Core/Shell Quantum Dots: Synthesis, Properties and Devices 3030465950, 9783030465957

This book outlines various synthetic approaches, tuneable physical properties, and device applications of core/shell qua

666 110 12MB

English Pages 332 [331] Year 2020

Report DMCA / Copyright

DOWNLOAD FILE

Polecaj historie

Core/Shell Quantum Dots: Synthesis, Properties and Devices
 3030465950, 9783030465957

Table of contents :
Preface
Contents
Synthesis, Properties, and Applications of II –VI Semiconductor Core/Shell Quantum Dots
1 Introduction
1.1 Classification of Core/Shell Quantum Dots
2 Synthesis of Core and Core/Shell Semiconductor QDs
2.1 Synthesis of Core QDs
2.1.1 Injection Method
2.1.2 Non-injection Method
2.2 Synthesis of Semiconductor Core/Shell QDs
2.2.1 Synthesis and Characterization of Type I Core/Shell QDs
2.2.2 Synthesis and Characterization of Type II Core/Shell QDs
2.2.3 Synthesis and Characterization of Reverse Type I Core/Shell QDs
3 Applications of Semiconductor Core/Shell QDs
3.1 Solar Cells
3.2 Light-Emitting Diodes (LEDs)
3.3 Applications in Biology
3.3.1 Biosensing
3.3.2 Gene and Drug Delivery
3.3.3 Therapy
4 Summary and Prospects
References
Design, Synthesis, and Properties of I-III-VI2 Chalcogenide-Based Core-Multishell Nanocrystals
1 Introduction
1.1 Classes of Core/Shell Nanocrystals
2 Design and Formation of Core-Multishell and Alloyed Shell Nanocrystals
2.1 Synthesis of Core/Shell Nanocrystals
2.2 Growth of Multishell Over Core Nanocrystals
2.3 Optical Properties of Mn2+ Ion-Doped Core and Core/Shell CuInS2 Nanocrystals
2.4 Photoluminescence Properties of Mn2+ Ion-Doped Core and Core/Shell CuInS2 Nanocrystals
2.5 XPS Analysis of Mn2+ Ion-Doped Core and Core/Shell CuInS2 Nanocrystals
2.6 Crystal Structure Analysis of Mn2+ Ion-Doped Core and Core/Shell CuInS2 Nanocrystals
2.7 Morphological Properties of Mn2+ Ion-Doped Core and Core/Shell CuInS2 Nanocrystals
2.8 Absorption Properties of Core-Multishell CuInS2/MnS/ZnS Nanocrystals
2.9 Photoluminescence Analysis of Core-Multishell CuInS2/MnS/ZnS Nanocrystals
2.9.1 Influence of Shell Concentration on PL Properties of CuInS2/MnS/ZnS Core-Multishell Nanocrystals
2.10 X-Ray Photoelectron Spectroscopy (XPS) Analysis of Core-Multishell CuInS2/MnS/ZnS Nanocrystals
2.11 Crystal Structure Analysis and the Influence of Shell Concentration on Crystal Structure of Core-Multishell CuInS2/MnS/ZnS Nanocrystals
2.12 Morphological Analysis of CuInS2/MnS/ZnS Core-Multishell Nanocrystals
2.13 UV-Vis Absorption Spectra of Cd2+-Doped Core and Core/Shell AgInS2 Nanocrystals
2.14 Photoluminescence Properties of Cd2+-Doped Core and Core/Shell AgInS2 Nanocrystals
2.15 Crystal Structure Analysis of Cd2+-Doped Core and Core/Shell AgInS2 Nanocrystals
2.16 Surface Morphology Analysis of Cd2+-Doped Core and Core/Shell AgInS2 Nanocrystals
2.17 Elemental Compositions of Cd2+-Doped Core and Core/Shell AgInS2 Nanocrystals
2.18 UV-Vis Studies of Core-Multishell and Alloyed AgInS2/CdS/ZnS Shell Nanocrystals
2.19 Photoluminescence Studies of Core-Multishell and Alloyed AgInS2/CdS/ZnS Shell Nanocrystals
2.20 XPS Spectra of Core-Multishell and Alloyed AgInS2/CdS/ZnS Shell Nanocrystals
2.21 Crystal Structure Analysis of Core-Multishell and Alloyed AgInS2/CdS/ZnS Shell Nanocrystals
2.22 Morphological Studies of Core-Multishell and Alloyed AgInS2/CdS/ZnS Shell Nanocrystals
3 Conclusions
References
Unique Luminescent Properties of Composition-/Size-Selected Aqueous Ag-In-S and Core/Shell Ag-In-S/ZnS Quantum Dots
1 Introduction
2 Aqueous Synthesis and Optical Properties of AIS and AIS/ZnS QDs
2.1 Optimization of the Synthesis of Aqueous AIS and AIS/ZnS QD Ensemble Colloids
2.2 Size Selection of AIS and AIS/ZnS QDs
2.3 Static PL of the Size-Selected AIS and AIS/ZnS QDs
2.4 Time-Resolved Photoluminescence of the Size-Selected GSH-Stabilized AIS and Core/Shell AIS/ZnS QDs
2.5 Single-Particle PL Measurements
3 Temperature-Dependent Luminescence of AIS and AIS/ZnS QDs
3.1 Temperature Range of 10–80 C
3.1.1 Variations of PL Intensity
3.1.2 Variations of the PL Band Maximum Position
3.1.3 Variations of the PL Decay Rate
3.2 Temperature Range of 10–300 K
4 Summary
References
Electronic and Optical Characteristics of Core/Shell Quantum Dots
1 Introduction
2 Theory
2.1 One Particle States in Spherical Core/Shell QD
2.2 Impurity States in Spherical Core/Shell QD
2.3 Two-Electron Sates in Thin Spherical Core/Shell QD: Two Electrons on a Sphere
2.4 Interband Optical Absorption in Spherical Core/Shell QD
2.5 Quadrupole Moment Created by Impurity Electron in the Core/Shell QD
2.6 Orbital and Spin Magnetic Moment Current in the Cylindrical Core/Shell QD
2.6.1 Cylindrical Nanolayer
3 Conclusion
References
Exciton –Phonon Interactions and Temperature Behavior of Optical Spectra in Core/Shell InP/ZnS Quantum Dots
1 Introduction
2 Nanocrystal Absorption and Derivative Spectrophotometry
3 Temperature Dependence of the Energy Gap
4 The Influence of Exciton–Phonon Interaction on the Energy of Optical Transitions
5 The Half-Width of the First Exciton Absorption Band
6 Temperature Evolution of the First Exciton Absorption Band of QD Ensemble
6.1 Static and Dynamic Disorder in Ensemble
6.2 The Behavior of the Exciton Line of an Individual Nanocrystal
6.3 QD Size Distribution
6.4 Contributions of Homogeneous and Inhomogeneous Broadening
6.5 Simulation of Experimental InP/ZnS Ensembles
6.6 Disorder Effects in Temperature Band Broadening
7 Conclusion
References
Thick-Shell Core/Shell Quantum Dots
1 Thick-Shell Core/Shell Quantum Dots
1.1 History of Thick-Shell Core/Shell Quantum Dots
1.2 Basic Characteristics of Thick-Shell Core/Shell Quantum Dots
2 Synthesis of Thick-Shell Core/Shell Quantum Dots
2.1 Successive Ionic Layer Adsorption and Reaction Method
2.2 Syringe Pump Injection
2.3 “Flash” Hot Injection
3 Optical Properties of Thick-Shell Core/Shell Quantum Dots
3.1 Fundamental Optical Properties
3.2 Exciton Dynamics Process
3.3 Quantum Confined Stark Effect
3.4 Optical Gain Performance
3.5 Stimulated Radiation from Thick-Shell Core/Shell Quantum Dots
4 Conclusion
References
Core/Shell Quantum-Dot-Sensitized Solar Cells
1 Introduction
2 Brief History of QDSCs
3 QDSC Architecture
3.1 Anode
3.2 Light Harvester
3.3 Electrolyte
3.4 Counter Electrode
4 Working Principle
4.1 Light Absorption
4.2 Carrier Separation
4.3 Carrier Transport
4.4 Recombination
5 Photovoltaic Characterizations
6 Core/Shell QDs
6.1 Classification of Core/Shell QD Systems
6.2 Synthesis of Core/Shell QDs
7 Photovoltaic Performance of Core/Shell QDSCs
7.1 Type-I Core/Shell QDs
7.2 Reverse Type-I Core/Shell QDs
7.3 Type-II Core/Shell QDSCs
7.4 Quasi-Type-II Core/Thick-Shell QDs (“Giant” Core/Shell QDs)
7.5 Core and Shell Interface Optimization
8 Conclusions
References
Core/Shell Quantum-Dot-Based Solar-DrivenPhotoelectrochemical Cells
1 Introduction
1.1 Conversion of Solar Energy into Hydrogen
1.2 Working Principle of Solar-Driven PEC Cell for Hydrogen Generation
2 Synthesis and Optical Properties of Core/Shell QDs
2.1 Synthesis of Core/Shell QDs
2.2 Optical Properties of Core/Shell QDs
2.2.1 Type I Core/Shell QDs
2.2.2 Type II Core/Shell QDs
2.2.3 Quasi-Type II Core/Shell QDs
2.3 Charge Dynamics of Core/Shell QDs
2.3.1 Type I Core/Shell QDs
2.3.2 Type II and Quasi-Type II Core/Shell QDs
2.3.3 Charge Dynamics from QDs to Semiconductor Films
3 Solar-Driven PEC Cells Based on Core/Shell QDs
3.1 Core/Thin-Shell QDs
3.2 “Giant” Core/Shell QDs
3.3 Core/Alloyed-Shell/Shell QDs
4 Heavy Metal-Free Core/Shell QD-Based PEC Cells
5 Conclusions and Perspectives
References
Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators
1 Introduction of Luminescent Solar Concentration
2 Band Alignment and Optical Properties of Core/Shell Quantum Dots
2.1 Exciton Dynamic in Core/Shell Quantum Dots
2.2 Stokes Shift in Core/Shell Quantum Dots
2.3 Quantum Yield in Core/Shell Quantum Dots
2.4 Stability of Core/Shell Quantum Dots
3 Luminescent Solar Concentrator Based on Core/Shell Quantum Dots
3.1 Near-Infrared PbS/CdS Core/Shell QD-Based LSCs
3.2 Visible CdSe/CdS Core/Shell QD-Based LSCs
3.3 Eco-Friendly InP/ZnO Core/Shell QD-Based LSCs
3.4 Cu-Based Ternary or Quaternary Eco-Friendly Core/Shell QD-Based LSCs
4 Conclusions and Perspectives
References
Index

Citation preview

Lecture Notes in Nanoscale Science and Technology 28

Xin Tong Zhiming M. Wang  Editors

Core/Shell Quantum Dots Synthesis, Properties and Devices

Lecture Notes in Nanoscale Science and Technology

Volume 28

Series editors Zhiming M. Wang, Chengdu, China Greg Salamo, Fayetteville, USA Stefano Bellucci, Frascati RM, Italy

Lecture Notes in Nanoscale Science and Technology (LNNST) aims to report latest developments in nanoscale science and technology research and teaching–quickly, informally and at a high level. Through publication, LNNST commits to serve the open communication of scientific and technological advances in the creation and use of objects at the nanometer scale, crossing the boundaries of physics, materials science, biology, chemistry, and engineering. Certainly, while historically the mysteries in each of the sciences have been very different, they have all required a relentless step-by-step pursuit to uncover the answer to a challenging scientific question, but recently many of the answers have brought questions that lie at the boundaries between the life sciences and the physical sciences and between what is fundamental and what is application. This is no accident since recent research in the physical and life sciences have each independently cut a path to the edge of their disciplines. As both paths intersect one may ask if transport of material in a cell is biology or is it physics? This intersection of curiosity makes us realize that nanoscience and technology crosses many if not all disciplines. It is this market that the proposed series of lecture notes targets.

More information about this series at http://www.springer.com/series/7544

Xin Tong • Zhiming M. Wang Editors

Core/Shell Quantum Dots Synthesis, Properties and Devices

Editors Xin Tong Institute of Fundamental and Frontier Sciences University of Electronic Science and Technology of China Chengdu, China

Zhiming M. Wang Institute of Fundamental and Frontier Sciences University of Electronic Science and Technology of China Chengdu, China

ISSN 2195-2159 ISSN 2195-2167 (electronic) Lecture Notes in Nanoscale Science and Technology ISBN 978-3-030-46595-7 ISBN 978-3-030-46596-4 (eBook) https://doi.org/10.1007/978-3-030-46596-4 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Colloidal semiconductor quantum dots (QDs) are small-sized fluorescent nanoparticles covered with surfactant ligands and dispersed in solution. The typical diameter of QDs in nanoscale dimension is comparable to that of the Bohr exciton radius of the material, which results in the quantum confinement effect that enables the size-dependent and tunable optical and electrical properties of QDs. This attractive feature allows the realization of optoelectronic materials with properties that are not available in conventional bulk semiconductors, making QDs as emerging nextgeneration semiconducting materials in various photovoltaic and light-emission devices. However, the extremely small size of QDs can result in a very high surfaceto-volume ratio and lead to a highly sensitive surface region, wherein the lower coordination of surface atoms is prone to induce the formation of the surface traps. These surface traps/defects can act as nonradiative recombination centers of photogenerated charge carriers, thereby reducing the photoluminescence (PL) efficiency and photo-/chemical/thermal stability of QDs, which are detrimental for the relevant optoelectronic applications. The growth of another semiconductor shell on core QDs to form core/shell QDs is one of the most promising approaches to efficiently passivate the surface defects/traps, leading to enhanced PL quantum yield, suppressed photobleaching, and photoblinking as well as largely improved photo-/chemical-/thermal-stability with respect to the organic ligands-capped bare QDs. More importantly, the band structure of core/shell QDs can be tailored via tuning the core and shell compositions and shell thickness etc., which is typically classified as type I, type II, and quasi-type II according to the spatial distribution of photogenerated charge carriers in the core/shell architecture. In type I core/shell QDs, both electron and hole are confined within the core, giving rise to the significantly enhanced PLQY and photostability that are favorable for light-emitting applications. The photogenerated electrons and holes are spatially separated in type II and quasi-type II core/shell QDs, leading to long-lived PL lifetime that is favorable for photovoltaic (PV) applications requiring efficient charge separation/transfer. These optimized and tunable optoelectronic properties render core/shell QDs as promising building blocks for various photonic technologies. v

vi

Preface

This book consists of the latest research progress in the rational synthesis, optoelectronic properties, and device applications of core/shell QDs. Specifically, Chaps. 1, 2 and 3 mainly focus on the investigations of synthesis of binary II-VI and ternary I-III-VI2 semiconductor core/shell QDs. Chapter 1 presents the synthesis and microstructural and optical properties of binary II-VI semiconductor core/shell QDs, such as CdSe/ZnS, CdSe/CdS, CdTe/ZnS etc., and the relevant applications of these II-VI semiconductor core/shell QDs are briefly introduced as well. Chapter 2 aims to deal with the synthesis of core/shell, core/multi-shell, and core/doped shell QDs using heavy metal-free ternary I-III-VI2 chalcogenides of CuInS2 and AgInS2 as core QDs. These core/shell structures can dramatically improve the optical and luminance properties of QDs, which act as an alternative to the existing toxic II-VI systems. In Chap. 3, a synthetic overview of stabilizing AgInS2 and AgInS2 /ZnS QDs in aqueous colloids by small multi-functional molecular ligands is illustrated, and the dependences of optical characteristics of the non-stoichiometric AgInS2 and AgInS2 /ZnS QDs on their composition and size as well as temperature are discussed. The optical and electrical properties of core/shell QDs are the main subjects of Chaps. 4, 5 and 6. A theoretical study of single-electron, two-electron, and impurity states as well as interband optical absorption and single-electron current in spherical and cylindrical core/shell QDs is presented in Chap. 4, showing the possibility of flexible manipulation of their spectral optical and spin characteristics. Chapter 5 studies the temperature-dependent optical properties in a wide temperature range for colloidal InP/ZnS core/shell QDs with different size distributions, and corresponding exciton-phonon interactions are determined by using derivative spectrophotometry approach. The Auger recombination in core/shell QDs can induce emission intermittency, which is detrimental for their optoelectronic applications such as lasers. Chapter 6 investigates the optical properties of thick-shell core/shell QDs, showing that the thick shells can suppress the Auger recombination and photoblinking and decrease the surface nonradiative channel and increase PLQYs and absorption cross-sections, thus resulting in superior optical gain performance and photostability. Chapters 7, 8 and 9 are dedicated to several representative solar energy conversion devices based on core/shell QDs. QDs-sensitized solar cells (QDSCs) are considered to be one of the most appealing approaches that directly convert solar radiation into electricity. Chapter 7 offers an overview of the recent progress in the development of type-I, type-II, quasi type-II core/shell QDs as promising light-harvesting materials to boost the performance of QDs-sensitized solar cells (QDSCs). Particularly, the band alignment between core and shell materials is demonstrated to efficiently tune the optoelectronic properties of QDs and reduce the carrier recombination within QDSCs. Colloidal QDs-based photoelectrochemical (PEC) cells are cost-effective devices showing remarkable solar-to-fuel conversion efficiency. The advances in core/shell QDs-based solar-driven PEC cells are presented in Chap. 8, highlighting the design of core/alloyed shell and heavy metal–free, near-infrared (NIR) core/shell QDs for enhanced optical absorption and optimized charge dynamics to achieve high performance QDs-based PEC devices.

Preface

vii

Luminescent solar concentrators (LSCs) with large-area sunlight collection can be employed for building-integrated PV devices. Chapter 9 reports the LSCs based on core/shell QDs with tailored optical properties including large Stokes shift and high PLQY, the relationship between QD’s structure/optical properties and the device performance, and a perspective on the remaining key issues and open opportunities in the field are provided as well. The editors are grateful to all the authors for their significant contributions and efforts to make this book a valuable guide to future optimization and developments of colloidal semiconductor QDs-based optoelectronic applications. We would like to thank Mr. Xin Li for his indispensable editorial assistance. Lastly, the editors acknowledge the financial support of the National Key Research and Development Program of China (2019YFB2203400), the “111 Project” (B20030), and the UESTC Shared Research Facilities of Electromagnetic Wave and Matter Interaction (Y0301901290100201). Chengdu, China

Xin Tong Zhiming M. Wang

Contents

Synthesis, Properties, and Applications of II –VI Semiconductor Core/Shell Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amar Nath Yadav, Ashwani Kumar Singh, and Kedar Singh

1

Design, Synthesis, and Properties of I-III-VI2 Chalcogenide-Based Core-Multishell Nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Renuga and C. Neela Mohan

29

Unique Luminescent Properties of Composition-/Size-Selected Aqueous Ag-In-S and Core/Shell Ag-In-S/ZnS Quantum Dots . . . . . . . . . . . . . Oleksandr Stroyuk, Oleksandra Raievska, and Dietrich R. T. Zahn

67

Electronic and Optical Characteristics of Core/Shell Quantum Dots . . . . . . 123 D. A. Baghdasaryan, H. T. Ghaltaghchyan, D. B. Hayrapetyan, E. M. Kazaryan, and H. A. Sarkisyan Exciton –Phonon Interactions and Temperature Behavior of Optical Spectra in Core/Shell InP/ZnS Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Sergey Savchenko, Alexander Vokhmintsev, and Ilya Weinstein Thick-Shell Core/Shell Quantum Dots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 Lei Zhang, Wenbin Xiang, and Jiayu Zhang Core/Shell Quantum-Dot-Sensitized Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Gurpreet Singh Selopal Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Ali Imran Channa, Xin Li, Xin Tong, and Zhiming M. Wang Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators . . . . . . . 287 Guiju Liu, Xiaohan Wang, Guangting Han, and Haiguang Zhao Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

ix

Synthesis, Properties, and Applications of II–VI Semiconductor Core/Shell Quantum Dots Amar Nath Yadav, Ashwani Kumar Singh, and Kedar Singh

Abstract Semiconductor core/shell quantum dots (QDs) are composed of at least two semiconducting materials having a structure like an onion. In the recent past, the synthesis of these systems impelled significant progress, as by growing an epitaxial shell, we can easily tune basic optical properties such as fluorescence quantum yield, emission wavelength, and carrier lifetime. The significance of developing an epitaxial shell over the surface of core QDs is making the nanocrystal less sensitive to environmental changes and photo-oxidation. Another advantage is the enhancement of fluorescence quantum yield by passivating surface trap states of core QDs. These properties are essential for the application of semiconductor core/shell QDs in light-emitting diodes, solar cell, and biological labelling. This chapter discusses the synthesis and microstructural and optical properties of mostly II–VI semiconductor, core/shell QDs. Moreover, various applications of core/shell QDs in solar cells, light-emitting diode, and biomedical have been also discussed in detail. Keywords Core/shell quantum dots · Photoluminescence · Semiconductors · Synthesis

1 Introduction In recent decades, semiconductor nanostructured materials are significantly cherished because they can link the gap between small molecules and bulk materials [1–4]. The nanostructured materials show distinct optical and electronic properties when their size varies in the range of 1–100 nm. With the variation of dimension, they can be classified as (1) two-dimensional, e.g., nanosheets or thin films or quantum wells; (2) one-dimensional, e.g., quantum wires; and (3) zero-dimensional,

A. N. Yadav · A. K. Singh · K. Singh () School of Physical Sciences, Jawaharlal Nehru University, New Delhi, India e-mail: [email protected] © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 X. Tong, Z. M. Wang (eds.), Core/Shell Quantum Dots, Lecture Notes in Nanoscale Science and Technology 28, https://doi.org/10.1007/978-3-030-46596-4_1

1

2

A. N. Yadav et al.

Fig. 1 Schematic representation of the idealized density of states for semiconductor nanostructure with reduced dimensionality (3d, 2d, 1d, and 0d represent three-dimensional, two-dimensional, one-dimensional, and zero-dimensional). For 3d bulk structure, the energy levels are continuous, whereas for 0d QDs energy levels are discrete. (Reprinted with permission from Ref. [5])

e.g., quantum dots. Thus, compared to the bulk semiconductor material, a quantum dot (QD) is zero-dimensional and has a limited number of atoms, displaying discrete energy states (Fig. 1) [5, 6]. Bulk semiconductor materials have continuous valance and conduction energy states with composition-dependent bandgap (Eg ), which is defined as minimal energy required to create an electron in conduction band from the valance band. In this process, the electron leaves a hole in valance band upon excitation energy higher than Eg . In the presence of the electric field, these opposite charge carriers may be mobilized and hence carry current. A bound state of electron-hole pair in their minimum energy state is called an exciton, and the distance between them is exciton Bohr radius (rB ). Further, the excited-state electron relaxes to annihilate exciton via radiative recombination of electron-hole and emit their energy in the form of a photon [7]. Two major factors can influence the unique properties of QDs: quantum confinement and surface effects. Quantum confinement: When the radius of semiconductor nanocrystal (r) becomes smaller or equal to exciton Bohr radius, i.e., r ≤ rB , then the motion of electrons and holes is spatially confined to the dimension of the QD. Also, in this condition, the energy difference between two levels of QD exceeds the value KB T (here KB is Boltzmann constant); this results in restriction of the mobility of electron and hole in the crystal dimensionality. In this regime, depending on the size of the nanocrystals, the QDs exhibit size-dependent absorption and emission with discrete electronic transitions [8, 9]. This effect is called quantum confinement effect. It can affect several properties of semiconductor QDs, including magnetic properties and conductivity. Nevertheless, the most exciting properties that arise from quantum confinement is the size- and shape-dependent optoelectronic properties. Surface effect: As the size of nanocrystal reduces, their surface-to-volume ratio increases. This leads to an increment in the free energy of the NCs, which makes NCs more dynamic and reactive as compared to their bulk counterparts. Henceforth, it modifies the materials basic properties including solubility, reactivity, evaporation and melting temperatures, plasticity etc. It also makes the NCs to easily disperse in solvents media and opens up a possibility to functionalize or modify their surface. This is the most beautiful property of colloidal NCs that can be used to

Synthesis, Properties, and Applications of II–VI Semiconductor Core/Shell. . .

(a)

3

(b)

(c) organic molecule

band offset Eg (core)

Eg (shell)

band offset

Fig. 2 Schematic representation of (a) organically passivated QD, (b) inorganically passivated QD (core/shell structure), and (c) energy band offsets of core/shell structure. (Reprinted with permission from Ref. [13])

build optoelectronic devices and targeting bio drugs. However, for very smaller nanocrystal, the surface-to-volume ratio is very high; results in most of the bonds associated with the surface are unsatisfied [10]. The energy levels associated with surface states lie in between valance and conduction band. Further, these surface states behave as fast non-radiative de-exciton channels and trap the photo-generated charge carriers. As a result, the optical and optoelectronic properties of QDs are significantly influenced by surface trap states [11, 12]. Therefore, passivation or capping of the surface is essential for the evolution of luminescent and stable QDs. Surface passivation of QDs can be carried out by two types: (1) organically and (2) inorganically (Fig. 2) [13]. Organic surface passivation involves organic molecules that bond with surface atoms and act as capping agent. The advantages of the organic passivation include monodispersity, colloidal suspension, and bioconjugation of QDs [14–16]. However, full coverage of surface atoms and simultaneous passivation of both cation and anion surface sites are still complicated because of shape distortion and larger size of the organic capping molecules [17, 18]. The second strategy is inorganically passivation, which involves full passivation of surface trap states. In this approach, surface passivation has been carried out by overgrowth of inorganic layer, particularly a second semiconductor. The resulting material is known as core/shell QDs [19]. In this case, fluorescence QY and photostability of the core/shell QDs drastically improved. It is also possible to tune the absorption and emission spectra of the material by choosing appropriate core and shell materials. The proper choice of shell material and its thickness are the essential terms that contribute to overall properties of QDs. If the core and shell structures have huge lattice mismatch, then this results in lattice strain that generates defect states within or at the core/shell interface. In addition, the thicker shell creates misfit dislocations, which also decreased fluorescence QY by the non-radiative process [20].

4

A. N. Yadav et al.

Fig. 3 Schematic diagram of band alignment for different types of core/shell structure: (a) type I, (b) type II, (c) quasi type II, and (d) reverse type I. The wave function represented by blue color is stands for electron wave function, whereas red color represents hole wave function. (Reprinted with permission from Ref. [21])

1.1 Classification of Core/Shell Quantum Dots Based on the energy bandgap offset, semiconductor core/shell QDs are generally categorized into three groups; type I, reverse type I, and type II (Fig. 3). In type I, the shell material has wider bandgap than the core material, i.e., valance and conduction band edge of the core lies in between shell material so that the charge carriers (electrons and holes) are restricted within the core. Further, in reverse type I, the bandgap of the core material is larger than the shell material. In this case, the photo-generated charge carriers partially or completely delocalized in the shell, and by changing shell thickness, the emission wavelength can be tuned. Finally, in type II, either valance or conduction band edge of shell material lies within the bandgap of the core material. Consequently, upon photoexcitation, the charge carriers are spatially isolated in a distinct region of core/shell heterostructure [19, 21]. In addition to above-discussed types of core/shell structure, there is an intermediate one identified as quasi type II (or type I1/2 ) core/shell QDs. The most studied quasi type II system is CdSe/CdS core/shell QDs, although it is a type I core/shell structure. However, the energy offset of the electron is minimal to confine it in the CdSe core, and subsequently the electronic wave function delocalized over the whole nanocrystal, while hole wave function remnant inside the core of the QD [19, 21].

2 Synthesis of Core and Core/Shell Semiconductor QDs Core/shell QDs have been mostly synthesized by two-step process: first the synthesis of core QDs and second subsequent shell growth process.

Synthesis, Properties, and Applications of II–VI Semiconductor Core/Shell. . .

Nucleation

Monodisperse Colloid Growth (La Mer)

0

Growth from Solution

Nucleation Threshold Injection

Concentration of Precursors (arbitrary units)

Fig. 4 Schematic illustration of nucleation and growth mechanism in nanocrystals based on La Mer model [23]. (Reprinted with permission from Ref. [22])

5

200

Ostwald ripening Staturation 400 600 Time (Seconds)

800

1000

2.1 Synthesis of Core QDs Colloidal semiconductor QDs have been generally synthesized by using three components such as solvents, precursor, and organic surfactant. In an inert atmosphere, heating of reaction mixture up to the required temperature changes the precursor into monomers (active atomic and molecular species) and is called nucleation. Further, the monomers are converted into nanocrystals whose continuous growth depends on the surfactant molecules. Thus, two steps are required for the formation of nanocrystals: (1) nucleation and (2) growth [22, 23]. In more than two decades, a large number of reviews have been reported related to the synthesis of II–VI and the other semiconductor QDs [13, 19, 21–24]. In general, two methods of non-injection and injection have been used for the synthesis of aqueous and nonaqueous semiconductor QDs by varying solvents, temperatures, and precursors (Fig. 4).

2.1.1

Injection Method

In 1993, Murray et al. developed a traditional approach for the synthesis of semiconductor QDs by fast injection of the precursor into a hot solvent [25]. In a typical synthesis protocol, they first prepared Cd and Se precursor solutions by mixing Cd(CH3 )2 and Se powder into tri-n-octyl phosphine (TOP) solvent, distinctively. These precursors were further injected quickly into hot trioctylphosphine oxide (TOPO) solution in a three-neck flask and under an inert (N2 ) atmosphere. Here, TOPO acts as stabilizing agent and allows the reaction mixture to heat at a high temperature generally up to 320 ◦ C. This synthesis technique was a model for the preparation of CdSe, CdS, and CdTe QDs with different sizes 1.5–11.5 nm.

6

A. N. Yadav et al.

Later in 2001, Peng et al. have used CdO as a Cd precursor in place of Cd(CH3 )2 . The reason behind is toxicity, explosive, and expensiveness of Cd(CH3 )2 in comparison to CdO [26, 27]. Moreover, the quality of CdSe QDs remains the same as in the case of Cd(CH3 )2 . Well ahead, due to environment issue, many efforts have been made for the synthesis of II–VI semiconductor QDs using “green” hot injection synthesis method [28, 29]. After that, the expensive and coordinating solvent TOPO has been replaced by low-cost and non-coordinating solvent octadecane (ODE) [30–32]. Further, Deng et al. simplified the reaction by using low-cost solvent oleic acid instead of TOP/TOPO [33]. As described above, the hot injection method is based on a fast injection of precursors into a hot solution containing another precursor followed by an instant homogenous reaction. In this method, it was hard to control the precise reaction temperature upon injection of the precursor solution. Thus, scale-up synthesis and reproducibility are difficult to achieve using this method. Therefore, for large-scale synthesis, another method “non-injection method” has been developed.

2.1.2

Non-injection Method

In this method, all reagents are mixed in a three-neck flask, and nucleation and growth are originated either chemically, thermally, or by physical impact (e.g., microwave irradiation) [34–36]. First, Pradhan et al. have developed a versatile and easy method for the synthesis of extremely good metal sulfide (CdS) nanoparticles using one-spot and low-temperature process [37]. In this method, heating of metal xanthate in hexadecylamine (HDA) gives rise to metal sulfide NPs even at low temperature 70 ◦ C. Moreover, by adjusting reaction temperature, NPs of various sizes could be obtained using this method. Later, Coe and his collaborators synthesized high-quality and monodispersed CdS QDs by one-spot non-injection method [34]. In this method, they have introduced two nucleation initiators, namely, tetraethylthiuram (I1 ) and 2,2 -dithiobisbenzothiazole (I2 ), in the reaction mixture. So separate nucleation and growth are accomplished, and the quality of CdS QDs is quite comparable with the injection method. Besides the abovementioned non-injection method, another non-injection way is single-source inorganic cluster approach. In this method, the single-source precursor (e.g., Li4 [Cd10 Se4 (SPh)16 ] for CdSe) was added in dodecylamine or hexadecylamine at 80 ◦ C under N2 . Further, after dissolving the cluster, the reaction mixture was heated up to 220 ◦ C for the growth of QDs. From this method, nucleation at relatively low temperature can be achieved, and reaction can be scaled to large quantities (1–50 g/L). Further, other QDs such as CdS, ZnSe, and CdSe/ZnS also can be prepared via this method [38].

Synthesis, Properties, and Applications of II–VI Semiconductor Core/Shell. . .

7

2.2 Synthesis of Semiconductor Core/Shell QDs For the growth of the shell, two points are most important: (1) choice of shell material and (2) thickness of the shell material. In the former one, generally, semiconductors with small lattice mismatch have been chosen for the core and shell structure. If both structures have a huge lattice mismatch, then it results in lattice strain and generates defect states at the core/shell structure or within the shell. The defect states can act as trap states for the charge carrier and can decrease PL QY [20]. Table 1 represents a list of material parameters for some selected bulk semiconductors. The second point is the thickness of shell material, which also plays a vital role in the properties of core/shell QDs. If the shell thickness is skinny, then passivation of surface traps of core QDs may be incomplete. On the other hand, if the shell thickness is very thick, then it results in the formation of defect states due to lattice strain. During shell growth, number monolayers of shell material are deposited on the surface of the core QDs. The required amount of shell thickness can be calculated as follows. The volume of the shell material for “m” monolayers of shell thickness can be calculated as Vshell =

 4   (rc + m × dML )3 − rc3 3

where Vshell is the volume of the shell material, rc is the radius of core QDs, and dML is the shell thickness for one monolayer (nm). If nshell is the amount of shell material (in moles) required to deposit “m” mL shell, then nshell =

Vshell × Dcore × NA × nQD MWcore

Table 1 Material parameter for bulk II–VI semiconductors [19] Semiconductor materials CdSe CdTe CdS ZnSe ZnTe ZnS

Structure (300 K) Wurtzite Zinc blende Wurtzite Zinc blende Zinc blende Zinc blende

Type II–VI II–VI II–VI II–VI II–VI II–VI

Egap (eV) 1.74 1.43 2.49 2.69 2.39 3.61

Lattice parameter (Å) 4.3/7.01 6.48 4.136/6.714 5.668 6.104 5.41

Density (kg m−3 ) 5810 5870 4820 5266 5636 4090

8

A. N. Yadav et al.

where Dcore is the density of core material, NA is the Avogadro number, nQD is the number of moles of core QDs in solution, and MWcore is the molecular weight of core material [39]. After calculations of the precise amount of shell thickness, deposition of shell has been carried out by a technique called successive ionic layer adsorption and reaction (SILAR) method [40]. By this technique, we can deposit one monolayer at a time by injecting cationic and anionic precursors into the core solution. After depositing one layer successively, one can deposit many monolayers of shell thickness on the surface of core QDs. By using this method, monodisperse and highly luminescent semiconductor core/shell QDs could be synthesized.

2.2.1

Synthesis and Characterization of Type I Core/Shell QDs

Type I core/shell QDs have been synthesized for the motive to increase fluorescence QY and to improve photostability of the QDs. The most studied II–VI semiconductor in type I is CdSe/ZnS core/shell QDs. First, Guyot-Sionnest group have grown ZnS shell of one to two monolayers onto 3 nm CdSe QDs by using growth temperature 300 ◦ C, and, in this case, they have found QY to be 50% [41]. For the synthesis of this system, they have used a hot injection method. First, they prepared Cd and Se stock solutions by dissolving dimethylcadmium and selenium shot in TOP. Further, using Cd and Se stock solutions, CdSe nanocrystal was synthesized. The growth of ZnS shell has been achieved by injection of Zn and S stock solutions which were developed by diethylzinc (Me2 Zn) and hexamethyldisilathiane [S(TMS)2 ] in TOP solution. Later, Bawendi group have reported a depth study on CdSe/ZnS core/shell QDs using low-temperature (140–220 ◦ C) synthesis [42]. Moreover, using HDA into TOPO/TOP, Weller group have been able to control the growth kinetics of CdSe/ZnS core/shell QDs and achieved lower particle size distribution with high QY up to 60% [43]. Moreover, the blue spectral region is also accessible using this material with extremely small size as reported by Kundra and co-authors [44]. They first synthesized CdSe “magic-sized” cluster in the solution of TOP, DDA, and nonionic acid at 80 ◦ C and further sequential growth of ZnS shell. In another report, Jun and Jang obtained similar spectral range with overgrowth of ZnS shell at 300 ◦ C by using zinc acetate and octanthiol as a precursor [45]. In this case, due to the low reactivity of precursors, the shell material spread into the core results in a remarkable blue shift of emission peak observed around 470 nm with QY up to 60%. In another work on the same material by Zhang group, they reached QY up to 95% by TOP-SILAR method (Fig. 5) [46]. This high QY is obtained by growing three monolayers of ZnS shell and even maintained after six monolayers of shell thickness (Fig. 6c). The second material in the type I system is CdSe/CdS core/shell QDs. In this case, the lattice mismatch between core and shell material is only 4% and exhibits different band alignment (holes have larger band offset than electrons). A detailed study on CdSe/CdS core/shell QDs with varying core diameters 2.3–3.9 nm and QY up to 50% was first reported by Peng and co-authors [47]. In this system, due to the

Synthesis, Properties, and Applications of II–VI Semiconductor Core/Shell. . .

