; TiO2 –Zn nanocomposite photoanode materials (PCE 1.34% for Zn-doped TiO2 as compared to undoped TiO2 of 0.66% PCE) [27]; and integration of 1D plasmonic Ag nanowires into the mesoporous TiO2 photoanode with an enhanced PCE of 9.41% from 7.5% [25]. A schematic for the incorporation of Ag nanowires-based plasmonic nanostructure in TiO2 layer is shown in Fig. 10 [25]. A modified two-step injection method was used to synthesize the Ag nanowires. These Ag nanowires of different concentrations were mixed in mesoporous TiO2 paste, which was then coated by the doctor blade technique on the top of a spin-coated TiO2 compact layer on FTO substrate. The J–V characteristics and IPCE curves for different concentrations of Ag nanowires in mesoporous TiO2 photoanode are shown in Figs. 11 and 12.

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Fig. 10 Schematic design of DSSCs with Ag nanowires plasmonic nanostructure-based mesoporous TiO2 photoanode. Reproduced with permission from journal Ref. [25], Copyright 2017, Elsevier Ltd Fig. 11 a J–V characteristic curve of DSSC and b IPCE curve for different concentrations of Ag nanowires in mesoporous TiO2 photoanode. Reproduced with permission from journal Ref. [25], Copyright 2017, Elsevier Ltd

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Fig. 12 a Short-circuit current density (J sc ) and power conversion efficiency (PCE) plotted as a function of Ag nanowires concentration in mesoporous TiO2 photoanode, and b open-circuit voltage (V oc ) and fill factor (FF) are plotted as a function of Ag nanowires concentration in mesoporous TiO2 photoanode. Reproduced with permission from journal Ref. [25], Copyright 2017, Elsevier Ltd

7.5 Tandem Dye-Sensitized Solar Cell A new class of solar cell named as “Tandem solar cell” based on dye sensitization has recently emerged and showed high light conversion efficiency as compared to simple DSSC. Conventional DSSC operates on the concept of photocurrent flowing through the circuit due to the electron injection in the conduction band of n-type photoanode materials as a result of the dye sensitizations (usually named as n-DSC). A similar concept is applicable for photocathode materials which operate in inverse mode to n-DSC are named as p-DSC [170]. In p-DSC, the dye molecules are excited by rapid electron transfer from a p-type semiconductor material to the dye molecules. If these p-DSCs and n-DSCs structures are combined together to construct a solar cell, then

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it is called as tandem solar cells (T-DSSCs) [170]. In a simple tandem solar cell, an electrolyte layer is sandwiched between one n-DSC and one p-DSC layer. Yanagida et al. [171] and Arakawa et al. [172] reported tandem DSCs sensitized by using N719 and black dye (BD) and achieved high PCE of 10.6% and 10.4%, respectively. Chae et al. [173] have reported a monolithic tandem solar cell of 6.1% PCE by combining a TiO2 -based n-DSC and a p-DSC consist of solution-processed CuInx Ga1−x Sey S1−y (CIGS) thin-film solar cell. A systematic design for DSSC/CIGS-based tandem solar cell is shown in Fig. 13a [173]. The transmittance curve of Y123 dye and IPCE of CIGS system is shown in Fig. 13b; photon flux of one sun irradiation and transmitted from different components of tandem solar cell is shown in Fig. 13c; and I–V curve of a single CIGS solar cell with and without a mask is depicted in Fig. 13d. The [Co(bpy)3 ]2+ /[Co(bpy)3 ]3+ ) redox couple, Y123 organic dye as a sensitizer, and well-matched counter electrode of PEDOT:PSS were used to achieve high durability of more than 1000 h in tandem solar cell. The high stability in the tandem solar cell was due to the less corrosion by the Co2+ /Co3+ electrolyte solution in comparison with iodide (I− /I− 3 ) redox couple. In a similar work by Moon et al. [174], a solution-processed tandem solar which consists of monolithic DSSC/CIGS system was fabricated with high PCE of 13.0%.

Fig. 13 a Design for a simple DSSC/CIGS tandem solar cell, b IPCE curve of CIGS single junction solar cell and transmittance curve of Y123-sensitized, TiO2 /PEDOT:PSS-coated FTO glass, c Photon flux of one sun irradiation (black line), transmitted photon flux from Y123-sensitized, TiO2 /PEDOT:PSS-coated FTO glass (filled black line), and intergraded photocurrent (blue line), and d current density (I–V) curve of a single CIGS solar cell with and without a mask. The figure is adopted from Ref. [173]. Open access journal; reproduced under Copyright 2016, nature scientific report

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Recently, Kwon et al. [175] have engineered a highly efficient two-terminal DSSC/Si monolithic tandem solar cell with PCE of 17.2%, which is of maximum efficiency reported among all DSSCs fabricated so far. The cell was fabricated by controlling and optimizing the optical properties of a PEDOT interfacial catalytic layer for CE and amount of dye adsorption on TiO2 NP’s surface. They individually optimized the top DSSC cell and bottom Si cell in order to achieve the best tandem solar cell. The top n-DSC was designed with coating of a mesoporous TiO2 layer on a FTO substrate. Porphyrin dye (SGT-021) for sensitization and an intermediate transparent PEDOT:FTS film as a catalytic layer for CE were used in top DSSC of tandem solar cell. The absorption band edge for porphyrin dye (SGT-021) is ~800 nm, whereas ~1200 nm for c-Si layers, both of which act as light absorption layers in tandem solar cell. When a solar light is irradiated on this tandem solar cell, the top nDSC based on dye-coated TiO2 photoelectrode mainly absorbs the short wavelengths light up to around 800 nm, whereas the low energy radiations up to 1200 nm wavelengths are absorbed by the bottom Si solar cell. The high efficiency of this tandem solar cell was attributed to the elimination of one TCO glass that is used as CE in conventional DSSC, low resistance, and high transparency of PEDOT interlayer and excellent current matching between the top and the bottom solar cell. In the bottom solar cell, a phosphorous spin-on-dopant (SOD) is spin coated on the front surface of the Si wafer for an n-type emitter and the back surface is coated with 2-mm-thick Al for a P+ back surface field (BSF) by e-beam evaporation, which is co-fired using rapid thermal annealing at 950 °C for 1 min. Then the surface passivation of an emitter surface was done by an 80-nm-thick SiNx thin film deposited by plasma-enhanced chemical vapor deposition (PECVD). An ITO thin film as a recombination layer was further deposited on the top of the SiNx passivation layer by using sputtering method. This ITO layer is desired for the recombination of electrons generated from the bottom Si cell and the holes coming from the top DSSC [175]. The design of this tandem solar cell is depicted in Fig. 14. Figure 15a shows the schematic of the bottom Si solar cell. Figure 15b shows the J–V characteristics of bottom Si solar Fig. 14 Design of the tandem solar cell reported by Kwon et al. [175]. Reproduced with permission from journal Ref. [175], Copyright 2016, The Royal Society of Chemistry

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Fig. 15 a Schematic design of the Si solar cells, b J–V curves for a single junction cell with passivation, c, d energy band diagrams and J–V curves, respectively, for the top and the middle silver grid of single junction solar cells

cell with emitter layer, SiNx passivation, and BSF layers, respectively. The current density is drastically increased with the coating of passivation and BSF layers on Si substrate. Figure 15c represents the energy band diagram for the top and the middle silver grid of single-junction solar cells. Ag ohmic contact layer of 300 nm thickness was introduced to reduce the energy barrier between the ITO and n-type Si interface down to 0.15 from 0.65 eV. Figure 15d shows the J–V characteristics of the bottom Si solar cell in two different configurations; one is with the Ag grid patterns formed between the n-type Si and ITO (emitter/Ag/ITO) as shown in Fig. 15a while another is the Ag grid patterns on top of the ITO layer (emitter/ITO). The high current density was observed in the case of Ag grid patterns formed between the n-type Si and ITO (emitter/Ag/ITO). Figure 16a–d shows the comparison of photovoltaic properties for various kinds of tandem solar cells. The few main contributions in the advancement of DSSCs have been summarized in Table 1.

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Fig. 16 Comparison of various kinds of tandem solar cells: a 4-terminal (external) and 2-terminal (internal) tandem solar cells, b IPCE curve of the masked Si solar cell and DSSC, c various tandem solar cells which are based on TiO2 photoanode in DSSCs, and d other hybrid structure types of tandem solar cells. Reproduced with permission from journal Ref. [175], Copyright 2016, The Royal Society of Chemistry

8 Merits and Demerits of DSSCs Most of the components in dye-sensitized solar cells do not have harmful materials. The materials used to fabricate the DSSC are easy to use, separate, and get back for recycling or even can be reused for DSSC panels. The electron injection and operation principle of DSSC are different than traditional Si solar cell. The major advantage over other commercially available silicon solar cells or thin-film solar cells, whose PCE generally lies between 5 and 17%, is the comparatively low-cost fabrication and to use like rooftop solar collectors owing to their lightweight and mechanical robustness. The different charge dynamics of the photocathode and photoanode inside the DSSCs allows it to work even in low intensity of light such as under cloudy skies and non-direct sunlight, whereas in other traditional Si solar cell or thin-film-based solar cells; usually, the PCE is reduced to a certain limit. The Si solar cells need to cover in a glass box for the safety reasons which causes heating inside the cell and results

Substrate and sheet resistance

FTO glass

FTO glass, TEC8, Pilkington, 8–10 /sq

FTO glass, 10–15 /sq

FTO glass

Reference No. and year

[142] 2019

[107] 2018

[25] 2017

[13] 2017

Pt/FTO

Poly(3,4-ethylenedioxythiophene) (PEDOT)-covered FTO conducting glass

8 μm of TiO2 (4 μm 30NRD + 4 μm scattering layer)

Pt/FTO

Oxygen and sulfur dual-doped (OS-doped) carbon

Counter electrode (CE)

Plasmonic Ag nanowire-based mesoporous TiO2 photoanode (10 μm) + TiO2 NPs 4 μm scattering layer

TiO2 paste

TiO2 -coated FTO electrode (total 20 mm in thickness with 16 mm absorber thick film and 4 mm for TiO2 light-scattering layer, respectively)

Photoanode

combination of D35:XY1 dyes (4:1)

0.3 mM N719 dye solution (1:1 vol. ratio of acetonitrile and TBP)

Phenothiazine dyes (PREDCN2) with 3,4-ethylenedioxythiophene (EDOT) units

N719 (cisdiisothiocyanatobis(2,29-bipyridyl-4, 49-dicaboxylato) ruthenium(II)bistetrabutylammonium))

Dye used

Table 1 Summary of important contributions of DSSC’s development (AM1.5–100 mW cm−2 )

Copper(II/I) complex-based electrolyte Cu(II/I)(tmby)2 TFSI2/1 (tmby, 4,4 ,6,6 -tetramethyl-2,2 -bipyridine; TFSI, bistrifluoromethane sulfonimidate)

Iodide/triiodide (I− /I− 3 ) electrolyte

3-propyl-1-methyl-imidazolium iodide (PMII, 1M), lithium iodide (LiI, 0.2 M), iodide (I2 , 0.05 M), and tert-butylpyridine (TBP, 0.5M) in acetonitrile/valeronitrile (85:15, v/v)

Iodide/triiodide (I− /I− 3 ) electrolyte

Electrolytes used

(continued)

11.3

9.4

8.3

10.2

Efficiency (PCE—%)

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Substrate and sheet resistance

FTO glass

FTO glass and Si substrate

FTO glass

Reference No. and year

[176] 2016

[175] 2016

[173] 2016

Table 1 (continued)

TiO2 paste

Mesoporous TiO2 (tandem solar cell)

One transparent layer (4 μm) that was printed with colloidal TiO2 paste (Dyesol DSL 30 NRD-T) and one light-scattering layer (4 μm) prepared by another paste (PST-400C, JGC Catalysts and Chemical Ltd.)

Photoanode

PEDOT:PSS, and Al-doped zinc oxide (AZO) window layer in bottom CIGS cell

PEDOT:FTS conductive polymer and ITO top layer on p-Si substrate (tandem solar cell)

Pt/FTO

Counter electrode (CE)

Y123

Porphyrin dye (SGT-021)

Cosensitized (D35/Dyenamo Blue) mixture of D35 and Dyenamo Blue in the ratio of 4:3 (0.1 mM: 0.075 mM) in tert-butanol: ACN (1:1 (v/v))

Dye used

17.2

6.8

Co(bpy)2+/3+ redox couple 3 (electrolyte solution of 0.22 M Co(bpy)3 (BCN4 )2 , 0.05 M Co(bpy)3 (BCN4 )3 , 0.1 M LiClO4 , and 0.8 M 4-tert-butylpyridine in acetonitrile) [Co(bpy)3 ]2+ /[Co(bpy)3 ]3+

(continued)

10.5

Efficiency (PCE—%)

TPAA/Co complex electrolye TPAA-tris(p-anisyl) amine

Electrolytes used

220 M. C. Mathpal et al.

Substrate and sheet resistance

FTO glass, 9-11 /sq.; Asahi glass

FTO glass, 10 /sq.

FTO glass, 15 /sq., nippon sheet Glass Co.

FTO glass

FTO glass, TEC8, 8 /sq.

FTO glass, 13 /sq.

Reference No. and year

[136] 2015

[9] 2014

[135] 2013

[17] 2013

[88] 2012

[22] 2011

Table 1 (continued)

TiO2 multilayers

Nanoporous TiO2 films (all solid-state DSSC)

Hierarchical TiO2 nanowire

Mesoporous TiO2 layer TiO2 paste (commercial P25 nanoparticles (Degussa AG, Germany) and ethyl cellulose in terpineol solvent

Mesoporous TiO2 films

Nanocrystalline-TiO2 thin layers (JGC Catalysts and Chemicals, PST-18NR9)

Photoanode

Pt/FTO

Pt/FTO

Pt/FTO

Pt/FTO

Graphene nanoplatelets/FTO glass

FTO/Au/GNP (GNP—graphene nanoplatelets)

Counter electrode (CE)

N719

N719

N719

N719 dye, Solaronix, Switzerland

SM315

Collaborative sensitization of carbazole/ hexyl-functionalized oligothiophene/ trimethoxysilyl-anchor dye (ADEKA-1) with Dibiphenylmonophenylamine (LEG4 dye)

Dye used

I− /I− 3 electrolyte solution

(continued)

11.05

10.2

7.34

I− /I− 3 electrolyte solution All solid-state, replacing liquid electrolyte by p-type direct bandgap semiconductor CsSnI3

5.8

13

[Co(bpy)3]2+/3+ cobalt(II/III) redox shuttle I− /I− 3 electrolyte solution

14.3

Efficiency (PCE—%)

Cobalt(III/II) tris(1,10 phenanthroline) complex [Co(phen)3 ]3+/2+

Electrolytes used

Basic Concepts, Engineering, and Advances … 221

Substrate and sheet resistance

FTO glass, 10 /sq.

FTO, 4 mm thickness, 10 /, nippon sheet glass)

FTO glass, 10 /sq.

FTO glass

FTO glass, 7-8 /sq.

FTO glass, 80 /sq.

Reference No. and year

[15] 2011

[4] 2011

[11] 2008

[95] 1997

[108] 1993

[1] 1991

Table 1 (continued)

Nanoporous poly(3,4-propylenedioxythiophene (PProDOT-coated FTO)

4.0 + 4.5 scattering layer with PProDoT

Anatase colloidal TiO2 NPs

Colloidal TiO2 NPs

TiO2 NPs and SnO2 TCO

Pt/FTO

Pt/FTO

Graphite/carbon

Pt/FTO

Pt/FTO

5 μm mesoporous TiO2 layer

Nanocrystalline-TiO2 (20 nm, paste A) and microcrystalline-TiO2 (400 nm, paste B)

Counter electrode (CE)

Photoanode

Trimeric ruthenium complex

N3 dye

Ruthenium complex 2,2’-bipyridine-4, 4’-dicarboxylate ligands

0.5 mM N-719 dye solution in a mixture of acetonitrile and tertbutyl alcohol (volume ratio, 1:1)

Y123, (3-{6-{4-[bis(2,4dihexyloxybiphenyl4-yl)amino-]phenyl}-4, 4-dihexylcyclopenta-[2,1-b:3, 4-b2]dithiphene-2-yl}-2cyanoacrylic acid)

YD2-o-C8 (zinc porphyrin dye)

Dye used

7.9

6.7

I− /I− 3 electrolyte solution

electrolyte solution

10



10

10.3

12.3

Efficiency (PCE—%)

I− /I− 3 electrolyte solution

I− /I3

0.6 M BMII, 0.03 M I2 , 0.10 M guanidinium thiocyanate and 0.5 M 4-tert-butylpyridine in a mixture of acetonitrile and valeronitrile (volume ratio, 85:15)

[Co(bpy-pz)2 ]3 +/2 +

Co(II/III)tris(bipyridyl) tetracyanoborate complexes redox shuttle

Electrolytes used

222 M. C. Mathpal et al.

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into the loss of overall efficiency, whereas in DSSC a thin conducting plastics layer on front side can solve the problem of heat dissipation to maintain the lower temperature and overall efficiency of DSSCs. In the current scenario, the most efficient DSSCs have a PCE of as high as 14.3% [136], or a DSSC based on tandem solar cell with a PCE of 17.2% [175] under continuous illumination of 1 sunlight energy (100 mW cm−2 ). The main challenges associated with the fabrication of highly efficient commercial DSSC are the absorption in a wide range of the solar spectrum (UV-visible to IR region); achieving high surface area; choosing suitable redox electrolyte solution (or a hole transport layer in solid-state DSSC) to avoid degradation of any component or electrolyte itself from other components; to reduce the recombination losses and improve the charge dynamics; avoiding degradation from the exposure of ultraviolet radiation; avoiding liquid electrolytes to prevent leakage and electrode corrosion but at the same time keeping in mind that solid-state DSSCs have low efficiency; enhancing the charge collection efficiency by improving the contacts between electrodes; and low stability as compared to traditional Si solar cell [61, 93]. However, the careful designing of DSSC can have high durability and work as long as 10–20 years. The iodine-based electrolyte solution for DSSC is corrosive enough to rust the resistant metals such as aluminum and stainless steel [52, 88]. The major disadvantage in DSSC is the use of liquid electrolyte, which can either easily freeze during winter season or if somehow heated in summer up to 80 °C (on a rooftop, for instance) then it can either stop functioning and damage or expand and rupture the sealing of the solar cell [88]. However, the DSSCs of high efficiency are not yet considered for large-scale production and field replacements, but an approach for further enhancement in the PCE of all solid-state DSSC (without liquid electrolyte) will make them attractive for commercialization. The high cost of Pt-based counter electrodes and ruthenium-based dye complexes is an additional disadvantage: though recently carbon, graphene layer, PEDOT:FTS, graphene nanoplatelet (GNP)-based CE, whereas Co and Cu complex-based dyes are emerged as good alternates to further reduce the cost of DSSC [9, 13, 57, 136, 175]. The DSSC-based tandem solar cells have shown remarkable improvements in PCE to be considered for future solar cell technologies [52, 175].

9 Summary and Future Directions So far, various approaches, such as increasing the photoanode specific surface area by using TiO2 NPs while increasing the red absorption through large particles by light-scattering effect, modifying dyes properties, improving the transport and conduction mechanism of electrolyte solution, enhancing catalytic activity of the counter electrodes, reducing molecular aggregation, using noble metal NPs for enhancing the absorption of light and reducing leakage, have been adopted to improve the efficiency of DSSCs. The characteristic of compact blocking layer to reduce the recombination losses should be carefully investigated. In addition, several research articles are dedicated in the direction to reduce the cost of DSSC by searching for an

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alternative of the traditional platinum counter electrode and ruthenium-based redox complexes. Porphyrin dye has shown promising absorption properties as an alternate of Ru complex-based expansive dyes. The absorption spectra of the dye sensitizer can be broadened by attaching strong electron donor and acceptor groups to the dyes through a π-bonded bridge. Iodine-based redox mediator is reported to be corrosive and unstable. To the date, cobalt and copper-based redox couples are found to be a better choice in the replacement of traditional iodine-based redox couple. Graphene film has got huge attention as a good replacement for Pt/FTO-based counter electrodes as it has several features including low-cost, high catalytic activity, low sheet resistance and a transmittance over 90% after doping. Different NIR dyes can be prepared and used in the combination of different semiconductors of different bandgaps to construct a multilayered photoanode to absorb the light in the entire region of solar spectrum. The active ongoing research to improve the efficiency of all solid-state DSSC will be a major breakthrough to enhance the durability and overall performance of the DSSCs by replacing wet-type DSSCs for outdoor applications. The surface plasmon resonance (SPR) effect can be exploited to enhance the efficiency of the solar cell by appropriate substitution of gold, silver, and copper nanoparticles at the metal/dielectric interface in photoanode. The rigorous investigation on shape and size effect of noble metal nanosize particles to enhance surface plasmon activity and the maximum absorbance in DSSC will be further helpful. So far, tandem solar cells have been very efficient so one can think to improve the efficiency by tuning the n-DSC and p-DSC junction for absorbing UV-visible and NIR light so that the net electron injection can be increased in DSSCs.

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Quantum Dot Solar Cells Hafeez Anwar, Iram Arif, Uswa Javeed, Huma Mushtaq, Khuram Ali and S. K. Sharma

Abstract Energy crisis has become one of the main hurdles in the path of development and technology due to the rapid reduction of fossil. Renewable energy resources have gained the great importance in last few decades. Solar technology is considered as a potential candidate for energy in future. Solar technology has evolved in different generations from single crystal semiconductor wafer to quantum dot solar cells. Quantum dots act as absorbing photovoltaic material instead of bulk materials like silicon or copper indium gallium selenide in quantum dot solar cell (QDSC). Quantum dots have tunable band gaps that depend on their size that makes them a promising candidate for multi-junction solar cells. The photovoltaic conversion efficiency of quantum dot solar cells is much higher as compared to traditional solar cells. Various types of quantum dots like CdSe, CdS, PbS, GaAs, CdTe, ZnSe and graphene are used in different designs of quantum dot solar cells. In this chapter, we discussed the quantum dot solar cells (QDSCs), their design along with their various architectures and materials selection approaches in detail. Keywords Solar cell · Quantum dots · Photovoltaic conversion · Band gap · Efficiency · Bulk materials

1 Introduction Climate change and depletion of traditional energy resources made man to search for other energy sources. Various renewable resources like geothermal, hydrothermal, nuclear, wind and biomass are introduced to replace the traditional one but to meet the need of today’s energy demand above mentioned technologies have no scalable capacity. H. Anwar (B) · I. Arif · U. Javeed · H. Mushtaq · K. Ali Department of Physics, University of Agriculture, Faisalabad 38040, Pakistan e-mail: [email protected] S. K. Sharma Department of Physics, Faculty of Science and Technology, The University of the West Indies, St. Augustine, Trinidad and Tobago © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_9

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Sun is the largest natural source of energy on earth. Energy provided by the sun in one minute is sufficient for the world’s need in one year [5]. In fact, the solar energy provided to earth in three days is as same as the amount of energy stored in all fossils. From centuries, man tried to get maximum energy from sun. The discovery of photoelectric effect in late nineteenth century laid the foundation of first solar cell where it was observed that when sunlight falls on metal surface it generates electric current [11]. It is predicted that after fifty years energy needs will be doubled [21]. Solar cell technology shows the most promising candidate to meet the energy demand of the world. A French physicist, A.E. Becquerel observed for the first-time photovoltaic effect in 1839 [28]. In 1883, Charles Fritts build up the first solar cell, a thin layer of gold has been coated on the selenium to produce solar cell [68]. In 1954, first silicon-based solar cell was developed by Daryl Chapin, Gerald Pearson and Calvin at Bell Labs with 4% efficiency that was later increased up to 11% [20]. From then, it has become one of the major research fields, and various kinds of solar cells are reported in past few decades. One of the major purposes is to develop such technology that must be cost effective and eco-friendly. Recently, a solar cell with record efficiency of 24.2% on a wafer measuring 244.62 cm2 is reported at very low production cost. This achievement boosts the solar technology as next major energy source of the world. In recent years, quantum dot solar cells emerged as a promising candidate and growing very fast. These solar cells can be fabricated with the help of cheap solutionphase process. Furthermore, these can work in a wide range of lighting conditions. For the next-generation solar cell applications, quantum dots are most favorable materials. As these can have ability to overcome the Shockley-Queisser power conversion efficiency limit of existing conventional single-junction solar cells. Also, due to tweaking and shape changing properties of quantum dots make these preferable than the conventional materials. In this chapter, we have discussed solar cell in general, its different generations and especially quantum dot solar cells and their various architectures in detail.

