Inorganic Flexible Optoelectronics Materials and Applications [1. Auflage] 9783527812967, 3527812962, 9783527812981, 3527812989, 9783527343959, 9783527813001

Citation preview

Inorganic Flexible Optoelectronics

Inorganic Flexible Optoelectronics Materials and Applications

Edited by Zhenqiang Ma and Dong Liu

Editors

University of Wisconsin-Madison Department of Electrical and Computer Engineering 1415 Engineering Drive Madison, WI 53706 United States

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Dr. Dong Liu

Library of Congress Card No.: applied for

Prof. Zhenqiang Ma

University of Wisconsin-Madison Department of Electrical and Computer Engineering 1415 Engineering Drive Madison, WI 53706 United States Cover Image: © Creative Commons,

© Oleksandra Korobova/Getty Images The image has previously been published in Zhang et al. Nature Communications volume 8, Article number: 1782 (2017).

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-34395-9 ePDF ISBN: 978-3-527-81296-7 ePub ISBN: 978-3-527-81298-1 oBook ISBN: 978-3-527-81300-1 Typesetting SPi Global, Chennai, India Printing and Binding

Printed on acid-free paper 10 9 8 7 6 5 4 3 2 1

v

Contents Preface xi 1

Flexible Inorganic Light Emitting Diodes Enabled by New Materials and Designs, With Examples of Their Use in Neuroscience Research 1 Hao Zhang, Philipp Gutruf, and John A. Rogers

1.1 1.2 1.3 1.4 1.5 1.6

Introduction 1 Flexible Micro-Inorganic LEDs (μ-ILEDs) 2 Flexible Quantum Dot LEDs (QLEDs) 7 Flexible Perovskite LEDs (PeLEDs) 16 Flexible 2D Materials-Based LEDs 20 Opportunities for Flexible Optoelectronic Systems in Neuroscience Research 24 Miniaturized Flexible LEDs and Detectors for Injectable Neural Probes 25 Wireless, Flexible Optoelectronic Systems for Genetically Modified Recording and Stimulation 28 Wireless, Battery-Free Optogenetic Stimulation Devices for Use in the Peripheral Nervous System 32 Conclusion 33 References 35

1.6.1 1.6.2 1.6.3 1.7

2

Flexible Light-Emitting Diodes Based on Inorganic Semiconductor Nanostructures: From Thin Films to Nanowires 41 Nan Guan and Maria Tchernycheva

2.1 2.2 2.2.1

Introduction 41 Flexible LEDs Based on Thin-Film Transfer 43 Conventional Approaches for Lift-Off and Transfer of Thin Crystalline Films 43 Thin Film Mechanical Transfer Using van der Waals Epitaxy on 2D Materials 47 Nanowire LEDs and Their Potential Advantages 50 Flexible LEDs Based on Inorganic Bottom-Up Nanowires 55

2.2.2 2.3 2.4

vi

Contents

2.4.1 2.4.1.1 2.4.1.2 2.4.2 2.4.3 2.4.4 2.5

LEDs Using a Direct Nanowire Growth on Flexible Substrates 55 ZnO Nanowire-Based Flexible LEDs 55 Nitride Flexible LEDs on Metal Foils 57 In-plane Transferred Nanowire LEDs 59 Vertically Transferred Nanowire LEDs 60 Novel Approaches for Nanostructure Lift-Off Using van der Waals Epitaxy 65 Conclusions 68 References 70

3

Flexible Photodetectors with Nanomembranes and Nanowires 79 Munho Kim, Jeongpil Park, Weidong Zhou, and Zhenqiang Ma

3.1 3.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.4 3.4.1 3.4.2 3.5 3.6 3.6.1 3.6.2 3.6.3 3.7 3.7.1 3.7.2 3.8

Introduction 79 Flexible Photodetectors 82 Performance Parameters 82 Responsivity 82 Detectivity 83 Photoconductive Gain (G) 84 Sensitivity (S) 84 Response Time 84 I on /I off Current Ratio 84 Fabrication of Donor Substrates for Transferrable NMs 84 Smart-Cut Technique 85 Epitaxial Growth Technique 86 Transfer Printing of Single Crystalline Semiconductor NMs 86 Semiconductor NM-Based Flexible Photodetectors 88 Si NM-Based Flexible Photodetectors 88 Ge NM-Based Flexible Photodetectors 94 III–V NM-Based Flexible Photodetectors 98 Fabrication of NW-Based Flexible Detectors 100 Synthesis of Single Crystal Si NWs 100 Assembly of NW-TFTs 101 Fabrication of Flexible Photodetectors Based on AgNWs/CdS NWs 101 Results and Discussion 103 I–V Curve Under Different Incident Light Densities 103 I–T Plot Under ON/OFF Switching of Light Source 104 Bending Performance 105 Response and Recovery Time 106 I–V Measurement Under Light and Dark Conditions 106 Sensitivity 108 UV Absorption of CdSe, P3HT, and Hybrid Photodetector 109 Responsivity 109 Conclusions and Outlook 111 References 112

3.9 3.9.1 3.9.2 3.9.3 3.9.4 3.9.5 3.9.6 3.9.7 3.9.8 3.10

®

Contents

4

2-D Material-Based Photodetectors on Flexible Substrates 117 Qin Lu, Wei Liu, and Xiaomu Wang

4.1 4.2 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.5

Introduction 117 Performance Metrics of Photodetectors 118 Working Mechanisms of 2D Photodetectors 120 Photovoltaic Effect 121 Photo-thermoelectric Effect 121 Bolometer Effect 123 Plasma-wave-Assisted Terahertz Detection 123 Photogating Effect 124 2D Photodetectors on Flexible Substrates 125 Photovoltaic Effect 125 Photothermal Effect 129 Plasma-wave-Assisted THz Detector 129 Photogating Detectors 135 Outlook and Perspectives 135 References 136

5

IV Group Materials-Based Solar Cells and Their Flexible Photovoltaic Technologies 143 Ying Chen, Ye Jiang, Yin Huang, and Xue Feng

5.1 5.2 5.2.1 5.2.1.1 5.2.1.2 5.2.2 5.2.3 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.2 5.4 5.4.1 5.4.2 5.4.3 5.5 5.6

Introduction 143 IV Group Materials-Based Solar Cells 144 Silicon-Based Solar Cells 144 Crystalline Silicon-Based Solar Cells 145 Amorphous Silicon-Based Solar Cells 149 Germanium-Based Solar Cells 151 Carbon-Based Solar Cells 153 Flexible Solar Cells Technology with Group IV Materials 155 Bottom-Up Method 155 Layer Transfer Method 156 SOI Method 156 Top-Down Method 157 Mechanics Analysis 161 Experimental Study on the Failure Modes 162 Theoretical Analysis of Shear Lag Model 163 Theoretical Mode Based on Fracture Mechanics 166 Applications 167 Conclusions 169 References 170

6

Thin-Film III–V Single Junction and Multijunction Solar Cells and Their Integration onto Heterogeneous Substrates 177 He Ding and Xing Sheng

6.1

Introduction 177

vii

viii

Contents

6.2 6.2.1 6.2.1.1 6.2.1.2 6.2.1.3 6.2.2 6.2.2.1 6.2.3 6.2.3.1 6.2.3.2 6.2.3.3 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.4 6.5 6.5.1 6.5.2 6.5.3 6.5.4 6.6

III–V Solar cells 178 Single Junction Solar Cells 178 GaAs 178 InP 179 InGaP 179 Double Junction Cells 180 InGaP/GaAs 180 Triple Junction Cells 182 InGaP/GaAs/Ge 182 InGaP/GaAs/InGaAs 182 InGaP/GaAs/InGaAsNSb 183 Thin-Film III–V Solar Cells on Flexible Substrates 183 Mechanical Spalling 184 Epitaxial Lift-Off 186 Mechanical Designs 190 Microcells with Luminescent Solar Concentrators 191 Applications 193 Future Generations 197 More Junctions 197 Mechanical Stack 197 Spectral Splitting 200 Photon Recycling 200 Conclusion 202 References 203

7

Novel Materials-Based Flexible Solar Cells 209 Dong Liu, Kwangeun Kim, Jisoo Kim, Jiarui Gong, Tzu-Hsuan Chang, and Zhenqiang Ma

7.1 7.1.1 7.1.2 7.1.2.1 7.1.2.2 7.1.2.3 7.1.3 7.1.3.1 7.1.3.2 7.1.4 7.1.5 7.2 7.2.1 7.2.2 7.2.2.1 7.2.2.2 7.2.3 7.2.3.1 7.2.3.2

Flexible Perovskites Solar Cells 209 Introduction 209 Preparation of Perovskites Materials 209 Solution Process Deposition Approaches 209 Vapor-Assisted Solution Deposition Approaches 211 Chemical Vapor Deposition Approaches 213 Flexible Perovskite Solar Cell 214 Sample Preparation 214 Performance Analysis 215 Stability Issues 221 Summary 223 Flexible CdTe/CdS Solar Cells 224 Introduction 224 Flexible CdTe/CdS Solar Cells on Metal Foil 224 Sample Preparation 224 Performance Analysis 225 Flexible CdTe/CdS Solar Cells on Polymer Film 227 Sample Preparation 227 Performance Analysis 228

Contents

7.2.4 7.2.4.1 7.2.4.2 7.2.5 7.2.5.1 7.2.5.2 7.2.6 7.3 7.3.1 7.3.2 7.3.3 7.3.4

Flexible CdTe/CdS Nanopillar Solar Cells 229 Sample Preparation 229 Performance Analysis 230 Flexible CdTe/CdS Solar Cells on Thin Glass 231 Sample Preparation 231 Performance Analysis 233 Outlook 237 Infrared Colloidal Quantum Dots Solar Cell 237 Introduction 237 Infrared PbS Quantum Dots Solar Cell 239 Surface Passivation and Air Stability 239 Conclusion 243 References 245 Index 255

ix

xi

Preface In conventional optoelectronics, inorganic semiconductors are employed as functional materials primarily due to their long-term stability under mechanical, electrical, and environmental stress as well as processing compatibility with existing infrastructures used for deposition, crystallization, doping, etc. However, the rigid form of the optoelectronic devices renders them difficult to be employed in many application situations. In contrast, mechanically flexible optoelectronic devices and systems can enable a much broader range of applications than what their rigid counterparts can do. Examples include rollable displays, deformable or implantable light sources for biological study and health care, skin sensors for a humanoid robot, energy harvesters embedded in clothing, flexible solar cells, etc. Organic materials can easily fulfill the mechanical flexibility requirement of flexible optoelectronic devices along with the additional advantages of low cost, compatibility with various flexible substrates, and large-scale manufacturing capability. However, organic materials suffer from oxidation, recrystallization, and temperature-induced degradation, which can jeopardize device performance in terms of electrical conductivity, interfaces quality, etc. This book focuses on flexible optoelectronics based on inorganic materials. The frontier developments and applications of flexible optoelectronics are comprehensively captured and elucidated within seven chapters, which cover all types of optoelectronic devices and all inorganic materials that have been used to develop the devices to date. The inorganic material forms include 3D (“bulk” and micros-sized “bulk” materials), quasi 2D (crystalline nanomembranes), 2D, 1D (nanowires), and 0D (quantum dots), ranging from Group IV to Group III–V, to II–VI, and to perovskites. The optoelectronic devices covered in this book include light emitting diodes (LED), photodetectors (PD), and photovoltaic devices. Some novel applications of flexible optoelectronic devices beyond their traditional ones (e.g. lighting, energy harvesting, and light detection), such as neuroscience research, are also included in the book. The book is intended as a comprehensive reference for advanced-level students and researchers with backgrounds in semiconductor materials and electronic devices, in particular, for those who are interested in flexible electronics and optoelectronics. Chapter 1, coauthored by Hao Zhang, Philipp Gutruf, and John A. Rogers, presents a comprehensive view of flexible LEDs made of various methods, including flexible micro-inorganic LEDs (μ-ILEDs), flexible quantum dot LEDs (QLEDs), flexible perovskite LEDs (PeLEDs),

xii

Preface

and flexible 2D materials-based LEDs. The emerging application research opportunities of flexible optoelectronic systems in neuroscience are also presented. Chapter 2, coauthored by Nan Guan and Maria Tchernycheva, is dedicated to nanowire-based flexible LEDs. Chapter 3, coauthored by Munho Kim, Jeongpil Park, Weidong Zhou, and Zhenqiang Ma, presents an overview of flexible photodetectors based on transferrable single crystalline semiconductor nanomembranes and nanowires, followed by Chapter 4, coauthored by Qin Lu, Wei Liu, and Xiaomu Wang, which is dedicated to 2D photodetectors. Chapter 5, coauthored by Ying Chen, Ye Jiang, Yin Huang, and Xue Feng, overviews the flexible photovoltaics employing Column IV materials. Flexible solar cells based on thin-film III–V materials are reviewed by He Ding and Xing Sheng in Chapter 6, where both single junction and multijunction solar cells are included. In Chapter 7, flexible solar cells employing perovskite materials and CdTe/CdS are reviewed by Dong Liu, Kwangeun Kim, Jisoo Kim, Jiarui Gong, Tzu-Hsuan Chang, and Zhenqiang Ma. We would like to thank all the contributing authors for their time and effort in preparing the contents of this book. Their pioneering and extensive work in the related fields has enabled the rapid developmental progress in the field witnessed in the last decade or so. We hope that this book can provide a handy reference to researchers during their continued explorations and further expansion of applications of this exciting field. We also hope that the book can be an inspiring introduction to those who are interested in entering this field, bringing with them new ideas. University of Wisconsin–Madison Madison, WI 53706, USA February 08, 2019

Zhenqiang (Jack) Ma Dong Liu

1

1 Flexible Inorganic Light Emitting Diodes Enabled by New Materials and Designs, With Examples of Their Use in Neuroscience Research Hao Zhang 1 , Philipp Gutruf 2 , and John A. Rogers 3 1 Northwestern University, Department of Materials Science and Engineering, 2145 Sheridan Road, Evanston, IL 60208, USA 2 University of Arizona Tucson, Department of Biomedical Engineering, 1230 N Cherry Ave., Tucson, AZ 85719, USA 3 Northwestern University, Simpson Querrey Institute for Nano/Biotechnology, Center for Bio-integrated Electronics, Departments of Materials Science and Engineering, Biomedical Engineering, Chemistry, Mechanical Engineering, Electrical Engineering and Computer Science, Civil and Environmental Engineering, 2145 Sheridan Road, Evanston, IL 60208, USA

1.1 Introduction Light emitting diodes (LEDs) represent essential components in nearly all solid-state lighting systems. Although conventional LEDs formed from epitaxial materials grown on rigid, brittle, and planar substrates are the most dominant technology, flexible LEDs continue to be of great interest, originating primarily from concepts in flexible, paper-like displays from the 1990s [1]. Flexible LEDs, as defined by their ability to be bent, twisted, and deformed in other ways, also serve as the basis for advanced optoelectronic technologies, ranging from next-generation displays in large or portable formats to wearable/implantable devices capable of intimate contact with soft, curvilinear bio-interfaces. Organic materials, including polymers and small molecules, are natural choices for flexible LEDs due to their favorable mechanical properties, their ability to provide multicolor light emission in ultrathin, lightweight films, and their low-temperature processability and associated compatibility with plastic substrates [1]. Challenges in performance degradation from photooxidation and other subtle effects and their limited color purity remain as key hurdles for organic light emitting diodes (OLEDs). By contrast, LEDs that exploit inorganic semiconductor materials as emissive layers outperform their organic counterparts in terms of brightness, lifetime, efficiency, and color purity. Recent progress in materials designs, fabrication concepts, and assembly approaches now enable high-performance, flexible classes of inorganic light emitting diodes (ILEDs). Integrating these ultrathin components with flexible electronics establishes the basis for system-level, advanced systems for deformable, high-brightness displays and for biomedical tools that provide diagnostic/therapeutic capabilities. The two main approaches toward flexible ILEDs use (i) microscale Inorganic Flexible Optoelectronics: Materials and Applications, First Edition. Edited by Zhenqiang Ma and Dong Liu. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

2

1 Flexible Inorganic Light Emitting Diodes

ILEDs fabricated from high-quality epitaxial materials grown on source wafers, subsequently released and assembled on flexible target substrates using the techniques of transfer printing, and bridged by structurally optimized interconnects (Section 1.2); and (ii) ILEDs formed with emissive layers composed of solution-processed semiconductors and/or low-dimensional nanomaterials. The former approach mainly relies on processing of well-established, high-performance III–V semiconductors with a novel set of techniques, while the latter deploys diverse classes of new materials, including colloidal semiconductor nanocrystals (or quantum dots [QDs], see Section 1.3), metal halide perovskites (Section 1.4), and two-dimensional (2D) materials (Section 1.5). This chapter summarizes the most recent advances and key remaining challenges associated with flexible ILEDs from both the materials and device perspectives. The focus is on their unique properties as candidates in flexible ILEDs and state-of-the-art devices design and performance. In addition, recent progress in integrating flexible ILEDs into system-level optoelectronic platforms for various applications highlights the current state of the field. The use of miniaturized, flexible ILEDs to optogenetically modulate neural activity (described in Section 1.6) represents one of the most recent cases.

1.2 Flexible Micro-Inorganic LEDs (𝛍-ILEDs) A combination of properties such as brightness, efficiency, color purity, and lifetime makes III–V semiconductor-based LEDs the most attractive candidates for solid-state lighting applications compared to almost all other options, including OLEDs [2, 3]. Existing techniques to incorporate commercial ILEDs into systems such as billboard-scale displays involve robotic, pick-and-place assembly of ILEDs diced from a wafer source, followed by device-by-device, (sub)millimeter-scale packaging, and interconnecting of these components with a collection of bulk wires and heat sinks [4]. These conceptually old techniques are ineffective for assembling ultrasmall (50 000 hours), and luminous efficiency (>200 lm/W) make InGaN LEDs one of the most widespread options in solid-state lighting applications [27]. Depending on the growth substrates, anisotropic wet etching [15] or laser lift-off [16, 17] can be used to separate lithographically defined, isolated μ-ILEDs from their growth wafers. Silicon is of particular interest as a growth substrate due to the availability of large, low-cost wafers and simple schemes for release based on anisotropic etching techniques. Specifically, the large difference (over 100 times) in etching rates of Si(110) compared to Si(111) in a hot potassium hydroxide bath [28, 29] allows for freely suspended, isolated μ-ILEDs where lithographically defined segments of InGaN serve as anchors (Figure 1.1f, inset in top panel) [15]. These approaches bypass the need for conventional laser lift-off techniques, thereby enabling high-throughput, parallel production of millions of devices in forms configured for transfer printing with micron-scale position accuracy. Additionally, the large bandgap of GaN enables a remarkably convenient means for metallization of interconnects, where backside exposure through the transparent active layers yields self-aligned traces without the need for photomasking. The ease in deterministic assembly of ultrathin, ultrasmall (100 μm × 100 μm) blue-emitting μ-ILEDs on target substrates and the straightforward methods for registration of electrical interconnects result in simple routes to large-area, flexible μ-ILED arrays (Figure 1.1f ) [15]. Laminating a thin layer of a down-converting phosphor embedded in PDMS and adding a diffuser film on top yield a uniform, white color emission over areas >100 times larger than that of a traditional LED die with the same amount of InGaN.

5

1 Flexible Inorganic Light Emitting Diodes

As an alternate substrate, sapphire is widely used to grow high-quality, state-of-the-art GaN epitaxial layers [17]. Here, separation of patterned μ-ILEDs from the sapphire requires a laser lift-off process. Via a dual transfer printing procedure, μ-ILEDs with dimensions as small as 25 μm × 25 μm can be made with the radiant efficiency up to ∼10% in this way. Transfer printing such μ-ILEDs on flexible substrates of polyethylene terephthalate (PET) yields systems with stable operation when strongly bent and twisted (Figure 1.1f, bottom panel) [17]. Moreover, introducing wireless powering components (e.g. a rectangular spiral inductor metal coil) leads to an integrated, implantable blue-emitting μ-ILED array with the ability to operate in a continuous or periodic mode, in a purely wireless manner [16]. This type of system served as the foundation for recent advances in implantable, wireless optogenetic tools for neuroscience, as described in Section 1.6. In the above cases, the assembly of μ-ILEDs relies on their release from a source wafer via selective etching or laser lift-off, followed by deterministic 100 %Increase in intensity

(c)

50

4.2%

2.8%

LED

2 mm

7.0%

Glucose 10 Water 0 Air

(a)

Glucose 7.0%

Deflate Inflate

Time (a.u.) Photocurrent (nA)

6

0.20

−10 V −5 V 0V

0.15 0.10

Electrode

0.05 0

0

4 8 Distance (mm)

12

Temperature sensor

μ-ILEDs

(b)

μ-IPDs 5 mm

Electrode

Figure 1.2 Flexible 𝜇-ILEDs: Examples of integrated devices in biomedical applications. (a) Measurement results from a representative refractive index microsensor. (Inset) The sensor integrates a μ-ILED array and molded plasmonic crystals and laminates directly on a flexible plastic tube with a sequence of glucose solutions passing through. Scale bar = 1 mm in inset. (b) Optical image of a proximity sensor with arrays of μ-ILEDs (4 × 6) and μ-IPDs, transfer printed on the fingertip region of a vinyl glove. The inset shows a plot of photocurrent as a function of distance between the sensor and an object (white filter paper) for different reverse biases and different voltages. Source: Reproduced with permission from Kim et al. [12]. Copyright 2010, Nature Publishing Group. (c) Optical image of a multifunctional balloon catheter in deflated and inflated states. The image shows arrays of temperature sensors (anterior), μ-ILEDs (posterior), and tactile sensors (facing downward). Source: Reproduced with permission from Kim et al. [13]. Copyright 2011, Nature Publishing Group.

1.3 Flexible Quantum Dot LEDs (QLEDs)

assembly via transfer printing. An alternative approach (Figure 1.1g) [22, 30] requires no sacrificial layers or transfer printing steps. Here, red-emitting AlGaInP epitaxial layers grown on GaAs wafers bond to a thin polyimide layer (PI, 25 μm, preprinted with bottom electrodes) via an anisotropic conductive film (ACF). Subsequent wet etching removes the entire GaAs wafer and exposes the epitaxial structures for further epoxy passivation and top electrode deposition. The resulting red-emitting, flexible vertical μ-ILEDs on PI substrate remain operational in the bent form (bending radius, 5 mm) (Figure 1.1h) [22]. A disadvantage of this approach is that it consumes the growth wafer, thereby preventing its reuse for additional cycles of growth. A key feature of these types of flexible μ-ILEDs is that they can conformally laminate onto curvilinear surfaces and find use in several unconventional applications, especially those in biomedical sensing, physiological monitoring, and clinical therapy. For example, a stretchable μ-ILED array integrated with molded plasmonic crystals and an external photodetector offers capabilities in the quantitative monitoring of changes in refractive indices of fluids that pass through tubing, which is of relevance for use in intravenous delivery systems for continuous monitoring of nutrient dosage (Figure 1.2a) [12]. In another example, a flexible device with both μ-ILEDs and micro-inorganic photodetectors (μ-IPDs) mounted on the fingertip of a glove serves as an optical proximity senor (Figure 1.2b) [12]. Encapsulation with biocompatible, PDMS layers renders these flexible μ-ILEDs waterproof and operational even when completely immersed in bio-fluids, allowing their use in implantable systems [12]. Flexible μ-ILEDs and a collections of other interconnected devices can be mounted on commercial balloon catheters, for the sensing of a variety of physiological parameters as well as electrically and/or thermally stimulating tissues at the bio-interface (Figure 1.2c) [13].