TOP

Zn

S

SILAR CdSe core

Activated CdSe

TOP Zn

9

S

TOP-SILAR CdSe/ZnS QDs

Multilayered CdSe/ZnS QDs

Fig. 5 Growth mechanism of multilayered CdSe/ZnS core/shell QDs by TOP-SILAR synthesis method. (Reprinted with permission from Ref. [46])

Fig. 6 TEM images of (a) CdSe core and (b) CdSe/ZnS core/shell QDs after three monolayers of Zn and S precursors. Insets of these figures are their high-resolution transmission electron microscope (HRTEM) image. (c) Variation of QY with increasing shell thickness, where the core is synthesized with three emitting colors green, orange, and red, respectively. Inset is photograph of CdSe/ZnS core/shell QDs under UV light with three monolayers of shell thickness. (Reprinted with permission from Ref. [46])

delocalization of electronic wave function over the whole core/shell, the absorption and emission spectra get more shifted in comparison to the CdSe/ZnS system. The precursors used in this system were dimethylcadmium and bis(trimethylsilyl sulfide) with solvent TOPO. Later, using SILAR technique the same group synthesized the same material by air-stable precursors cadmium oleate and elemental sulfur in the

10

A. N. Yadav et al.

low-cost, low toxic, and high boiling solvent octadecane [40]. The SILAR technique was further extended to synthesize “giant” core/shell QDs as reported by Klimov group for the synthesis of giant CdSe/CdS core/shell QDs [48]. The shell thickness was 6 nm by growing 19 monolayers of CdS, and the QY was achieved up to 40%. Recently, Manna group have reported the synthesis of extremely luminescent giant CdSe/CdS core/shell QDs by a fast continuous injection method [49]. In this report, they have synthesized CdSe core of diameter between 2.8 and 5.5 nm and CdS shell thickness of 7–8 nm (~20 monolayers of CdS). Interestingly, the QY was maintained up to 90% by using this defect-free QD. The purpose of growth of the giant shell was to make nanocrystal independent from environment and surface chemistry. Further, a thicker shell protects the QDs from photobleaching and photoblinking. However, due to their larger size, they have poor size distribution and broad PL emission. Therefore, another method was required that could meet all the criteria. Later, Bawendi group synthesized high-quality CdSe/CdS core/shell QDs by optimizing growth rate of the shell material, using cadmium oleate and octanthiol as shell precursors [50]. The obtained core/shell QDs have very narrow emission peak with high PL QY (Fig. 7) and high uniformity (Fig. 8). Moreover, in contrast to previous studies, in this case, photoblinking too much suppressed with growing a relatively thin shell (2.4 nm). CdSe/ZnSe is another type I core/shell system, where electrons conduction band offset is larger than the holes valance band offset. The lattice mismatch, in this case, is significant as 6.3%; however, the anion-type structure is suitable for epitaxial shell growth. In the early time of synthesis, the QY of this system is very low ( aB , quantization of the motion of the center of mass of the exciton occurs. The energy spectrum of such a quasiparticle resembles a hydrogen-like one but does not coincide with it. In the limit of the strong quantum confinement when aB >> a, it is implied that an electron and a hole never form bound hydrogen-like states. Therefore, their motion in the first approximation is separately analyzed [21]. Thus, in spite of regarding elementary excitation at a

172

S. Savchenko et al.

quantum dot as an exciton, the latter itself does not reflect the term accepted for bulk semiconductors [48]. In this connection, the bandgap for QDs corresponds to the first-transition energy in the exciton absorption spectrum. It should be stressed that the terms such as electron–phonon interaction and exciton–phonon interaction are used with respect to QDs as interchangeable ones due to the specifics of elementary excitations in zero-dimensional objects. At quantum dots, along with the size, the temperature factor affects the bandgap. In this case, an important characteristic is the temperature coefficient β = dEg /dT. For most materials, the latter is thought to be a constant with a negative value [49]. Let us look into the existing relations to describe the dependence Eg (T). The simplest case describes the temperature change in the bandgap using a linear model: Eg (T ) = Eg (0) − βT .

(2)

Within this representation, the magnitude of β is determined by the slope of the linear part of the experimental temperature dependence Eg (T). However, at low temperatures, it can be essentially nonlinear. The linear–quadratic relation proposed by Varshni and widely used for describing Eg (T) appears as [50]: Eg (T ) = Eg (0) −

α1 T 2 , α2 + T

(3)

where α 1 and α 2 are empirical parameters that have no specific physical meaning. The α 2 constant having the dimension [α 2 ] = [T] is assumed to be close in magnitude to the Debye temperature. In the limit of high temperatures, when T >> α 2 , it follows from Eq. (3) that α 1 ≈ β. In some cases, the α 1 and α 2 coefficients are negative. Therefore, the physical interpretation of the recorded dependencies meets certain difficulties. Nevertheless, despite a shortage of information extracted, Eq. (3) quite satisfactorily describes the experimentally observed shape of the Eg (T) temperature characteristic, which was tested on a large number of different objects [49]. Within the single-phonon approximation and second-order perturbation theory, the temperature dependence of the bandgap can be represented as the relation [51]:  −1 Eg (T ) = Eg (0) − AF n , where n = exp (ω/kT ) − 1 .

(4)

Here AF is the Fan parameter, depending on the microscopic properties of the material, eV; n is the Bose–Einstein factor for phonons with an average energy ω; k is the Boltzmann constant, eV/K. Earlier, it was shown in [49] that Eq. (4) reduces to the form of Eq. (2) in the limit of high temperatures (kT >> -hω). In the process, the temperature coefficient can be written as

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

β∞ = AF

k . ω

173

(5)

It should be pointed out that Eq. (4) explicitly accounts for no contribution of thermal dilation of the lattice. As shown in [51, 52], this contribution to the total temperature change in the bandgap amounts to a magnitude of the order of 20%, and it can be neglected in the first approximation. In addition, at higher temperatures, the contribution of thermal expansion to the shift in energy levels is expected to be also proportional to n [52]. In this case, the calculated value of the AF parameter takes into account both internal (electron–phonon interaction) and external (thermal expansion) contributions to the Eg (T) dependence. Note that the AF parameter has an energy dimension and coincides in magnitude with a change in the bandgap at a temperature when the average number of phonons responsible for the displacement of the energy levels of the band edges is unity. The microscopic expression for the Fan parameter was written in [53] in the following way: ⎡ 3 3 ⎤   1  1 13 1  1 2 2⎥ mh 1 ⎢ me AF = √ (m0 ω) 2 + − ⎣ ⎦. 4πε ε∞ ε0 m0 m0 2 e2

(6)

Here e is the electron charge; ε is the dielectric constant; ε0 and ε∞ are the static and high-frequency permittivities, respectively; m0 is the mass of a free electron; me and mh are the effective masses of an electron and a hole, respectively. Equation (6), in which all quantities are tabular, can be used for estimating the fan parameter in the case of the lack of experimental data on the dependence Eg (T). The paper [49] demonstrated the relationship between the Varshni relation (3) and the Fan expression (4). Expanding in a series the right-hand side of Eq. (4) in the limit of kT >> -hω to the quadratic terms in temperature, we can arrive at an expression identical to Eq. (3) with the following coefficients: α1 = AF ·

k , ω

α2 =

ω , 2k

AF = 2α1 α2 .

(7)

Consequently, if the condition T >> 2α 2 holds, the Varshni coefficients α 1 and α 2 should contain information on the effective phonon energy. In [54], a semiempirical relation was proposed, the terms of which explicitly take into account effects of both thermal expansion and electron–phonon interaction: 4

Eg (T ) = Eg (0) − U1 T

U2



ω − U3 ω coth 2kT



5 −1 ,

(8)

where U1 , U2 , and U3 are temperature-independent parameters. The second term on the right-hand side of Eq. (8) represents thermal expansion; the third term is responsible for electron–phonon interaction. It is easy to see that the second and

174

S. Savchenko et al.

third terms in the right-hand sides of the expressions (4) and (8), respectively, coincide, with AF = 2U3 -hω. To approximate experimental data, the authors of [55] resorted to the relation: Eg (T ) = a − b (1 + 2 n) ,

(9)

where a–b = Eg (0) and b is a parameter characterizing the force of the electron– phonon interaction [56]. After comparing expressions (4) and (9), it can be written that AF = 2b. The authors of [57] proposed to describe the Eg temperature dependence of semiconductors as follows: Eg (T ) = Eg (0) − 2Sω n ,

(10)

where S is the Huang–Rhys factor proportional to the force of the electron–phonon interaction [28, 29]. Comparing Eqs. (4) and (10), we can see that AF = 2Sω. Thus, it is obvious that expressions (4), (8), (9), and (10) are similar to each other and allow describing the Eg temperature dependencies for the materials to obtain quantitative information on the effective phonon energy. The above models of the Eg (T) dependence are traditionally used to describe the properties of both bulk materials and low-dimensional objects. For example, papers [28, 29] applied relation (10) for delineating the temperature influence on the maxima of the photoluminescence bands of various-size InP/ZnS nanocrystals. Paper [56] analyzed the temperature dependencies of the energies of various optical transitions in indium phosphide nanowires using the PL excitation spectra and Eq. (9). In general case, there are four temperature-dependent factors that affect the positions of energy levels in quantum dots: dilation of the lattice, thermal expansion of the envelope function, mechanical strain, and electron–phonon coupling [58, 59], with the electron–phonon contribution dominating both for quantum dots [58] and for bulk materials [49]. In this regard, it seems more justified to use models that explicitly take into account phonon statistics and make it possible to extract information about the effective energy of phonons, the interactions with which determine the observed shifts of energy levels.

4 The Influence of Exciton–Phonon Interaction on the Energy of Optical Transitions The temperature dependencies of the absorption of nanocrystals were studied for InP/ZnS films. The OA spectra were measured at T = 6.5–296 K using a setup based on a Shimadzu UV-2450 spectrophotometer and a Janis model CCS-100/204N closed-cycle helium cryostat. The temperature was adjusted by a LakeShore Model 335 controller equipped with a DT-670B-CU diode temperature sensor. The vacuum

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

175

Fig. 4 The temperature dependencies of the OA of the QD-1 (a and b) and QD-2 (c and d) films in various spectral regions corresponding to transitions E1 and E2 . The dotted line indicates the direction of shift of the spectral features

inside the cryostat was produced using a HiCube 80 Eco turbo pump station and maintained during measurements at a level of less than 7·10−5 mbar. The OA spectra were recorded at fixed temperatures: 6.5 K, in the range of 10–100 K with a step of 10 K, and in the range of 100–296 K with a step of 20 K. Figure 4 sketches the OA spectra of the QD-1 and QD-2 InP/ZnS films, measured at various temperatures. To explore the features of the QD-1 absorption spectrum, corresponding to the E1 and E2 transitions, two samples were prepared by precipitating solutions with concentrations of 40 and 20 g/l. This made it possible to tune the optical density range on the desired spectral region. Curves 8 and 9 in Fig. 2a refer to the absorption of these samples at room temperature. Figure 4a, b displays the temperature dependencies of the QD-1 absorption for the above spectral regions. To study the QD-2 ensemble, we used a sample obtained by precipitating a solution of the initial concentration. The resulting absorption of the QD-2 ensemble is depicted by curve 8 in Fig. 2b. This film was characterized by a rather low optical density D < 0.1 in the region of the first InP exciton band. To enhance the optical density in this range, we made several attempts to raise the initial concentration. This was done in two ways: either by evaporating or by repeatedly depositing the solution on a substrate. However, the foregoing procedure led to an

176

S. Savchenko et al.

Fig. 5 The OA second-derivative spectra for determining the optical transition energies E1 (a), E2 (b) of QD-1 and E1 (c), E2 (d) of QD-2 at the indicated temperatures

increase in scattering only. Figure 4c, d shows the temperature evolution of the QD2 absorption curves in different wavelength ranges. As can be understood from the figure, the QD-1 and QD-2 ensembles, when cooled, demonstrate spectral shifts of the maxima and shoulders towards higher energies. Besides, the corresponding optical density grows. The inset in Fig. 4a illustrates these changes in the first QD-1 exciton absorption band in more detail. For determining the E1 and E2 energies of the optical transitions in QD-1 and QD-2 at various temperatures, the derivative spectrophotometry method was used. In Fig. 5, the second-derivative spectra for absorption curves at different temperatures serve as an example; they are designated as dotted lines. The minima in the derivative spectra allow accurately tracking the quantitative changes in the spectral parameters of the QD absorption bands as temperature varies [60]. In Fig. 6, square-shaped symbols represent the temperature dependencies for the E1 and E2 optical transition energies in QDs at hand. It can be seen that the magnitudes of E1 and E2 increase with decreasing T. Their behavior is characteristic of the temperature-induced change in the width of the optical gap in bulk semiconductor crystals [38, 49 and references therein]. At low temperatures, a range of unchangeable energies is observed. As becomes clear from the figure, temperatures of reaching this range for QD-1 are lower than those for QD-2. The solid lines show the approximation of the experimental data using Fan relation

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

177

Fig. 6 The temperature dependencies of the E1 and E2 energies for QD-1 (a and b) and QD2 (c and d). Comparison of E1 shift for the QDs and bulk InP (e). Square-shaped symbols are the experimental estimates; solid lines denote the approximation according to Fan expression (4); dashed lines are the approximation within the linear model (2); short dashed lines indicate the temperature dependence; dash-dotted lines denote the limit level β ∞ according to Eq. (5); round symbols present the value of temperature coefficient β when approximated by a linear model

(4). The dashed lines represent the description within the linear model (2). As a comparison, the obtained parameters and independent literature data are tabulated in Table 3.

178

S. Savchenko et al.

Table 3 Parameters of the E1 (T) and E2 (T) temperature dependencies for QD-1 and QD-2 QD-1

E1

QD-2

E2 E1 E2

InP nanowires [56] Bulk InP

E(0) (eV) AF , eV 2.72 0.09 0.06* [28] 0.14* [29] 4.21 0.07 2.50 1.06 4.19 0.27 1.50 0.09*

h¯ ω (meV) 15 13 [28] 23 [29] 14 59 37 21

1.42 [38] 0.05* [64] 14* [64]

β ∞ (10−4 eV/K) 5.13 3.83* [28] 5.20* [29] 2.37 3.84 (120–330 K) 4.08 9.03 9.97 (260–330 K) 15.56 3.76 5.31 (200–330 K) 6.47 2.20* 3.78* S 2.98

β (10−4 eV/K) 4.76 (90–296 K)

1.78* 4.6 [38]

3.06* [64]

*Our estimate using data from the cited work

Temperature behavior of the first exciton absorption band position in QD-1 is governed by the interaction with effective phonons of 15-meV energy corresponding to the energy of longitudinal acoustic vibrations (LA) in bulk InP [61, 62]. Overall, the values of the -hω effective phonon energy, the AF Fan and S = AF /2-hω Huang– Rhys parameters (see above) characterizing the E1 and E2 transitions, respectively, are consistent with the values for InP nanowires [56] (the length is of about 1 μm, and the diameter is of about 100 nm accordingly to the TEM image, but the authors do not give their values) and InP/ZnS quantum dots with 2.3-nm [28] and 2.1-nm [29] diameters, see Table 3. In QD-2, the shift of the E1 and E2 exciton transitions is due to the interaction with phonon modes having different effective energies [63]. The Huang–Rhys factor S for the first InP exciton absorption band is 2.4 times larger than that for the E2 transition. This means stronger exciton–phonon interaction for the E1 band. Figure 6e compares the data as a shift E1 (T) = E1 (T) − E1 (0) for the E1 transition of the QD-1 and QD-2 ensembles with the findings for the temperature behavior of exciton absorption levels in InP single crystals [64]. In Fig. 6, the short dotted line shows the dependence of dE/dT for the corresponding optical transitions. It characterizes the change in the β-coefficient with temperature. The dash-dotted line meets the limit value of the coefficient calculated using Eq. (5) and designated as β ∞ . The value of β indicated by the red circle was computed by approximating the experimental temperature dependencies of E1 and E2 within the linear model (2). In the general case, this coefficient depends on the interval T used for the linear approximation. This is because the dependencies involved exhibit a nonlinear character in the low-temperature region. The appropriate temperature ranges for the data satisfactorily described by the βconstant coefficient are listed in Table 3. The coefficient value for QD-1 is consistent with the reference data for bulk InP and significantly exceeds them in the case of QD-2. The relation β ≈ β ∞ being fulfilled for QD-1, it can be argued that the considered temperature ranges for the bands under study completely satisfy the high-temperature condition kT >> -hω.

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

179

5 The Half-Width of the First Exciton Absorption Band Change in the half-width of the characteristic bands reflects the temperature influence on the optical absorption spectra of condensed matter, as well. Figure 7 illustrates the normalized OA spectra of QD-1 and QD-2 at various temperatures. It should be noted that, with decreasing T, the low-energy edge of the exciton absorption band shifts towards higher energies without changing the slope. In other words, the shape of the band is temperature-independent. The insets in Fig. 7 confirm the above statement. The spectra are shifted along the energy axis for visual clarity of their identical shape in the low-energy part at different temperatures. In Fig. 8a, b, solid lines represent the experimental dependencies of the optical density on the photon energy for QD-1 at various temperatures in the range studied. It is worth pointing out that, along with the shift of the first exciton absorption

Fig. 7 Normalized OA spectra of QD-1 (a) and QD-2 (b) for different T. The insets illustrate the identity of the shape of the first exciton absorption band in QDs. The dashed line is a Gaussian approximation for estimating HG

Fig. 8 Temperature behavior of the first exciton absorption band of QD-1. An estimate of the Hvis half-width (a) and HG (b) is illustrated. The dashed line shows the approximation by a Gaussian band. The insets illustrate the changes in optical density at the maximum of the exciton band [65]

180

S. Savchenko et al.

band of the nanocrystal ensemble towards higher energies, an increase in the visible maximum Dmax of optical density is observed when the temperature drops from 296 K to 6.5 K (see the inset in Fig. 8a). In this case, the shape of the band has no changes. The estimation of the H half-widths was made in two ways: (1) visual Hvis was estimated by the energy position of Dmax and (2) HG by using the Gaussian approximation of the low-energy part of the spectrum. The calculation of the Hvis value was carried out over the low-energy part of the spectrum. In doing so, the width at half height was graphically determined and multiplied by two conditionals upon that the band was said to be symmetric (see Fig. 8a). The Hvis (T) calculated dependence denoted by blue solid squares is shown in Fig. 9. It can be seen that the value equal to Hvis = 390 ± 10 meV remained constant within the indicated error above the entire temperature range under consideration. As was stated earlier, a number of factors affect the position and half-width of the bands during the optical absorption of QDs. To substantiate a quantitative estimate of H, the low-energy wing of the exciton band was approximated by a Gaussian function accounting for these contributions (see Fig. 8b). The centers of the bands were set at energies E1 obtained using data of the second-order derivative spectrophotometry [61, 63]. The inset in Fig. 8b displays the change in the optical density DG corresponding to the maximum of the approximating Gaussian. As it becomes clear from the insets in Fig. 8a, b, the temperature dependencies are quite consistent with each other. The HG values derived from the described analysis of the experimental curves are presented in Fig. 9 – they are denoted black solid circles. For the QD-1 ensemble, the HG = 290 ± 20 meV half-width similar to Hvis (T) does not change within the experimental error over the entire temperature range. In this case HG < Hvis , which corresponds to the half-width of the luminescence band of the quantum dots explored [63, 66, 67]. For the QD-2 exciton band, the estimation was carried out using the second method. An example of an approximating Gaussian is presented in the inset in Fig. 7b for 6.5 K. As in the case of QD-1, the half-width does not change with temperature and amounts to HG = 370 ± 30 meV [63]. Fig. 9 The values of H for various InP modifications [65]. 1, Hvis (T) [61]; 2, HG (T) [61]; 3, InP/ZnS [15]; 4, InP/ZnS [4]; 5, 3.2 ÐÏ InP/ZnS [6]; 6, 2.3 ÐÏ InP/ZnS [28]; 7, InP/ZnS [5]; 8, PL H(T) InP/ZnS [29]. Inset: 9, InP single crystal [64]; 10, InP nanowires [56]; 11, InP single crystal [69]

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

181

The temperature-independent half-width of the exciton band was previously observed in the optical absorption spectra of CdSSe quantum dots with an average size of 2.3 nm in a glassy matrix [68]. The values of H = 154 meV for the first exciton band corresponding to the lowest electron–hole pair transitions almost did not change upon cooling from 300 K to 20 K. As the authors indicate, this is a consequence of the dominance of temperature-independent inhomogeneous broadening mainly related to the f (a) size distribution of nanocrystals. In Fig. 9, different symbols show the values of the visual half-width for the first exciton absorption peak; we calculated using independent data: for InP/ZnS quantum dots of various sizes [4–6, 15, 28], nanowires [56], and bulk InP singlecrystals [64, 69]. It can be seen that the Hvis value at room temperature (see dependence 1 in Fig. 9) is consistent with the independent literature data on InP/ZnS core/shell quantum dots (see symbols 3–7 in Fig. 9, the average nanocrystal size, if known, indicated in the caption to the figure). In bulk InP single crystals, the corresponding characteristic varies from 2 meV to 12 meV in the temperature range of 5–300 K [69] (see the inset in Fig. 9). Thus, the half-width of the exciton band at 6.5 K in InP/ZnS various-size quantum dots made by different manufacturers is more than 100 times higher than the similar characteristic for bulk InP single crystals. The large value of the half-width of the exciton absorption band in InP/ZnS quantum dots and its temperature independence evidence the inhomogeneous nature of the broadening of the first exciton absorption band in the nanocrystals. In [16], the authors note the dominant contribution of inhomogeneous broadening to the full spectrum width for InP/ZnS QDs. They make this conclusion based on an analysis of the stationary absorption and luminescence spectra within the Kennard–Stepanov relation. It is also known that the composition and thickness of layers in multiple quantum wells have an influence on the inhomogeneous width of the exciton absorption peak [70]. It should be emphasized that the temperature-independent behavior is observed not only in low-dimensional systems but also typical of disordered massive solids [71, 72]. In amorphous and vitreous states, the properties of optically active point defects are significantly affected by inhomogeneous spectrum broadening that is associated with the nonequivalence of centers’ local surrounding [73]. By analogy with the disordered state, the nonequivalence of the excitons’ surrounding being created due to the absorption of photons in each individual nanocrystal of QD ensemble takes place. Overall, inhomogeneous broadening in the absorption spectra is typical of ensembles of nanocrystals and is a consequence of the size, shape, stoichiometry, defect concentration, charge state, local environment distributions, etc. of nanocrystals. All these factors cause essential differences in the potentials of each individual nanocrystal [74]. Therefore, the energy structure of elementary excitations changes, which ultimately manifests itself in the inhomogeneous broadening of the exciton absorption bands. To quantitatively analyze this effect, a model was proposed that describes well the experimentally observed behavior of the first exciton absorption band and allows one to estimate the influence of inhomogeneities in the ensembles of nanocrystals by a magnitude of the broadening of optical spectra.

182

S. Savchenko et al.

6 Temperature Evolution of the First Exciton Absorption Band of QD Ensemble 6.1 Static and Dynamic Disorder in Ensemble The half-width h of an exciton optical absorption peak of a single nanocrystal at any temperature can be represented as follows: h(T ) = h0 + Δh (T )

(11)

Here, the first term reflects the natural line width at zero temperature. The second summand accounts for the effects initiated by its temperature broadening. When looking into a system of identical nanocrystals with the same energy e1 of the first exciton transition, the corresponding absorption line homogeneously broadens under the influence of temperature. Within the exciton–phonon interaction, the corresponding contribution is given by [28, 29, 75]:  −1 Δh (T ) = σ T + ALO exp (ωLO /kT ) − 1

(12)

where σ is the exciton–acoustic phonon coupling coefficient, eV/K, and the ALO value reflects the force of interaction between excitons and longitudinal optical LO-phonons with an energy -hωLO . In this case, the Δh value considered herein quantitatively characterizes the dynamic disorder, which provides a temperaturedependent contribution to the broadening of energy levels due to lattice vibrations [61]. In real systems, nanocrystals in an ensemble possess different energies e1 due to the scatter in the characteristics of QDs [74]. As a result, the H half-width of the exciton absorption band for the ensemble turns out to be larger than h for individual nanocrystals even at zero temperature. This occurs because of overlapping the closely spaced peaks with different energies e1 . In this case, the optical absorption band is inhomogeneously broadened, and the ΔI quantity quantitatively characterizes the static disorder, which provides a temperature-independent contribution to the broadening of energy levels [72, 76]. The static disorder is caused by the f (X) distribution in the parameters of QDs in the ensemble tested. With increasing temperature, the bands of individual nanocrystals broaden in accordance with Eq. (12). This, in turn, affects the H width of the ensemble’s exciton band as a whole, which leads to a homogeneous contribution of T (X, T) to the band broadening. The temperature evolution of the H half-width of the optical absorption band of the ensemble of nanocrystals with a certain distribution f in parameters can be written as follows [65]: H (X, T ) = h0 + ΔI (X) + ΔT (X, T ) .

(13)

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

183

In this case, the broadening is governed by the influence of both static and dynamic types of atomic disorder. In what follows, we will call it temperature broadening.

6.2 The Behavior of the Exciton Line of an Individual Nanocrystal To simulate the temperature behavior of an exciton band of an ensemble of nanocrystals, it is necessary to know the change in the parameters of their individual components. For this, the temperature dependencies should be preassigned for the shift of the maximum of individual bands, their broadening, and area changes. The individual spectral components for an ensemble of QDs are assumed to behave identically, depending on temperature. The shift of the e1 centers of individual Gaussian peaks corresponding to the energy positions of the maxima of the absorption peaks of individual nanocrystals of size a was modeled in accordance with Fan expression (4). As was claimed above, this model well describes the shift of the first exciton absorption peak of the QD ensemble for the following values of the parameters given in Table 4. The indicated dependence is shown in Fig. 10, curve 1. Thus, the model adopted and

Table 4 Parameters of temperature evolution of the exciton peak in an InP/ZnS single nanocrystal Shift [61] Broadening [29, 77]

E1 (0) (eV) 2.72 h0 (meV) 6.3

AF (eV) 0.09 σ (μeV/K) 172

ALO (meV) 60

h¯ ω (meV) 15 h¯ ωLO (meV) 40

Fig. 10 Model’s dependencies of the temperature behavior of the exciton absorption peak of a single nanocrystal. (a) Shift (line 1 calculated according to Eq. (4)) and broadening (line 2 calculated according to Eq. (11)) of the exciton peak for an InP/ZnS individual nanocrystal. The red triangle is the experimental data from [77]. (b) Change in area. Symbols are the experimental estimates; solid line is the approximation according to Eq. (14)

184

S. Savchenko et al.

the experiment performed share the same shift magnitude for e1 in the band of an individual nanocrystal and for E1 in the band of the ensemble as a whole. The h(T) dependence was established through relations (11) and (12). Eq. (12) includes the parameter values taken from [29], where they were determined by approximating the H(T) dependence of the luminescence band of an ensemble of InP/ZnS nanocrystals with an average size of 2.1 nm in the temperature range of 2–510 K. The values are given in Table 4. The value of H = 73 meV for an individual InP/ZnS core/shell nanocrystal at room temperature was found in [77] using photon-correlation Fourier spectroscopy (see triangle in Fig. 10). The close value of H(300) = 68 meV was previously obtained for InP nanocrystals when analyzing the size dependence of the bandgap [78]. Figure 10 (curve 2) illustrates the h(T) dependence calculated for a single nanocrystal in the ensemble. The value of the fundamental half-width h0 in the model adopted amounts to 6.3 meV. The line width at 6.5 K is h = 7.4 meV and exceeds the corresponding value for a single crystal (2 meV). The values specified are in complete agreement with the data of [68]. The latter emphasizes that the half-width of optical bands in quantum dots, measured using the spectral hole burning technique, is three times larger than that for bulk analogs. The temperature-dependent area of the individual components corresponding to individual nanocrystals is assumed to change in the same manner as the experimentally observed area S of the integral peak of the QD ensemble. With the half-width H remaining constant over a wide range of temperatures, the S(T) dependence curve reflects the behavior of the optical density DG (T) at the maximum of the exciton band. The S(T) experimental graph was built by evaluating the area of the Gaussians used for approximating the first exciton band in the optical absorption spectra measured (see Fig. 8). When simulating the temperature behavior of the area of the optical component of an individual nanocrystal, the empirical formula was utilized for the relative change in sr (T). This formula well describes the experimentally observed dependence S(T): sr (T ) =

 −1 S(T ) = 1 − B1 exp (B2 /T ) − 1 . S(0)

(14)

The values of the area beneath the Gaussian curves for the QD-1 ensemble, normalized to S(0), and the corresponding model curve represented by expression (14) that imitates the sr (T) dependence are shown in Fig. 10b. The values of empirical parameters calculated during the approximation are S(0) = 0.22 eV, B1 = 0.68, and B2 = 282.92 K. It can be seen that the area of the exciton peak area decreases almost twice as the temperature increases in the range studied.

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

185

6.3 QD Size Distribution A number of papers regard the dispersion of QDs in size a as the dominant factor of inhomogeneous broadening [18, 22, 25, 26, 74]. Therefore, for definiteness, it is the influence of this factor, X = f (a), that will be further associated with the static disorder. The relationship between the size variation in the QD ensemble and the width of the observe exciton absorption band is due, first and foremost, to quantumsize effects. The estimate of the f (a) distribution function in the QD ensemble was brought about using Eq. (1) and Gaussian curves intended for approximating the exciton absorption bands at 6.5 K (see Fig. 8). In Eq. (1), the bandgap width for a bulk InP single crystal was accepted as Eg (0) = 1.42 eV, which corresponds to the value at 2 K [69]. The maximum position of the Gaussian band on the energy axis corresponds to the energy e1 of the first exciton absorption peak of a particular nanocrystal in the ensemble. Thus, the energy was recalculated into the radius of the corresponding nanocrystal through the transformation e1 → a. In the process, the optical density at certain energy is assumed to be proportional to the number of nanocrystals of the corresponding size in the ensemble. Figure 11 compares the normalized distributions f1 (a) and f2 (a) thus calculated for the QD-1 and QD-2 ensembles, respectively, and experimental data taken from independent works [79, 80]. The average values of the nanocrystal radii in the respective distributions are a¯ 1 = 2.1 nm and a¯ 2 = 2.3 nm. To characterize the quality of the size distributions, we computed the relative width parameter using the following formula: δ=

Fig. 11 Size distributions and the δ parameter of the studied InP/ZnS quantum dots in comparison with the data from independent works [65]: 1, [79]; 2, [80]

Δa 100% a

(15)

186

S. Savchenko et al.

Table 5 Estimation of the size of nanocrystals according to Eq. (1) at different temperatures

Temperature (K) 6.5 296

a¯ 1 (nm) 2.08 2.26

a¯ 2 (nm) 2.10 2.32

where a is the width at half-height for the f (a) dependence. The δ-values obtained are shown in Fig. 11 in brackets. It can be seen that in the ensembles concerned a2 > a1 , f1 (a) and f2 (a) are quite typical of InP/ZnS nanocrystals. It should be noted that such an approach in estimating the size distribution is most appropriate for the absorption band since the latter’s position is not affected by the Stokes shift, against the luminescence band. In addition, expression (1) does not explicitly take into account the temperature factor. The latter is included only through the Eg quantities for QDs and bulk semiconductors. However, the nature of the influence of temperature on the behavior of the bandgap in the zerodimensional and three-dimensional cases may be different. This leads to an error in estimating both the size itself and the distribution in size. We calculated the sizes of nanocrystals both for room temperature and for 6.5 K in accordance with the experimental data obtained. The results are listed in Table 5. For the QD-2 ensemble, where the dependence E1 (T) differs significantly from that of bulk InP, the divergence in size estimates is larger in magnitude than for QD-1. Thus, to reduce a temperature influence on the spectral position and half-width of the band, the average size and size distribution need to be estimated at low temperatures. Then the T (f, T) and E1 (T) contributions can be neglected. Varying H of the absorption band underlies the simulation of the ensembles with different size distributions f (a). In the case of a monodisperse QD ensemble, the absorption band is formed by a single Gaussian-shaped component with the energy e1 , half-width h, and the area s. The size distribution process leads to dramatically raising the number of such components due to differences in the first exciton transition energy to finally form a Gaussian-shaped band with energy E1 , half-width H, and area S.

6.4 Contributions of Homogeneous and Inhomogeneous Broadening The influence of the degree of static disorder on broadening of the first exciton absorption band can be analyzed premised on the proposed model by comparing the homogeneous and inhomogeneous contributions for samples with different f (X) distribution. The contribution of homogeneous broadening can be quantified as CH =

ΔT (X, T ) . H (X, T )

The contribution of inhomogeneous broadening, in turn, can be written as

(16)

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

CI =

ΔI (X) . H (X, T )

187

(17)

Apart from the dimensional criterion δ, we also employed the optical criterion Q that reflects the ratio of the homogeneous and inhomogeneous contributions to the band broadening: Q=

CH ΔT (X, T ) . = CI ΔI (X)

(18)

In this case, it does not matter which factor causes the inhomogeneous broadening since we are dealing with their complex influence on the optical characteristics. For the QD ensembles, this criterion is integral and monotonously increases with decreasing static disorder in the system. Thus, the criterion can serve as a universal tool of comparing the quality of low-dimensional systems with distributed parameters.

6.5 Simulation of Experimental InP/ZnS Ensembles Simulation of the temperature behavior of the exciton absorption band for a QD ensemble with a given size distribution of f (a) included three stages. At first, the OA band at zero temperature was formed by a set of Gaussian components. Each of the latter had a half-width h0 and corresponded to a subset of nanocrystals of the same size in the ensemble. The second stage predicted the behavior of an individual Gaussian as the temperature increases. That is, the position of the e1 maximum, the h half-width, and the s area were changed according to the assigned functional dependencies (4), (12), and (14), respectively. At the end, at a fixed temperature, the Gaussian components were summed up and, thus, a model optical exciton absorption band was formed for the QD ensemble with parameters E1 , H, and S. The model proposed was intended for describing the temperature behavior of the first exciton absorption band of the experimentally studied QD-1 ensemble with a size distribution of δ = 11.1%. The band with a half-width of 290 meV was approximated by Gaussian components with an h = 7.4 meV half-width, which corresponds to a temperature of 6.5 K. The minimum number of components to describe the experimental data with high accuracy (adj. R2 > 0.999) amounted to N = 180. Then, the exciton peaks of individual nanocrystals were simulated for various experimental temperatures in the range 6.5–296 K. Figure 12 illustrates the temperature evolution of such an individual spectral component. It can be seen that the maximum shifted by e1 = 112 meV to the region of lower energies in the temperature range of 6.5–296 K. Simultaneously, the area s decreased by 1.8 times, and h increased by almost ten times from 7.4 meV to 73 meV. After summing up the individual spectral components at different temperatures, the temperature behavior of the model integrated band was found to describe well

188

S. Savchenko et al.

Fig. 12 Visualization of the temperature change in the absorption bands corresponding to individual nanocrystals in the ensemble. For clarity, the optical density is normalized to the maximum value for a component at 6.5 K [65]

Fig. 13 Experimental optical absorption spectra (symbols) and simulation of the first exciton absorption band (solid lines) for QD-1 at specified temperatures [65]

the exciton absorption band experimentally observed for the QD-1 ensemble across the entire temperature range (see Fig. 13). The resulting shifts in the position of the model absorption band for the ensemble and a change in its area occur by the same amount as in the experiment. It is important to underscore that the temperature behavior of the indicated integral parameters coincides with the behavior of the optical components for single nanocrystals. Thus, good agreement between the experimental and simulation results confirms the validity of the assumptions made earlier. The half-width of the total exciton absorption band for the InP/ZnS QD ensemble remains unchanged within the experimental error in the temperature range of 6.5–296 K (see the dependence for δ = 11.1% in Fig. 14 [hollow squares]). At the same time, the h half-width of the optical peak for each of the nanocrystals in the distribution varies by almost ten times. The proposed model correctly reproduces the temperature behavior of the first exciton absorption band for the InP/ZnS QD

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

189

Fig. 14 Simulated H(T) dependencies of the exciton band of ensembles of nanocrystals with different size distributions (Table 6). Hollow symbols indicate the experimental dependence for the QD-1 ensemble [65]

ensemble and can be applied for analyzing the influence of the size distribution of nanocrystals on the broadening of optical spectra.

6.6 Disorder Effects in Temperature Band Broadening Analyzing the influence of the degree of static disorder on the processes of homogeneous and inhomogeneous broadening of the absorption band involved the investigation of a number of QD ensembles with different half-widths H of the first exciton band (see Table 6). The value of N indicates the number of Gaussian components used for simulation of the corresponding integrated band. Moreover, the latter’s maximum was at an energy of 2.72 eV, which conforms to the experimental position for the ensemble of nanocrystals under study at 6.5 K. This means that the computational experiments save the average particle size standing, and only the size distribution changes, which is reflected in a change in H. Figure 14 presents the simulation results of the temperature change in the halfwidth of the exciton absorption band for ensembles of nanocrystals with different degrees of static disorder, i.e., by different distribution of f (a), in the temperature range of 6.5–296 K. It can be seen that as the width of f (a) for the nanocrystals in the ensemble diminishes, the homogeneous broadening of the model band is more pronounced. In the case of the band experimentally observed (the half-width is 290 meV), the contribution of homogeneous broadening is CH = 3% and does not exceed the experimental error of 7%. Reducing the degree of static disorder, the contribution of homogeneous broadening rises. The findings of CT , CI , and Q calculated using Eqs. (16), (17), and (18) at room temperature for the model range of samples are shown in Table 6. As is clear from the table, as δ decreased, the behavior of the H exciton absorption band of the ensemble tended to be the characteristic of the only homogeneously broadened component. Otherwise speaking, H behaved as

190

S. Savchenko et al.

Table 6 The simulation results of the temperature behavior of the first exciton absorption band of InP/ZnS quantum dot ensembles with different size distributions (data are shown in Fig. 14) Curve 1 2 3 4 5 6 7 8 9 10 Single QD

H(6.5 K) (meV) 290 242 193 145 113 81 64 48 32 16 7.4

N 180 150 120 90 70 50 40 30 20 10 1

δ (%) 11.1 9.2 7.3 5.5 4.3 3.1 2.4 1.8 1.2 0.6 0

CI (%) 94.9 93.3 90.5 85.5 79.4 68.7 59.6 47.9 32.4 13.0 0

CH (%) 3.0 4.2 6.4 10.6 15.9 25.5 33.9 44.9 59.7 78.5 89.9

Q 0.03 0.05 0.07 0.12 0.20 0.37 0.57 0.94 1.84 6.02 –

matching to a monodisperse ensemble. On the other hand, the increase in the degree of static disorder in the ensemble led to lesser and lesser impact of the homogenous broadening of the peak of each individual nanocrystal on the width of the integrated band with increasing temperature. The simulated regularities also explain the temperature behavior of the H for the QD-2 ensemble. For the latter, the half-width of the optical absorption band amounts to 370 meV, which corresponds to a wider size distribution (δ = 17.3%) than for QD-1. The contribution of CT to the temperature broadening in such an ensemble is even less than in the case of QD-1. It is worth emphasizing that the work in [29] reports on a change observed in the half-width of the PL band of an ensemble of InP/ZnS nanocrystals. In particular, for a sample with a half-width of ≈0.24 eV at 300 K, the temperature broadening in the range of 2–300 K was ≈0.08 eV (see curve 8 in Fig. 9). This may be due to the transfer of excitation energy over the ensemble of quantum dots from small to larger [36]. As a consequence, a certain size subset of nanocrystals participates in the radiative process, which leads to a decrease in the inhomogeneous broadening contribution to the H value of the luminescence band of the ensemble. In this case, the dynamic disorder contribution to the temperature broadening increases. The analysis conducted indicates some limitations that should be considered when analyzing the temperature evolution of the exciton absorption bands in the ensembles of semiconductor nanocrystals. In describing the temperature curves for the shift of the maxima, the values of the extracted fundamental parameters of the exciton–phonon interaction are physically justified. As was shown in the framework of the model proposed, the degree of static disorder in the system affects slightly these dependencies. The shift of the maximum of the integrated band reflects the displacement of the optical component of the peak for an individual nanocrystal. However, when considering the temperature variations in the band half-width for a QD ensemble, the static disorder plays a significant role. When forming, the integrated absorption band losses the homogeneous broadening of the individual

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

191

nanocrystal components due to their overlap. The contribution of static disorder distorts the corresponding parameter values of the exciton–phonon interaction, while they characterize the ratio of the homogeneous and inhomogeneous broadening processes. It is, therefore, mandatory to carry out estimates of them carefully. Thus, for making an infallible estimate of the parameters of dynamic disorder, an analysis of the temperature broadening processes in optical spectra needs to run accounting for the contribution of inhomogeneous broadening factors.