2 Solar Cell and Generations of Solar Cell In photovoltaic effect, generation of electrons occurs when the sunlight falls on the material; this causes the flow of electricity. Solar cell is a solid-state device in which electrons moved from valance to conduction band that results building up the voltage between electrodes. There are mainly three generations in which solar cells have been classified. Research has been conducted in all three generations but commercially, first-generation solar cells’ production is over 85% of all cells [23]. These generations are discussed in the following section and Fig. 1 shows the schematic diagram of all generations of solar cells.

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Fig. 1 The schematic diagram of the generations of solar cells

2.1 First-Generation Solar Cell Generally, these are silicon-based photovoltaic cells. The efficiency of these cells is due to their pentavalent nature; till now, 33% theoretical limiting efficiency has been achieved by single-junction silicon [2]. Sand is the main source of silicon but extracting silicon before its growth of crystal makes it expensive. First-generation solar cell usually based on silicon [6]. First-generation silicon-based solar cells further classified into four categories for commercial applications i.e., monocrystalline silicon cell, polycrystalline silicon cell, ribbon silicon solar cell [22], amorphous silicon cell and hybrid silicon cell [30]. To fabricate a solar cell, silicon must be purified [16]. Mostly casting method is used for the production of silicon [43]. This process involves the following steps: molten silicon is poured to the mold and after setting it, wafers are cut [57]. It is less expensive method to produce polycrystalline as compared to the other methods such as photolithography and MCZ method. [7]. To produce solar cell, n-type or p-type silicon is obtained by doping which produces p-n junction [39]. Temperature diffusion process is used for doping. In thin-film solar cell, dopant impurity is added during deposition. When photon falls on it, electrons are knocked out by these photons and energy is absorbed by silicon, and hole is generated. The movement of holes and electrons are toward respective electrode. In this way, current is generated in solar cells. The schematic diagram of first-generation solar cell is given below in Fig. 2.

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Fig. 2 The schematic diagram of first-generation solar cell

2.2 Second-Generation Solar Cells The second-generation solar cells are known as thin-film solar cells, these are cheaper than the first-generation solar cell [6], production cost of CdTe thin-film solar cell is less than 1$ per peak watt [54]. Thin-film solar cells are less efficient, but these have been using commercially since 2008. Thin-film solar cell is required less amount of material for deposition. Mostly used second-generation solar cells are based on amorphous silicon, microcrystalline silicon and cadmium telluride. Cadmium telluride thin-film solar cell is 30% less costly as compared to the copper indium gallium selenide solar cell, but if the cadmium is released from the cell, it will be toxic [65]. Amorphous silicon (a–Si) solar cells are non-toxic and have greater ability to absorb light in visible region but due to disorder in structure, it is bad conductor for charge carrier. a-Si has higher band gap of 1.6 eV as compared to crystalline silicon (1.1 eV) [49]. Thin film of material is an effective way in reducing the size of the cell design. Thin-film solar cells are less efficient than silicon (wafer-based), but thin-film solar cells are cost effective. Most common second-generation solar cells are based on cadmium telluride (CdTe), amorphous silicon (a–Si) and micromorphous silicon. In CdTe solar cell, thin layer of CdTe (semiconductor layer) absorb and change sunlight to electricity. The schematic diagram of second-generation solar cell is given in Fig. 3. The disadvantage of CdTe solar cell is that if the cadmium present in it released then the cell becomes toxic. But this is the low-cost solar cell i.e., its cost is 30% less than the copper indium gallium selenide (CIGS) and 40% less than amorphous silicon technology (a–Si). Amorphous silicon (a–Si) solar cell is very promising as amorphous silicon present in a huge amount, and unlike CdTe, it is non-toxic in nature. It is low cost, can be flexible and also it needs low processing temperature. The structure of this cell is consisted of amorphous or microcrystalline silicon and it formed a p-i-n architecture. The thin layer of amorphous silicon is photo-electrically active and it absorbed more light as compared to the crystalline cells. Because it is amorphous in nature,

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Fig. 3 The schematic diagram of second-generation solar cell

so it is bad conductor of charge carriers due to the dangling bond and irregular arrangement of atoms. It shows a wider band gap.

2.3 Third-Generation Solar Cells In these days, focus of researches is shifted toward third-generation solar cells. Thirdgeneration solar cells are also known as organic solar cells. In these solar cells, p-n junction is not formed as compared to other generations of solar cell. There are many types of third-generation solar cells including, dye-sensitized solar cells, perovskite solar cells and quantum dot solar cells etc. [73]. The working of all type of thirdgeneration solar cells is almost same [42]. Research on sensitizer-based solar cells was started in 1970s. Dye-sensitized solar cell (DSSC) is based on a photo electrochemical system. It consists of a photo anode or working electrode, a cathode or counter electrode and electrolyte along with organic dyes. The generation of electrons occurs due to sunlight that hits dye layer; electrons are collected for powering a load at cathode. These electrons are re-entered in the electrolyte while reducing triiodide to iodide after passing through the external circuit [26]. The schematic diagram of third-generation solar cells is given in Fig. 4. Later on, organic dyes are replaced by inorganic sensitizers i.e., quantum dots and perovskite materials. Till now, 21% efficiency is achieved that is a great milestone. In the next sections, we will discuss about the quantum dots, quantum dot solar cells and its design. Also, we will discuss about material selection in quantum dot solar cells.

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Fig. 4 The schematic diagram of third-generation solar cell

3 Quantum Dots Quantum dots are the semiconductor nanostructures mostly assembled from the group II-VI or III–V of the periodic table [61]. Quantum confinement is most common term in nano world, this is important because of the change in structure of atom due to influence of very small length scale on energy band structure. It further corresponds to the quantum confinement regime i.e., 1–25 nm for semiconductors. Due to geometrical constraints, electron adjusts its energy in response to change in particle size. This is known as the quantum size effect. It becomes much important when the properties of the materials are size dependent. Moreover, quantum confinement changes the concept of continuous energy bands of bulk materials to discrete energy levels. Schematic diagram as shown in Fig. 5 represents that the semiconductor shows the discrete energy spectrum which is the main difference to bulk semiconductor. In quantum confined structure, motion of charge carriers are restricted by potential barrier in one or more directions. There are three categories of quantum confined structure on the basis of confinement direction, i.e., quantum well, quantum wire and quantum dots. Quantum confined structure’s types are given below.

Structure

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0

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Those charge carriers who are remained confined to the three dimensions these electrons show a discrete energy spectrum. When the two dimensions are confined then quantum wires are formed. In quantum well, electrons are confined to move in a plane. This shows that energy levels change from continuous to discrete. The density of charge carriers is higher near conduction and valance bands. These possess tunable optical properties as compared to others due to their threedimensional (3D) quantum confinement. This quantum confinement is due to the artificial electrostatic potentials [55], presence of semiconductors surfaces and undersurfaces among hybrid nanostructures [51] that binds the electrons when the size is of few nanometers i.e., less than Bohr exciton radius [12]. In semiconductors, the conduction and valance bands are separated by band gap. The motion of electron hole present in these bands has been restricted due to quantum confinement. The charge carrier confinement energy of quantum dots increases that is observed as blue shift in their optical properties [14]. The energy spectrum of quantum dots is discrete in nature and its corresponding wave function is spatially localized within the quantum dot that may extend over few folds in crystal lattice [9]. A small number of elementary particles i.e., electrons and holes in conduction and valance bands ranging of the order 1–100 present in quantum dots that limits its size up to few nanometers [44]. A quantum dot contains only 10–50 atoms in diameter that leads only 100–100,000 atoms within its volume whereas self-assembled quantum dots have range only between 10 and 50 nm [10]. The interface present in hybrid nanostructures like core-shell particles also give rise to the quantum confinement that provides them extraordinary optical properties. The band gap of the shell is larger than the core particles that helps to improve the optical properties of core particle and separates it from surrounding medium [13]. A schematic diagram of core-shell quantum dot is shown in Fig. 6. For the construction of a realistic solar cell, core-shell quantum dots are most important. The core-shell quantum shell contains materials which have high crystal quality, well-controlled hetero interface and surface passivation. There are different categories of core-shell quantum dots exist, depending on the energy gap between valance band and conduction band in semiconductor. In type I, the core has a large band gap than the shell and vice versa for inverse type I. In type II, the conduction or valance band of core could be present within the band gap of the shell. The physico-chemical and optical properties of quantum dots are different as compared to its bulk form. They have tunable band gap ranging from ultraviolet to the infrared that depends on its size [62]. As the size of quantum dots decreases, its spectrum shifts toward the blue, i.e., higher energy [45, 58]. Generally, the synthesis approaches for the quantum dots are divided into two categories i.e., ex situ preparation of Quantum dots and in situ preparation of Quantum dots.

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Fig. 5 Discrete energy level of semiconductor

Fig. 6 A schematic diagram of core-shell quantum dot

4 Quantum Dot Solar Cells Quantum dot solar cells (QDCs) belong to the third-generation solar cells. The quantum dot solar cells show low resistance as compared to conventional solar cell. As in conventional solar cells when semiconductors are absorbed light, this shows much higher resistance as compared to the metals. Silicon, cadmium telluride and copper indium gallium selenide are absorbed photons, and electrons jumps from valance to conduction band, but those electrons which move higher than the conduction band, relax and give off phonon and heat up the solar cell without providing energy. This damages the cell and decreases the cell performance. The quantum dots solar cells generate more electrons as compared to the conventional solar cell for absorption of each photon. The changing in size and shape of the quantum dots, it can change

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the wavelength of the absorbed light. In quantum dot solar cell (QDSC), quantum dots are used as photovoltaic material instead of cadmium telluride, copper indium gallium selenide or silicon wafer. Till now, 16.6% efficiency is achieved for quantum dot solar cells. There are various types of quantum dot solar cell (QDSC) depending upon the captivating photovoltaic material. These are discussed in the following section.

4.1 Metal-Semiconductor Junction Solar Cell (Schottky Cell) Metal-semiconductor junction solar cells are also known as Schottky junction quantum dots solar cells. These are the first kind of quantum dot solar cells (QDSCs) that achieved the efficiency up to 1% [38]. Quantum dots in the form of thin film are sandwiched between the indium-doped tin oxide (ITO) counter electrode and metallic electrode. The ITO-based counter electrode acts as a photo electrode. A schematic diagram of metal-semiconductor junction solar cell is shown in Fig. 7. The design of cell based on the transparent conducting oxide (TCO) having large work function such as indium-doped tin oxide with the p-type quantum dot film to create an ohmic contact [24]. Then, a suitable band-bending to extract electrons is created by evaporating a metal having low work function such as magnesium and aluminum [63]. This kind of solar cells have few limitations such as short diffusion length and immobilization of the Fermi level due to defect states that affect the open-circuit voltage [66]. These issues can be resolved by post treatments, optimization of material synthesis and hole selective contacts. Till now, 5.2% efficiency is achieved [40].

Fig. 7 A schematic diagram of metal-semiconductor junction solar cell

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4.2 Quantum Dot-Sensitized Solar Cells The working principle of quantum dot-sensitized solar cells (QDSCs) is almost same as dye-sensitized cells (DSCs) with a slight exception that is quantum dots are used in quantum dot-sensitized solar cells (QDSCs) instead of dye as a current injector [27]. The structure of the three important parts of the quantum dot-sensitized solar cell includes counter electrode, photoanode having layer of nanostructured TiO2 with quantum dots layer on it and electrolyte. Photoanode consists of films of semiconductor which increase the power conversion efficiency. There are two main features of photoanode: (i) It provides smooth path for the transfer of the photoexcited electrons, which move toward conducting substrate and then to external circuit. (ii) It offers enough surface area to load enough quantum dots for light absorption. In short, a good photoanode must have fast electron transport rate for high electron collection efficiency and a large surface area for quantum dots for light absorption. 7.55% power conversion efficiency is obtained by controlling photoanode. To transfer charge between counter electrode and photoanode, redox electrolyte plays role of medium. It helps both in stability and efficiency of the quantum dot solar cell. I− /I3− (iodide/triiodide) are not stable in chalcogenide quantum dot-based quantum dot solar cell. Therefore, aqueous solution of polysulfide (S2− /Sn2− ) redox couple is used in quantum dot solar cells as it provides stability to QDSCs. Counter electrode in quantum dot solar cells transfer electrons from external circuit and catalyzing the reduction reaction of oxidized electrolytes at electrolyte/CE interface. The quantum dot-sensitized solar cell is shown schematically in Fig. 8.

Fig. 8 The structure of the quantum dot-sensitized solar cell

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In dye-sensitized solar cell (DSC), dyes do not directly play any role in recombination process. The electrons firstly, injected into nanostructured TiO2 and holes are regenerated within a time scale of life of nanostructured TiO2 , whereas in the quantum dot-sensitized solar cell, quantum dots directly participated in the recombination process [25].

4.3 p-I-n Quantum Dot Solar Cell This architecture of solar cell is also known as multi quantum well solar cell. The three-dimensional (3D) array of quantum dots in core region of p+ –I–n+ structure is used in this configuration. The interspacing of the quantum dots is small to achieve the strongest electronic coupling that allows the long-range electron transportation and this three-dimensional (3D) array of the quantum dots acts as photo electrode [48]. The schematic diagram of multi quantum well solar cell is shown in Fig. 9. The miniband states present in quantum dots solar cells are delocalized and quantized and slow down the cooling that allows the transportation and collection of charge carriers at their respective contacts (p or n). It permits to enhance the photo potential in solar cell. The delocalized quantized 3-D miniband states slow down the cooling of charge carriers, allows the transport of charge carriers and their collection at the respective p and n contacts to produce. The limitations of this kind of solar cells are the nature of electronic states as a function of inter dot spaces, transportation properties, shape, order and disorder of arrays and orientation of quantum dots [47].

Fig. 9 Schematic diagram of p-I-n quantum dot solar cell (adapted from [27]

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4.4 Polymer-Semiconductor Structure Configuration The hybrid polymer-semiconductor quantum dot solar cells were firstly introduced by Huynh in 2002 with the efficiency of 1.7% [27]. A junction is created between organic polymer and semiconductor quantum dots. The efficiencies of these solar cells are not much higher but they are cost effective and stable as compared to other configurations. Polymers are attached to the electrode, when light falls on it holes are generated. The polymer present in the solar cell is not completely responsible for the recombination process due to the presence of quantum dots that have much higher absorption [50]. The donor present in solar cell have much higher lowest unoccupied molecular orbitals as compared to acceptor and the Fermi level of cathode is slightly lower than the conduction band of acceptor (quantum dots) that allows the electrons to flow from polymer to cathode. A schematic diagram of hybrid polymer-semiconductor quantum dot solar cell is shown in Fig. 10. The role of polymers in recombination process is still not completely clear, there is a general rule followed in this regard, i.e., the II–VI and IV–VI quantum dots are capped by small chain quantum dots (Quantum dots) to enhance the photovoltaic performance of the cell. The limitations of these solar cells are unbalanced charge mobilities [31].

Fig. 10 A schematic diagram of hybrid polymer-semiconductor quantum dot solar cell

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Fig. 11 A schematic and energy diagram of heterojunction quantum dot solar cell

4.5 Depleted Heterojunction Quantum Dots Solar Cells The heterojunction quantum dot solar cells are mostly based on lead chalcogenide quantum dots. The highest efficiency achieved by these solar cells is 12.35% [17]. These solar cells overcome the limitation of Schottky quantum dot solar cell, i.e., low built-in voltages. The structure of these solar cell consists of a quantum dot layer that is sandwiched between metal electrode and electron transporting layer usually TiO2 is used for this purpose. The electrons mostly transport toward TiO2 layer instead of evaporated metal contact that creates an inverse polarity [15] and holes transfer from TiO2 layer to quantum dots is restricted that allows better charge separation. A schematic and energy of heterojunction quantum dot solar cell is shown in Fig. 11.

5 Material Selection in Quantum Dot Solar Cells 5.1 CdSe-Based Quantum Dot Solar Cell CdSe quantum dots are coupled with comparatively stable and large band gap metal oxide, i.e., TiO2 to produce quantum dot solar cells. CdSe Quantum dots are synthesized by using pyrolysis of organometallic reagents by injection into a hot coordinating solvent; it provides controlled growth of CdSe quantum dots. 0.08% efficiency of the solar cell is obtained under 100 mW/cm2 under solar illumination [29]. Different-sized CdSe quantum dots have been assembled on TiO2 nanoparticles and nanotubes by using a bifunctional linker molecule. CdSe Quantum dots inject electrons in TiO2 (as shown in Fig. 12) and produce photocurrent in photo electrochemical solar cell. It is interesting to note that by controlling the size of the CdSe Quantum dots photo electrochemical response can be tuned and by facilitating transportation of charge through TiO2 nanotube architecture, photo conversion efficiency can be improved [8].

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Fig. 12 To Harvest Light TiO2 film Functionalized with CdSe quantum dots

Sensitized-type solar cell which was based on TiO2 inverse opal and CdSe quantum dots are used as the sensitizer. By using chemical bath deposition method, CdSe quantum dots were grown at appropriate place on TiO2 inverse opal electrode. Improvement in the photovoltaic performance can be observed by modifying surface with zinc sulfide and fluoride ions [18]. With the increase in the adsorption time, the size of the CdSe increases. To obtain the best photovoltaic conversion efficiency, there is a specific adsorption time for CdSe. Under a solar illumination of 100 mW/cm2 for CdSe quantum dot solar cell, 2.02% efficiency is observed [59]. Light harvesting ability can be enhanced by stacking quantum dots of PbS, CdS and CdSe on mesoporous layer of TiO2 in different combination. Solar efficiency of 6.2% has been achieved by three layers of PbS/CdS/CdSe quantum dot solar cell while solar cell efficiency of two layers of quantum dots solar cells consisted of PbS/CdS and CdS/CdSe was obtained 5.8 and 4.2% respectively [41]. Two steps ion exchange process utilized to obtain CdSe/ZnO at lower temperature. Analyses showed that greater size of ZnO quantum dots is obtained which enhances the light harvesting. Quantum dots solar cell which consisted of CdSe/ZnO showed 5.08% efficiency [52]. The performance of the quantum dot-sensitized solar cell (QDSC) might be increased with the presence of both CdS and CdSe quantum dots. Fill factor of CdS is greater than the CdSe-DSSC as for electron injection it provides greater driving force [34].

5.2 CdS-Based Quantum Dot Solar Cell CdS-based quantum dot solar cells absorb light in the visible region [71]. Quantum dot solar cell as compared to other conventional solar cell decreases the dark current and enhances the efficiency of the solar cell [60]. Anodic oxidation in a NH4 F

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organic electrolyte is used to synthesis highly ordered TiO2 nanotube films. Sequential chemical bath method is used to deposit CdS Quantum dots onto TiO2 nanotubes. Fill factor of 0.578 and 4.15% cell efficiency are achieved by this system. The high efficiency is due to geometry of the film which permits the faster transfer of photo generated electrons from CdS quantum dots to TiO2 substrate [64]. CdS quantum dot-sensitized solar cell based on modified polysulfide electrolyte shows 3.2% energy conversion efficiency with high photo voltage of V oc = 1.2 V under air mass 1.5 G illumination and fill factor is 0.89 (shown in Fig. 13). CdS quantum dots are attached to nanoporous TiO2 by using thioglycolic acid (TGA) [35]. Mercaptosuccinic acid (MSA) modified the surface of CdS Quantum dots and it behaves as N3 dye in terms of adsorption legend which attaches onto the bare TiO2 substrate. Modification on the TiO2 surface can also be made by using linker molecules to render a surface with amine or thiol group. It is observed that the power conversion efficiency of CdS sensitized TiO2 electrode is 20%, 13% TiO2 film which is modified by 3-mercaptopropyl trimethoxysilane and 6% for TiO2 film modified by 3-aminopropyl-methyl diethoxysilane. Moderate absorption rate of MSA-CdS Quantum dots using carboxylic acid/TiO2 provide the efficient assembly of quantum dots on to TiO2 film [60]. Successive ionic layer adsorption and reaction (SILAR) method is used to deposit undoped and Pb-doped CdS Quantum dots on TiO2. In the visible region, 2% Pbdoped CdS Quantum dots show highest absorption rate and show 1.19% power conversion efficiency [70]. Quantum dot solar cell is competitive only when it shows the higher power conversion efficiency, the quantum dot solar cell based on Mndoped- CdS/CdSe quantum dot on TiO2 substrate shows improved efficiency of 5.4% [56].

Fig. 13 Schematic diagram of the CdS quantum dot solar cell

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5.3 PbS-Based Quantum Dot Solar Cell It is a low-cost new-generation solar cell. It is interesting to note that the power conversion efficiency of the PbS-based quantum dot solar cell increases to 8.45% by introducing of the conjugated polymer as anode buffer layer. The open-circuit voltage also increases dramatically due to modification of anode [37]. In fabrication of PbS colloidal quantum dot-based solar cell, fullerene derivative is used as the electron transporting layer. Structure of quantum dot films is modified by thiol treatment and oxidation process and quantum dot solar cell shows fill factor of 62%. Under one sun AM 1.5 illumination, the power conversion efficiency is 1.3% [79]. By using lead sulfide quantum dots, a large area which is present in between TiO2 and organic charge transport material is sensitized to visible light. The incident photon-to-electron efficiency is up to 45% and under simulated sunlight of 10 mW/cm2 , 0.49% energy conversion efficiency is observed [53]. The colloidal QD solar cell which is prepared by using solution processing method with ZnO/PbSEDT/carbon shows remarkable photovoltaic response. It shows greater power conversion efficiencies (5.9%), fill factor (51.8%) as compared to Au-coated colloidal quantum dot solar cell [3]. PbS quantum dots developed by non-hot injection synthetic route simplifies the fabrication of Pb-chalcogenide QD solar cell. PbS QD of size 2.2 nm is produced having band gap 1.7 eV. Photoluminescence quantum yield of chloride-terminated Quantum dots is higher as compared to the other conventional synthesis techniques and these showed improved air stability but the solar cell performance of chloridepassivated PbS Quantum dots is much better than traditional synthesis [77], Fig. 14. As on large scale, it is not economic to deposit many layers of film in solar cell. A simple and efficient way of deposition is to replace the methanol with acetonitrile (ACN). During the solid-state ligand-exchange process, the film cracks generated from the volume loss, this problem can effectively be cured by using ACN which enable thick and dense deposition of film with less deposition steps. CPVT-certified

Fig. 14 Preparation of PbS quantum dots and the photovoltaic performance of PbS quantum dot solar cell (Reproduced with the permission of [77]

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IPCE of 11.21% is achieved which is highest efficiency reported for PbS Quantum dots solar cells [36].

5.4 InP-Based Quantum Dots Solar Cells As InP has ideal band gap (1.35 eV) and it is also environment friendly. It harvests more light than other materials used in photovoltaic cells. By using one pot nucleation doping method, Sn-doped InP quantum dots are synthesized. Sn-doped InP quantum dots are used as sensitizer in quantum dot solar cell. Capping ligand induced selfassembly is used to load quantum dots on mesoporous TiO2 film. Sn-doped InP quantum dot solar cell shows power conversion efficiency of 3.54% under AM 1.5 G. With the increase in size of the Sn:InP Quantum dots, the absorption peak is gradually red-shifted and also broadened. Sn:InP quantum dots have nearly spherical shape and lattice spacing is equal to those of InP Quantum dots as shown in Fig. 15 [74]. By using an intermediate band within the band gap, efficiency beyond the Shockley-Queisser limit can be attained. InP quantum dot is in In0.49 Ga0.51 P host is most promising for intermediate band solar cell. Metal organic chemical vapor deposition (MOCVD) is used for the growth of InP quantum dots. With both 3 and 5 layers of InP Quantum dots in the i-region, several n-i-p In0.49 Ga0.51 P solar cells are grown. Enhancement of 0.11 mA/cm2 , in sub-band gap is observed due to absorption and collection from InP Quantum dots [32], Fig. 16. InGaP-based InP quantum dot solar cells are also intermediate band solar cells. To fabricate this system, solid-source molecular beam epitaxy is used and optical absorption of InP is observed as ~850 nm which is greater than optical absorption range of the host InGaP solar cell. Thermal carrier escape still occurred despite the deep confinement of electrons in InGaP-based InP QD solar cells. This solar cell shows efficiency 0.38% [1].