1.3 Flexible Quantum Dot LEDs (QLEDs) QDs are semiconductor nanocrystals with sizes in the quantum-confined regime (usually in the range of 2–20 nm) [31, 32]. The discrete, quantized energy levels lead to well-known size-dependent optical properties (Figure 1.3a) [33]. The most enabling features of QDs as LED emitters are their broad spectral tunability (from UV to near infrared [NIR]) and narrow emission peaks (full width at half maximum [FWHM] smaller than 30 nm). Additionally, as with organic materials, QDs in colloidal form can be well dispersed in solvents and are amenable to a series of low-cost, solution-based techniques to assemble into large-scale solid-state materials [34, 41]. These attributes, especially their exceptional color purity and extended color gamut, motivate research into electroluminescent (EL) quantum dot-based light emitting diodes (QLEDs) as alternatives to OLEDs, with initial work published more than two decades ago [42, 43]. On the other hand, applications that harness optically induced emission (or photoluminescence [PL]) of QDs in backlighting for liquid crystal displays or as downconverters in solid-state lighting sources have evolved into mass-produced consumer products, including the Samsung Quantum Dot TV and Amazon Kindle Fire HDX 7 Tablet [44].

7

2.5

520

3.1

560

3.4

590

4.0

610

5.2

640

6.3

CdSe/ZnS

(c)

0.8

sRGB Rec. 2020 Adobe RGB

0.6

y

480

PbS/CdS

(b)

Normalized photoluminescence

(a)

0.4

0.2

0.5

1.0

1.5 2.0 Energy (eV)

2.5

3.0

0.0 0.0

QLEDs OLEDs 0.2

0.4 x

0.6

0.8

Figure 1.3 Flexible QLEDs: Materials designs, fabrication techniques, and bendable QLEDs. (a) (Top) Schematic diagram of bandgap and (bottom) emission color as a function of size of CdSe QDs. Source: Reproduced with permission from Goesmann and Feldmann [33]. Copyright 2010, John Wiley & Sons. (b) Photoluminescence (PL) spectra of CdSe/ZnS and PbS/CdS core/shell colloidal QDs, demonstrating the size- and composition-dependent tunability of QD emission color. Source: Reproduced with permission from Shirasaki et al. [34]. Copyright 2013, Nature Publishing Group. (c) Representative RGB color spaces (solid lines) and chromaticity points of RGB QLEDs (squares) and cutting-edge OLED products (triangles) relative to the CIE 1931 chromaticity diagram. While the state-of-the-art OLEDs can only cover sRGB or Adobe RGB color space with the help of optical engineering, QLEDs easily meet the current standards and satisfy Rec. 2020. Source: Reproduced with permission from Pietryga et al. [35]. Copyright 2016, American Chemical Society. (d) Band engineering of QDs by forming heterostructures. Representative transmission electron microscopy (TEM) images and band structures of (Left) core/shell (continuously graded CdSe/Cdx Zn1−x Se/ZnSe0.5 S0.5 ) QDs. Scale bar in the TEM image: 10 nm. Source: Reproduced with permission from Lim et al. [36]. Copyright 2018, Nature Publishing Group. (Right) Representative TEM image and band structures of DHNRs. Scale bars in the TEM image and the inset: 50 and 5 nm. Source: Reproduced with permission from Oh et al. [37]. Copyright 2014, Nature Publishing Group. (e) Intaglio transfer printing for high-resolution RGB QLEDs. (Left) The PL image showing aligned RGB pixels (2460 ppi with the pixel size of 6 μm). (Right) The PL image of the RGB QD patterns via multiple aligned transfer printings. Source: https://creativecommons.org/licenses/by/4.0/, [38]. (f ) Composite fluorescence images of electrohydrodynamic jet (E-jet) printed dual-color QD patterns. Inset shows an optical microscope image of a metal-coated glass nozzle (5 μm inner diameter at the tip) and a target substrate during the E-jet printing. Source: Reproduced with permission from Kim et al. [39]. Copyright 2015, American Chemical Society. (g) A typical device structure and energy band diagram of flexible QLED using inorganic/organic hybrid charge transporting layers. (h) Flexible full color QLED with RGB pixels (inset) patterned by transfer printing onto polyethylene naphthalate substrate. Inset: optical image of simultaneous electroluminescence emission of RGB patterned QDs. Source: Reproduced with permission from Kim et al. [40]. Copyright 2011, Nature Publishing Group. (i) Optical image of the flexible white QLEDs made by Intaglio transfer printing under the bias. Bending radius is 1 cm. Source: https://creativecommons.org/licenses/by/4.0/, [38].

(e)

(d)

(f)

Fluoresc E-jet printed

10 μm

E-jet nozzle

1st QD line 2nd QD line

10 nm

3rd QD line

QD solution

Energy (eV)

ZnSe0.5 S0.5

CdSe

CdxZn1–xSe

5.0 2.4

6.0

1.75

7.0

5 mm

CdS ZnSe

CdSe

(g)

2460 ppi (6 μm)

2 cm

2.0 3.0

Energy (eV)

15 μm

4.0

4.0 5.0

(h)

E-jet printed green QD

Bending R = 1 cm

100 μm

(i)

2.3 3.4 4.1 3.9 PEDOT TFB 4.6 ITO

5.0

6.0

4.3 Al

QD 5.3 6.2

TiO2

7.0 8.0

Figure 1.3 (Continued)

7.8

300 μm

5 mm

10

1 Flexible Inorganic Light Emitting Diodes

Progress in QLEDs can be found in several recent reviews [34, 35, 45, 46]. This section begins with a brief introduction to the unique optical properties of QDs and focuses on the most recent advances of flexible QLEDs and their applications in high-resolution displays as well as wearable, skin-mounted devices. A wealth of well-established synthetic protocols (primarily wet chemistries) can yield QDs with tunable emission properties spanning the entire visible spectrum and NIR. A combination of QD sizes, compositions, heterostructures, as well as surface chemistry offers superior control of QD emission characteristics: (i) Spectral tunability. Cadmium selenide (CdSe) QDs with different sizes (2.5–6.3 nm, smaller than the Bohr radius of bulk CdSe) exhibit different colors as a result of size-dependent bandgaps (Figure 1.3a) [33, 47]. The spectral tunability also comes from changes in compositions/stoichiometries, as exemplified by CdSe- and PbS-based QDs (Figure 1.3b) [34]. Efficient emission in the NIR regime afforded by IV–VI [48, 49], III–V [50, 51], and other QDs provides a distinct advantage over organic-based fluorophores. (ii) Color purity. High-quality, monodisperse QDs (size distribution within 5%) exhibit narrow emission (FWHM < 30 nm compared to 40–60 nm in OLEDs) and thus a wider color gamut, meeting the Rec. 2020 standard for ultrahigh definition TV (Figure 1.3c) [34, 35, 45]. (iii) Brightness. As with conventional LEDs, the brightness or the radiant efficiency of QLEDs is closely related to EQE, which depends on the injection of charge carriers, light emission (quantified by quantum yield, defined as the ratio of radiative recombination rate to the sum of rates of radiative and nonradiative recombination), and light out-coupling. Advanced synthetic procedures enable the formation of heterostructures, such as core–shell QDs [36, 52, 53], double heterojunction nanorods (DHNRs) [37, 54, 55], and many others, in a precisely controlled manner at the nanometer scale (Figure 1.3d). Core–shell QDs with near unity quantum efficiency are the most widely used materials in QLEDs [56, 57]. More sophisticated, anisotropic DHNRs feature two larger bandgap semiconductors (CdS and ZnSe) with type II band offset surrounding and in contact with a smaller bandgap (CdSe) emitting center [37]. This material design allows independent control over the electron and hole processes, and more interestingly, increases the upper limit on light out-coupling due to the anisotropic optical properties [54]. Details of the underlying mechanism that correlates heterostructure designs to enhanced EQEs can be found in a recent review [35]. The optimized compositional, structural, and surface control of QDs yields QLEDs with high efficiencies and brightness on par with the state-of-the-art OLEDs (record EQE and brightness for QLEDs: red: 20.2% [58], 106 000 cd/m2 [59]; green: 14.5% [60], 218 800 cd/m2 [61]; blue: 10.7% [60], 7600 cd/m2 [62]). Fabrication of emissive QD layers in monochromic QLEDs typically exploits spin-casting processes. These same techniques are not, however, amenable to the fabrication of RGB pixelated, full color displays due to cross-contamination/redissolution that can arise during sequential steps in spin-casting. Transfer printing methods, similar to those described for μ-ILEDs, provide effective routes to pixelating QDs [38, 53, 63–65]. Here, an elastomeric stamp (typically PDMS) delivers a uniform QD film (either spin-cast on a stamp

1.3 Flexible Quantum Dot LEDs (QLEDs)

[65] or peeled off from a donor substrate [40, 63]) to a target substrate in a deterministic, parallel manner. In one example, this solvent-free method enables placement of RGB pixels (46 μm × 96 μm) across a 4-in. display with 320 × 240 pixels (corresponding to 100 pixels per inch, or 100 ppi) [40]. The stamps can lead to discrepancies between the designed and printed QD patterns, especially for high-resolution geometries and small pixel sizes (e.g. below 35 μm) [38]. An alternative form of transfer printing process addresses this limitation with the use of an intaglio trench that allows high-resolution (2460 ppi), full color displays with uniform, ultrasmall RGB pixels (as small as 5 μm, Figure 1.3e) [38]. The high fidelity follows from the gentle contact between the QD thin film on the stamp and the intaglio trench, and subsequent slow delamination. During this process, cracks occur at the sharp edges of the trenches and only the noncontacting part of the QD layer (with sharp edges) remains on the stamp. The printing yields approach ∼100%, independent of pixel sizes. In addition to transfer printing of the QD layer, schemes now exist for transfer printing of multilayer assemblies (e.g. QD emitting layer/electron transport layer/cathode layer) from a donor substrate to a receiver substrate pre-coated with hole transport layer and anodes, all enabled by the use of a sacrificial fluoropolymer coating [64]. The most enabling feature of this type of multilayer transfer printing is the ability to independently tailor band alignments between the charge transporting layers and QD layers that emit at different wavelengths. As an example, green QDs/TiO2 and red QDs/ZnO pixels can be sequentially assembled on the same substrate for optimized device performance. Additionally, inkjet printing [39] or 3D printing [66] also provides useful routes to patterning QDs with elaborate designs. For example, inkjet printing enables sequential printing of QDs with different colors in programmable patterns with uniform thicknesses and ultrasmall pixel sizes (5 μm), in a fully automatic manner (Figure 1.3f ) [39]. The high performance of QLEDs originates from rational materials design and advanced techniques to assemble ultrathin, high-definition QD emitting pixels. The resulting capabilities serve as the basis for recent advances in flexible QLEDs as next-generation displays as well as their integration with other flexible electronic components for skin-mounted and bio-interfaced applications. An optimized flexible QLED device structure (Figure 1.3g) typically includes a QD emitting layer sandwiched between two hybrid charge transport layers (an inorganic ZnO or TiO2 electron transport layer and an organic hole transport layer such as poly[(9,9-dioctylfluorenyl2,7-diyl)-co-(4,4′ -(N-(4-sec-butylphenyl))diphenylamine)], or TFB) [38, 40, 63, 67–69]. This device structure utilizes solution-based deposition of inorganic TiO2 or ZnO sol–gel nanoparticles at temperatures that are compatible with flexible plastic substrates, in a way that leads to balanced electron/hole injection rates. The first demonstration of flexible QLEDs involved transfer printed RGB pixels on a polyethylene naphthalate (PEN) substrate (Figure 1.3h) [40], with no appreciable changes in luminous efficiency or current–voltage characteristics at a bending radius of 3 cm. Flexible white QLEDs with mixed QD active layers [70] or sequentially stacked RGB layers [63] require sophisticated control of the ratios of QDs of different colors and suffer from low efficiencies due to the inevitable energy transfer between different QDs. Pixelated white QLEDs enabled by intaglio transfer printing circumvent these issues to allow for excellent device

11

12

1 Flexible Inorganic Light Emitting Diodes

performance (true white emission with a maximum EQE of ∼1.5%) and stable operation at different bending angles (up to 135∘ ) (Figure 1.3i) [38]. In early examples, flexible QLEDs were typically fabricated on plastic substrates with relatively large thickness (e.g. in the range of hundreds of micrometers for PET [71]), limiting their minimum bending radius to several tens of millimeters [45]. The use of thin tapes of polyimide (Kapton) allows for highly flexible and mechanically robust QLEDs, capable of mounting on and removal from the curved surfaces of many objects [72]. The high efficiency (EQE up to ∼4%) and brightness (over 20 000 cd/m2 ) largely remain (over 90% of the original brightness) after bending onto a 4 mm diameter rod for 300 cycles. In another example, a double layer composed of parylene and epoxy serves as an ultrathin (∼1.1 μm) substrate that is also biocompatible and waterproof [38]. The ultrathin form factor of QLEDs (Figure 1.4a, in total ∼2.6 μm) enables various deformations (bending, folding, or crumpling) and conformal integration on human skin as wearable tattoo-like devices. The EQE and brightness (EQE ∼ 2.35% at 4.5 V and brightness ∼14 000 cd/m2 at 7 V, Figure 1.4b) are among the highest of reported wearable LEDs and remain stable after 1000 cycles of uniaxial stretching to strains of 20% [38]. Device structure engineering and heterogeneous QD designs further improve the EL performance of flexible QLEDs. Introducing an interfacial layer of polyethylenimine ethoxylated (PEIE) between the green QD emitting layer and hole transport layer in an inverted architecture upshifts the valance band maximum of QDs and favors hole injection, leading to record-high EQE (15.6%) and current efficiency (65.3 cd/A) on glass substrates (Figure 1.4c,d) [71]. Flexible QLEDs on PET using a similar device structure also show a maximum EQE of 8.4% and a current efficiency of 35.1 cd/A, both of which are the highest values reported for flexible QLEDs [71]. On the other hand, green-emitting QDs with relatively thick shells (2 nm thicker compared to conventional core/shell QDs) show drastically suppressed nonradiative Auger recombination, leading to flexible QLEDs with the highest reported brightness (44 719 cd/m2 at 9 V, Figure 1.4e) [69]. The exceptional electronic and mechanical properties of flexible QLEDs facilitate their integration with other emerging flexible electronic platforms into system-level, skin-mounted devices [68, 69, 73, 74]. One representative example is a smart sensor system capable of monitoring and storing information related to pressure, temperature, and movements, and displaying them in QLED arrays (Figure 1.4f ) [69]. Flexible red-emitting QLEDs can also operate as light sources in wearable optical sensors for photoplethysmography (PPG) [73]. A single-step process of transfer printing of the multilayers (Al/TiO2 /QD/TFB/graphene/PEN) to a prestrained PDMS substrate, followed by buckling process, results in highly stretchable QLEDs. These devices show no degradation in performance when stretched at 70% strain or folded to a 35 μm bending radius of curvature. Together with an array of PbS QD-based photodetectors, the integrated device can be wrapped around a fingertip to provide in situ monitoring of PPG pulses (Figure 1.4g) [73]. In a broader context, a light-responsive QLED represents an important, recent advance in this field [55]. The ability to combine both efficient photocurrent generation and high electroluminescence within a single system follows from the unique band diagrams in heterogeneous DHNRs (Figure 1.5a). DHNRs contain

ITO

ZnO NPs PEDOT:PSS and TFB

Parylene and epoxy

(a)

2.6 μm

Encap.

30

1E4 1E3

20

1E2

10 1E1 0

1E0 0

2 4 6 Voltage (V)

Brightness (cd/m2)

QD

Current density (mA/cm2)

Li/Al

Parylene and epoxy

1E3 1E2

20%

1E1

5 mm

5 mm

0 0

1E1

1E2

1E3

Cycle number

(b)

Figure 1.4 Flexible QLEDs: State-of-the-art devices and their applications in integrated, wearable systems. (a) Exploded view of the ultrathin, tattoo-like wearable QLED. Inset shows a cross-sectional SEM image in which the thicknesses of the encapsulation and active layers are shown. (b) (Left) The current density–voltage–luminance (J–V–L) characteristics of the ultrathin, wearable QLEDs shown in (a). (Right) Stable brightness in multiple stretching experiments (20%, 1000 times). The inset shows photographs of buckled and stretched ultrathin red QLEDs. Source: https://creativecommons.org/licenses/by/4.0/, [38]. (c) Cross-sectional TEM and stacking sequence of a highly efficient, inverted QLED device with a polyethylenimine ethoxylated (PEIE) interlayer. Top right shows an operating QLED in highly bent state. (d) Current efficiency–EQE–luminance characteristics of inverted QLEDs without and with 15.5 nm thick PEIE interlayer. Source: Reproduced with permission from Yang et al. [72]. Copyright 2014, American Chemical Society. (e) (Left) Photographs of an ultrathin QLED display based on CdSe/ZnS core/shell QDs with thick shell, in rolled condition and mounted on skin. (Right) J–V–L characteristics of the device. (f ) Photographs of (Left) the touch sensor integrated with the ultrathin QLED display and (Right) the integrated wearable system subjected to external heat. Source: Reproduced with permission from Kim et al. [69]. Copyright 2017, John Wiley & Sons. (g) (Left) Skin-mounted photoplethysmographic (PPG) sensor composed of QLEDs during LED operation at 8.4 V and QD photodetectors wrapped around the finger of a subject. (Right) Real-time PPG signal pulse wave measured by a stretchable QD photodetector using the stretchable QLED or an indium tin oxide (ITO)-based rigid QLED as a light source. Source: Reproduced with permission from Kim et al. [73]. Copyright 2017, American Chemical Society.

10−1 100

4 2 103 10 101 10 Luminance (cd/m2)

105

Touch sensor

Figure 1.4 (Continued)

2

Current density (A/cm )

Current efficiency (cd/A)

103

0.6

102 0.3

101 2

4 6 Voltage (V)

8

100 10

LED on the back Rigid LED Stretchable LED

1.0

PD on the front

0.8 t2

0.6 P1

0.4

P2

0.2

Heat

t1

0

1 cm

(f)

104

0.9

0

Amplitude (a.u.)

Csensed

R RC delay measure

105

(e)

Temperature increase

MCU

1.2

106

L–V C/SConventional L–V C/SThick

0

10−4 106

(d)

Touch sensor

5.2 V Vpp

10

−8

10

5 mm

−2

10

100 nm

(c)

100

−6

I–V I–V

2

ITO

102

10−4

1.5

Rollable

106 104

100

10−2

Al MoOx Poly-TPD PEIE QDs ZnO NPs ITO

108

Luminescence (cd/m )

MoOx PEIE/Poly-TPD QDs ZnO NPs

no PEIE 15.5 nm PEIE

102

EQE (%)

104

Al

0

(g)

0.2

0.4 0.6 Time (s)

0.8

1.0

[DHNR] e

e ZnO

TFB:F4TCNQ

PD mode: reverse ITO bias

PEDOT :PSS

1.3 Flexible Quantum Dot LEDs (QLEDs)

Light detection

Al Se

Light emission

h

Cd

h

Iinj Iext

S

LED mode: (a) forward bias

Cd

Control circuit

1.2 Pulse: 3.5 V, 100 μs

Photocurrent (a.u.)

EL intensity (a.u.)

2 nm

(b)

100

10−1

10−2

τDecay < 0.8 μs

10−3

0 (c)

Se

Zn

50

Light on

1.0 0.8

6.5 μs

0.6 6.6 μs

0.4 0.2 0.0 −20

100 150 200 250 Time (μs)

0

(d)

20 40 Time (μs)

60

80

Encapsulation Anisotropic conductive film Al electrode ZnO

Laser pointer

DHNRs

TFB:F4TCNQ PEDOT:PSS Cr/Au electrode ITO electrode Glass substrate

(e)

(f)

Figure 1.5 Light responsive, dual-functional DHNR LEDs. (a) Energy band diagram of DHNR-LED along with directions of charge flow for light emissive (orange) and detection (blue) and a schematic of a DHNR. (b) A high magnification scanning transmission electron microscopy (STEM) images of DHNRs. (c,d) Transient EL showing decay time and photocurrent in response to illumination by a blue LED source driven by 3 V, 50 μs square-wave voltage pulses. (e) Schematic of a 10 × 10 DHNR-LED array. (f ) Photographs of a light-responsive LED array with a laser pointer illuminating and turning on pixels along the path outlined by the orange arrows. (g) Automatic brightness control at the single-pixel level in response to an approaching white LED bulb (Left) or a finger (Right) that blocks ambient light. Source: Reproduced with permission from Oh et al. [55]. Copyright 2017, The American Association of Advancement of Science.

15

16

1 Flexible Inorganic Light Emitting Diodes

(g)

Auto brightness control

Touchless user interface

External light

Figure 1.5 (Continued)

type-I heterojunctions between the CdSe QDs and two surrounding materials (CdS and ZnSe), which also form type-II offsets by themselves (Figure 1.5b). This band diagram (Figure 1.5a) allows separate control of injection of electron and holes and the more advanced, switchablility between light-emitting and light-detection modes by forward or reverse bias. Figure 1.5c,d demonstrates the temporal response of the dual-functioning DHNR-LEDs. Both EL characteristics (e.g. EQE of 8.0% at 1000 cd/m2 under 2.5 V bias) and photoresponsivity (e.g. 200 mA/W) of this dual-functional device compare favorably to state-of-the art QLEDs and commercial silicon photodetectors. A multilayered, 10 × 10 pixel device (Figure 1.5e) programmed by a circuit board demonstrates the “writing” action in response to laser excitation (Figure 1.5f ). The switchability of the dual modes and the fast response enable their use in touchless displays with automatic brightness control (Figure 1.5g) as well as direct display-to-display data communication systems. Although the reported DHNR-LEDs are fabricated on a glass substrate, the same material/device designs can be extended to flexible light-responsive LEDs.