7 Conclusion In conclusion, we note that the chapter has explored the peculiarities of optical absorption spectra in the temperature range of 6.5–296 K for ensembles of InP/ZnS core/shell colloidal quantum dots with average particle radii of 2.1 (QD-1) and 2.3 (QD-2) nm. The approaches of derivative spectrophotometry are shown to make it possible to successfully determine the spectral positions of hidden exciton absorption bands of the nanocrystals. When formed by the precipitation method, the films exhibit collective effects in the QD ensembles. This can be evidenced by shifting the absorption bands for the shell, as compared to those for the solution. For describing the temperature dependence of the energy gap in semiconductor nanocrystals, well-known models were considered. It is revealed that the shift in the position of the first exciton absorption band E1 with temperature is due to the interaction with the effective vibrational modes of 15- and 59-meV energies for QD-1 and QD-2, respectively. The H half-width of the band for these ensembles is temperature-independent and amounts to 290 and 370 meV. A model is proposed that allows one to quantitatively describe the temperature behavior of the first exciton absorption band of InP/ZnS QD ensembles, as well as to analyze the contributions of homogeneous and inhomogeneous broadening of this band. The numerical analysis conducted proved that the halfwidth of the exciton absorption band for the ensembles remains unchanged in the temperature range of 6.5–296 K, even accounting for the tenfold homogeneous broadening of spectral components for individual nanocrystals. The influence of static disorder associated with the δ quality factor of the size distribution in the QD ensembles on the processes of broadening of model bands is analyzed. It is shown that, when going over from a monodisperse ensemble of nanocrystals to a δ = 11.1% distribution (for the QD-1 sample), the homogeneous contribution to the integrated broadening drops from 90% to 3%. Thus, processes of inhomogeneous broadening due to the high-degree static disorder in the QD ensembles tested determine the observed temperature independence of the exciton band half-width. The approach proposed allows examining various low-dimensional systems whose optical characteristics are sensitive to the distribution of structural parameters.

192

S. Savchenko et al.

Acknowledgments This research was supported by RFBR according to the research project no. 18-32-00664 and Act 211 Government of the Russian Federation, contract no. 02.A03.21.0006.

References 1. Alivisatos, A.P.: Semiconductor clusters, nanocrystals, and quantum dots. Science. 271, 933– 937 (1996). https://doi.org/10.1126/science.271.5251.933 2. Efros, A.L., Efros, A.L.: Interband absorption of light in a semiconductor sphere. Sov. Phys. Semicond. 16, 772–775 (1982) 3. Brus, L.E.: Electron-electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state. J. Chem. Phys. 80, 4403–4409 (1984). https://doi.org/10.1063/1.447218 4. Xie, R., Battaglia, D., Peng, X.: Colloidal InP nanocrystals as efficient emitters covering blue to near-infrared. J. Am. Chem. Soc. 129, 15432–15433 (2007). https://doi.org/10.1021/ ja076363h 5. Lee, S.-H., Lee, K.-H., Jo, J.-H., Park, B., Kwon, Y., Jang, H.S., Yang, H.: Remote-type, highcolor gamut white light-emitting diode based on InP quantum dot color converters. Optic. Mater. Express. 4, 1297–1302 (2014). https://doi.org/10.1364/OME.4.001297 6. Song, W.-S., Lee, S.-H., Yang, H.: Fabrication of warm, high CRI white LED using noncadmium quantum dots. Opt. Mater. Express. 3, 1468–1473 (2013). https://doi.org/10.1364/ OME.3.001468 7. Kamat, P.V.: Quantum dot solar cells. Semiconductor nanocrystals as light harvesters. J. Phys. Chem. C. 112, 18737–18753 (2008). https://doi.org/10.1021/jp806791s 8. Scher, J.A., Bayne, M.G., Srihari, A., Nangia, S., Chakraborty, A.: Development of effective stochastic potential method using random matrix theory for efficient conformational sampling of semiconductor nanoparticles at non-zero temperatures. J. Chem. Phys. 149, 014103 (2018). https://doi.org/10.1063/1.5026027 9. Rempel, A.A., Kozlova, E.A., Gorbunova, T.I., Cherepanova, S.V., Gerasimov, E.Y., Kozhevnikova, N.S., Valeeva, A.A., Korovin, E.Y., Kaichev, V.V., Shchipunov, Y.A.: Synthesis and solar light catalytic properties of titania-cadmium sulfide hybrid nanostructures. Catal. Commun. 68, 61–66 (2015). https://doi.org/10.1016/j.catcom.2015.04.034 10. Savchenko, S.S., Vokhmintsev, A.S., Weinstein, I.A.: Luminescence parameters of InP/ZnS@AAO nanostructures. AIP Conf. Proc. 1717(4943471), (2016). https://doi.org/ 10.1063/1.4943471 11. Savchenko, S.S., Vokhmintsev, A.S., Weinstein, I.A.: Optical properties of InP/ZnS quantum dots deposited into nanoporous anodic alumina. J. Phys. Conf. Ser. 741, 012151 (2016). https:/ /doi.org/10.1088/1742-6596/741/1/012151 12. Klimov, V.I., Mikhailovsky, A.A., Xu, S., Malko, A., Hollingsworth, J.A., Leatherdale, C.A., Eisler, H.-J., Bawendi, M.G.: Optical gain and stimulated emission in nanocrystal quantum dots. Science. 290, 314–317 (2000). https://doi.org/10.1126/science.290.5490.314 13. Anc, M.J., Pickett, N.L., Gresty, N.C., Harris, J.A., Mishra, K.C.: Progress in non-Cd quantum dot development for lighting applications. ECS J. Solid State Sci. Technol. 2, R3071–R3082 (2013). https://doi.org/10.1149/2.016302jss 14. Resch-Genger, U., Grabolle, M., Cavaliere-Jaricot, S., Nitschke, R., Nann, T.: Quantum dots versus organic dyes as fluorescent labels. Nat. Methods. 5, 763–775 (2008). https://doi.org/ 10.1038/nmeth.1248 15. Hussain, S., Won, N., Nam, J., Bang, J., Chung, H., Kim, S.: One-pot fabrication of high-quality InP/ZnS (core/shell) quantum dots and their application to cellular imaging. ChemPhysChem. 10, 1466–1470 (2009). https://doi.org/10.1002/cphc.200900159 16. Reiss, P., Protiere, M., Li, L.: Core/shell semiconductor nanocrystals. Small. 5, 154–168 (2009). https://doi.org/10.1002/smll.200800841

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

193

17. Brichkin, S.B.: Synthesis and properties of colloidal indium phosphide quantum dots. Colloid J. 77, 393–403 (2015). https://doi.org/10.1134/S1061933X15040043 18. Song, W.-S., Lee, H.-S., Lee, J.C., Jang, D.S., Choi, Y., Choi, M., Yang, H.: Amine-derived synthetic approach to color-tunable InP/ZnS quantum dots with high fluorescent qualities. J. Nanopart. Res. 15, 1750 (2013). https://doi.org/10.1007/s11051-013-1750-y 19. Brichkin, S.B., Spirin, M.G., Tovstun, S.A., Gak, V.Y., Mart’yanova, E.G., Razumov, V.F.: Colloidal quantum dots InP@ZnS: inhomogeneous broadening and distribution of luminescence lifetimes. High Energy Chem. 50, 395–399 (2016). https://doi.org/10.1134/ S0018143916050064 20. Ayupova, D., Dobhal, G., Laufersky, G., Nann, T., Goreham, R.V.: An in vitro investigation of cytotoxic effects of InP/ZnS quantum dots with different surface chemistries. Nano. 9, 135 (2019). https://doi.org/10.3390/nano9020135 21. Gaponenko, S.V.: Optical Properties of Semiconductor Nanocrystals. Cambridge University Press, Cambridge (1998) 22. Weller, H.: Colloidal semiconductor Q-particles: chemistry in the transition region between solid state and molecules. Angew. Chem. Int. Ed. Engl. 32, 41–53 (1993). https://doi.org/ 10.1002/anie.199300411 23. Empedocles, S.A., Bawendi, M.G.: Quantum-confined stark effect in single CdSe quantumconfined stark effect in single CdSe nanocrystallite quantum dots. Science. 278, 2114–2117 (1997). https://doi.org/10.1126/science.278.5346.2114 24. Ekimov, A.I.: Optical properties of semiconductor quantum dots in glass matrix. Phys. Scr. 39, 217–222 (1991). https://doi.org/10.1088/0031-8949/1991/T39/033 25. Murray, C.B., Norris, D.J., Bawendi, M.G.: Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites. J. Am. Chem. Soc. 115, 8706–8715 (1993). https://doi.org/10.1021/ja00072a025 26. Efros, A.L., Rosen, M., Kuno, M., Nirmal, M., Norris, D., Bawendi, M.: Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: dark and bright exciton states. Phys. Rev. B. 54, 4843–4856 (1996). https://doi.org/10.1103/PhysRevB.54.4843 27. Sugisaki, M., Ren, H.-W., Nishi, K., Masumoto, Y.: Optical properties of InP self-assembled quantum dots studied by imaging and single dot spectroscopy. Jpn. J. Appl. Phys. Part. 1 Regul. Pap. Short Notes Rev. Pap. 41, 958–966 (2002) 28. Narayanaswamy, A., Feiner, L.F., Van Der Zaag, P.J.: Temperature dependence of the photoluminescence of InP/ZnS quantum dots. J. Phys. Chem. C. 112, 6775–6780 (2008). https://doi.org/10.1021/jp800339m 29. Narayanaswamy, A., Feiner, L.F., Meijerink, A., Van Der Zaag, P.J.: The effect of temperature and dot size on the spectral properties of colloidal InP/ZnS core-shell quantum dots. ACS Nano. 3, 2539–2546 (2009). https://doi.org/10.1021/nn9004507 30. Biadala, L., Siebers, B., Beyasit, Y., Tessier, M.D., Dupont, D., Hens, Z., Yakovlev, D.R., Bayer, M.: Band-edge exciton fine structure and recombination dynamics in InP/ZnS colloidal nanocrystals. ACS Nano. 10, 3356–3364 (2016). https://doi.org/10.1021/acsnano.5b07065 31. Norris, D.J., Bawendi, M.G.: Measurement and assignment of the size-dependent optical spectrum in CdSe quantum dots. Phys. Rev. B. 53, 16338–16346 (1996). https://doi.org/ 10.1103/PhysRevB.53.16338 32. Talsky, G.: Derivative Spectrophotometry: Low and Higher Order. VCH, Weinheim (1994) 33. O’Haver, T.C., Begley, T.: Signal-to-noise ratio in higher order derivative spectrometry. Anal. Chem. 53, 1876–1878 (1981). https://doi.org/10.1021/ac00235a036 34. Savitzky, A., Golay, M.J.E.: Smoothing and differentiation of data by simplified least squares procedure. Anal. Chem. 36, 1627–1639 (1964). https://doi.org/10.1021/ac60214a047 35. Ku´s, S., Marczenko, Z., Obarski, N.: Derivative UV-VIS spectrophotometry in analytical chemistry. Chem. Anal. 41, 899–929 (1996) 36. Mi´ci´c, O.I., Jones, K.M., Cahill, A., Nozik, A.J.: Optical, electronic, and structural properties of uncoupled and close-packed arrays of InP quantum dots. J. Phys. Chem. B. 102, 9791–9796 (1998). https://doi.org/10.1021/jp981703u

194

S. Savchenko et al.

37. Fu, H., Zunger, A.: Excitons in InP quantum dots. Phys. Rev. B Condens. Matter Mater. Phys. 57, R15064–R15067 (1998). https://doi.org/10.1103/PhysRevB.57.R15064 38. Babichev, A.P.: Handbook of Physical Quantities. In: Grigor’ev, I.S., Meilikhov E.Z. (eds.) Energoatomizdat, Moscow (1991). (in Russian) 39. Baskoutas, S., Terzis, A.F.: Size-dependent band gap of colloidal quantum dots. J. Appl. Phys. 99, 013708 (2006). https://doi.org/10.1063/1.2158502 40. Fu, H., Wang, L.W., Zunger, A.: Applicability of the k•p method to the electronic structure of quantum dots. Phys. Rev. B. 57, 9971–9987 (1998). https://doi.org/10.1103/ PhysRevB.57.9971 41. Kayanuma, Y., Momiji, H.: Incomplete confinement of electrons and holes in microcrystals. Phys. Rev. B. 41, 10261–10263 (1990). https://doi.org/10.1103/PhysRevB.41.10261 42. Pellegrini, G., Mattei, G., Mazzoldi, P.: Finite depth square well model: applicability and limitations. J. Appl. Phys. 97, 073706 (2005). https://doi.org/10.1063/1.1868875 43. Baskoutas, S., Terzis, A.F.: Size dependent exciton energy of various technologically important colloidal quantum dots. Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 147, 280–283 (2008). https://doi.org/10.1016/j.mseb.2007.09.041 44. Weller, H., Koch, U., Gutierrez, M., Henglein, A.: Photochemistry of colloidal metal sulfides. 7. Absorption and fluorescence of extremely small ZnS particles (the world of the neglected dimensions). Berichte der Bunsengesellschaft/Physical Chemistry Chemical Physics. 88, 649– 656 (1984). https://doi.org/10.1002/bbpc.19840880715 45. Zhang, Y., Ma, M., Wang, X., Fu, D., Gu, N., Liu, J., Lu, Z., Ma, Y., Xu, L., Chen, K.: First-order hyperpolarizability of ZnS nanocrystal quantum dots studied by hyper-Rayleigh scattering. J. Phys. Chem. Solids. 63, 2115–2118 (2002). https://doi.org/10.1016/S00223697(02)00259-7 46. Kho, R., Torres-Martınez, C.L., Mehra, R.K.: A simple colloidal synthesis for gram-quantity production of water-soluble ZnS nanocrystal powders. J. Colloid Interface Sci. 227, 561–566 (2000). https://doi.org/10.1006/jcis.2000.6894 47. Norris, D.J., Sacra, A., Murray, C.B., Bawendi, M.G.: Measurement of the size dependent hole spectrum in CdSe quantum dots. Phys. Rev. Lett. 72, 2612–2615 (1994). https://doi.org/ 10.1103/PhysRevLett.72.2612 48. Woggon, U., Gaponenko, S.V.: Excitons in quantum dots. Phys. Status. Solidi B Basic Res. 189, 285–343 (1995). https://doi.org/10.1002/pssb.2221890202 49. Vainshtein, I.A., Zatsepin, A.F., Kortov, V.S.: Applicability of the empirical Varshni relation for the temperature dependence of the width of the band gap. Phys. Solid State. 41, 905–908 (1999). https://doi.org/10.1134/1.1130901 50. Varshni, Y.P.: Temperature dependence of the energy gap in semiconductors. Physica. 34, 149– 154 (1967). https://doi.org/10.1016/0031-8914(67)90062-6 51. Fan, H.Y.: Temperature dependence of the energy gap in semiconductors. Phys. Rev. 82, 900– 905 (1951). https://doi.org/10.1103/PhysRev.82.900 52. Skettrup, T.: Urbach’s rule derived from thermal fluctuations in the band-gap energy. Phys. Rev. B. 18, 2622–2631 (1978). https://doi.org/10.1103/PhysRevB.18.2622 53. Fan, H.Y.: Photon-Electron Interaction: Crystals Without Fields. Springer, Berlin (1967). https://doi.org/10.1007/978-3-642-46074-6_3 54. Manoogian, A., Wooley, J.C.: Temperature dependence of the energy gap in semiconductors. Can. J. Phys. 62, 285–287 (1984). https://doi.org/10.1139/p84-043 55. Viña, L., Logothetidis, S., Cardona, M.: Temperature dependence of the dielectric function of germanium. Phys. Rev. B. 30, 1979–1991 (1984). https://doi.org/10.1103/PhysRevB.30.1979 56. Zilli, A., De Luca, M., Tedeschi, D., Fonseka, H.A., Miriametro, A., Tan, H.H., Jagadish, C., Capizzi, M., Polimeni, A.: Temperature dependence of interband transitions in wurtzite InP nanowires. ACS Nano. 9, 4277–4287 (2015). https://doi.org/10.1021/acsnano.5b00699 57. O’Donnell, K.P., Chen, X.: Temperature dependence of semiconductor band gaps. Appl. Phys. Lett. 58, 2924–2926 (1991). https://doi.org/10.1063/1.104723 58. Chen, L., Bao, H., Tan, T., Prezhdo, O.V., Ruan, X.: Shape and temperature dependence of hot carrier relaxation dynamics in spherical and elongated CdSe quantum dots. J. Phys. Chem. C. 115, 11400–11406 (2011). https://doi.org/10.1021/jp201408m

Exciton–Phonon Interactions and Temperature Behavior of Optical Spectra. . .

195

59. Olkhovets, A., Hsu, R.-C., Lipovskii, A., Wise, F.W.: Size-dependent temperature variation of the energy gap in lead-salt quantum dots. Phys. Rev. Lett. 81, 3539–3542 (1998). https:// doi.org/10.1103/PhysRevLett.81.3539 60. Savchenko, S.S., Vokhmintsev, A.S., Weinstein, I.A.: Temperature dependence of the optical absorption spectra of InP/ZnS quantum dots. Tech. Phys. Lett. 43, 297–300 (2017). https:// doi.org/10.1134/S1063785017030221 61. Savchenko, S.S., Vokhmintsev, A.S., Weinstein, I.A.: Temperature-induced shift of the exciton absorption band in InP/ZnS quantum dots. Optic. Mater. Express. 7, 354–359 (2017). https:// doi.org/10.1364/OME.7.000354 62. Alfrey, G.F., Borcherds, P.H.: Phonon frequencies from the Raman spectrum of indium phosphide. J. Phys. C Solid State Phys. 5, L275–L278 (1972). https://doi.org/10.1088/00223719/5/20/002 63. Savchenko, S.S., Vokhmintsev, A.S., Weinstein, I.A.: Effect of temperature on the spectral properties of InP/ZnS nanocrystals. J. Phys. Conf. Ser. 961, 012003 (2018). https://doi.org/ 10.1088/1742-6596/961/1/012003 64. Turner, W.J., Reese, W.E., Pettit, G.D.: Exciton absorption and emission in InP. Phys. Rev. 136, A1467–A1470 (1964). https://doi.org/10.1103/PhysRev.136.A1467 65. Savchenko, S.S., Weinstein, I.A.: Inhomogeneous broadening of the exciton band in optical absorption spectra of InP/ZnS nanocrystals. Nano. 9, 716 (2019). https://doi.org/10.3390/ nano9050716 66. Savchenko, S.S., Vokhmintsev, A.S., Weinstein, I.A.: Spectral features and luminescence thermal quenching of InP/ZnS quantum dots within 7.5–295 K range. Optics InfoBase Conference Papers Part F107-NOMA 2018. 140118 (2018). https://doi.org/10.1364/ NOMA.2018.NoW1J.4 67. Savchenko, S.S., Vokhmintsev, A.S., Weinstein, I.A.: Photoluminescence thermal quenching of yellow-emitting InP/ZnS quantum dots. AIP Conf. Proc. 2015(020085), (2018). https://doi.org/ 10.1063/1.5055158 68. Woggon, U., Gaponenko, S.V., Uhrig, A., Langbein, W., Klingshirn, C.: Homogeneous linewidth and relaxation of excited hole states in II–VI quantum dots. Adv. Mater. Opt. Electron. 3, 141–150 (1994). https://doi.org/10.1002/amo.860030121 69. Vaganov, S.A., Seisyan, R.P.: Temperature-dependent integral exciton absorption in semiconducting InP crystals. Tech. Phys. Lett. 38, 121–124 (2012). https://doi.org/10.1134/ S1063785012020174 70. Lee, D., Johnson, A.M., Zucker, J.E., Feldman, R.D., Austin, R.F.: Room temperature excitonic absorption in CdZnTe/ZnTe quantum wells: contributions to exciton linewidth. J. Appl. Phys. 69, 6722–6724 (1991). https://doi.org/10.1063/1.348895 71. Weinstein, I.A., Zatsepin, A.F., Shchapova, Y.V.: The phonon-assisted shift of the energy levels of localized electron states in statically disordered solids. Physica B. 263–264, 167–169 (1999). https://doi.org/10.1016/S0921-4526(98)01213-7 72. Weinstein, I.A., Zatsepin, A.F.: Modified Urbach’s rule and frozen phonons in glasses. Phys. Status Solidi C. 1, 2916–2919 (2004). https://doi.org/10.1002/pssc.200405416 73. Skuja, L.: Defect studies in vitreous silica and related materials: optically active oxygendeficiency-related centers in amorphous silicon dioxide. J. Non-Cryst. Solids. 239, 16–48 (1998). https://doi.org/10.1016/0925-8388(96)02241-4 74. Salvador, M.R., Graham, M.W., Scholes, G.D.: Exciton-phonon coupling and disorder in the excited states of CdSe colloidal quantum dots. J. Chem. Phys. 125, 184709 (2006). https:// doi.org/10.1063/1.2363190 75. Rudin, S., Reinecke, T.L., Segall, B.: Temperature-dependent exciton linewidths in semiconductors. Phys. Rev. B. 42, 11218–11231 (1990). https://doi.org/10.1103/PhysRevB.42.11218 76. Weinstein, I.A., Zatsepin, A.F., Kortov, V.S.: Effects of structural disorder and Urbach’s rule in binary lead silicate glasses. J. Non-Cryst. Solids. 279, 77–87 (2001). https://doi.org/10.1016/ S0022-3093(00)00396-3 77. Cui, J., Beyler, A.P., Marshall, L.F., Chen, O., Harris, D.K., Wanger, D.D., Brokmann, X., Bawendi, M.G.: Direct probe of spectral inhomogeneity reveals synthetic tunability of

196

S. Savchenko et al.

single-nanocrystal spectral linewidths. Nat. Chem. 5, 602–606 (2013). https://doi.org/10.1038/ nchem.1654 78. Mi´ci´c, O.I., Curtis, C.J., Jones, K.M., Sprague, J.R., Nozik, A.J.: Synthesis and characterization of InP quantum dots. J. Phys. Chem. 98, 4966–4969 (1994). https://doi.org/10.1021/ j100070a004 79. Arias-Cerón, J.S., González-Araoz, M.P., Bautista-Hernández, A., Sánchez Ramírez, J.F., Herrera-Pérez, J.L., Mendoza-Álvarez, J.G.: Semiconductor nanocrystals of InP@ZnS: synthesis and characterization. Superf. y Vacío. 25, 134–138 (2012) 80. Shen, W., Tang, H., Yang, X., Cao, Z., Cheng, T., Wang, X., Tan, Z., You, J., Deng, Z.: Synthesis of highly fluorescent InP/ZnS small-core/thick-shell tetrahedral-shaped quantum dots for blue light-emitting diodes. J. Mater. Chem. C. 5, 8243–8249 (2017). https://doi.org/ 10.1039/c7tc02927f

Thick-Shell Core/Shell Quantum Dots Lei Zhang, Wenbin Xiang, and Jiayu Zhang

Abstract Due to the Auger effect, traditional core/shell quantum dots exhibit emission intermittency, which affects the application of quantum dots on lasers. A thick shell could effectively inhibit the Auger nonradiation process, which makes the quantum dots have high optical gain. In addition, the thick shell can effectively eliminate the influence of the external environment on the excitons in the nucleus, thus greatly improving the optical and chemical stability of the quantum dots. Therefore, a number of research groups, including our research group, have conducted extensive research on thick-shell core/shell quantum dots. This chapter will review the important works of this kind of quantum dots according to the following contents. Keywords Core-thick shell · Quantum dots · Auger non-radiation · Optical gain · Chemical stability

1 Thick-Shell Core/Shell Quantum Dots 1.1 History of Thick-Shell Core/Shell Quantum Dots In recent decades, quantum dots (QDs), which are a kind of semiconductor nanocrystals with particle size similar to the exciton Bohr radius, have attracted much attention due to their unique physical, chemical, and luminescent properties. However, the existence of a large number of active atoms on the QDs’ surface can easily cause surface defects, which will seriously affect the photoluminescence

L. Zhang · W. Xiang · J. Zhang () Advanced Photonics Center, Southeast University, Nanjing, China e-mail: [email protected]; [email protected]; [email protected] © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 X. Tong, Z. M. Wang (eds.), Core/Shell Quantum Dots, Lecture Notes in Nanoscale Science and Technology 28, https://doi.org/10.1007/978-3-030-46596-4_6

197

198

L. Zhang et al.

quantum yields (PL QYs) and nonlinear optical properties of the QDs. In addition to adjusting the nucleation and growth dynamics of quantum dots and controlling the size and shape of quantum dots, the surfactants introduced in the preparation process also play the role of surface ligands, which can not only provide enough space or electrostatic repulsion force to make quantum dots disperse stably in solution and prevent aggregation, but also passivates the surface trap states, so as to improve the optical properties of quantum dots. Although the existence of surface organic ligands can improve the surface properties and QYs, the optical stability of such soft passivated QDs is relatively poor, and the surface ligand may also introduce new defects on the QDs. In semiconductor QDs, the quantum confinement effect leads to the discrete energy level structure, different energy levels correspond to different angular momentums, and the exciton Auger recombination (AR) process no longer needs to meet the dynamic quantity conservation [1–3]. Therefore, the AR process of excitons in QDs is no longer constrained by the potential barrier and even the carriers in the ground state can produce nonradiative AR. The nonradiative AR is about much faster than the PL relaxation process. This ultrafast nonradiative process will consume a large number of high-energy excitons and seriously hinder the particle number inversion distribution of exciton state in QDs, thus limiting the optical gain lifetime and gain bandwidth and reducing the optical gain performance. For practical luminescent applications, inorganic shells are usually grown on the surface of QDs to form core/shell heterostructure semiconductor QDs [4]. First, this kind of inorganic shells can not only effectively passivate the surface trap states of QDs but also isolate the surrounding environment to avoid the direct interference for QDs. Therefore, the photophysical and photochemical stability of QDs are improved significantly while obtaining high PL QYs. Second, the shell thickness has a great influence on the exciton dynamics (AR and multiexciton process) of QDs, which can adjust the optical properties of core/shell QDs by changing the shell thickness according to the application requirements [5]. Therefore, the core/shell QDs have important potential applications in the fields of light-emitting diode, fluorescence labeling, solar cell, and microcavity lasers. The optical properties of QDs can be further improved with the increase of the shell thickness. For CdSe/CdS QDs, when the shell thickness is greater than 10 CdS monolayers (MLs), the quantum dots are called “giant” QDs [5, 6]. In this kind of thick-shell QDs, the nonradiative Auger process and the surface/interface states can be effectively suppressed, so as to reduce the nonradiative loss of excitons and enhance the gain performance of the QDs. At the same time, the PL blinking would be reduced or even inhibited, and longtime laser radiation would not induce a photobleaching phenomenon. Moreover, with the further increase of the shell thickness, ultrathick-shell QDs would form the dot-in-bulk QD system, which could show PL or stimulated emission from thick CdS shells due to the effect of the hole potential barrier [7, 8].

Thick-Shell Core/Shell Quantum Dots

199

Fig. 1 Schematic illustration of the different types of core/shell heterostructures (top row) and their energy band diagrams (bottom row) with possible electron (red curves) and hole (blue curves) wave function

1.2 Basic Characteristics of Thick-Shell Core/Shell Quantum Dots Core/shell heterogeneous quantum dots are usually composed of two or more semiconductor materials. Generally, core/shell heterostructures are classified according to the energy band relative position and arrangement of core/shell semiconductor materials, mainly including type-I, type-II and quasi-type-II core/shell heterostructure, as shown in Fig. 1. Type-I core/shell QDs refer to the shell material with a higher conduction band energy level than the core material and the shell material with a lower valence band energy level than the core material. That is to say, the highest valence band energy level and the lowest conduction band energy level of the nuclear material are all within the forbidden band of the shell material, such as CdSe/ZnS, CdSe/CdS, CdS/ZnS, et al. The most commonly used shell materials are CdS, ZnS, and ZnSe with a wide band gap. The emission of QDs is mainly determined by the band gap of the core QDs. The biggest advantage of this structure is that it can inorganically passivate the QD core surface. Most of the photogenerated carriers are confined in the core. The excitons are blocked by the shell, which reduces the probability of being trapped by the external surface defects. The QYs and PL stability of QDs are improved obviously. In the structural design of type-I core/shell QDs, in addition to the band gap meeting the above requirements, the lattice mismatch between the core material and

200

L. Zhang et al.

the shell material should also be considered. For example, the lattice mismatch of CdS and CdSe is about 4%. More than ten layers of CdS shell can be epitaxially grown on the surface of a CdSe core [9]. When the lattice constant difference is large, the epitaxial growth is difficult to be carried out. For example, the lattice mismatch of ZnS and CdSe is ~12%, and only a few layers of epitaxial growth can be completed. Thicker shell growth will lead to lattice dislocation and result in defects by the stress release in the interface or shell, thus inducing the decrease of the PL QYs [10, 11]. Although the lattice constants of CdS and ZnSe are smaller than ZnS, they are easier to grow as thicker shells on the surface of the CdSe core, but their band gap width is also smaller, which is not as good as the ZnS material in improving the photochemical stability of the CdSe core. Whereas ZnS has a large bandwidth, the lattice mismatch with the CdSe core is too large, which is difficult to meet the high-quality epitaxial growth of thicker shells. In order to solve these problems, D.V. Talapin et al. used CdS or ZnSe as the buffer layer of a ZnS shell epitaxy [10]. In 2005, R.G.Xie et al. further developed such design idea by introducing an alloyed buffer layer with lattice constant between CdS and ZnS [11]. This kind of alloy buffer structure with gradual change of component proportion realizes the gradual change of lattice constant from core to shell, reduces the lattice mismatch between core/shell materials, and effectively eliminates the defect states induced by lattice stress release at the core/shell interface or shells. At the same time, due to the smooth interface potential barrier, the excitons can rapidly relax from the shell to the core, thus improving the exciton recombination efficiency in the cores [12]. The structure design of the alloy buffer layer greatly improves the optical properties and promotes the extensive research of the thick-shell quantum dots. In type-II core/shell QDs, the energy levels of the QD core material and the shell material are staggered, so the electrons are confined in the material with lower conduction band, while the holes are confined in the material with higher valence band. The emission of QDs is at the core/shell material interface, which is determined by the energy levels of core and shell materials. So the exciton properties would be controlled by both the energy levels of core and shell materials. The QDs with type-II structure have photoelectric properties that are neither core nor shell materials, so the type-II core/shell structure is called the “energy band engineering” of QDs. In 2003, Bawendi group first reported CdTe/CdSe and CdSe/ZnTe type-II core/shell QDs [13]. The confinement of electron/hole wave function in different materials would reduce the radiation recombination probability between electrons and holes. So this kind of heterostructure QDs usually has lower QYs and longer lifetime. The type-II core/shell structure realizes the complete separation of carriers in quantum dots, which is of great significance for the applications of QDs in fields of solar cells, photocatalysis, and photodetectors. In addition to the core/shell heterostructure with carrier common confinement (type-I core/shell structure) and carrier separation (type-II core/shell structure), a quasi-type-II core/shell structure is that one carrier confinement in the core material, and the other carrier can be distributed in the core/shell materials. Such core/shell heterostructures have conduction band or valence band with similar energy, such as CdSe/CdS and CdSe/ZnSe. They have very high PL QYs. For example, the quantum

Thick-Shell Core/Shell Quantum Dots

201

yield of CdSe/CdS quantum dots can be up to 100% [14]. Due to the weakening of the quantum confinement effect, electrons in the core of quasi-type-II core/shell QDs can continuously diffuse to the shells, which is different from type-I core/shell QDs, so the PL and absorption spectra gradually redshift with the growth of the shells. It is worth noting that the degree of superposition of the electron hole wave function in quasi-type-II core/shell QDs can be controlled by changing the thickness of the shell. Among them, CdSe/CdS core/shell quantum dots are widely studied as a classical model. Generally, with the increase of the shell thickness of CdSe/CdS core/shell quantum dots, the quantum dots transition from a structure similar to type-I to a quasi-type-II structure and then further change to a structure similar to type II. In this process, the electron wave function in the QDs gradually diffuses to the CdS shells, while the hole is still mainly confined in the CdSe core, so the degree of overlap of the electrons and hole wave function gradually decreases. In the thickshell QDs, in addition to the reduced superposition of the exciton wave functions, the Coulomb repulsion interaction among the multiexcitons is also conducive to the suppression of the nonradiative Auger process and the improvement of the optical gain performance of the QDs. At the same time, the Coulomb interaction between the multiexcitons could be controlled by changing the size of the core and shell thickness, thus causing the relative blue-, red-, and none-shift of the amplified spontaneous emission (ASE) relative to the peak of spontaneous emission [15]. For the past 10 years, the optical properties of thick-shell QDs have attracted more attention from researchers, whether it is the design concept of alloyed buffer derived from type-I core/shell QDs or the adjustment of the distribution of exciton wave function through the regulation of core size and shell thickness in quasi-type-II core/shell QDs. In addition to the influence of the band gap between different core/shell materials on the band structure, the thick shells can also greatly increase the absorption cross section of the QDs. For example, the single-photon (∼1.6 × 10−13 cm2 ) and twophoton (~1.1 × 105 GM) absorption cross section of the thick-shell CdSe/CdS QDs is at least one order of magnitude higher than that of the CdSe core QDs [16, 17]. High absorption cross section contributes to the generation of more photogenerated carriers in the shells, which could relax to the CdSe cores rapidly and construct the population inversion, thus reducing the optical gain thresholds. Besides, the thick shells could effectively avoid the influence of the external environment on the luminescent center of the QD core, thus enhancing the photochemistry and photophysical stability of the QDs [6]. However, in the ultrathick-shell QDs, the stress-release-induced defect states would lead to the degradation of the optical properties, such as the decrease of QYs and more serious PL blinking with the increase of the shell [18]. Therefore, the most excellent optical properties of thickshell QDs can be obtained with appropriate shell thickness.

202

L. Zhang et al.

2 Synthesis of Thick-Shell Core/Shell Quantum Dots 2.1 Successive Ionic Layer Adsorption and Reaction Method With the development of high-temperature organometallic synthesis, the size and shape of core/shell heterostructures can be controlled, and the types of material combinations are far more than that synthesized by the low-temperature approach. In 2003, X. G. Peng group applied the “successive ionic layer adsorption and reaction” (SILAR) method for the epitaxial growth of shell materials on the surface of QDs and obtained high-quality CdSe/CdS core/shell QDs with different shell thicknesses [19]. Soon, Peng group measured the extinction coefficients of some binary quantum dots in different sizes, which paves a foundation for the accurate calculation of the shell thickness [20]. The thickness of the shell can be controlled more accurately by calculating the amount of precursor needed to grow each shell monolayer on the surface of the QD core and then injecting the anion and cation precursor alternately. The injection amount of the shell precursor is just completely adsorbed on the surface of the QDs, which successfully avoids the problem of selfnucleation of the shell precursor. In the synthesis process, the growth defects can be reduced by the annealing process for each single molecular shell layer, so as to improve the optical properties of quantum dots. The SILAR method is simple and easy to operate, which has become the most commonly used method to synthesize thick-shell quantum dots. In 2008, Hollingsworth et al. synthesized ultrathick-shell CdSe/CdS QDs with 19 CdS MLs by the SILAR method [6]. Compared with the traditional quantum dots, the thick-shell quantum dots exhibit the high optical and chemical stability. The QYs of the QDs after purification for several times are nearly unchanged (±5%). At the same time, there is no photobleaching under laser irradiation for several hours. However, the volume of the shell precursor increases with the increase of the shell thickness, indicating that the chemical potential of the monomer in the solution is too high, which makes the quantum dots tend to anisotropically grow, and the shape of the QDs is not easy to control. Then, in 2007, the researchers report the improved SILAR and proposed the thermal cycling alternate ionic layer adsorption and reaction (TC-SILAR) method [21]. This method uses the principle of crystal growth kinetics. For the same precursor concentration, low precursor activity in a low-temperature environment helps in the uniform adsorption of monomers on the surface of quantum dots; a high-temperature environment helps to improve the crystallinity of quantum dots and reduce lattice defects. The TC-SILAR method is not only used for the preparation of various complex core/shell QDs, but also for the synthesis of metal nanocrystals and rare-earth-doped core/shell crystals [22]. With the development of synthesis methods, the growth of the core/shell heterostructure with controllable size and shape has been realized, and the types of material combinations have become diverse, such as CdSe/CdS/ZnS core/shell/shell structure, CdSe/CdZnS core/alloy-shell, and CdSe/Cdx Zn1-x S core/shell/gradient shell structure, while the optical properties have also been greatly improved.