5.5 Graphene Quantum Dot-Based Solar Cell Graphene quantum dots show promising properties in quantum confinement and edge effect. Due to its unique optoelectronic properties, it is used in photovoltaic devices. Graphene quantum dots are used as sensitizer in dye-sensitized solar cell [78]. Hybrid zinc oxide/graphene electrodes in quantum dot solar cells are more efficient than TiO2 /ZnO films. From Fig. 17, it is observed that for both materials, absorption peaks lie in the ultraviolet region and hybrid dots are red-shifted. A luminescence quenching effect of graphene shell is about 72% observed by photoluminescence measurement. System consisting of these hybrid electrodes shows power conversion efficiency of 35% which is quite higher than that is obtained by ZnO electrode as shown in Fig. 16 [67].

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Fig. 15 a UV-vis spectra of Sn: InP Quantum dots (Sn/In ¼/0.1) with various absorption wavelengths, b XRD patterns of InP and Sn: InP Quantum dots. c HR-TEM images of Sn:InP Quantum dots and d InP Quantum dots. Insets are HR-TEM images and column charts of the size distribution (Reproduced with the permission from [74] Fig. 16 Schematic illustration of the structure of InGaP-based InP quantum dot solar cell

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Fig. 17 Optical properties of hybrid ZnO/G Quantum dots as compared to pristine ZnO nanoparticles. a UV-Vis spectrum of the colloidal solutions in DMF (24 mg/ml). b PL spectrum of CQD films (300 nm) prepared by spin coating (Reproduced with the permission from [67]

Zinc oxide nanowires are coupled with the graphene quantum dots (which synthesized by direct chemical method) prove their potential as light harvesting material in photovoltaic cells and show 0.8 V open-circuit voltage [19]. The efficiency of n-type Si heterojunction solar cells is enhanced about 16.5% due to the presence of graphene quantum dots. In depletion region, more electrons are absorbed for effective carrier separation, due to photon down conversion phenomenon of the graphene quantum dots. Fill factor increases from 70.29 to 72.51% [69].

5.6 Other Quantum Dot Solar Cell PbSe quantum dots show enhancement in the photocurrent due to multiple exciton generation (MEG). PbSe quantum dots that are synthesized by a direct cation exchange show air stability and sustain their photoluminescence quantum yield under ambient condition for more than thirty days [76]. Improvement in the efficiency of InAs/GaAs quantum dot solar cell (from 11.3 to 17.0%) is observed by direct silicon doping in InAs Quantum dots. Molecular beam epitaxy is used to grow five stacked quantum dots in i-region of the devices. Open-circuit voltage also increases to 0.84 V due to the Si-doping [75]. Five layers of InAs/GaAs Quantum dots with different levels of Si-doping are placed in the middle of the i-region are shown in Fig. 18. To increase photocurrent in the solar cell, quantum dots are embedded into the intrinsic layer of p-i-n solar cell. It is observed that the presence of defects increases carrier extraction from InAs [72]. Another approach of quantum dot solar cell is growth of In(Ga)As/GaAs quantum dot on the silicon substrate by using molecular beam epitaxy. By inserting forty layers of In(Ga)As/GaAs quantum dots in heterojunction pin GaAs/n+ -Si spectral response increases up to 1200 nm [4]. GaSb/GaAs

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Fig. 18 Structure of the QD solar cells. Five layers of InAs/GaAs Quantum dots with different levels of Si-doping are placed in the middle of the i-region

quantum dot solar cell showed enhanced infrared spectral response as from 0.9 to 1.36 µm as compared to GaAs cell which is considered to be reference [33]. For broad absorption and efficient carrier extraction type II, ZnSe/CdS quantum dot solar cell are fabricated. In type II, ZnSe/CdS system minimal conduction band edge in CdS shell and maximal valence band in ZnSe core i.e., these conduction and valence band lie in the different semiconductor, which provide a smaller band gap for a broad absorption spectrum [46].

6 Conclusion Renewable energy resources especially photovoltaic have become the promising solution for the energy crisis. Many countries are adding solar technology in main grid system. Third-generation solar cells like DSSC, perovskite solar cells, organic solar cells and quantum dot solar cells have dominated the research from past few decades. Quantum dots solar cells overcome the limitations of dye-sensitized solar cells and also cost effective. Various types of quantum dots are used to enhance the efficiency of solar technology. To overcome the world energy crisis, it is necessary to implement the transformative technology.

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Acknowledgements H. Anwar is grateful to Pakistan Science Foundation for funding under the project PSF-NSF/Eng/P-UAF (05).

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Organometal Halide Perovskite-Based Materials and Their Applications in Solar Cell Devices Amna Bashir and Muhammad Sultan

Abstract Perovskite Solar Cells (PSCs) based on organometal trihalide materials have gained enormous attention for photovoltaic applications due to its outstanding optical and electronic properties such as high absorption coefficient long carrier diffusion lengths, long carrier mobility and unique defect physics. As a result, the power conversion efficiency (PCE) of PSCs rapidly enhanced from 3.8 to 24% through the advancement made in processing methods, compositional tuning, and interface engineering. The dominant PSCs architecture has been evolved; n–i–p and p–i–n with mesoporous and planar heterojunction. In both configurations, i.e. planar or mesoporous, the perovskite material is sandwiched between electron and hole transporting layers and top electrode. The basic function of charge transporting layers is to improve charge collection efficiency and reduce charge recombination at interfaces. In the following chapter, we present the critical survey of the recent progress in perovskite absorber and charge transporting materials for the exceptionally higher PCE of perovskite devices. Furthermore, numerous fabrication techniques and device architectures are summarized. Keywords Photovoltaics · Third-generation solar cells · Perovskite materials · Perovskite solar cells · Heterojunction

1 Introduction The perovskite solar cells are evolved from dye-sensitized solar cells (DSSC). The mesoscopic DSSC was first reported by Micheal Gratzel and O’ Ragan in 1991 [1]. A power conversion efficiency (PCE) of 7% was reported using Ruthenium dye absorbed on mesoscopic film of nanocrystalline TiO2 [1]. This discovery opened a new frontier in the race of developing the solar energy harvesting technologies. A characteristic liquid DSSC consists of a photoanode, which is usually a glass A. Bashir Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan A. Bashir · M. Sultan (B) Nanoscience & Technology Department, National Centre for Physics, Islamabad 44000, Pakistan e-mail: [email protected] © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_10

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coated with the transparent conducting oxide (TCO), a mesoporous TiO2 with the sensitized dye absorbed onto it, and electrolyte (typically an iodine/iodide redox couple), and a platinized counter electrode. The schematics representation and the working principle of DSSC and device architecture are shown in Fig. 1 [1]. After the absorption of photons, the electrons are excited from Highest Occupied Molecular Orbital (HOMO) to Lowest Unoccupied Molecular Orbital (LUMO) of dye. Meanwhile, the electrons are injected from LUMO into the conduction band of the TiO2 , which is acting as electron transporting layer. The electrons are collected at corresponding electrode via diffusion process through mesoporous TiO2 layer. At counter electrode, the electrolyte is reduced by the electrons approaching through the external circuit [2, 3]. Until now, an impressive efficiency up to 13% has been achieved for the devices made in the laboratory and 10% for the prototype modules [4, 5]. However, electrolyte leakage, instability and its corrosive nature are the main difficulties towards the commercialization of liquid DSSC [6]. Subsequently, Batch et al. in [7] first reported solid-state dye-sensitized solar cells (ss-DSSC) with PCE of 0.74% [7]. In 2011, a record PCE of 7.4% has been achieved by optimizing the various components of ss-DSSC [8]. In 2012, thin-film photovoltaics technologies have been revolutionized with the substation of dye with the organic-inorganic hybrid perovskite (PS) absorber [9, 10]. Perovskite solar cells (PSCs) based on organometal trihalide material (MAPbI3 ) have engrossed enormous research interest for solar cell applications [10–12], due to their outstanding optical and electronic properties such as high absorption coefficient [13], long carrier diffusion lengths [14, 15], long carrier mobility and unique defect physics. [16] As the result, the power conversion efficiency (PCE) of PSCs rapidly enhanced from 3.8% to 22.1% [17] through the advancement made in processing methods via compositional engineering [18], interface engineering [19, 20]. The advantage of PSCs over conventional silicon solar cells is their low cost and simple manufacturing process. However, for the conventional silicon solar cells, the

Fig. 1 Schematic representation of, a components of solid-state dye-sensitized solar cell (ssDSSC), b working principle of ss-DSSC, reproduced with permission from royal society of chemistry, Ref. [92]

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manufacturing process is multistep, and each step being operated at high temperature (>1000 °C) in highly evacuated chamber. However, synthesis of perovskite material is simple by wet chemical method in non-evacuated ambient conditions.

2 Perovskite Material for Solar Cell Applications The PSCs based on the light harvester and charge carrier conductor that is actually an organic-inorganic lead or tin halide compound also called perovskite compound. The perovskite refers to the class of material that has the general formula of ABX3 , where A and B are the cations having different atomic radii, and X is an anion bond to both. The crystal structure of methylammonium lead halide perovskite is shown in Fig. 2. In methylammonium lead halide (CH3 NH3 PbX3 ), the CH3 NH3 + cation ion is surrounded by the octahedra of PbX6 . The X ions are highly mobile and can be localized through the whole crystal structure. The activation energy of X ions is 0.6 eV, which is dependent on the axial to axial, equatorial to axial or equatorial to equatorial bond position of halide anions. With the CH3 NH3 PbI3, the record PCE od >20% has been achieved successfully after an extensive research [11]. This PCE is higher than the traditional DSSC and organic solar cells [17, 19]. The perovskite material CH3 NH3 PbI3 and CH3 NH3 PbBr3 firstly reported as light harvesting material for solar cells by Kojima et al. [17] with the record efficiency of 3.81%. Later, this PCE had been further improved to 6.54% by introducing the CH3 NH3 PbI3 quantum dots (QDs) spin-coated TiO2 surface using perovskite precursor solution [21]. The PCE of PSCs had been improved further up to 9.7% by the development of mesoscopic heterojunction solar cells with the use Fig. 2 Typical crystal structure of perovskite materials, based on the data of Ref. [93]

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of CH3 NH3 PbI3 as light harvester [21]. These perovskite materials were produced by reaction between methylammonium iodide and lead iodide, and then deposited on the TiO2 surface having the thickness of micrometre. The 2,2 ,7,7 -tetrakis (N,Ndi-p-methoxyphenyl-amine)-9,9 -spirobifluorene (spiro-OMeTAD)was used as hole transporting layer (HTL). Later, Lee et al. [17] have reported comparatively low cost, solution-processable PSCs with the improved PCE up to 10.9%. This type of solar cells employed mesoporous alumina (Al2 O3 ) as a chemically inactive layer which acts as scaffold for perovskite material and also forces the electron to move within the perovskite film [17]. Liu et al. [22] have proved that the nanostructure is not an essential requirement to obtain higher performance with perovskite material. They fabricated the planar heterojunction (PHJ) solar cells with vapour deposited perovskite film and achieve PCE of 15% under one sun illumination conditions [22]. This achievement has demonstrated that the PSCs with simple structure can attain higher power conversion efficiencies, therefore avoiding pointless complications related with the use of nanostructures. The efficiencies of PSCs have been further improved by the Zhou et al. [19] group. They use yttrium (Y) doped TiO2 as electron transport layer (ETL) to boost up the carrier concentration and modified indium tin oxide (ITO), yielding an PCE of 19.3% [19]. Jeng et al. [23] developed a new method to construct inverted planar heterojunction (PHJ) PSCs. The device architecture for PHJ solar cells can be of two types depending on the p–i–n heterojunction scheme employed: (a) normal devices having poly(3,4-ethylenedioxythiophene) polystyrene sulphonate (PEDOT:PSS) acting as a hole transporting layer, perovskite as intrinsic layer, and PCBM or fullerene derived [6, 6]-phenyl-C61 -butyric acid methyl ester as an electron transporting layer [23], (b) an inverted structure with compact TiO2 as n-doped, perovskite as undoped or intrinsic and organic semiconductor as p-doped region [24]. Nie et al. [25] have employed solution-based hot casting technique to fabricate highly uniform pinhole free film of perovskite material having millimetre-sized crystalline grain, approaching PCE of 18% [25]. The power conversion efficiency has been increased further to 18.3% by growing perovskite film with large grain size on several HTLs, and decreasing recombination of carrier at grain boundaries [26]. The advantages of exploiting conventional and inverted PHJ structure are that the mesoscopic metal oxide layer can be dispensed, that leads to the fabrication process more facile [27]. Figure 1 depicted the device architecture of conventional and inverted PHJ perovskite solar cells. The electron and hole transporting layers act as an interfacial modifier, while metallic layer and TCO layer function as conducting electrode. The perovskite layer acts as light harvester and transports the charges till their arrival to the corresponding electrodes. Thus, research has been focussed on the development of techniques to prevent the recombination of charges, including interfacial engineering which is the main technique to suppress the charge recombination at electrodes and perovskite interface [28, 29].

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3 Compositional Aspect of Perovskite Material The most frequently considered perovskite materials for photovoltaics (PV) applications are CH3 NH3 PbI3 , CH3 NH3 PbI3−x Clx , CH3 NH3 PbBr3, CH3 NH3 Pb(I1−x Brx )3, HC(NH2 )2 PbI3, CH(NH2)2 Pb(I1−x Brx )3, CH3 NH3 SnI3. The properties of perovskite material can be tuned further by substituting various cations and anions at the AMX3 sites of the perovskite.

3.1 ‘A’ Site The electronic properties of perovskite material are not directly affected by the A cation. However, it has been demonstrated via simulation that the crystal structure of perovskite material is affected by the size of A cation, which ultimately changing the electronic properties of the perovskite material [30]. A three-dimensional symmetrical crystal structure can be achieved by substituting a ‘A’ site vacancy with a small cation like caesium (Cs), rubidium (Rb), methylammonium (MA) and formamidinium (FA) [31]. Methylammonium (MA): The most frequently used cation in organic-inorganic hybrid perovskite material is the methylammonium (MA). The PSCs employing MA cation in hybrid perovskite material has attained the efficiencies of more than 15%. The MA cation is most commonly used in methylammonium lead triiodide (CH3 NH3 PbI3 or MAPbI3 ) perovskite material. The MAPbI3 develops the tetragonal crystal structure rather than the cubic structure, which might be due to the very small size of MA+ cation. The resulting band gap of 1.51–1.55 eV is obtained which is greater than the optimized Shockley–Queisser limit for mono-junction device [32, 33]. The band gap was further tuned by replacing the MA+ cation with the bigger cation, which improves the light absorption throughout the spectrum. The replacement of smaller cation with the bigger one imparted the better crystal symmetry which results in smaller band gap and improved light absorption. The other cations that have been explored are ethylammonium (EA), formamidinium (FA) and caesium (Cs). The replacing MA+ cation with bigger cation “of having larger ionic radii” such as EA results in distortion of three-dimensional crystal structure. The ethyl ammonium lead triiodide (CH3 CH2 NH3 PbI3 ) develops a two-dimensional orthorhombic crystal structure having band gap of 2.2 eV. A relatively lower PCE of 2.2% was recorded for PSCs employing CH3 CH2 NH3 PbI3 as light harvester under standard illumination conditions [31]. Formamidinium (FA): FA cation has been estimated to result in a better symmetry than that of MA and therefore has been broadly investigated. The FA crystal possesses advantageous bad-gap of 1.43–1.48 eV. However, Formamidinium lead triiodide (FAPbI3 ) develops a yellow colour hexagonal one-dimensional non-perovskite polymorph at room temperature showing serious band coordination with TiO2 [31]. This

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results in lower photovoltaic performance of PSCs using FAPbI3 as light harvester [31]. It has been anticipated further that the complete removal of this non-perovskite polymorph state of FAPbI3 would increase the PCE of FA-based devices even more than that of MA-based PSCs. Caesium (Cs): The Cs-based perovskite was first explored by the Scaife et al. [32]. It was investigated that the caesium tin triiodide (CsSnI3 )-based perovskite showed highly intense photoluminescence (PL) peak for near infrared region of the solar spectrum [34]. The four different crystal structure of CsSnI3 that was present independently at room temperature was investigated by Chung et al. [35]. The small size of Cs as compared to MA results in a higher band gap of 1.73 eV for CsPbI3 over MA PbI3 (1.57 eV), more than the appropriate bang-gap of 1.4 eV [33] suitable for PV applications. Mixed A cations perovskites: Mixed A cations perovskites combine the strong characteristics of individual components. It was first reported by the Pellet et al. [29]. They modify the band gap of mixed A cation perovskite material by changing its composition. They reported the mixed A cations perovskite, i.e. methylammonium formamidinium lead triiodide (MAx FA1−x PbI3 ) with the different compositional ratio of MA to FA and ultimately achieving the PCE of 14.9% [29]. The improved PCE of these devices compared with MA-based PSCs is due to the higher light absorption by perovskite layer in red region of solar spectrum. Mei et al. realize perovskite solar cells built on 5-aminovaleric acid (5-AVAI)x MA1−x PbI3 perovskite material with reduced defects [36]. This results in improved carrier lifetime and reduced recombination as compared to the unchanged MAPbI3 . A PCE of 12.8% was reported with enhanced stability under light and ambient conditions.

3.2 ‘M’ Site The M site of organic-inorganic hybrid perovskite material is usually occupied by the metal cations of group IV A. The most commonly used ions of group IV A in +2 oxidation state are Lead (Pb), Tin (Sn) and Germanium (Ge). The most extensively used metal of group IVA is the lead metal, as it showed much better performance and stability as compared to the tin metal (Sn) [37, 38]. However, Ge has not often been explored in +2 oxidation state due to its mercurial characters in this state [39]. Due to the inert electron pair effects, the +2-oxidation state becomes less stable as we move down the group IV A, i.e. from Pb to Ge. However, reduction in band gap is observed due to increase in ionic characters moving down the group. Hypothetically, MASnX3 shows band-gap value of 1.2–1.4 eV as compared to MAPbX3 that has the band-gap value of 1.6–1.8 eV. However, the drawback of MASnX3 is the instability of Sn in +2 oxidation state. It is readily oxidized in air producing volatile SnI4 compound. MASnX3 -based PSCs showed better current density (Jsc ) than the solar devices based on MAPbX3 , yet its unstable nature has hindered its further development [38]. Mixed M cations: The band gap has been changed effectively by changing the ratio of Pb to Sn in CH3 NH3 Sn1−x Pbx I3 perovskite-based devices [31, 40]. It has been

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observed that by increasing the Sn contents up to x = 0.5, a quasi cubic tetragonal structure is formed similar to the pure MASnI3 [31]. By increasing the Sn contents further, a tetragonal structure has been observed. A tetragonal structure with increased absorptivity and decreased band gap was observed by further increasing the Sn contents [31]. A lot of development has been accomplished in dual metal hybrid perovskite devices as elaborated elsewhere [41].

3.3 ‘X’ Site Changing the halide ion is the best method to probe the performance characteristics of hybrid perovskite photovoltaics. In moving down the group VIIA, atomic size increases, absorption spectra shifted towards longer wavelength, and a decrease in energy is observed [40]. Iodide (I): The PSCs based on iodide ions are the most important one reaching PCE over 20% [23, 42]. The electronegativity value of iodine (2.66) is closer to that of lead (2.33) thus leading to the most stable perovskite structure. The bond between iodine and lead is neither covalent nor ionic rather mixed characters. Although iodide forms the most basic components of hybrid halide perovskite, its instability towards humidity is a problem. Therefore, more study is encouraged to realize iodide substitute and mixed halide perovskite. Chloride (Cl): The PSCs containing Cl-based perovskite material showed better efficiencies as incorporation of Cl in perovskite material results in longer carrier diffusion lengths and better charge carrier lifetimes [43]. MAPbCl3 exhibits the cubic structure at ambient temperature; nevertheless, blended CH3 NH3 PbI3−x Clx produced remarkably aligned crystalline arrangement [44]. Furthermore, X-ray diffraction (XRD) studies have confirmed the arrangement of CH3 NH3 PbI3−x Clx similar to MAPbI3 , having maximum 3–4% Cl to I ratio [45]. Dar et al. [46], however, showed that Cl remains unconverted into CH3 NH3 PbI3−x Clx as PbCl2 that works as nucleation vacancies leading to improved surface masking of perovskite layer [46]. It is proposed that crystallization starts with the formation of complex ion agglomerates due to the limited solubility of the precursor containing chlorine. Chlorine allows freezing of redundant organic components by restructuring of components ions amid crystal growth, that in-turn ascertain the crystal and grain structure of perovskite material. Thus, chlorine incorporation controls the morphological evolution of the film and hence plays a role in tuning material characteristics and solar cell efficiency. Bromide (Br): Bromine is effectively used to tune the band gap of the perovskite material [33]. It has been proposed by the Moscon et al. [43] that the introduction of Br ions in iodide-based perovskite materials results in increased in band gap of material as the result of crystal distortion [43]. Pure MAPbBr3 showed cubic crystal structure at room temperature [44]. The PSCs based on CH3 NH3 PbI3−x Brx showed superior efficiency and stability by changing the band gap through chemical combination [47]. Moreover, Eperon et al. established the result of changing the I:Br ratios on

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mixed halide FAPbI3−x Brx perovskite, representing an variable band gap from 1.48 to 2.23 eV with a range of film colour [33]. The highest PCE achieved was 14.2% with higher Jsc of 23 mA/cm2 . Complex ions: Lately, mixed perovskite MAPbI3−x (BF4 )x was synthesized by the Nagane et al. [48]. They replaced the X ion with the BF4 − molecular ion [48]. The MAPbI3−x (BF4 )x perovskite showed better electrical conductivities and improved response to photons under one sun illumination conditions in contrast to MAPbI3 . Because of strong hydrogen bond between halide BF4 − ion and MA ion, a lower degree of vitalization of MA ion is expected [30].