1.4 Flexible Perovskite LEDs (PeLEDs) Metal halide perovskites (ABX3 , where A is an alkali metal or organic cation, B is typically Pb or other group IV cations, X are halide anions or their mixtures) have attracted tremendous attention in the last several years, primarily due to their rapidly increasing photovoltaic (PV) power conversion efficiencies, from 3) junctions that are difficult to grow monolithically and realize both lattice matching and current matching. Emerging concepts such as spectral splitting, luminescent concentrators, and photon recycling and the corresponding fabrication and integration processes are proposed to further enhance the photon conversion efficiencies and approach theoretical limits. Because of their excellent performances and recently developed fabrication schemes, III–V based thin-film solar cells could potentially provide a viable solution to cost-effective electricity generation. By taking advantage of their high efficiencies, light weight and convenient uses, thin-film microscale III–V solar cells

References

and arrays on flexible substrates will serve as a high-performance remote power supply for our daily uses. Cell modules can be integrated with costumes, automobiles, buildings, aircraft, satellites, etc. On the other hand, the microscale PV cells will find applications in miniaturized systems, for example, being integrated with integrated circuits comprising batteries, sensors, transistors, and actuators for multifunctional sensing, diagnosis, and therapy in biomedicine.

References 1 Green, M.A., Hishikawa, Y., Warta, W. et al. (2017). Progress in Photo-

voltaics: Research and Applications 25: 668–676. 2 Nowak, S. (2016). Trends 2016 in Photovoltaic Applications: Survey Report

of Selected IEA Countries Between 1992 and 2015. Report IEA-PVPS. 3 Powell, D.M., Fu, R., Horowitz, K. et al. (2015). Energy & Environmental Sci-

ence 8: 3395–3408. 4 Singh, G.K. (2013). Energy 53: 1–13. 5 Shahrjerdi, D., Bedell, S.W., Bayram, C. et al. (2013). Advanced Energy Mate-

rials 3: 566–571. 6 Pagliaro, M., Ciriminna, R., and Palmisano, G. (2008). ChemSusChem 1:

880–891. 7 Sheng, X., Corcoran, C.J., He, J.W. et al. (2013). Physical Chemistry Chemical

Physics 15: 20434–20437. 8 Kayes, B.M., Zhang, L., Ding, I.K., and Higashi, G.S. (2014). IEEE Journal of

Photovoltaics 4: 729–733. 9 Rogers, J.A., Someya, T., and Huang, Y.G. (2010). Science 327: 1603–1607. 10 Cheng, C.W., Shiu, K.T., Li, N. et al. (2013). Nature Communications 4. 11 van Leest, R.H., Mulder, P., Gruginskie, N. et al. (2017). IEEE Journal of Pho-

tovoltaics 7: 702–708. 12 Song, K., Han, J.H., Lim, T. et al. (2016). Advanced Healthcare Materials 5:

1572–1580. 13 Shockley, W. and Queisser, H.J. (1961). Journal of Applied Physics 32:

510–519. 14 Polman, A., Knight, M., Garnett, E.C. et al. (2016). Science 352: 4424. 15 Steiner, M.A., Geisz, J.F., Garcia, I. et al. (2013). Journal of Applied Physics

113: 123109. 16 Mattos, L.S., Scully, S.R., Syfu, M. et al. (2012). New module efficiency

17 18 19 20

record: 23.5% under 1-sun illumination using thin-film single-junction GaAs solar cells. In: 2012 38th IEEE Photovoltaic Specialists Conference, 3187–3190. Yablonovitch, E., Gmitter, T., Harbison, J.P., and Bhat, R. (1987). Applied Physics Letters 51: 2222–2224. Ando, K. and Yamaguchi, M. (1985). Applied Physics Letters 47: 846–848. Coutts, T.J. and Naseem, S. (1985). Applied Physics Letters 46: 164–166. Keavney, C., Haven, V., and Vernon, S. (1990). Emitter structures in MOCVD InP solar cells. In: IEEE Conference on Photovoltaic Specialists, 141–144.

203

204

6 Thin-Film III–V Single Junction

21 Wanlass, M. (2017). Systems and methods for advanced ultra-high-

performance InP solar cells. Google Patents. 22 Yin, X., Battaglia, C., Lin, Y. et al. (2014). ACS Photonics 1: 1245–1250. 23 Schubert, M., Gottschalch, V., Herzinger, C.M. et al. (1995). Journal of

Applied Physics 77: 3416–3419. 24 Karam, N.H., King, R.R., Haddad, M. et al. (2001). Solar Energy Materials &

Solar Cells 66: 453–466. 25 King, R.R., Fetzer, C.M., Colter, P.C. et al. (2002). High-efficiency space

26 27 28 29 30 31 32 33 34

35 36 37 38

39 40

41

and terrestrial multijunction solar cells through bandgap control in cell structures. In: Conference Record of the Twenty-Ninth IEEE Photovoltaic Specialists Conference, 776–781. Pla, J., Barrera, M., and Rubinelli, F. (2007). Semiconductor Science and Technology 22: 1122–1130. Geisz, J.F., Steiner, M.A., Garcia, I. et al. (2013). Applied Physics Letters 103: 041118. Yang, W.Q., Becker, J., Liu, S. et al. (2014). Journal of Applied Physics 115: 203105. Takamoto, T., Ikeda, E., Kurita, H., and Ohmori, M. (1997). Applied Physics Letters 70: 381–383. Sugiura, H., Amano, C., Yamamoto, A., and Yamaguchi, M. (1988). Japanese Journal of Applied Physics 27: 269–272. Olson, J.M., Kurtz, S.R., Kibbler, A.E., and Faine, P. (1990). Applied Physics Letters 56: 623–625. Takamoto, T., Ikeda, E., Kurita, H. et al. (1997). Japanese Journal of Applied Physics 36: 6215–6220. NREL (2013). NREL Reports 31.1% Efficiency for III-V Solar Cell. https:// www.nrel.gov/news/press/2013/2226.html (accessed 12 January 2019). Jain, N., Schulte, K.L., Geisz, J.F. et al. (2017). GaInAsP/GaInAs tandem solar cell with 32.6% one-sun efficiency. In: 2017 IEEE 44th Photovoltaic Specialist Conference (PVSC). Yamaguchi, M., Okuda, T., Taylor, S.J. et al. (1997). Applied Physics Letters 70: 1566–1568. Baudrit, M. and Algora, C. (2010). Physica Status Solidi A: Applications and Materials Science 207: 474–478. King, R., Law, D., Edmondson, K. et al. (2007). Applied Physics Letters 90: 183516. King, R.R., Haddad, M., Isshiki, T. et al. (2000). Metamorphic GaInP/GaInAs/Ge solar cells. In: Conference Record of the IEEE Photovoltaic Specialists Conference, 982–985. Sasaki, K., Agui, T., Nakaido, K. et al. (2013). AIP Conference Proceedings 1556: 22–25. Wojtczuk, S., Chiu, P., Zhang, X.B. et al. (2010). InGaP/GaAs/InGaAs 41% concentrator cells using bi-facial epigrowth. In: Proceedings of the 35th IEEE Photovoltaic Specialists Conference, 1259–1264. Dimroth, F., Guter, W., Schone, J. et al. (2009). Metamorphic GaInP/GaInAs/Ge triple-junction solar cells with > 41% efficiency. In:

References

42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70

34th IEEE Photovoltaic Specialists Conference. (PVSC), Philadelphia, PA, 1933–1937. Sharp. (2013) http://www.sharp-world.com/corporate/news/130614.html. Derkacs, D., Jones-Albertus, R., Suarez, F., and Fidaner, O. (2012). Journal of Photonics for Energy 2: 8. Wiemer, M., Sabnis, V., and Yuen, H. (2011). 43.5% efficient lattice matched solar cells. In: SPIE Solar Energy + Technology, vol. 8108, 5. SPIE. Bruel, M., Aspar, B., and AubertonHerve, A.J. (1997). Japanese Journal of Applied Physics Part 1 36: 1636–1641. Cho, E.C., Green, M.A., Xia, J. et al. (2004). Applied Physics Letters 84: 2286–2288. Bedell, S.W., Shahrjerdi, D., Hekmatshoar, B. et al. (2012). IEEE Journal of Photovoltaics 2: 141–147. Bedell, S.W., Fogel, K., Lauro, P. et al. (2013). Journal of Physics D: Applied Physics 46. Yoon, J., Jo, S., Chun, I.S. et al. (2010). Nature 465: 329–U380. Thouless, M.D., Evans, A.G., Ashby, M.F., and Hutchinson, J.W. (1987). Acta Metallurgica et Materialia 35: 1333–1341. Suo, Z. and Hutchinson, J.W. (1989). International Journal of Solids and Structures 25: 1337–1353. Shahrjerdi, D. and Bedell, S.W. (2013). Nano Letters 13: 315–320. Kim, Y., Cruz, S.S., Lee, K. et al. (2017). Nature 544: 340–343. Konagai, M., Sugimoto, M., and Takahashi, K. (1978). Journal of Crystal Growth 45: 277–280. Essig, S., Allebé, C., Remo, T. et al. (2017). Nature Energy 6: 17144. Ahrenkiel, R.K., Dunlavy, D.J., Keyes, B. et al. (1989). Applied Physics Letters 55: 1088–1090. Geisz, J.F., Friedman, D.J., Ward, J.S. et al. (2008). Applied Physics Letters 93: 123505. Moon, S., Kim, K., Kim, Y. et al. (2016). Scientific Reports 6: 30107. van Niftrik, A.T.J., Schermer, J.J., Bauhuis, G.J. et al. (2008). Journal of the Electrochemical Society 155: D35–D39. Lee, J.W., Pearton, S.J., Abernathy, C.R. et al. (1995). Journal of the Electrochemical Society 142: L100–L102. Yoon, J., Baca, A.J., Park, S.I. et al. (2008). Nature Materials 7: 907–915. Lee, J., Wu, J.A., Shi, M.X. et al. (2011). Advanced Materials 23: 986–991. Lee, J., Wu, J., Ryu, J.H. et al. (2012). Small 8: 1851–1856. Yamaguchi, M. and Luque, A. (1999). IEEE T. Electron. Dev. 46: 2139–2144. Swanson, R.M. (2000). Progress in Photovoltaics 8: 93–111. Yoon, J., Li, L.F., Semichaevsky, A.V. et al. (2011). Nature Communications 2. Mousazadeh, H., Keyhani, A., Javadi, A. et al. (2009). Renewable and Sustainable Energy Reviews 13: 1800–1818. Sheng, X., Shen, L., Kim, T. et al. (2013). Advanced Energy Materials 3: 991–996. Sansregret, J., Drake, J., Thomas, W., and Lesiecki, M. (1983). Applied Optics 22: 573–577. Sheng, X., Yun, M.H., Zhang, C. et al. (2015). Advanced Energy Materials 5.

205

206

6 Thin-Film III–V Single Junction

71 Hyldahl, M.G., Bailey, S.T., and Wittmershaus, B.P. (2009). Solar Energy 83:

566–573. 72 Wu, J.J. and Kortshagen, U.R. (2015). RSC Advances 5: 103822–103828. 73 Currie, M.J., Mapel, J.K., Heidel, T.D. et al. (2008). Science 321: 226–228. 74 Barnham, K., Marques, J.L., Hassard, J., and O’Brien, P. (2000). Applied

Physics Letters 76: 1197–1199. 75 Meinardi, F., Ehrenberg, S., Dhamo, L. et al. (2017). Nature Photonics 11: 76 77 78 79 80

81 82 83

84 85 86 87 88

89 90 91 92 93 94 95

177–185. Bronstein, N.D., Yao, Y., Xu, L. et al. (2015). ACS Photonics 2: 1576–1583. Kim, J., Hwang, J., Song, K. et al. (2016). Applied Physics Letters 108: 253101. Il Park, S., Shin, G., Banks, A. et al. (2015). Journal of Neural Engineering 12. Chiu, P., Law, D., Woo, R. et al. (2014). IEEE Journal of Photovoltaics 4: 493–497. King, R.R., Fetzer, C.M., Law, D.C. et al. (2006). Advanced III–V multijunction cells for space. In: Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4, 1757–1762. Kim, B.J., Kim, D.H., Lee, Y.Y. et al. (2015). Energy & Environmental Science 8: 916–921. Bereuter, L., Williner, S., Pianezzi, F. et al. (2017). Annals of Biomedical Engineering 45: 1172–1180. Takamoto, T., Washio, H., and Juso, H. (2014). Application of InGaP/GaAs/InGaAs triple junction solar cells to space use and concentrator photovoltaic. In: IEEE 40th Photovoltaic Specialist Conference (PVSC), 0001–0005. Dimroth, F., Tibbits, T.N.D., Niemeyer, M. et al. (2016). IEEE Journal of Photovoltaics 6: 343–349. Luque, A. (2011). Journal of Applied Physics 110: 031301. Friedman, D.J. and Kurtz, S.R. (2002). Progress in Photovoltaics 10: 331–344. Zhang, Y., Wang, Q., Zhang, X.B. et al. (2016). Chinese Physics Letters 33. King, R.R., Boca, A., Hong, W. et al. (2009). Band-gap-engineered architectures for high-efficiency multijunction concentrator solar cells. In: 24th European Photovoltaic Solar Energy Conference and Exhibition, Hamburg, Germany, vol. 21, 55. France, R.M., Geisz, J.F., Garcia, I. et al. (2016). IEEE Journal of Photovoltaics 6: 578–583. Zhao, L., Flamand, G., and Poortmans, J. (2010). AIP Conference Proceedings 1277: 284–289. Sheng, X., Bower, C.A., Bonafede, S. et al. (2014). Nature Materials 13: 593–598. Cotal, H., Fetzer, C., Boisvert, J. et al. (2009). Energy & Environmental Science 2: 174–192. Hopkinson, M., Martin, T., and Smowton, P. (2013). Semiconductor Science and Technology 28: 090301. Wang, Z.C., Van Gasse, K., Moskalenko, V. et al. (2017). Light: Science & Applications 6. Mohseni, P.K., Behnam, A., Wood, J.D. et al. (2014). Advanced Materials 26: 3755–3760.

References

96 Xiong, K., Lu, S., Dong, J. et al. (2010). Solar Energy 84: 1975–1978. 97 Huang, Q.L., Wang, J.Z., Quan, B.G. et al. (2013). Applied Optics 52:

2312–2319. 98 Mojiri, A., Taylor, R., Thomsen, E., and Rosengarten, G. (2013). Renewable

and Sustainable Energy Reviews 28: 654–663. 99 Goetzberger, A., Goldschmidt, J., Peters, M., and Löper, P. (2008). Solar

Energy Materials & Solar Cells 92: 1570–1578. 100 Arbabi, E., Arbabi, A., Kamali, S.M. et al. (2017). Optica 4: 625–632. 101 Ganapati, V., Steiner, M.A., and Yablonovitch, E. (2016). IEEE Journal of

Photovoltaics 6: 801–809. 102 Ganapati, V., Ho, C.S., and Yablonovitch, E. (2015). IEEE Journal of Photo-

voltaics 5: 410–417. 103 Garcia, I., Kearns-McCoy, C.F., Ward, J.S. et al. (2014). Applied Physics

Letters 105.

207

209

7 Novel Materials-Based Flexible Solar Cells Dong Liu 1 , Kwangeun Kim 1 , Jisoo Kim 1 , Jiarui Gong 1 , Tzu-Hsuan Chang 1,2 , and Zhenqiang Ma 1 1 University of Wisconsin-Madison, Department of Electrical and Computer Engineering, Madison, WI, 53706, USA 2 National Taiwan University, Department of Electrical Engineering, Taipei, Taiwan

7.1 Flexible Perovskites Solar Cells 7.1.1

Introduction

Perovskites are a class of oxides with lattice structure repeatedly of ABX3 (Figure 7.1) that exhibit exceptional solar energy harvesting capabilities. Their physical properties such as magnetic, ferroelectric, and two-dimensional electronic conductivity have been broadly researched in the recent 10 years [2, 3]. It was first identified by the geologist Gustav Rose, who named CaTiO3 , an archetypal perovskite, after the Russian mineralogist Count Lev Aleksevich von Perovski back in the 1830s [4]. In a typical ABX3 structure, a perovskite-type oxide has X serving as an anion and A and B as cations of different sizes. In general, atoms located in position A are larger than those in position B. Cations, located in A site, are composed of atoms with a large ionic radius and have 12 coordination to cation B atoms, and cations with a smaller ionic radius have six coordination and occupy B sites. In other words, A and X form a cubic closest packing, and B is contained in the octahedral voids in the packing. To form the stable ABX3 structures, the relation of ionic radii (rA , rB , and rO ) needs to satisfy the formula: 0.8√< t < 1.0, and rA > 0.090 nm, rB > 0.051 nm, where the tolerance factor(t) = 2 (rA + rO )∕(rB + rO ) [5]. 7.1.2 7.1.2.1

Preparation of Perovskites Materials Solution Process Deposition Approaches

For laboratory scale preparation, spin-coating is the most used process. The capability of spin-coating, as illustrated in Figure 7.2, has been a major advantage in preparing perovskite solar cells (PSCs), and offers wafer-scale perovskites film preparation at low cost. In general, premixed solutions of perovskites materials Inorganic Flexible Optoelectronics: Materials and Applications, First Edition. Edited by Zhenqiang Ma and Dong Liu. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

7 Novel Materials-Based Flexible Solar Cells

Figure 7.1 Structure of Perovskite (ABX3 ). Source: Tanaka and Misono 2001 [1]. Reproduced with permission of Elsevier.

A

B X

Mixture at 70 °C Hot-casting

210

FTO/PEDOT

Spin-coating or ‘‘quenching’’

FTO/PEDOT

Cooling on glass

FTO/PEDOT GLASS

Substrate temperature 170 °C

Figure 7.2 Processing scheme for perovskite thin film using spin-coating and hot-casting approaches [6].

are dissolved in the solvent together and cast on the substrate surface through spin-casting. This convenient process can support the scaling capability and can enable preparation of the film up to windows size and has been constantly used in the semiconductor industry. In contrast to the sophisticated and commercialized single crystal Si solar cells that have their photon-conversion efficiency close to 20% for decades, perovskites solar cells have easily achieved up to 13% in just a few years. The major limitation of the solution-based approaches is the grains size of the perovskite, which is usually limited to a few micrometers. To increase the grain size, an improved hot-casting approach is designed by W. Nie et al. In the hot-casting approach, mixtures of lead-based source material, lead iodide (PbI2 ), and the precursor methylamine hydrochloride (MACl) are dissolved in solution and preheated to 70∘ and cast to 170∘ substrates that are made of conductive FTO, fluorine-doped tin oxide, and poly(3,4-ethylenedioxythiophene) (PEDOT), a transparent conductive polymer. It has been shown that the grains size of the perovskites can be improved to millimeters based on the hot-casting approach

7.1 Flexible Perovskites Solar Cells

Figure 7.3 (a) Schematic illustration of slot-die coating with a gas-quenching process for the fabrication of pinhole-free perovskite layer. (b) Photographs of perovskite film with different drying condition. Source: Hwang et al. 2015 [8]. Reproduced with permission of John Wiley & Sons.

Nitrogen

(a)

(b)

and the reported efficiency of the perovskite film can be improved to 18%, close to that of the current commercial Si-based solar cell process. Hot-casting solution approaches have opened the door to large-scale PSCs at production costs much cheaper than the current single crystal solar cell markets. Slot-die coating is a promising technique to scale up to commercial levels. The film thickness can be controlled precisely by concentrations and viscosities of inks [7]. Hwang et al. reported a roll-to-roll (R2R) production process based on slot-die coating with two major advantages [8]. First, unlike spin-coating, the solution precursor for slot-die coating will not be wasted. As shown in Figure 7.3a, the solution precursor comes out through the first slot-die head. Also, the second head is connected to high-pressure nitrogen to quickly dry the perovskite film. Figure 7.3b shows a comparison of the perovskite film condition with different drying conditions. In the case of a slow drying sample, the small ions move easily to form overgrown crystals, and then some areas on substrates are devoid of material and pinholes can be formed. Compared to the slow drying step, a gas-quenched sample forms a dense and uniform perovskite film. 7.1.2.2

Vapor-Assisted Solution Deposition Approaches

Preparation of perovskites films from a mixture of the source components in solvents suffers from some major drawbacks because of the rapid reaction rate and the lack of proper solvents. This may induce pinholes and cracks in the deposited films during the solvent-removal process. A modified approach that combines the benefits of both solutions-based and vapor-deposited approaches in sequential steps has been developed to improve the film quality. For example, as shown in Figure 7.4, the depositions on perovskites film, CH3 NH3 PbI3 , are separated into the solution-based coating of the component, PbI2 , and later

211

5 Current density (mA/mc2)

CH3NH3I vapor

0 –5 –10 –15 –20

12.1%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Voltage (V) PbI2

Perovskite

Perovskite photovoltaics

Figure 7.4 Schematic of perovskite film formation through vapor-Assisted solution process. Source: Chen et al. 2014 [9]. Reproduced with permission of ACS.

7.1 Flexible Perovskites Solar Cells

vapor-assisted CH3 NH3 I deposition. In this revised process, a smooth and uniform PbI2 can be first deposited on the surface of the substrate and formed into a densely packed film without pinholes. In the second process, second source components, CH3 NH3 I, that do not share the same solvent as PbI2 , can be carried in the system through evaporation process. The reaction of CH3 NH3 I and PbI2 can be well controlled by the flow rate and chamber pressure of the vapor in the second step. With better control of the deposition process, the cracks and pinholes density are significantly reduced and perovskite films with better surface-roughness can be achieved. 7.1.2.3

Chemical Vapor Deposition Approaches

So far, the preparation of perovskite materials involves the use of the solution-based process, which ultimately leads to polycrystalline perovskite films and the film quality is subject to the environmental conditions, for example, humidity. The perovskite film produced by this approach has short lifetime and unavoidable defects in it. To make the perovskites have compatible crystalline quality as the traditional semiconductor material, Si, vapor deposition process in high vacuum can effectively exclude the damage from humidity. With well-controlled environment, high-quality single crystal perovskite films can be deposited through physical vapor deposition approaches if a lattice-matched substrate is selected. As shown in Figure 7.5, two precursors that contain the components of CH3 NH3 I and PbI2 are placed in the vacuum chamber respectively. The material carrying boats are preheated to the evaporation point and carried by the back-flowing argon gas. The reaction happens, and the latter gets deposited on the substrate site that is located at the lowered temperature point. The chemical reaction is insulated from the ambient, and ultrahigh-quality perovskite films can be deposited through this approach. However, it remains challenging to scale up the vapor-deposited approaches to larger scale, and maintaining the vacuum condition will cost much more than the solution-based approaches. But it is still a good approach to study the ultimate solar energy conversion performance of the perovskites during the development stages. The balance between the Substrate

PbX2 (X = I and CI)

MAI

Argon gas Reaction zone

Zone 2 low temperature

Zone 1 high temperature

Figure 7.5 Schematics of the perovskite film fabrication using MAI and PbX2 sources deposited onto a c-TiO2 -coated FTO glass substrate, which is performed in a chemical vapor deposition (CVD) furnace. Source: https://creativecommons.org/licenses/by/4.0/, [10].