Thick-Shell Core/Shell Quantum Dots

203

2.2 Syringe Pump Injection The synthesis method of thick-shell QDs is chosen according to the precursor activity and crystal structure growth requirements. In the preparation of phasepure CdSe/CdS core/shell QDs, the higher growth quality and excellent optical properties of QDs are related to the slower precursor injection and higher reaction temperature. Moreover, the simultaneous and continuous injection of anionic and cationic precursors by syringe can further save manpower and time cost. In 2013, Bawendi group reported the synthesis of such high-quality CdSe/CdS core/shell QDs in an optimized process that maintains a slow growth rate of the shell through the use of low-activity octanethiol and cadmium oleate as precursors. The cation and anion precursor were injected into the core/shell QDs of phasepure wurtzite CdSe/CdS by syringe at 310 ◦ C high temperature [23]. The alloyed interface layer was gradually formed during the growth of CdS shells at high temperature, which helped minimize defects within the shell and on the surface/interface, thus obtaining the high optical quality QDs. In 2014, Christodoulou and Moreels et al. synthesized phase-pure wurtzite CdSe/CdS giant-shell nanocrystals with a CdS shell thickness of up to 7–8 nm (equivalent to about 20 MLs of CdS) [24]. They take sulfur powder and cadmium oleate as precursors and injected the precursors continuously through a syringe at a high temperature of 300 ◦ C. The average diameter of QDs could be up to 18.1 nm over the growth course of 4 h. Both the core and shell have a wurtzite crystal structure, yielding epitaxial growth of the shell and nearly defect-free crystals. As a result, the PL QYs is as high as 90%. The results show that the slow injection of precursor is beneficial to obtain high optical performance ultrathick-shell QDs.

2.3 “Flash” Hot Injection With the trend of industrial production and application of QDs, thick-shell core/shell QDs have become a main choice because of its excellent photostability. In order to, as far as possible, save time on the preparation of thick-shell QDs, the researchers put forward some experimental scheme for the rapid synthesis of thick-shell QDs. In 2014, Cirillo group developed a “flash” method to rapidly synthesize thick-shell CdSe/CdS core/shell QDs [25]. It only takes 3 min and the shell thickness can reach ~20 MLs. However, the rapid shell growth results in the occurrence of excessive defects, with the PL QYs at only 15%. A thick-shell heterostructure also plays an important role in Mn-doped core/shell QDs. In order to avoid lattice exclusion for Mn2+ dopants, the reaction temperature should not be too high, and the growth time of shells should not be too long. In 2015, Xu et al. studied the flash preparation of Mn-doped CdS/ZnS QDs with thick shells [26]. When the temperature is over 300 ◦ C, the crystallization of the shell is improved, while the defect states are decreased. In addition, the high precursor

204

L. Zhang et al.

concentration and few QD core seeds could promote the growth of thick shells. In this synthesis system, high concentration of oleic acid can be used to avoid the nucleation of ZnS shell materials. The thick ZnS shell was overcoated at the injection and growth temperature of 340 and 315 ◦ C, respectively, and ZnS shell with thickness up to 18 MLs could be grown within only 9 min. The PL QYs of CdS/ZnS QDs with a thick shell of a 14 ZnS molecular layer doped with manganese can reach 36%. The experimental results of optical stability show that the thick-shell QDs have high optical stability and narrow size distribution. The narrower size distribution and higher crystallinity of Mn-doped QDs can be prepared by nucleation doping at high reaction temperature [27]. Especially, for the Mn2+ -doped QD crystal nucleus prepared by the high-temperature hot injection method, the distribution of the Mn2+ ion is close to the inner layer of the crystal nucleus. The nucleus diffusion is relatively matched with the high temperature of shell growth; it can promote the diffusion of the Mn2+ ion and ensure a higher rate of exciton energy transfer to the Mn2+ ion, which is conducive to obtaining a higher PL QY [28]. The perfect combination strategy of hot-injection nucleation doping and optimized “flash” synthesis goes beyond the combination strategy of one-pot growth doping and typical “flash” synthesis, which led to an increase in PL QYs of giant Mn-doped CdS/ZnS QDs (~18 MLs) from ~20% to 40%. The PLQY was enhanced to 45% by light annealing. Using traditional LED as the reference, these simply-encapsulated QDs exhibited the higher photostability. Therefore, in order to find an efficient way to prepare QD materials that can meet the practical application requirements, the synthesis approaches for thick-shell core/shell QDs are also always in development.

3 Optical Properties of Thick-Shell Core/Shell Quantum Dots 3.1 Fundamental Optical Properties In the thick-shell core/shell QDs, the inorganic wide band gap shells can passivate the surface defects of QDs and reduce the probability of exciton trapping on the surface/interface, thus improving the optical stability of the QDs. First of all, the thick shells can passivate the surface defect states and reduce the nonradiative loss from the surface trapping of exciton. As shown in Fig. 2, with the increase of the shell thickness, the single exciton relaxation process in the thickshell CdSe/CdS QDs gradually tends to the single exponential dynamic behavior, which can be well fitted by the single exponential function. χ2 is smaller than 1.2, suggesting the acceptability of the PL decay fitting, thus showing that the thick CdS shells effectively reduce the surface states related to the exciton nonradiative recombination [16]. Therefore, the QYs of single exciton in thick-shell CdSe/CdS

Thick-Shell Core/Shell Quantum Dots

205

Fig. 2 PL decay trace of the CdSe cores and their corresponding core/shell QDs with different shell thicknesses. The solid line stands for the single-exponential fittings using the function I(t) = Aexp(−t/τ) + B [16]

Fig. 3 Integrated PL intensities of CdZnSeS cores and CdZnSeS/ZnS (3, 11, and 17 ML) core/shell QDs as a function of exposure time [30]

QDs could be as high as 100% due to the effective suppression of the exciton nonradiative process [14]. Secondly, the ultraviolet (UV) light stability of CdSe/CdS core/shell QDs is greatly enhanced with increasing shell thickness [29]. It is considered that the thicker shell can effectively prevent the surface-related recombination and improve the photostability of the QDs. Similarly, the shell thickness is also important for the optical stability of CdZnSeS/ZnS alloy core/shell QDs [30]. As shown in Fig. 3, the integrated PL intensities were normalized to the initial values of each sample (CdZnSeS cores and with 3, 11, and 17 MLs). The core emission experienced a large drop (by ∼70%) after 3 h of UV illumination and decreased to 17% of the initial intensity after illumination for 136 h. In contrast, the photostability of

206

L. Zhang et al.

CdZnSeS/ZnS QDs with 3 MLs of ZnS was significantly improved, and only a small drop (by ∼20%) could be found after 136 h of illumination. However, as the illumination time was continuously increased, the PL intensity constantly decreased and retained only 47% of its initial intensity after 660 h of UV exposure. Contrary to the QDs with 3 MLs, the PL intensity of the sample with 11 MLs of ZnS slightly increased during light irradiation, and the enhanced PL was maintained after 660 h of illumination, which was about 1.3 times stronger than the initial intensity. The enhanced emission may be due to photo-oxidation-induced passivation of defect sites on QDs. The sample with 17 MLs of ZnS underwent less pronounced decrease of the emission intensity (25% decrease). These data demonstrated that a thick ZnS shell could effectively improve the photostability of QDs. This is mainly because the coating of the shell makes the exciton wave function of the QDs far away from the surface defects. The smaller overlap between the exciton and the surface trap wave function in thicker shells leads to lesser sensitivity of QDs to the external environment. The thick-shell core/shell green QDs with 11 ML ZnS exhibit the best optical stability. However, in ultrathick-shell CdZnSeS/ZnS QDs coated with 17 ML ZnS, with the further increase of the ZnS shell, the lattice dislocations are generated at the core/shell interface and release part of the stress, which leads to the generation of interface defects and the reduction of QYs.

3.2 Exciton Dynamics Process In 2009, Klimov group reported a kind of “giant” QDs to suppress AR. The “giant” QD consists of a small-size CdSe core (3–4 nm) and a thick CdS shell (more than 10 ML CdS) with a wide band gap growing on the surface of the core, as shown in Fig. 4a [31]. Thick-shell CdSe/CdS QDs exhibit a significant

Fig. 4 (a) A schematic of “giant” CdSe/CdS quantum dots; (b) spatial probability distribution of the hole and electron. The inset shows a contour plot of the electron-hole overlap integral [31]

Thick-Shell Core/Shell Quantum Dots

207

(orders of magnitude) suppression of Auger decay rates, thus greatly improving the radiative recombination efficiency of multiexcitons in QDs. The biexciton QYs of the ultrathick-shell CdSe/CdS QDs with 19 ML CdS and the thick-shell CdSe/CdS QDs with gradient shell can be as high as 100% [32, 33]. Klimov et al. analyzed the physical mechanism behind this huge inhibition and believed that it was the result of at least three factors: 1. Giant volume In general, the Auger lifetime is directly proportional to the quantum dot volume [34]. In giant QDs, the electron wave function expands in a huge space, which will lead to the increase of the lifetime of the biexciton. 2. Partial separation of electron and hole wave function In giant QDs, the electron delocalization to the shell region and the hole is mainly confined in the core (Fig. 4b) with the increase of shell thickness. Therefore, the spatial separation of electron and hole wave function can reduce the Coulomb interaction between electrons and holes and thus help to reduce the Auger rates [35]. 3. Alloy interface The alloy interface properties of the giant QDs are very similar to those of the epitaxial grown quantum dots. Although the synthesis temperature is not high, the long reaction time (about 40 h for the synthesis of QDs with 10 ML CdS in shell thickness) makes the anions diffuse significantly at the core/shell interface and form a transition alloy layer with a smoothed interface potential, which would contribute to the diffusion of electron wave function, thus inhibiting the Auger process of excitons [36]. Generally, the biexciton dynamics in the thick-shell CdSe/CdS QDs could be extracted by subtracting the low-fluence (average QD occupancy N   1) PL traces from the “tail-normalized” traces measured for N  slightly above unity [34]. The biexciton lifetimes (τ2 ) were derived from the obtained decay traces by fitting function. The biexciton lifetime can reach 29 ns with increasing shell thickness from 0 to 19 MLs [37]. The biexciton Auger lifetime (τ2A ) can be further estimated using these results. τ2A can be obtained from τ2A −1 = τ2 −1 −τ2r −1 . The measured biexciton lifetime, τ2 , is limited by both biexciton radiative lifetime (τ2r ) and biexciton Auger lifetime (τ2A ) (here nonradiative decay channels other than AR are neglected). The τ2r in QDs is shorter than τ1r by a factor β (range from 3 to 4), which relates to the relative number of recombination routes [36]. The obtained values of τ2A indicate more than a thousandfold increase in the Auger lifetime with increasing shell thickness. It changes from 67 ps for the core-only QDs to 102– 593 ns (depending on β) for the QDs with 19 ML CdS shell, which indicates that the nonradiative AR of excitons is effectively suppressed. In addition, the PL intermittence of quantum dots usually comes from the photoionization of excitons and the trapping of excitons by defect states. Therefore,

208

L. Zhang et al.

Fig. 5 Average number of excitons per particle (N ) as a function of pump intensity measured for CdSe/CdS QDs (5, 11, and 15 MLs). The solid lines are on behalf of their theoretical relationships. The dashed lines denote the variation trend of the number of higher-order excitons at higher pump intensities [17]

the PL blinking can be obviously suppressed in the thick-shell QDs, because the thick-shell structure can effectively reduce the surface defect states and AR. Particularly, in wurtzite CdSe/CdS QDs, there are almost no defects in the core/shell interface due to the alloy interface layer formed at high temperature for a long reaction time. Thereby, the phase-pure CdSe/CdS QDs have a very low non-blinking volume threshold (~390 nm3 ), which is significantly lower than the traditional thickshell QDs (~750 nm3 ) [5, 23]. Furthermore, the complicated fast multiexciton dynamics in phase-pure wurtzite CdSe/CdS QDs can be analyzed from the view of the superradiation mechanism [17]. The relationships between the average number of excitons per QD (N ) and pump intensity can be obtained by fitting PL decay traces using a superradiance d decay function: I(t) = Im csch2 ( t+τ τr ) + Bexp(−αt) + C, as shown in Fig. 5. For all of the QD samples (with 5, 11, and 15 ML CdS), there was a common characteristic that the N value was proportional to pump intensity within a certain range of pump intensity. At the same time, the data points were almost in perfect conformity with their own theoretical linear relationships (solid lines). However, when the pump intensity continued to increase, the average number of excitons (>6) in QDs deviated from the theoretical values and started to exhibit different saturation trends (dashed lines), because the nonradiation losses of excitons increase with the pump intensity. At the same time, the ASE measurements of CdSe/CdS QDs (11 MLs) prove that the fitting results of N  (2, 7.6, and 15.8) at the three ASE thresholds are basically conformed to the theory (2, 6, and 14), [38] further indicating the correctness of the fitting scheme. These results give us more insights into the multiexciton states in the fast PL decay process and provide a new way from the view of superradiance mechanism to study the multiexciton dynamics of CdSe/CdS QDs.

Thick-Shell Core/Shell Quantum Dots

209

3.3 Quantum Confined Stark Effect In 1984, the quantum confinement stark effect was first observed in quantum well structures [39]. In 1997, Bawendi group first reported the quantum confinement stark effect of a single CdSe QD [40]. The variation of PL spectra of CdSe QD with the applied electric field was studied at a temperature of 10 K. Under the applied electric field, the excited electron-hole pairs were polarized. In the experiment, a spectral shift of two orders of magnitude wider than the line could be observed. At the same time, it was also observed that the local field around the quantum dot changes with the time, resulting in the spectra shift with time. The random local electric field mainly originates from the charged state generated by the surface trapping states of excitons, AR process, and the ionization of exciton under a strong electric field. In 2012, Park et al. studied the quantum confinement stark effect of a single quantum dot with different heterostructures at room temperature [41]. The research results show that the initial separation distance between electrons and holes in a quantum dot determines the strength of the quantum confinement stark effect under an external electric field. Semiconductor QDs are always suggested as promising candidates for electric field (voltage) sensing via the quantum-confined Stark effect (QCSE) [40–43]. However, the charged carriers on or near the QD-surface-induced randomly oriented local electric field would lead to spectral broadening in the ensemble, which is difficult for QDs to exhibit an obvious ensemble Stark effect for potential applications. If the local field is eliminated, QCSE in QD ensembles can be readily observable at room temperature, and the response of the absorbing state can be possibly characterized in the ensemble measurement. The phase-pure wurtzite CdSe/CdS core/shell QDs have been proven to efficiently inhibit AR and the consequent PL blinking [23, 37]. Hence, the thick-shell wurtzite CdSe/CdS QDs can inhibit the random local field induced by nonradiative AR or defect states in favor of the applications based on QCSE, such as optical switch and optoelectronic sensors. Figure 6a shows the PL spectra of thick-shell CdSe/CdS QDs (11 MLs) with increasing field strength [44]. The PL spectra showed an obvious Stark shift in the peak position with a redshift of ∼2.3 nm. The spectral shifts could be well fitted by a purely quadratic function of the electric field (Fig. 6b). At the same time, the PL intensity showed remarkable quenching of ∼17% and the PL spectra slightly broadened by ∼6%, as expected features of QCSE in semiconductor nanoparticles. Figure 6c shows the absorption spectra of thick-shell CdSe/CdS QDs under different electric fields. On increasing the electric field strength, the redshift (∼2.1 nm) in the first exciton absorption peak also exhibited quadratic dependence on the external electric field (Fig. 6d), concomitant with ∼9% decrease in the absorption intensity and ∼10% increase in the full width at half maximum. The results provide a possibility to efficiently tune the absorption properties by an external field, which is meaningful for the applications of QD-based QCSE in electroabsorption modulators. However, the field-dependent optical properties in the

210

L. Zhang et al.

Fig. 6 (a) PL spectra of thick-shell CdSe/CdS QDs (11 MLs) in the QCSE device. (b) The Stark shift versus electric field for the PL spectra in (a). (c) Absorption spectra of thick-shell CdSe/CdS QDs under the electric field. The spectra are vertically shifted for clarity. (d) The Stark shift in the first excitonic absorption peak for the spectra in (c) [44]

moderate-shell (5 ML) CdSe/CdS QDs showed nearly no spectral shifts, while the shifts for ultrathick-shell (15 ML) CdSe/CdS QDs were not as obvious as that for the thick-shell QDs. The time-dependent PL measurements at the single-particle level revealed that moderate-shell and ultrathick-shell QDs were likely to suffer from the randomly local electric field, stemmed from a large number of deep surface traps and strain-induced defect states, respectively. Therefore, the study suggested that thickshell CdSe/CdS QDs could minimize the adverse impact of the random local electric field induced by the charged exciton states, which is crucial for the applications of QCSE in QD ensembles.

3.4 Optical Gain Performance The suppression of AR can greatly improve the QDs’ optical gain performance. At the same time, the large absorption cross section of the thick-shell QDs is also

Thick-Shell Core/Shell Quantum Dots

211

Fig. 7 Band structure of a CdSe/CdS core/shell g-QD features an energetic barrier in the valence band associated with an interfacial ZB CdS layer, which separates a thick WZ CdS outer layer from the ZB CdSe core [8]

helpful to reduce the threshold of ASE. Therefore, the ultralow gain threshold (~26 μJ cm−2 ) and larger optical gain bandwidth (>500 meV) is realized in the “giant” CdSe/CdS QDs with 11 ML CdS [31]. In order to suppress the Auger process and improve the photostability of QDs, further studies show that the quantum dots need to be coated with thicker inorganic shells [6, 33]. However, there is a conventional problem in the synthesis of ultrathick CdSe/CdS core/shell QDs. In the schematic diagram shown in Fig. 7, the CdSe core has a zinc-blende phase crystal structure at the usual synthesis method, the first layers of CdS shells will grow into zinc-blende phase due to their epitaxial growth on the surface of CdSe core, and then the outer CdS shells will change into the wurtzite phase due to the higher growth temperature and long reaction time. The CdS shell with zinc-blende phase plays a role of hole potential barrier in the valence band, which will block the relaxation rate of the hole from the shells to the core. Therefore, the radiative recombination of excitons among the high energy levels of shells becomes competitive. Under the excitation of strong subpicosecond pulses, these ultrathick-shell QDs exhibit an ASE from CdS shells similar to the bulk CdS material, but ASE cannot be realized in the quantum-confined CdSe cores [7, 8]. For quantum dot laser applications, the ASE from the quantum confinement core is better than that from the shells. In 2005, Liao et al. reported that the interfacial potential barrier in a thick-shell CdSe/CdS QDs could be removed by fs laser annealing and consequently transferred ASE from a bulk-like CdS shell to a quantum-confined CdSe core [8]. To obtain the high-performance ASE from the quantum-confined CdSe core, the interface hole barrier can be completely eliminated in phase-pure wurtzite CdSe/CdS core/shell QDs [37]. Emission from the phase-pure wurtzite CdSe/CdS (4 and 11 ML) QD film edge with increasing pump intensity exhibits a clear transition from spontaneous emission to ASE through an abrupt increase in output intensity and spectral narrowing (Fig. 8a–e). In the QDs with a 4 ML CdS shell, the ASE peak (654 nm) is redshifted (∼11 nm) with respect to the spontaneous emission peak, because of attractive exciton-exciton interaction. In the QDs with an 11 ML CdS shell, the longer-wavelength ASE peak (635 nm) corresponding to the first (1S) electron quantization shell of the CdSe core is blueshifted (∼14 nm), because of the

212

L. Zhang et al.

Fig. 8 (a–c) Per-pulse pump intensity-dependent emission spectra of a close-packed film of CdSe/CdS core/shell QDs with 4, 11, and 19 ML CdS shell, respectively. Emission intensity versus pump intensity at the positions of ASE and spontaneous peaks observed for CdSe/CdS QDs with (d) 4 and 11 ML and (e) 19 ML CdS shell. (f) ASE thresholds for biexcitonic gain as a function of CdS shell thickness [37]

formation of quasi-type-II band alignment and subsequent repulsive exciton-exciton interaction. As the pump intensity further increases, two shorter-wavelength ASE peaks (at 580 and 535 nm) corresponding to optical transitions involving the second (1P) and the third (1D) quantization shells also appear. Within a simple particle-ina-box model [38], the third electron-quantized level becomes a population inversion only if the QD contains at least 14 excitons. High-order multiexcitons exhibit high emission efficiencies and contribute to optical gain, indicating that AR is effectively suppressed. The ASE threshold for biexcitonic gain (Fig. 8f) decreased dramatically with the increase of the CdS shell toward 11 MLs, which results from increased absorption cross section and passivated surface, in addition to suppressed Auger process. The ASE thresholds are increased with further shell growth because strain-induced defects will weaken the suppression of AR. In addition, the ultrathick shells would more reduce wave function overlap between shell-based electron states and corelocalized hole states, therefore slowing the relaxation of holes into the core. At the same time, both nonradiative recombination involving surface and interface trap states and radiative recombination involving shell-based states become competitive. In the QDs with a 19 ML CdS shell, the ultrathick shell slows the relaxation to a degree which is sufficient for obtaining population inversion of shell-based states and then observed an ASE from the CdS shell instead of the CdSe core.

Thick-Shell Core/Shell Quantum Dots

213

The transient spectra obtained with a streak camera exhibit clearly the evolution of ASE emission of QDs with the pump intensity [37]. At low pump intensity, only a broad long-decayed spontaneous peak was observed. As the intensity increases, a narrower ASE peak appears. As expected, the emission lifetime is shortened to near the camera time resolution of ~10 ps, which is more than 3 orders of magnitude faster than the Auger lifetime and implies highly efficient ASE. Noticeably, at the position of the gain emission band, the optical gain lifetime exceeds 1000 ps, which is primarily limited by the multiexciton AR [45]. The extremely long gain lifetime (>1000 ps) of the phase-pure QDs is about an order of magnitude longer than that of thin-shell QDs (>100 ps) [46] and more than 2 times greater than that of conventional giant QDs [47] and similar dot/rods (>400 ps) [48]. As the pump intensity further increases, the bandwidth of optical gain of the QDs exceeds 170 nm because of the participation of higher-order multiexcitons. These results imply that using single-size QDs one can tune the lasing color almost over the entire range of visible wavelengths by simply adjusting the laser cavity. In 2018, Klimov group reported population inversion and optical gain in continuously graded colloidal CdSe/Cdx Zn1 − x Se/ZnSe0.5 S0.5 core/shell QDs under the direct-current electrical pumping [49]. The considerable suppression of Auger decay can be outpaced by electrical injection. The special device architecture allows them to produce high current densities up to ~18 A cm−2 without damaging either the QDs or the injection layers. The quantitative analysis of electroluminescence and current-modulated transmission spectra indicates that the population inversion of the band-edge states was achieved with current densities of 3–4 A cm−2 .

3.5 Stimulated Radiation from Thick-Shell Core/Shell Quantum Dots Particularly, for the CdSe/CdS core/shell structure, both thick CdS shells and alloyed interfacial layers with a smooth confinement potential have been proven to be efficient ways to suppress AR. Thick-shell CdSe/CdS QDs with quasi-typeII band alignment, concurring strong electron delocalization, efficiently decrease the rate of AR and simultaneously reduce or even suppress blinking [5, 6, 31]. Therefore, the ASE regime will be realized in thick-shell QDs with a lowered gain threshold. Besides, CdSe-based QDs show almost temperature-independent optical gain performance because of the well-separated electronic states [50]. All of these properties clearly state that thick-shell CdSe/CdS QDs have many incomparable merits that make them a superior laser gain medium. Several optical microstructures have been applied as resonant cavities in laser devices, such as microring [37, 51], microsphere resonators [52, 53], and distributed feedback (DFB) gratings [46, 54], which have been used as optical feedback structures in QD lasers. With the assistance of these resonant cavities, the laser performance can be improved markedly. Among the available options, DFB structures

214

L. Zhang et al.

Fig. 9 The dashed frame shows the fabrication process of DFB structures by LIA. Atomic force microscopy (AFM) images of the DFB structures. Lasing emission spectra of the DFB devices [16]

have the advantages of single-mode emission, low threshold, high quality factor, and tunable lasing wavelength by changing the grating period and thickness of the gain layer. Nanoimprint lithography and soft lithography are common approaches to construct DFB gratings. Furthermore, DFB lasers can be fabricated by laser interference ablation (LIA) based on thick-shell CdSe/CdS core/shell QDs; the schematic of the experimental setup for fabricating DFB QDs gratings is shown in Fig. 9 [16]. The prepared films were exposed to a laser interference pattern for about 15 min, and their surfaces were structured to acquire periodic relief patterns with uniform structure distribution and good repeatability due to the laserinduced ablation effect. The single-mode laser emission with high performance of the QDs’ distributed feedback structure is realized under single-photon and twophoton excitation. The laser emission peaks are all at 645 nm, the linewidth is about ~1 nm, and the laser threshold is 0.028 and 1.03 mJ cm−2 , respectively. In 2015, Park et al. reported random lasers of thick-shell CdSe/CdS core/shell QDs with alloyed interface layer that realized single-mode lasing emission with low threshold of ~18 μJ cm−2 [55]. In 2015, Moreels et al. fabricated ultralow threshold (~10 μJ cm−2 ) nanorod microcavity lasers based on a coffee-ring microstructure by using CdSe/CdS quantum dot-in-rods as gain medium. The laser linewidth was as narrow as ~0.79 nm, and the quality factor was estimated to be as high as ~1000 according to Q = λ/Δλ, where λ and Δλ are the wavelength and line width of the lasing peak, respectively [56]. The fabrication process of the self-assembled microstructure is very simple, which can achieve high-performance laser emission. Liao et al. also reported self-assembled phase-pure thick-shell CdSe/CdS QDs with low-threshold gain performance into a coffee-ring microcavity (Fig. 10a) and studied its laser emission properties [37]. The evaporation dynamics of the droplets

Thick-Shell Core/Shell Quantum Dots

215

Fig. 10 (a) Optical microscopy image of a CdSe/CdS QD (11 ML CdS shell) coffee ring. The scale bar represents 200 μm. (b) AFM image of the bottom left part of the coffee ring shown in panel a. (c) Per-pulse pump intensity-dependent emission spectra of the QD coffee ring shown in panel a. The inset shows the emission intensity versus pump intensity at the position of the lasing peak [37]

are governed by the “coffee-ring effect,” which leads to the formation of welldefined micrometer-size rings. The AFM image of the microlaser (Fig. 10b) shows that the ring has a full width at half maximum of 7.7 μm and a height of 350 nm. As a result, the microlaser displays single-mode operation (at 636 nm, with line width of ∼0.3 nm) and an ultralow threshold of ∼2 μJ cm−2 (Fig. 10c). The quality factor of the microlaser is as high as ∼2000, indicating the high quality of the coffee-ring microlaser. With the development of synthesis technology and exciton dynamics theory of QDs, the relevant research tends to the single exciton low-threshold lasing. In 2019, Klimov group reported single-mode lasing with low thresholds using thick-shell, multilayered CdSe/Cdx Zn1-x Se QDs combined with second-order DFB resonators [57]. An extremely low and sub-single-exciton lasing threshold ≈0.3 was achieved by charging the sample with about three extra electrons per dot on average.

216

L. Zhang et al.

The result is more than fourfold reduction compared with the neutral QDs. Furthermore, they also show a reversible tuning of the lasing threshold by controlling the degree of QD charging. Their approaches facilitate the development of solutionprocessable lasing devices, thereby extending the reach of lasing technologies into areas not accessible with traditional or epitaxially grown semiconductor materials.

4 Conclusion In summary, the optical properties of quantum dots are improved greatly in thickshell heterostructures compared to traditional core/shell quantum dots. Thick shells not only efficiently decrease the rate of AR and simultaneously reduce or even suppress blinking, but also decrease the surface nonradiative channel and increase PL QYs and absorption cross sections. Therefore, thick-shell QDs exhibit superior optical gain performance and photostability, thus contributing to the application of quantum dots in the fields of microlaser, LED, optoelectronics sensors, and fluorescence labeling.

References 1. Norris, D.J., Sacra, A., Murray, C.B., et al.: Measurement of the size dependent hole spectrum in CdSe quantum dots. Phys. Rev. Lett. 72(16), 2612–2615 (1994) 2. Wang, L.W., Califano, M., Zunger, A., et al.: Pseudopotential theory of Auger processes in CdSe quantum dots. Phys. Rev. Lett. 91(5), 056404 (2003) 3. Kharchenko, V.A., Rosen, M.: Auger relaxation processes in semiconductor nanocrystals and quantum wells. J. Fluoresc. 70, 158–169 (1996) 4. Reiss, P., Protière, M., Li, L.: Core/Shell semiconductor Nanocrystals. Small. 5(2), 154–168 (2009) 5. Ghosh, Y., Mangum, B.D., Casson, J.L., et al.: New insights into the complexities of shell growth and the strong influence of particle volume in nonblinking “giant” core/shell nanocrystal quantum dots. J. Am. Chem. Soc. 134(23), 9634–9643 (2012) 6. Chen, Y., Vela, J., Htoon, H., et al.: “Giant” multishell CdSe nanocrystal quantum dots with suppressed blinking. J. Am. Chem. Soc. 130(15), 5026–5027 (2008) 7. Galland, C., Brovelli, S., Bae, W.K., et al.: Dynamic hole blockade yields two-color quantum and classical light from dot-in-bulk nanocrystals. Nano Lett. 13(1), 321–328 (2013) 8. Liao, C., Fan, K., Xu, R., et al.: Laser-annealing-made amplified spontaneous emission of “giant” CdSe/CdS core/shell nanocrystals transferred from bulk-like shell to quantum-confined core. Photonics Res. 3(5), 200–205 (2015) 9. Mahler, B., Spinicelli, P., Buil, S., et al.: Towards non-blinking colloidal quantum dots. Nat. Mater. 7(8), 659–664 (2008) 10. Talapin, D.V., Mekis, I., Götzinger, S., et al.: CdSe/CdS/ZnS and CdSe/ZnSe/ZnS core-shellshell nanocrystals. J. Phys. Chem. B. 108(49), 18826–18831 (2004) 11. Xie, R., Kolb, U., Li, J., et al.: Synthesis and characterization of highly luminescent CdSe-Core CdS/Zn0.5 Cd0.5 S/ZnS multishell nanocrystals. J. Am. Chem. Soc. 127(20), 7480–7488 (2005) 12. Pinchetti, V., Meinardi, F., Camellini, A., et al.: Effect of core/shell interface on carrier dynamics and optical gain properties of dual-color emitting CdSe/CdS nanocrystals. ACS Nano. 10(7), 6877–6887 (2016)

Thick-Shell Core/Shell Quantum Dots

217

13. Kim, S., Fisher, B., Eisler, H., et al.: Type-II quantum dots: CdTe/CdSe (core/shell) and CdSe/ZnTe (core/shell) heterostructures. J. Am. Chem. Soc. 125(38), 11466–11467 (2003) 14. Javaux, C., Mahler, B., Dubertret, B., et al.: Thermal activation of non-radiative Auger recombination in charged colloidal nanocrystals. Nat. Nanotechnol. 8(3), 206–212 (2013) 15. Cihan, A.F., Kelestemur, Y., Guzelturk, B., et al.: Attractive versus repulsive excitonic interactions of colloidal quantum dots control blue- to red-shifting (and non-shifting) amplified spontaneous emission. J. Phys. Chem. Lett. 4(23), 4146–4152 (2013) 16. Zhang, L., Liao, C., Lv, B., et al.: Single-mode lasing from “Giant” CdSe/CdS core-shell quantum dots in distributed feedback structures. ACS Appl. Mater. Interfaces. 9(15), 13293– 13303 (2017) 17. Zhang, L., Li, H., Liao, C., et al.: New insights into the multiexciton dynamics in phase-pure thick-Shell CdSe/CdS quantum dots. J. Phys. Chem. C. 122(43), 25059–25066 (2018) 18. Omogo, B., Gao, F., Bajwa, P., et al.: Reducing blinking in small core−multishell quantum dots by carefully balancing confinement potential and induced lattice strain: the “goldilocks” effect. ACS Nano. 10(4), 4072–4082 (2016) 19. Li, J.J., Wang, Y.A., Guo, W., et al.: Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction. J. Am. Chem. Soc. 125(41), 12567–12575 (2003) 20. Yu, W.W., Qu, L., Guo, W., et al.: Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals. Chem. Mater. 15(14), 2854–2860 (2003) 21. Blackman, B., Battaglia, D.M., Mishima, T.D., et al.: Control of the morphology of complex semiconductor nanocrystals with a type II heterojunction, dots vs peanuts, by thermal cycling. Chem. Mater. 19(15), 3815–3821 (2007) 22. Li, X., Shen, D., Yang, J., et al.: Successive layer-by-layer strategy for multi-Shell epitaxial growth: Shell thickness and doping position dependence in upconverting optical properties. Chem. Mater. 25(1), 106–112 (2013) 23. Chen, O., Zhao, J., Chauhan, V.P., et al.: Compact high-quality CdSe-CdS core-shell nanocrystals with narrow emission linewidths and suppressed blinking. Nat. Mater. 12(5), 445–451 (2013) 24. Christodoulou, S., Vaccaro, G., Pinchetti, V., et al.: Synthesis of highly luminescent wurtzite CdSe/CdS giant-shell nanocrystals using a fast continuous injection route. J. Mater. Chem. C. 2(17), 3439–3447 (2014) 25. Marco, C., Tangi, A., Raquel, G., et al.: “Flash” synthesis of CdSe/CdS core–shell quantum dots. Chem. Mater. 26(2), 1154–1160 (2014) 26. Xu, R., Liao, C., Zhang, H., et al.: “Flash” synthesis of “giant” Mn-doped CdS/ZnS nanocrystals for high photostability. RSC Adv. 5(108), 88921–88927 (2015) 27. Xu, R., Huang, B., Wang, T., et al.: Bright and high-photostable inner-Mn-doped core/giantshell quantum dots. Superlattice. Microst. 111, 665–670 (2017) 28. Chen, H.Y., Maiti, S., Son, D.H., et al.: Doping location-dependent energy transfer dynamics in Mn-doped CdS/ZnS nanocrystals. ACS Nano. 6(1), 583–591 (2012) 29. Huang, B., Xu, R., Zhuo, N., et al.: “Giant” red and green core/shell quantum dots with high color purity and photostability. Superlattice. Microst. 91, 201–207 (2016) 30. Huang, B., Yang, H., Zhang, L., et al.: Effect of surface/interfacial defects on photostability of thick-shell CdZnSeS/ZnS quantum dots. Nanoscale. 10(38), 18331–18340 (2018) 31. Garcia-Santamaria, F., Chen, Y.F., Vela, J., et al.: Suppressed Auger recombination in “giant” nanocrystals boosts optical gain performance. Nano Lett. 9(10), 3482–3488 (2009) 32. Park, Y.S., Malko, A.V., Vela, J., et al.: Near-unity quantum yields of biexciton emission from CdSe/CdS nanocrystals measured using single-particle spectroscopy. Phys. Rev. Lett. 106(18), 187401 (2011) 33. Nasilowski, M., Spinicelli, P., Patriarche, G., et al.: Gradient CdSe/CdS quantum dots with room temperature biexciton unity quantum yield. Nano Lett. 15(6), 3953–3958 (2015) 34. Klimov, V.I., Mikhailovsky, A.A., McBranch, D.W., et al.: Quantization of multiparticle Auger rates in semiconductor quantum dots. Science. 287(5455), 1011–1013 (2000)