4 Structure, Phase Transformation and Electronic Characteristics of Perovskite Materials Perovskites refer to a class of materials that have the general formula of ABX3 , where A and B are the two cations with very different atomic radii and X an anion. The A cation is always occupied the cubooctahedral site of perovskite material, while B cation is always present in an octahedral site. The oxidation states of A and B cations depend mainly on the nature of X anion. If X = Oxygen, then A and B sites are commonly filled by the divalent and tetravalent cations. However, when X = halogen, then A and B sites are occupied by the monovalent and divalent cations [49]. Perovskite material is broadly studied as light harvesting material for PV devices because one can substitute a variety of elements having varied oxidation states at A and B sites of this material. The most meritorious CH3 NH3 PbI3 has CH3 NH3 + occupying A site, while Pb+2 present at B site as demonstrated in Fig. 2. The optical, electronic and structural properties of perovskite material are directly influenced by the ion arrangement and crystal constituents. The formability and stability of perovskites are determined by a dimensionless factor called Goldschmidt tolerance factor (t) given below, t=

0.707(rA + rX ) rB + rX

where r A is the radius of cation A, r B is the radius of cation B, and r X is the radius of X anion. A cubic structure is predicted for the perovskite material containing transition metal cation and oxide anion with t = 1, whereas octahedral distortion is observed when t < 1 [50]. When t < 1, the loss in symmetry is observed which in turn effects the electronic properties of perovskite material [50]. For the perovskite material, the stable crystal structure is formed when tolerance factor is 0.813 < t < 1.107 [51]. It is reported in literature that cation with ionic radii of 1.66 and 2.50 Å is suitable for the formation of stable APbX3 (X = Cl, Br, I) type perovskite material. The crystal structure of perovskite material has been confirmed by the XRD studies. The reported crystal structures for perovskite material through XRD studies are cubic (Pm3 m), tetragonal (14/mcm) and orthorhombic (Pnma) [52]. At ambient

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temperature, perovskite material possesses cubic crystal structure, which undergoes series of phase transformation with decreasing temperature [53]. Perovskite crystal structure becomes more symmetric as temperature increases: at room temperature, MAPbI3 attains tetragonal structure, whereas MAPbBr3 and MAPbCI3 acquire cubic structures [54]. Phase transition in methylammonium (MA), formamidinium (FA), lead (Pb), Tin (Sn) and perovskite has been studied by Kanatzidis et al. [31]. They pointed out the three different transition states: at high temperature α state exists, while at intermediate temperature β state and at low-temperature gamma (γ ) state exist as depicted in Table 1 [31]. It is clear from Table 1 that the FA-based perovskite materials do not undergo phase transformation even at high temperatures; this property is highly substantial to fabricate solar cells for field operation. Thus, phase transformations that occur due temperature and pressure variation are the significant factor to be considered for practical application of PSCs. In Table 2, the selective physical, electronics and spectroscopic properties of perovskite material used in PSCs are summarized. All perovskite materials have long carrier diffusion length with the almost equal exciton binding energy and thermal energy [55]. Furthermore, owing to very small exciton binding energy, both free electrical charges and infirm bound exciton coexists [56]. All perovskite materials possess direct band-gap and high absorption coefficient, i.e. 1.5 × 104 cm−1 at 550 nm [56]. The average diffusion length (LD) for MAPbCI3−X Clx has been calculated using relation LD = (Dτe )1/2 where D and τe are the coefficient of diffusion and recombination lifetime in the inexistence of a quenching components, respectively [56]. The values of D and τe were determined by the Stranks et al. by using photoluminescence quenching measurements. There reported values for MAPbCI3−X Clx are more than 1 μm (Table 2).

5 Band-gap Tuning of Perovskite Materials Hybrid perovskite materials exhibit larger absorption coefficient allowing the usage of very thin films for light absorption [57]. Perovskite material with the thickness of 500 nm absorbed the solar radiation that is equivalent to the absorption by 2 μm thickness of required by the typical silicon solar cells [30]. The perovskite material like MAPbI3 and MASnI3 has direct band gap that ranges from 1.50 to 1.55 eV as determined from the ultraviolet photoelectron and ultraviolet-visible spectrophotometer [17, 22, 28]. Nevertheless, the band gap of 1.55 eV is not enough for panchromatic absorption due to the relatively shorter absorption wavelength limited to 800 nm [28]. To increase the absorption of energy towards higher wavelength without cooperating for absorption coefficient, we can change the band gap of perovskite material. There are two possible ways to attain this: (a) replacing the MA cation with other organic cations, since replacement of MA with another cation alter the bond length and bond angle in M–X–M of ABX3 without changing the valence band maximum [58]. For example, the decreased in band gap of about 0.07 eV has been detected by replacing the MA cation with FA cation. The absorption wavelength is also increased by 40 nm

Temperature (K)

400 293 162–172

178.8 172.9–178.9 236.9 155.1–236.9 149.5–155.1 15%. This type of architecture categorizes further into two types; one is the normal with the normal configuration (n–i–p) and other with the inverted configuration (p–i–n) as depicted in (Fig. 5). The normal planar heterojunction consists of glass/TCO/ETL/perovskite/HTL/metal, while inverted heterojunction contains glass/TCO/HTL/perovskite/ETL/metal [30]. Generally, HTL/ETL acts as selective charge extraction layers [30]. The silver (Ag) and Gold (Au) are the normally acting as top electrode which collects the holes, though Ag is less commonly used as top electrode owing to its chemical sensitivity towards perovskite material. Recently, Nickel (Ni) and carbon have also been used as hole collector due to their improved stability as compared to the Ag and Au [36]. The most important architecture in PSCs is the mesoscopic structure with n–i–p configurations. Many high-efficiency solar cells implement this device configuration. The normal mesoscopic n–i–p architecture is shown in Fig. 5a. This architecture consists of FTO acting as cathode, a thin compact layer of TiO2 (thickness is about 50–70 nm) acting as ETL, a mesoporous layer of metallic oxide (normally TiO2 or Al2 O3 , 150–300 nm thickness) acting as scaffold for perovskite material, a perovskite absorber layer having thickness of about 300 nm, an HTL layer with thickness of 150–200 nm, and top electrode with the thickness of 50–100 nm (normally Au or Ag). Previously, the perovskite film used as light harvester in PSCs has the thickness of more than 500 nm. But, since the grain expansion of perovskite restricted in the structure pores, a considerable measure of the substance exists in amorphous phase [75], resultant comparatively lower open-circuit voltage (Voc ) and short-circuit current density (Jsc ) [74]. However, by decreasing the thickness of mesoporous film

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to 150–200 nm remarkably enhanced the device efficiency because of improved crystallinity of perovskite film [24]. Further reducing the thickness below 150 nm results in poor Jsc and device performance, Yang et al. reported the PCE of 20.2% with the mesoscopic structure consisting of perovskite nanocrystals in-built in the preceding ETM layer with the top uniform and compact perovskite layer [76]. The planar n–i–p architecture is demonstrated in Fig. 5b. Now highly efficient PSCs can be fabricated without a mesoporous layer by careful supervision of growth of perovskite film, the perovskite interface, carrier transport films and electrodes [19]. Zhou et al. [19] fabricated the highly efficient planar n–i–p PSCs and reported the PCE of 19.3% [19]. In the inverted architecture (p–i–n) the order in which the films are deposited is inversed in contrast to n–i–p architecture as shown in Fig. 5c. In these types of PSCs, a 50–80 nm thick HTL is deposited first on the top of TCO, later on the intrinsic perovskite layer of 300 nm thickness is deposited on the top of HTL, followed by the deposition of ETL of about 10–60 nm thickness. Finally, the whole device stack is completed by the deposition of back contact of metal (Au/Ag/Al). Dong et al. first fabricated the planar p–i–n PSCs and attained PCE of 18.9% [77]. With further progress in the p–i–n solar cells architecture, the specific contact substitutes have been extended from organic to inorganic matter [78, 79].

7 Challenges Related with the PSCs and Their Potential Solutions There are certain challenges that are related to the PSCs. The challenges related to the PSCs and their possible solutions are discussed below.

7.1 J-V Hysteresis The abnormal current-voltage hysteresis has been observed on PSCs. In J-V hysteresis, the efficiency of solar cells is anomalously dependent on the voltage scan rate and scan speed (Fig. 6a). The three possible reasons for J-V hysteresis have been suggested by the researchers [80], namely ferroelectricity (Fig. 6b) [81], ion migration (Fig. 6c) [68], [82], and unbalance charge collection rates [83] (Fig. 6b). Recently, a more consideration has been paid to the ferroelectric polarization of perovskite material as their ferroelectric properties may be critical in enhancing the power conversion efficiencies and stability [65]. Ferroelectricity influences the separation of photoexcited electrons and holes as well as recombination rate (Fig. 6b) [84]. Simulations implied that the internal electric field pertaining to microscopic polarized regions produce hysteresis in the current-voltage features of perovskite devices because of alteration in the electron-hole reunion in the material volume [85]. According to Xia.

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Fig. 6 a J−V hysteresis shown for various scan rates and scan directions [82]. b Ferroelectric behaviour of CH3 NH3 PbI3 perovskite [97]. c Schematic that depicts the effect of ion drifting in perovskite SCs [68]. d A considerable J − V hysteresis has been observed when the perovskite solar cell is cooled down to 175 K [98]. Figures are reproduced with permission from royal chemical society, ACS publications and nature publications under creative commons licence

et al. [86], perovskite material shows strong ionic characters, and in the presence of electric field it gets polarized. Furthermore, a study based on impedance revealed that higher value of dielectric constant at lower frequencies is the cause of J-V hysteresis and this mainly arises due to the dipolar, ionic and electronic contributions [87]. Snaith et al. proposed that the abnormal hysteresis may be due to the ferroelectric behaviour of the perovskite material owing to which material gets weakly polarized, capacitive effects, defects that are present on the surface of absorber [88]. Furthermore, in case of mesoporous architecture PSCs, the hysteresis strongly dependent on the crystal growth within mesoporous structure as well as thickness of mesoporous layer [88]. The J-V hysteresis is also dependent on the charge transport rate at perovskite/ETL and perovskite/HTL interface. The inverted p–i–n PSCs showed significant smaller hysteresis, likely because of uniform charge transport and passivation of perovskite/HTL or perovskite/ETL interface [81, 89] (Fig. 6d). The ferroelectric response of CH3 NH3 PbI3 has been probed by analysing the polarization electric field, though it was strongly affected by leakage current and hence cannot be treated as an incontestable evidence to the electric response [81]. Kutes et al. for the first time demonstrated the existence of ferroelectric domain in high-quality CH3 NH3 PbI3 thin films fabricated by a solution processing method.

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They reported the size of the ferroelectric domains to be approximately to the size of the grains (approximately 100 nm).

7.2 Long-Term Stability Another issue that is related to the PSCs is the long-term stability of these devices. A further progressive consideration is needed to understand the degradation mechanism, which is eventually necessary for the outdoor applications of PSCs. The reactivity of perovskite films towards water has been confirmed by the standard negative Gibbs Free energy for CH3 NH3 PbI3 degradation with humidity [90]. CH3 NH3 PbI3 perovskite undergoes hydrolysis in the presence of moisture due to its hygroscopic nature. The decomposition takes place through the following paths as given below; CH3 NH3 PbI2(s) → CH3 NH3 I(aq) + PbI2(s)

(1)

CH3 NH3 I(aq) → CH3 NH2(aq) + HI(aq)

(2)

4HI(aq) + O2(g) → 2I2(s) + 2H2 O(I)

(3)

2HI(s) → I2(s) + H2(g)

(4)

The reaction (Eq. 1) shows that CH3 NH3 + is separated from the structure of perovskite and compound with the iodine to form aqueous CH3 NH3 Iand solid PbI2 salt. The CH3 NH3 I further decompose into aqueous methylamine and hydroiodic acid (Eq. 2). Furthermore, hydrogen iodide (HI) can undergo further decomposition to give water and I2 through a redox reaction due to the presence of oxygen (Eq. 3). The HI also undergoes photochemical reaction in the presence of UV light to form I2 and H2 gas. The perovskite degradation takes place even at lowest humidity level of 55%, and film colour change from dark black to yellow is an indicator of this phenomenon [47]. Snaith et al. [91] fabricated the PSCs with enhanced thermal and moisture stability by replacing the organic HTL with single-walled carbon nanotubes (SWCNTs). The SWCNTs was modified with the insulating polymeric matrix [91]. The PSCs using this HTL showed improved thermal stability and performance as compared to the PSCs based on organic HTL. Mei et al. [60] reported the PSCs without using HTL and contain double film of mesoporous TiO2 /ZrO2 scaffold. The contact layer of 10micron thickness was a critical player in improving the resistance to water and the cell registered a certified PCE with stability over 1000 h in full sunlight and ambient air conditions.

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8 Conclusions This chapter has demonstrated the complete summary of gradual progress and limits in the field of PSCs. This chapter also summarized the basics of perovskite materials with a focus to explain their widespread applications in photovoltaics solar devices. We have also discussed the recent progress on the compositional engineering and interface features of perovskite layer and electric contacts, as these properties are critical to improve the power conversion efficiencies of PSCs. Such progressive bulk and interface engineered architecture will be favourable for increasing perovskite grain size and crystallinity, removing surface and bulk defects, refining film morphology and enhancing film uniformity. Interface engineering improved charge carrier dynamics across solar cell junction, charge extraction and transportation, and thus helps to enhance the device performance. Furthermore, the perovskite film deposition techniques such as solution processing, vapour deposition and vapour assisted solution processing have been discussed in detail. The perovskite film quality, crystallinity and uniformity are strongly dependent on the deposition method employed. The perovskite film morphology has been controlled by solvent engineering, hence been an important player in improving the PCE of PSCs. Moreover, the band-gap tuning of perovskite material has been discussed in detail. Finally, the problems that are associated with the perovskite solar cells such as J-V hysteresis have been demonstrated. These are the main factors that are preventing the further advancement of PSCs. The next major issue which has been identified is the long-term thermal and chemical stability of perovskite SCs, which requires to be focussed urgently to launch them in the market for outdoor photovoltaic applications.

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83. Bergmann VW, Weber SA, Ramos FJ, Nazeeruddin MK, Grätzel M, Li D, Domanski AL, Lieberwirth I, Ahmad S, Berger R (2014) Nat Commun 5:5001 84. Liu S, Zheng F, Koocher NZ, Takenaka H, Wang F, Rappe AM (2015) J Phys Chem lett 6(4):693–699 85. Kim BJ, Kim DH, Lee Y-Y, Shin H-W, Han GS, Hong JS, Mahmood K, Ahn TK, Joo Y-C, Hong KS (2015) Energy Environ Sci 8(3):916–921 86. Xia WS, Li LX, Ning PF, Liao QW (2012) J Am Ceram Soc 95(8):2587–2592 87. Dualeh A, Moehl T, Tétreault N, Teuscher J, Gao P, Nazeeruddin MK, Grätzel M (2014) ACS Nano 8(4):4053 88. Snaith HJ, Abate A, Ball JM, Eperon GE, Leijtens T, Noel NK, Stranks SD, Wang JT-W, Wojciechowski K, Zhang W (2014) J Phys Chem lett 5(9):1511–1515 89. Kim H-S, Park N-G (2014) J Phys Chem lett 5(17):2927–2934 90. Niu G, Li W, Meng F, Wang L, Dong H, Qiu Y (2014) J Mater Chem A 2(3):705–710 91. Habisreutinger SN, Leijtens T, Eperon GE, Stranks SD, Nicholas RJ, Snaith HJ (2014) Nano Lett 14(10):5561–5568 92. Krishna A, Grimsdale AC (2017) J Mater Chem A 5(32):16446–16466 93. Sutton RJ, Filip MR, Haghighirad AA, Sakai N, Wenger B, Giustino F, Snaith HJ (2018) ACS Energy Lett 3(8):1787–1794 94. Jung HS, Park NG (2015) Small 11(1):10–25 95. Im J-H, Kim H-S, Park N-G (2014) APL Mater 2(8):081510 96. Malinkiewicz O, Roldán-Carmona C, Soriano A, Bandiello E, Camacho L, Nazeeruddin MK, Bolink HJ (2014) Adv Energy Mater 4(15):1400345 97. Fan Z, Xiao J, Sun K, Chen L, Hu Y, Ouyang J, Ong KP, Zeng K, Wang J (2015) J Phys Chem lett 6(7):1155–1161 98. Bryant D, Wheeler S, O’Regan BC, Watson T, Barnes PR, Worsley D, Durrant J (2015) J Phys Chem Lett 6(16):3190–3194 99. Poglitsch A, Weber D (1987) J Chem Phys 87(11):6373–6378 100. Hirasawa M, Ishihara T, Goto T, Uchida K, Miura N (1994) Phys B 201:427–430 101. Tanaka K, Takahashi T, Ban T, Kondo T, Uchida K, Miura N (2003) Solid State Commun 127(9–10):619–623 102. Abrusci A, Stranks SD, Docampo P, Yip H-L, Jen AK-Y, Snaith HJ (2013) Nano Lett 13(7):3124–3128

Effect of Oxygen Vacancies in Electron Transport Layer for Perovskite Solar Cells Mohamad Firdaus Mohamad Noh, Nurul Affiqah Arzaee and Mohd Asri Mat Teridi

Abstract Metal oxide-based electron transport layer (ETL) is one of the vital components in conventional n-i-p type perovskite solar cells (PSC) that enables efficient electron extraction and transport within the device. Nonetheless, native point defect associated with oxygen vacancies that naturally exist in the ETL materials such as TiO2 , SnO2 and Nb2 O5 could deteriorate the overall performance of the PSC. In this chapter, the intentional and unintentional formation of oxygen vacancies during the fabrication process of the ETL will be firstly clarified. The properties of oxygen vacancies in the viewpoint of structural, optical and electrical as well as surface wettability will also be profoundly elaborated to provide valuable insight on the impact of oxygen vacancies in the ETL towards efficiency, hysteresis and stability of PSC devices. Keywords Perovskite solar cells · Electron transport layer · Metal oxide · Point defects · Oxygen vacancy

1 Introduction Perovskite solar cells (PSC) have appeared as a shining star among the third generation solar cells owing to their excellent solar-to-power conversion efficiency (PCE) of beyond 20% [1–3]. Now, the development of PSC has entered a new phase which is aiming for vast fabrication for future commercialization but there are still several issues related to long-term stability and hysteresis behaviour of the device that require some degree of attention [4]. Many factors have been acknowledged as the major causes for the instability of PSC and among those, existence of humidity in ambient environment is the most crucial one [5, 6]. It is known that PSC is made up of several layers with electron transport layer (ETL) being one of the most essential components for boosting the overall performance of the device [7]. The ETL is necessary for enabling efficient transfer of the M. F. M. Noh · N. A. Arzaee · M. A. M. Teridi (B) Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia e-mail: [email protected] © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_11

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photoexcited electrons from the perovskite layer and to block the transition of holes from the perovskite to the FTO substrate. This will prevent charge recombination in the device that could deteriorate the short-circuit current density (JSC ) as well as open-circuit voltage (VOC ). The ETL could also accelerate the transport of extracted charges to external circuit via FTO. To date, the ETL is built based on inorganic or organic material where the former is usually implemented in conventional structured devices, while the latter is widely used in inverted structured devices. For devices having conventional architecture, the ETL used is typically made up of wide band gap metal oxide such as TiO2 [8], SnO2 [9], and ZnO [10]. All of these metal oxides are chosen as ETL for PSC not only due to their electrical properties including excellent charge transfer and transport ability, but also attributed to the compatible conduction band and valence band with respect to perovskite as well as outstanding thermal, chemical and photochemical stability. Perfectly bonded atoms in metal oxides (i.e. metal oxides without defects) render the material incapable to conduct electrical charges [11]. Hence, nonstoichiometric defects related to oxygen vacancies are important characteristics for metal oxides that govern a range of their properties including electrical conductivity, diffusion kinetics and electronic structure [12, 13]. Together with other point defects such as cation interstitial, cation vacancies and oxygen vacancies are the most common defects that occurs in the lattice of nonstoichiometric undoped metal oxides. In relative to other defect species, oxygen vacancies are well-known as the predominant one that play pivotal role in determining the properties of metal oxides [14]. Figure 1a and b schematically illustrate the formation of oxygen defects on the surface of TiO2 during

Fig. 1 Schematic diagram of oxygen vacancies. a Formation of doubly ionized oxygen vacancy on the surface. Reproduced with permission from [15]. Copyright 2011, with permission from American Chemical Society. b Oxygen vacancies in the crystal lattice of TiO2

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the interaction with oxygen as well as the oxygen defects in the lattice. The removal of an oxygen ion from the surface creates oxygen vacancy which is fully ionized at high temperatures. Meanwhile, the removal of one neutral oxygen atom from the crystal lattice leads to the formation of one point defect and two under-coordinated Ti3+ [15, 16]. Nevertheless, extremely high concentration of oxygen vacancies in the ETL of PSC could result in poor device efficiency. To make things worse, oxygen vacancies have been found as another factor that may accelerate the degradation process of perovskite layer and adversely affect the hysteresis behaviour of the PSC [17]. Even though investigation on this topic is not substantial up to now, the issue should not be overlooked as it is still seen as one of the vital aspect that requires urgent attention to allow mass production of highly efficient, stable and hysteresis-free PSC device in near future. Therefore, in this chapter, the formation and properties of oxygen vacancies in metal oxide-based ETL as well as their impact towards the performance of PSC are clarified.

2 Formation of Oxygen Vacancies During Fabrication During the fabrication process of metal oxide-based ETL, oxygen vacancies may be formed either intentionally or unintentionally. Parameters that may lead to the formation or elimination of oxygen vacancies in pristine metal oxides include the quantity of oxygen that is available during the fabrication, precursor annealing, fabrication method adopted and introduction of dopant. Meticulous control on these fabrication parameters could modify the concentration of oxygen defect generated. Other strategies in manipulating the defect density have also been reported for other solar energy-based technologies such as photocatalysts [18, 19] and dye-sensitized solar cells [20, 21] but this chapter only focuses on the development that has been conducted for PSC devices.

2.1 Oxygen Quantity The presence or more specifically quantity of oxygen is the most critical aspect in determining the amount of oxygen vacancies in the fabricated metal oxide thin films. Intentionally tuning the quantity of oxygen present in environment during fabrication is the easiest approach to regulate the density of oxygen deficiency in the film. This includes annealing the metal precursor at different environmental condition or in contrasting oxygen pressure, applying plasma treatment and adjusting the oxygen flux during direct current magnetron sputtering (DCMS) or radio frequency magnetron sputtering (RFMS) process. Influence of annealing will be elaborated further in next section. From the viewpoint of plasma treatment, Zhang et al. [22] demonstrated that applying O2 plasma treatment towards pre-annealed compact TiO2 layer would

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decrease the intrinsic point defects, i.e. oxygen vacancies particularly at the surface of TiO2 . On the other hand, inverse phenomenon was observed upon treating the sample by Ar plasma where oxygen vacancies in TiO2 were remarkably amplified. Meanwhile, increasing the value of oxygen flux in DCMS or RFMS process would also reduce the oxygen deficiency in compact metal oxide thin film providing that other deposition parameters, such as sputtering time, total pressure, etc., are kept constant [23, 24]. Here, it must be noted that oxygen flux even in small amount is required during the sputtering process if pure metal is used as target. However, pure Ar environment may be applied only if the target is already in the form of oxide metal.

2.2 Annealing Indeed, formation of metal oxide is thermodynamically favourable since the enthalpy of formation of most metal oxides is typically lower in comparison to the metalbased precursor like metal chloride, metal nitride, etc. Yet, annealing process is a crucial step in the fabrication of both compact and mesoporous metal oxide layers as energy is required for activating the breaking and formation of the bonds during the synthesis process. In addition, most metal oxides require high temperature thermal treatment in order to function well as ETL. For instance, TiO2 must be annealed at temperature range of 450–500 °C to allow nanocrystallite formation [25]. The reason lies in the fact that TiO2 shows excellent electrical conductivity only after it has turned into nanocrystallites [7]. Some metal oxides, such as SnO2 and ZnO indicate higher performance after annealing at temperature of less than 200 °C as a result of their distinguished electron recombination resistance [26, 27]. Therefore, it can be generalized that annealing process is a vital procedure for enhancing the electrical properties of the ETL which eventually affects the overall performance of PSC device. It is worth mentioning that the thermal treatment may also influence the crystallinity, morphology, electrical and optical properties of the metal oxide layer which should be taken into consideration during the fabrication of highly efficient device [7, 28]. Very low or very high temperature applied during processing of metal oxide may induce the generation of oxygen vacancy defects in the crystal lattice or on the surface of the film [17, 29, 30]. Under low-temperature operation, the energy supplied is usually insufficient to fully convert the metal-based precursor to metal oxide especially when precursor based on metal with lower oxidation state is employed for the synthesis. The low-temperature treatment subsequently slows down the rate of oxygen vacancies annihilation. For instance, the use of SnCl2 instead of SnCl4 as precursor easily creates non-oxidized Sn2+ in the low-temperature-synthesized SnO2 [29]. This is due to the fact that Sn and Cl atoms in SnCl2 are held together by ionic bonds and hence, it is hypothesized that the forces that held the atoms together are relatively strong [31]. Therefore, low-temperature processing would limit the heat energy needed to break the ionic bonds and impede successful reaction between Sn and O atoms. This concept is also applicable for other metal oxides. On the other

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hand, exposing the metal oxide to elevated temperatures intensifies the vacancy concentration almost linearly. As reported by Wendt et al. [32], oxygen vacancies in TiO2 increase upon annealing the material up to approximately 676 °C. This could be attributed to slight decomposition of metal oxide during thermal treatment which leads to oxygen vacancies formation. Palumbo et al. [33] demonstrated that annealing of TiO2 at temperature up to 500 °C generates oxygen vacancies which are reducible through decreasing the processing temperature to 400 °C. Applying lower temperature treatment, however, has adverse effect in terms of organic residue on the surface and crystallinity of TiO2 film. The condition applied during annealing of metal oxide could also alter the density of oxygen vacancy sites in the film. For example, it was shown that annealing of TiO2 in ambient air has greater potential to promote oxygen vacancies formation [34]. This could be attributed to the relatively low percentage of oxygen in environment which reduces the probability of successful collision and reaction between oxygen molecules and metal precursors. Although the oxidation of TiO2 begins from the surface and gradually penetrates into the bulk of the film, the deficiency of oxygen still appears throughout the film (i.e. both at the surface and in the bulk). This defect, however, could be alleviated by either introducing oxygen environment annealing or prolonging the annealing duration [32, 34]. As depicted by XPS spectra in Fig. 2, shoulder peak centred at 457.3 eV which corresponds to Ti4+ bound to an oxygen vacancy (Ti4+ –VO ) declines as annealing of TiO2 film was carried out in oxygen environment instead of ambient air. Further reduction was detected upon extending the duration of annealing process which verifies the impact of environmental condition towards the generation of oxygen vacancies.