213

214

7 Novel Materials-Based Flexible Solar Cells

performance of PSCs and the film preparation approaches depends on the competition from the commercialized Si solar cells in terms of production cost. 7.1.3 7.1.3.1

Flexible Perovskite Solar Cell Sample Preparation

High power-conversion efficiency (PCE) solar cells have been fabricated largely on rigid glass substrates, and solar cells on rigid substrates continue to hold the record for the highest PCE. Owing to the lack of flexibility and capability of lowering the processing temperature in the traditional rigid solar cell materials, such as Si and III–V, the major advance in flexible solar cells comes from the development of organic polymer-based chemicals, which have poor electrical properties. However, low-temperature processible PSCs have gained considerable attention in the photovoltaics (PVs) research community and have shown great promises toward commercialization. In the early stage of their development, flexible PSCs have already proved to be several times better than the traditional organic solar cells and close to the rigid semiconductor-based ones. Rigid glass substrates are normally used to fabricate PSCs that have good optical transmittance, heat resistance, corrosion resistance, and good contact with the transparent conductive films [11]. However, for bendable, wearable, conformable, and R2R production of PSCs, the devices must utilize flexible substrates that are highly mechanically robust. Thus, glass substrates that have fragile characteristics are not suitable to use for flexible PSCs. Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films are promising flexible substrates because of their bendability, low cost, deactivation to usual solvents, and high optical transparency [12]. Also, polyimide (PI) is another plastic substrate that can endure high temperature processing. Moreover, a colorless PI film was released that can increase optical transmittance compared to the previous brown color PI film [13]. Park et al. reported that the bendability and transmittance of PI/ITO films are even higher than that of the PET/ITO reference [13]. In addition, Noland Optical Adhesive 63 (NOA 63) substrate can be used for flexible PSCs with its shape memory characteristics [14]. Lastly, other types of flexible PSCs substrates are metallic foils [15, 16] and fibrous materials [17, 18] with good device performance. Metallic foils have good electrical and thermal stability compared to plastic substrates [19]. Lee et al. [20] investigated the dependence of device performance on conductive foil substrate and different thickness of ultra-thin metal film. Ti foils were usually used as substrates in flexible solar cells because of their good corrosion resistance and high strength–weight ratio [21]. Cu [22] and stainless steel foils [17, 23] have been also used as metal substrates in flexible PSCs. These fiber-shaped PSCs have the advantage of absorbing light and being flexible to use in wearable devices. Flexible substrates used in the fabrication of flex-PSCs can be broadly categorized as indium oxide based, such as ITO (indium tin oxide), and non-indium oxide based. Indium oxide-based materials have been widely used in optoelectronic applications such as display, and solar cells. To be specific, ITO has highly

7.1 Flexible Perovskites Solar Cells

conductive property and transparent characteristics. ITO-based substrates were reported to exhibit good transmittance in visible and near infrared regions of the light spectrum [7]. However, like ITO, indium oxide-based materials are expensive and do not match the low-cost market demand. Moreover, ITO film is not suitable for curvature bending because of its brittle characteristic [24]. Therefore, replacement of indium oxide-based material is necessary for production. The results of ITO and ITO-free flexible PSCs are shown in Tables 7.1 and 7.2 respectively. Comparing both tables, the average PCE of flexible solar cells including indium oxide material such as ITO is higher than that of non-indium oxide material. For depositing perovskite films, the method most used is spin-coating, and CH3 NH3 Pb3 (MAPBI3 , methylammonium lead iodide) is used to form the perovskite film in most research groups. In the case of flexible PSCs including indium oxide material, many substrates are used such as PET, PEN, and metal, but for flexible PSCs excluding indium oxide material, PET is a major substrate to use. In the case of fibrous solar cell, most groups fabricate without indium oxide material. As mentioned above, the first priority of flexible photovoltaic cells is the cost advantage over that of rigid photovoltaics owing in part to the ease of production in an R2R processing that allows for manufacturing of large product volumes and throughputs within a short duration [80]. This would significantly reduce the production cost of flex-PSCs by requiring less overhead cost as compared with rigid solar cells. Moreover, non-indium oxide-based electrodes that are low cost materials compared to indium oxide-based electrodes have far better mechanical robustness [81]. Hence, the ultimate replacement of indium oxide-based electrodes and metal-based counter electrodes would guarantee mechanically resistant flex-PSCs that can withstand rigorous mechanical deformation over a long-term. 7.1.3.2

Performance Analysis

In order to reduce the cost of commercializing flexible PSCs, the R2R process is the most important technique that has a high production rate [8]. Also, several characteristics such as portability, bendability, lightweight, conformability, and wearability are essential for use in the real world. Robust mechanical tolerance is required to maintain device performance after applying physical deformation of bending, compressing, stretching, twisting, and even crumpling. As shown in Figure 7.6, many researchers have studied performance analysis. Fiber-shaped flexible PSC has been subjected to deformation of tying and twisting in Figure 7.6a. However, the author reported that this fiber-shaped solar cell retained 90% of its performance. The only 10% reduction in PCE appeared after 400 cycles due to a slight decline in J sc [18]. Figure 7.6b shows the schematic drawing and photographs of a stretchable PSC. Sinusoidal waves and localized high-aspect-ratio ridge wrinkles form on relaxation of the pre-stretched elastomer. The bottom figure shows 0% compression on the left and 50% compression on the right. The authors mentioned that even 50% compression, V oc, and fill factor (FF) do not change significantly as a function of compression as the illuminated active area shrinks [82]. The crumpling test of the shape recoverable device is shown in Figure 7.6c. Thermal annealing process

215

Table 7.1 Summary of the performance of non-indium oxide flexible PSCs.

PCE (%)

Perovskite deposition method

References

80.0

14.00

Spin-coated

[25]

68.6

12.20

Spin-coated

[26] [27]

V oc (V)

Jsc (mA/cm2 )

FF (%)

PET/Ag-mesh/PH1000/PEDOT:PSS/CH3 NH3 PbI3 /PCBM/Al

0.91

19.5

PET/AZO/ZnO/C60 /CH3 NH3 PbI3 /Spiro-MeOTAD/MoO3 /In2 O3 :H embedded with Ni/Al grid

1.08

16.1

PET

PET/graphene/P3HT/CH3 NH3 PbI3 /PC71 BM/Ag

1.04

18.6

59.4

11.48

Spin-coated

PET/PEDOT:PSS/PEI/CH3 NH3 PbI3 /Spiro-MeOTAD/Au

0.95

17.2

59.7

9.73

Spin-coated

[28]

PET/MSA-PEDOT:PSS/CH3 NH3 PbI3 /PCBM/C60 /LiF/Ag

0.87

17.2

57.0

8.60

Spin-coated

[29]

PET/Ag nano-network/graphene oxide/PEDOT:PSS/CH3 NH3 PbI3 /PFN-P1/PCBM/Al

0.94

12.7

66.2

7.92

Spin-coated

[30] [31]

PET/PEDOT:PSS/CH3 NH3 PbI3 /PCBM/Al

0.8

15.0

60.0

7.60

Spin-coated

PET/AZO/Ag/AZO/PEDOT:PSS/Poly-TPD/CH3 NH3 PbI3 /PCBM/Au

1.04

14.3

57.0

7.00

Spin-coated

[32]

PET/SWCNT/CH3 NH3 PbI3 /PCBM/Al

0.71

11.8

56.0

5.38

Spin-coated

[33]

PET/PEDOT:PSS/CH3 NH3 PbI3 /PCBM/TiO2 /Al

0.75

15.8

41.0

4.90

Spin-coated

[34]

1.00

21.7

0.8

16.80

Spin-coated

[35]

NOA 63/PEDOT:PSS/CH3 NH3 PbI3 /PCBM/Eutectic Ga–In blend

0.92

16.6

70.5

10.75

Spin-coated

[14]

NOA 63/MoO3 /Au/PEDOT:PSS/CH3 NH3 PbI3 /PCBM/DMD (MoO3 /Au/Ag/MoO3 /Alq3 )

0.83

13.9

60.7

6.96

Spin-coated

[36]

0.96

22.5

59.2

12.80

Spin-coated

[22]

PEN PEN/graphene-Mo/PEDOT:PSS/CH3 NH3 PbI3 /C60 /BCP/LiF/Al NOA

Metal Cu/CuI/CH3 NH3 PbI3 /ZnO/Ag nanowires

Ti-foil/c-TiO2 /mp-Al2 O3 /CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/ PEDOT:PSS/PET with Ni mesh

0.98

17.0

61.0

10.30

Spin-coated

[16]

PEN/ITO/TiO2 /CH3 NH3 PbI3 /CNT

0.91

15.9

65.6

9.49

Solutiondip coated (Fibrous)

[37]

[15]

Ti/TiO2 nanotubes/CH3 NH3 PbI3 /Spiro-MeOTAD/CNT

0.99

14.4

68.0

8.31

Spin-coated

Ti/TiO2 /CH3 NH3 PbI3 /Spiro-MeOTAD/Ag nanowires

0.92

16.5

49.0

7.45

Spin-coated

[38]

Ti/c-TiO2 /meso–TiO2 /CH3 NH3 PbI3 /CNT

0.85

14.5

56.0

7.10

Solutiondip coated (Fibrous)

[18]

Ti/c-TiO2 /mp-TiO2 /CH3 NH3 PbI3 /Spiro-MeOTAD/ultra-thin Ag

0.89

9.5

72.8

6.15

Spin-coated

[20]

Ti/c-TiO2 /meso–TiO2 /CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/Au

0.71

12.3

60.9

5.35

Solutiondip coated (Fibrous)

[39]

5.22

Solutiondip coated (Fibrous)

[40]

Ti/TiO2 nanotube/CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/CNT sheet

Ti/dimple c-TiO2 /mp-TiO2 /CH3 NH3 PbI3 /Spiro-MeOTAD/Ag NW

0.73

12.0

44.0

3.85

Solutiondip coated (Fibrous)

[41]

Stainless steel/c-TiO2 /mp-TiO2 /CH3 NH3 PbI3 /Spiro-MeOTAD/CNT sheet

0.66

10.2

48.7

3.30

Solutiondip coated (Fibrous)

[17]

CNT fiber/c-TiO2 /mp-TiO2 /CH3 NH3 PbI3−x Clx /P 3 HT/SWCNT/Ag NW

0.62

8.8

56.4

3.03

Homemade heat-assisted coating (Fibrous)

[42]

Table 7.2 Summary of the performance of indium oxide flexible PSCs.

Structures (substrate categorized)

V oc (V)

Jsc (mA/cm2 )

FF (%)

PCE (%)

Perovskite deposition method

References

PET PET/ITO/ss-IL/(HC[NH2 ]2 PbI3 )0.85 (CH3 NH3 PbBr3 )0.15 /Spiro-OmeTAD/Au

1.07

22.7

66.2

16.09

Spin-coated

[43]

PET/ITO/TiO2 /CH3 NH3 Pb(I1−x Brx )3 /PTAA/Au

1.11

20.8

69.0

15.88

Spin-coated

[44]

PET/ITO/Nb2 O5 /(HC(NH2 )2 PbI3 )0.85 (CH3 NH3 PbBr3 )0.15 /spiro-OMeTAD/Au

1.12

23.5

63.1

15.56

Spin-coated

[45]

PET/ITO/ZnSnO4 /CH3 NH3 Pb3 /PTAA/Au

1.05

21.6

67.0

15.30

Spin-coated

[46]

PET/ITO/TiO2 /CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/Au

1.03

20.9

70.0

15.07

Evaporation and powder

[47]

PET/ITO/Li:SnO2 /CH3 NH3 Pb3 /Spiro-MeOTAD/Au

1.02

20.6

76.3

14.78

Spin-coated

[48]

PET/ITO/Al2 O3 /CH3 NH3 Pb3 /Spiro-MeOTAD/Au

1.00

22.8

67.0

14.60

Spin-coated

[19]

PET/ITO/NiOx /CH3 NH3 Pb3 /PCBM/Bis-C60 /Ag

1.00

20.7

70.5

14.53

R2R

[13]

PET/ITO/NiOx /CH3 NH3 Pb3 /C60 /Bis-C60 /Ag

1.00

20.9

69.6

14.19

Spin-coated

[49]

PET/ITO/PEIE/CDIN/CH3 NH3 Pb3 /Spiro-MeOTAD/Ag

1.02

19.7

70.4

14.15

Spin-coated

[50]

PET/ITO/PEDOT:PSS/PEI-HI/CH3 NH3 PbI3−x Clx /MAPbI3 /PCBM/LiF/Ag

19.01

1.1

68.0

13.80

Blading-coated

[51]

PET/ITO/TiO2 /CH3 NH3 PbI3−x Clx /PTAA/Au

0.91

21.3

69.0

13.50

Spin-coated

[52]

PET/ITO/NiOx /CH3 NH3 Pb3 /PCBM/Ag

1.04

18.7

68.9

13.43

Spin-coated

[53]

PET/ITO/SnOx /BK-TiO2 /CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/Au

1.03

19

68.0

13.40

Spin-coated

[54]

PET/IZO/TiO2 /CH3 NH3 PbI3 /Spiro-MeOTAD/Ag

1.05

18.2

70.0

13.20

Spin-coated

[55]

PET/ITO/[HC(NH2 )2 PbI3 ]0.9 [CH3 NH3 PbCl3 ]0.1 /Spiro-MeOTAD/Au

0.96

20.7

64.0

12.70

Spin-coated

[56]

PET/ITO/PEDOT:PSS/CH3 NH3 Pb3 /PCBM/Ca/Al

0.99

17.2

72.0

12.25

Evaporation and dipping

[57]

PET/ITO/c-TiO2 /me-TiO2 /CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/Au

0.66

33.7

77.3

12.10

Spin-coated

[58]

PET/ITO/Zn2 SnO4 /PCBM/CH3 NH3 Pb3 /Spiro-MeOTAD/Ag

1.05

17.4

63.8

11.61

Spin-coated

[59]

PET/ITO/PEDOT:PSS/CH3 NH3 Pb3 /PCBM/Ag

0.89

19.4

68.3

11.29

Blading-coated

[60]

PET/ITO/ZnO/CH3 NH3 Pb3 /Spiro-MeOTAD/Ag

1.03

13.4

73.9

10.20

Spin-coated

[61]

PET/ITO/FPI-PEIE/PCBM/CH3 NH3 Pb3 /Spiro-MeOTAD/Au

1.07

17.8

53.0

10.00

Spin-coated

[62]

PET/ITO/graphite/ZnO-QDs/CH3 NH3 Pb3 /Spiro-MeOTAD/Ag

0.94

16.8

62.0

9.73

Spin-coated

[63]

PET/ITO/PEDOT:PSS/CH3 NH3 Pb3 /PCBM/Bis-C60 /Ag

0.86

14.6

75.0

9.43

Spin-coated

[64]

PET/ITO/PEDOT:PSS/CH3 NH3 PbI3−x Clx /PCBM/Bis-C60 /Al

0.86

16.5

64.0

9.20

Spin-coated

[65]

PEN PEN/ITO/PEDOT:PSS/CH3 NH3 Pb3 /C60 /BCP/LiF/Al

0.97

21.5

83.0

17.30

Spin-coated

[35]

PEN/ITO/Zn2 SnO4 /CH3 NH3 Pb(I0.9 Br0.1 )3 /PTAA/Au

1.10

20.4

73.0

16.50

Spin-coated

[43]

PEN/ITO/UV-Nb:TiO2 /CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/Au

1.04

20.2

76.0

16.01

Spin-coated

[66]

PEN/ITO/C60 /CH3 NH3 Pb3 /Spiro-MeOTAD/Au

1.02

23.2

67.3

16.00

Spin-coated

[67]

PEN/ITO/W(Nb)Ox /CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/Ag

0.98

21.4

75.0

15.65

Spin-coated

[68]

PEN/ITO/ZnO/CH3 NH3 Pb3 /PTAA/Au

1.10

18.7

76.0

15.50

Spin-coated

[69] [70]

PEN/ITO/PhNa-1T/CH3 NH3 Pb3 /PCBM/Ag

1.03

18.4

77.4

14.70

Spin-coated

PEN/ITO/SnO2 /PCBM/CH3 NH3 Pb3 /Spiro-MeOTAD/Au

1.08

20.6

63.0

14.00

Spin-coated

[71]

PEN/ITO/PEIE/C60 /CH3 NH3 Pb3 /Spiro-MeOTAD/Ag

1.02

17.9

73.0

13.30

Spin-coated

[72]

PEN/ITO/PEIE/PCBM/CH3 NH3 Pb3 /LN-P3HT/Au

0.88

22.0

69.6

13.12

Spin-coated

[73]

PEN/ITO/TiOx /CH3 NH3 PbI3−x Clx /Spiro-MeOTAD/Ag

0.95

21.4

60.0

12.20

Spin-coated

[24]

PEN/ITO/NiOx /CH3 NH3 Pb(I0.9 Cl0.1 )3 /PCBM/BCP/Ag

1.04

17.7

64.2

11.84

Spin-coated

[74]

PEN/ITO/PEIE/PCBM/CH3 NH3 Pb3 /Co–P3HT/Au

1.01

19.8

59.3

11.84

Spin-coated

[75]

PEN/ITO/CH3 NH3 PbI3 /Spiro-MeOTAD/MoO3 /Ag

0.96

17.4

56.0

11.34

Spin-coated

[76]

PEN/FTO/TiO2 /PCBM/CH3 NH3 Pb3 /PTAA/Au

0.99

18.7

60.0

11.10

Spin-coated

[77]

PEN/ITO/PEDOT:PSS/FASnI3 /C60 /BCP/Ag

0.31

16.1

62.6

3.12

Spin-coated

[78]

1.10

17.6

79.4

15.20

R2R

[13]

Ti/TiO2 nanowire/CH3 NH3 PbI3 /PEDOT/PEN-ITO

0.94

21.7

63.0

13.07

Injected

[21]

Ti/c-TiO2 /mp-TiO2 /CH3 NH3 PbI3 /Spiro-MeOTAD/ITO

0.10

18.5

61.0

11.01

Spin-coated

[79]

PI PI/ITO/ZnO/CH3 NH3 Pb3 /PTAA/Au Metal

Ultrathin solar cell

Before crumpling 0s

Elastomer Thermal annealing 5s

(a)

(b)

10 s

(c)

Normalized efficiency (η/η0)

1.1

CPI/ITO PET/ITO

1.0 0.9 12 mm

0.8

10 mm

8 mm

6 mm

4 mm

2 mm

0.7 0.6 Flat

(d)

12

10

8

6

4

2

Curvature radius (mm)

Figure 7.6 (a) Images of fiber-shaped flex-PSC undergoing tying (top) and twisting (bottom). Source: Qiu et al. 2016 [18]. Reproduced with permission of John Wiley & Sons. (b) Schematic diagram and photographs of a stretchable perovskite solar cell. 0% compression (left picture) and 50% compression (right picture). Source: Kaltenbrunner et al. 2015 [82]. Reproduced with permission of Springer Nature. (c) Recovering of crumped Noland Optical Adhesive 63 through heating at 80 ∘ C for 10 seconds. Source: Park et al. 2015 [14]. Reproduced with permission of John Wiley & Sons. (d) Normalized power conversion efficiency of flexible devices, which have CPI and PET substrates versus bending radius (R). Source: Park et al. 2017 [13]. Reproduced with permission of Elsevier.

7.1 Flexible Perovskites Solar Cells

on a hot plate at 80 ∘ C for 10 seconds can recover the device. After recovery from crumpling, flexible PSCs still function with a slightly dropped PCE (from 10.2% to 6.1%) [14]. Curvature radius (mm) is an important parameter in the bending test of flexible PSCs. Figure 7.6d shows the PCEs of both flexible PSCs–PET and colorless-polyimide (CPI) substrates, bending the devices to different curvature radii. Both flexible PSCs exhibit mechanical stability, even under the bending radius of 2 mm [13]. Bending cycle test has been carried out not only in these references but also by various research groups when fabricating flex-PSCs. Most groups have carried out more than 500 bending cycles and reported variance of device performance parameters. Variance of PCE is less than 10% [46, 48, 56, 61, 83–85]; some groups reported even no PCE variance after 1000 bending cycles [25, 67]. It has been established that the perovskite film can withstand rigorous bending test. 7.1.4

Stability Issues

So far, PSCs have demonstrated incredible performances that are compatible to the state-of-art in the industry. However, long-term stability is a major drawback of this technique. There are many factors affecting the lifetime of the perovskites, including intrinsic crystalline integrity and insulations. The intrinsic crystalline integrity, including the crystalline quality, impurities, and materials, dominates the maximum lifetime a perovskite can have; on the other hand, the insulations decide the actual operation duty-duration of the devices. However, the perovskites are known to easily degrade in the presence of moisture, gas [86], and ultraviolet radiation [87, 88]. The fabrication process conditions are even more critical for the solution-processed PSCs since the fabrication unavoidably introduces moisture, gas, and impurities during the fabrication procedure. For the flexible PSCs, proper encapsulation plays an even more vital role in the stability of the perovskite solar cells. As shown in Figure 7.7, the perovskite film encapsulated in two flexible materials can be accidently exposed to the ambient condition if the devices are bent or twisted. The moisture and gas can infiltrate the devices through the exposed region and gradually degrade the perovskite films. The devices with incomplete passivation, demonstrated in Figure 7.8, show tens of days of lifetime reduction compared to the full encapsulation devices. Besides, the presence of high operating temperature has been reported to facilitate the degradation of the perovskites [89, 90]. It has been studied that the PCE was boosted to a maximum of 19.3%, where the commercialized Si solar cells stay around 21%, in a planar geometry if PSCs were fabricated in stability-controlled conditions [80]. In addition, UV radiation in the ambient conditions can trigger internal structure degradation without the presence of water, as illustrated in Figure 7.9. The presence of TiO2 in the hole transport material (HTM), as shown in Figure 7.9, can serve as the UV catalyst to degrade the perovskite materials. In this degrading process, iodine-based perovskite materials can be reduced and form acido iodidrico (HI), with the UV photogenerated free carriers that migrated from the TiO2 layer. The formation of HI is not reversible and will gradually decompose the integrity of the perovskite film. To avoid the degradation, it has been reported that insertion of a surface blocking layer, Sb2 S3 in Figure 7.9, is essential to block the self-catalyst degradation process.

221

222

7 Novel Materials-Based Flexible Solar Cells

(a) Flexible barrier encapsulant film with integrated adhesive

Flexible perovskite solar cell device

(b)

Figure 7.7 Schematic of (a) “partial” and (b) “complete” encapsulation architectures. Source: Weerasinghe et al. 2015 [55]. Reproduced with permission of Elsevier.