218

L. Zhang et al.

35. Jain, A., Voznyy, O., Hoogland, S., et al.: Atomistic design of CdSe/CdS core-shell quantum dots with suppressed Auger recombination. Nano Lett. 16(10), 6491–6496 (2016) 36. Garcia-Santamaria, F., Brovelli, S., Viswanatha, R., et al.: Breakdown of volume scaling in Auger recombination in CdSe/CdS heteronanocrystals: the role of the core-shell interface. Nano Lett. 11(2), 687–693 (2011) 37. Liao, C., Xu, R., Xu, Y., et al.: Ultralow-threshold single-mode lasing from phase-pure CdSe/CdS core/shell quantum dots. J. Phys. Chem. Lett. 7(24), 4968–4976 (2016) 38. Efros, A.L., Efros, A.L.: Interband absorption of light in a semiconductor sphere. Semiconductors. 16(7), 772–775 (1982) 39. Miller, D.A.B., Chemla, D.S., Damen, T.C., et al.: Band-edge electroabsorption in quantum well structures: the quantum-confined stark effect. Phys. Rev. Lett. 53(22), 2173–2176 (1984) 40. Empedocles, S.A., Bawendi, M.G.: Quantum-confined stark effect in single CdSe nanocrystallite quantum dots. Science. 278(5346), 2114–2117 (1997) 41. Park, K.W., Deutsch, Z., Li, J.J., et al.: Single molecule quantum-confined stark effect measurements of semiconductor nanoparticles at room temperature. ACS Nano. 6(11), 10013– 10023 (2012) 42. Kuo, Y., Li, J., Michalet, X., et al.: Characterizing the quantum-confined stark effect in semiconductor quantum dots and nanorods for single-molecule electrophysiology. ACS Photonics. 5(12), 4788–4800 (2018) 43. Achtstein, A.W., Prudnikau, A.V., Ermolenko, M.V., et al.: Electroabsorption by 0D, 1D, and 2D nanocrystals: a comparative study of CdSe colloidal quantum dots, nanorods, and nanoplatelets. ACS Nano. 8(8), 7678–7686 (2014) 44. Zhang, L., Lv, B., Yang, H., Xu, R., et al.: Quantum confined stark effect in ensemble of phasepure CdSe/CdS quantum dots. Nanoscale. 11(26), 12619–12625 (2019) 45. Xu, Y., Chen, Q., Zhang, C., et al.: Two-photon-pumped perovskite semiconductor nanocrystal lasers. J. Am. Chem. Soc. 138(11), 3761–3768 (2016) 46. Todescato, F., Fortunati, I., Gardin, S., et al.: Soft-lithographed up-converted distributed feedback visible lasers based on CdSe-CdZnS-ZnS quantum dots. Adv. Funct. Mater. 22(2), 337–344 (2012) 47. Marceddu, M., Saba, M., Quochi, F., et al.: Auger recombination and optical gain in CdSe/CdS nanocrystals. Nanotechnology. 23(1), 015201 (2012) 48. Zavelani-Rossi, M., Lupo, M.G., Tassone, F., et al.: Suppression of biexciton Auger recombination in CdSe/CdS Dot/Rods: role of the electronic structure in the carrier dynamics. Nano Lett. 10(8), 3142–3150 (2010) 49. Lim, J., Park, Y.S., Klimov, V.I.: Optical gain in colloidal quantum dots achieved with directcurrent electrical pumping. Nat. Mater. 17(1), 42–49 (2018) 50. Moreels, I., Raino, G., Gomes, R., et al.: Nearly temperature-independent threshold for amplified spontaneous emission in colloidal CdSe/CdS quantum dot-in-rods. Adv. Mater. 24(35), OP231–OP235 (2012) 51. Kiraz, A., Chen, Q., Fan, X.: Optofluidic lasers with aqueous quantum dots. ACS Photonics. 2(6), 707–713 (2015) 52. Chan, Y., Steckel, J.S., Snee, P.T., et al.: Blue semiconductor nanocrystal laser. Appl. Phys. Lett. 86(7), 073102 (2005) 53. Snee, P.T., Chan, Y., Nocera, D.G., et al.: Whispering-gallery-mode lasing from a semiconductor nanocrystal/microsphere resonator composite. Adv. Mater. 17(9), 1131–1136 (2005) 54. Saliba, M., Wood, S.M., Patel, J.B., et al.: Structured organic−inorganic perovskite toward a distributed feedback laser. Adv. Mater. 28(5), 923–929 (2016) 55. Park, Y.S., Bae, W.K., Baker, T., et al.: Effect of Auger recombination on lasing in heterostructured quantum dots with engineered core/shell interfaces. Nano Lett. 15(11), 7319–7328 (2015) 56. Stasio, F.D., Grim, J.Q., Lesnyak, V., et al.: Single-mode lasing from colloidal water-soluble CdSe/CdS quantum dot-in-rods. Small. 11(11), 1328–1334 (2015) 57. Kozlov, O.V., Park, Y., Roh, J., et al.: Sub–single-exciton lasing using charged quantum dots coupled to a distributed feedback cavity. Science. 365(6454), 672–675 (2019)

Core/Shell Quantum-Dot-Sensitized Solar Cells Gurpreet Singh Selopal

Abstract Colloidal quantum dots (QDs) exhibit size-/shape-/compositiondependent optoelectronic properties due to quantum confinement and considered as building blocks for several optoelectronic devices. However, QDs possess a high density of surface trap states, which act as non-radiative carrier recombination centers, thereby reducing the overall performance of the optoelectronic device. Surface passivation of QDs by the epitaxial growth of an outer shell of different materials or composition, so-called core/shell QDs, has proven to be an effective approach to reduce the surface trap states as well as to tune their optoelectronic properties. Resulting core/shell QDs offer broader absorption spectrum, improved quantum yield (QY), and prolonged photoluminescence (PL) lifetime with better thermal, chemical, and photophysical stability compared to core QDs. In addition, these optoelectronic properties can be controlled by tuning the size and shape of core QDs, thickness and compositions of the shell layer, and electronic band edge alignment between core and shell. This chapter provides a comprehensive overview of the recent development in QDs-sensitized solar cells (QDSCs) based on different types of colloidal core/shell QDs including type-I, type-II, and quasitype-II core/shell QDs as light-harvesting materials. Keywords Core/shell quantum dots · Band alignment · Carrier dynamic · Quantum dot sensitized solar cells

G. S. Selopal () Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, People’s Republic of China Institut National de la Recherché Scientifique, Centre Énergie, Matériaux et Télécommunications, Montreal, QC, Canada e-mail: [email protected] © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 X. Tong, Z. M. Wang (eds.), Core/Shell Quantum Dots, Lecture Notes in Nanoscale Science and Technology 28, https://doi.org/10.1007/978-3-030-46596-4_7

219

220

G. S. Selopal

1 Introduction The world’s major dependency on fossil fuels for energy is causing its rapid depletion and environmental pollution and global warming [1, 2]. Therefore, the scientific community is putting their best efforts to substitute fossil fuels with renewable, clean energy sources like wind, hydro-energy, biomass, and solar energy. Among all, solar energy is considered as one of the most promising, reliable, clean, and economical renewable energy resources due to several reasons. For instance, the amount of solar energy that reaches the surface of the earth in 1 h is larger than the energy consumed by the whole world in 1 year [3]. This highlights that the efficient utilization of solar energy is of great importance to fulfil future energy demands. There are several approaches such as photovoltaic (PV), photoelectrochemical, and photothermal that are communally available to utilize this promising energy source. Among these renewable technologies, PV is considered as the most appealing approach that converts solar radiation directly into electricity based on the photoelectric effect. In addition, PV devices are lightweight and can be used everywhere without the need of any transmission line and also the large-scale application of PV devices can partially address the growing energy demand and environment-related issues [4]. The PV devices are mainly categorized into three generations based on the nature of the materials, cost per watt, and obtainable maximum photoconversion efficiency (PCE). In brief, first-generation PV devices are based on single-crystal silicon features with high PCE of 24.4% (close to the theoretical limit) [5] due to the broad absorption spectrum range and high carrier mobility. To date, silicon solar cell technology covers more than 90% of the PV market [6]. However, the high production cost of single-crystal silicon for first-generation PV increases the overall cost of the electricity compared to the electricity produced from fossil fuels. Thus, with the aim to reduce the production cost of the first-generation PV, the second-generation solar cells were developed based on thin-film technology. Amorphous silicon (a-Si), copper indium gallium selenide (CIGS), copper indium selenide (CIS), and cadmium telluride (CdTe) are the most commonly reported second-generation solar cells. Although the PCE of the second-generation solar cells is comparable to the silicon solar cells, still they possess less than 10% share of the PV market due to the problem of the module scale implementation and their limited stability. In the last few decades, third-generation PV technology attained great attention due to the possibility of achieving high PCE while maintaining low production cost with the application of novel nanomaterials and device architecture. In addition, utilization of inexpensive and environment-friendly materials and large-area fabrication are considered as motivation assets for third-generation PV technology. Due to these appealing features, third-generation PV devices can represent perhaps the future of solar cell technology, and even these devices are at the pre-commercial stage. Currently, dyesensitized solar cells (DSCs), organic (or polymer) solar cells (OSCs), perovskite solar cells (PSCs), and quantum dot (QD)-based solar cells belong to the thirdgeneration PV technology. Among all, QD-based solar cells are of particular interest

Core/Shell Quantum-Dot-Sensitized Solar Cells

221

Fig. 1 Schematic illustration of QD-based solar cell configurations and energy band diagram: (a) QDSC, (b) Schottky junction solar cell, (c) QD polymer hybrid solar cell, and p-n junction solar cells: (d) heterojunction and (e) homojunction. (Reproduced with permission from Ref. [9]. Copyright 2015, American Chemical Society)

due to unique and promising optoelectronic properties of QDs as a light-harvesting material such as size-, shape-, or composition-dependent absorption edge, high absorption coefficient, high photoluminescence quantum yield (PL QY), and large intrinsic dipole moment. Also the possibility of multiple exciton generation with single-photon absorption and hot electron extraction before thermalization, which could boost the PCE of QD-based solar cells beyond the Shockley–Queisser limit of 32.9% for single-absorber-based solar cells [7, 8]. Based on the device architecture, QD-based solar cells can be categorized into four types such as p-n junction solar cells (homo- or heterojunction), hybrid QD-polymer solar cells, Schottky junction solar cells, and QD-sensitized solar cells (QDSCs) [9]. Figure 1 displays the device architecture of different types of QD-based solar cells and electronic band alignment with carrier separation dynamic at interfaces of corresponding devices. Among these, QDSCs have attracted increasing attention due to simple, low-cost fabrication and great potential to obtain high PCE, which is confirmed from the increasing trend of the number of publications in the last few years (Fig. 2a). In addition, QDSCs borrowed their architecture from their analogous counterpart DSCs and can possibly be utilized for large area scalability. In the last few years, significant progress in the PCE has occurred and recorded value reached more than 13% due to the development of novel materials and device architecture (Fig. 2b) [10].

2 Brief History of QDSCs The present form of QDSC was developed by the concept of sensitization of the semiconductor. For the first time, in 1986, Gerischer et al. [11] introduced the idea of sensitization of a wide-bandgap semiconductor (e.g., TiO2 ) with

222

G. S. Selopal

Fig. 2 (a) Number of published articles per year based on the keywords “quantum dot sensitized solar cell” data is collected from ISI web of knowledge. (b) Trend of record PCE of liquid junction QDSCs based on a standard two-electrode configuration and tested under one sun illumination (AM 1.5, 100 mW/cm−2 ) from 2003 to 2019 [6]

narrow-bandgap materials (e.g., CdS) and demonstrated the enhancement in the photocurrent of the electrode due to the broader light absorption range. Later on, Vogel et al. [12] fabricated a three-electrode-based photoelectrochemical cell based on TiO2 mesoporous-film-sensitized quantum-sized CdS particles and reported the open-circuit voltage (Voc ) of 395 mV, short-circuit current density (Jsc ) of 175 mAm−2 , and fill factor (FF) of 0.75 under the monochromatic light illumination (λ = 460 nm). In 1998, Zaban et al. [13] reported for the first time a complete QDSC by using a nanoporous TiO2 film sensitized with pre-synthesized InP colloidal QDs, with iodine/iodide redox couple as an electrolyte and Pt as a counter electrode (CE). Yu et al. [14] reported a PCE of 0.3% for QDSCs based on InAs QDs. These reports highlight that more attention should be paid toward other device components, such as redox couple electrolyte and CE, to fabricate efficient QDSCs. For instance, the metal chalcogenide (CdS, CdSe, PbS, PbSe, etc.) QDs applied as light harvesters are not stable in an iodine/iodide redox couple electrolyte due to their chemical incompatibility. In 2007, a significant breakthrough occurs in PCE of QDSCs by replacing the iodine /iodide redox couple electrolyte with a polysulfide electrolyte. Diguna et al. [15] reported a PCE of 2.7% for QDSCs based on CdSe QD-sensitized TiO2 inverse opal electrode with polysulfide electrolyte. In addition, they highlight that the thin overcoating of the ZnS layer on the CdSe QD-sensitized TiO2 photoanode enhances the device performance due to reduced carrier recombination. Thereafter, Lee et al. [16] demonstrated the concept of TiO2 sensitization by using a bifunctional linker molecule, followed by a chemical bath deposition (CBD) process for CdSe layer growth and Au-coated conducting glass

Core/Shell Quantum-Dot-Sensitized Solar Cells

223

as CE and reported a PCE of 2.9%. Later on, the same group used a CdS/CdSe cosensitized photoanode via successive ion layer adsorption and reaction (SILAR) and boost the PCE of QDSCs over 4% [17]. During this period, the scientists realized that the sulfur species of the polysulfide electrolyte have a poisoning effect over noble metal-based CEs due to the adsorption of S2− onto the surface of the CE, which reduces its catalytic performances. In addition, the high cost of noble metals increases the overall production cost of the device that is an issue for its large-scale commercialization. In this context, Gimenez et al. [18] explored Cu2 S CE prepared by treating brass with HCl and demonstrated that the PCE of QDSCs is significantly improved from 0.65% with Pt CE to 1.83% with Cu2 S CE. In 2011, Zhang et al. [19] reported the PCE of 4.92% by using Cu2 S/brass CE with optimized TiO2 film electrode sensitized with CdS/CdSe QDs. Fan et al. [20] prepared efficient CE based on mesoporous carbon nanofibers and reported a PCE of 4.81%. Afterward, Cu2 S CEs prepared by different approaches have achieved considerable attention to enhance the performance of QDSCs [21–26]. These studies demonstrated that performance of QDSCs can be improved by using a suitable combination of polysulfide-based redox couple electrolytes (which guarantee for improved QD stability) [27] with Cu2 S CE (better catalytic activity toward polysulfide electrolyte) [28]. In the last few years, a significant improvement in PCE from 5% to more than 13% has been reported due to advanced development in the material design, device architecture through interface engineering, and in-depth understanding of carrier mechanism of QDSCs. Among all, it is worth mentioning, Kamat et al. [29] reported a PCE of 5.4% for QDSCs fabricated by using Mn-doped CdS/CdSe QDsensitized TiO2 mesoscopic film as photoanode and Cu2 S/graphene oxide (GRO) as CEs with polysulfide as the electrolyte. Zhong et al. [30] synthesized core– shell colloidal CdTe/CdSe QD and fabricated QDSCs with PCE of 6.76%. The design and development of ternary- or quaternary-alloyed and core/shell QDs leads to a significant breakthrough in the PCE of QDSCs. For example, in 2014, a new record of PCE as high as 7.04% has been reported for QDSCs based on type-I CuInS2 /ZnS core/shell QDs [31]. Then Zhao et al. [32] fabricated QDSCs using ternary a CdSex Te1−x QD-sensitized TiO2 mesoporous film with additional double layer capping of ZnS and SiO2 to reduce the carrier recombination at the TiO2 /QDs/electrolyte interface. The resulting QDSCs exhibit a certified PCE of 8.21% under one sun illumination. In 2016, Du et al. [33] synthesized a mesoporous carbon (MC) deposited on titanium (Ti) mesh and used as CEs. QDSCs based on MC-Ti CEs yield a PCE of 11.51% by using a CdSex Te1−x QD-sensitized TiO2 mesoporous film as photoanode and polysulfide as the electrolyte. This PCE value was further enhanced to 12.45% by exploring Zn-Cu-In-Se QDs as light harvester and nitrogen-doped MC-Ti as CE [34]. Recently, Pan et al. [35] fabricated QDSCs by employing a co-sensitization approach based on Zn-Cu-In-Se and Zn-Cu-In-Se colloidal QDs and reported an average PCE of 13.18%. This significant improvement in QDSC performance makes this promising technology one step toward commercialization and competitive with the other emerging solar cell technology. Figure 2b displays the overall progress in PCE of QDSCs based on sandwich-type two-electrode configuration

224

G. S. Selopal

under one sun illumination (AM 1.5G). All calculated photovoltaic parameters of the corresponding QDSCs are reported in Table 1.

3 QDSC Architecture The device architecture of QDSCs is analogous to DSCs, in which the dye molecule is replaced by semiconductor QDs as a light-harvesting material. Typically, QDSCs consist of a wide-bandgap semiconductor-based anode as the electron transport layer; QDs as a light harvester; a hole transport medium, so-called electrolyte; and CE (Fig. 1a).

3.1 Anode Anode is mainly composed of a wide-bandgap semiconductor film deposited on a transparent conducting oxide glass [e.g., fluorine-doped tin oxide (FTO) or indiumdoped tin oxide (ITO)], which acts as a scaffold to support QDs, extract the electron from the photoexcited QD, and then transport them toward the conducting oxide glass substrate. For the ideal anode, a semiconductor film should have a high specific surface area, high electron mobility, appropriate band alignment with the QDs, and compatibility with the other device components. Usually, the thickness of the electron transport layer is slightly lower compared to the thickness used in DSCs due to the high extinction coefficient of semiconductor QDs. However, the optimization of the thickness of the electron transport layer can be varied according to the nature, structure of the materials, and design of the device architecture. There are different types of wide-bandgap semiconductors that can be used as an electron transport layer: (i) The TiO2 semiconductor is most communally used as an electron transport layer due to several promising features such as low cost, non-toxic, and chemically stable materials [42, 43]. Usually, a standard TiO2 mesoporous electron transport layer is composed of two layers: The first layer is the 20-nmsized TiO2 nanoparticles with high specific surface area to load the QDs, so-called active layer, and second layer is the 200–400-nm-sized nanoparticles, so-called scattering layer, to increase the time spent within the photoanode and enhance the possibility of harvesting more sunlight photon by QDs. To date, QDSCs based on the TiO2 nanoparticle electron transport layer yield the highest PCE. However, the photoinjected electron undergoes transport within nanoparticles and the mesoporous film faces grain boundaries of nanoparticles, therefore increasing the possibility of electron recombination. Thus, with the aim to facilitate faster electron transport, one-dimensional (1D) nanostructures of TiO2 such as nanorods, nanowires, and nanotubes have been synthesized and applied as the electron transport layer [44– 46]. These 1D nanostructures improve the electron transport within the photoanode and reduce the carrier recombination. However, the lower amount of QD loading

Year 1998 2002 2006 2007 2008 2009 2011 2012 2013 2013 2014 2015 2015 2016 2016 2016 2017 2018 2019

QDs InP PbS InAs CdSe CdS/CdSe CdS/CdSe CdS/CdSe Mn-CdSe/CdSe CdSex Te1−x CdTe/CdSe CuInS2 /ZnS CdSex Te1−x CdSex Te1−x CdSex Te1−x CdSex Te1−x ZCISe ZCISe ZCISe-CdSe ZCISe-ZCIS

Electrolyte I− /I3 − Spiro-OMeTAD Co2+ /Co3+ Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide Polysulfide

CE Pt – Pt Pt Au Au Cu2 S/brass Cu2 S-RGO Cu2 S/brass Cu2 S/brass Cu2 S/brass Cu2 S/brass Cu2 S/FTO Cu2 S/brass MC-Ti MC-Ti N-MC-Ti MC-Ti MC-Ti

Voc (V) – 0.24 0.35 0.71 0.503 0.514 0.575 0.558 0.571 0.606 0.586 0.653 0.702 0.720 0.807 0.745 0.759 0.752 0.762

Jsc (mAcm−2 ) – – 1.75 7.51 11.66 16.80 13.68 20.70 19.35 19.59 20.65 20.78 20.78 21.04 20.69 25.49 25.67 27.39 25.93 FF 0.685 – 0.48 0.50 0.49 0.49 0.63 0.47 0.58 0.57 0.58 0.61 0.64 0.64 0.69 0.63 0.64 0.62 0.66

PCE (%) – 0.49 0.30 2.7 2.9 4.22 4.92 5.42 6.36 6.76 7.04 8.21 9.28 9.73 11.51 11.91 12.45 12.75 12.98

Ref. [13] [36] [14] [15] [16] [17] [19] [29] [37] [30] [31] [32] [38] [39] [33] [40] [34] [41] [35]

Table 1 Summary of the development in photovoltaic performance of QDSCs based on two-electrode sandwich configuration, tested under one sun illumination (AM 1.5G)

Core/Shell Quantum-Dot-Sensitized Solar Cells 225

226

G. S. Selopal

is due to a limited specific area of 1D nanostructures that reduces the overall PCE of QDSCs. (ii) ZnO is also widely studied as an electron transport layer due to its higher electron mobility (1–5 cm2 V−1 s−1 ) compared to TiO2 (0.1–4 cm2 V−1 s−1 ) and position of the conduction band (CB) edge, and it is relatively easy to grow at different morphologies (nanowires, nanotubes, and neotheropods) via the wetchemical approach [47, 48]. However, the overall PCE of QDSCs based on ZnO is still lower than that of QDSCs based on TiO2 due to several issues. In addition, the chemical instability of ZnO as the electron transport layer in QDSCs reduces the long-term stability. Furthermore, the dissolution of ZnO in the acidic medium limits the possibility to attach the bi-linker groups to load pre-synthesized colloidal QDs. Despite these issues, it is reported that the highest obtained Voc of QDSCs based on ZnO is 0.836 V, which is 0.2 V higher than the Voc of QDSCs based on TiO2 due to its CB edge position [49]. This highlights that ZnO possesses a great potential to be applied as an electron transport layer to fabricate high-performing QDSCs by optimizing the QD structure and interfacial engineering by surface passivation. (iii) SnO2 is also considered as promising electron transport materials in QDSCs due to its excellent physical, chemical, optical, and electrical properties such as less sensitive toward UV light and better chemical stability and electron mobility compared to both TiO2 and ZnO [50, 51]. However, the PCE of the QDSCs based on SnO2 electron transport layer is still lower than QDSCs based on TiO2 .

3.2 Light Harvester QDs are considered as the heart of QDSCs and applied as light harvester materials. Ideal QDs should have broader light absorption range, high absorption coefficient, and suitable band alignment with the electron transport layer and hole transport materials that allows the efficient separation of exciton generated with the absorption of a photon. In addition, the stability of QDs in the electrolyte and under light illumination and heat is also considered as a crucial requirement to fabricate efficient and stable QDSCs. Generally, there are several approaches available to deposit QDs (in situ and ex situ) on the mesoporous electron transport layer depending on the nature of QDs. The most commonly used QDs are classified into four categories: (i) binary QDs (InP, InAs, CdS, CdSe, CdTe, PbS, and Ag2 S) [12–15, 52, 53]; (ii) alloyed QDs (II–VI, CdSx Se1−x , CdSex Te1−x , etc.; I, III, VI, CuInS2 , AgInS2 , CuInSe2 , etc.) [32, 37–40]; (iii) doped QDs (CIS/Mn:CdS; Mn-CdSex Te1-x ; Co2+ ion-doped CdS/CdSe, etc.) [29, 54], and (iv) core/shell QDs (type-I, CuInS2 /ZnS, CdSeTe/CdS, etc.; reverse type-I, CdS/CdSe; and type-II, CdSex Te1−x /CdS; more details about the core/shell QDs are reported in Sect. 6) [30, 31, 55].

Core/Shell Quantum-Dot-Sensitized Solar Cells

227

3.3 Electrolyte The space between the two electrodes of a sandwich cell structure is filled with the redox couple electrolyte, which regenerates the oxidized QDs to its original form and acts as hole transport media during cell operation. For highly efficient devices, the redox couple electrolyte possesses the following properties: (i) high solubility and diffusion coefficient of the solvent used in electrolyte preparation for better charge carrier transport, (ii) oxidized and reduced forms that are stable and highly reversible, and (iii) chemically inert toward the other cell components and spectrally toward the visible light spectrum (absence of light absorption from the visible light spectrum). The most commonly used liquid electrolyte is polysulfide redox couple (S2− /Sn2 ), which is composed of Na2 S, S, and NaOH or KCL [15–17, 29–32]. Also, Co2+ /Co3+ , (CH3 )4 N2 S/(CH3 )4 N2 Sn , Fe2+ /Fe3+ , and Fe(CN)6 3− / Fe(CN)6 4− redox couple electrolytes are used to improve the performance of QDSCs [14, 56–58]. However, leakage and volatile nature of the liquid electrolytes reduce the stability of QDSCs. To solve these issues, quasisolid-state [gel polysulfide electrolyte using gelators: Dextran, sodium polyacrylate (PAAS), and sodium carboxymethylcellulose (CMC-Na)] and solid-state (SpiroOMeTAD, poly (3-hexylthiophene), CuSCN, etc.) electrolytes are also used in QDSCs [36, 59–61].

3.4 Counter Electrode CE is a thin layer of catalytic active materials deposited on the conducting substrate, which mainly speeds up the reduction of the oxidized electrolyte. Several parameters of the CE that effect the overall device performance such as: (i) electro-catalytic activity, (ii) chemical stability in redox couple electrolyte, and (iii) morphology and surface roughness of exposed facet. The FF of the device is significantly influenced by the properties of CE, so by improving the CE properties, we can improve the performance of QDSCs. For efficient CE, there should be less charge transfer resistance (Rct ) from the CE to the electrolyte, which reduces the series resistance leading to improvement in FF, which results in higher PCE. There are several types of materials used as CEs in QDSCs: (i) noble metals (Pt and Au) [15, 17], (ii) metal chalcogenides (Cu2 S/brass, Cu2 S/FTO or ITO, Cu2 Se, CoS, FeS, NiS, etc.) [19, 38, 62–64], (iii) carbon materials (active carbon, conductive carbon black, mesoporous carbon nanofibers, mesoporous carbon supported on Ti mesh, etc.) [33, 34], and (iv) composites (Cu2 S-RGO, CuInS2 /carbon composite, PbS/carbon black, etc.) [29, 65, 66].

228

G. S. Selopal

Fig. 3 Schematic illustration: (a) device structure and working principle of QDSC composed of a photoanode as electron transport layer, QDs as light harvester, redox couple electrolyte as hole transport medium, and counter electrode; (b) current density–voltage (J-V) curves of QDSCs under one sun illumination (AM 1.5G, 100 mW/cm−2 )

4 Working Principle The working principle of QDSCs is quite different from conventional p-n junction solar cells due to the difference in their fundamental physical phenomena behind their operation and device construction. In the conventional p-n junction solar cells, light absorption and charge transport occur within the same region of the material, whereas in the QDSC, light is absorbed by the QDs and transport occurs through the metal oxide electrode and electrolyte. In addition, carrier separation in QDSC mainly occurs due to energetic and entropic forces at the anode/QD/electrolyte interface, whereas in the conventional p-n junction solar cell, it is controlled by the electric field across the p-n junction. A complete one cycle of QDSC operation is shown in Fig. 3a and consists of the following process:

4.1 Light Absorption The absorption of a light photon (hν ≥ Eg of QD) by QDs via an electronic excitation process (1). In brief, an electron–hole pair is generated with the absorption of photon and electron jumps from the higher energy levels of valence band (VB) to the lowest energy levels of CB. During this process, QD is get transferred from the ground state (QD) to the excited state (QD∗ ) as shown below: QD + hϑ → QD∗

Core/Shell Quantum-Dot-Sensitized Solar Cells

229

4.2 Carrier Separation Now the photogenerated electron–hole pair in QD with the absorption of photon is separated through the injection of the electron from the CB energy level of excited QD to CB of metal oxide (2) and a hole transport from the VB energy level of excited QD to the electrolyte (3), as shown below: QD∗ → QD+ + e− CB 2QD+ + S2− → 2QD + S The electron injection process occurs on the femtosecond scale, whereas the hole transport process occurs on the microsecond range.

4.3 Carrier Transport After the electron–hole separation, the injected electrons travel through a mesoporous electron transport layer via diffusion (4) to the transparent conducting oxide substrate. Several theoretical and experimental studies have been carried out to understand the exact electron transport mechanism through the mesoporous electron transport layer. In brief, the concept of electron transport by a built-in electric field has been discarded due to the small size of the nanoparticles of the mesoporous electron transport layer and the screening effect of the surrounding electrolyte. Thus, electron transport is assumed to occur via the diffusion process and is considered as the most realistic [67, 68]. The electron diffusion process involves trapping and de-trapping of the electron from one particle to another [69]. It has been demonstrated that an electron undergoes 103 –106 trapping/de-trapping events to reach the transparent conducting oxide [70]. These events (electron trapping/detrapping) occur through the localized energy trap states having different depths and lies just below the CB of the electron transport layer. These energy states (traps/detraps) play a significant role during electron transport. At low light intensity, deep traps participate in the electron transport causing low diffusion coefficient, whereas at higher light intensity (≈ solar illumination intensity), deep traps are filled and shallow traps contribute to electron transport resulting in a high diffusion coefficient [71]. Finally, these electrons are collected at the conducting substrate and reach the CE through the external load. On the other hand, the hole in the VB energy level of excited QD is scavenged by the polysulfide redox couple electrolyte (3) and regenerates the QD. The oxidized electrolyte reaching the CE is immediately reduced by gaining the electron that reaches at CE through an external circuit (5), hence completing one cycle. This electrolyte reduction is catalyzed by catalytic CE materials as shown below:

230

G. S. Selopal

Sx 2− + 2e− (CE) → Sx−1 2− + S2− The time scale at which the electron and hole transport occurs is in 100 microseconds and less than 10 microseconds, respectively.

4.4 Recombination Apart from these favorable carrier-transport processes in QDSCs, there are series of non-radiative carrier recombination paths during their transport (Fig. 3a). These recombination processes depend on various factors such as the morphology of the mesoporous electron transport network, type and nature of QDs, electrolyte composition, and CE and are responsible for the loss in overall PV performance of QDSCs. The most predominant recombination processes occur at the electron transport layer/QD/electrolyte interface and within the QDs, for example, (i) recombination of the photoinjected electron in the CB of the electron transport layer with the oxidized species in the electrolyte at the electron transport layer/electrolyte interface (6), (ii) recombination of the electron in the CB of the electron transport layer with hole in the VB of QDs at the electron transport layer/QD interface (7), (iii) recombination inside the QDs through surface trap states (8), and (iv) recombination of the excited electrons in the CB of QDs with the oxidized species in the redox couple electrolyte at the QD/electrolyte interface (9) [72–74].

5 Photovoltaic Characterizations The key parameters of the PV performance of QDSC are obtained from J-V measurements taken under standard one sun illumination conditions (AM 1.5 G). The intensity of the solar simulator spectrum (AM 1.5 G) should be optimized using a standard silicon solar cell (100 mWcm−2 , one sun) that allows the systematic comparisons of the obtained results from different research labs. The J-V curve is obtained by applying an external potential and measuring the current, then plotted as the current density (current/area of the device) vs voltage (Fig. 3b). From the J-V curves, Jsc and Voc values can be obtained from the intercepts of the J-V curves at the y-axis (when V = 0) and x-axis (when J = 0), respectively. There is a point in a J-V curve which intercepts the maxima of the photocurrent (Imp ) and photovoltage (Vmp ) drawn and their product gives maximum obtainable power (Pmax ). The FF is defined as the ratio of the Pmax to the product of the Voc and Jsc (theoretical value), which is in the range of 0–1. The overall PCE of the QDSCs can be calculated from the ratio of the Pmax to the total power of the incident light (Pin ) as given below:

Core/Shell Quantum-Dot-Sensitized Solar Cells

PCE =

231

Pmax VMP × JMP F F × Voc × Jsc = = Pin Pin Pin

Apart from these PV parameters, incident photon to current conversion efficiency (IPCE) is also an important measurement to know the conversion efficiency of the solar cells at particular wavelength and is the combination of the light-harvesting efficiency (LHE), electron injection efficiency (ηinj ) from the QDs to the CB of electron transport layer, and photogenerated charge collection efficiency (ηcol ) [75]: IPCE = LHE (λ) .ηinj (λ) .ηcol (λ) The Isc measured under short-circuit conditions is the integrated sum of IPCE (λ) measured over the entire solar spectrum:  Isc (λ) =



IPCE (λ) Isun (λ) dλ 0

where Isun (λ) is the incident irradiance as a function of the wavelength λ (nm). This integrated current Isc (λ) is used to cross-check with the result obtained with I-V measurements. The IPCE is also called external quantum efficiency (EQE). In addition, the absorbed photon to current conversion efficiency (APCE) is the ratio of the number of generated electrons (Ne ) and the number of absorbed photons (Na ). The APCE is also called the internal quantum efficiency (IQE) and reflects the efficiency of absorbed photons by a QD being converted into photocurrent excluding the effect of the light-harvesting efficiency of the photoanode. The APCE can be calculated by the following equation: APCE (λ) =

Ne IPCE (λ) = ϕinj ϕcc = Na LHE (λ)

Carrier dynamics measurements of QDSCs can be performed using charge extraction (CE), transient photocurrent decay (TCD) and photovoltage decay (TVD), electrochemical impedance spectroscopy (EIS), open-circuit voltage decay (OCVD), intensity-modulated photocurrent spectroscopy (IMPS), and photovoltage spectroscopy (IMVS).

6 Core/Shell QDs QDSCs have the possibility to boost the PCE beyond the Shockley–Queisser limit of 32.9% due to promising optoelectronic properties of QDs as light-harvesting materials. However, the record value of PCE (≥ 13%) of QDSCs is still lower than that of the calculated theoretical limit of PCE [76]. This substantially lower value of PCE of QDSCs is mainly attributed to undesirable non-radiative carrier

232

G. S. Selopal

recombination during the operation under illumination (Sect. 4.4). It can be seen in Fig. 2 that the major non-radiative recombination processes [(7), (8), and (9)] in QDSCs are mainly dependent on the nature of the QDs, in particular, the ability of exciton generation with the absorption of a light photon, efficient dissociation of photogenerated exciton at the interface of QD/carrier scavengers (TiO2 or electrolyte), and regeneration of QDs for the next cycle [72, 77]. Usually, QDs process a high density of surface traps due to high surface-to-volume ratio, which promotes these non-radiative recombination processes [(7), (8), and (9)] through surface trap states that act as recombination center and reduces the overall performance of QDSCs. Also, the bandgap tuning of QDs by tailoring their size has several issues. In particular, in solar cell application, QDs with a narrow bandgap offer a broader light-harvesting range, but the efficiency of the photogenerated electrons from the CB of QDs to the CB of the electron transport layer is due to unfavorable band alignment between QDs and carrier scavengers [78]. Thus, there is an urgent need to design and synthesize high-quality QDs for the fabrication of high-efficiency and stable QDSCs. Till date, different approaches have been reported to minimizing these non-radiative recombinations. Among all, the core/shell architecture of QDs is considered the most effective strategy [79–82]. In this core/shell architecture, the surface of the core QDs is passivated by the growth of the shell of different materials and thickness. The core/shell QDs offer a reduced density of surface traps, broader light absorption, better thermal and photochemical/physical stability to the surrounding environment, and longer PL lifetime compared to core QDs [80, 82, 83]. In addition, these optoelectronic properties of the core/shell QD system can be tuned by the appropriate design and selection of the core and shell materials, thickness, and composition. Thus, the band alignment between the core and shell materials and understanding of carrier dynamics of the core/shell QD system are crucial to fabricate high-performing QDSCs.

6.1 Classification of Core/Shell QD Systems The core/shell QD systems are typically classified into three systems, such as type-I, reverse type-I, and type-II, based on the relative band alignment between the CB and VB edges of the core and shell materials (Fig. 4) [84]. In type-I core/shell QDs, the bandgap of the shell materials is larger than that of the core materials and both the CB and VB edges of the core are located in the bandgap of the shell materials. Therefore, both electrons and holes are confined in the core region (Fig. 4a). The shell is used not only to passivate the active surface of the core QD but also improves its optoelectronic properties such as PLQY and photophysical/chemical stability toward its surrounding environment [85–89]. However, the PL emission of the type-I core/shell QD system depends on the nature of the core materials as both electron and hole are localized in the core region. The reverse type-I core/shell QD system is opposite to the type-I core/shell QDs, in which the bandgap of the shell materials is lower than that of core materials (Fig. 4b). Both electrons and holes

Core/Shell Quantum-Dot-Sensitized Solar Cells

233

Fig. 4 Schematic illustration of the electronic band alignment for different types of core/shell QD systems and below electron and hole wave functions correspond to the position of the CB and VB of the core (center) and shell materials (outer), respectively: (a) type-I, (b) reverse type-I, (c) type-II, (d) quasi-type-II

are partially or entirely delocalized in the shell region that depends on the thickness of the shell layer [90, 91]. The absorption spectrum of the reverse type-I core/shell QD system can be tuned by changing the shell thickness that allows the efficient extraction of the photogenerated electrons and improves the electron injection rate [78, 92]. In type-II core/shell QD system, either the CB edge or VB edge of the shell is located in the bandgap of the core (Fig. 4c) [93, 94]. The electrons and holes are spatially separated into different regions of the core and shell [95, 96] This spatial carrier separation in type-II core/shell QDs leads to a substantial shift of the emission wavelength and prolonged PL lifetime decay with shell growth. The emission of the type-II core/shell QD system can be controlled by the effective band offset of core and shell materials. In addition, these core/shell QDs, there is a particular case of core/shell structured QDs called quasi-type-II in which one type of carrier is confined in the core, whereas the other is delocalized over the entire core/shell structure (Fig. 4d). In the case of “giant” core/shell QDs (shell thickness ≥ 1.5 nm to several tens of nm), type-I core/shell QDs can be transferred to quasi-type-II core/shell QDs, by tuning the thickness and composition of the shell layer.

6.2 Synthesis of Core/Shell QDs The size-/shape-/composition-dependent optoelectronic properties of QDs allow to design and synthesize novel colloidal QDs as per requirements for the fabrication of

234

G. S. Selopal

Fig. 5 Schematic representation of the synthesis of colloidal core/shell QDs. First, the core QDs can be synthesized via the hot-injection approach (left side), then core/thin shell, core/thick shell, and core/alloyed shell/shell structure (right side) synthesized by the growth of shell over the core QDs either via cation exchange/SILAR or a combination of both the cation exchange and SILAR approach

efficient and stable optoelectronic devices. In the last few decades, considerable efforts have been devoted to developing novel wet chemical approaches to synthesize colloidal QDs with uniform size distribution to replace the expensive and complicated physical methods. Core/shell QDs are commonly synthesized in a twostep approach as shown in Fig. 5: (i) first, the synthesis of high-quality colloidal core QDs via the well-established hot-injection approach. In brief, one of the reaction precursor is heated to a high temperature that depends on the nature of the precursor and then subsequent injection of another precursor at room temperature. These mixed precursors decomposed to form nucleation seeds and the slight lowering in reaction temperature allows the growth of seeds into QDs. The QDs synthesized by the hot-injection approach are monodisperse with a narrow size distribution, high QY, and better chemical/photostability. The size of the colloidal QDs can be tuned by varying the reaction temperature, duration, and precursor concentration [97, 98]. Then the as-synthesized core QDs are purified and re-dispersed in the organic solvent for the shell growth. (ii) Several approaches have been reported for shell growth over the pre-synthesized core QDs. The cation exchange and SILAR approaches are the most preferred approaches for shell growth. In the cation exchange approach, the mixture of the cationic and anionic precursor is injected in the core QD solution at the growth temperature and only the new cationic precursor of the shell material is gradually replaced by the cation of the core QDs during the shell growth. The overall size of the as-synthesized core/shell QDs via the cation exchange method does not change significantly compared to core QDs (Fig. 5). While in the SILAR method, the shell is grown over the pre-synthesized core QDs

Core/Shell Quantum-Dot-Sensitized Solar Cells

235

by alternative injection of cationic and anionic precursors. The thickness of the shell layer can be tuned by changing the growth temperature, duration, and amount of shell precursor. The overall size of core/shell QDs synthesized via the SILAR approach is increased and depends on the number of grown shell layers. In addition, the combination of cation exchange and SILAR is used to engineer the interface between the core and shell QDs via the synthesis of the core/alloyed shell/shell QDs, which offer better optoelectronic properties compared to simple core/shell QDs (Fig. 5). In these approaches, the shell growth temperature (T2 ) is generally lower than the temperature used for the core QD synthesis to avoid uncontrolled ripening of the core QDs and homogeneous nucleation of the shell material. In addition, the concentration of core QD dispersion is required to inject shell precursor amount to obtain the desired shell thickness.