Fig. 2 X-ray photoelectron spectroscopy (XPS) analysis of Ti 2p3/2 peak in TiO2 film annealed in air and O2 environment for 0.5 and 2 h. Main peak centred at 458.8 eV (light blue) and shoulder peak 4+ centred at 457.3 eV (red) correspond to Ti4+ (2p/3) state and Ti bound to an oxygen vacancy, respectively. Reproduced with permission from [34]. Copyright 2017, with permission from American Chemical Society

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2.3 Fabrication Method The choice of fabrication technique may also unintentionally manipulate the type and amount of defect states such as oxygen vacancies and perhaps low-oxidation state metal ions in the as-prepared metal oxide-based ETL. For instance, Noh et al. [17] discovered that aerosol-assisted chemical vapour deposition (AACVD) which represents vapour-based deposition method could eliminate the oxygen vacancies in SnO2 thin film. On the contrary, high amount of oxygen vacancies appears upon employing solution-based deposition method which is represented by spin coating technique in the same study. Such phenomenon could be related to the film formation mechanism induced by the fabrication environments. In the study conducted by Noh et al. [17], SnO2 thin film was fabricated at low temperature rather than high temperature as most PSC devices have been verified to perform efficiently using low-temperature-treated SnO2 as ETL [35]. As described in the previous section, low-temperature processing could induce oxygen vacancies generation due to the inadequate energy supply for complete metal oxidation. Nonetheless, for deposition using AACVD technique, the reaction between Sn-based precursor and oxygen (O2 ) occurred readily on the surface of the substrate. Since O2 gas is supplied excessively during the deposition process, complete reaction between Sn-based precursor and O2 is able to take place resulting in the formation of SnO2 . It should be emphasized that O2 molecules react right after the chemisorption of Sn-based precursor which lead to the growth of SnO2 layer and inhibit the formation of oxygen vacancies in the film. Conversely, oxygen vacancies may be created upon utilizing spin coating technique especially during annealing of the spin-coated precursor solution. The mechanism for oxygen vacancies generation can be described as follows. During annealing step, solvent and by-products start to evaporate and leave Sn-based intermediates on the surface of the substrate. As the sample is continuously heated, the Sn-based intermediates are converted to SnO and then, the SnO particles are oxidized by O2 molecules in environment which eventually produces SnO2 film. Since O2 molecules are able to infiltrate only from the surface of the layer which is directly exposed to air, this may cause incomplete oxidation of SnO to SnO2 particularly at the region near to FTO. Consequently, high amount of oxygen vacancies is produced. The schematic diagram in Fig. 3 depicts the effect of fabrication methods towards oxygen defects formation in SnO2 films. Therefore, it could be generalized that solution-based deposition techniques that comprise of annealing step similar or almost similar to spin coating method such as dip coating, spray pyrolysis, etc., may lead to the formation of oxygen vacancies in the oxide-based thin film.

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Fig. 3 Schematic diagram of a fabrication procedure and b film formation in spin-coated-SnO2 thin film which generates oxygen vacancies. c Fabrication procedure and d film formation in AACVD-SnO2 thin film which annihilate oxygen vacancies. Reproduced with permission from [17]. Copyright 2018, with permission from Elsevier

2.4 Doping Doping is an effective approach in tuning the concentration of oxygen vacancies. For the development of PSC device, a series of materials ranging from metal cations (from monovalent to pentavalent) to non-metal anions such as Li+ , Co2+ , Fe3+ , Nb5+ , Ta5+ and F− , to name but a few, have been successfully adopted as dopant in the host metal oxide-based ETL [36–42]. Successful doping could only take place if the dopant is capable to diffuse as interstitial ion through the crystal lattice of host material during the fabrication process [16]. In principle, the density of oxygen defects typically shows decreasing trend initially up to a certain degree and followed by an increment as the doping concentration increases. Here, it is worth mentioning that extremely high amount of doping concentration would lead to formation of metal oxides nanocomposite. Appropriate quantity of dopant could reduce the oxygen deficiencies to a desired amount which would be beneficial for the application in PSC. However, the optimal percentage of dopant introduced depends on the oxidation state of the dopant and density of defect in the ETL. For instance, Giordano et al. [36] and Liu et al. [43] introduced Li+ ions into TiO2 crystal lattice by thermal diffusion and noticed remarkable depletion of oxygen vacancies which they attributed this outcome to the partial reduction of Ti4+ to Ti3+ induced by Li+ . In other words, the presence of Li+

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Fig. 4 Elimination of oxygen vacancies in TiO2 crystal lattice by introduction of Fe3+ dopant. Reproduced with permission from [38]. Copyright 2017, with permission from Royal Society of Chemistry

stabilized the Ti4+ bound to oxygen vacancy (Ti4+ –VO ) to Ti3+ through the removal of the dangling bonds. Gu et al. [38] incorporated 1 mol% of Fe3+ as dopant into TiO2 and found that two Fe3+ ions can replace two adjacent Ti4+ –VO as displayed in Fig. 4. Comparable result was also observed by Pathak et al. [16] in which case they developed Al3+ -doped TiO2 using 0.3 mol% of Al3+ as ETL. Substitution of one Ti4+ by Al3+ is in fact energetically unfavourable but it is believed that Al3+ tend to replace two Ti4+ accompanied by a release of the bridging oxygen. Therefore, in TiO2 crystal containing defect sites, substitution of Al3+ cations with Ti3+ and oxygen vacancies is claimed to be thermodynamically and electronically favourable [44]. Since both Fe3+ and Al3+ dopants are trivalent cations, the substitution of Ti4+ –VO causes the dopant to form bond with only three adjacent oxygen atoms and therefore removes the oxygen defect sites from the crystal lattice. Most researches described here employed very low amount of dopant (less than or equal to 1%) in the host metal oxides because such quantity could assist in reducing the oxygen deficiencies which is highly advantageous for the performance of PSC device. The use of high percentage of dopant, however, would foster the generation of free charge carriers owing to the formation of new oxygen vacancy sites. This argument has been proven by Wu et al. [45] where they demonstrated that inclusion of 10% Fe as dopant in anatase TiO2 nanoparticles lead to substitution of Fe3+ for Ti4+ which deliberately creates oxygen vacancies. Such defect is favourable for boosting the photocatalytic activity of TiO2 but is actually detrimental towards PSC performance. This topic will be further clarified in the subsequent section of this chapter. It should also be noted that reduction of oxygen vacancies is not the only critical factor that determines the optimal performance of the ETL, but also morphology, electrical properties and electronic structure which could also be affected by doping must be taken into account [46–48]. Therefore, the quantity of dopant should be optimized by considering every single aspect to balance the entire properties of the ETL to realize

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maximum cell performance. The impact of doping towards the overall properties of metal oxides, however, would not be elaborated further in this chapter.

3 Properties of Metal Oxide with Oxygen Vacancies Loss of oxygen in the metal oxide crystal may have decent effect on the structural properties and surface wettability of the film. However, tuning the properties of ETL is of great importance in order to achieve highly efficient and stable PSCs because the existence or specifically concentration of oxygen vacancies in the metal oxides is majorly responsible for determining the optical and electrical properties of the ETL.

3.1 Structural Properties Oxygen defect usually has a negligible influence towards the crystal phase of a particular metal oxide. However, metal oxide film that consists of greater concentration of oxygen vacancy sites would typically have larger lattice constant and vice versa. As stated by Du et al. [23], the lattice constant of anatase TiO2 film increases as higher amount of oxygen vacancies exist in the film. This characteristic manifested itself in the shifting of X-ray diffraction (XRD) peak towards a lower diffraction angle particularly at Bragg angle of 25.37° which is assigned to the (101) plane (Fig. 5).

Fig. 5 a Wide and b high-resolution XRD spectra of TiO2 film prepared via sputtering technique with different oxygen flux. Lower oxygen flux represents higher density of oxygen vacancies. Reproduced with permission from [23]. Copyright 2017, with permission from Royal Society of Chemistry

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It is widely recognized that oxygen binds the cations in TiO2 film and hence, the cation–cation overlap reduced as a consequence of underdeveloped oxygen content which subsequently results in lattice constant enlargement. The occurrence of lattice distortion in metal oxide could also be ascribed to the difference in ionic radius between two states of metal cation. For instance, the ionic radius of Ti4+ is 0.61 Å which is slightly lower than that ionic radius of Ti3+ which has a value of 0.64 Å [16]. Another example is Sn4+ and Sn2+ which have ionic radius of 0.69 Å and 0.62 Å, respectively [29, 49]. Oxygen vacancy defects also have slight impact towards the morphological properties of the film, such as surface roughness and layer thickness. For example, the thickness and roughness of TiO2 thin film fabricated using DCMS technique exhibited increasing trend up to a certain maximum value and then decreasing again with increasing density of oxygen defect. The cause of such phenomenon is still vague, but it is postulated that greater oxygen vacancies accelerate the growth rate of TiO2 layer [23].

3.2 Wettability In the fabrication of PSCs, wettability is actually an important characteristic for the ETL as it can influence the resistance of the sample towards humidity-induced degradation. In other words, wettability of metal oxide-based ETL could affect the long-term stability of the device especially during exposure to ambient air. However, most researches nowadays usually do not consider wettability as a critical issue since the fabrication process is mostly conducted in glove box with ultra-low humidity level and the completed devices are properly encapsulated [50–54]. Yet, for realising commercializable perovskite devices with high stability which can be fabricated in high-humidity condition, wettability of ETL must be taken into consideration. Less wettable (i.e. less hydrophilic or more hydrophobic) ETL is highly desirable for PSC devices as it would improve the resistance of the sample towards humidity and ultimately enhance the long-term stability of PSC. Wettability of a surface could be influenced by the surface roughness, crsytallinity and presence of oxygen vacancies [55–61]. It has been reported that greater amount of oxygen vacancies would increase the hygroscopic nature of particular metal oxide layer which in turn reduces the value of water contact angle. As unveiled in Fig. 6, SnO2 film with low- and highdefect sites have water contact angle of 93.9° and 63.3°, respectively. The presence of oxygen vacancy encourages dissociative adsorption of water at the vacancy sites. This is due to the fact that the single oxygen atom of water heals the vacancy, while the other two hydrogen atoms bind to the oxygen atoms on the surface of metal oxide to form hydroxyl groups [55, 56]. Since the hydroxyl groups that were created on the film surface are polar molecules, they are able to form oxygen bonds with the subsequent water molecules which consequently promotes the surface hydrophilicity of metal oxide [17].

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Fig. 6 Water contact angle measurement of SnO2 surface with a low amount and b high amount of oxygen vacancies. Reproduced with permission from [17]. Copyright 2018, with permission from Elsevier

3.3 Optical Properties Optical properties are another pivotal feature for metal oxide ETL as it affects the absorption ability of the subsequent perovskite layer which in turn determine the photogeneration rate of charge carriers. To achieve maximum device efficiency, metal oxide ETL must possess high light transmittance (i.e. wide band gap) and compatible electronic configuration (i.e. lower conduction band minimum (CBM) and valence band maximum (VBM) with respect to perovskite light harvester) [7, 62–64]. These properties are in part determined by the defect sites in the film and thus, meticulous control on the capacity of oxygen vacancies in metal oxide is necessary to acquire the desirable optical properties [65]. As displayed in Fig. 7, the colour of metal oxides such as TiO2 and SnO2 becomes darker as the amount of oxygen vacancies increases implying that the optical properties including light absorption, transmittance and reflectance are affected by the defects. In another report by Guo et al. [66], they detected that sputter-deposited Nb2 O5 layer possessing high oxygen vacancies

Fig. 7 a TiO2 thin films with different density of oxygen vacancies (from left to right: low to high oxygen vacancies). Reproduced with permission from [22]. Copyright 2016, with permission from American Chemical Society b SnO2 powders with different density of oxygen vacancies (left: commercial SnO2 with low oxygen vacancies; right: SnO2 with high oxygen vacancies or Sn3 O4 ). Reproduced with permission from [29]. Copyright 2017, with permission from Elsevier

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exhibits a relatively low light transmittance of less than 70% in comparison to sample with low oxygen defect which achieved greater transparency of above 80%. High percentage of defect sites (i.e. oxygen vacancies) on the surface and in crystal lattice of metal oxide results in band gap narrowing [67, 68]. This causes greater light absorption by the metal oxide which is observable from the red-shift in absorbance spectra and reduction in light transmittance ability of the thin film (Fig. 8a) [17, 34]. Moreover, metal oxide with high oxygen defects apparently implies extended absorption tail into visible light region which is basically originated from the electron transitions associated with surface oxygen vacancies-induced electronic states near the band edge (Fig. 8b). In other words, oxygen vacancies introduced new energy level in the band gap near the band edge of metal oxide which enable photons with lower energy to excite electrons trapped in localized states within the band gap [69]. The amount of low energy photon that can be absorbed increases with the increasing concentration of oxygen vacancies [17]. Strong absorption in the visible region renders the metal oxides darker in colour. Low oxygen vacancies in metal oxide layer are, therefore, required to widen the optical band gap in order to allow more solar light passing through the ETL and being absorbed by the perovskite layer to generate larger photocurrent [34].

Fig. 8 a Light transmittance spectra (inset is Tauc Plot for band gap determination) of TiO2 film annealed in air and O2 environment for different duration to regulate the concentration of oxygen vacancies. Amount of oxygen vacancies follows the ascending order: O2 2 h < O2 0.5 h < Air 0.5 h. Reproduced with permission from [34]. Copyright 2017, with permission from American Chemical Society b Absorbance spectra (inset is Tauc Plot for band gap determination) of SnO2 film fabricated using AACVD and spin coating. AACVD and spin coating represent low and high oxygen vacancies, respectively. Reproduced with permission from [17]. Copyright 2018, with permission from Elsevier

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3.4 Electrical Properties Oxygen vacancies are advantageous towards the electrical conductivity of metal oxide because the defect band assists in improving the conductivity by declining the material band gap [70, 71]. This means that the resistivity of metal oxide declines with increasing number of oxygen vacancies and hence, the electron mobility in the bulk of ETL also improves [22, 72]. Despite the ETL having superior electron transport ability, the presence of oxygen defect sites at the interface between metal oxide-based ETL and perovskite light active layer severely deteriorates the charge extraction process. Consequently, the electrons accumulated near the interface at perovskite side and recombination of electron–hole pairs surges, plummeting the device performance. The argument is supported by electrochemical impedance spectroscopy (EIS) analysed by Du et al. as unveiled in Nyquist plot of Fig. 9a [23]. In this analysis, the size of semicircle at high-frequency region and another incomplete semicircle at low-frequency section corresponds to the electron transfer resistance and the charge recombination at the ETL/perovskite interface, respectively [73, 74]. Apparently, samples with lower concentration of oxygen vacancies indicate smaller electron transfer resistance and enhanced recombination resistance, giving rise to charge carrier extraction and transport. Performance enhancement also manifested itself in significant improvement of electron lifetime upon lowering the oxygen deficiencies in the ETL as displayed in time-resolve photoluminescence spectra (Fig. 9b) [75].

Fig. 9 a Nyquist plot of EIS spectra for PSC based on sputtered compact TiO2 ETL using different oxygen flux (higher oxygen flux represents lower oxygen vacancies). Reproduced with permission from [23]. Copyright 2017, with permission from Royal Society of Chemistry. b Time-resolve photoluminescence spectra of perovskite/ETL fabricated at different oxygen pressure (higher oxygen pressure represents lower oxygen vacancies). Reproduced with permission from [22]. Copyright 2016, with permission from American Chemical Society

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4 Effect of Oxygen Vacancies Towards Device Performance The ongoing struggle in the development of PSC has one main target, i.e. attaining highly efficient and stable devices at low fabrication cost for rapid commercialization. Voluminous brilliant approaches have been proposed to reach that goal and controlling oxygen vacancies would become another critical topic that can serve the purpose. Since oxygen vacancies show significant impact on the properties of the metal oxide, its presence in the ETL indisputably influences the overall efficiency, hysteresis behaviour and long-term stability of the developed device which will be elucidated profoundly in the following section.

4.1 Efficiency Considerable strategies that are focusing on enhancing the efficiency of PSC have been put in place including properly regulating the magnitude of oxygen vacancies in metal oxide-based ETL [76–79]. Many studies have provided concrete evidences that high oxygen deficiencies in ETL are detrimental towards the overall performance of PSC. Eliminating or markedly minimizing the oxygen vacancies could improve the short-circuit current density (JSC ), open-circuit voltage (VOC ) and fill factor (FF) of the devices, ultimately ameliorating the solar-to-power conversion efficiency (PCE). Depending on the fabrication procedures adopted and type of PSC device developed, the relative enhancement of device performance could range from 0% to 15, 22, 24 and 40% for JSC , VOC , FF and PCE, respectively [34, 40, 72]. All of these increments are undoubtedly attributed to the properties of ETL which change with the intensity of oxygen vacancies. An example of device performance with respect to amount of oxygen vacancies in TiO2 ETL is observable in Fig. 10a where in this case, the efficiency of PSC measured through reverse scan mode improved from 12.51 to 15.40% upon reducing the capacity of oxygen vacancies in the sputter-fabricated compact TiO2 ETL [23]. As described previously, electrical properties of ETL are one of the paramount aspects that allow the enhancement of PSC performance. In principle, ETL facilitates the collection of generated electrons from perovskite layer by allowing the injection of electrons to its conduction band and independently transferred them to the FTO. However, since the oxygen vacancies that are located close to the ETL/perovskite interface inhibit efficient injection of electrons, it encourages the charge recombination particularly non-radiative recombination at the interface which diminishes the VOC value. Sluggish electron transfer and high charge recombination processes at the interface also lead to an obvious reduction of the amount of extracted charges to the external circuit, leading to a lower JSC [22]. In addition, ETL containing high oxygen deficiencies also possesses higher free electron concentration which could easily recombine with holes in perovskite layer.

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Fig. 10 a J–V diagram and b IPCE spectra of planar perovskite solar cells based on sputtered TiO2 compact layer as the function of oxygen flux. High oxygen flux corresponds to low oxygen vacancies. Reproduced with permission from [23]. Copyright 2017, with permission from Royal Society of Chemistry

Oxygen vacancies also narrow the optical band gap of the ETL, resulting in greater generation of electrons in the ETL upon light absorption. Similarly, these free electrons are unwanted as they would further magnify the charge recombination rate. Besides, smaller band gap tends to increase the opacity of ETL, obstructing the light to reach perovskite layer and in the end, reduces the photocurrent generation owing to less photoexcitation of electron–hole pairs [26, 80]. As exhibited in Fig. 10b, incident photon-to-current efficiency (IPCE) spectra of PSC consisting high vacancy sites declines in comparison to that device with low oxygen defects. On top of that, the contrasting wettability of ETL induced by oxygen defects could also influence the growth of perovskite grains. Less wettable ETL (more hydrophobic layer due to less oxygen vacancies) is preferable as it has lower surface tension dragging force which increases grain boundary mobility and fosters the formation of larger and well-defined perovskite grains [81]. The improved morphology of perovskite hinders charge recombination at grain boundaries and gives a boost to the PCE of solar cell. Therefore, diminishing oxygen vacancies is certainly beneficial for improving the efficiency of PSC device.

4.2 Hysteresis Hysteresis effect in power generation that accompanies the J–V measurement of PSC device is one of the serious issues that needs further scrutiny, since the reported device efficiency along with current hysteresis may cast doubts on the reliability of this technology as the measured values may probably not represent the exact device performance [54]. In contrast to inverted structured (p-i-n) PSCs, the conventional type (n-i-p) PSCs usually suffers from stronger hysteresis behaviour due to the scan speed and scan direction (forward bias or reverse bias). In most cases, conventional

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Fig. 11 a Reduction of device hysteresis upon eliminating oxygen vacancies in TiO2 through Li doping (black = undoped-TiO2 , red = Li doped-TiO2 ) Reproduced with permission from [36]. Copyright 2016, with permission from Springer Nature b Schematic illustration of oxygen vacancies migration in the ETL during J–V measurement. Reproduced with permission from [22]. Copyright 2016, with permission from American Chemical Society

PSCs would exhibit better device performance upon applying reverse scanning mode (i.e. from VOC to JSC ) in relative to forward-scanning mode (i.e. from JSC to VOC ). This hysteresis effect stemmed from several factors where the presence of oxygen vacancies in ETL belongs to that group. In general, PSCs with high density of oxygen vacancies exhibit greater hysteresis effect as shown in Fig. 11a. The impact of interfacial oxygen vacancies towards device hysteresis has been thoroughly investigated by Zhang et al. through experiments and simulations [22]. From transient absorption measurement and first-principles calculations, it was revealed that oxygen vacancies could migrate within the oxide-based electrode under electrical field during the voltage scans which lead to different photovoltaic performance as the voltage is swept in different directions (Fig. 11b). The direction of migration of oxygen vacancies, either away from or towards the interface of ETL, is predictable with the aids of external electric field. Under reverse scan, an external electric field pointing from perovskite light absorber to oxide-based ETL is exerted on the PSC which consequently causes the oxygen vacancies to diffuse from the perovskite/ETL interface to the bulk of the ETL [82]. This in turn slows down the recombination dynamics and subsequently escalating the power output. On the other hand, the oxygen vacancies would migrate to the perovskite/ETL interface upon employing forward-scanning mode due to the inverse electric field. The accumulation of oxygen vacancies at the interface minimize the electron collection efficiency and substantially accelerate the charge recombination rate at the interface. As a result, the same measured PSC device indicates poorer power output under forward scan. It is also important to note that the electron–hole recombination rates at the perovskite/ETL interface depend strongly on the positions of oxygen vacancies where the difference in recombination rates could reach up to one order of magnitude.