(a)

(b)

1 cm

1 cm 0

70

140

250

500

650 Time (h)

Figure 7.8 Optical transmission photographs showing the loss of Ca film (dark area) of “partially” encapsulated (a) and “completely” encapsulated (b) Ca films as a function of storage time at ambient conditions. Source: Weerasinghe et al. 2015 [55]. Reproduced with permission of Elsevier.

In summary, to modulate the stability of flexible PSCs, many factors should be taken into consideration from material preparation, structure designs, and long-term encapsulation approaches. The solutions also need to endure the repeatedly mechanical bending and are suitable with the encapsulated flexible substrates.

7.1 Flexible Perovskites Solar Cells

CH3NH3Pbl3 2l–

Light

e–-h+

Light

h+

l2 3HI

2e–

e–

e–-h+

l– TiO2 Sb2S3 CH3NH3Pbl3

3H+ + 3CH3NH2

Overnight TiO2

+

3CH3NH3

Overnight 3HI (b)

TiO2 Sb2S3 CH3NH3Pbl3

Pbl2 (a)

TiO2

3CH3NH2

Figure 7.9 Degradation scheme of CH3 NH3 PbI3 perovskite solar cells during light exposure test: (a) TiO2 /CH3 NH3 PbI3 and (b) TiO2 /Sb2 S3 /CH3 NH3 PbI3 . Source: Ito et al. 2014 [91]. Reproduced with permission of ACS.

7.1.5

Summary

Flexible PSCs are one of the promising technologies in next-generation photovoltaics. Many researchers have achieved high PCE that can be compared to crystalline silicon solar cell, which is a major solar cell in commercial market. With low cost and fast development of manufacturing options, flexible PSCs have the possibility for mass production and commercialization in the near future. The major advantages of flexible PSCs are power/weight, portability, wearability, and large-scale fabrication by R2R process. Firstly, we have briefly introduced the perovskite material and its different preparation methods. Next, we have summarized the recent development of flexible PSCs categorized by substrates, followed by performance analysis. Finally, stability issue has been discussed. In the case of solar cells, scale up to a large area is needed. Owing to the need for scale up for use in the real world, R2R techniques should be developed together with increasing PCE. Moreover, module fabrication of flexible PSCs with encapsulation is also needed to be developed in the near future. Currently, PCE from modules are relatively lower than PCE from cells. Cell to module PCE drop should be decreased to commercialize like silicon rigid solar cell. If so, flexible PSCs can be much used in real life.

223

224

7 Novel Materials-Based Flexible Solar Cells

7.2 Flexible CdTe/CdS Solar Cells 7.2.1

Introduction

CdTe thin-film solar cells have been represented as an important photovoltaic (PV) structure due to direct and small bandgap (1.45 eV), high absorption coefficient, low cost, high chemical stability, and multiple deposition technologies [92, 93]. Conventional CdTe solar cells are fabricated on glass substrate. However, the glass constitutes most of the weight and thickness of the solar cells [94]. Also, the glass is rigid, heavy, and fragile, which leads to deployment difficulties in space applications with high costs around tens of millions of dollars [95]. These issues can be minimized if the rigid thick glass substrates are substituted by the flexible thin substrates such as metal foils, polymers, and thin glasses, which requires better understanding of the cell manufacturing process, PV fundamental, and materials property. Then, thin-film flexible CdTe solar cells are feasible with several advantages. For example, the flexible solar cells on foils can be fabricated using R2R process, which increases material utilization, fabrication scalability, and production rate [96]. The polymer substrates enable the solar cells to be constructed in both superstrate and substrate configurations for higher efficiency and low cost production. To date, there exist issues of poor adhesion between the CdTe films and flexible foils and optical absorption of polymer substrates [97, 98]. In this chapter, recent progress and development of flexible CdTe/CdS solar cells will be introduced. Detailed performance enhancement of the flexible solar cells on thin glass, metal foil, and polymer will be discussed. Also, interconnection and nanopillar structures of flexible solar cells will be described. Alternative design considerations for high efficiency and low cost of flexible solar cells will be discussed. Lastly, outlook for viable flexible CdTe solar cells will be proposed.

7.2.2 7.2.2.1

Flexible CdTe/CdS Solar Cells on Metal Foil Sample Preparation

CdTe solar cells were grown on Corning 7059 glass, 50 mm Mo foil, and 30 mm steel foil substrates [99]. A 60/230 nm Ti/TiN layer was deposited on steel foil by DC sputtering. A 600 nm Mo back contact was deposited by DC sputtering on all cells, and then 150 nm MoO3 and 50 nm Te layers were deposited by evaporation. 4–6 mm CdTe was deposited by evaporation on the substrates at a temperature of 350 ∘ C, followed by 400 nm CdCl2 deposition and annealing at 435 ∘ C. Cu was deposited on the CdTe by high-vacuum evaporation and annealed at 400 ∘ C for diffusion. 50–100 nm CdS was grown on the Cu-doped CdTe thin film by chemical bath deposition (CBD). Intrinsic ZnO (i-ZnO) and ZnO:Al were deposited for bilayer front contacts by radio frequency (RF) sputtering. Ni/Al grid was additionally deposited for front contacts and 85 nm MgF2 was coated for antireflection. The solar cells were finally annealed at 190–250 ∘ C. The thickness of the evaporated Cu layer was between 0.1 and 10 Å, which contain 8 × 1013 and 8 × 1015 Cu atoms/cm2 .

7.2 Flexible CdTe/CdS Solar Cells

7.2.2.2

Performance Analysis

Figure 7.10a shows the cross-sectional scanning electron microscope (SEM) image of CdTe solar cell in the superstrate configuration, which has been a mainstream structure in solar cell manufacturing over 40 years [100]. In this configuration, the types of substrates of the solar cells are restricted to the transparent ones since the light goes on the substrates first and CdTe/CdS junctions. On the other hand, in the substrate configuration, the flexible metal foil substrates are available for the solar cells due to the top incidence of light (Figure 7.10b,c). This configuration enables manufacturing through R2R process with low cost. Still, the substrate configuration exhibits lower efficiency than the superstrate configuration, although it has lower production cost. Figure 7.11a,b show the electron beam-induced current (EBIC) measurements of Cu-free and Cu-doped flexible CdTe solar cells, representing the depth-dependent efficiency

Back contact

Front contact n-CdS

p-CdTe p-CdTe

n-CdS Front contact Substrate (a)

Back contact Substrate (b)

(c)

Figure 7.10 Structure of CdTe solar cells. Cross-sectional scanning electron microscope (SEM) images of CdTe solar cells in (a) superstrate and (b) substrate configurations. The substrate structure enables the use of metal foils as substrates. Mo/MoOx is used for back contact and i-ZnO/ZnO:Al is used for front contact for the substrate configuration. Scale bar = 1 μm. The illumination direction is indicated as yellow arrows. (c) Photograph of the flexible CdTe solar cells on metal foil substrate. Source: Reprinted with permission from Kranz et al. [99]. Copyright 2013, Springer Nature.

225

100 90 80 (c) 70 60 50 Vbi 40 30 Normalized current (%)

(a) Collection

Front contact

Generation

(b)

CdS Vbi

Collection

5

10

15

20

CdTe EC

EV

Back contact

Cu-free

Generation

EC

EV

Back contact

Front contact

7 Novel Materials-Based Flexible Solar Cells

Cu-doped

226

(d)

(μm)

Figure 7.11 Electron beam-induced current (EBIC) measurements of CdTe solar cells. EBIC measurements of (a) Cu-free and (b) Cu-doped CdTe solar cells (1 × 1015 Cu atoms/cm2 ). The current is collected at the back side of Cu-free solar cell while the current is collected more efficiently at the front side of the Cu-doped solar cell. Energy band diagrams for (c) Cu-free and (d) Cu-doped CdTe solar cells. Compared to the Cu-free solar cell band bending, the energy bands of Cu-doped solar cell are bent at the front contact, in which efficient carrier collection is feasible. E c , E v , and V bi denote conduction band, valence band, and built-in voltage, respectively. Source: Reprinted with permission from Kranz et al. [99]. Copyright 2013, Springer Nature.

of solar cells. Effective carrier generation is preferred by the incident sunlight and most of the incoming sunlight (90%) is absorbed at the interface of CdTe/CdS layers. In a Cu-doped solar cell, higher carrier generation is observed in a region near the CdS layer, while in a Cu-free solar cell, higher carrier generation is observed in a region close to the back contact. As a result, Cu-doping helps the effective region of carrier collection shift to region of carrier generation, with increase in the efficiency [99]. Figure 7.11c,d show the energy band diagrams for Cu-free and -doped flexible CdTe solar cells, supporting the EBIC measurement results in Figure 7.11a,b. In the band diagram of Cu-free solar cell, the energy band near the interface of CdS/CdTe is almost flat, and close to the back contact it is bent, leading to efficient carrier collection near the back contact. In contrast, in the band diagram of Cu-doped solar cell, the energy band near the interface of CdS/CdTe and at the front contact is bent, resulting in efficient carrier collection in the region of carrier generation by the incoming sunlight. The Cu-doped CdTe/CdS solar cell on glass substrate in substrate configuration showed an efficiency of 13.6% (Figure 7.12a). The open-circuit voltage (V oc ) and FF of the solar cell are 852 mV and 75.3%, respectively, which reaches the record cells in superstrate configuration with V oc of 857 mV and FF of 79.0% [101]. Figure 7.12b exhibits the quantum efficiency (QE) measurements of CdTe solar cells in substrate configurations on glass (green), Mo foil (red), and steel foil (blue) substrates. The decrease in the QE below 550 nm is because of the absorption of the CdS layer, which implies the possible increase in the QE with reduced thickness of CdS layer.

7.2 Flexible CdTe/CdS Solar Cells

Current density (mA/cm2)

0

–5

–10

–15

–20

–25 0

200

400 600 Voltage (mV)

(a)

800

Quantum efficiency (normalized)

1.0

0.8

0.6

0.4

0.2

0.0 400 (b)

500

600 700 Wavelength (nm)

800

900

Figure 7.12 Electrical characteristics of CdTe solar cells. (a) Current density–voltage and (b) quantum efficiency (QE) measurement of CdTe solar cells on glass (green), Mo foil (red), and steel foil (blue) substrates. The decrease in the QE for wavelengths below 550 nm is due to the CdS layer absorption, which indicates the possible increase in the QE with reduced thickness of CdS layer. Source: Reprinted with permission from Kranz et al. [99]. Copyright 2013, Springer Nature.

7.2.3 7.2.3.1

Flexible CdTe/CdS Solar Cells on Polymer Film Sample Preparation

CdTe/CdS solar cells were grown on PI substrates in superstrate configuration [102]. As the first step, 100 nm MgF2 was applied by e-beam evaporation as antireflection coating on the light incident side of the PI substrates. For the aluminum-doped zinc oxide (ZnO:Al) transparent conducting oxide (TCO)

227

7 Novel Materials-Based Flexible Solar Cells

CdS

TCO

CdTe

Active layers

CdCl

Activation

Scribing

Scribing

Foil

Scribing

228

Bc

Back contact

Transperent conductive oxide

Figure 7.13 Concept of roll-to-roll coating for production of CdTe solar cell module. The high speed and low temperature are merits for the roll-to-roll coating process. Source: Reprinted with permission from Perrenoud et al. [102]. Copyright 2011, Elsevier.

growth for the front contact layer, RF sputtering was applied at 300 ∘ C. Then, intrinsic ZnO (i-ZnO) was deposited as a highly resistive transparent layer. The CdS layer was deposited by evaporation at 165 ∘ C and annealed at 420 ∘ C for 30 nm, followed by the deposition of CdTe layer at 350 ∘ C. A 400 nm CdCl2 layer was deposited on CdTe/CdS film by evaporation and annealed at 420 ∘ C for 20 minutes. A Cu/Au layer was used for back contact. The measurements for transmittance (T) and reflectance (R) were performed with a Shimadzu UV 3600 spectrometer. The current–voltage and spectral response (SR) measurements of the solar cells were performed according to the IEC 60904-1 Ed.2 and IEC 60904-8 Ed2. The short-circuit current density (J sc ) was obtained from external quantum efficiency (EQE) under AM1.5G using the reference spectrum IEC 609004-4 Ed.2 (Figure 7.13). 7.2.3.2

Performance Analysis

The optical properties of PI with different thickness were investigated in order to clarify the effects of optical loss due to absorption in the PI substrates of CdTe solar cells in superstrate configuration. With the PI films with the thicknesses of 7.5, 12.5, 25, and 50 mm, the transmittance (T) and reflectance (R) were measured, based on the relationship T(𝜆) = (1 − R(𝜆))e−𝛼(𝜆)x , where T(𝜆) is the measured transmittance, R(𝜆) the measured reflectance, 𝛼(𝜆) the absorption coefficient, and x the PI thickness [102]. The spectra of T and T/(1 − R) of PI films are shown in Figure 7.14, in which the T and R measurements determine the absorption coefficient of PI films as 5 × 103 cm−1 in the range 300–400 nm. The complete absorption below 400 nm is due to the absorption coefficient of the PI films. The loss of the solar cells on PI substrates consists of reflection and absorption factors. The reflection loss is defined by the measured reflectance

7.2 Flexible CdTe/CdS Solar Cells

100 12.5 μm

T (%) and T/(1 − R) (%)

7.5 μm

80

7.5 μm

25 μm 25 μm

60

12.5 μm 50 μm 50 μm Jsc PIA limit Pl mA/cm2 μm – 30.1 7.5 25.5 22.9 12.5 19.3 25 12.5 50 (range 300–845 nm)

40

20

0 400

600

800

1000

Wavelength (nm)

Figure 7.14 Transmittance of polyimide (PI) films. Transmittance (T) (solid lines) and T/(1 − R) (dashed lines) of PI films with various thickness. The inset shows the Jsc PIA limit of CdTe solar cells on the PI films. The Jsc PIA limit is obtained after subtracting absorption of photons in the PI films. Source: Reprinted with permission from Perrenoud et al. [102]. Copyright 2011, Elsevier.

weighed with the absorption. The reflection loss was calculated from the 7.5, 12.5, 25, and 50 mm PI films as 4.2, 3.4, 2.8, and 1.5 mA/cm2 , respectively. This loss can be reduced with the antireflection coating for the front side and the ZnO for the back side. The absorption loss can be reduced through luminescent down shifting [103, 104]. A total of 11 solar cells were electrically integrated in series. The optical loss due to the interconnection area was 15%. The electrical characteristic of the interconnected solar cells is shown in Figure 7.15. The inset shows the performance parameters of the solar cells. The short-circuit current density (J sc ) of the module was obtained from the average value of individual single cells. The efficiency of the solar module for the total area and active area are 8.0% and 9.4%, respectively. For performance improvement, the implementation of high mobility TCO and stable back contact is expected. 7.2.4 7.2.4.1

Flexible CdTe/CdS Nanopillar Solar Cells Sample Preparation

The growths of CdS nanopillar and CdTe thin film were performed in a 1 in. quartz furnace [105]. CdS power was placed in the first heating zone as the source and the CdS nanopillar was grown by the template-assisted Vapor-Liquid-Solid method with the 50 sccm H2 transport gas with a 15 Torr chamber pressure. The anodic alumina membrane (AAM) growth template was placed with electroplated Au seeds in the second heating zone. The temperatures of the first and second heating zones were 700 and 550 ∘ C, individually. The growth was carried out for 30 minutes. A 400–600 nm CdS nanopillar was exposed after the etching in 1 N NaOH at RT for 50–60 nm. The CdTe thin film was then deposited on the CdS nanopillar. For the CdTe deposition, the CdS nanopillar array was placed

229

7 Novel Materials-Based Flexible Solar Cells

0 31.9 cm2 module (11 cells) –10 Current (mA)

230

Tot. area (cm2) Voc (V) FF (%) Isc (mA) η (tot. area) (%)

–20 –30

31.9 8.35 59.3 51.5 8.0

–40 –50 0

2

4 Voltage (V)

6

8

Figure 7.15 Electrical characteristics of a monolithically integrated flexible CdTe solar cell module. Eleven cells are interconnected in series and width/length of each cell is 0.5 cm/5.8 nm. The interconnection loss is around 15%. The efficiency of integrated solar cells on PI film is 8.0%. Source: Reprinted with permission from Perrenoud et al. [102]. Copyright 2011, Elsevier.

in the second zone and the CdTe power was placed in the upper flow zone as the source with the 19 mTorr pressure. The temperatures of the first and second heating zones were 650 and 400 ∘ C, individually. The growth was carried out for 50 minutes. A 1/13 nm Cu/Au bilayer was deposited as the top contact by evaporator. The solar cell was encapsulated with polydimethylsiloxane (PDMS) for the flexible modules. 7.2.4.2

Performance Analysis

Among several factors determining the efficiency of thin-film solar cells, the absorption and minority carrier lifetime in the materials mainly decide the energy conversion efficiency [106]. In this context, the device structure with thickness comparable to the light absorption depth and short minority carrier lifetime is preferred. It is also known that 3-dimensional device structure can enhance the light absorption efficiency [107]. These results reveal that the use of CdS nanopillar structure can enhance the energy efficiency of the solar cells. Figure 7.16a and b depict the energy band diagram of a CdTe/CdS photovoltaic and the schematic of carrier generation in the solar nanopillar (SNOP) array, respectively. The merit of the fabrication method using AAM template is the capability to achieve the growth of single crystalline CdS nanopillars on an amorphous substrate (Figure 7.16c). Figure 7.17a,b display the SEM images of AAM template and CdS nanopillar array, in which the highly periodic AAM with pores and CdS nanopillars is formed. The transmission electron microscope (TEM) image of CdTe/CdS SNOP interface exhibits the single crystalline property of CdS nanopillar with a near 1 : 1 stoichiometric composition and polycrystalline property of CdTe layer (Figure 7.17c). Figure 7.18a,b show the

7.2 Flexible CdTe/CdS Solar Cells

EC EF

EV (a)

CdTe

n-CdS

CdS

p-CdTe

AAM Au

(c)

(b)

Cu/Au Superstrate

Electron

Hole

Al CdS nanopillar

CdTe

Figure 7.16 Structures of CdTe/CdS solar nanopillar (SNOP) cells. (a) Band diagram of CdTe/CdS SNOP cells for photovoltaic. (b) Cross-sectional schematics of CdTe/CdS SNOP cells with enhanced carrier collection efficiency. (c) Fabrication process of SNOP cells. Source: Reprinted with permission from Fan et al. [105]. Copyright 2009, Springer Nature.

SNOP cells encapsulated with bending property. Finite-element simulation was performed to investigate the effects of strain on the performance of SNOP cells, in which maximum 0.01% strain exists in the CdS nanopillar (Figure 7.18c,d). The electrical characteristics and conversion efficiency of the SNOP cells under different bending radius are shown in Figure 7.18e,f, respectively. The performance of SNOP cells was maintained under the bending conditions. Further research is required to optimize the performance and low-cost production. 7.2.5 7.2.5.1

Flexible CdTe/CdS Solar Cells on Thin Glass Sample Preparation

Flexible CdTe solar cells were constructed on an alternative substrate, Corning Willow Glass [108, 109]. The flexible glass has several advantages such as amenability to high temperature process and high transparency. The first sample

231

232

7 Novel Materials-Based Flexible Solar Cells

2 μm

500 nm

(a)

(b)

(c)

Figure 7.17 SNOP cell fabrication stages. (a) SEM image of highly periodic anodic alumina membrane (AAM) with perfectly ordered pores. (b) SEM image of CdS nanopillar array formed by the etching of AAM. (c) Transmission electron microscope (TEM) measurement of CdTe thin film/CdS nanopillar interface. The inset shows the diffraction pattern with symmetric spots of single crystalline CdS nanopillar and multi-rings of polycrystalline CdTe thin film. Source: Reprinted with permission from Fan et al. [105]. Copyright 2009, Springer Nature.

shows 14.05% efficiency with the 5.1 cm radius of bending curvature without degradation of solar cell performance. The second sample achieves a 16.42% efficiency with a sputtered CdS:O for window layer and ZnTe:Cu/Au for back contact. The CdTe thin film for both samples is identical. The sputtered CdS:O improved the uniformity of the window layer and short-circuit current density (J sc ). The co-evaporated ZnTe:Cu back contact increased the open circuit voltage (V oc ) and FF. For the solar cell fabrication, Corning Willow Glass was first cut into 100 μm × 38.1 mm × 38.1 mm. SnO2 :F and SnO2 bilayers were deposited as TCO layers at 550 ∘ C by metal–organic chemical vapor deposition (MOCVD). A 100 nm CdS:O layer was deposited by RF sputtering using CdS target. The CdTe thin film was deposited by close-spaced sublimation (CSS) at the CdTe

7.2 Flexible CdTe/CdS Solar Cells

CdTe CdS nanopillar AAM In contact PDMS

Cu/Au contact

(a)

(b)

0.04

(mm)

εχ

0.02 0

–0.02 –0.04 –0.6 –0.08 4

J (mA/cm2)

40 20 0

Bending radius ∞ 91 mm 67 mm 54 mm 48 mm 39 mm 31 mm 27 mm

12 8 (mm)

16

3 2 1 0

–1

–2 –3

0

(d)

1

2

3

–4

4

(μm) 4

r

α

Efficiency (%)

0

(c)

4

2.0 0.5 1.0 0.5 0 –0.5 –1.0 –1.5 –2.0

εx (×10–4)

4 2 0 –2 –4 –6 –8 –10

0.06

(μm)

0.08

3 2 L R

1 –20

D α

0 –1.0

(e)

–0.5

0 0.5 Voltage (V)

1.0

2

(f)

3

4 5 6 7 8 Bending radius (cm)

9

10

Figure 7.18 Flexible SNOP cells. (a) Structure of SNOP cell embedded in PDMS. (b) Optical image of bendable SNOP cell. (c) Simulation of strain in the bendable SNOP cell with 4 mm PDMS, in which maximum 0.01% strain exists in the CdS nanopillar (d). (e) Electrical characteristics of SNOP cells under the various bending radii. (f ) Efficiency measurements on bending of substrate. The insets shows the measurement set up for flexible module. Source: Reprinted with permission from Fan et al. [105]. Copyright 2009, Springer Nature.

source temperature of 660 ∘ C for 2.5 minutes. Co-evaporation of ZnTe and Cu was performed for the deposition of ZnTe:Cu back contact, followed by Au. 7.2.5.2

Performance Analysis

Figure 7.19a shows the transmittance of 100 μm thick Corning Willow Glass with the other data from rigid 1.1 mm Corning 7059 and 3.8 mm soda lime glasses and 7.5 μm thick PI foil, which were measured using a spectrophotometer. Throughout the ultraviolet (UV) and visible region, all glass substrates have high transmission ratio. Compared to the other rigid glass substrates, the flexible Willow Glass substrate has stable transmission ratio below 300 nm due to the thinness of Willow Glass, whereas the PI foil has no transmission below 400 nm because of high absorption of PI foil. Figure 7.19b shows the current density–voltage characteristics of the flexible CdTe solar cells on Willow Glass. The efficiency of 14.05%

233

7 Novel Materials-Based Flexible Solar Cells

100

Transmittance (%)

80

60

40

Willow glass 7059 glass Soda lime glass Polyimide, 7.5 μm

20

0 200

300

(a)

400

500

600

700

800

900

Wavelength (nm)

Figure 7.19 Flexible CdTe solar cells on thin-film glass. (a) Transmittance of various substrates for solar cells: Willow Glass, Corning 7059 glass, soda-lime glass, and PI. The transmittance is measured by N%K spectrophotometer. (b) National renewable energy laboratory (NREL)-certified 14.05% efficiency of flexible CdTe solar cells on Willow Glass. A 100 nm MgF2 antireflection layer is deposited prior to certification. Source: Reprinted with permission from Rance et al. [108]. Copyright 2014, AIP Publishing.