7 Photovoltaic Performance of Core/Shell QDSCs The major undesirable non-radiative recombination processes of QDSC solar cells are mainly associated with QDs. Therefore, high-quality QDs with reduced surface trap states and efficient carrier dynamics are required to fabricate efficient and stable QDSCs. In the last few years, core/shell QDs achieved considerable attention to be applied as light-harvesting materials due to their versatile optoelectronic properties (discussed in Sect. 6). All three types of core/shell QDs (type-I, reverse type-I, and type-II) have been widely applied as a light harvester in QDSCs and are discussed below.

7.1 Type-I Core/Shell QDs In type-I core/shell QDs, the CB and VB edges of shell materials are higher than that of core materials as discussed in Sect. 6.1 and both electron and hole are confined in the core region (Fig. 4a). The shell layer passivates the surface of the core QDs that reduces the surface traps and improves the photophysical/chemical stability and optoelectronic properties. For example, the carrier dynamics measurements of typeI CdSe/ZnS core/shell QDs as a function of ZnS shell thickness highlight that the ZnS shell layer acts as a tunnelling barrier for the electron and hole transfer can lower their transfer rates due to reduced electronic coupling between the type-I core/shell QDs and carrier scavengers [83]. Thus, optimization of the shell layer thickness with efficient carrier transfer rates should be required for the fabrication of high-performance QDSCs. In this context, Pan et al. [31] synthesized heavy metal-free CuInS2 core QDs via the hot-injection method at 180 ◦ C and then a thin shell layer of ZnS was overcoated via the partial cation exchange approach. Figure 6a–b displays the TEM images of CuInS2 core QDs and CuInS2 /ZnS core/shell QDs, which confirm the comparable size of both core and core/shell QDs with an

236

G. S. Selopal

Fig. 6 TEM images of as-synthesized (a) CuInS2 QDs and (b) CuInS2 /ZnS core/shell QDs. Inset displays the cation exchange process for the synthesis of CuInS2 /ZnS core/shell QDs. Optical characterizations: (c) UV−vis absorption and (d) PL emission spectra with emission wavelength = 400 nm. (e) J-V curves of QDSCs based on CuInS2 and CuInS2 /ZnS core/shell QDs; (f) IPCE spectra of corresponding QDSCs. Electrochemical impedance spectroscopy characterization of the QDSCs based on CuInS2 and CuInS2 /ZnS core/shell QDs: (g) recombination resistance (Rrec ) on applied voltage (Vappl ). (h) Nyquist plots of both cells at −0.55 V forward bias. Reproduced with permission from Ref [31]. Copyright 2014, American Chemical Society

average size of 5.1 ± 0.4 nm. Then bi-linker molecule mercaptopropionic (MPA)capped water-soluble CuInS2 /ZnS core/shell QDs were prepared by phase transfer via the ligand exchange approach. The absorption spectra of resulting CuInS2 /ZnS core/shell QDs extend to the near-infrared region of 850 nm (Fig. 6c). As can be seen, the PL QY of the CuInS2 /ZnS core/shell QDs is increased more than tenfold with the addition of a thin ZnS shell layer over the CuInS2 QDs due to reduced surface traps (Fig. 6d). The QDSC-based MPA-capped water-soluble CuInS2 /ZnS core/shell QDs yield a PCE of 7.04% under one sun illumination (AM 1.5G), which is 40% higher than the PCE of QDSCs with CuInS2 core QDs (Fig. 6e). This enhanced photovoltaic performance of QDSCs is mainly attributed to improved optoelectronic properties and reduced surface trap states with the surface passivation of CuInS2 QDs with a thin ZnS shell layer. The IPCE response of QDSCs based on CuInS2 /ZnS core/shell QDs (75%) is significantly higher compared to QDSCs based on CuInS2 QDs (60%) between the 350- and 700-nm range (Fig. 6f). The effect of the ZnS shell layer on the carrier dynamics at the QD/TiO2 /electrolyte interface of QDSCs is evaluated using EIS at different applied voltages. It can be seen that the calculated Rrec value of QDSCs based on CuInS2 /ZnS core/shell QDs is significantly (threefold) higher than that of the QDSCs based on CuInS2 QDs, which confirms the reduced carrier recombination (Fig. 6g). At particular forward bias voltage (−0.55 V, close to the Voc of the device), comparison of Nyquist plots between QDSCs based on CuInS2 /ZnS core/shell QDs and CuInS2 QDs shows the clear difference in the carrier dynamics of the respective devices (Fig. 6h).

Core/Shell Quantum-Dot-Sensitized Solar Cells

237

Yang et al. [55] also reported the synthesis of type-I CdSeTe/CdS core/shell QDs via a combination of two approaches: hot injection for the synthesis of CdSeTe core QDs and CdS shell layer grown by the SILAR method at moderate temperature condition. Then as-synthesized oil-soluble QDs were transferred to thioglycolic acid (TGA)-capped water-soluble QDs through the ligand exchange process and anchored on TiO2 mesoporous film via the self-assembly deposition technique. QDSCs based on type-I CdSeTe/CdS core/shell QDs yield a PCE of 8.02%, which is 13% higher than the PCE of QDSCs based on CdSeTe QDs. This significant improvement in the PCE of QDSCs is ascribed to reduced carrier recombination within QDs and QD/TiO2 /electrolyte interface with the grown CdS shell layer over CdSeTe QDs. In addition, the capping of a QD/TiO2 photoanode with SiO2 and amorphous TiO2 layers was applied to further improve the interface between the QDs/TiO2 and electrolyte. Resulting QDSCs achieved a PCE of 9.48%, which was a new record of the PCE for liquid-junction QDSCs. Overall, these studies highlight that in the case of type-I core/shell QDs, overcoating of the shell layer on core QDs improves the optoelectronic properties and photochemical/physical stability due to reduced surface trap states. Although the performance of the QDSCs is enhanced due to these appealing features of type-I core/shell QDs, the shell layer acts as a tunnelling barrier for the carrier separation process and reduces their transfer rates.

7.2 Reverse Type-I Core/Shell QDs In type-I core/shell QDs, both electrons and holes are confined in the core region and the shell layer acts as a tunnelling barrier that reduces the carrier transfer rates (Fig. 4b). Thus, to solve these issues, another type of core/shell QDs in which the CB and VB gap of the core materials is higher than that of shell materials is the so-called “reverse type-I” core/shell QDs (also called “inverted type-I” core/shell QDs). Among the various reverse type-I core/shell QDs, CdS/CdSe core/shell QDs have been widely studied and applied as a light harvester in QDSCs due to several appealing features of the constituents: CdS offers higher Voc and CdSe offers broader light absorption near 720 nm. In addition, charge carriers (both electron and hole) are delocalized in the CdSe shell region that allows efficient extraction of photogenerated electrons and holes by carrier scavengers and enhances their injection rates [99]. In the last few years, reverse type-I CdS/CdSe core/shell QD-based solar cells achieved great attention due to the above-mentioned promising optoelectronic properties [100–103]. In these studies, reverse type-I CdS/CdSe QDs were directly grown over the mesoporous wide-bandgap semiconductors (e.g., TiO2 or SnO2 ) via SILAR or chemical bath deposition. However, it is hard to control the size and shape of the directly grown CdS/CdSe QDs due to the uncontrollability of nucleation and growth processes of the QDs on the mesoporous film surfaces. In addition, these QDs are not offered the exact inverted type-I core/shell configuration.

238

G. S. Selopal

Thus, the overall PCE of QDSCs based on directly grown CdS/CdSe QDs is in the range of 2–4%. To solve these issues, high-quality pre-synthesized reverse type-I colloidal QDs were developed and their carrier dynamics were studied [99, 104]. Both fluorescence lifetime decay and femtosecond transient absorption (TA) spectroscopy measurements of reverse type-I CdS/CdSe core/shell QDs demonstrated that both electron and hole are localized in the CdSe shell region and suitable for the fabrication of high-efficiency QDSCs. Pan et al. [78] synthesized high-quality reverse type-I CdS/CdSe core/shell QDs by the hot-injection approach for the CdS core, followed by the SILAR method for the CdSe shell of tunable thickness. It can be seen that both CdS core and CdS/CdSe core/shell QDs with tunable CdSe shell thickness show a nearly spherical shape with narrow size distribution (Fig. 7a–e). To improve the QD loading, as-synthesized oil-soluble high-quality CdS/CdSe core/shell QDs were transferred to water-soluble QDs via the ligand exchange approach by using a bifunctional mercaptopropionic acid (MPA) linker molecule. As a proof of concept, water-soluble MPA-capped CdS/CdSe core/shell QDs were loaded to TiO2 mesoporous film. It is clear that absorption spectra profiles of CdS/CdSe core/shell QDs with different CdSe shell thicknesses anchored to TiO2 is consistent with MPA-capped QD aqueous solution (Fig. 7g). This highlights that the high-temperature synthesis method has better control over the size and structure of QDs, which is absent in the directly grown reverse type-I CdS/CdSe QDs. QDSCs based on MPA-capped CdS/CdSe core/shell QDs with optimized absorption onset reported a new record PCE of 5.32% under one sun illumination (Fig. 7h). The trend of the IPCE values of the corresponding QDSCs between the 400- and 650-nm range is consistent with J-V measurements (Fig. 7i). This significant improvement in the QDSC performance is mainly ascribed to the extended light-harvesting range, efficient charge injection, and suppressed charge recombination in reverse type-I CdS/CdSe core/shell QDs compared to core QDs.

7.3 Type-II Core/Shell QDSCs Type-II core/shell QDs are composed of a core QD passivated by the shell layer with the CB or VB edge located in the bandgap of the core material (Fig. 4c). As discussed previously, the spatial separation of electron and hole wave function enables the fast electron injection rate from the QDs to the electron transport layer and reduces the carrier recombination processes as the shell layer acts as a tunnelling barrier for the hole localized in the core region. In addition, the prolonged lifetime of the charge carrier in type-II core/shell QDs can create a dipole moment through the accumulation of a negative charge in the electron transport layer (e.g., TiO2 or ZnO) and a positive charge in the core of the QDs. This dipole moment leads to the upshift in the CB of the electron transport layer and resulting solar cells yield higher opencircuit photovoltage compared to core QD-based solar cells [30, 105]. A significant redshift of the absorption edge of type-II core/shell QDs due to the exciplex state offers a new route to broaden the light absorption range by reducing the effective

Core/Shell Quantum-Dot-Sensitized Solar Cells

239

Fig. 7 TEM images of as-synthesized (a) CdS core QDs with size of 2.9 nm; (b)–(e) CdS/CdSe core/shell QDs with sizes of 4.6, 5.7, 6.1, and 6.7 nm. (f) Schematic illustration of band edge alignment of reverse type-I CdS/CdSe core/shell QD-sensitized TiO2 -based solar cell. (g) Absorption spectra of MPA-capped water-soluble CdS/CdSe QD-sensitized TiO2 films of 4-μm thickness. Inset displays photographs of corresponding photoanode films. (h) J-V curves of QDSCs based on reverse type-I CdS/CdSe core/shell QDs; (i) IPCE spectra corresponding to QDSCs. (Reproduced with permission from Ref [78]. Copyright 2012, American Chemical Society)

bandgap [106, 107]. All these appealing features of the type-II core/shell QDs make them as promising light-harvesting materials for QDSCs and other optoelectronic devices. For the first time, Ning et al. [108] synthesized type-II ZnSe/CdS core/shell QDs and fabricated QDSCs with prominent absorbed photon to current conversion efficiency and PCE of 0.27%. Luo et al. [109] designed and developed a microwaveassisted aqueous approach to synthesize type-II CdSex Te1-x /CdS core/shell QDs. Resulting QDSC-based CdSex Te1-x /CdS core/shell QDs show a PCE of 5.04% under one sun illumination (AM 1.5 G).

240

G. S. Selopal

Later, Bang et al. [110] also synthesized environmentally friendly type-II ZnTe/ZnSe core/shell QDs that are explored as light-harvesting materials in solar cell application. However, the PCE of the QDSCs based on these type-II core/shell QDs is still lower than that of the predicted theoretical limit of PCE for typeII core/shell QDs due to limited loading of colloidal QDs in the mesoporous electron transport layer. Then, Wang et al. [30] synthesized type-II CdTe/CdSe core/shell QDs synthesized via the combination of the hot-injection and SILAR methods. Briefly, first, CdTe core QDs of the average size of 2.7 ± 0.2 nm were synthesized via the hot-injection method in which CdO-tetradecylphosphonic acid (TDPA) reacts with trioctylphosphine (TOP)-Te in the 1-octadecene medium at high temperature (Fig. 8a). As-synthesized CdTe core QDs were purified and dispersed in a chloroform solution. Then CdSe shell layers were grown over the CdTe core QDs via the SILAR approach. After the three cycles of CdSe shell layers, an average size of 4.9 ± 0.3 nm of CdTe/CdSe core/shell QDs was obtained (Fig. 8b). Finally, the oil-soluble type-II CdTe/CdSe core/shell QDs were transferred to MPA-capped water-soluble QDs. The absorption spectrum of CdTe core QDs shows the first excitonic absorption peak at 540 nm and PL emission peak at 558 nm, whereas with the growth of three CdSe shell layers, CdTe/CdSe core/shell QDs show broadening of the absorption spectrum toward a longer wavelength with first excitonic peak at 785 nm and PL emission peak redshift from 558 to 795 nm (Fig. 8c). QDSCs based on MPA-capped type-II CdTe/CdSe core/shell QDs reported impressive PCE of 6.75%, a new record for type-II core/shell QD-based QDSCs. However, the relatively low value of Voc still needs to be improved to further boost the PCE of the QDSCs based on type-II core/shell QDs. Thus, Jiao et al. [111] explored the novel ZnTe/CdSe type-II core/shell QDs as a light-harvesting material in the fabrication of QDSCs. As-synthesized ZnTe/CdSe type-II core/shell QDs show an average size of 5.3 nm with ZnTe core of 3.2 nm and CdSe shell of 2.1 nm (Fig. 8d). Figure 8e displays the TEM image of QD-sensitized mesoporous TiO2 film. It can be seen that ZnTe/CdSe type-II core/shell QDs display better optoelectronic and relative band alignment compared to the CdTe/CdSe core/shell QDs. In particular, ZnTe/CdSe core/shell QDs show much higher CB offset potential (1.22 eV) compared to the (0.27 eV) CdTe/CdSe core/shell QDs (Fig. 8f). The higher band offset of ZnTe/CdSe than CdTe/CdSe QDs produces an increase of charge accumulation at QD/TiO2 interfaces under illumination, which results in creating a stronger dipole moment and leads to a greater upward shift in the CB edge of QD-sensitized TiO2 photoanodes and enhances the Voc of QDSCs. In addition, the reduced effective bandgap of type-II ZnTe/CdSe core/shell QDs broadens the light absorption spectrum from the visible to the NIR region (Fig. 8g). Resulting QDSCs based on the MPA-capped aqueous type-II ZnTe/CdSe core/shell QDs yield a PCE of 7.17%, which is 8% higher than that of previously designed type-II CdTe/CdSe core/shell QDs (Fig. 8h). This significant improvement in the PCE of QDSCs based on type-II ZnTe/CdSe core/shell QDs is mainly derived from the enhanced Voc of 0.646 V compared to 0.597 V for CdTe/CdSe QD-based QDSCs through the band engineering approach. The IPCE of the respective QDSCs shows an ~ 75% in the range of 360–680 nm for both CdSe, CdTe/CdSe and ZnTe/CdSe,

Core/Shell Quantum-Dot-Sensitized Solar Cells

241

Fig. 8 Structural and optical characterization: TEM images of as-synthesized QDs: (a) CdTe core QDs; (b) CdTe/CdSe core/shell QDs; and (c) normalized absorption (solid lines) and PL spectra (dashed line) with emission wavelength of 360 nm of CdTe (black lines) and type-II CdTe/CdSe core/shell QDs (red lines). Reproduced with permission from Ref [30]. Copyright 2013, American Chemical Society. TEM images: (d) as-synthesized ZnTe/CdSe core/shell QDs with 3 monolayers of CdS shell; (e) ZnTe/CdSe core/shell QD-sensitized TiO2 mesoporous film. (f) Schematic diagram of the bandgap and band offsets (in eV) for the interfaces between bulk ZnTe/CdSe and CdTe/CdSe core/shell QDs. (g) Absorption spectra of ZnTe/CdSe, CdTe/CdSe, and CdSe QD-sensitized TiO2 films with a 6.0-μm transparent layer. Inset display of the photographs of corresponding QD-sensitized TiO2 film electrodes. Photovoltaic performance of QDSCs based on ZnTe/CdSe and reference CdTe/CdSe and CdSe QDs: (h) J-V curves of champion cells; (i) IPCE curves of corresponding devices. Reproduced with permission from Ref [111]. Copyright 2015, American Chemical Society

but the QDSCs based on CdTe/CdSe and ZnTe/CdSe core/shell QDs also show IPCE response in the broader range of 350–900 nm (Fig. 8i).These results highlight that the band engineering of QDs through the core/shell (type-I, reverse type-I, and type-II) approach is the most effective to improve the optoelectronic properties of the QDs and hence to enhance the performance of QDSCs based on the respective core/shell QDs.

242

G. S. Selopal

7.4 Quasi-Type-II Core/Thick-Shell QDs (“Giant” Core/Shell QDs) The performance of solar energy conversion devices depends on the efficiency of photogenerated electrons injected from the photoexcited QDs to the wide-bandgap mesoporous electron transport layer. In particular, to fabricate high-performing QDSCs, understanding of separation and injection of photogenerated carriers (electron and hole) at the QD/scavenger interface is a key point. In core/shell QDs, the band alignment between core and shell materials depends on the nature of the constitute materials (type-I, reverse type-I, and type-II, as discussed above) and can be tuned by tailoring the core size and shell thickness. The tunable shell thickness of core/shell QDs allows us to precisely tune the carrier transfer rates from the photoexcited QDs to the carrier scavengers. In addition, there is a possibility of formation of quasi-type-II core/shell QD system with the increase of the shell thickness (1.5 nm up to tens of nm) in which one type of carrier is confined in the core, whereas the other is delocalized over the entire core/shell structure (Fig. 4d). This spatial separation reduces the carrier recombination and hence boosts the performance of QDSCs. In this context, Selopal et al. [112] designed and synthesized “giant” CdSe/CdS core/shell QDs with a CdSe core of 1.65 nm in size and a CdS shell of tunable shell thickness from 0.66 to 4.51 nm to investigate the carrier dynamics as a function of CdS shell thickness and photovoltaic performance of QDSCs based on respective “giant” CdSe/CdS core/shell QDs (Fig. 9a). Typically, the first CdSe core QDs were synthesized by the hot-injection approach. Then CdSe/CdS core/shell QDs were prepared by growing a thick CdS shell layer over CdSe core QDs by the SILAR approach at 240 ◦ C under continuous N2 flow. The diameter of the as-synthesized CdSe core QDs is 3.30 ± 0.29 nm (Fig. 9b). The final diameter of CdSe/CdS core/shell QDs increases with the growth of 2, 6, and 13 monolayers of CdS at 4.6 ± 0.5 nm, 7.2 ± 0.5 nm and 12.3 ± 1.1 nm, respectively (Fig. 9c–d). The electron (ψ e (r)) and hole (ψ h (r)) wave function of CdSe/CdS core/shell QDs with tunable CdSe shell thickness is calculated by solving the stationary Schrödinger equation in spherical geometry. Results demonstrated with the increase of CdS shell thickness, the ψ e (r) shows enhanced leakage of the electrons into the shell region, while the ψ h (r) displayed that the hole is confined in the CdSe core (Fig. 9e). This highlights that with the increase of CdS shell thickness, typeI CdSe/CdS core/shell QDs displays the quasi-type II band alignment and spatial overlap area between electron and hole wave functions decreasing from 94% for CdSe core QDs to 57% for “giant” CdSe/CdS core/shell QDs with CdS shell thickness of 4.51 nm. This significantly enhances the electron lifetime of “giant” CdSe/CdS core/shell QDs compared to CdSe core QDs, which is further confirmed by the trend of electron lifetime calculated from the transient PL decay [112]. To investigate the effect of the CdS shell layer on the carrier transfer of CdSe/CdS core/shell QDs, as-synthesized CdSe/CdS core/shell QDs with different shell thicknesses were grafted on mesoporous semiconductor metal oxide film,

Core/Shell Quantum-Dot-Sensitized Solar Cells

243

Fig. 9 (a) Schematic illustration of CdSe/CdS core/shell QDs with tunable CdS shell thickness (yellow color), with a constant CdSe core (red color) radius of 1.65 nm. TEM images of assynthesized: (b) CdSe core, (c) CdSe/CdS core/shell with 6-shell layers, and (d) CdSe/CdS core/shell with 13-shell layers. (e) Spatial probability distribution [ρ (r)] of electron and hole wave functions of the corresponding QDs as a function of the QD radius (R + H, nm). (f) PL intensity decay for the core/shell QDs of different shell thicknesses. Calculated carrier dynamics parameters as a function of CdS shell thickness, from the electron lifetime measurement of core/shell QDs: (g) electron lifetime and (h) electron transfer rate. (i) Current density–voltage curves of QDSCs based on CdSe/CdS core/shell QDs as a function of CdS shell thickness; (j) open-circuit voltage decay as the function of time; (k) electron lifetime as a function of Voc of the corresponding QDSCs. (Reproduced with permission from Ref [112]. Copyright 2017, John Wiley and Sons Ltd)

244

G. S. Selopal

measuring the transient PL decay rates (Fig. 9f). Results demonstrated that with the increase of CdS shell thickness from 0 to 4.51 nm, the electron lifetime increases from 20 ± 2 to 36 ± 1 ns, respectively (Fig. 9g). This enhanced the electron lifetime value with the growth of the CdS shell that is mainly ascribed to the reduced spatial electron–hole overlap due to the leakage of the electron in the CdS shell region and the hole confined in the CdSe core region. On the other hand, the electron transfer rate decreases from 2.2 ± 0.1 × 107 s−1 to 0.6 ± 0.1 × 107 s−1 with the CdS shell thickness increasing from 0.0 to 4.51 nm, respectively (Fig. 9h). This is mainly attributed to large barrier potential of the thick CdS shell and undesirable carrier confinement at the CdSe core and CdS shell interface due to large lattice mismatch of 4.4%. QDSCs were fabricated based on these CdSe/CdS core/shell QDs with tunable shell thickness from 0.66 to 4.51 nm. The photovoltaic performance of QDSCs was initially increasing with the increase in CdS shell thickness from 0.0 to 1.96 nm and then decreases with the further increase in CdS shell thickness from 1.96 to 4.51 nm (Fig. 9i). Briefly, QDSCs based on CdSe/CdS core/shell QDs with a shell thickness of 1.96 nm yield a PCE of 3.01% under one sun illumination (AM 1.5 G, 100 mWcm−2 ), which is significantly higher than bare CdSe (PCE = 1.22%). This significant improvement in the PCE is mainly attributed to reduced carrier recombination at the QD/TiO2 /electrolyte interface and enhanced electron lifetime (Fig. 9j–k) and broadening of the light absorption spectrum compared to CdSe core QDs [112]. Recently, Ghosh et al. [113] also synthesized CdSe/CdS core/shell QDs with tunable CdS shell thickness and investigated the modulation of carrier dynamics with shell thickness by using ultrafast transient absorption (TA) and correlated the PV performance of QDSCs. TA measurement demonstrated that with the increase in CdS shell thickness, carrier recombination reduces due to the decoupling of the electron–hole increases (electron delocalized in the shell region, while the hole confined in the core region) and carrier cooling time enhances from 500 fs (100%) for bare CdSe QDs to biexponential 1 ps (72%) and 6 ps (28%) for CdSe/CdS core/shell QDs (6ML). However, the electron transfer decreases from 750 fs (−67%) to 1.75 ps (−69%), 2.5 ps (−65%), and 3.5 ps (−63%) for CdSe, CdSe/CdS (2 ML), CdSe/CdS (4 ML), and CdSe/CdS (6 ML), respectively, as the CdS shell layer acts as a potential barrier for the electron transfer. Furthermore, carrier dynamics measurement of “giant” CdSe/CdS core/shell QDs (CdSe core = 1.65 nm and shell thickness = 4.3 nm) coupled with TiO2 demonstrated that the electron transfer rate is 1.5- to 1.9-fold lower than that of CdSe core QDs, whereas the hole transfer rate is four- to fivefold lower than that of CdSe core QDs [114]. This difference in the decrease in the value of electron transfer rate compared with the decrease in hole transfer rate for “giant” CdSe/CdS core/shell QDs leads to the formation of quasi-type-II band alignment [115, 116]. In addition to thick shell core/shell with constant core size, PbS/CdS core/shell QDs with different PbS core size and CdS shell thicknesses were synthesized [117]. Carrier dynamics of as-synthesized PbS/CdS core/shell QDs were investigated by coupling QDs with different types of wide-bandgap semiconductor metal oxide films such as SiO2 , TiO2 , and SnO2 and evaluated the trend of electron and hole transfer rates by using transient PL spectroscopy. Results demonstrated that the fast

Core/Shell Quantum-Dot-Sensitized Solar Cells

245

charge injection rate for QD-sensitized TiO2 mesoporous film is in the range of 110– 250 ns and for sensitized SnO2 mesoporous film is in the range of 100–170 ns for PbS core with diameters in the 3–4.2-nm range and CdS shell thickness of 0.3 nm of PbS/CdS core/shell QDs. Therefore, the optimization of the shell thickness and core size of core/shell QDs is a crucial factor to tune the carrier dynamics and broaden the absorption spectrum toward a longer wavelength with a suitable electron transfer rate and hole tunnelling through a shell barrier.

7.5 Core and Shell Interface Optimization A core/shell heterostructured system offers promising optoelectronic properties and superior photophysical/chemical stability due to reduced surface trap/defect states by the surface passivation of core QDs with the shell of different materials and thickness [114, 118–119]. However, core/shell QDs suffer from the formation of interfacial defects at the core and shell sharp interface during the shell growth due to lattice mismatch between the core and shell materials (e.g., 4.4% for CdSe and CdS) [120]. These interfacial defects cause the undesirable carrier recombination during the device operation and reduce the overall performance [121]. In addition, the growth of the shell layer over the core QDs reduces the electron and hole transfer rate as the shell layer acts as an energy barrier potential for the electron and hole injection process [122]. Thus, understanding and optimization of these interfacial strains between the core and shell are required to fabricate efficient core/shell QD-based solar cells. In this context, interfacial engineering of core/shell QDs by addition of an interfacial layer between the core and shell has attracted great attention to reduce the lattice mismatch and modulate the band alignment to improve the electron and hole transfer rate and reduce the undesirable carrier recombination. Recently, different types of core/alloyed layer/shell QD systems were synthesized and their structural and optoelectronic properties were studied through several spectroscopic and theoretical measurements. Among all, “giant” core/shell QDs with interfacial layers such as PbS/CdS/(CdS)n [123], CdSe/Pbx Cd1-x S/CdS [124], and CdSe/CdSex S1-x /CdS [125–128] are the most commonly studied due to their special feature and suitability for a broad range of applications in optoelectronic devices. These core/shell QDs with alloyed interfacial layers can be synthesized either by the combination of the cation exchange and SILAR approach or only by the SILAR approach depending on the final structure and size of QDs (Fig. 4). For example, “giant” PbS/CdS/(CdS)n core/shell/shell QDs were synthesized by the combination of the cation exchange and SILAR approach [123]. For the dualcolor-emission PbS/CdS/(CdS)n core/shell/shell QDs, the first PbS core QDs were synthesized by hot injection using OLA as ligands and purified using ethanol and re-dispersed in toluene. Then PbS/CdS core/thin-shell QDs were synthesized by two-step cation exchange from PbS QDs. After the growth of a thin CdS shell, a thick CdS shell layers were grown over the PbS/CdS core/thin-shell QDs via

246

G. S. Selopal

the SILAR approach by adding equal molar ratio Cd and S source precursors. On the other hand, single-color-emission core/shell/shell QDs were synthesized using the Cd and S precursors with a molar ratio of 1:0.8. Then PbS/CdS/CdS core/shell/shell QDs were washed with ethanol and re-dispersed in toluene for characterizations and device applications. Selopal et al. [126] also design and synthesized CdSe/(CdS)6 core/shell QDs and CdSe/CdSex S1−x /CdS core/shell QDs with different numbers of monolayers of interfacial layer CdSex S1−x (x = 0.5 or x = 0.9 ~ 0.1). Figure 10a–c displays the schematic of band alignment of CdSe/(CdS)6 core/shell QDs (denoted by CS) and CdSe/CdSex S1−x /CdS core/shell QDs with x = 0.5 or x = 0.9 ~ 0.1. Briefly, the first CdSe core QDs of diameter 3.3 nm were synthesized by hot injection. Then four cycles of CdSe1−x Sx -alloyed interfacial layer (x = 0.5) were grown over the CdSe core QDs via the SILAR approach by adding the mixture of 0.2 M Se and S in ODE instead of 0.2 M S. Finally, two monolayers of CdS shell were grown over the CdSe/(CdSe0.5 S0.5 )4 core/alloyed shell QDs via the SILAR approach at 240 ◦ C under continuous N2 flow to synthesize CdSe/(CdSe0.5 S0.5 )4 /(CdS)2 (denoted as CSA1). Similarly, CdSe/(CdSex S1−x )5 /(CdS)1 (denoted as CSA2) were synthesized via the SILAR approach by growing five interfacial layers of CdSex S1−x (x = 0.9 ~ 0.1) and one monolayer of CdS. It can be seen that the final diameter of as-synthesized CS core/shell QDs is 7.2 ± 0.5 nm, whereas for CSA1 core/alloyed shell/shell QDs is 7.4 ± 0.7 nm and CSA2 core/graded alloyed shell/shell QDs is 7.6 ± 1.3 nm (Fig. 10d–f). The clearly visible lattice fringes shown in the high-resolution TEM (HRTEM) image of corresponding QDs [inset of each Fig. 10d–f] confirm the high crystallinity of each QDs. The effect of the interface engineering on the carrier dynamics was carried out by transient PL spectroscopic decay measurements. Results demonstrated that addition of CdSex S1−x interfacial layers between the CdSe core and CdS shell leads not only to improve the electron and hole transfer rates (Fig. 10h–i), but also to broaden the absorption spectrum compared to the reference “giant” CdSe/CdS core/shell QDs. However, the electron lifetime of core/shell QDs decreases with the incorporation of CdSex S1−x interfacial layers from 29 ± 0.4 ns for CS QDs to 24 ± 0.4 ns and 14 ± 0.4 ns for CSA1 and CSA2 QDs (Fig. 10g). This decrease in the electron lifetime values is mainly attributed to the favorable band alignment of CSA1 and CSA2 QDs that allows the leakage of both electron and holes in the shell region, whereas in CS QDs only electrons leak into the shell region. This enhanced possibility of the holes leaking into the shell region is mainly attributed to reduced overall interfacial barrier potential with the incorporation of CdSex S1−x interfacial layers that could slow down or even block hole transfer from the CdSe core into the shell region. This leads to the fast electron–hole recombination, thus decreasing the overall electron lifetime values for CSA1 and CSA2 QDs compared to CS QDs. This increased electron and hole wave function overlapping was confirmed by the theoretical simulation. QDSCs were fabricated using these specially designed core/shell QDs as a light harvester. Resulting QDSCs yield a PCE of 3.08% for CS QDs, 5.52% for CSA1, and 7.14% for CSA2 QDs under one sun simulated sunlight (AM 1.5 G) (Fig. 10j), which is consistent with optoelectronic properties

Core/Shell Quantum-Dot-Sensitized Solar Cells

247

Fig. 10 Schematic illustration of internal interfacial structures and carrier confinement potentials of QDs: (a) CdSe/(CdS)6 (R = 1.65 nm, H = 1.96 nm), (b) CdSe/(CdSex S1−x )4 /(CdS)2 (x = 0.5 for all monolayers, R = 1.65 nm, H1 = 1.39 nm, H2 = 0.66 nm), and (c) CdSe/(CdSex S1−x )5 /(CdS)1 (x = 0.9–0.1, R = 1.65 nm, H3 = 0.82 nm, H4 = 0.33 nm). R is radius of CdSe QDs and H is the shell thickness. TEM images of as-synthesized QDs: (d) CdSe/(CdS)6 , (e) CdSe/(CdSex S1−x )4 /(CdS)2 , and (f) CdSe/(CdSex S1−x )5 /(CdS)1 . Inset of each figure displays the HR-TEM images of corresponding QDs. Comparison of the calculated parameters from the transient PL curves of each types of QDs: (g) electron lifetime, (h) electron transfer rate, and (i) hole transfer rate. (j) Comparison of current density versus voltage curves of QDSCs based on these QDs under one sun irradiation (AM 1.5G, 100 mW/cm−2 ); (k) electron lifetime (τ ) as a function of Voc , calculated from transient photovoltage decay measurements. (l) Variation of Jsc (mA/cm2 ) (black color, left) and PCE (%) (red color, right) of QDSCs with QD structure. (Reproduced with permission from Ref [126]. Copyright 2019 Elsevier)

of the respective QDs. The calculated electron lifetime of QDSCs from the transient photovoltage decay measurements confirms the reduced recombination in QDSCs by tailoring the structure of core/shell QDs via adding the alloyed and graded alloyed interfacial layers at the core/shell interface (Fig. 10k). The trend of Jsc and PCE as a function of the structure of different types of QDs is shown in Fig. 10l. Zhou et al. [129] reported the synthesis of CdS/CdSe core/shell QDs with a CdSx Se1−x interfacial layer between the CdS core and CdSe shell. Resulting graded alloyed CdS/CdSx Se1−x /CdSe core/shell QD-based QDSCs yield a PCE of 5.06%, which is 14% higher than the conventional CdS/CdSe core/shell QD-based QDSCs

248

G. S. Selopal

(4.41%). This enhanced photovoltaic performance of QDSCs is mainly attributed to improved charge transfer rate and reduced recombination with the passivation of the interfacial defects and interphase strains. Therefore, the results of these studies demonstrated that the interface engineering of core/shell QDs is a promising approach to tune the carrier dynamics of QDs without alerting their size and shape.

8 Conclusions In conclusion, this chapter presents an overview of the recent progress in the development of different types of colloidal core/shell QDs including type-I, typeII, and quasi-type-II core/shell QDs as promising light-harvesting materials to boost the performance of QDSCs. In particular, we described how the band alignment between core and shell materials plays a major role to tune the optoelectronic properties of QDs and to reduce the carrier dynamics within the QDs and at the QD/carrier scavenger (metal oxide and electrolyte) interfaces in QDSCs. Although, in the last few years, due to considerable development in materials design, device architecture and technology advance, the performance of QDSCs has reached above 13%. This record value of PCE is still lower than that of expected theoretical value due to several unrevealed recombination processes in the different components (mesoporous anode, electrolyte, and CEs) and interfaces of QDSCs (QDs/TiO2 or ZnO, QDs/electrolyte, electrolyte/TiO2 or ZnO, and CE/electrolyte). Thus, the optimization of each component and interfaces of QDSCs along with engineering of QDs is required to make further breakthroughs in the performance of QDSCs. Acknowledgments The author acknowledges funding support from the UESTC, China and INRS-EMT, Canada. The Author is greatful to Dr. Gopal Singh for his valuable discussion. The Author also thanks to Mrs. Ravneet Kaur for her support.