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4.3 Stability Another major concern in the field of PSC is to fabricate a highly stable device since organic compound in the perovskite light absorber degrades at an alarming rate upon exposure to moisture [54, 83]. To make things worse, some other external and internal factors such as UV light, oxygen, high temperature and trapped charges could also accelerate the degradation process of perovskite layer [84–88]. Oxygen vacancies in the ETL made of metal oxide are also another reason for the poor stability of the device that was often overlooked nowadays. Several research groups have discovered that PSC employing ETL with high oxygen vacancies degrade much faster than that PSC with low oxygen defects ETL. As described earlier in this chapter, one of the reasons is that oxygen vacancies intensify the hydrophilic nature of the metal oxide film. Consequently, this layer is prone to absorb more moisture from environment particularly when the device is prepared in ambient air or encapsulation step is neglected which expedite the degradation process of perovskite [17]. Furthermore, Ahn et al. [87] claimed that the trapped charges at perovskite/ETL interface could accelerate the degradation of perovskite in the presence of moisture. Therefore, it is natural to deduce that oxygen vacancy defects in metal oxide-based ETL is one of the major causes for degradation of PSC, since the defects impede efficient charge transfer and induce charge accumulation in the perovskite layer. From the above discussion, it seems like the most convenient way to mitigate this problem is by applying encapsulation on the device to prevent the perovskite layer from exposure towards humid air. Nonetheless, Leijtens and his team [88] revealed that encapsulated PSC degraded much faster than non-encapsulated device counterparts which they attributed this effect to the UV-radiation from sunlight and oxygen vacancies defect (or Ti3+ surface states) in mesoporous TiO2 layer. This is because oxygen molecules from environment are needed for healing or passivating the Ti3+ surface states on TiO2 , as illustrated in Fig. 12. The oxygen vacancies which are deep electron-donating sites interact with oxygen that adsorbs to the vacancy sites forming O2 − -Ti4+ charge transfer complex. Absorption of UV light by TiO2 creates electron–hole pair and the hole in the valence band of TiO2 could recombine with the electron at the oxygen adsorption site which simultaneously setting free the neutral oxygen molecule. This unoccupied oxygen vacancy sites serve as trap states [89]. Upon photoexcitation of perovskite layer, the electrons from perovskite are injected either directly into the deep trap states or initially into the conduction band and finally become deeply trapped. Since the trapped electrons are immobile, they would recombine with holes in the hole-transport layer [90]. In oxygen-free encapsulated device, the O2 − –Ti4+ complexes are depleted and the density of deep electronic traps rises, amplifying the recombination process until a level where the cell turns malfunctioning after several hours. As mentioned before, omitting encapsulation step assists in TiO2 passivation due to exposure to oxygen but at the same time, moisture in environment would react irreversibly with perovskite causing rapid degradation of the layer. Other preventive measures have also been suggested to address UV light-induced degradation issue

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Fig. 12 Schematic diagram illustrating the mechanism for degradation of perovskite due to UV light. Reproduced with permission from [88]. Copyright 2013, with permission from Springer Nature

such as blocking UV light illumination from reaching the device and replacing the TiO2 with other metal oxide as ETL. However, the former could reduce the total light absorption capability of the device which decreases the JSC , whereas the latter requires much deeper studies to success which may delay the commercialization of PSC. Hence, eliminating oxygen vacancies in metal oxide-based ETL while preserving the device from exposure to humidity is considered to be the most convenient solution to alleviate the issues and ensure good stability of PSC [23].

5 Conclusion Perovskite solar cells have experienced unprecedented advancements in terms of efficiency, approaching and overtaking those of early generation solar cells technology. One of the major contributors towards such achievement is the rapid progress in the development of electron transport layer. As research on enhancing the capability of ETL continues, point defects associated with oxygen vacancies in metal oxide-based ETL which could act as traps for photoexcited electrons have been recognized as one of the crucial parameters that governs the performance of a PSC. Even though the

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metal oxide shows superior electrical properties especially the electrical conductivity in the presence of high oxygen vacancies, the defect is found to be detrimental towards the generated efficiency, hysteresis behaviour and long-term stability of PSC devices. Low transparency, high hydrophobicity, enhanced recombination rate and diffusion of the defect sites are the key characteristics of oxygen vacancies that negatively affect the device performance. Thus, carefully tuning the density of defect in ETL either through modification of fabrication protocols or introduction of dopant is of great importance to ameliorate device stability while preserving excellent charge transfer and transport by the ETL. Profound investigation regarding this topic is urgently needed in the future to realize large-scale production of highly efficient and stable PSC device for commercialization. Acknowledgements The authors would like to thank Universiti Kebangsaan Malaysia (UKM) for the financial support through the internal grant DIP-2016-003.

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Solar Cells and Optoelectronic Devices in Space Khuram Ali, Syedda Shaher Bano, Hasan M. Khan and S. K. Sharma

Abstract Optoelectronic devices including solar cells have been widely used in space and are extremely sensitive to substantially shorter wavelength electromagnetic radiations, e.g., gamma ray. Electrons and secondary photons are produced when gamma rays pass through the matter and this phenomenon can be described as Compton effect. When a photon interacts with a device, it removes the primary electron from an atom. That primary electron in each ionizing collision produces swift secondary electron which may have nearly as much kinetic energy as the primary photon. This secondary electron dissipates its energy as kinetic energy which results in the ionization and excitation of the atoms in the absorber. This kinetic energy eventually is dissipated in the medium as heat and imparted to the atom in order to displace it from its normal site producing vacancy-interstitial pair. Ultimately, lattice periodicity changes and give rise to additional energy levels and alter the electrical properties of optoelectronic devices. Keywords Defects · Radiation damage · Gamma irradiation · Solar cells · LEDs · Photodiodes

1 Introduction Space cells have significant importance in atmospheric applications, extensively important in communications, transmissions, investigation of resources, space improvement applications and technical research. The use of space solar cells in satellites is important as power sources [1]. The fundamental purposes to use and K. Ali (B) · S. S. Bano Nano-Optoelectronics Research Laboratory, Department of Physics, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan e-mail: [email protected] H. M. Khan Department of Physics, The Islamia University of Bahawalpur, Bahawalpur, Pakistan S. K. Sharma Department of Physics, Faculty of Science and Technology, The University of the West Indies, St. Augustine, Trinidad and Tobago © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_12

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develop space photovoltaic cells are to increase radiation resistance (life), reduce cost and mass of arrays and photovoltaic cells and also enhance the conversion efficiency. Both single junction and multi-junction solar cells are important in space applications. However, multiple solar cells such as gallium arsenide/indium gallium phosphide are important in space applications due to greater radiation resistance and high conversion efficiency as compared to silicon solar cell [2]. Since, in the early 1990s the power requirements in satellites have increased significantly. Furthermore, for the enhancement of space applications, the improvements in radiation resistance and conversion efficiency of solar cells are required [3]. The large amount of solar energy almost 50–60% is vanished on its way through the atmosphere of earth by the special effects of absorption and reflection. However, the space-based photovoltaic system helps to avoid these losses by converting the sunlight to microwaves outside the atmosphere. The space photovoltaic system is important due to the production of large amount of energy in the entire earth with very less ecological effect. Such as the output of the sun is very large that is 2.3 billion but the earth receives only one part from the sun. The largest available source of potential energy on earth is space solar power [4]. Space solar energy system is important because it does not release greenhouse gases while ethanol, coal, gas and oil plants do. Also, space cells do not produce dangerous waste like nuclear power plants [5].

1.1 Radiations in Space Gamma radiations are electromagnetic radiations consisting of high-energy photons with high frequency and shorter wavelengths. Gamma radiations are present around us which are the natural phenomena since the earth has come into existence [6]. Hundreds of years ago scientists have succeeded in investigating the main source of emission of gamma radiations from unstable nuclei. This process is also known as gamma decay and produced due to the transition of unstable nuclei from its highenergy level to low-energy level [7, 8]. Another major source for the emission of gamma radiations is the galactic center [9]. Galactic center consists of the complex region containing massive X-rays, binding systems, star clusters, black holes and massive stars. The presence of such dark matter inside the galactic center leads to the excessive rate of matter annihilation which results in the emission of gamma radiations [10, 11]. A detailed study of gamma radiations is observed from the galactic central region. Gamma radiations-based maps indicate the existence of gamma radiations at galactic center. Owing to the high penetrating power of the exposure of these radiations on different devices of electronics has been a major course of study for many scientists [12]. Gamma radiations even have high penetrating power to such an extent that it can easily penetrate in the human body [13, 14]. In order to prevent the penetration of gamma radiations, it must be covered by highly dense materials such as uranium or lead [15]. Figure 1 shows the penetrating power of gamma radiations in different objects [16]. It is clear that alpha particles have the least penetrating power as compared to other radiations. These radiations are even

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Fig. 1 Penetrating power of radiations in paper, aluminum sheet and lead

blocked by few inches of papers. Irradiation of the alpha particles can produce many harmful effects. On the other hand, beta radiations are more like electrons and have slightly greater penetrating power as compared to alpha particles. Beta radiations can pass through the sheet of 3 mm thick aluminum [17, 18]. Figure 1 demonstrates the penetrating power of gamma radiations through lead. The penetrating power of gamma radiations is more when lead is thinner in diameter. With the increase in thickness, the transmission of gamma radiations starts to decrease. Gamma radiations are the most powerful electromagnetic ionizing radiations. These radiations are emitted from the radioactive materials. These ionizing radiations have very high penetrating power. They may even penetrate into a thickness of several centimeters distance. High ionizing gamma radiations can pass through the lead and can be easily detected from other side as shown in Fig. 1. During gamma rays interaction with matter, atomic electrons are knocked out in a way that it donates all of its energy and terminates its existence [19, 20]. Some of the energy is being used in overcoming the binding energy of the electron and remaining is transferred to the free electron in the form of kinetic energy [21]. In order to conserve the momentum, the minute amount of energy remains with the atom. This phenomenon is termed as photoelectric absorption. Photons having visible light ejects electron from the metal surface. Photoelectric effect is important because it can detect the gamma radiations interaction with the atoms using full peak values [22].

2 Radiation Environment and Devices in Space Optoelectronic devices are strongly affected by extreme radiation environment. These radiations may produce ionization and displacement damage affects [23]. These types of radiations are widely distributed in space and cannot be neglected [24, 25]. In order to avoid ionization and displacement damage affects, these particles need to understand and evaluate using standard practical approach. Exposure of these

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radiations produces defects in optoelectronic devices [3, 26]. These defects create additional energy levels in the band gap that alters the structure of semiconductor devices. These energy levels affect the electrical properties of different optoelectronic devices. Electron-hole pairs are generated near the energy band gap and produce leakage current. [27, 28]. There are many factors that contribute toward the recoupling of electron-hole pairs with the radiation-induced centers and tunneling of charge carriers. Different types of radiation-induced defects that may occur in semiconductor-based devices are shown in Fig. 2 [29]. When crystal lattice is irradiated by gamma radiations then lattice defects are produced in the crystal’s geometry [30]. When the incident particles pass through the solid, elastic and inelastic collisions are produced within the atoms of the semiconductors. The formation and elimination of electron-hole pairs in the semiconductor devices produce no net effect [31]. This process is termed as generation and recombination, respectively. Electron-holes pairs are formed by generation are removed by recombination [32]. Such mechanisms are basically restoring phenomena of nature. The excess and deficit charge carriers are stabilized within the semiconductor. Radiation environment produces damage defects in the semiconductor devices [33]. Generation and re-combinations can affect device efficiency. Extreme environmental conditions may cause other draw-backs, i.e., tunneling, traps and compensation. Carrier lifetime of semiconductors is decreased by electrons held in energy gaps. Most of the electrons pass through the energy gaps by compensation that eventually reduces the semiconductors stability [31, 34]. To reduce the impact of this defect, compensation-capacitors are installed into system format. Tunneling is the most critical defect among all the defects that have been discussed so far. Without changing the energy level tunneling causes the jumping of electrons across these energy levels. This defect can cause the device instability and significantly affects the working of optoelectronic devices [35].

Fig. 2 Different effects due to defect centers in the silicon band gap. Figure reprinted/adapted with permission from Ref. [29]

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2.1 Displacement Damage Mechanisms When gamma radiation interacts with atoms of different elements they knock out electrons. This displacement causes many defects in the standard working phenomena of devices. The topic under discussion helps in understanding the displacement damage effects in electronic devices [24, 36]. When gamma radiations are incident on silicon, there is probability of increased carrier recombination due to radiationinduced levels in the silicon band gaps. In case of silicon new analysis and simulations are being used in order to observe the cluster defects [37]. NIEL describes the non-ionizing energy loss per unit distance being traveled by incident particles in the silicone device. Normally carriers are generated at radiation-induced centers and after that recombination of these charge carriers carried out. This work largely emphasizes on the experimental situations in which NIEL is kept constant with the penetration depth. It is optimized in such a way that the range of high-energy particles is greater than the thickness of devices [38]. Cluster and subclusters both terms are used for small defect regions [39]. Clusters in irradiated silicon have been investigated by many scientists with experimental findings [40]. It is observed that radiation-induced degradation increases with the thermal generation rate. In case of the irradiated silicon, all the silicon devices are independent of NIEL [41]. Radiation-induced displacement damage defects depend upon the time (after irradiations) and bombardment conditions. Many factors such as irradiations temperature, measurement temperature and particle types are also responsible for the device degradation [42]. Radiation defects have great influence on the optical and electrical behavior of many semiconductor devices [43, 44]. Displacement damage can also affect the efficiency of solar cells [45]. Number of simulation models is being used for the evaluation of the irradiated solar cells. There is a chance of catastrophic failure due to high fluence rates due to carriers’ removal effects. These effects have been observed by many scientists and developed a model, which totally agrees with the experimental results [46]. Figure 3 shows the high-frequency voltage and capacitance measurements of irradiated and non-irradiated silicon. It is observed that after irradiation there is a gradual decrease in the value of capacitance.

2.2 Ionization Damage Mechanisms Gamma rays photons deposit energy in many devices by the process of ionization. The energy that is used for the purpose of ionization is termed as “ionization dose.” However, during whole exposure the energy observed is known as total ionizing dose [48]. The ionizing effects are produced by different kinds of ionizing radiations such as ultraviolet radiations, X-rays, gamma rays and by secondary recoil particles [49]. Ionizing effects are produced in the bulk materials. These can be stated as enhancement of conductivity by the production of excess amount of charge carriers

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Fig. 3 C–V characteristics at a frequency of 10 kHz on a p-type MOS capacitor and proton irradiation fluences of 1 × 1013 and 1 × 1014 protons/cm2 . Figure reprinted/adapted with permission from Ref. [47]

and trapped charge carriers [50]. Compton and pair production take place under high-energy photons like gamma rays. Many free electrons are produced which have sufficient energy to move toward forbidden energy gaps and create electron-hole pairs [51]. So the conductivity of bulk materials enhanced. The pairs generated in semi-conductive material may diffuse or recombine according to the electric field present between them [52]. Because of the trapping of the charge in the silicon dioxide, the damage is caused at the edges (interfaces) of silicon and silicon dioxide. When the gamma radiations are incident on the silicon structure electron-hole pairs are generated. Some of the generated pairs combine with other charge carriers while only few electrons leave the silica [53]. The mobility of electrons is greater than the holes; therefore, there is very little chance of movement of holes toward silicon through the interfaces. On the other hand, holes are responsible for charge built up in silicon dioxide. During gamma irradiation of silicon dioxide, the amount of charge generated in the silica depends upon the electric field across the oxides [25]. Figure 4 demonstrates the curve between time and partial pressure of oxygen. It is shown that linear values are obtained with constant oxidation rate. Owing to the tunneling affect some of the traps can recombine and accelerates under the induced electric field. Due to rearrangements of the bonds at the edges of silicon and oxides, new interface states are produced. Due to radiation exposure of electronic components, charges are trapped within these states and irregularities are produced. These unwanted irregularities, greatly influence the electric properties of electronic devices [54]. With the decrease in the thickness of silicon dioxide

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Fig. 4 Radiation aging during the predicted oxygen partial pressure. Figure reprinted/adapted with permission from Ref. [58]

effect of these traps becomes more dominant specifically in submicron technologies. When energetic particles are allowed to incident on the optoelectronic device, it loses its energy in the form of ionizing and non-ionizing radiations as they travel inside the materials [55]. As a consequence of this energy loss, electron-hole pairs are generated and atoms start to displace. The defects which are produced as a result of these incident radiations named as vacancies and interstitials [56]. Vacancy is actually defined as the void space generated in the absence of an atom from its normal lattice position. If the movement of that atom occurs into non-lattice positions then the defect which is produced is called interstitial defect. The combination of the interstitial and the adjacent vacancy is termed as Frenkel pair. In irradiated silicone devices, the large amount of vacancies may occur and different types of defects can be produced [57].

3 Interaction with Matter Since the last few decades interaction of gamma rays with matter is of major importance for researchers. Gamma radiations are considered as the ionizing radiations, scattered by the nuclei and electrons [55]. As a result, radiation fields with positive ions and negatively charged electrons are generated [59]. The interaction of gamma radiations with the matter may occur in the form of photonuclear, photoelectric, pair

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production and Compton effects. In addition many affects such as Rayleigh scattering, Thomson scattering and photofission may also occur as a result of photonic interaction of gamma radiations with matter [60, 61]. The existence of these affects may occur in different forms and needs to be discussed in detail. Depending upon the quantum mechanical properties of gamma ray photons, different types of scattering may occur. In the vicinity of the nucleus electron and positron pairs can also be generated. Photoelectric effect helps in knocking out the electrons, whereas knocking of the elementary particles from the nucleus deals with the photonuclear effect [62]. Gamma radiations are produced during the process of decay of many radioactive isotopes. Gamma ray bursts are the main source of generation of gamma radiation fields and mostly present in space at cosmic scale. In addition, terrestrial gamma rays are present in the atmosphere and are different from the gamma rays that are present on the surface of earth [63]. The kinetic energy of the gamma photon having zero rest mass can be calculated with the help of the formula given below:  E = h f = hc λ

(1)

In this equation, h represents the Planck’s constant, c is speed of light, and wavelength of electromagnetic radiations is λ and f is the frequency of gamma radiations.

3.1 Photoelectric Effect In these phenomena, gamma photon interacts with the electron (orbiting in the outer shell of an atom). During interaction, gamma radiation transfers its energy to the outermost electron and as a result of this energy; the electron is knocked out from the orbit of atom [64]. After the emission of electron a void space or a vacancy is generated and then filled by the outermost electron. During the transition process of electron, the energy is radiated in the form of soft electromagnetic (EM) radiations which falls in the ultraviolet, X-rays and visible regions of the EM spectrum [65]. Photoelectric interaction has indirect relationship with the gamma photon energy and has a direct relation with the number of electrons present in the elements. It is observed that the photoelectric effect is larger for elements having high atomic numbers and decreases with the rise of energy [66]. Photoelectric effect always takes place by the secondary emission as atoms cannot exist in the form of excited state. In most cases, atom emits X-rays and returns to the ground state. The emission of Auger electrons is carried out from the outer most electronic shells and results in the emission of the secondary radiations which are used in the detection of gamma radiations [67]. Photons energy in mega electron volts (MeV) and percentage of total interactions is shown in Fig. 5. The graph shows that at lower photon energies photoelectric effect is dominant, whereas Compton effect becomes more dominant as the photon energy values increase from 0.1 to 0.15 MeV.

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Fig. 5 Photoelectric effect and Compton effect. Figure reprinted/adapted with permission from Ref. [68]

3.2 Photo Fission of Nuclei If high-energy photons collide with the CaF2 or fluorite, then gamma photons having energy greater than 5 MeV can be generated. During these phenomena nuclei of the thorium and uranium becomes unstable to an extent that they can produce fission reaction. Energetic X-rays having energy greater than 8 MeV are produced by the particle accelerators. In case of uranium and thorium, the values of threshold energies do not differ from one nucleotide to the other nucleotide under the area of the mass numbers. Gamma photon having energy greater than 16 MeV are unable to produce fission in lead [69].

3.3 Compton Scatterings Compton scattering is a major phenomenon in understanding the interaction of gamma radiations with matter. In this process, a gamma ray photon strikes with the free electron and elastically scatters. Momentum and the energy are not conserved if the striking photon is completely absorbed by the electrons at rest. It is a matter of fact that the electron in the matter is not free or at rest; however, if the energy of incident photon is much greater than the binding energy of the electrons then the state of electron is either at rest or is free. In Compton scattering gamma radiations interact with the loosely bound electrons being scattered by an appropriate loss in amount of energy [70]. Figure 6 illustrates the Compton scattering by gamma rays in the presence of incident laser light. Laser photon strikes with the electron having an initial energy of E e ,i at an angle θ p . As a result of Compton scattering photon energy is converted into gamma energy (E γ ).

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Fig. 6 Laser Compton scattering schematically. Figure reprinted/adapted with permission from Ref. [71]

3.4 Dose Rate Effects Dose rate effects for silicon and germanium have been investigated by many researchers. It involves the study of Cobalt-60 (60 Co) gamma irradiation of electronic devices with energy greater than 1.43 MeV. It was observed from this study that the calculated dose rates greatly depend upon the primary source of the radiations [72]. It has been found that the silicon and germanium negligibly depend upon the dose rates [73]. Many experiments have been performed in calculating the dose rate effects. In case of silicon and germanium, there is a gradual decrease in the value of biased current at low voltage, up to the dose rate of 5-kilo rad (Si). On the other hand increase in the value of biased current is observed for values of dose rates greater than 5-kilo rad (Si). Many physical methods are also adopted for optoelectronic devices to understand the dose rate affects produced by the ionizing radiations. Previous studies show that the emitter-based junctions in optoelectronic devices highly depend upon the low dose rates [74].

4 Influence of Gamma Radiations on the Solar Cells, LED’s and Photodiodes 4.1 Effect on Solar Cells Most of the optoelectronic devices, such as solar cells, LED’s, photodiodes, etc., are significantly influenced by gamma irradiations. This is due to the fact that the production or absorption of light in a solid medium is greatly influenced by the presence of defects inside the medium. Phototransistors are designed in such a way that the base region absorbs maximum light when exposed to it. Solar cells are frequently used in

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most of the electronic systems and in satellites as power sources. Solar cells are highly sensitive to the electromagnetic radiations having shorter wavelengths like gamma rays and X-rays. These radiations are largely present in space and help in studying the radiation-induced defects in different optoelectronic devices. These types of radiations are highly energetic and can produce structural as well as electronic defects in semiconductors. These defects ultimately alter the output power and reduce the efficiency of solar cells. The radiations produced by the gamma rays, neutrons or any other charged particles may produce large number of energy levels in the band gaps. Undesired energy levels are produced by these gamma radiations. These unwanted energy levels significantly affect the electrical behavior of the solar cells and produce electron-hole pairs near the mid gap. Generation of electron-hole pairs increases the possibility of leakage current [75]. Figure 7 demonstrates the output parameters of cadmium telluride (CdTe) solar cell. Output parameters such as open-circuit voltage, fill factor and efficiency were observed as a function of absorber layer thickness for various back surface recombination velocities. In addition gamma radiation also affect the solar cell devices such as in the form of trapping of hole pairs,

Fig. 7 Output parameters of the CdTe solar cell after irradiation. Figure reprinted/adapted with permission from Ref. [77]

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compensations, and tunneling of the charge carriers at the edges of the devices. These defects ultimately become responsible for altering the desired output characteristics of the solar cells [76].

4.1.1

Compton Scattering in Silicon Solar Cells

During the passage of gamma radiations through the matter, electrons and secondary photons are emitted and the mechanism is termed as Compton scattering. A large number of the electrons are generated as a result of this Compton scattering affect. Other effects such as pair production and photoelectric affect are almost negligible in this process. In the silicon solar cells, the photon collides with the cell and ejects the primary electrons from the atoms. Each of these primary electrons generates the secondary electrons which exhibit as kinetic energy greater than that of the primary photon. The secondary electron then excites the atoms in the absorber region after the dissipation of kinetic energy [78]. This kinetic energy is dissipated in the form of heat and transferred to the atom. The remaining kinetic energy then relocates the atoms from its normal position and thus generates the vacancies inside the solar cell. Under the higher electric field, the presence of defects increases the thermal production of the charge carriers by decreasing the potential barriers. This phenomenon is termed as the Poole Frenkel effect [78]. So, increased kinetic energy is dissipated as heat energy into the solar cell devices [79, 80].