7 6 5 Current (mA)

234

4

Device area = 0.25 cm2 Irradiance = 1000 W/m2 Voc = 0.8217 V

3

Isc = 6.0248 mA Jsc = 24.334 mA/cm2

2

Fill factor = 70.26% Efficiency = 14.05%

1 0 –1 –0.2 (b)

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

was measured using GaAs reference diode calibrated by a National renewable energy laboratory (NREL). A 100 nm MgF2 layer was deposited by evaporation for antireflection. The high efficiency of the flexible solar cell is mainly due to the high photocurrent of 24.3 mA/cm2 , which is related to the high transmission property of Willow Glass. Figure 7.20 shows the electrical characteristics of the flexible CdTe solar cells under the bending conditions. In all compressive and tensile conditions, the V oc of solar cells was decreased, and the V oc was recovered as the strain was removed from the solar cells. Figure 7.21 shows the current–voltage characteristic of flexible CdTe solar cell on Willow Glass with a sputtered CdS:O

7.2 Flexible CdTe/CdS Solar Cells

0

Current density (mA/cm2)

–5

Voc Jsc FF n Inital 809 23.4 61.9 11.7 Flex 776 23.8 64.7 11.9

Voc Jsc FF n Inital 811 22.3 68.1 12.3 Flex 778 23.3 71 12.9 Flex. 24hrs 778 22.6 69.8 12.3 Relax 803 21.9 69 12.1

Flex. 24hrs 781 23.3 67.3 12.3 Relax 803 23 66.4 12.2

–10

Compression, rbend = 3.9 cm

Compression, rbend = 5.1 cm

–15

–20

–25 0.0 (a)

0.2

0.4 0.6 Voltage (V)

0.8

0.2 (b)

0.4 0.6 Voltage (V)

0.8

Voc Jsc FF n Inital 808 24.5 64.8 12.8 Flex 771 26.1 65.2 13.1 Flex. 24hrs 771 25.8 65.3 13 Relax 803 24.5 65.7 12.9

Tension, rbend = 5.7 cm

0.2 (c)

0.6 0.4 Voltage (V)

0.8

Figure 7.20 Bending tests of flexible CdTe solar cells. Electrical performance of flexible CdTe solar cells with the bending radius of (a) 5.1 cm, (b) 3.9 cm, and (c) 5.7 cm. The insets show the bending configurations (compression or tension). Source: Reprinted with permission from Rance et al. [108]. Copyright 2014, AIP Publishing.

window layer and co-evaporated ZnTe:Cu/Au back contact. The NREL-certified efficiency of 16.4% is the highest for flexible CdTe solar cells. The increase in the FF, compared to the 14.05% of solar cell, is ascribed to the evaporated ZnTe:Cu back contact. Figure 7.22a shows the comparison of QE between the 16.4% and previous 14.0% solar cells. The QE of solar cells with the sputtered CdS:O window

235

7 Novel Materials-Based Flexible Solar Cells

2.0

1.5

Current (mA)

Device area = 0.070 25 cm2 Irradiance = 1000.0 W/m2

1.0

Voc = 0.8313 V Isc = 1.7927 mA Jsc = 25.519 mA/cm2

0.5

Fill factor = 77.43% Efficiency = 16.42% 0.0

–0.5 –0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 7.21 Flexible CdTe solar cells with 16.42% efficiency. NREL-certified 16.42% efficiency of flexible CdTe solar cells on Willow Glass. A 100 nm MgF2 antireflection layer is deposited by thermal evaporator prior to certification. Source: Reprinted with permission from Mahabaduge et al. [109]. Copyright 2015, AIP Publishing.

Quantum efficiency (%)

100 80 60 40 14.0% 16.4%

20 0 300

400

(a)

500

600

700

800

900

Wavelength (nm) Co-evaporated ZnTe:Cu Sputtered ZnTe:Cu

2

1015 NA(cm–3)

236

6

1.25 μm

4

2.15 μm

2

1014 6

0.5 (b)

1.0

1.5

2.0

w (μm)

2.5

3.0

Figure 7.22 Performance enhancement of CdTe solar cells. (a) Enhanced QE (16.42%) of CdTe solar cells with sputtered CdS:O. (b) Acceptor density profile of evaporated (and sputtered) ZnTe:Cu back contact used for 16.42% solar cell. The arrow indicates the zero bias point with depletion width. Source: Reprinted with permission from Mahabaduge et al. [109]. Copyright 2015, AIP Publishing.

7.3 Infrared Colloidal Quantum Dots Solar Cell

layer is improved by the increased transmission of CdS:O layer. The slightly low QE at around 600–700 nm is attributed to the thickness of SnO2 :F layer for the solar cells with sputtered CdS:O. Figure 7.22b shows the net acceptor density (N A ) over depletion width (w) for two types of solar cells. Both profiles show the U-shape properties of CdTe/CdS thin-film solar cells [110], in which the carrier density of the solar cell with the co-evaporated ZnTe:Cu back contact is higher [111]. Further enhancement in the efficiency of the CdTe solar cells is viable with more ideal Cu profile and transparent front contact [109]. 7.2.6

Outlook

Over the past decades, the field of solar cells is rapidly growing at a rate of 10–40% [112]. It is recognized as a critical energy solution for sustainable development. The widespread adoption is attributed to this consideration. Based on the previous discussion, novel device structures are applied to obtain more efficient and cost-effective flexible CdTe solar cells. For the flexible solar cells to compete with the rigid ones, the ability to produce solar cell modules with comparable efficiency and cost to the rigid structures is the critical challenge. Also, the ability to maintain reliability (longevity and less degradation) is another key challenge for further investigations. In addition, strategies to discover new markets such as space area and consumer electronics are necessary.

7.3 Infrared Colloidal Quantum Dots Solar Cell 7.3.1

Introduction

Solar energy is one of the promising energy sources for the near future. However, solar energy currently represents a minute portion of the current energy mix. There are several challenges that require to be solved before using solar energy widely. One of the challenges comes from the fact that the Sun’s broad spectrum (Figure 7.23a) covers the whole range of the visible and a considerable portion of the infrared regions. Half of the solar energy reaching the Earth’s surface lies in the infrared region. This means we also need solar cells with a broad absorption band. The simplest solution is to make a solar cell with multiple junctions, which increases the PCE of the solar cell. From the Sun’s power spectrum reaching the Earth, we can find the optimal bandgap choices for each number of junctions (Figure 7.23b). In the multijunction solar cells, the first large-bandgap cell absorbs high-energy photons in the sunlight, generating a correspondingly large Voc; and then the following cells absorb the lower-energy photons, contributing another portion to the Voc. And we can see that the three infrared wavelengths are adopted for the optimal design for a 4-junction solar cell, which shows the great importance of infrared solar cells. Another challenge comes from the disadvantages of the single-crystal solar cells: high expense and low physical flexibility. Therefore, solar cell materials that

237

7 Novel Materials-Based Flexible Solar Cells

Spectral power (W/m2 nm)

2

1.5

1

0.5

0 500

(a) Power-conversion efficiency (%)

238

2000

2500

70 60 50 40 30 20

Eg = 1.13 eV

Eg = 0.94 eV 1.64 eV

Eg = 0.71 eV 1.16 eV 1.83 eV

Eg = 0.71 eV 1.13 eV 1.55 eV 2.13 eV

10 0 1

(b)

1000 1500 Wavelength (nm)

2

3 4 Number of junctions

Infinite

Figure 7.23 The Solar power spectrum at the surface of Earth and the corresponding optimal design for solar cells. (a) The unconcentrated AM1.5 solar spectrum. (b) The maximum PCE that an n-junction (n = 1, 2, 3, …) solar cell can achieve for unconcentrated AM1.5 illumination. The optimal bandgap(s) for the corresponding junction(s) are indicated within the bars of the plot. Source: Reproduced with permission from Sargent [113]. Copyright 2009, Nature Publishing Group.

are compatible with large-area application, such as amorphous silicon, copper indium gallium selenide, cadmium telluride, and photochemical, have been the hot topics of recent research [114]. Colloidal quantum dots (CQDs) provide a possible solution to the previous challenges. CQDs are nanoscale semiconductor particles (active light absorbers) synthesized in and processed from solution. The solvent evaporates during the photovoltaic manufacturing process. Being generally noncrystalline or polycrystalline, they have no issue of lattice matching from which the single-crystal tandem solar cells suffer, nor the requirement for a rigid crystalline substrate on which the crystal to grow. This makes it possible for CQD inks to be painted onto flexible substrates for large-scale, low-cost fabrication of solar cells, offering promise with respect to the cost per area. CQDs also exhibit quantum size effect

7.3 Infrared Colloidal Quantum Dots Solar Cell

tunability, tuning the absorption range of the CQDs by changing the size of the nanocrystals. This allows the absorption range of CQD films covers most of the infrared region of the solar spectrum, giving access to half of the available solar energy lying in the infrared which is unusable by other traditional materials. 7.3.2

Infrared PbS Quantum Dots Solar Cell

The first infrared-sensitive CQD solar cell is reported by Prof. Edward H. Sargent’s group in 2005 [115]. In their report, quantum dot nanocrystals of PbS, chosen to sensitize the conjugated polymer poly[2-methoxy-5-(2′ -ethylhexyloxyp-phenylenevinylene)] (MEH-PPV) whose absorption spectrum lies between ∼400 and ∼600 nm, were tuned to have absorption peaks from ∼800 to ∼2000 nm [115]. In order to achieve charge separation between the polymer and the nanocrystal, the band structure of the polymer has to be carefully selected. Owing to the higher hole mobility in conjugated polymers than electron mobility, it requires a band alignment in favor of photogenerated holes transferring from the nanocrystal to the polymer; that is, the ionization energy (the energy difference between the vacuum and the valence band for semiconductors) of the polymer should be larger than that of the nanocrystal for the best performance. The bulk ionization energy of PbS is ∼4.95 eV, while that of most conjugated polymers is larger than ∼5.3 eV. MEH-PPV was intentionally chosen for its low ionization energy, which is between ∼4.9 and ∼5.1 eV [115]. The devices consist of a sandwich structure of glass, ITO, PPV (hole transport layer, ∼40 nm), MEH-PPV/PbS nanocrystal blend (90% nanocrystal by weight, 100–150 nm), and an upper magnesium contact (150 nm Mg/100 nm Al/10 nm Au) (Figure 7.24b). The PPV layer brings several benefits in addition to acting as a hole transport layer: it improves electrical stability with a smooth and pinhole-free pre-layer, eliminating the catastrophic shorts from the upper contact directly to the ITO layer; and it suppresses the dark current by introducing an injection barrier in the valence band at the ITO contact. The dark current is 216 nA at a bias of 5 V and 144 nA at −5 V (Figure 7.24a) [115]. Figure 7.25 shows the dependence of absorbance spectrum on the nanocrystals, each tuned to a different part of the infrared spectrum. No bias is applied during the measurement. All the absorption at wavelength longer than 600 nm is assigned only to PbS nanocrystals [115]. 7.3.3

Surface Passivation and Air Stability

As shown before, CQD films make possible large-area fabrication and bandgap tuning. However, the large ratio of surface area to volume for the nanocrystals makes CQD films vulnerable to high trap state densities if the surfaces of nanocrystals are untreated, leading to the trap-induced recombination of carriers and poor device performance [116]. Meanwhile, some applications require

239

7 Novel Materials-Based Flexible Solar Cells

40

Dark 2.7 mW 13.6 mW 54.5 mW 207 mW

20

0

Current (μA)

–20

Current (nA)

–40

–60

–80

Dark current

250 200 150 100 50 0 –50 –100 –150

–6 –4 –2 0 2 4 Applied bias (V)

–100 –5

–4

–3

–2

–1

(a)

0

2

1

3

4

6

5

Applied bias (V) Dark 13.6 mW 68.2 mW 123 mW 207 mW

500

250

0.36 V Current (nA)

240

0 PPV MEH-PPV Nanocrystal

–250

Mg

352 nA ITO –500 Ef –750 Not to scale –0.2 (b)

0.0

0.2

0.4

0.6

0.8

Applied bias (V)

Figure 7.24 Dark current and photocurrent versus applied bias at the ITO electrode with device band structure. (a) Main panel: dark current and photocurrent results for a sample with ∼90% by weight nanocrystals in the polymer/nanocrystal blend. Inset: dark current for the main panel. (b) Main panel: dark current and photocurrent curves near zero bias, demonstrating the photovoltaic effect; these data were obtained from a different sample from that shown in (a) and represent the best results in that report for short-circuit current and open-circuit voltage. Inset: proposed simplified band diagram depicting the relative energy alignments after the magnesium electrode has been deposited and the sample has reached equilibrium. Source: Reproduced with permission from McDonald et al. [115]. Copyright 2005, Nature Publishing Group.

500

1000 1500 Wavelength (nm)

Solid lines

Symbols

800

2000

Absorbance (a.u.)

Photocurrent (a.u.)

Photocurrent MEH-PPV Abs. Nanocrystal Abs.

Absorbance (a.u.)

Photocurrent (a.u.)

7.3 Infrared Colloidal Quantum Dots Solar Cell

1000

1200

1400

1600

1800

Wavelength (nm)

Figure 7.25 Photocurrent spectral responses and absorption spectra. Main panel: photocurrent spectral response (symbols) and the corresponding absorption spectra (solid line) for three different samples. The absorption peaks are tuned to 955 (black), 1200 (red), and 1355 nm (blue). Inset: extended spectral response for the sample centered at 955 nm, indicating the response in the region below ∼600 nm where both the polymer and nanocrystal are excited. Also shown are the absorption spectra of the polymer and the nanocrystal. Source: Reproduced with permission from McDonald et al. [115]. Copyright 2005, Nature Publishing Group.

high-quality and high-stability CQD solids that are controllably n-type or p-type [117]. Unfortunately, n-type nanocrystals are prone to oxidation within minutes of air exposure, which also requires surface passivation of the nanocrystals to repel the air attack. There are several different methods to passivate the nanocrystals. Here we introduce two of them. The first option is called ligand passivation (Figure 7.26). PbS CQDs’ Pb2+ -rich surface are initially with deprotonated oleic acid (OA) after their synthesis in solution. In the organic route, 1,2-ethanedithiol (EDT) substitutes the OA ligands and binds to Pb2+ on the surface. There are many choices of the organic ligands. Short alkylthiols, aromatic thiols, alkylamines, and mercaptocarboxylic acids (MPA) have all shown promising achievement of effective passivation [119–123]. However, the vulnerability to oxidation and thermal degradation of

241

242

7 Novel Materials-Based Flexible Solar Cells

EDT OA (I) Cd–TDPA Organic ligand

CTAB

S1 S2 (II)

Atomic passivation

Figure 7.26 Organic and atomic ligand passivation strategies. The molecular structures of EDT, OA, Cd-TDPA, and CTAB are shown as insets. Colors are green (lead), yellow (sulfur), cyan (carbon), white (hydrogen), red (oxygen), gray (cadmium), blue (bromine), and purple (nitrogen). Source: Reproduced with permission from Tang et al. [118]. Copyright 2011, Nature Publishing Group.

these large organic ligands have motivated an inorganic passivation route, which can also be depicted as atomic ligand passivation, to achieve improved passivation and higher packing density. In the atomic route, along with the OA ligands, a cadmium–tetradecylphosphonic acid (Cd-TDPA) complex covers the PbS CQD surface to passivate the S2− anions as the first step (Figure 7.26 S1). Then, a solid-state halide anion treatment, such as employing cetyltrimethylammonium bromide (CTAB), introduces Br− to cap the surface cations (Figure 7.26 S2) to form all-inorganic, halide anion passivated PbS CQDs. The metal chalcogenide complexes (MCC), showing impressive field effect transistor electron mobilities, are one of the new and highly promising inorganic ligands. A device was reported using atomic passivation with 6% solar PCE in 2011 [118]. The second option is called hybrid passivation. Instead of using solid-state ligand exchange, a solution-based approach achieves better control over the charge neutrality on the surface of the nanocrystals, reducing the number of the mid-gap trap states. Halide anions in the solution phase are adopted to bind hard-to-access cations on the PbS CQDs surfaces. Meanwhile, intentionally chosen metal cations (such as CdCl2 dissolved in a mixture of TDPA and

7.3 Infrared Colloidal Quantum Dots Solar Cell

Lead Iodide O2

Chloride Oxygen

Figure 7.27 Surface engineering of CQD solids for air stability. To realize air-stable n-type CQDs, a complete and robust ligand shell would protect the surface against the attack by oxygen. The ligands are chosen so that they can completely passivate the dangling bonds on the CQD surface, while at the same time sterically inhibit oxidative attack. Source: Reproduced with permission from Ning et al. [117]. Copyright 2014, Nature Publishing Group.

oleylamine) bind unpassivated anions on the surface, suppressing the density of valence-band-associated trap states. A device with 7.0% solar PCE was reported in 2012 using this technique [124]. The further development of the hybrid approach is using the molecular halides to passivate the PbS CQDs. A redox reaction (PbS + I2 = PbI2 + S) helps to passivate the PbS CQDs surfaces with iodine molecules (I2 ) dissolved in a nonpolar solvent. This process is conducted in the solution before the solid-state ligand exchange, which can make the CQDs better passivated, resulting from the optimized reactivity and longer carrier diffusion length in the film. A 9.9% solar PCE was reported in 2015 using this approach [125]. Due to the shallow ionization potentials of chalcogenides, they are vulnerable to oxidation and losing electrons, which results in the loss of n-type character when these materials are exposed to the air. By adopting a complete ligand shell, the CQDs’ surface is protected from the effects of direct exposure to oxygen. It is shown that iodide (Figure 7.27), with its large atomic radius, can better protect the surface of CQDs, along with the benefits of small steric hindrance for complete coverage of the nanocrystal surface [117]. 7.3.4

Conclusion

The infrared CQD solar cell has been an active field in the past 10 years (Figure 7.28). It makes it possible to increase the solar PCE by multijunction solar cells. The CQD solar cell provides us with the possibility to have access to the rich energy staying in the infrared of the solar spectrum.

243

Figure 7.28 Solar cell efficiency chart from NREL. The progress of CQD solar cell is shown on the bottom right of the chart. Source: Reproduced with permission from NERL [126]. Copyright 2018, NREL.

References

References 1 Tanaka, H. and Misono, M. (2001). Advances in designing perovskite cata-

lysts. Current Opinion in Solid State and Materials Science 5 (5): 381–387. 2 Cohen, R.E. (1992). Origin of ferroelectricity in perovskite oxides. Nature

358: 136. 3 Peña, M.A. and Fierro, J.L.G. (2001). Chemical structures and performance

of perovskite oxides. Chemical Reviews 101 (7): 1981–2018. 4 Hazen, R.M. (1988). Perovskites. Scientific American 258: 77–81. 5 Glazer, A.M. (1975). Simple ways of determining perovskite structures. Acta

Crystallographica Section A: Foundations 31 (6): 756–762. 6 Nie, W., Tsai, H., Asadpour, R. et al. (2015). High-efficiency

7

8

9

10

11

12

13

14

15

16

solution-processed perovskite solar cells with millimeter-scale grains. Science 347 (6221): 522–525. Di Giacomo, F., Fakharuddin, A., Jose, R., and Brown, T.M. (2016). Progress, challenges and perspectives in flexible perovskite solar cells. Energy & Environmental Science, https://doi.org/10.1039/C6EE01137C 9 (10): 3007–3035. Hwang, K., Jung, Y.-S., Heo, Y.-J. et al. (2015). Toward large scale roll-to-roll production of fully printed perovskite solar cells. Advanced Materials 27 (7): 1241–1247. Chen, Q., Zhou, H., Hong, Z. et al. (2014). Planar heterojunction perovskite solar cells via vapor-assisted solution process. Journal of the American Chemical Society 136 (2): 622–625. Tavakoli, M.M., Gu, L., Gao, Y. et al. (2015). Fabrication of efficient planar perovskite solar cells using a one-step chemical vapor deposition method. Scientific Reports 5: 14083. Lam, J.-Y., Chen, J.-Y., Tsai, P.-C. et al. (2017). A stable, efficient textile-based flexible perovskite solar cell with improved washable and deployable capabilities for wearable device applications. RSC Advances, https://doi.org/10.1039/ C7RA10321B 7 (86): 54361–54368. Zardetto, V., Brown, T.M., Reale, A., and Di Carlo, A. (2011). Substrates for flexible electronics: a practical investigation on the electrical, film flexibility, optical, temperature, and solvent resistance properties. Journal of Polymer Science Part B: Polymer Physics 49 (9): 638–648. Park, J.-I., Heo, J.H., Park, S.-H. et al. (2017). Highly flexible InSnO electrodes on thin colourless polyimide substrate for high-performance flexible CH3 NH3 PbI3 perovskite solar cells. Journal of Power Sources 341: 340–347. Park, M., Kim, H.J., Jeong, I. et al. (2015). Mechanically recoverable and highly efficient perovskite solar cells: investigation of intrinsic flexibility of organic–inorganic perovskite. Advanced Energy Materials 5 (22): 1501406. Wang, X., Li, Z., Xu, W. et al. (2015). TiO2 nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode. Nano Energy 11: 728–735. Troughton, J., Bryant, D., Wojciechowski, K. et al. (2015). Highly efficient, flexible, indium-free perovskite solar cells employing metallic substrates.