References 1. Bickerstaff, K., Walker, G.: Public understandings of air pollution: the ‘localisation’ of environmental risk. Glob. Environ. Chang. 11, 133–145 (2001) 2. Nayak, P.K., Garcia-Belmonte, G., Kahn, A., Bisquert, J., Cahen, D.: Photovoltaic efficiency limits and material disorder. Energy Environ. Sci. 5, 6022–6039 (2012) 3. Morton, O.: A new day dawning?: Silicon Valley sunrise. Nature. 443, 19–22 (2006) 4. Holdren, J.P.: Science and technology for sustainable well-being. Science. 319, 424–434 (2008) 5. Zhao, J., Wang, A., Greeen, M.A., Ferrazza, F.: 19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells. App. Phys. Lett. 73, 1991– 1993 (1998) 6. Pan, Z., Rao, H., Mora-Sero, I., Bisquert, J., Zhong, X.: Quantum dot-sensitized solar cells. Chem. Soc. Rev. 47, 7659–7702 (2018)

Core/Shell Quantum-Dot-Sensitized Solar Cells

249

7. Nozik, A.J., Beard, M.C., Luther, J.M., Law, M., Ellingson, R.J., Johnson, J.C.: Semiconductor quantum dots and quantum dot arrays and applications of multiple Exciton generation to third-generation photovoltaic solar cells. Chem. Rev. 110, 6873–6890 (2010) 8. Semonin, O.E., Luther, J.M., Choi, S., Chen, H.-Y., Gao, J., Nozik, A.J., Beard, M.C.: Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell. Science. 334, 1530–1533 (2011) 9. Kim, M.R., Ma, D.: Quantum-dot-based solar cells: recent advances, strategies, and challenges. J. Phys. Chem. Lett. 6, 85–99 (2015) 10. Wang, D., Yin, F., Du, Z., Han, D., Tang, J.: Recent progress in quantum dot-sensitized solar cells employing metal chalcogenides. J. Mater. Chem. A. 7, 26205–26226 (2019) 11. Gerischer, H., Lubke, M.: A particle size effect in the sensitization of TiO2 electrodes by a CdS deposit. J. Electroanal. Chem. 204, 225–227 (1986) 12. Vogel, R., Pohl, K., Weller, H.: Sensitization of highly porous, polycrystalline TiO2 electrodes by quantum sized CdS. Chem. Phys. Lett. 174, 241–246 (1990) 13. Zaban, A., Micic, O.I., Gregg, B.A., Nozik, A.J.: Photosensitization of Nanoporous TiO2 electrodes with InP quantum dots. Langmuir. 14, 3153–3156 (1998) 14. Yu, P., Zhu, K., Norman, A.G., Ferrere, S., Frank, A.J., Nozik, A.J.: Nanocrystalline TiO2 solar cells sensitized with InAs quantum dots. J. Phys. Chem. B. 110, 25451–25454 (2006) 15. Diguna, L.J., Shen, Q., Kobayashi, J., Toyoda, T.: High efficiency of CdSe quantum-dotsensitized TiO2 inverse opal solar cells. Appl. Phys. Lett. 91, 023116 (2007) 16. Lee, Y.-L., Huang, B.-M., Chien, H.-T.: Highly efficient CdSe-sensitized TiO2 photoelectrode for quantum-dot-sensitized solar cell applications. Chem. Mater. 20, 6903–6905 (2008) 17. Lee, Y.-L., Lo, Y.-S.: Highly efficient quantum-dot-sensitized solar cell based on cosensitization of CdS/CdSe. Adv. Funct. Mater. 19, 604–609 (2009) 18. Gimenez, S., Mora-Sero, I., Macor, L., Guijarro, N., Lana-Villarreal, T., Gomez, R., Diguna, L.J., Shen, Q., Toyoda, T., Bisquert, J.: Improving the performance of colloidal quantum-dotsensitized solar cells. Nanotechnology. 20, 295204 (2009) 19. Zhang, Q., Guo, X., Huang, X., Huang, S., Li, D., Luo, Y., Shen, Q., Toyoda, T., Meng, Q.: Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes. Phys. Chem. Chem. Phys. 13, 4659–4667 (2011) 20. Fang, B., Kim, M., Fan, S.-Q., Kim, J.H., Wilkinson, D.P., Ko, J., Yu, J.-S.: Facile synthesis of open mesoporous carbon nanofibers with tailored nanostructure as a highly efficient counter electrode in CdSe quantum-dot-sensitized solar cells. J. Mater. Chem. 21, 8742–8748 (2011) 21. Selopal, G.S., Concina, I., Milan, R., Natile, M.M., Sberveglieri, G., Vomiero, A.: Hierarchical self-assembled Cu2 S nanostructures: fast and reproducible spray deposition of effective counter electrodes for high efficiency quantum dot solar cells. Nano Energy. 6, 200–210 (2014) 22. Jiang, Y., Yu, B.B., Liu, J., Li, Z.H., Sun, J.K., Zhong, X.H., Hu, J.S., Song, W.G., Wan, L.J.: Boosting the open circuit voltage and fill factor of QDSSCs using hierarchically assembled ITO@Cu2 S nanowire array counter electrodes. Nano Lett. 15, 3088–3095 (2015) 23. Zhang, H., Wang, C., Peng, W.X., Yang, C., Zhong, X.H.: Quantum dot sensitized solar cells with efficiency up to 8.7% based on heavily copper-deficient copper selenide counter electrode. Nano Energy. 23, 60–69 (2016) 24. Wang, S.X., Shen, T., Bai, H.W., Li, B., Tian, J.J.: Cu3 Se2 nanostructure as a counter electrode for high efficiency quantum dot-sensitized solar cells. J. Mater. Chem. C. 4, 8020–8026 (2016) 25. Ghosh, D., Halder, G., Sahasrabudhe, A., Bhattacharyya, S.: A microwave synthesized Cux S and graphene oxide nanoribbon composite as a highly efficient counter electrode for quantum dot sensitized solar cells. Nanoscale. 8, 10632–10641 (2016) 26. Selopal, G.S., Chahine, R., Mohammadnezhad, M., Navarro-Pardo, F., Benetti, D., Zhao, H., Wang, Z.M., Rosei, F.: Highly efficient and stable spray assisted nanostructured Cu2 S/Carbon paper counter electrode for quantum dots sensitized solar cells. J. Power Sources. 436, 226849 (2019)

250

G. S. Selopal

27. Chakrapani, V., Baker, D., Kamat, P.V.: Understanding the role of the sulfide redox couple (S2–/Sn 2–) in quantum dot-sensitized solar cells. J. Am. Chem. Soc. 133, 9607–9615 (2011) 28. Radich, J.G., Dwyer, R., Kamat, P.V.: Cu2 S reduced graphene oxide composite for highefficiency quantum dot solar cells. Overcoming the redox limitations of S2 –/Sn 2– at the counter electrode. J. Phys. Chem. Lett. 2, 2453–2460 (2011) 29. Santra, P.K., Kamat, P.V.: Mn-doped quantum dot sensitized solar cells: a strategy to boost efficiency over 5% J. Am. Chem. Soc. 134, 2508–2511 (2012) 30. Wang, J., Mora-Seró, I., Pan, Z., Zhao, K., Zhang, H., Feng, Y., Yang, G., Zhong, X., Bisquert, J.: Core/shell colloidal quantum dot exciplex states for the development of highly efficient quantum-dot-sensitized solar cells. J. Am. Chem. Soc. 135, 15913–15922 (2013) 31. Pan, Z., Mora-Sero, I., Shen, Q., Zhang, H., Li, Y., Zhao, K., Wang, J., Zhong, X., Bisquert, J.: High-efficiency “green” quantum dot solar cells. J. Am. Chem. Soc. 136, 9203–9210 (2014) 32. Zhao, K., Pan, Z., Mora-Sero, I., Canovas, E., Wang, H., Song, Y., Gong, X., Wang, J., Bonn, M., Bisquert, J., Zhong, X.: Boosting power conversion efficiencies of quantum-dot-sensitized solar cells beyond 8% by recombination control. J. Am. Chem. Soc. 137, 5602–5609 (2015) 33. Du, Z., Pan, Z., Fabregat-Santiago, F., Zhao, K., Long, D., Zhang, H., Zhao, Y., Zhong, X., Yu, J., Bisquert, J.: Carbon counter-electrode-based quantum-dot-sensitized solar cells with certified efficiency exceeding 11%. J. Phys. Chem. Lett. 7, 3103–3111 (2016) 34. Jiao, S., Du, J., Du, Z., Long, D., Jiang, W., Pan, Z., Li, Y., Zhong, X.: Nitrogen-doped mesoporous carbons as counter electrodes in quantum dot sensitized solar cells with a conversion efficiency exceeding 12%. J. Phys. Chem. Lett. 8, 559–564 (2017) 35. Pan, Z., Yue, L., Rao, H., Zhang, J., Zhong, X., Zhu, Z., Jen, A.K.-Y.: Boosting the performance of environmentally friendly quantum dot-sensitized solar cells over 13% efficiency by dual sensitizers with cascade energy structure. Adv. Mater. 31, 1903696 (2019) 36. Plass, R., Pelet, S., Krueger, J., Grätzel, M., Bach, U.: Quantum dot sensitization of organic−inorganic hybrid solar cells. J. Phys. Chem. B. 106, 7578–7580 (2002) 37. Pan, Z., Zhao, K., Wang, J., Zhang, H., Feng, Y., Zhong, X.: Near infrared absorption of CdSex Te1–x alloyed quantum dot sensitized solar cells with more than 6% efficiency and high stability. ACS Nano. 7, 5215–5222 (2013) 38. Ren, Z., Wang, J., Pan, Z., Zhao, K., Zhang, H., Li, Y., Zhao, Y., Mora-Sero, I., Bisquert, J., Zhong, X.: Amorphous TiO2 buffer layer boosts efficiency of quantum dot sensitized solar cells to over 9%. Chem. Mater. 27, 8398–8405 (2015) 39. Ren, Z., Wang, Z., Wang, R., Pan, Z., Gong, X., Zhong, X.: Effects of metal oxyhydroxide coatings on photoanode in quantum dot sensitized solar cells. Chem. Mater. 28, 2323–2330 (2016) 40. Du, J., Du, Z., Hu, J.S., Pan, Z., Shen, Q., Sun, J., Long, D., Dong, H., Sun, L., Zhong, X., Wan, L.J.: Zn-Cu-In-Se quantum dot solar cells with a certified power conversion efficiency of 11.6%. J. Am. Chem. Soc. 138, 4201–4209 (2016) 41. Wang, W., Feng, W., Du, J., Xue, W., Zhang, L., Zhao, L., Li, Y., Zhong, X.: Cosensitized quantum dot solar cells with conversion efficiency over 12%. Adv. Mater. 30, 1705746 (2018) 42. O’Regan, B., Grätzel, M.: A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature. 353, 737–740 (1991) 43. Bai, Y., Mora-Seró, I., De Angelis, F., Bisquert, J., Wang, P.: Titanium dioxide nanomaterials for photovoltaic applications. Chem. Rev. 114, 10095–10130 (2014) 44. Chen, C., Xie, Y., Ali, G., Yoo, S.H., Cho, S.O.: Improved conversion efficiency of CdS quantum dots-sensitized TiO2 nanotube array using ZnO energy barrier layer. Nanotechnology. 22, 015202 (2011) 45. Liu, B., Sun, Y., Wang, X., Zhang, L., Wang, D., Fu, Z., Lin, Y., Xie, T.: Branched hierarchical photoanode of anatase TiO2 nanotubes on rutile TiO2 nanorod arrays for efficient quantum dot-sensitized solar cells. J. Mater. Chem. A. 3, 4445–4452 (2015) 46. Qiu, Q., Li, S., Jiang, J., Wang, D., Lin, Y., Xie, T.: Improved Electron transfer between TiO2 and FTO interface by N-doped anatase TiO2 nanowires and its applications in quantum dot-sensitized solar cells. J. Phys. Chem. C. 121, 21560–21570 (2017)

Core/Shell Quantum-Dot-Sensitized Solar Cells

251

47. Zhang, Q., Dandeneau, C.S., Zhou, X., Cao, G.: Zno nanostructures for dye-sensitized solar cells. Adv. Mater. 21, 4087–4108 (2009) 48. Djurisic, A.B., Chen, X., Leung, Y.H., Ng, A.M.C.: ZnO nanostructures: growth, properties and applications. J. Mater. Chem. 22, 6526–6535 (2012) 49. Xu, J., Yang, X., Yang, Q.-D., Wong, T.-L., Lee, S.-T., Zhang, W.-J., Lee, C.-S.: Arrays of CdSe sensitized ZnO/ZnSe nanocables for efficient solar cells with high open-circuit voltage. J. Mater. Chem. 22, 13374–13379 (2012) 50. Park, N.G., Kang, M.G., Ryu, K.S., Kim, K.M., Chang, S.H.: Photovoltaic characteristics of dye-sensitized surface-modified nanocrystalline SnO2 solar cells. J. Photochem. Photobiol. A. 161, 105–110 (2004) 51. Xiong, L., Guo, Y., Wen, J., Liu, H., Yang, G., Qin, P., Fang, G.: Review on the application of SnO2 in perovskite solar cells. Adv. Funct. Mater. 28, 1802757 (2018) 52. Chava, R.K., Kang, M.: Ag2 S quantum dot sensitized zinc oxide photoanodes for environment friendly photovoltaic devices. Mater. Lett. 199, 188–191 (2017) 53. Yang, J., Zhong, X.: CdTe based quantum dot sensitized solar cells with efficiency exceeding 7% fabricated from quantum dots prepared in aqueous media. J. Mater. Chem. A. 4, 16553– 16561 (2016) 54. Wang, J., Li, Y., Shen, Q., Izuishi, T., Pan, Z., Zhao, K., Zhong, X.: Mn doped quantum dot sensitized solar cells with power conversion efficiency exceeding 9%. J. Mater. Chem. A. 4, 877–886 (2016) 55. Yang, J.W., Wang, J., Zhao, K., Izuishi, T., Li, Y., Shen, Q., Zhong, X.H.: CdSeTe/CdS type-I Core/Shell quantum dot sensitized solar cells with efficiency over 9%. J. Phys. Chem. C. 119, 28800–28808 (2015) 56. Li, L., Yang, X.C., Gao, J.J., Tianand, H.N., Zhao, J.Z.: Highly efficient CdS quantum dotsensitized solar cells based on a modified polysulfide electrolyte. J. Am. Chem. Soc. 133, 8458–8460 (2011) 57. Evangelista, R.M., Makuta, S., Yonezu, S., Andrews, J., Tachibana, Y.: Semiconductor quantum dot sensitized solar cells based on ferricyanide/ferrocyanide redox electrolyte reaching an open circuit photovoltage of 0.8 V. ACS Appl. Mater. Interfaces. 8, 13957–13965 (2016) 58. Choi, H., Nicolaescu, R., Paek, S., Ko, J., Kamat, P.V.: Supersensitization of CdS quantum dots with a near-infrared organic dye: toward the design of panchromatic hybrid-sensitized solar cells. ACS Nano. 5, 9238–9245 (2011) 59. Levy-Clement, C., Tena-Zaera, R., Ryan, M.A., Katty, A., Hodes, G.: CdSe-sensitized pCuSCN/nanowire n-ZnO heterojunctions. Adv. Mater. 17, 1512–1515 (2005) 60. Im, S.H., Lim, C.S., Chang, J.A., Lee, Y.H., Maiti, N., Kim, H.J., Nazeeruddin, M.K., Grätzel, M., Seok, S.I.: Toward interaction of sensitizer and functional moieties in hole-transporting materials for efficient semiconductor-sensitized solar cells. Nano Lett. 11, 4789–4793 (2011) 61. Park, J., Heo, J., Im, S., Kim, S.: Highly efficient solid-state mesoscopic PbS with embedded CuS quantum dot-sensitized solar cells. J. Mater. Chem. A. 4, 785–790 (2016) 62. Quan, L., Li, W., Zhu, L., Geng, H., Changand, X., Liu, H.: A new in-situ preparation method to FeS counter electrode for quantum dots-sensitized solar cells. J. Power Sources. 272, 546– 553 (2014) 63. Faber, M., Park, K., Caban-Acevedo, M., Santra, P., Jin, S.: Earth-abundant cobalt pyrite (CoS2 ) thin film on glass as a robust, high-performance counter electrode for quantum dotsensitized solar cells. J. Phys. Chem. Lett. 4, 1843–1849 (2013) 64. Chen, X., Li, Z., Bai, Y., Sun, Q., Wangand, L., Dou, S.: Room-temperature synthesis of Cu2−x E (E=S, se) nanotubes with hierarchical architecture as high-performance counter electrodes of quantum-dot-sensitized solar cells. Chemistry. 21, 1055–1063 (2015) 65. Yang, Y., Zhu, L., Sun, H., Huang, X., Luo, Y., Li, M., Meng, Q.: Composite counter electrode based on nanoparticulate PbS and carbon black: towards quantum dot-sensitized solar cells with both high efficiency and stability. ACS Appl. Mater. Interfaces. 4, 6162–6168 (2012)

252

G. S. Selopal

66. Zhang, X., Huang, X., Yang, Y., Wang, S., Gong, Y., Luo, Y., Li, D., Meng, Q.: Investigation on new CuInS2 /carbon composite counter electrodes for CdS/CdSe cosensitized solar cells. ACS Appl. Mater. Interfaces. 5, 5954–5960 (2013) 67. Luque, A., Hegedus, S.: Handbook of photovoltaic science and engineering. Wiley (2011) 68. Katoh, R., Furube, A.: Electron injection efficiency in dye-sensitized solar cells. J Photochem Photobiol C: Photochem Rev. 20, 1–16 (2014) 69. Hagfeldt, A., Gratzel, M.: Light-induced redox reactions in nanocrystalline systems. Chem. Rev. 95, 49–68 (1995) 70. Benkstein, K.D., Kopidakis, N., Van de Lagemaat, J., Frank, A.J.: Influence of the percolation network geometry on electron transport in dye-sensitized titanium dioxide solar cells. J. Phys. Chem. B. 107, 7759–7767 (2003) 71. Hagfeldt, A., Grätzel, M.: Molecular photovoltaics. Acc. Chem. Res. 33, 269–277 (2000) 72. Mora-Sero, I., Gimenez, S., Fabregat-Santiago, F., Gomez, R., Shen, Q., Toyoda, T., Bisquert, J.: Recombination in quantum dot sensitized solar cells. Acc. Chem. Res. 42, 1848–1857 (2009) 73. Hines, D.A., Forrest, R.P., Corcelli, S.A., Kamat, P.V.: Predicting the rate constant of electron tunneling reactions at the CdSe–TiO2 interface. J. Phys. Chem. B. 119, 7439–7446 (2015) 74. Zhao, K., Pan, Z.X., Zhong, X.H.: Carbon counter-electrode-based quantum-dot-sensitized solar cells with certified efficiency exceeding 11%. J. Phys. Chem. Lett. 7, 406–417 (2016) 75. Gentilini, D., D’Ercole, D., Gagliardi, A., Brunetti, A., Reale, A., Brown, T., Di Carlo, A.: Analysis and simulation of incident photon to current efficiency in dye sensitized solar cells. Superlattice. Microst. 47, 192–196 (2010) 76. Sanehira, E.M., Marshall, A.R., Christians, J.A., Harvey, S.P., Ciesielski, P.N., Wheeler, L.M., Schulz, P., Lin, L.Y., Beard, M.C., Luther, J.M.: Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells. Sci Adv. 3, eaao4204 (2017) 77. Hetsch, F., Xu, X.Q., Wang, H.K., Kershaw, S.V., Rogach, A.L.: Semiconductor nanocrystal quantum dots as solar cell components and photosensitizers: material, charge transfer, and separation aspects of some device topologies. J. Phys. Chem. Lett. 2, 1879–1887 (2011) 78. Pan, Z.X., Zhang, H., Cheng, K., Hou, Y.M., Hua, J.L., Zhong, X.H.: Highly efficient inverted type-I CdS/CdSe Core/Shell structure QD-sensitized solar cells. ACS Nano. 6, 3982–3991 (2012) 79. Peng, X.G., Schlamp, M.C., Kadavanich, A.V., Alivisatos, A.P.: Epitaxial growth of highly luminescent CdSe/CdS core/shell nanocrystals with photostability and electronic accessibility. J. Am. Chem. Soc. 119, 7019–7029 (1997) 80. Reiss, P., Protiere, M., Li, L.: Core/Shell semiconductor nanocrystals. Small. 5, 154–168 (2009) 81. Smith, A.M., Nie, S.M.: Semiconductor nanocrystals: structure, properties, and band gap engineering. Acc. Chem. Res. 43, 190–200 (2010) 82. Chaudhuri, R.G., Paria, S.: Core/shell nanoparticles: classes, properties, synthesis mechanisms, characterization, and applications. Chem. Rev. 112, 2373–2433 (2012) 83. Zhu, H.M., Song, N.H., Lian, T.Q.: Controlling charge separation and recombination rates in CdSe/ZnS Type I core−shell quantum dots by shell thicknesses. J. Am. Chem. Soc. 133, 8762–8771 (2011) 84. De Geyter, B., Justo, Y., Moreels, I., Lambert, K., Smet, P.F., Van Thourhout, D., Houtepen, A.J., Grodzinska, D., Donega, C.D., Meijerink, A., Vanmaekelbergh, D., Hens, Z.: The different nature of band edge absorption and emission in colloidal PbSe/CdSe core/shell quantum dots. ACS Nano. 5, 58–66 (2011) 85. Dabbousi, B.O., RodriguezViejo, J., Mikulec, F.V., Heine, J.R., Mattoussi, H., Ober, R., Jensen, K.F., Bawendi, M.G.: (CdSe)ZnS Core−Shell quantum dots: synthesis and characterization of a size series of highly luminescent nanocrystallites. J. Phys. Chem. B. 101, 9463–9475 (1997) 86. Lambert, K., De Geyter, B., Moreels, I., Hens, Z.: PbTe|CdTe core|shell particles by cation exchange, a HR-TEM study. Chem. Mater. 21, 778–780 (2009)

Core/Shell Quantum-Dot-Sensitized Solar Cells

253

87. Huang, K., Demadrille, R., Silly, M.G., Sirotti, F., Reiss, P., Renault, O.: Internal structure of InP/ZnS nanocrystals unraveled by high-resolution soft X-ray photoelectron spectroscopy. ACS Nano. 4, 4799–4805 (2010) 88. Ivanov, S.A., Piryatinski, A., Nanda, J., Tretiak, S., Zavadil, K.R., Wallace, W.O., Werder, D., Klimov, V.I.: Type-II core/shell CdS/ZnSe nanocrystals: synthesis, electronic structures, and spectroscopic properties. J. Am. Chem. Soc. 129, 11708–11719 (2007) 89. Chen, Y., Vela, J., Htoon, H., Casson, J.L., Werder, D.J., Bussian, D.A., Klimov, V.I., Hollingsworth, J.A.: “Giant” multishell CdSe nanocrystal quantum dots with suppressed blinking. J. Am. Chem. Soc. 130, 5026–5027 (2008) 90. Shariati, M.R., Samadi-Maybodi, A., Colagar, A.H.: Dual co-catalyst loaded reverse type-I core/shell quantum dots for photocatalytic antibacterial applications. J. Mater. Chem. A. 6, 20433–20443 (2018) 91. Balet, L.P., Ivanov, S.A., Piryatinski, A., Achermann, M., Klimov, V.I.: Inverted core/shell nanocrystals continuously tunable between type-I and type-II localization regimes. Nano Lett. 4, 1485–1488 (2004) 92. Kim, S., Park, J., Kim, T., Jang, E., Jun, S., Jang, H., Kim, B., Kim, S.W.: Reverse Type-I ZnSe/InP/ZnS core/shell/shell nanocrystals: cadmium-free quantum dots for visible luminescence. Small. 7, 70–73 (2010) 93. Jones, M., Kumar, S., Lo, S.S., Scholes, G.D.: Exciton trapping and recombination in Type II CdSe/CdTe nanorod heterostructures. J. Phys. Chem. C. 112, 5423–5431 (2008) 94. Dorfs, D., Franzl, T., Osovsky, R., Brumer, M., Lifshitz, E., Klar, T.A., Eychmuller, A.: TypeI and type-II nanoscale heterostructures based on CdTe nanocrystals: a comparative study. Small. 4, 1148–1152 (2008) 95. Piryatinski, A., Ivanov, S.A., Tretiak, S., Klimov, V.I.: Effect of quantum and dielectric confinement on the exciton−exciton interaction energy in Type II core/shell semiconductor nanocrystals. Nano Lett. 7, 108–115 (2007) 96. Tong, X., Zhou, Y.F., Jin, L., Basu, K., Adhikari, R., Selopal, G.S., Tong, X., Zhao, H.G., Sun, S.H., Vomiero, A., Wang, Z.M., Rosei, F.: Heavy metal-free, near-infrared colloidal quantum dots for efficient photoelectrochemical hydrogen generation. Nano Energy. 31, 441– 449 (2017) 97. Peng, Z.A., Peng, X.G.: Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor. J. Am. Chem. Soc. 123, 183–184 (2001) 98. Hines, M.A., Scholes, G.D.: Colloidal PbS nanocrystals with size-tunable near-infrared emission: observation of post-synthesis self-narrowing of the particle size distribution. Adv. Mater. 15, 1844–1849 (2003) 99. Ning, Z., Tian, H., Qin, H., Zhang, Q., Ågren, H., Sun, L., Fu, Y.: Wave-function engineering of CdSe/CdS core/shell quantum dots for enhanced electron transfer to a TiO2 substrate. J. Phys. Chem. C. 114, 15184–15189 (2010) 100. Lee, H.J., Bang, J., Park, J., Kim, S., Park, S.-M.: Multilayered semiconductor (CdS/CdSe/ZnS)-sensitized TiO2 mesoporous solar cells: all prepared by successive ionic layer adsorption and reaction processes. Chem. Mater. 22, 5636–5643 (2010) 101. Yang, Z., Chen, C.-Y., Liu, C.-W., Chang, H.-T.: Electrocatalytic sulfur electrodes for CdS/CdSe quantum dot-sensitized solar cells. Chem. Commun. 46, 5485–5487 (2010) 102. Hossain, M.A., Jennings, J.R., Koh, Z.Y., Wang, Q.: Carrier generation and collection in CdS/CdSe-sensitized SnO2 solar cells exhibiting unprecedented photocurrent densities. ACS Nano. 5, 3172–3181 (2011) 103. Zhu, G., Pan, L., Xu, T., Sun, Z.: CdS/CdSe-cosensitized TiO2 photoanode for quantum-dotsensitized solar cells by a microwave-assisted chemical bath deposition method. ACS Appl. Mater. Interfaces. 3, 3146–3151 (2011) 104. Maity, P., Debnath, T., Ghosh, H.N.: Ultrafast charge carrier delocalization in CdSe/CdS Quasi-Type II and CdS/CdSe inverted Type I core–shell: a structural analysis through carrierquenching study. J. Phys. Chem. C. 119, 26202–26211 (2015) 105. Kim, S., Fisher, B., Eisler, H.J., Bawendi, M.: Type-II quantum dots: CdTe/CdSe (core/shell) and CdSe/ZnTe (core/shell) heterostructures. J. Am. Chem. Soc. 125, 11466–11467 (2003)

254

G. S. Selopal

106. Lo, S.S., Mirkovic, T., Chuang, C.H., Burda, C., Scholes, G.D.: Emergent properties resulting from type-II band alignment in semiconductor nanoheterostructures. Adv. Mater. 23, 180–197 (2011) 107. Scholes, G.D., Jones, M., Kumar, S.: Energetics of photoinduced electron-transfer reactions decided by quantum confinement. J. Phys. Chem. C. 111, 13777–13785 (2007) 108. Ning, Z., Tian, H., Yuan, C., Fu, Y., Qin, H., Sun, L., Agren, H.: Solar cells sensitized with type-II ZnSe-CdS core/shell colloidal quantum dots. Chem. Commun. 47, 1536–1538 (2011) 109. Luo, J., Wei, H., Li, F., Huang, Q., Li, D., Luo, Y., Meng, Q.: Microwave assisted aqueous synthesis of core-shell CdSe(x)Te(1-x)-CdS quantum dots for high performance sensitized solar cells. Chem. Commun. 50, 3464–3466 (2014) 110. Bang, J., Park, J., Lee, J.H., Won, N., Nam, J., Lim, J., Chang, B.Y., Lee, H.J., Chon, B., Shin, J., Park, J.B., Choi, J.H., Cho, K., Park, S.M., Joo, T., Kim, S.: ZnTe/ZnSe (core/shell) Type-II quantum dots: their optical and photovoltaic properties. Chem. Mater. 22, 233–240 (2010) 111. Jiao, S., Shen, Q., Mora-Seró, I., Wang, J., Pan, Z., Zhao, K., Kuga, Y., Zhong, X., Bisquert, J.: Band engineering in core/shell ZnTe/CdSe for photovoltage and efficiency enhancement in exciplex quantum dot sensitized solar cells. ACS Nano. 9, 908–915 (2015) 112. Selopal, G.S., Zhao, H.G., Tong, X., Benetti, D., Navarro-Pardo, F., Zhou, Y.F., Barba, D., Vidal, F., Wang, Z.M., Rosei, F.: Highly stable colloidal “giant” quantum dots sensitized solar cells. Adv. Funct. Mater. 27, 1701468 (2017) 113. Dana, J., Maiti, S., Tripathi, V.S., Ghosh, H.N.: Direct correlation of excitonics with efficiency in a core–shell quantum dot solar cell. Chemistry. 24, 2418–2425 (2018) 114. Adhikari, R., Jin, L., Navarro-Pardo, F., Benetti, D., AlOtaibi, B., Vanka, S., Zhao, H.G., Mi, Z.T., Vomiero, A., Rosei, F.: High efficiency, Pt-free photoelectrochemical cells for solar hydrogen generation based on “giant” quantum dots. Nano Energy. 27, 265–274 (2016) 115. Brovelli, S., Schaller, R.D., Crooker, S.A., Garcia-Santamaria, F., Chen, Y., Viswanatha, R., Hollingsworth, J.A., Htoon, H., Klimov, V.I.: Nano-engineered electron–hole exchange interaction controls exciton dynamics in core–shell semiconductor nanocrystals. Nat. Commun. 2, 280 (2011) 116. Abdi, F.F., Van de Krol, R.: Nature and light dependence of bulk recombination in Co-PiCatalyzed BiVO4 photoanodes. J. Phys. Chem. C. 116, 9398–9404 (2012) 117. Zhao, H., Fan, Z., Liang, H., Selopal, G.S., Gonfa, B.A., Jin, L., Soudi, A., Cui, D., Enrichi, F., Natile, M.M., Concina, I., Ma, D., Govorov, A.O., Rosei, F., Vomiero, A.: Controlling photoinduced electron transfer from PbS@CdS core@shell quantum dots to metal oxide nanostructured thin films. Nanoscale. 6, 7004–7011 (2014) 118. Pal, B.N., Ghosh, Y., Brovelli, S., Laocharoensuk, R., Klimov, V.I., Hollingsworth, J.A., Htoon, H.: ‘Giant’ CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: strong influence of shell thickness on light-emitting diode performance. Nano Lett. 12, 331 (2012) 119. Navarro-Pardo, F., Zhao, H., Wang, Z.M., Rosei, F.: Structure/property relations in “giant” semiconductor nanocrystals: opportunities in photonics and electronics. Acc. Chem. Res. 51, 609–618 (2017) 120. Smith, A.M., Mohs, A.M., Nie, S.: Tuning the optical and electronic properties of colloidal nanocrystals by lattice strain. Nat. Nanotechnol. 4, 56–63 (2009) 121. Pinchetti, V., Meinardi, F., Camellini, A., Sirigu, G., Christodoulou, S., Bae, W.K., De Donato, F., Manna, L., Zavelani-Rossi, M., Moreels, I., Klimov, V.I., Brovelli, S.: Effect of core/shell interface on carrier dynamics and optical gain properties of dual-color emitting CdSe/CdS nanocrystals. ACS Nano. 10, 6877–6887 (2016) 122. Jiao, S., Wang, J., Shen, Q., Li, Y., Zhong, Z.H.: Surface engineering of PbS quantum dot sensitized solar cells with a conversion efficiency exceeding 7%. J. Mater. Chem. A. 4, 7214– 7221 (2016)

Core/Shell Quantum-Dot-Sensitized Solar Cells

255

123. Jin, L., Sirigu, G., Tong, X., Camellini, A., Parisini, A., Nicotra, G., Spinella, C., Zhao, H.G., Sun, S.H., Morandi, V., Zavelani-Rossi, M., Rosei, F., Vomiero, A.: Engineering interfacial structure in “giant” PbS/CdS quantum dots for photoelectrochemical solar energy conversion. Nano Energy. 30, 531–541 (2016) 124. Adhikari, R., Basu, K., Zhou, Y.F., Vetrone, F., Ma, D.L., Sun, S.H., Vidal, F., Zhao, H.G., Rosei, F.: Heterostructured quantum dot architectures for efficient and stable photoelectrochemical hydrogen production. J. Mater. Chem. A. 6, 6822–6829 (2018) 125. Bae, W.K., Padilha, L.A., Park, Y.S., McDaniel, H., Robel, I., Pietryga, J.M., Klimov, V.I.: Controlled alloying of the core–shell interface in CdSe/CdS quantum dots for suppression of Auger recombination. ACS Nano. 7, 3411–3419 (2013) 126. Selopal, G.S., Zhao, H.G., Liu, G.J., Zhang, H., Tong, X., Wang, K.H., Tang, J., Sun, X.H., Sun, S.H., Vidal, F., Wang, Y.Q., Wang, Z.M., Rosei, F.: Interfacial engineering in colloidal “giant” quantum dots for high-performance photovoltaics. Nano Energy. 55, 377–388 (2019) 127. Zhao, H.G., Liu, G.J., Vidal, F., Wang, Y.Q., Vomiero, A.: Colloidal thick-shell pyramidal quantum dots for efficient hydrogen production. Nano Energy. 53, 116–124 (2018) 128. Zhao, H.G., Liu, J.B., Vidal, F., Vomiero, A., Rosei, F.: Tailoring the interfacial structure of colloidal “giant” quantum dots for optoelectronic applications. Nanoscale. 10, 17189–17197 (2018) 129. Zhou, R., Wan, L., Niu, H.H., Yang, L., Mao, X.L., Zhang, Q.F., Miao, S.D., Xu, J.Z., Cao, G.Z.: Tailoring band structure of ternary CdSx Se1-x quantum dots for highly efficient sensitized solar cells. Sol. Energy Mater. Sol. Cells. 155, 20–29 (2016)

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells Ali Imran Channa, Xin Li, Xin Tong, and Zhiming M. Wang

Abstract Colloidal quantum dots (QDs) are promising for a variety of optoelectronic applications due to their size/shape/composition optical properties. Among various QDs, the core/shell QDs have demonstrated improved optical properties and enhanced photo-/chemical stability with respect to the bare QDs. Specifically, the band structure of the core/shell QDs can be tailored by choosing the appropriate core and shell material, changing the chemical composition, or varying the shell thickness. In this chapter, we review the recent advances of the synthetic approaches, optical properties engineering, and charge dynamics of core/shell QDs with type I, type II, and quasi-type II band structure and highlight their application in solar-driven photoelectrochemical (PEC) cells. The device performance of these core/shell QD-based PEC cells is discussed in detail and their prospective developments toward future device optimizations and commercialization are proposed as well. Keywords Colloidal quantum dots · Core/shell architecture · Band structure engineering · Photoelectrochemical cell · Hydrogen generation

1 Introduction The energy consumption of the modern world has been increasing steadily over the past few decades [1, 2]. The use of fossil fuels may not be able to meet the required energy needs in an environmentally sustainable way for decades to

Author Contribution Authors Ali Imran Channa and Xin Li have been equally contributed to this chapter. A. I. Channa · X. Li · X. Tong () · Z. M. Wang () Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, People’s Republic of China e-mail: [email protected]; [email protected] © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 X. Tong, Z. M. Wang (eds.), Core/Shell Quantum Dots, Lecture Notes in Nanoscale Science and Technology 28, https://doi.org/10.1007/978-3-030-46596-4_8

257

258

A. I. Channa et al.

come. The solar energy incident on earth in 1 h is more than the total energy consumed by the world in 1 year, suggesting that the solar energy conversion is a promising approach to address the future energy needs [2–4]. Hydrogen is a promising high-density fuel with no toxic effects on the environment [5, 6]. During the combustion of hydrogen, water is the only by-product that is generated. Solardriven hydrogen generation via water splitting has become one of the hot topics after the pioneering work by Fujishima and Honda [7]. Fujishima et al. have reported that a semiconductor material (i.e., TiO2 ) could be used for the production of hydrogen via photoelectrochemical (PEC) water splitting [7]. TiO2 possesses numerous merits such as considerable photo-/chemical stability, inertness toward biological/chemical corrosion, and nontoxicity and cost-effectiveness [8]. However, due to the wide bandgap of TiO2 (~3.2 eV), the solar energy conversion was limited to the ultraviolet (UV) region of the solar spectrum [7]. To establish a sustainable and efficient solar energy conversion system, semiconductor materials with a narrow bandgap and broad light absorption spectrum are required [9, 10]. Moreover, efficient charge carrier separation/transfer and long-term stability are vital parameters to achieve high-efficiency solar energy conversion [11, 12]. Semiconductor colloidal quantum dots (QDs) are nanocrystals with size typically smaller than ~20 nm with numerous outstanding features such as size-dependent optical bandgap due to quantum confinement effect [13], high absorption coefficients [14, 15], cost-effective synthesis [16], facile and large-scale production, etc. [17, 18]. To date, colloidal QDs have been applied for various applications such as QD-sensitized solar cells (QDSCs) [19], light-emitting diodes (LEDs) [20], photodetectors [21], luminescent solar concentrators (LSCs) [22], and PEC cells for hydrogen generation [23]. Among the various types of QDs, the core/shell QDs have been demonstrated as promising building blocks in applications due to their favorable physical/chemical and optical properties [24]. In general, the surface of the bare QD is very sensitive to the ambient environment (light, moisture, oxygen, temperature, etc.); such sensitivity can induce the dangling/broken bonds, leading to the degradation of QDs and affecting its chemical stability as well as the optical properties [25, 26]. The organic ligands on the surface of bare QDs can offer a good degree of surface passivation. However, the stability of organic ligand passivated QDs is also dependent on the ligand stability itself, which cannot guarantee longterm stability [27]. During various QD-based device fabrication processes, the organic ligands can be damaged; hence, the ligands offer inefficient passivation of both anionic and cationic surface sites. Moreover, the surface-related defects/traps can result in non-radiative charge recombination for poor charge separation/transfer capability. This effect is detrimental for device applications which require better charge separation and transfer characteristics as well as for the long-term stability of QDs [28]. An elegant approach to solve this problem is to overcoat the bare QD’s surface with a second inorganic semiconductor material, to form core/shell structured QDs, wherein the bare QDs are effectively passivated through an inorganic shell. Several types of colloidal core/shell QDs have been developed with different morphologies including spherical core/shell, dot-in-rod, and rod/rod core/shell QDs [29]. In

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells

259

Fig. 1 Schematic diagrams of type I, type II, and quasi-type II band structures. (Adapted from Ref. [31])

contrast to the bare QDs, core/shell QDs allow further tuning of the physical properties, such as the photoluminescence quantum yield (PLQY) and excitonic lifetime, which are considered as important parameters for the application of QDs in optoelectronic devices. The band alignment of the two materials (i.e., core and shell) can be tuned and different carrier-localization regions can be obtained which affects the QD’s optoelectronic properties [30]. Hence, different kinds of bandengineered core/shell QDs can be targeted for diverse device applications. The typical band structures of core/shell QDs with type I, type II, and quasi-type II band alignment (as shown in Fig. 1) can be obtained via tuning the core size, chemical composition, or thickness of the shell. The electronic band structure of bare QDs is simple and the excitations or de-excitations of the charge carriers occur within the bandgap. However, the growth of the shell on the bare QDs alters the band structure and creates new pathways for the de-excitation process of charge carriers. For instance, in type I core/shell QDs, a wide bandgap semiconductor material is coated on the core of the narrow-bandgap material, in which the conduction band (CB) and valence band (VB) of the core reside within the bandgap of the shell material. Therefore, the excitation and de-excitation of charge carriers occur within the bandgap of the core [31]. In the type II band structure, the CB and the VB of the shell stagger with that of the core, allowing the spatial separation of electrons and holes, with holes residing in the core and electrons into the shell [32]. This type of core/shell structure often exhibits long-lived lifetime of excitons [33]. Quasi-type II band alignment is a special case in which either the CB or the VB of the shell and core materials possess a small band offset. This band structure allows the existence of one type of charge carrier in the entire core/shell structure and another type of carrier confined in the core [34, 35]. Core/shell QDs with both type II and quasi-type II band structures have demonstrated efficient spatial separation of photoinduced electrons and holes, leading to efficient charge separation and transfer in QD-based PEC systems, which are beneficial to boost the efficiency of QD-based solar-driven PEC cells for hydrogen generation [36, 37].