4.2 Effect on Photodiodes and LED’s Many optoelectronic devices such as LED’s and photodiodes are a course of major discussion in the work. To study the radiation-resistant properties, photodiodes and LED’s are exposed by 60 Co gamma rays at different angles. Dose rates are normally chosen in connection with the available dose in space environment [81]. The results of the earlier investigations show that there is a significant decrease in the output light of LEDs and photodiodes. It was observed that non-radiative recombination centers are mainly responsible for decreasing the output parameters. Due to an increase of the density of the non-radiative recombination centers there is a reduction in the lifetime of minority charge carriers. However, in case of the LEDs and photodiodes, the radiative recombination is relatively smaller as compared to other electronic devices [82]. Before irradiation with gamma rays, LEDs and photodiodes show a short lifetime of the minority charge carriers and higher current densities. In case of photodiodes, the irradiation phenomena produce dark current in great extent [83]. Influence of the gamma rays on the photodiodes has been analyzed by many researchers. It was observed that after irradiation there is an increase in resistance and as a result voltage is dropped across the photodiodes. Different types of radiation-induced defects in the form of shunt resistance dark current has been observed [84]. Light-emitting diodes

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Fig. 8 Signal to noise ratio of photodiode with fluence of gamma radiations at different temperatures. Figure reprinted/adapted with permission from Ref. [85]

made of gallium nitrides (GaN) endure considerable attraction from the past few years owing to its wide energy band gap and radiation-resistant electrical properties. Figure 8 shows the signal to noise ratio (SNR) of photodiode with fluence of gamma radiations at different temperatures. The signal to noise ratio is calculated for quantum efficiency against the fluence of gamma radiations at different temperatures. Two curves are obtained at temperatures above 100 K. It is observed that the quantum efficiency of photodiodes is decreased by increasing the temperature.

5 Conclusion and Future Prospectus The purpose of the present work was to investigate the gamma radiation interaction with optoelectronic devices. During gamma rays interaction with these devices, atomic electrons are knocked out and kinetic energy is produced. This kinetic energy eventually is dissipated in the medium as heat and imparted to the atom in order to displace it from its normal site producing vacancy-interstitial pair. Ultimately, lattice periodicity changes and give rise to additional energy levels and alter the electrical properties of optoelectronic devices. A major challenge in the electronic industry is to recognize the relationship among the characteristics and defects of the optoelectronic devices. For successful operation and development of the aerospace systems, radiation-resistant materials need to be employed. This study emphasizes on the understanding of radiation tolerance of optoelectronic devices. This demands a thorough and detailed study of the distribution of defects by using simulation and laboratory tests. One of the most adopted techniques is the relocation of radiationinduced defects in nanoscale structures. It is observed that output parameters of irradiated optoelectronic devices are influenced by the increased value of resistance.

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This effect can be analyzed by using long-to-short range 3D models [86]. In some cases, improvement is being carried out by increasing the n layer thickness of the LED’s and photodiodes. This method is based on the principle that the given intensity of light and wavelength changes with the reverse current. With increased thickness of the layers, the total reverse currents in irradiated photodiodes can be made negligible. Si solar cells can be optimized with lithium (Li) incorporation. This can improve the radiation tolerance and the minority carrier diffusion length. Li has the ability to enhance radiation-resistant properties of solar cells by diffusing to and combining with radiation-induced point defects. Radiation tolerance of integrated electronic circuits is the main area of study for high-energy physics experiments. It is observed that the radiation-resistant techniques are also useful in submicron technologies and have a significant role in the future research and development of aerospace missions [87]. Acknowledgements This work was supported by the Higher Education Commission (HEC) of Pakistan [Grant No: 8615/Punjab/NRPU/R&DHEC/2017]; and the National Centre for Physics (NCP), Islamabad, Pakistan for providing assistance and Associate Membership to Dr. Khuram Ali.

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Multi-junction (III–V) Solar Cells: From Basics to Advanced Materials Choices Khuram Ali, Afifa Khalid, Muhammad Raza Ahmad, Hasan M. Khan, Irshad Ali and S. K. Sharma

Abstract Solar cell efficiency can be associated with the ability of the solar cell to produce the maximum amount of electricity from a light energy source. There are many uses of multi-junction solar cells based upon likewise in satellites and space vehicles. Physically based two-dimensional methods under ultra-high concentration above 1000 suns, the important limiting factors of multi-junction solar cells can be investigated. The single-junction solar cells that are merged with silicon and GaAs solar cells lead to the great importance due to 30% limit of intrinsic efficiency. In the present chapter, we have discussed the basic physics and operation of solar cells with multiple-junction cell designs of different types of materials, with a particular focus on the GaInP/GaAs/Ge tandem cells. Further, their performance based on different parameters will be discussed along with future consideration for developing most advances in high efficiency III–V multi-junction solar cells. Keywords Multi-junction · Anti-reflection coating · Conversion efficiency · Limiting factors · Concentrated photovoltaic · Tandem solar cells

K. Ali (B) · A. Khalid Nano-Optoelectronics Research Laboratory, Department of Physics, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan e-mail: [email protected] M. R. Ahmad Centre for Advanced Studies in Physics (CASP), GC University Lahore, Lahore, Pakistan H. M. Khan Department of Physics, The Islamia University of Bahawalpur, Bahawalpur, Pakistan I. Ali Department of Physics, Bahauddin Zakariya University, Multan, Pakistan S. K. Sharma Department of Physics, Faculty of Science and Technology, The University of the West Indies, St. Augustine, Trinidad and Tobago © Springer Nature Switzerland AG 2020 S. K. Sharma and K. Ali (eds.), Solar Cells, https://doi.org/10.1007/978-3-030-36354-3_13

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1 Introduction The solar cell is a device which converts the energy of light (photons) directly into electricity. It is a form of photoelectric cell whose electrical characteristics vary when exposed to light. The solar cell devices can be combined to form modules which are also known as solar panels. The power conversion efficiency is an important parameter which refers to the portion of energy in the form of sunlight that can be converted into the electricity by the solar cell. It is the need of hour to new types of solar cells which have better conversion efficiency. For this, there are new group of solar cells known as multi-junction solar cells which are based on the combination of multiple p–n junctions and made up of different semiconductor materials [1]. These junctions are used for improving the solar cell conversion efficiency of sunlight into electrical energy. In this new generation of solar cells, a large spectrum of lights can be used for high efficiency. Multi-junction solar cell is composed of various types of photovoltaic junctions that stack over each other to give better efficiency [2]. Different individual solar cells having different band gaps of physical properties and energies are held together that are able to catch and change a big range of photon wavelengths into electrical energy with better efficiency. Multiple-junction solar cells have the ability to give two times more energy as compared to conventional solar cells [3]. By increasing the amount of multiple-junction solar cells, large amount of energy with high efficiency can be generated. In this method, light is absorbed in the panel through reflectors which lead to the idea of concentrated photovoltaic (CPV) [4]. Generally, four regions are given to the solar cell concentration. Multi-junction solar cells deal with thermal management and face tracking problems, by increasing the solar concentration level. However, the strength of tunnel junction can be affected by increasing the current densities [5]. The important purpose of photovoltaic studies is to enhance the ability of solar cells. In August 2009, a press released by Spectrolab declared that a triple-junction solar cell concentrator has an effectiveness of 41.6% [6]. In the light of this press release, the triple-junction solar cells of III–V semiconductor materials are brought under trial by Fraunhofer Institute of Solar Energy (ISE) and announced the efficiency of 41.1% [7]. This efficiency is studied under spectral conditions such as under AM1.5 direct and under 454 KW/m2 intensity of light. Thus, this cell is designed for increasing the characteristics of solar concentrator set-up [8]. The concentrator photovoltaic devices exhibit 40% altering efficiencies in the solar cells of triple junction derived from III–V semiconductor type materials, which is the present position of proficiency [9]. There are many uses of multi-junction solar cells likewise in satellites and space vehicles. These applications are studied under spectral conditions of one sun of International Organization for Standardization (ISO) [10]. However, under the hard environment, these applications cannot be studied, because protons and electrons can destroy the solar cell in space due to high energy and rough environment [11]. Humans can obtain a significant amount of electrical power through solar energy without developing nuclear fission reactions and fossil fuels such as oil, natural gas,

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and coal. Multiple-junction solar cells or photovoltaic have many benefits such as they can produce broad range of electricity which can be used in many applications [12]. These are also less costly, economically, and electrically suitable devices. Conversion efficiencies in excess of 50% can be achieved by III–V semiconductor multi-junction photovoltaic cells [13]. These types of photovoltaic cells have capabilities of applications in terrestrial and space. From the past few years, MJ solar cells have become important photovoltaic cells. Two-junction solar cells of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) had been investigated by some researchers since 1982. The group related to this research also proved the conversion efficiency of 20.2% in 1987, connected with a sub-cell of dual hetero tunnel junction structure [14, 15]. In 1990, MJ solar cells with super high conversion efficiency were investigated by research and development project in Japan to obtain 40% of conversion efficiency and developed resourceful technologies subjected by the support of New Energy and Industrial Technology Development Organization (NEDO) [16]. The concentrated photovoltaic CPV system depends on the ratio of concentration which vividly enhances the MJ solar cell‘s production scale and decreases its cost for terrestrial uses because this system corresponds to the powerful device for multiple-junction solar cells [17]. In the fiscal year of 2001, the multiple photovoltaic cells were achieved for the development of cost-effective and great efficiency MJ concentrated cells and its modules, through the support of research and development project under NEDO [18].

1.1 Limiting Factors Involved in Multi-junction Solar Cells In the physically based two-dimensional methods under ultra-high concentration above 1000 suns, the important limiting factors of multi-junction solar cells can be investigated [19]. In the electrical activity of multi-junction solar cells, series resistance is the main factor. This factor can be studied and examined in different ways such as top solar cell’s window layer; the band dis-continuities are found at multi-junctions and the electrical characteristics of the emitter [20]. The structure of multi-junction solar cells can be formed under ultra-high concentration, i.e., >1000 suns, so the amount of concentrator photovoltaic (CPV) system can be decreased. The process done under ultra-high concentration is also important for optical properties of concentrators [21]. The structures of multi-junction solar cells that are formed under ultra-high concentrations (>1000 suns) are attracting the great importance at the present time with the increasing efficiency of 35%. The way to working of solar cell and the release of heat are also important factors involved in the multi-junction solar cell. The loss of the series resistance is the main disadvantage of ultra-high concentrators [22, 23]. However, in physical operations, the solar rays having properties of light spectrum, size, or quantity and the match of solar ray band gap with junction are the main factors in which the conversion of energy of solar cell is based [24]. The air mass coefficient is a common term which is used to characterize the performance of photovoltaic

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system under standardized conditions such as temperature, weatherly, surrounding conditions, and geomorphology. It is referred to using the syntax “AM” followed by a number. The photovoltaic multi-junction devices refer the suitable air conditions, better efficiency with AM1.5 spectrum, and suitable micro-surroundings [25]. The temperature and intensities can also be the factors of multi-junction solar cells. The multi-junction solar cells are connected in series with tunnel junctions and contain three sub-cells spreading through organic vapor phase [26]. These three sub-cells that connect in series are able to work as a single diode to define the procedure of multi-junction solar cell. This procedure is used under the factors of intensity and temperature which leads to estimate the design of multi-junction solar cells [27].

2 Multi-junction Solar Cells Design The multi-junction solar cell (MJSC) devices are the third generation solar cells which exhibit better efficiency and have potential to overcome the Shockley–Queisser limit (SQ limit) of 31–41% [28]. Mostly the MJSCs are based on multiple semiconducting materials, and these semiconductors are stacked on top of each other having different energy gaps, which is similar to that concept of sub-cells [29]. The MJSC due to these sub-cells has different energy gaps for handling the spectral matching of singlejunction solar cell. The sub-cells of different energy gaps have ability to absorb the photons from different segments of the solar spectrum. The more photons of light can be absorbed by the upper part of the sub-cell so the upper level is produced with high-energy band gaps [29]. The design framework of multiple-junction solar cells should be perfectly chosen, to obtain the better efficiencies of solar cells. The drift diffusion model defines the perfect combination of the design framework, with all factors of semiconductor materials (III–V) likewise conduction band (CB) and valence band (VB) density measured as factor of energy gap and effective mass [30]. The design of generic material set, with the explained model of non-radiative and radiative losses, provides the basic information of multi-junction solar cell. In multijunction solar cells, the use of suitable anti-reflective coating is assumed for better efficiency [31]. It is thoroughly difficult to calculate design of full spectrum. For the accurate combination of multi-junction solar cell, there is a well-ordered method to describe the design of a spectrum. This method is used to obtain better efficiency of spectrum design under standard test condition (STC), at any location to maximize the energy [32]. The multi-junction solar cell shown in Fig. 1 is used to increase the efficiency. The conversion efficiency can be increased to discover the band gaps by using spectral functions [33].

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Fig. 1 Schematic diagram of multi-junction solar cells. Figure reprinted/adapted with permission from Ref. [34]

2.1 GaAs/Si Tandem Solar Cell In the photovoltaic research, the multi-junction solar cells that consist of silicon are very important. The single-junction solar cells that are merged with silicon and GaAs solar cells lead to the great importance due to 30% limit of intrinsic efficiency [35]. For non-concentrating solar cells, the Si-based multi-junction provides better path to exceed the efficiency limit to 30%. The use of gallium arsenide (GaAs) and germanium materials is going less. The Si-based method can be done with fewer amounts and is less costly. This method has great importance due to the requirement of better efficiencies for non-concentrating photovoltaic systems [36]. Silicon-based solar cells are less expensive due to the use of silicon at the lower level of the cell. Thus, these cells have gained the novel status in their use. Solar cell, with a lower silicon cell and two junctions have 1.7–1.8 eV band gap of its upper cell and 45% efficiency limit, is described by method of explained balance limit [37]. The merger of GaAs with silicon is still less and is considered non-suitable because it has a 1.41 eV band gap. However, the efficiency of silicon-based GaAs is greater as compared to single-junction solar cell. Thus, the efficiency of siliconbased GaAs tandem solar cell with the arrangement of non-current matched four terminals is 42% and for two terminals of current-matched arrangement is 39% [38]. The silicon-based gallium arsenide (GaAs) solar cells have been becoming developed electronic components. Thus, in the laboratory conditions and suitable environment, Si-based GaAs solar cells can be obtained with efficiency greater than 30% with four terminals arrangement and have attained technologically importance [39]. The silicon-based multi-junction solar cells with appropriate band gap have been achieving a significant importance in the present industry. These days, multi-junction solar cell with silicon as a bottom cell is giving the contemporary structure in industries. The high efficiencies and great performance in multi-junction arrangements

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Fig. 2 Schematic diagram of multi-junction tandem solar cells. Figure reprinted/adapted with permission from Ref. [43]

are achieved by using III–V semiconductor materials [40]. The multi-junction solar cells that are put together with current match two terminals and same band gaps are considered to obtain the better efficiencies. The band gap of the upper cell should be 1.75–1.79 eV in the two-junction silicon-based solar cell which exhibits good combination and can provide 29.79% efficiency with silicon bottom cell [41]. In III–V semiconductor solar cells, the combination of gallium indium phosphorous (GaInP) with gallium indium arsenide (GaInAs) has been used to obtain 31.5% efficiency. The efficiency limit of 25.2% has been recognized for perovskite materials that are now in the procedure of growth [42]. Figure 2 shows two- and four-terminal siliconbased GaAs tandem solar cells. In two-terminal tandem cell with thickness of 200 nm GaAs is placed at the top with Si bottom cell. In four-terminal solar cell, 1 µm GaAs top cell is placed. The sub-cells are separated by zero-thickness insulation coating.

3 Multi-junction Solar Cell Performance The multi-junction solar cell (MJSC) consists of multiple p–n junctions of different semiconductor materials. These semiconductor materials absorb a wide range of wavelengths and improve electrical energy conversion efficiency [44]. The multijunction solar cells (MJSCs) are instrumental in concentrated photovoltaic (CPV) and space photovoltaic systems. The idea of CPV is used for optical light concentrators which increase the incident power on solar cells. The driving force for the material and technological development of MJSCs is the need for higher conversion efficiency. In CPV systems, the conversion efficiency is further increased owing to the use of concentrated light and therefore any efficiency gain that can be made by using more suitable materials and advanced design would lead to significant gain in

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overall system efficiency [45]. This CPV module uses four-junction III–V-based solar cells. The record CPV efficiency for lattice-matched GaInP/GaAs/GaInNAsSb solar cell is 44%. On the other hand, the best lattice-matched GaInP/GaAs/Ge exhibits a peak efficiency of 43.3% under concentration and 34.1% at 1 sun. Efficiencies as high as 50% have been predicted for cells with a larger number of junctions and high concentration. To this end, a promising approach is to integrate dilute nitrides and standard GaInP/GaAs/Ge. Yet, so far, such heterostructures have exhibited low current generation [46]. In addition to these higher efficiencies, tracking that allows CPV systems can produce a larger amount of energy throughout the day in sunny regions, notably during the late part of the day when electricity demand is at the peak. At the same time and in contrast to concentrated solar power (CSP), the size of the installations can be scaled over a wide range, i.e., from kW to multi-MW. This way the local demands are adapted. Some CPV systems also disturb a smaller land area, since the trackers with relatively narrow pedestals are not closely packed [47]. In some cases, this could allow for continued use of the land for other purposes, for example, agriculture, although the relevant benefits of CPV versus flat plate PV in this case is still an active area of research. Finally, heliostat concentrator photovoltaic (HCPV) could provide an advantage over traditional c-Si technology in hot climates because of the lower temperature coefficient.

3.1 Current Density–Voltage Curves People are little impressed from the efficiency which is almost 25% for singlejunction record cells and more than 30% in the case of tandems. Nobody bothers to ask this sort of question for the latest sports car even though the efficiency of typical combustion engines in cars is probably not much higher. Cells made from multiple materials layers can have multiple band gaps and will therefore respond to multiple light wavelengths, capturing and converting some of the energy that would otherwise be lost to relaxation as described above [48]. For instance, if one had a cell with two band gaps in it, one tuned to red light and the other to green, then the extra energy in green, cyan, and blue light would be lost only to the band gap of the green-sensitive material, while the energy of the red, yellow, and orange would be lost only to the band gap of the red-sensitive material. Following analysis similar to those performed for single-band gap devices, it can be demonstrated that the perfect band gaps for a two-gap device are at 1.1 and 1.8 eV. Traditional single-junction cells have a maximum theoretical efficiency of 33.16%. Theoretically, an infinite number of junctions would have a limiting efficiency of 86.8% under highly concentrated sunlight [49]. Currently, the best laboratory tested results of traditional crystalline silicon (c-Si) solar cells have efficiencies between 20 and 25%, while the best laboratory tested results of multi-junction cells have demonstrated performance over 46% under concentrated sunlight. Commercial examples of tandem cells are widely available at 30%

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under one-sun illumination and improve to around 40% under concentrated sunlight. However, this efficiency is gained at the cost of increased complexity and manufacturing price [50]. To date, their higher price and higher price-to-performance ratio have limited their use to special roles, notably in aerospace where their high power-toweight ratio is desirable. In terrestrial applications, these solar cells are emerging in concentrator photovoltaics (CPVs), with a growing number of installations around the world [51]. Tandem fabrication techniques have been used to improve the performance of existing designs. In particular, the technique can be applied to lower cost thin-film solar cells using amorphous silicon, as opposed to conventional crystalline silicon, to produce a cell with about 10% efficiency that is lightweight and flexible. This approach has been used by several commercial vendors, but these products are currently limited to certain niche roles, like roofing materials [52]. The current–voltage (I–V ) characteristics of single-junction GaInNAs SC, for AM1.5G real-sun illumination, are shown in Fig. 3a. Measurements were done with and without a 900-nm long-pass filter inserted before the solar cell. The filter was used for simulating the light absorption into top junctions present in a multi-junction device. The open-circuit voltage (V oc ) and short-circuit current (J sc ) values for the GaInNAs solar cells were 0.416 V and approximately 40 mA/cm2 and 0.368 V and approximately 10 mA/cm2 , without and with a long-pass filter, respectively. The spectral behavior of PL and EQE is shown in Fig. 3b. The band gap of the GaInNAs was estimated from the PL peak maximum wavelength to be approximately 1 eV [53]. Examples of the measured PL spectra for GaInNAsSb structures with different amounts of Sb are presented in Fig. 4a. As it can be seen, the band gap of GaInNAsSb can be decreased down to 0.83 eV (1500 nm). The I–V characteristics of a GaInNAsSb SC with E g = 0.9 eV measured under real-sun excitation at AM1.5G are presented in Fig. 4b [56]. The data showed the calculated values of the W oc for selected GaInNAs and GaInNAsSb single-junction solar cells (SCs). For GaInNAs solar cell with E g = 1 eV,

Fig. 3 I–V characteristics of single-junction GaInNAs solar cell (a) and EQE and PL spectra of GaInNAs (b). Figure reprinted/adapted with permission from Ref. [54]

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Fig. 4 Measured photoluminescence spectra of GaInNAsSb SCs (a) and I–V characteristics of 0.9-eV GaInNAsSb SC (b)

the W oc was 0.58 V, and for GaInNAsSb with E g = 0.90 eV, the W oc was 0.59 V. The best W oc achieved so far from GaInNAs single-junction SCs is 0.49 V. The observations made here are in accordance with previously published reports which indicate that the Sb-based solar cells have a slightly higher W oc values compared to GaInNAs SCs [55]. The optimal band gap for GaInNAsSb junction of the triple- and four-junction solar cells depends on the target spectrum and the performance of the sub-junctions. In a four-junction cell, it would be beneficial to have slightly larger band gaps for the top junctions, especially for the AM1.5G spectrum. The GaInP/GaAs top cells have already been well optimized and that is the reason why the band gap shifting is probably not the best practical step to start with, especially because the W oc values of top junctions with larger band gaps increase easily [56].

3.2 Band Gap Conversion Efficiency There are several reasons why III–V multi-junction solar cells reach the highest efficiencies of any photovoltaic technology. The III–V solar cells are composed of compounds of elements from group III and V of the periodic table. In the corresponding multi-junction devices, several solar cells made of different III–V semiconductors are stacked with decreasing band gaps from top to bottom [56]. This reduces thermalization losses as photons are mostly absorbed in layers with a band gap close to the photon’s energy. Moreover, transmission losses are reduced as the absorption range of the multi-junction solar cell is usually wider than for single-junction devices. Finally, the use of direct band gap III–V semiconductors facilitates a high absorption

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of light even in comparably thin layers. In addition, the efficiency increases when operated under concentrated illumination due to a linear increase of short-circuit current and logarithmic increase of voltage [57]. The most common III–V multi-junction solar cell in space and terrestrial concentrator systems is a lattice-matched Ga0.50 In0.50 P/Ga0.99 In0.01 As/Ge triple-junction solar cell. The device is typically grown with high throughput in commercial metal organic vapor phase epitaxy (MOVPE) reactors. All semiconductors in this structure have the same lattice constant as the Ge substrate, which facilitates crystal growth with high material quality [58]. However, its band gap combination is not optimal as the bottom cell receives significantly more light than the upper two cells resulting in about twice the photocurrent of the upper two sub-cells. Nevertheless, a record efficiency for this triple-junction concentrator solar cell 41.6% (AM1.5d, 364 suns) was achieved in 2009. Various approaches are under investigation to further increase in solar cell efficiencies. So far achieved record cell efficiencies are above 41%. This uses different elements from the wide range of technology building blocks available for III–V multi-junction solar cells [59]. The fill factor (FF) is essentially a measure of quality of the solar cell. It is calculated by comparing the maximum power to the theoretical power (PT ) that would be output at both the open-circuit voltage and short-circuit current together. In practical terms, for two solar panels of the same physical size, if one has a 21% efficiency rating and the other has a 14% efficiency rating, the 21% efficient panel will produce 50% more kilowatt hours (kWh) of electricity under the same conditions as the 14% efficient panel [60].

3.3 Spectral Distribution Effects It is well known that triple-junction CPV cell performance depends on spectral conditions, and therefore, atmospheric conditions such as aerosol optical depth (AOD), perceptible water vapor (PWV), and optical air mass (AM) can have an impact on cell and module performance. Although improved models are needed to accurately predict energy production for CPV systems, it is not clear what, if any, spectral parameters should be included in these models [61]. As spectral data is not readily available, one must justify requiring this data by a significant increase in model accuracy. To further compound this issue, CPV system technology varies significantly, and it is probable that spectral data could improve energy predictions for one technology but have little impact on alternate technology [62]. The part of this chapter explores the impact of AOD (at 500 nm) PWV, optical AM on module performance, in conjunction with the SMARTS spectral model, cell quantum efficiency (QE) data (InGaP/InGaAs/Ge optimized for a G173/AM1.5 spectrum), and spectral measurements from a PG S-100 Direct Normal Spectral Radiometer. Integrating the QE data with both modeled and measured spectra provides a means to predict changes in module current as AOD, PWV, and optical AM changes [63].