245

246

7 Novel Materials-Based Flexible Solar Cells

17

18 19

20

21

22

23

24

25 26

27 28

29

30

Journal of Materials Chemistry A, https://doi.org/10.1039/C5TA01755F 3 (17): 9141–9145. Qiu, L., Deng, J., Lu, X. et al. (2014). Integrating perovskite solar cells into a flexible fiber. Angewandte Chemie International Edition 53 (39): 10425–10428. Qiu, L., He, S., Yang, J. et al. (2016). Fiber-shaped perovskite solar cells with high power conversion efficiency. Small 12 (18): 2419–2424. Wei, J., Li, H., Zhao, Y. et al. (2016). Flexible perovskite solar cells based on the metal–insulator–semiconductor structure. Chemical Communications, https://doi.org/10.1039/C6CC04840D 52 (71): 10791–10794. Lee, M., Jo, Y., Kim, D.S., and Jun, Y. (2015). Flexible organo-metal halide perovskite solar cells on a Ti metal substrate. Journal of Materials Chemistry A, https://doi.org/10.1039/C4TA06011C 3 (8): 4129–4133. Xiao, Y., Han, G., Zhou, H., and Wu, J. (2016). An efficient titanium foil based perovskite solar cell: using a titanium dioxide nanowire array anode and transparent poly(3,4-ethylenedioxythiophene) electrode. RSC Advances, https://doi.org/10.1039/C5RA23430A 6 (4): 2778–2784. Abdollahi Nejand, B., Nazari, P., Gharibzadeh, S. et al. (2017). All-inorganic large-area low-cost and durable flexible perovskite solar cells using copper foil as a substrate. Chemical Communications, https://doi.org/10.1039/ C6Cc07573H 53 (4): 747–750. He, S., Qiu, L., Fang, X. et al. (2015). Radically grown obelisk-like ZnO arrays for perovskite solar cell fibers and fabrics through a mild solution process. Journal of Materials Chemistry A, https://doi.org/10.1039/ C5TA01532D 3 (18): 9406–9410. Kim, B.J., Kim, D.H., Lee, Y.-Y. et al. (2015). Highly efficient and bending durable perovskite solar cells: toward a wearable power source. Energy & Environmental Science, https://doi.org/10.1039/C4EE02441A 8 (3): 916–921. Li, Y., Meng, L., Yang, Y. et al. (2016). High-efficiency robust perovskite solar cells on ultrathin flexible substrates. Nature Communications 7: 10214. Pisoni, S., Fu, F., Feurer, T. et al. (2017). Flexible NIR-transparent perovskite solar cells for all-thin-film tandem photovoltaic devices. Journal of Materials Chemistry A, https://doi.org/10.1039/C7TA04225F 5 (26): 13639–13647. Liu, Z., You, P., Xie, C. et al. (2016). Ultrathin and flexible perovskite solar cells with graphene transparent electrodes. Nano Energy 28: 151–157. Chen, L., Xie, X., Liu, Z., and Lee, E.-C. (2017). A transparent poly(3,4-ethylenedioxylenethiophene):poly(styrene sulfonate) cathode for low temperature processed, metal-oxide free perovskite solar cells. Journal of Materials Chemistry A, https://doi.org/10.1039/C6TA10588B 5 (15): 6974–6980. Sun, K., Li, P., Xia, Y. et al. (2015). Transparent conductive oxide-free perovskite solar cells with PEDOT:PSS as transparent electrode. ACS Applied Materials & Interfaces 7 (28): 15314–15320. Lu, H., Sun, J., Zhang, H. et al. (2016). Room-temperature solutionprocessed and metal oxide-free nano-composite for the flexible transparent bottom electrode of perovskite solar cells. Nanoscale, https://doi.org/10 .1039/C6NR00011H 8 (11): 5946–5953.

References

31 Poorkazem, K., Liu, D., and Kelly, T.L. (2015). Fatigue resistance of a flexi-

32

33

34

35

36

37

38

39

40

41

42 43

44

45

ble, efficient, and metal oxide-free perovskite solar cell. Journal of Materials Chemistry A, https://doi.org/10.1039/C5TA00084J 3 (17): 9241–9248. Roldán-Carmona, C., Malinkiewicz, O., Soriano, A. et al. (2014). Flexible high efficiency perovskite solar cells. Energy & Environmental Science, https://doi.org/10.1039/C3EE43619E 7 (3): 994–997. Jeon, I., Chiba, T., Delacou, C. et al. (2015). Single-walled carbon nanotube film as electrode in indium-free planar heterojunction perovskite solar cells: investigation of electron-blocking layers and dopants. Nano Letters 15 (10): 6665–6671. Dianetti, M., Di Giacomo, F., Polino, G. et al. (2015). TCO-free flexible organo metal trihalide perovskite planar-heterojunction solar cells. Solar Energy Materials and Solar Cells 140: 150–157. Yoon, J., Sung, H., Lee, G. et al. (2017). Superflexible, high-efficiency perovskite solar cells utilizing graphene electrodes: towards future foldable power sources. Energy & Environmental Science, https://doi.org/10.1039/ C6EE02650H 10 (1): 337–345. Ou, X.-L., Feng, J., Xu, M., and Sun, H.-B. (2017). Semitransparent and flexible perovskite solar cell with high visible transmittance based on ultrathin metallic electrodes. Optics Letters 42 (10): 1958–1961. Qiu, L., He, S., Yang, J. et al. (2016). An all-solid-state fiber-type solar cell achieving 9.49% efficiency. Journal of Materials Chemistry A, https://doi.org/ 10.1039/C6TA03263J 4 (26): 10105–10109. Lee, M., Ko, Y., Min, B.K., and Jun, Y. (2016). Silver nanowire top electrodes in flexible perovskite solar cells using titanium metal as substrate. ChemSusChem 9 (1): 31–35. Hu, H., Yan, K., Peng, M. et al. (2016). Fiber-shaped perovskite solar cells with 5.3% efficiency. Journal of Materials Chemistry A, https://doi.org/10 .1039/C5TA09280A 4 (10): 3901–3906. Deng, J., Qiu, L., Lu, X. et al. (2015). Elastic perovskite solar cells. Journal of Materials Chemistry A, https://doi.org/10.1039/C5TA06156C 3 (42): 21070–21076. Lee, M., Ko, Y., and Jun, Y. (2015). Efficient fiber-shaped perovskite photovoltaics using silver nanowires as top electrode. Journal of Materials Chemistry A, https://doi.org/10.1039/C5TA02779A 3 (38): 19310–19313. Li, R., Xiang, X., Tong, X. et al. (2015). Wearable double-twisted fibrous perovskite solar cell. Advanced Materials 27 (25): 3831–3835. Shin, S.S., Yang, W.S., Yeom, E.J. et al. (2016). Tailoring of electron-collecting oxide nanoparticulate layer for flexible perovskite solar cells. The Journal of Physical Chemistry Letters 7 (10): 1845–1851. Mali, S.S., Hong, C.K., Inamdar, A.I. et al. (2017). Efficient planar n-i-p type heterojunction flexible perovskite solar cells with sputtered TiO2 electron transporting layers. Nanoscale, https://doi.org/10.1039/C6NR09032J 9 (9): 3095–3104. Feng, J., Yang, Z., Yang, D. et al. (2017). E-beam evaporated Nb2 O5 as an effective electron transport layer for large flexible perovskite solar cells. Nano Energy 36: 1–8.

247

248

7 Novel Materials-Based Flexible Solar Cells

46 Shin, S.S., Yang, W.S., Noh, J.H. et al. (2015). High-performance flexible

47

48

49

50

51

52

53

54

55

56

57

58

59

perovskite solar cells exploiting Zn2 SnO4 prepared in solution below 100 ∘ C. Nature Communications 6: 7410. Yang, D., Yang, R., Zhang, J. et al. (2015). High efficiency flexible perovskite solar cells using superior low temperature TiO2 . Energy & Environmental Science, https://doi.org/10.1039/C5EE02155C 8 (11): 3208–3214. Park, M., Kim, J.-Y., Son, H.J. et al. (2016). Low-temperature solution-processed Li-doped SnO2 as an effective electron transporting layer for high-performance flexible and wearable perovskite solar cells. Nano Energy 26: 208–215. Zhang, H., Cheng, J., Li, D. et al. (2017). Toward all room-temperature, solution-processed, high-performance planar perovskite solar cells: a new scheme of pyridine-promoted perovskite formation. Advanced Materials 29 (13): 1604695. Zhu, Z., Xu, J.-Q., Chueh, C.-C. et al. (2016). A low-temperature, solution-processable organic electron-transporting layer based on planar coronene for high-performance conventional perovskite solar cells. Advanced Materials 28 (48): 10786–10793. Yao, K., Wang, X., Xu, Y.-x., and Li, F. (2015). A general fabrication procedure for efficient and stable planar perovskite solar cells: morphological and interfacial control by in-situ-generated layered perovskite. Nano Energy 18: 165–175. Qiu, W., Paetzold, U.W., Gehlhaar, R. et al. (2015). An electron beam evaporated TiO2 layer for high efficiency planar perovskite solar cells on flexible polyethylene terephthalate substrates. Journal of Materials Chemistry A, https://doi.org/10.1039/C5TA07515G 3 (45): 22824–22829. Yin, X., Chen, P., Que, M. et al. (2016). Highly efficient flexible perovskite solar cells using solution-derived NiOx hole contacts. ACS Nano 10 (3): 3630–3636. Kogo, A., Ikegami, M., and Miyasaka, T. (2016). A SnOx –brookite TiO2 bilayer electron collector for hysteresis-less high efficiency plastic perovskite solar cells fabricated at low process temperature. Chemical Communications, https://doi.org/10.1039/C6CC02589G 52 (52): 8119–8122. Weerasinghe, H.C., Dkhissi, Y., Scully, A.D. et al. (2015). Encapsulation for improving the lifetime of flexible perovskite solar cells. Nano Energy 18: 118–125. Xu, X., Chen, Q., Hong, Z. et al. (2015). Working mechanism for flexible perovskite solar cells with simplified architecture. Nano Letters 15 (10): 6514–6520. Chen, Y., Chen, T., and Dai, L. (2015). Layer-by-layer growth of CH3 NH3 PbI3−x Clx for highly efficient planar heterojunction perovskite solar cells. Advanced Materials 27 (6): 1053–1059. Lucarelli, G., Di Giacomo, F., Zardetto, V. et al. (2017). Efficient light harvesting from flexible perovskite solar cells under indoor white light-emitting diode illumination. Nano Research 10 (6): 2130–2145. Liu, X., Chueh, C.-C., Zhu, Z. et al. (2016). Highly crystalline Zn2 SnO4 nanoparticles as efficient electron-transporting layers toward stable inverted

References

60

61

62

63

64

65

66

67

68

69

70

71

72

and flexible conventional perovskite solar cells. Journal of Materials Chemistry A, https://doi.org/10.1039/C6TA05745D 4 (40): 15294–15301. Wu, H., Zhang, C., Ding, K. et al. (2017). Efficient planar heterojunction perovskite solar cells fabricated by in-situ thermal-annealing doctor blading in ambient condition. Organic Electronics 45: 302–307. Liu, D. and Kelly, T.L. (2013). Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nature Photonics 8: 133. Kim, J.H., Chueh, C.-C., Williams, S.T., and Jen, A.K.Y. (2015). Room-temperature, solution-processable organic electron extraction layer for high-performance planar heterojunction perovskite solar cells. Nanoscale, https://doi.org/10.1039/C5NR04250J 7 (41): 17343–17349. Ameen, S., Akhtar, M.S., Seo, H.-K. et al. (2015). An insight into atmospheric plasma jet modified ZnO quantum dots thin film for flexible perovskite solar cell: optoelectronic transient and charge trapping studies. The Journal of Physical Chemistry C 119 (19): 10379–10390. Jung, J.W., Williams, S.T., and Jen, A.K.Y. (2014). Low-temperature processed high-performance flexible perovskite solar cells via rationally optimized solvent washing treatments. RSC Advances 4 (108): 62971–62977. You, J., Hong, Z., Yang, Y. et al. (2014). Low-temperature solution-processed perovskite solar cells with high efficiency and flexibility. ACS Nano 8 (2): 1674–1680. Jeong, I., Jung, H., Park, M. et al. (2016). A tailored TiO2 electron selective layer for high-performance flexible perovskite solar cells via low temperature UV process. Nano Energy 28: 380–389. Yoon, H., Kang, S.M., Lee, J.-K., and Choi, M. (2016). Hysteresis-free low-temperature-processed planar perovskite solar cells with 19.1% efficiency. Energy & Environmental Science, https://doi.org/10.1039/ C6EE01037G 9 (7): 2262–2266. Wang, K., Shi, Y., Gao, L. et al. (2017). W(Nb)Ox -based efficient flexible perovskite solar cells: from material optimization to working principle. Nano Energy 31: 424–431. Heo, J.H., Lee, M.H., Han, H.J. et al. (2016). Highly efficient low temperature solution processable planar type CH3 NH3 PbI3 perovskite flexible solar cells. Journal of Materials Chemistry A, https://doi.org/10.1039/C5TA09520D 4 (5): 1572–1578. Jo, J.W., Seo, M.-S., Park, M. et al. (2016). Improving performance and stability of flexible planar-heterojunction perovskite solar cells using polymeric hole-transport material. Advanced Functional Materials 26 (25): 4464–4471. Chen, Z., Yang, G., Zheng, X. et al. (2017). Bulk heterojunction perovskite solar cells based on room temperature deposited hole-blocking layer: suppressed hysteresis and flexible photovoltaic application. Journal of Power Sources 351: 123–129. Ha, J., Kim, H., Lee, H. et al. (2017). Device architecture for efficient, low-hysteresis flexible perovskite solar cells: replacing TiO2 with C60 assisted by polyethylenimine ethoxylated interfacial layers. Solar Energy Materials and Solar Cells 161: 338–346.

249

250

7 Novel Materials-Based Flexible Solar Cells

73 Park, M., Park, J.-S., Han, I.K., and Oh, J.Y. (2016). High-performance

74

75

76

77

78

79

80

81

82

83

84

85

86

flexible and air-stable perovskite solar cells with a large active area based on poly(3-hexylthiophene) nanofibrils. Journal of Materials Chemistry A, https://doi.org/10.1039/C6TA03164A 4 (29): 11307–11316. Liu, Z., Zhu, A., Cai, F. et al. (2017). Nickel oxide nanoparticles for efficient hole transport in p-i-n and n-i-p perovskite solar cells. Journal of Materials Chemistry A, https://doi.org/10.1039/C7TA01593C 5 (14): 6597–6605. Jung, J.W., Park, J.-S., Han, I.K. et al. (2017). Flexible and highly efficient perovskite solar cells with a large active area incorporating cobalt-doped poly(3-hexylthiophene) for enhanced open-circuit voltage. Journal of Materials Chemistry A, https://doi.org/10.1039/C7TA03541A 5 (24): 12158–12167. Gao, L.-L., Liang, L.-S., Song, X.-X. et al. (2016). Preparation of flexible perovskite solar cells by a gas pump drying method on a plastic substrate. Journal of Materials Chemistry A, https://doi.org/10.1039/C6TA00230G 4 (10): 3704–3710. Ryu, S., Seo, J., Shin, S. et al. (2015). Fabrication of metal-oxide-free CH3 NH3 PbI3 perovskite solar cells processed at low temperature. Journal of Materials Chemistry A, https://doi.org/10.1039/C5TA00011D 3 (7): 3271–3275. Xi, J., Wu, Z., Jiao, B. et al. (2017). Multichannel interdiffusion driven FASnI3 film formation using aqueous hybrid salt/polymer solutions toward flexible lead-free perovskite solar cells. Advanced Materials 29 (23): 1606964. Lee, M., Jo, Y., Kim, D.S. et al. (2015). Efficient, durable and flexible perovskite photovoltaic devices with Ag-embedded ITO as the top electrode on a metal substrate. Journal of Materials Chemistry A, https://doi.org/10.1039/ C5TA03240G 3 (28): 14592–14597. Hashmi, G., Miettunen, K., Peltola, T. et al. (2011). Review of materials and manufacturing options for large area flexible dye solar cells. Renewable and Sustainable Energy Reviews 15 (8): 3717–3732. Popoola, I.K., Gondal, M.A., and Qahtan, T.F. (2018). Recent progress in flexible perovskite solar cells: materials, mechanical tolerance and stability. Renewable and Sustainable Energy Reviews 82: 3127–3151. Kaltenbrunner, M., Adam, G., Głowacki, E.D. et al. (2015). Flexible high power-per-weight perovskite solar cells with chromium oxide–metal contacts for improved stability in air. Nature Materials 14: 1032. Xi, J., Wu, Z., Xi, K. et al. (2016). Initiating crystal growth kinetics of 𝛼-HC(NH2 )2 PbI3 for flexible solar cells with long-term stability. Nano Energy 26: 438–445. Di Giacomo, F., Zardetto, V., D’Epifanio, A. et al. (2015). Flexible perovskite photovoltaic modules and solar cells based on atomic layer deposited compact layers and UV-irradiated TiO2 scaffolds on plastic substrates. Advanced Energy Materials 5 (8): 1401808. Liu, T., Kim, D., Han, H. et al. (2015). Fine-tuning optical and electronic properties of graphene oxide for highly efficient perovskite solar cells. Nanoscale, https://doi.org/10.1039/C5NR01433F 7 (24): 10708–10718. Zhao, Y. and Zhu, K. (2014). Optical bleaching of perovskite (CH3 NH3 )PbI3 through room-temperature phase transformation induced by ammonia.

References

87 88 89

90

91

92 93

94

95 96

97

98 99

100

101 102

103

Chemical Communications, https://doi.org/10.1039/C3CC48522F 50 (13): 1605–1607. Green, M.A., Ho-Baillie, A., and Snaith, H.J. (2014). The emergence of perovskite solar cells. Nature Photonics 8: 506. Grätzel, M. (2014). The light and shade of perovskite solar cells. Nature Materials 13: 838. Niu, G., Guo, X., and Wang, L. (2015). Review of recent progress in chemical stability of perovskite solar cells. Journal of Materials Chemistry A, https://doi.org/10.1039/C4TA04994B 3 (17): 8970–8980. Poglitsch, A. and Weber, D. (1987). Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter-wave spectroscopy. The Journal of Chemical Physics 87 (11): 6373–6378. Ito, S., Tanaka, S., Manabe, K., and Nishino, H. (2014). Effects of surface blocking layer of Sb2 S3 on nanocrystalline TiO2 for CH3 NH3 PbI3 perovskite solar cells. The Journal of Physical Chemistry C 118 (30): 16995–17000. Basol, B.M. (1988). Electrodeposited CdTe and HgCdTe solar cells. Solar Cells 23 (1–2): 69–88. Aliyu, M.M., Islam, M.A., Hamzah, N.R. et al. (2012). Recent developments of flexible CdTe solar cells on metallic substrates: issues and prospects. International Journal of Photoenergy. Seth, G.B.L.A., McClure, J.C., Singh, V.P., and Flood, D. (1999). Growth and characterization of CdTe by close spaced sublimation on metal substrates. Solar Energy Materials and Solar Cells 59 (1–2): 35–49. Tiwari, A. N. (2006). Low cost flexible solar cells: prospects and challenges. Presented at the European Energy Fair. Ferekides, C. (2017). The next generation CdTe technology- Substrate foil based solar cells (in English), no. Final Report DE-FG36-08GO18023, 2017/03/22. Khrypunov, A.R.G., Kurdesau, F., Bätzner, D.L. et al. (2006). Recent developments in evaporated CdTe solar cell. Solar Energy Materials and Solar Cells 90 (6): 664–677. Xavier Mathew, J.P.E., Romeo, A., and Tiwari, A.N. (2004). CdTe/CdS solar cells on flexible substrates. Solar Energy 77 (6): 831–838. Kranz, L., Gretener, C., Perrenoud, J. et al. (2013). Doping of polycrystalline CdTe for high-efficiency solar cells on flexible metal foil. Nature Communications 4: 2306. Buecheler, L.K.S., Perrenoud, J., and Tiwari, A.N. (2012). CdTe solar cells. In: Encyclopedia of Sustainability Science and Technology (ed. R.A. Meyers). Springer-Verlag. Green, M.A., Emery, K., Hishikawa, Y. et al. (2013). Solar cell efficiency tables (version 41). Progress in Photovoltaics 21 (1): 1–11. Perrenoud, B.S.J., Buecheler, S., and Tiwari, A.N. (2011). Fabrication of flexible CdTe solar modules with monolithic cell interconnection. Solar Energy Materials and Solar Cells 95: S8–S12. Efthymios Klampaftis, D.R., McIntosh, K.R., and Richards, B.S. (2009). Enhancing the performance of solar cells via luminescent down-shifting of

251

252

7 Novel Materials-Based Flexible Solar Cells

104

105

106 107

108 109

110

111

112

113 114 115

116

117 118 119

the incident spectrum: a review. Solar Energy Materials and Solar Cells 93 (8): 1182–1194. Toshiro Maruyama, R.K. (2001). Transformations of the wavelength of the light incident upon CdS/CdTe solar cells. Solar Energy Materials and Solar Cells 69 (1): 61–68. Fan, Z., Razavi, H., Do, J.-W. et al. (2009). Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates. Nature Materials 8 (8): 648. Alan Fahrenbruch, R.B. (1983). Fundamentals Of Solar Cells, 1e. Joshua, H.A.A., Spurgeon, M., and Lewis, N.S. (2008). A comparison between the behavior of nanorod array and planar Cd(Se, Te) photoelectrodes. The Journal of Physical Chemistry C 112 (15): 6186–6193. Rance, W.L., Burst, J.M., Meysing, D.M. et al. (2014). 14%-efficient flexible CdTe solar cells on ultra-thin glass substrates. Applied Physics Letters 104. Mahabaduge, H.P., Rance, W.L., Burst, J.M. et al. (2015). High-efficiency, flexible CdTe solar cells on ultra-thin glass substrates. Applied Physics Letters 106: 133501. Li, J. V., Duenow, J. N., Kuciauskas, D. et al. (2012). Electrical characterization of Cu composition effects in CdS/CdTe thin-film solar cells with a ZnTe:Cu back contact. Presented at the 2012 IEEE 38th Photovoltaic Specialists Conference (PVSC) PART 2, Austin, TX, USA. Burst, J. M., Rance, W. L., Barnes, T. M. et al. (2012). The effect of CdTe growth temperature and ZnTe:Cu contacting conditions on CdTe device performance. Presented at the 2012 38th IEEE Photovoltaic Specialists Conference, Austin, TX, USA. Reese, M.O. and Barnes, T.M. (2017). Flexible glass in thin film photovoltaics. In: Flexible Glass: Enabling Thin, Lightweight, and Flexible Electronics, Chapter 7 (ed. S.M. Garner). Wiley Scrivener Publishing LLC. Sargent, E.H. (2009). Infrared photovoltaics made by solution processing. Nature Photonics 3 (6): 325. Green, M.A., Hishikawa, Y., Warta, W. et al. (2017). Solar cell efficiency tables (version 50). Progress in Photovoltaics 25 (7): 668–676. McDonald, S.A., Konstantatos, G., Zhang, S. et al. (2005). Solution-processed PbS quantum dot infrared photodetectors and photovoltaics. Nature Materials 4 (2): 138. Gur, I., Fromer, N.A., Geier, M.L., and Alivisatos, A.P. (2005). Air-stable all-inorganic nanocrystal solar cells processed from solution. Science 310 (5747): 462–465. Ning, Z., Voznyy, O., Pan, J. et al. (2014). Air-stable n-type colloidal quantum dot solids. Nature Materials 13 (8): 822. Tang, J., Kemp, K.W., Hoogland, S. et al. (2011). Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nature Materials 10 (10): 765. Klem, E.J.D., Shukla, H., Hinds, S. et al. (2008). Impact of dithiol treatment and air annealing on the conductivity, mobility, and hole density in PbS colloidal quantum dot solids. Applied Physics Letters 92 (21): 212105.