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It is useful to begin an analysis of the spectral sensitivity of CPV modules by briefly mentioning characteristics of an InGaP/InGaAs/Ge cell in relation to the solar spectrum and key spectral parameters. The InGaP (top junction) is generally responsive in the range of 350–700 nm, the InGaAs (middle junction) from 500 to 1000 nm, and the Ge (bottom junction) from 800 to 1800 nm. The three junctions are wired in series, and therefore, the cell operates based on the junction that is generating the least current. Manufactures typically design cells such that the top and middle junctions alternate in the role of limiting current, while the bottom junction always has surplus current. The top and middle junctions are grown to be current matched for the G173/AM1.5 direct reference spectrum, a design point that is suggested for optimum energy production at many CPV appropriate sites [64]. The spectrum is very dynamic, and changes in optical AM, PWV, and AOD all impact CPV performance. Optical AM, the relative path length through the atmosphere, varies from 1 to 5 when CPV produces the majority of its energy. As optical AM increases, there is an increased Rayleigh scattering of blue light and the top junction predominantly loses current. PWV, cm of condensed water vapor in the vertical direction, absorbs solar radiation in bands around 720, 820, 940, 1100, 1380, 1870, and 3200 nm. Absorption bands corresponding to the bottom junction are the strongest, but because the bottom junction does not limit this device’s performance, PWV has little effect on cell performance [65]. However, absorption by water vapor does decrease the measured direct normal irradiance (DNI). Consequently, as PWV increases, DNI decreases more than power, resulting in an increase in efficiency. It is interesting to note that changes in PWV have stronger impact in dry climates due to the fact that an increase from 0 to 0.4 cm of PWV decreases solar radiation by 10%, while an increase from 0.4 to 4 cm is required to decrease radiation another 10%. AOD measurements quantify the number of particles/aerosols in the vertical direction that results in radiation attenuation in the range of 400–2000 nm. The rate of attenuation decreases with wavelength but is a complex function of quantity and size distribution of particles. An increase in AOD typically results in the top junction decreasing in current more than the middle junction [66]. The part of the chapter briefly explains the spectral sensitivity of CPV modules with triple-junction cells on sun for multiple months in Golden, CO. The simple model of the atmospheric radiative transfer of sunshine (SMARTS) model is used with quantum efficiency (QE) data, optical air mass (AM), PWV, and AOD measurements to predict variation in Isc/DNI over 9 months. Ultimately, this approach provides a reasonable estimate of long-term variation in Isc/DNI. When SMARTS is used in the same approach to predict intraday changes, the results are less useful. This is likely due to small intraday changes in PWV and AOD being within the measurement uncertainty of these parameters coupled with uncertainties and assumptions made when applying the SMARTS model. At the same time, using measured spectra rather than SMARTS spectra, intraday modeling indicated the correct movements in Isc/DNI but with a reduced magnitude. In examining CPV module performance, data show clearly defined AM response peaks for Isc/DNI and efficiency while confirming that FF is minimized when Isc/DNI peaks. The module metrics, Photovoltaics for Utility Scale Applications (PVUSA) power ratings,

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performance ratio, and DNI weighted efficiency, all show monthly variations on the order of 5–10%, while Isc/DNI shows variation of 15%. In a simple approach, the overall spectral sensitivity of three modules is quantified by fitting an AM correction factor to nine months of data and then applying these correction factors in a yearly energy prediction. Ultimately, the AM correction factor results in a 7% drop in annual energy over a fixed efficiency module and 2% difference between CPV modules with the differing spectral sensitivities [67].

3.4 Anti-reflection Coating in Multi-junction Solar Cells An anti-reflection (AR) coating is a type of optical coating which is applied to the surface of multi-junction solar cell for reducing the reflection of light which improves the performance of the solar cell. This anti-reflective coating (ARC) is very much needed as the reflection of bare silicon solar cells is over 30%. For the thin AR Coating, silicon nitride or titanium oxide is generally used. The anti-reflecting coatings are also used in various systems such as telescopes and microscopes for improving the contrast of the image. In these systems, AR eliminates the stray light.

3.4.1

Principle of Anti-reflection Coating

Many coatings consist of transparent thin-film structures with alternating layers of contrasting refractive index. Layer thicknesses are chosen to produce destructive interference in the beams reflected from the interfaces and constructive interference in the corresponding transmitted beams. This makes the structure’s performance change with wavelength and incident angle, so that color effects often appear at oblique angles. Figure 5 exhibits the experimental process of anti-reflection coating [68]. Fig. 5 Coating structure diagram of quadruple anti-reflection coating structure. Figure reprinted/adapted with permission from Ref. [69]

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Mechanism

Anti-reflection coatings are used to reduce reflection from surfaces. Whenever a ray of light moves from one medium to another (such as when light enters a sheet of glass after traveling through air), some portion of the light is reflected from the surface (known as the interface) between the two media [70]. A number of different effects are used to reduce reflection. The simplest is to use a thin layer of material at the interface, with an index of refraction between those of the two media. The reflection is minimized when n1 =



n0ns

(1)

where n 1 is the index of the thin layer, and n 0 and n s are the indices of the two media. The optimum refractive indices for multiple coating layers at angles of incidence other than 0° are given. Such coatings can reduce the reflection for ordinary glass from about 4% per surface to around 2%. These were the first type of anti-reflection coating known, having been discovered by Lord Rayleigh in 1886. He found that old, slightly tarnished pieces of glass transmitted more light than new, clean pieces due to this effect [71]. Practical anti-reflection coatings rely on an intermediate layer not only for its direct reduction of reflection coefficient, but also use the interference effect of a thin layer. If the layer’s thickness is controlled precisely such that it is exactly one-quarter of the wavelength of the light in the layer (a quarter-wave coating), the reflections from the front and back sides of the thin layer will destructively interfere and cancel each other. In practice, the performance of a simple one-layer interference coating is limited by the fact that the reflections only exactly cancel for one wavelength of light at one angle and by difficulties finding suitable materials. For ordinary glass (n ≈ 1.5), the optimum coating index is n ≈ 1.23. Few useful substances have the required refractive index. Magnesium fluoride (MgF2 ) is often used, since it is hard-wearing and can be easily applied to substrates using physical vapor deposition, even though its index is higher than desirable (n = 1.38). With such coatings, reflection as low as 1% can be achieved on common glass and better results can be obtained on higher index media [72]. Further reduction is possible by using multiple coating layers, designed such that reflections from the surfaces undergo maximum destructive interference. By using two or more layers, broadband anti-reflection coatings which cover the visible range (400–700 nm) with maximum reflectivities of less than 0.5% are commonly achievable. Reflection in narrower wavelength bands can be as low as 0.1%. Alternatively, a series of layers with small differences in refractive index can be used to create a broadband anti-reflective coating by means of a refractive index gradient. The antireflective coating on a solar cells helps to increase the amount of light absorbed into the cell [73]. From the previous conception of photovoltaic systems, the anti-reflection layers have been broadly studied. If there are no anti-reflection coatings, then it would result

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in unsuitable reduction into large refractive indexes of semiconductors (n nearly four). The largest efficiency of about 43.49% is calculated for photovoltaic cells presented by solar cells of multiple junctions. Multi-junction solar cells are generally affected through a spectral bandwidth as compared to silicon [74]. However, these solar cells are mostly demanding for semiconductors and exhibit anti-reflection coating designs. Collectively, these types of solar cells exhibit series connection. However, the anti-reflective coating (ARC) mechanism results into restricted current (photogenerated) in a specific area of the spectrum for the related sub-cell, so this results into decreased efficiency of the cell. The consideration of angular distribution is important to properly design the concentrator solar cell with anti-reflection coating [75]. According to the model of Henry, multi-junction solar cells exhibit particularly greater limiting efficiencies. In comparison to the single-junction solar cells, in multiple-junction solar cells, the conversion of the solar spectrum into electrical energy is obtained at very large scale. Because of the larger range of solar spectrum, this type of photovoltaic devices needs a very wide band anti-reflection coating [76]. The comparatively simple interference dual layer coatings are used to sufficiently obtain control of reflection for customary single-junction solar cells, for example, silicon, gallium arsenide, and indium phosphide, and also for double junction solar cells, for example, GaAs/AlGaAs or gallium arsenide/InGaP [77]. This is probable for these solar cells, because of the comparatively thin range of the solar spectrum. The range of solar spectrum is almost doubled, with the development of multijunction solar cells consisting of photovoltaic germanium substrates. Because of the requirement of current matching, multi-junction cells that are connected in series have higher demands for the performance of anti-reflection coatings. For this purpose, the reconsideration is required for the performance of anti-reflection coatings and also the information of technologies required for the performance of these solar cells is important [78]. To allow the greater conversion efficiencies in MJ solar cells, the factors that include are costly materials, complex in design, and contain extra steps in processing. The solar cells of these types are broadly used in space applications where cost of device contains one component only of the total cost. These types of devices have been enabling for worldly economical applications by integrated with concentrated photovoltaic (CPV) systems, these systems concentrate the big area of sunlight on the solar cell. CPV systems also give permission to the equivalent decrease in the area of solar cell and usually generate nearly 500X rise in intensity. So by increasing intensity, the efficiency of solar cell also increases which is the advantage also, until the internal resistance restricts the device [79]. The design of multi-junction solar cell exhibits different band gaps in three subcells which absorbs the solar spectrum of different parts. The sufficient efficiency is obtained in this type of solar cell by refining the lattice match and current match between the sub-cells. The design for anti-reflection coating in single-junction solar cells is developing at large scale and widely studied in the literature. However, appropriate design of anti-reflection coating for multi-junction solar cell is extra challenging as compared to single-junction solar cells, because of some reasons. For example,

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firstly, as compared to single-junction solar cells, MJ solar cells can change more solar spectrum bandwidth into electrical energy. So the more wideband anti-reflection coating is required [80]. However, commonly the solar cell‘s bandwidth is probably increased, by adding the more multiple junctions in the cell. Now secondly, the AR coating design for MJ solar cells is challenging due to series connection between the sub-cells of multiplejunction solar cells by which the performance of anti-reflection coating requires for further demands [81]. The main objective of SJ solar cells is to provide the considerable amount of light possible for the device. While the main objective in case of MJ solar cells is to obtain matching current in relation to the light dispersion among all sub parts of solar cell. However, thirdly, multiple junctions with series inter-connection exhibit variations in the design of anti-reflecting coating, which corresponds to the rise in mismatch current [82].

4 Material Choice and Growth Photovoltaic effect occurred when the p–n junction solar cells of III–V type semiconductor materials transform the light rays into electrical energy. In 1954, the 6% efficient silicon-based solar cells were invented. Photovoltaic systems have been utilized in various applications such as electronic calculators, characteristics of remote locations, and in space operations to supply power [83]. The occurrence of large amount of power from the sun to earth exists every day. It is possible to calculate the power, because in photovoltaic device the area of device and the energy produced are directly proportional to each other. The one approach is occurred to estimate the percentage that can generate the use of energy in the country, by supposing the photovoltaic devices of 15% efficiency that executed with commercial tools. This research shows that the land area of Belgium is required 24% that is the least felicitous country, the land area of UK is required almost 11% which is the less felicitous country, while the land area of Australia is required 0.2% which is the most felicitous country [84]. The more research on photovoltaic solar cells has shown that photovoltaic panels can generate the power that is equivalent to the total usage of energy in UK. A number of benefits that are investigated by building photovoltaic solar cells like the efficient matching between use of power and power matching, land usage minimization, and the photovoltaic panels used in built structure are less costly. The building integration photovoltaic solar cells fabricate the Saturn modules used by Northumbria University to show the appearance of 40 KW power of Northumberland building. Each module is based on 36 silicon crystalline solar cells that are attached with each other [85]. These cells are attached in series to extend the output voltage. The silicon-based solar cells are the most progressed solar cells and have 1.1 electron-volt energy band gaps in polycrystalline and crystalline form. However, the need of purity for semiconductor grade is greater than grade of solar cell that carries the decrease of cost in feedstock and beneficial for micro-electronics industry [86].

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To cover the broader region of wavelength in range of 300–1800 nano-meters (nm), multiple-junction cells with various band gaps are stacked over each other in a cycle. Different factors can play an important role in selecting the material for solar cells such as band gap energies, impurity defects, lattice matching, thermal and electrical properties [87].

4.1 Metal Organic Chemical Vapor Deposition The triple p–n junction solar cells are used in worldly applications of concentrator photovoltaic solar cells as well as in space and are the solar cells of greatest efficiency. In commercial reactors of large scale, these cells are formed by utilizing metal organic chemical vapor deposition. By increasing the growth rate of metal organic chemical vapor, deposition can increase the throughput of reactor and decrease the process time in the triple-junction solar cells. The process time of triple-junction solar cells is largely directed by the growth of top cell (indium gallium phosphide) and middle cell (indium gallium arsenide). By metal organic chemical vapor deposition growth process, the higher growth processes are integrated and this also leads to the formation of cheaper triple-junction solar cells and decrease in the time of process nearly 20% [88]. In multi-junction solar cells, the method of metal organic chemical vapor deposition (MOCVD) can also be used in combination with molecular beam epitaxy (MBE). For example, highly efficient triple-junction solar cells of gallium indium phosphide, gallium arsenide, and gallium indium nitride arsenide are fabricated by these methods. By combining the MOCVD and MBE methods, the combined advantages are obtained such as for III–V semiconductor materials rapid growth rate is obtained by metal organic chemical deposition process and dilute nitrides are grown by high quality of molecular beam epitaxy [89]. In this process, firstly, GaInNAs is put as a bottom cell and grown by molecular beam epitaxy then MOCVD process deposited the structure. Triple-junctions solar cells are displayed at 0AM and have nearly 29% conversion efficiency. This tends to the production of less costly and high-efficient concentrated multi-junction photovoltaic solar cells [90]. The progress of III–V compound semiconductor solar cells depends upon the improvement in the innovative growth techniques. These technologies include such as generally molecular epitaxy beam, MOCVD, and metal organic epitaxy vapor phase and related technologies [91]. To achieve multiple-junction cells with high conversion efficiency, highly accurate growth epitaxial technique is required. This corresponds to the small difference between lattice match of the nearby cell layers. The atomic level of semiconducting materials is uniquely controlled by growth technologies. So the growth of new devices is possible such as compositions of multiple alloys and pseudo-morphic alloys [92].

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Fig. 6 Schematic diagram of GaAs based on Schottky junction photovoltaic cells. Figure reprinted/adapted with permission from Ref. [96]

4.2 Gallium Arsenide Solar Cells In photovoltaic solar systems, gallium arsenide (GaAs) is the most frequently utilized III–V semiconductor material. GaAs solar cells have efficient growth techniques such as electron mobility as well as band gap [93]. The present investigation on single p–n junction devices shows that gallium arsenide is almost 30% efficient. The researchers at IRDEP observed the perfectly gained photoluminescence samples of gallium arsenide photovoltaic solar cells from the ISE institute of Fraunhofer. In this process, 532 nm ray was passed to irradiate the whole field of vision from microscope and collected the photoluminescence signals from various points. The hyper-spectral images were obtained, and the dimensions of these images can be increased a couple of square meters [94]. The p–n junctions of GaAs were manufactured by molecular beam that is same as substrate. The voltage–current properties were calculated under various conditions such as in the dark and contain 1.5AM. It has 4.5% total efficiency. The emission peak of electroluminescence calculated is about 1–4 electron-volt so these results correspond in the photovoltaic applications of nanowires. Figure 6 shows Schottky junction solar cell based on gallium arsenide [95]. In GaAs solar cells, the large amount of photons are absorbed in the device at first 5µm. The thickness of gallium arsenide is more than 5 µm that gives a little bit more output of power from device. The photons of energies are greatly absorbed by gallium arsenide, because it is a semiconductor of direct band gap [97].

4.3 Gallium Indium Phosphide Solar Cells In the industry of photovoltaic devices, the main objective is to obtain the less costly solar cells. The efficiency of single junctions is reduced because the solar spectrum of these cells can absorb only a certain wavelength. In contrast, the solar spectrum of MJ solar cells is divided into smaller pieces, so they can absorb sunlight to the matched solar spectrum pieces. MJ solar cells exhibit alloys of III–V semiconductor materials [98]. These solar cells provide greater efficiencies than SJ solar cells. Because, with greater optical sensitivity and by ideal grouping of band gaps, the

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concentration of photons increases, which gives rise to large number of electron hole pairs and thus increases the device efficiency. Hence, many important organizations of research spent their money to design the projects of III–IV semiconductor MJ solar cells. The most prosperous sustainable root of energy is the sunlight, affecting the surface of earth each day, having intensity of almost 0.1 W/cm2 and in excess of 15,000 exajoules (EJ) [99]. This huge amount of energy is 10,000 times larger than the energy used by the world daily, that is, 1.3 electron-joules. For the network of electricity, the electricity produced by the photovoltaic system is still unsuitable. This restriction has solutions such as increasing the efficiency of multiple-junction III–V semiconductor materials with perfect band gaps choice and greater optical sensitivity, so the total absorption of solar cell increases [100]. In solar cells of single junction, the replacement between the voltage and current cannot give the greater conversion efficiency. Higher current is obtained by solar cell which generates by using semiconductor of one material with small band gap. But in such type of materials the light absorption is good in visible spectrum region, but the production of open circuit high voltage is limited [101]. In small band gap materials, the voltage of open circuit is restricted by higher dark currents. This problem has a solution, by utilizing multiple band gap system having upper layer with larger band gap, and solar spectral region is split to the sensitivity of matched spectrum. The MJ solar cells were established and generated by utilizing III–V semiconductor materials on silicon substrates and having interlayers of silicon germanium. Without adjusting the design of solar cell, the open-circuit voltage greater than 2 V is obtained to increase the performance of double junction [102]. The positions and lattice parameters of upper and lower cells should be carefully matched in the material to decrease the mismatch dislocations in lattice-matched solar cells. It is significant to study the connection between lattice material performance and band gap to obtain higher performance. The maximum efficiency can be achieved by energies of ideal band gap in lattice-matched solar cells. Although in III–IV semiconductor compounds, there is no substrate material with lattice-matched lower cell, which is cost-effective and available at commercial scale. The three-junction lattice-matched system includes examples such as GaAs/InGaP/germanium and twojunction lattice-matched materials are GaAs/AlGaAs, InGaAs/InP, and GaAs/InGaP [103]. Sharp organizations have obtained the highest conversion efficiency of solar cell which is 36.8% approximately by utilizing triple-junction solar cell compounds. This structure of cell exhibits three stacked layers. The solar cell compounds using layers of photo-absorption is prepared from compounds containing two or more components such as gallium and indium. These solar cell compounds are used in space satellites due to their higher conversion efficiencies [104].

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4.4 Germanium Solar Cells In space for potential characteristics, the multi p–n junction solar cells that stacked mechanically have been growing at IMEC. In these mechanically layered solar cells, the germanium solar cells are being used in hybrid lighting devices as stand-alone solar cells and developed in mechanical stack cells as a bottom cell. The general spectrum is applicable in germanium solar cells for absorbance, so these solar cells can also be used in thermo-photovoltaic systems. The use of less costly method and substrates is necessary for cheaper and efficient thermo-photovoltaic system. The use of germanium solar cells is important in these systems because of its low cost and low band gap [105]. In the past decades, the remarkable researches have been started on photovoltaic germanium solar cells. In initial researches, the monolithically layered multi-junction solar cells have been studied, where the junction of germanium is achieved by diffusion of arsenide by layer produced by metal organic chemical vapor deposition from gallium arsenide. The single-junction solar cells have been investigated recently in various researches at spectrum 1.5AM with efficiency 6% and at concentration of light presented with 13% efficiency [106]. The surface passivation is a valuable characteristic of single-junction solar cells with greater efficiency. The germanium oxide attracts the water that it is a hygroscopic, because of this reason it devalued the electrical characteristics and became the difficult element in past. The materials such as amorphous silicon, silicon dioxide, silicon nitride, and gallium arsenide have been investigated for the characteristics of surface passivation. The surface passivation characteristics of these materials were not contained much data at that time. It is significant to use materials that can accumulate by less cost in future [107]. The schematic Fig. 7 shows amorphous intrinsic silicon germanium (i-a-SiGe:H) Fig. 7 Schematic diagram of MJ solar cells with silicon germanium cell. Figure reprinted/adapted with permission from Ref. [108]

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films with U-type coated glass substrate for amorphous silicon germanium (aSiGe:H) multijunction solar cells. The performance of photovoltaic cell is depreciated, through the production of misfit dislocations in dense gallium arsenide and lattice mismatch of 0.079% between germanium and gallium arsenide [109]. For the better performance of solar cell with germanium substrate, all layers of cell can be exactly lattice-matched by almost 0.99% addition of indium in GaAs/GaInP layers of solar cell. Through this addition of indium, the lattice mismatch and dislocations disappeared as presented by above figure. The factors including open-circuit voltage and short-circuit current were improved by addition of almost 0.99% indium. In addition, longer wavelength of absorption edge is also obtained [110].

5 Future Consideration for Developing Advanced Multi-junction Solar Cells Multi-junction III–V compound tandem semiconductor solar cells have capability for obtaining higher conversion efficiencies, that is, in excess of 40%. These solar cells are also capable for terrestrial and space applications. The above schematic plot is the example of chronological progress in space solar cells target with conversion efficiency of 0AM. Figure 8 shows the significance and development of multi-junction solar cells with greater efficiency. As compared to single-junction silicon solar cells, the MJ solar cells with two and three junctions such as InGaP/gallium arsenide/germanium solar cells have been established newly in space applications due to their conversion higher efficiencies and higher resistance radiation [112]. The R&D project of superb high-efficient solar cell has been developed in Japan in 1990 fiscal year; this project was the long-term goal to the initial twenty-first century. The progress in solar cells technologies with super high efficiency is carried out and determined in this project, and the main objective is to achieve 40% conversion efficiency and generating innovative technologies [113]. MJ solar cell technology manufacturing programs include some struggles on this type of solar cells for the use in space into profitable production through Spectrolab and TEC-STAR. In August 1997, the commercial production of satellite was launched based on two-junction solar cells of InGaP/GaAs on germanium solar cell substrate. However, indium gallium phosphide single junction and GaInP/gallium arsenide multiple-junction cells were originated to become radiation resistant as compared to silicon and gallium arsenide SJ cells. Thus, MJ solar cells are broadly used in space applications [109]. For the use of solar cells at large scale with higher efficiency, it is important to make them cost-effective and develop their conversion efficiencies. The above schematic plot shows the estimated accurately and theoretically single- and multijunction solar cell’s conversion efficiencies versus experimentally measured conversion efficiencies. Thus, concentrator photovoltaic three- and four-junction solar cells have greater capability to achieve 40% super high conversion efficiency [114]. The

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Fig. 8 Conversion efficiencies plotted for different types of solar cells per year. Figure reprinted/adapted with permission from Ref. [111]

already established system of InGaP/GaInAs/Ge solar cell on a germanium cell substrate three-junction arrangement will become broadly used system. There is also a system with four-junction arrangement which theoretically exhibits 42% conversion efficiency under condition of one-sun 0AM. This system exhibits arrangement of germanium substrate as a bottom cell, the upper cell with energy band gap E g is equal to 2.0 eV, the second layer of solar cell is gallium arsenide, and third layer of cell exhibits energy band gap of 1.05 eV. Under condition of 1.5AM five hundred suns, this system has the capability to generate in excess of 52% conversion efficiency [115]. The solar concentrator cells are looking to correspond to reduce the electricity cost in the future. From one of the future consideration applications, there also exist three-junction solar cells on metal or silicon substrates. For terrestrial uses, the concentration process of multiple-junction cells is important. The CPV systems have also ability to reduce the cost. Thus, for general photovoltaic applications and costeffective production of electricity, crystalline silicon and multi-junction concentrator solar cells are probably used [116].

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Acknowledgements This work was supported by the Higher Education Commission (HEC) of Pakistan, [Grant No: 8615/Punjab/NRPU/R&D/HEC/2017] and the National Centre for Physics (NCP), Islamabad, Pakistan, for providing Associate Membership to Dr. Khuram Ali.

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