References

120 Koleilat, G.I., Levina, L., Shukla, H. et al. (2008). Efficient, stable infrared

121 122 123

124 125

126

photovoltaics based on solution-cast colloidal quantum dots. ACS Nano 2 (5): 833–840. Luther, J.M., Law, M., Beard, M.C. et al. (2008). Schottky solar cells based on colloidal nanocrystal films. Nano Letters 8 (10): 3488–3492. Talapin, D.V. and Murray, C.B. (2005). PbSe nanocrystal solids for n- and p-channel thin film field-effect transistors. Science 310 (5745): 86–89. Pattantyus-Abraham, A.G., Kramer, I.J., Barkhouse, A.R. et al. (2010). Depleted-heterojunction colloidal quantum dot solar cells. ACS Nano 4 (6): 3374–3380. Ip, A.H., Thon, S.M., Hoogland, S. et al. (2012). Hybrid passivated colloidal quantum dot solids. Nature Nanotechnology 7 (9): 577. Lan, X., Voznyy, O., Kiani, A. et al. (2015). Passivation using molecular halides increases quantum dot solar cell performance. Advanced Materials 28 (2): 299–304. NERL. (2018). Best Research-Cell Efficiencies. https://www.nrel.gov/pv/assets/ images/efficiency-chart.png.

253

255

Index a acido iodidrico (HI) 221 Advillin (Advillin-ChR2) 33 Ag-/Cds-NW’s 103 alkylamines 241 alkylthiols 241 aluminum doped zinc oxide (ZnO:Al) 224, 225, 227 amorphous silicon based solar cells 149–151 anisotropic conductive film (ACF) 7 anodic alumina membrane (AAM) 229, 230, 232 antireflection coating (ARC) layer 90, 189 aromatic thiols 241 atomically thin 2D materials 47

b backside reflector (BSR) 192, 201 (2,2′ ,2′′ -(1,3,5-benzinetriyl)-tris (1-phenyl-1-H-benzimidazole)) 19 bi-color blue-green flexible LED 64 bio-medical domain 41 black phosphorus (BP) transistors 129 blue and green μ-ILEDs 28 blue-emitting μ-ILEDs 5, 6 blue μ-ILED 28 bolometric process 123 bottom-up epitaxy 50–51 bottom-up InP nanowire arrays 61 buffer hole-injection layer (Buf-HIL) 19

bulk ionization energy 239 Buried oxide (BOX) 87, 89, 98

c cadmium selenide (CdSe) QDs 8, 10, 16, 193 cadmium-tetradecylphosphonic acid (Cd-TDPA) 242 carbon based solar cells 153–155 CdTe thin film solar cells 224 cetyltrimethylammonium bromide (CTAB) 242 channel mode fracture 184 channel rhodopsins 46 chemical mechanical polishing (CMP) 86 chemical vapor deposition approaches 213–214 CH3 NH3 PbI3 perovskite solar cells 223 close-spaced sublimation (CSS) 232 clothing-integrated photovoltaics 167 coefficient of thermal expansion (CTE) 94 colloidal perovskite QDs 19 colloidal quantum dots (CQDs) 43, 238, 239, 241–244 color rendering index (CRI) 56, 65 complementary metal-oxide-semiconductor (CMOS) process 52, 94, 117, 144, 184 compound semiconductors 98, 121, 188

Inorganic Flexible Optoelectronics: Materials and Applications, First Edition. Edited by Zhenqiang Ma and Dong Liu. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

256

Index

controlled spalling technology (CST) 183, 184 core-shell nanowire LEDs 51, 53, 54, 63 core-shell QDs 10, 12, 13 corning willow glass 231, 232 crossed-nanowire LED arrays 60 crystalline indium phosphide (InP) 179 crystalline silicon based solar cells HIT 146 light trapping structure 148–149 PERC 145–146 crystalline silicon (c-Si) wafer 146, 150, 157 curvilinear extended light sources 41 CVD-graphene 48, 61, 67, 126, 128, 131

d direct mechanical peel-off of polymerembedded nanowires 43 double-heterostructure (DH) single-junction solar cells 180 double junction cells 180–182 InGaP/GaAs 180–182 dual transfer printing procedure 6 dye-sensitized solar cells (DSSCs) 126, 127, 143

e e-beam evaporation 89, 90, 97, 227 efficiency droop 50, 51 electrochemical etching 86, 149 electroluminescent (EL) QD-based LEDs (QLEDs) 7–16, 19, 34 electron backscatter diffraction 48 epitaxial graphene 48 epitaxial growth technique 86 epitaxial lift-off (ELO) 42, 44, 46, 47, 69, 179, 183, 186–189, 191, 193, 202 epitaxial lift-off/transfer printing 42 1,2-Ethanedithiol (EDT) 241, 242 external quantum efficiency (EQE) 5, 10, 12, 13, 16, 19, 42, 50, 53, 88, 89, 93, 96, 119, 129, 131, 181, 228

external quantum efficiency (EQE) spectra 93, 181

f figures of merits (FOMs) 118, 120 flexible all-graphene based devices 23 flexible blue emitters 46 flexible blue LED 45 flexible CdTe/CdS solar cells nanopillar solar cells performance analysis 230–231 sample preparation 229–230 polymer film performance analysis 228–229 sample preparation 227–228 thin glass performance analysis 233–237 sample preparation 231–233 flexible lateral p-i-n photodetector array 88, 89 flexible LEDs 1, 20, 23, 25–28, 41–70 flexible LEDs based on inorganic bottom-up nanowires nitride flexible LEDs on metal foils 57–59 ZnO nanowires 55–57 flexible LEDs based on thin film transfer conventional approaches for lift-off and transfer of thin crystalline films 43–46 thin film mechanical transfer using van de Waals epitaxy 47–49 flexible III-V LEDs 46 flexible light sources and displays 41, 42 flexible micro-inorganic LEDs (μ-ILEDs) 2–7, 10, 25, 28, 31, 34 flexible nitride nanowire LEDs 62, 63, 65 flexible nitride thin-film LEDs 45 flexible optoelectronic systems in neuroscience research genetically modified recording and stimulation 28–31 injectable neural probes 25–28 peripheral nervous system 32–33

Index

flexible perovskite LEDs (PeLEDs) 16–20 flexible perovskite solar cell performance analysis 215, 221 sample preparation 214–215 flexible photodetectors nanomembranes (NM) 82 nanowires (NW) 81 performance parameters detectivity 83 I on/I off current ratio 84 photo conductive gain (G) 84 response time 84 responsivity 82–83 sensitivity (S) 84 flexible quantum dot LEDs (QLEDs) 7–16, 19 flexible red-emitting QLEDs 12 flexible silicon solar cells bottom-up method layer transfer method 156 SOI method 156–157 top-down method SOM exfoliate method 157 wet etching method 157 flexible SNOP cells 233 flexible solar cells applications 167–169 failure mechanism fracture mechanics 166 Park’s experimental study 162–163 shear lag model 163–166 flexible perovskite solar cells 214–221 performance analysis 215, 221 stability issues 221–222 flexible perovskite solar cell sample preparation 214–215 perovskites materials chemical vapor deposition approaches 213–214 solution process deposition approaches 209–211 vapor-assisted solution deposition approaches 211–213

flexible substrates 214–221 2D photodetectors photogating detectors 135 photothermal effect 129 photovoltaic effect 125–126 plasma-wave-assisted THz detector 129–131 flexible transfer-printed blue InGaN LEDs 46 flexible 2D materials-based LEDs 20–23 flip-chip and vertical thin-film LEDs 45 fluorescent calcium indicators 24 4 junction (4J) based cells 181, 197, 200 4-junction solar cell 237 frame-assisted membrane transfer (FAMT) 98 fully-printed PeLEDs 20 fully transparent flexible nanowire LED 64

g GaAs/monolayer graphene/GaAs 48 GaAs/multilayer graphene/GaAs 48 GaAs photovoltaic devices 44 gallium arsenide (GaAs) 159, 178, 179 GaN core-shell micro-rod arrays 61 GaN-InGaN-GaN sandwich structure 45 genetically targeted calcium indicators (GCAMP1-7) 25, 28, 29, 31 genetically targeted sensor (GCaMP6) 28, 31 Ge NM-based flexible photodetectors 94–98 Ge on polyimide (GOP) 94, 109 germanium based solar cells 151–153 germanium-on-insulator (GeOI) 85 graphene 19, 20, 23, 43, 47, 48, 61, 65–67, 81, 118–126, 129, 131, 135, 136 graphene-based photodetector 129 green gap 50 green QDs/TiO2 and red QDs/ZnO 11

257

258

Index

h halide anions 16, 242 healthcare systems 177 hetero-epitaxial nanowires 51 heterogeneous DHNRs 12 heterojunction with intrinsic thin layer (HIT) solar cell 144, 146 hexagonal CVD-graphene nanodomains 67 high-quality, monodisperse QDs 10 high-quality nanowires 60 hole-transport-material (HTM) 221 hybrid passivation 242 hydrochloric acid (HCl) 44, 188 hydrofluoric acid (HF) 3, 186 hydrogenated amorphous silicon (a-Si:H) 146, 149

i iBeam materials 57 indium gallium phosphide (InGaP) 179–180 indium oxide flexible-PSCs 218–219 indium tin oxide (ITO) 239 infrared colloidal quantum dots solar cell PbS 239, 241 surface passivation and air stability 239–243 InGaN/GaN core/shell nanowires 66 InGaN/GaN flexible visible LEDs 44 InGaN/GaN LEDs 42, 45, 46, 52, 57, 62 InGaN/GaN thin film LEDs 57 InGaN LEDs 5, 45, 46, 48, 49, 63 injectable neural probes 25 ink-jet printing strategy 23 inorganic flexible LEDs 43 inorganic LEDs (ILEDs) 1, 42, 69, 196 inorganic semiconductor 1, 41–70, 79, 81, 92 inorganic semiconductor LEDs 42 inorganic thin film flexible LEDs 43 in-plane transferred nanowire LEDs 59–60 vertically transferred nanowire LEDs 60–65 InP NM 85, 98, 99

internal quantum efficiency (IQE) 5, 53, 119 intrinsic crystalline integrity 221 intrinsic ZnO (i-ZnO) 224, 228

k Kapton 12

l Langmuir-Blodgett technique 60 laser lift-off technique 5, 45 layer transfer method 42, 156 lead iodide (PbI2 ) 210, 215 ligand passivation 241, 242 light emitting diodes (LEDs) 1, 81 light-responsive QLED 12 light trapping structure 148–149 low aspect ratio nanowires 43, 65 luminescent dyes 191 luminescent solar concentrators (LSCs) 191–193 luminophores 191

m MAPbBr3 17, 19, 20 mass-produced n-type ZnO nanowires 59 MBE-grown InGaN LED film 45 mechanical spalling 184–186 mercaptocarboxylic acids (MPA) 241 metal-assisted chemical etching (MACE) 149, 151 metal chalcogenide complexes (MCC) 242 metal halide perovskites 2, 16 metal-insulator-semiconductor (MIS) photodetectors Ge 94–98 metallic foils 214 metal organic chemical vapor deposition (MOCVD) 3, 45, 57, 63, 66, 67, 86, 151, 179, 182, 197, 232 metalorganic-hydride vapor phase epitaxy 52 metal–organic vapor deposition (MOCVD) 179 metal-organic vapor-phase epitaxy (MOVPE) method 48, 181

Index

metal oxide semiconductor field effect transistor (MOSFET) 89, 90 metal-semiconductor-metal (MSM) photodetectors Ge 97 methylamine hydrochloride (MACl) 210 methyl ammonium lead bromide (MAPbBr3 ) 19 microcrystalline silicon (mc-Si:H) 151 micro-inorganic photodetectors (μ-IPDs) 6, 7, 34 micro-LED displays 50 micro-pyramids 43 micro-scale GaAs based thin-film LEDs 46 microscale ILEDs 2 micro-sized surgery robots 41 micro-transfer printing 44–46 micro-transferred InGaN/GaN LEDs 46 miniaturized flexible LEDs 25–28 mixed-mode fracture theory 162, 166 molecular beam epitaxy (MBE) 45, 51–53, 65, 67, 86, 182, 197 MOVPE AlN film 48 MOVPE nanowire LEDs 52 μ-ILED-based probes 25 μ-ILEDs involved red-emitting AlInGaP epitaxial structure 2 multijunction (MJ) 177, 178, 180, 181, 201, 202, 237, 243 multijunction (MJ) solar cells 181, 201, 202 multiple photolithography 3 multi quantum wells (MQW) 4, 16, 21, 49

n nanocrystal lumophores 193 nanomembranes (NM) III-V 98–100 donor substrates epitaxial growth 86 Smart-Cut technique 85–86 flexibility and bendability of 79 Ge 94–98 Si 88–93

®

thickness of 79 transfer printing of 86–88 nano printing process 148 Nano scale photon detection 81 nanostructure lift-off using van der Waals epitaxy 65–68 nanowire(s) 43 nanowire-based flexible LEDs 43 nanowire LEDs 50–57, 59–66, 69 nanowire membrane formation 61 nanowires (NW) based flexible detectors critical parameters for 110 fabrication of AgNWs/CdS 101–103 single crystal Si NWs 100 TFTs assembly of 101 length and diameter of 83 properties of 81 National renewable energy laboratory (NREL) 179, 181, 234–236, 244 net acceptor density (N A ) 237 neutral mechanical plane (NMP) 3 nickel (Ni) 184 nitride flexible LEDs 41 nitride flexible LEDs on metal foils 57–59 in-plane transferred nanowire LEDs 59–60 nitride nanowires 51, 52, 57, 61 noise equivalent power (NEP) 83, 119 non-indium oxide flexible-PSCs 216 N-type crystalline silicon solar cells 144

o oleic acid (OA) 241 I–V measurement 104, 106–108 dark and light conditions 108 open-circuit voltages (V oc ) 146, 150, 178, 179, 182, 189, 191, 201, 202, 226, 232, 240 optimized flexible QLED device structure 11 opto-electronic detectors 81 optogenetics 24–26, 28, 31–33, 41, 46, 196

259

260

Index

organic light emitting diodes (OLEDs) 1, 7, 10, 19, 42 organic semiconductors 42, 59, 79, 92

p Panasonic’s HIT-IBC solar cells 146 passivated emitter and rear cell (PERC) 144, 145 PbS-based QDs 10 perovskites 2, 16–20, 209–223 perovskite semiconductors 16 perovskites materials chemical vapor deposition approaches 213–214 solution process deposition approaches 209–211 vapor-assisted solution deposition approaches 211–213 phosphor-converted flexible white nanowire LED 65 phosphor-converted white LEDs 52, 64 phosphor-free flexible InGaN/GaN nanowire LEDs 65 phosphors 5, 52, 63–65, 180, 191 photo conductive gain (G) 82, 84, 135 photocurrent, light density 103 photodetectors 81 performance metrics 118–120 2D materials 117–118 photogating detectors 135 photogating effect 124, 135 photon recycling 178, 179, 181, 200–202 photosensitivity of device 106 photo thermal effect 129 photothermoelectric effect 121–124 photovoltaic (PV) 16, 178 photovoltaic effect 121, 123, 125, 129, 240 photovoltaic (PV) power conversion efficiencies 16 piezo-phototronic effect 57, 61, 69 plasma-assisted MBE technique 52 plasma-wave-assisted THz detector 123, 129–131 p-ohmic metal 188

polydimethylsiloxane (PDMS) 3, 46, 82, 135, 162, 190, 230, 231 poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) layer 55, 161 polyethylene naphthalate (PEN) 8, 11, 131, 135, 214 poly(ethylene oxide) (PEO) 20 polyethylene terephthalate (PET) 6, 55, 82, 125, 129, 189, 214 polyimide (PI) 214 polyimide (PI) foil 233 polyimide or polydimethylsiloxane (PDMS) 3–5, 7, 10, 12, 46, 61, 63–65, 82, 101, 190 polyimide (PI) substrates 7, 126, 227, 228 polymer LEDs 42 poly[2-methoxy-5-(2′ -ethylhexyloxy-pphenylenevinylene)] (MEH-PPV) 239 poly(p-phenylenevinylene) (PPV) 239 post-lift-off GaAs substrates 44 power consumption 79, 118, 120, 121 power-conversion efficieny (PCE) 16, 126, 155, 178, 179, 214, 220, 238 processable perovskite solar cells (PSCs) 143, 209, 211, 214, 215, 221

q QD emission characteristics 10 quantum-confined Stark effect 50, 51 quantum dot LEDs 43 quantum dots (QDs) 2, 7, 117, 191 quantum efficiency (QE) 5, 20, 42, 50, 53, 89, 118, 119, 131, 149, 235

r reactive ion etching (RIE) 87, 98, 148, 149 red-emitting AlGaInP epitaxial layers 7 red-emitting, flexible vertical μ-ILEDs 7 redox reaction 243 reduced graphene oxide (rGO) 23, 126, 155

Index

reflectance (R) 228 remote epitaxy 47, 48 remote’ homoepitaxy method 188 resonant regime of plasma-wave photodetection 131 roll-to-roll (R2R) 136, 151, 211, 228

s sapphire 6, 45, 47–49, 63 scanning electron microscope (SEM) 4, 53, 56, 59, 60, 67, 93, 102, 103, 149, 162, 202, 225, 230 self-assemble GaN nanowire LEDs 57 self-assembled MOCVD core-shell nanowire InGaN LEDs 63 self-organized conducting polymer (SOCP) 19 semiconductor NM 79, 85–88 semiconductor-on-insulator 85, 86 semiconductor on metal (SOM) exfoliate method 157 sensitivity (S) 34, 41, 81–84, 108, 110, 111, 118, 123 separation by implantation of oxygen (SIMOX) technology 157 series-connected amorphous silicon (a-Si:H) 146, 149–151 series-connection through apertures formed on film (SCAF) 151 serpentine-shaped interconnects 3 shear lag model 162, 165 Shockley-Queisser (SQ) theory 19, 178 short circuit current density (J SC ) 126, 146, 149, 179, 189, 228, 229, 232 Si NM-based flexible photodetectors 88–93 silicon based solar cells amorphous 149–151 crystalline 145–149 PV process 144 silicon-germanium alloys (SiGe) 152 silicon-on-insulator (SOI) 65, 86, 117, 156, 184 single crystalline NMs 81, 86 single crystal Si NWs 100 single-crystal solar cells 211, 237

single junction solar cells GaAs 178–179 InGaP 179–180 InP 179 Si photonics 51, 117 smart sensor system 12 Smart-Cut technique 85–86, 94, 183 soft lithographic techniques 46 SOI method 156–157 solution process deposition approaches 210–211 spectral response (SR) 109, 180, 228, 241 spectral splitting 200 stretchable μ-ILED array 7 substrate etching 42 sun’s power spectrum 237 surface tension-assisted (STA) 188

®

t thin crystalline films 43–46 thin-film inorganic solar cells 193 thin film mechanical transfer 47–49 thin-film III–V solar cells, flexible substrates ELO 186–189 mechanical designs 190–191 mechanical spalling 184–186 3D finite-difference time-domain (3D FDTD) technique, Si NM 90 III–V NM-based flexible photodetectors 98–100 III–V solar cells applications 193–196 double junction cells InGaP/GaAs 180–82 future generations mechanical stack 197–200 more junctions 197 photon recycling 200–202 spectral splitting 200 single junction solar cells GaAs 178–179 InGaP 179–180 InP 179 triple junction cells InGaP/GaAs/Ge 182

261

262

Index

III–V solar cells (contd.) InGaP/GaAs/InGaAs 182–183 InGaP/GaAs/InGaAsNSb 183 top-down etched Si microwire arrays 61 transfer 11, 32, 42, 47, 48, 61 transfer printing 2, 3, 5–7, 10–12, 28, 34, 42, 86–88, 191 transition metal dichalcogenides (TMDs) 20, 21, 118 transmission electron microscope (TEM) 152, 230, 232 transmittance (T) 126, 214, 215, 228, 234 transparent conducting oxide (TCO) 126, 227, 229, 232 transparent conductive electrodes (TCE) 126 triple junction cells InGaP/GaAs/Ge 182 InGaP/GaAs/InGaAs 182–183 InGaP/GaAs/InGaAsNSb 183 TrpV1 promoter 33 2D emitting materials 20 2D FAPbBr3 17, 19 2D materials-based photodetectors bolometric process 123 flexible substrates photogating detectors 135 photo thermal effect 129 photovoltaic effect 125–126 plasma-wave-assisted THz detector 129–131 nano-photonic techniques 136 photogating effect 124 photothermoelectric effect 121–123 photovoltaic effect 121 plasma-wave-assisted mechanism 123 plasma-wave-assisted Terahertz Detection 123 2D nanoplatelets 16

u ultraviolet (UV) 57, 111, 118, 180, 221, 233 ultraviolet (UV) light 180

unmanned aerial vehicles (UAVs) 177 US National Renewable Energy Laboratory (NREL) 179 UV absorption 109 CdSe, P3HT and hybrid photodetector spectra 109

v van der Waals epitaxy 43, 47, 48, 65–69 van der Waals (vdW) heterojunctions 23, 121, 136 vapor-assisted solution deposition approaches 211–213 vertically transferred nanowire flexible light sources 61 vertically transferred nanowire LEDs 60–65 nanostructure lift-off using van der Waals epitaxy 65–68 very high bond (VHB) film 135

w waterproof flexible LEDs 46 wet etching 5, 7, 45, 85, 89, 94, 98, 157, 161, 182, 186 wet etching method 45, 157 white nanowire LED 53, 54, 65 wireless, battery-free optogenetic stimulation devices 32–33 wireless, flexible optoelectronic systems 28–31 wurtzite crystal structure 57

y yellow nanophosphors

53, 65

z ZnO nanowire LEDs 56, 57 ZnO nanowire/p-polymer LED array 57 ZnO nanowires-based flexible LEDs 55–57 ZnO nanowires/poly (9, 9-dioctylfluorene) (PFO) hybrid structure 56 ZnO/organic hybrid white LEDs 56

WILEY END USER LICENSE AGREEMENT Go to www.wiley.com/go/eula to access Wiley’s ebook EULA.