260

A. I. Channa et al.

1.1 Conversion of Solar Energy into Hydrogen The overall world’s energy resources include fossil fuel, nuclear fuel, and the renewable ones. Among these sources, the renewable energy resources are available throughout the years. However, nonrenewable energy resources are predicted to be eventually depleted. In addition, the immense use of nonrenewable sources has polluted the environment by producing huge quantity of toxic gases in the atmosphere (e.g., COx , NOx , SOx , Cx Hy ) and radioactive pollution [38]. Therefore, the hazardous effects of nonrenewable energy sources as well as their risk of being depleted have led to the exploitation of various renewable energy sources including wind, water, sun, and biomass. From these renewable energy sources, chemical fuel such as hydrogen gas can be produced by thermal, electrical, photonic, and biochemical energy. Hydrogen can be generated via thermal energy, playing a main role in steam reforming of fossil fuels, which is not suitable due to the utilization of fossil fuels. Hydrogen generation via water splitting is not a spontaneous process and requires an external energy source to establish a sustainable water splitting reaction. Hydrogen can also be obtained by using different methods with solar energy as an external driving force to establish a sustainable water splitting reaction, as demonstrated in Fig. 2. Thermolysis is the direct splitting of water at high temperatures for hydrogen production, though it offers a drawback due to the possibility of a rapid back reaction of H2 and O2 at higher temperatures, thus preventing the thermolysis method from being a viable approach. Actually, this method was initially developed to utilize the waste heat from nuclear reactors [39]. Biomass conversion into hydrogen via solar energy is not considered as a reasonable approach due to its low conversion efficiency. It implies that in order to achieve higher efficiency to generate substantial amounts of hydrogen, the system must be designed in large inappropriate dimensions. However, if the biomass from the waste by-product is used for this purpose, then this could perhaps be the least expensive technique for hydrogen generation. Wind energy and photovoltaic (PV) systems connected to electrolyzers have shown to be adaptable and reliable approaches for hydrogen production in the near future. An electrolyzer is a type of device that splits water into H2 and O2 by electricity. Currently, these systems are being used in industries to produce highpurity hydrogen and are very expensive. Researches are being devoted to develop small and inexpensive electrolyzer systems for individual use. In contrast to the wind and PV systems coupled to the electrolyzers, a photolysis system consists of a single unit combining the two separate steps of electrical generation and electrolysis into a single step. The photolysis systems include photoelectrolysis and photobiologically based systems and depend purely on solar energy to split water [40].

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells

261

Fig. 2 Methods for sustainable hydrogen generation

1.2 Working Principle of Solar-Driven PEC Cell for Hydrogen Generation PEC hydrogen generation via photoelectrolysis of water is a promising technique to achieve considerable hydrogen production in an environmentally sustainable manner [41, 42]. This technique exhibits promising features such as simple architecture, cost-effectiveness, and easy large-scale implementation. Figure 3 shows a schematic diagram of a typical PEC cell, which exhibits the key components including a light-sensitive semiconductor as a working electrode, a counter electrode, and an electrolyte. When the PEC cell is exposed to light, electron–hole pairs are generated in the semiconductors upon the absorption of light. The photoexcited electrons from the CB of the semiconductor migrate to the counter electrode under an external applied electric field. The electrons then reduce water at the counter electrode/electrolyte interface to produce hydrogen. The holes however oxidize water at the (photoanode) working electrode/electrolyte interface for oxygen evolution [43]. In case if the working electrode is a photocathode, then the process will be reversed, i.e., hydrogen will be generated at the photocathode side, whereas the oxygen evolution reaction will occur at the counter electrode side [44]. The

262

A. I. Channa et al.

Fig. 3 Scheme of a typical PEC cell for solar-driven hydrogen evolution

following reactions explain the complete chemical processes [45]. 2γ → 2e− + 2h+ Photon − induced electron − hole pair generation Working electrode/electrolyte interface : H2 O + 2h+ →

1 O2 (gas) + 2H+ 2 (1)

Counter electrode/electrolyte interface : 2H+ + 2e− → H2 (gas)

(2)

1 O2 (gas) + H2 (gas) 2

(3)

G = +237.18 kJ mol−1 Standard Gibbs free energy

(4)

Overall reaction : H2 O + 2γ →

Vrev =

G = 1.23 V Standard reversible potential nF

(5)

Vop = Vrev + ηa + ηc + η" + ηsys Operating voltage with overpotential losses (6) Here γ represents the photon energy, e− is electron, h+ is a hole, and G is the standard Gibbs free energy. Gibbs free energy is the minimum energy required for the water decomposition reaction to occur under a standard temperature of

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells

263

Fig. 4 Schematic diagram of a representative modern three-electrode PEC cell

25 ◦ C and a pressure of 1 bar. The standard Gibbs free energy change required for water splitting reaction is +237.18 kJ mol−1 . The positive sign of Gibbs free energy (G) deduces that the energy must be provided to the system for the electrolysis of water. Vrev in Eq. 5 is the standard reversible potential, i.e., the minimum electrical potential required to carry out the reversible photoelectrolysis. The minimum reversible potential for initiating a reversible photoelectrolysis is given as 1.23 eV. However, this potential does not include the overpotential losses; therefore, the water splitting reaction cannot occur at this potential. Vop represented in Eq. 6 is the operational voltage and ηa , ηc , η" , and ηsys are the overpotentials related to the anode, cathode, and ionic conductivity of the electrolyte and other system losses, respectively [45]. “n” denotes the total number of electrons involved in the reaction for water photoelectrolysis. “F” is the Faraday constant with a value of 96484.34 C/mol. During the operation of a PEC cell, it is necessary to maintain a fixed or controlled potential on the electrodes. However, it is difficult for electrodes to maintain a constant potential while passing current simultaneously. To resolve this issue, the passage of current and maintaining a fixed potential are separated by the introduction of an additional electrode called reference electrode [46]. The reference electrode with a known reduction potential serves as a half cell and provides a reference for measuring and controlling the potential of the working electrode with no current passing through it. However, the counter electrode passes all the current needed to balance the current observed at the working electrode [46]. Hence, the modern voltammetry system consists of three electrodes, the working electrode, reference electrode, and counter electrode, as shown in Fig. 4.

264

A. I. Channa et al.

2 Synthesis and Optical Properties of Core/Shell QDs 2.1 Synthesis of Core/Shell QDs Generally, the bare QDs have large surface-to-volume ratio due to their very small size, typically around or less than ~20 nm [47], rendering the surface a very important factor for determining the optical properties of the QDs. The organic ligands attached on the surface of the QDs can stabilize them by passivating the dangling bonds [47]. However, there exists a weak bonding between the organic ligands and the QDs’ surface, which can be affected by various parameters such as light exposure, temperature, and humidity, thus consequently affecting the properties which are highly dependent on the surface conditions including the optical properties. The surface deformations can generate surface-related defects that act as the non-radiative recombination centers, henceforth reducing both the PLQY and photostability of the bare QDs [16]. The bare QDs, therefore, face many challenges for their practical applications. One of the solutions to address this problem is the formation of core/shell structured QDs. In such type of QDs, the bare QDs are coated by another inorganic shell, and such efficient surface passivation by the inorganic shell can lead to improved PLQY and photostability compared to the bare QDs [47–49]. Moreover, the formation of the shell on bare QDs not only improves the optical properties but also creates a new band structure, which can be tuned by the choice of composition and thickness of the shell materials. In this case, reduced non-radiative recombination and efficient spatial separation of the electron–hole pairs can be achieved, which is the important parameter for achieving high-efficiency PV devices [50]. Core/shell QDs are mainly categorized as type I, type II, and quasi-type II depending upon the relative band alignment of the CB and VB of the shell material with respect to the core QD, as discussed in the Introduction [49]. The growth of the core/shell QDs can be achieved by injecting the cationic and anionic solutions of shell precursors together or separately in parts into the core QD solution [16, 51–53]. Jo et al. have reported the growth of CuGaS2 (CGS)/ZnS core/shell QDs by using the injection of preprepared cationic–anionic solutions for the ZnS shell [54]. The CGS core QDs were first synthesized at 180 ◦ C for 5 min, and then two different kinds of ZnS shell solutions prepared beforehand were injected. The first ZnS shell solution was prepared by dissolving 4 mmol of Zn (OAc)2 in 4 mL of oleic acid (OA), 2 mL of 1-dodecanethiol (DDT), and 2 mL of 1-octadecene (ODE) at 190 ◦ C. The second ZnS shell solution was synthesized by dissolving 8 mmol of Zn (St)2 in 4 mL of DDT and 8 mL of ODE. The first stock solution was injected into the pre-synthesized CGS core QDs at 220 ◦ C; after an interval of 30 min, the second stock solution was then injected at 250 ◦ C; and the reaction was held for 60 min [54]. The obtained CGS/ZnS core/shell QDs exhibited white luminescence with PLQY of about ~75%. A similar methodology was also employed for the growth of CGS/ZnS by Jalalah et al. with slight modifications in the chemical composition and reaction parameters [55].

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells

265

Successive ion layer adsorption and reaction (SILAR) is a well-known synthetic technique to prepare the core/shell QDs [56]. In this technique, the cationic and anionic solutions of shell precursors are alternatively injected into the core QD solution with a suitable time interval in between [56]. For example, Xie et al. first synthesized the Cu-doped InP core QDs at 210 ◦ C [57]. In order to grow the ZnSe shell, they used 0.1 M Zn stearate and 0.1 M trioctylphosphine (TOP)-Se solutions. Both these solutions were injected into the InP core solution at 150 ◦ C at an interval of 10 min followed by the increase of the temperature to 220 ◦ C for the growth of the ZnSe shell [57]. Similarly, they injected the second round of cationic and anionic precursor solutions to increase the ZnSe shell thickness at 150 ◦ C and then increased the temperature to 220 ◦ C for the growth of the second shell layer [57]. Tessier et al. also synthesized InP/ZnE (E = S, Se) core/shell QDs; they first synthesized InP core QDs at 180 ◦ C [58] and then used Zn stearate dissolved in ODE and TOP-(Se, S) as the shell precursor solutions for the reaction. The TOP-(Se, S) solution was injected dropwise at the same temperature and left for 60 min under continuous stirring. After 60 min, the temperature was increased to 200 ◦ C and maintained for 60 min, followed by injection of the Zn stearate solution at 200 ◦ C and increasing the temperature to 220 ◦ C and maintained for 30 min [58]. In this way, they consequently obtained a thick layer of ZnSe(S) shell on the InP core QDs. The shell on the core can be grown by a two-step method as well. In this method, the core QDs are grown first and purified to get rid of the unreacted precursor and then redispersed in an organic solvent such as ODE and degassed to evacuate the humidity and air. The reaction mixture is then heated to the optimum shell growth temperature whereupon the shell precursor solution is injected dropwise. Such method is reported by Dabbousi et al.; they first synthesized the CdSe core QDs through hot-injection method [51]. The purified CdSe QDs were then redispersed in a mixed solvent of ODE and octadecylamine (ODA) and heated to the shell growth temperature (240 ◦ C) after the conventional degassing process and N2 purging. The Zn and S precursor mixture solutions with identical molarity were slowly injected into the core QD solution [51]. The main advantage of this overgrowth technique is that the shell thickness can be precisely controlled. The shell thickness of 1.5 nm was obtained in the above-reported CdSe/ZnS core/shell QDs [51]. Using this technique, numerous core/shell QDs with well-controlled shell thickness have been grown such as CdTe/CdSe, CdSe/ZnTe, CdSe/Zn1-x Cdx S, PbSe/PbS, and InP/ZnS [52, 53, 59, 60]. Specifically, the as-synthesized core/shell QDs with very thick shell, typically known as the “giant” core/shell QDs, possess outstanding properties such as higher chemical and photostability as compared to both bare QDs and thinshell QDs [56, 61, 62]. The band structure in such “giant” core/shell QDs can be suitably tailored by controlling the shell thickness and composition to obtain longlived lifetime of photoexcited charge carriers as compared to the pure and thin-shell QDs [56, 61, 62]. Moreover, the existence of type II or quasi-type II band alignment in these “giant” core/shell QDs makes them excellent candidates for PV applications [63, 64]. Cation exchange method is another method to grow a very thin shell on the core to passivate the surface defects [65]. In this method, the overall size of the

266

A. I. Channa et al.

Fig. 5 HRTEM images of PbTe/CdTe core/shell QDs along (a) (111), (b) (100), and (c) (211) directions. (Adapted from Ref. [67])

core/shell QDs remains almost the same as the initial size of the core QDs [66– 68]. The reason behind this is the gradual replacement of cationic constituents of the core material by cations of the shell material. Pietryga et al. demonstrated the formation of PbSe/CdSe core–shell QDs via the cation exchange method [69]. Bare PbX (X = S, Se, Te) QDs were synthesized and purified to get rid of the unreacted precursor and then redispersed in toluene. The surface treatment of the PbX QDs was performed at 100 ◦ C by injecting Cd oleate solution which was separately prepared beforehand at 155 ◦ C under a N2 environment. Then the Cd oleate was injected into PbX QDs at 100 ◦ C [69]. Lambert et al. have also reported the synthesis of PbTe/CdTe core/shell QDs via the cation exchange method; the representative high-resolution transmission electron microscope (HRTEM) images of PbTe/CdTe core/shell QDs are displayed in Fig. 5, wherein the clear different lattice fringes from the core and the shell, which were observed along (100) and (211) orientations of PbTe/CdTe core/shell QDs, indicate the formation of the core/shell structure [67]. Growth of a very thin shell of ZnS (~0.1 nm) on CuInSex S2 − x via cation exchange has also been reported by Tong et al. [66]. After the synthesis, CuInSex S2 − x QDs were purified and redispersed in ODE. A 0.25 M Zn oleate solution was used for the surface treatment of CuInSex S2 − x QDs in ODE at 100 ◦ C for 10 min to obtain the CuInSex S2 − x /ZnS core/shell QDs [66].

2.2 Optical Properties of Core/Shell QDs 2.2.1

Type I Core/Shell QDs

Numerous type I core/shell QDs, including CdSe/ZnS [51, 70], CdS/ZnS [71], and InP/ZnS [72] core/shell/shell QDs [73, 74], have been reported. In type I core/shell QDs, both the electrons and the holes are well confined within the core QDs due to the wider bandgap of the shell material. Therefore, the optical properties of such core/shell structures are not very sensitive to the surrounding environments and are maintained even after multiple purifications, film deposition, or ligand

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells

267

Fig. 6 (a) Absorption and (b) emission spectrum of type I CdSe/ZnS core/shell QDs. (Adapted from Ref. [76])

exchange [72, 73, 75]. Figure 6a, b presents the absorption and emission spectra of type I CdSe/ZnS core/shell QDs with variable ZnS shell thickness [76]. With the increasing ZnS shell thickness, a slight redshift in both the absorption (Fig. 6a) and photoluminescence (PL) spectrum (Fig. 6b) of CdSe/ZnS core/shell QDs can be observed, which was attributed to the extension of the electron and hole wave functions into the shell material [51, 76]. The ZnS coating on CdSe suppressed the defects in the CdSe core and enhanced the PLQY from 10% to over 40% in CdSe/ZnS (1.7 MLs) (inset in Fig. 6b). Another study based on CdSe/ZnS core/shell QDs with type I band structure revealed that the formation of type I band structure maintains the original optical properties of the bare QDs, including the absorption onset and the PL peak position, while the core/shell structure exhibited enhanced PLQY compared to the bare QDs [70]. Figure 7 shows the absorption and emission spectrum of bare CdSe and CdSe/ZnS core/shell QDs, in which the CdSe/ZnS core/shell QD showed the absorption and PL features at the same peak positions as the bare CdSe QDs, but with much enhanced peak intensities.

2.2.2

Type II Core/Shell QDs

In type II core/shell QDs, the lowest-energy CB electrons and VB holes are localized in different regions within the QD (i.e., one type of charge carrier resides in the core, whereas another in the shell). A new band for the interfacial charge transfer (CT) between the two materials is formed in addition to the electronic transitions of individual materials [77]. This CT band is situated at the lower band energy than the individual bandgaps of the core and shell materials, extending both the absorption onset and the emission spectrum of the type II core/shell QDs to longer wavelength as compared to core QDs. Figure 8a shows the UV–vis absorption and emission spectra of CdTe core QDs and CdTe/CdSe core/shell QDs in heptane. CdTe core QDs exhibited characteristic first (1 s) exciton peak at around ∼560 nm of wavelength. However, CdTe/CdSe core/shell QDs exhibited a new emission at ∼650 nm with diminished previous 1 s exciton peak. As compared to bare CdTe

268

A. I. Channa et al.

Fig. 7 (a) Absorption spectrum of the CdSe QDs (dotted line) and the CdSe/ZnS core/shell QDs (solid line), PL spectrum of the CdSe/ZnS core/shell (solid line). (b) PL spectrum of CdSe QDs (dotted line) and CdSe/ZnS core/shell QDs (solid line). (Adapted from Ref. [70])

core QDs, the CdTe/CdSe core/shell QDs also exhibited broad absorption spectrum extended to near infrared (NIR). In addition, the emission spectrum of CdTe/CdSe core/shell QDs was observed to be considerably red-shifted (∼800 nm) as compared to the CdTe core QDs (∼580 nm) [77]. It is also demonstrated that tunable optical properties can be obtained by varying the core size and shell thickness [52, 78]. Typically, in type II core/shell structures, the absorption spectrum exhibits the transition of charge carriers between both materials (core and shell). As shown in Fig. 8a, four absorption bands of B1, B2, B3, and C can be observed at 770, 650, 500, and 560 nm, respectively, for CdTe/CdSe core/shell QDs. These bands are more obvious in the TA spectrum (blue spectrum in Fig. 8a) at the initial delay times (such as 1 ps) exhibiting bleaches at these bands. These bleaches can be assigned to the filling of CB electron levels [77]. For instance, the absorption band B1 centered at ∼770 nm (near the emission peak position) can be assigned to the CT transition between the CdTe (core) VB edge (1sh ) and CdSe (shell) CB edge (1se ) [77]. The TA spectra speculate B1, B2, and B3 transitions to follow exactly the same bleach formation strategy and decay kinetics, which suggest that these transitions are sharing identical electron level. The C band corresponds to transition between a core-localized VB 1 s hole level and a delocalized CB electron level. Type II band alignment can control the excited state dynamics of photoexcited electrons and holes. After the excitation, the electron–hole pairs undergo charge separation and localize in different regions such as electrons in the shell and holes in the core [36]. Figure 8c, d shows the TA spectra of CdTe/CdSe core/shell QDs, exhibiting bleaches at B1, B2, and B3 with a time constant of 0.7 ps. The formation

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells

269

Fig. 8 (a) UV–vis absorption and emission spectra of CdTe core and CdTe/CdSe core/shell QDs. (b) Band structure diagram of CdTe/CdSe core/shell QDs illustrating the transitions from different energy levels. (c) Transient absorption spectra of CdTe/CdSe core/shell QDs at indicated delay time windows after 400-nm excitation. (d) Transient decay kinetics at indicated transitions in (c). (Adapted from Ref. [77])

of these bleaches refer to the separation of excited electrons at the shell, which is consistent with other similar core/shell structures with type II band alignment [79–82]. The PL lifetime of QDs with a type II core/shell structure has also been observed to be lengthened with respect to the bare core QDs. The extended lifetimes of hundreds of ps, nanoseconds, or even in the scale of microseconds have been reported [36, 83, 84]. A recent study on CdTe/CdSe core/shell QDs has revealed that the exciton lifetime depends on the volume and electron–hole overlap integral [84]. Therefore, in type II core/shell structures, due to the reduction in the electron–hole overlap region and increased volume, the lifetime can be significantly prolonged [84].

2.2.3

Quasi-Type II Core/Shell QDs

In a typical quasi-type II band structure, a small CB and large VB offset exists between the core and the shell materials. The holes with the lowest energy are localized in the core, whereas the electrons in both core and shell [63, 83, 85].

270

A. I. Channa et al.

Fig. 9 (a) Schematic energy level diagram and lowest energy electron and hole wave functions in CdSe/CdS (core/shell) quasi-type II QDs. (b) Static absorption and emission spectra with transient absorption spectrum (at 1 ns, green line) spectra of CdSe/CdS core/shell QDs. (c) Transient absorption spectra and (d) bleach formation kinetics of T0 (575 nm) and T1 (475 nm) bands of CdSe/CdS QDs at indicated delay times (0–3 ps) after 400 nm excitation. (Adapted from Ref. [90])

Examples of quasi-type II core/shell QDs include CdSe/CdS [86], CdSe/ZnSe [87], PbSe/CdSe [88], Zn- CuInSe2 /CuInS2 [89], CuInSex S2 − x /CdSeS/CdS [37], etc. It has been observed that due to the existence of a small CB offset, no noticeable interfacial CT band is formed in case of quasi-type II core/shell structures. The theoretical calculation studies for CdSe/CdS core/shell QDs by Zhu et al. revealed the existence of quasi-type II band alignment [90]. Figure 9a displays the schematic energy level diagram of quasi-type II band alignment for CdSe/CdS core/shell QDs, in which 1 s electrons are delocalized throughout the whole particle (core and shell), whereas 1 s holes are strongly confined within the core [90]. Figure 9b shows the absorption spectrum obtained for CdSe/CdS core/shell QDs with weak absorption band at 575 nm (denoted as T0 ) and strong absorption band with onset around ∼475 nm (denoted as T1 ). These bands are further clearly observed in the TA spectrum (Fig. 9c). The T0 band being closer to the emission spectrum can be ascribed to the transition between the lowest energy CB electron (1se ) and VB hole (1sh ) levels in the core/shell structure (1se -1sh ). In Fig. 9b

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells

271

(TA spectrum at 1 ns), the bleaches at both the T1 and T0 bands suggest these transitions to involve the 1se level. The T1 band therefore can be assigned to the transition between the delocalized 1se level and the lowest energy hole level above the VB band edge of the CdS shell [90]. This can be further supported by identical relaxation kinetics of T0 and T1 bleaches, as illustrated in Fig. 9d [90]. In summary, similar to type II core/shell QDs, the quasi-type II core/shell QDs can also exhibit prolonged lifetime due to the reduction of the electron–hole overlap integral [91, 92].

2.3 Charge Dynamics of Core/Shell QDs 2.3.1

Type I Core/Shell QDs

The charge transfer from different core/shell QDs coupled with semiconductor films or molecular acceptors has been widely studied [93–96]. An investigation of type I CdSe/ZnS core/shell QDs and CdSe-sensitized QDSCs has revealed almost similar power conversion efficiencies (PCE) from both CdSe core and CdSe/ZnS core/shell QDs [97], while some studies have also demonstrated the enhanced PCE of the QDSCs via coating ZnS overlayers on the QD-sensitized photoanode surface [98– 100]. This suggests that the ZnS surface coating plays a crucial role in achieving efficient charge separation/transfer in QDSCs. The CdSe/ZnS core/shell QD– anthraquinone (AQ) complex was studied as a model type I QD–electron acceptor system by Lian et al. [76]. In this work, the charge separation (kCS ) and charge recombination (kCR ) kinetics as a function of ZnS shell thickness were measured by TA spectroscopy (Fig. 10a). Both kCS and kCR rates exhibited exponential decay with variable shell thickness (Fig. 10b), mathematically represented as follows: k(d) = k0 exp(−βd), where k0 denotes charge separation or recombination rate of bare CdSe QDs and β represents decay constant. The findings of this study suggested that the ZnS shell in CdSe/ZnS type I QDs slows down the electron and hole transfer rates and the carrier extraction is controlled by the surface carrier density [101– 103]. It is worth noting that the decrease in the charge recombination rate is faster compared to the charge separation rate with increasing shell thickness, which can be ascribed to heavier hole effective mass in the ZnS shell [76]. As the rates of charge separation are considerably faster than excited state relaxation, a high charge separation yield can be achieved via increasing the shell thickness.

2.3.2

Type II and Quasi-Type II Core/Shell QDs

In type II and quasi-type II core/shell QDs, such as CdTe/CdSe (type II) and CdSe/CdS (quasi type II) core/shell QDs, the electron is either localized in the shell (in case of type II) or delocalized over both the core and shell (in case of quasitype II). Therefore, the electron holds greater chance to overlap with the adsorbed

272

A. I. Channa et al.

Fig. 10 (a) The kinetics of the excited QD population (NQD∗ (t)) for the QD−AQ complexes with different shell thicknesses (x-axis is logarithmic in the right panel and linear in the left panel). (b) Plot of the logarithm of charge separation (red circles) and recombination (blue triangles) rates as a function of the ZnS shell thicknesses. (Adapted from Ref. [76])

electron acceptors coupled with the shell surface. The hole is confined in the core for both cases (type II and quasi-type II), due to which electronic coupling of the hole at the surface is reduced. As compared to bare QDs, the type II and quasi-type II QDs can exhibit enhanced or retained charge separation rate (kCS ) but reduced charge recombination rates (kCR ). A study based on CdTe/CdSe core/shell QDs (type II) has revealed that type II carrier distribution can facilitate charge separation and significantly prolonged carrier lifetimes compared to bare CdSe QDs [77]. Likewise, quasi-type II CdSe/CdS core/shell QDs exhibited almost similar ultrafast charge separation rate as the CdSe core, while the charge recombination rate lowered to about three orders of magnitude [90]. This is different from the case of type I QDs (such as CdSe/ZnS) in which both charge separation and recombination rates are reduced [76]. The type II QDs with the internal electron–hole separation are similar to the molecular donor–acceptor complexes which were employed to enhance the efficiency of dye-sensitized solar cells [104, 105]. Thus, the type II or quasi-type II core/shell QDs, with their built-in charge separation properties, are very promising for light-harvesting applications in PV and photocatalytic devices.

2.3.3

Charge Dynamics from QDs to Semiconductor Films

In QD-based PV devices such as a QDSC or PEC photoanode, QDs are normally attached with a wide-bandgap metal oxide semiconductor such as TiO2 , which acts as an electron transport layer as well as a vessel for QDs [19, 106]. As the QDs are surrounded by the organic ligands on the surface, the contact of QD-TiO2 is indirect and linked together via the organic ligands. In this case, the photoexcited electrons have to tunnel through an insulating barrier (i.e., linker organic molecule) even though the linker molecules are important in attaching and increasing the

Core/Shell Quantum-Dot-Based Solar-Driven Photoelectrochemical Cells

273

loading capacity of QDs onto the substrate and to achieve a homogenous distribution [107]. The direct contact of QD–TiO2 can alleviate the electron tunnel barrier [108]. Watson et al. studied the effect of linker molecule length on the electron transfer from QDs to TiO2 and demonstrated much decreased electron transfer rates for long-chain linker molecule in QD–TiO2 assemblies [109]. In addition, Bisquert et al. have demonstrated that the enhanced concentration of QDs on TiO2 can lead to the increased incident photon-to-current efficiency (IPCE) for QDSCs [110]. From these studies, it can be deduced that the charge recombination can possibly occur at the QD–TiO2 interface due to poor charge transfer characteristics of these linker molecules. The long lifetime of charge carriers and high charge carrier mobility can minimize the charge recombination losses. In this regard, the band alignment of the core/shell QDs can be tuned to exhibit spatial charge separation for extended PL lifetime [111]. The energy band alignment can be tuned by changing the core QD size/composition and the shell thickness [111]. For example, CdSe/CdS core/shell QDs (with shell thickness 1.5 nm), by increasing the shell thickness, the electrons exhibit increased leakage from the core to the shell region, while the holes remain confined in the core, and the band alignment of the core/shell structure changes from type I to quasi-type II structure [74]. Similarly in CuInSe2 /CuInS2 core/thick-shell QDs, with the increase of CuInS2 shell thickness, the QDs also exhibit a quasi-type II band alignment structure [75]. The quasi-type II band alignment is advantageous for optoelectronic devices, as this structure leads to an efficient electron-hole separation, an increased PL lifetime, and a strong PL red shift, leading to enlarged Stokes shift, which is a dominant factor for the fabrication of high-quality LSCs [60, 73–74, 76].

2.2 Stokes Shift in Core/Shell Quantum Dots The Stokes shift is the spectral difference between the first excitonic absorption peak and maximum emission, as shown in Fig. 3 [58]. The reabsorption energy loss can be largely reduced by the increase of Stokes shift, especially for large-scale LSCs.

Fig. 3 (a) Normalized absorption and PL spectra of PbS/CdS QDs with core radius of 1.5 nm and 2.3 nm with tunable CdS shell thickness (H, 0–0.7 nm). (b) Approximate electronic band structure of a core/shell PbS/CdS QD. (c) Stokes shift of QDs with various shell thicknesses as a function of the PL peak position (related to the core size diameter from 3 to 6 nm). (Ref. [58]. Copyright 2016, Wiley-VCH)

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

293

Several strategies have been proposed to increase the Stokes shift of QDs, containing the design and synthesis of ternary I-III-VI2 semiconducting nanocrystals (e.g., CuInSex S2–x QDs) [77–78], doped core/shell nanocrystals (e.g., Mn2+ -doped ZnSe/ZnS QDs) [79–80], and core/shell QDs (e.g., CdSe/CdS, PbS/CdS QDs) [58, 60, 81]. As mentioned above, coating QDs with a wide-bandgap semiconductor to form a quasi-type II structure can increase the Stokes shift due to the spatial separation of e-h wave function [58]. As shown in Fig. 3, the PbS/CdS core/shell QDs exhibit a larger Stokes shift compared to the PbS core QDs. Meanwhile, the Stokes shift could be tunable by varying core size and shell thickness. For example, in PbS/CdS QDs with a larger core size (R = 2.3 nm), there is a very slight variation of the Stokes shift by tuning the shell thickness. In contrast, for the QDs with a core radius of 1.5 nm, the Stokes shift exhibits a significant increase with the increase of shell thickness. The PbS/CdS core/shell QDs with small core size and thicker shell layer have larger Stokes shift (200–250 meV) than that of core QDs or core/thinshell QDs. With the increase of the shell thickness, more electrons are delocalized in the shell region and the absorption becomes dominated by the thick shell, leading to an increase of the Stokes shift [58]. Similar to the core/thick-shell quasi-type II QDs, core/thick shell type I QDs could also have large Stokes shift due to the leakage of the electron. In the thick-shell QDs, the absorption of the QDs is dominant in the shell materials, while the emission is governed by both the shell and core. For example, the thick-shell CdSe/Cd1-x Znx S QDs have a PL peak at 628 nm and the onset of absorption at 460 nm, which corresponds to a Stokes shift of 720 meV [47]. It is sufficiently large to reduce the reabsorption energy loss in large-scale LSCs.

2.3 Quantum Yield in Core/Shell Quantum Dots Core/shell QDs are usually more suitable for optoelectronic applications due to their greater environmental tolerance and higher QY compared to bare QDs, which often suffer from the surface oxidation. The QY is defined as the ratio between the emitted photons and absorbed photons by QDs. It is also a very important factor to determine the optical efficiency in LSCs [68, 82–84]. The QY of the QDs are strongly affected by the structure of the QDs. The presence of shell materials usually can give a very well surface passivation of the core, thus enhancing the optical properties of QDs. For example, for the PbS/CdS QDs, as the CdS shell thickness increases, the QY increases first, due to the surface passivation, and then decreases quickly because of the lattice mismatch, which leads to defect-related non-radiative decay. The maximum QY of 33% was obtained for the QDs in water with optimized shell thickness of 0.7 nm [68]. When the QDs were dispersed in toluene, the QY is increased to 67% after a 0.7-nm CdS-shell coating [85]. For the thicker-shell QDs (H > 1 nm), the introduction of defects causes a significant drop of QY (Fig. 4) [68]. For the synthesis of CdSe/CdS QDs via a successive ionic layer adsorption reaction (SILAR) approach, long annealing times and asymmetric distribution of annealing times between cadmium and sulfur precursors significantly affect QY in both thin-

294

G. Liu et al.

Fig. 4 QY of PbS/CdS QDs before and after water transfer as a function of CdS shell thickness. (Ref. [68]. Copyright 2011, The Royal Society of Chemistry)

and thick-shell regimes [82]. Hanifi et al. [84] synthesized CdSe/CdS core/shell QDs by a SILAR approach using oleylamine, which helps to maintain a high ligand surface coverage, reduce the surface traps, and preserve a high radiative efficiency. With the increase of CdS shell thickness, the QY increases first and then tends to be stable. For the QDs with a 2-nm-thick shell, the QY is 90 ± 2%, and when the shell thickness increases to 4 nm, the QY consistently exceeds 97 ± 3% [84].

2.4 Stability of Core/Shell Quantum Dots The stability of the QDs plays an important role in the potential applications of QD-based optoelectronic devices. The photostability of the QDs is usually characterized by the variation of PL intensity under continuous illumination. In general, the core/shell QDs show much better stability as compared to the shellfree QDs due to the improved surface passivation of the core QDs [68, 74–75]. For example, the PbS/CdS core/shell QDs exhibit markedly better photostability under continuous ultraviolet (UV) illumination and better thermal stability under heat treatment than the shell-free PbS QDs dispersed in organic solvent (Fig. 5a, b) [85]. Compared to the CdZnSeS alloy cores, the CdZnSeS/ZnS core/shell QDs with 3, 11, and 15 monolayers of the ZnS shell show better photostability [86]. When the shell thickness further increased to 17 MLs, the PL intensity decreases due to the increased interfacial defects caused by stress release, but the decrease rate is much slower than that of the QDs coated with 3 MLs of ZnS. Hence, the thick shell could effectively improve the photostability of QDs. A similar phenomenon was also found in the system of InP/ZnS QDs. For the InP/ZnS QDs with thin shell, the PL intensity shows a rapid decrease within 10 h of UV illumination, while for the thick-shell QDs, even after 25 h of continuous irradiation, little photodegradation can be seen [87]. The core/thick-shell QDs can be engineered to

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

295

Fig. 5 (a) The variation in the PL intensity of PbS and PbS/CdS QDs in ODE after 4 h of continuous illumination. (b) The PL spectra of PbS and PbS/CdS QDs before and after heat treatment in ODE (PbS, 100 ◦ C for 5 min; PbS/CdS, 150 ◦ C for 30 min). (Ref. [85]. Copyright 2011, The Royal Society of Chemistry)

exhibit outstanding chemical/photostability for the fabrication of high-performance optoelectronic devices [69, 74, 88–89].

3 Luminescent Solar Concentrator Based on Core/Shell Quantum Dots 3.1 Near-Infrared PbS/CdS Core/Shell QD-Based LSCs As mentioned above, QDs especially core/shell QDs usually have high QY and good stability due to the appropriate surface passivation. The Stokes shift can be tuned by controlling the absorption and emission spectra which are determined by the band alignment of core/shell QDs. Usually, the quasi-type II core/shell QDs are excellent emitters for near-infrared (NIR) wavelength, which is well suited for harvesting solar radiation. It is able to realize the fabrication of high-transparency colorless LSCs. In addition, the NIR wavelength emitted by QDs perfectly match with a typical Si PV cell which exhibits high PCE in the 400–1000-nm range. Zhou et al. [58] synthesized the NIR PbS/CdS QDs by hot injection for PbS QDs and flowing a cation exchange approach for core/shell QDs. The absorption and emission spectra can be well controlled by tuning the size of the core and the thickness of the shell (Fig. 3). The PbS/CdS QDs with small core size and large shell thickness have larger Stokes shift due to the efficient leakage of electrons [58, 68]. The PbS/CdS QDs with high QY and large Stokes shift can alleviate the reabsorption, making them promising candidates as emitters for high-efficiency large-area LSCs.

296

G. Liu et al.

Zhou et al. [58] fabricated the PbS QDs and PbS/CdS QD-based LSCs (5 × 1.5 × 0.2 cm3 ) by embedding the QDs in a polymer matrix. Typically, the monomer precursors of lauryl methacrylate and ethylene glycol dimethacrylate were first mixed at a mass loading of ~20%. Then the UV initiator [diphenyl(2,4,6trimethylbenzoyl)phosphine oxide] was added in the above solution to form a clear solution. Subsequently, the solution was mixed with the dried QDs powder and sonicated until a clear solution was obtained. The mixture was further injected into a mold consisting of two glass slides separated by a flexible rubber spacer and illuminated by a UV lamp to form an LSC. With the increase of the optical path, the normalized PL peak position of PbS QD-based LSC shows a redshift due to the strong overlap between the absorption and emission spectra, which causes strong reabsorption (Fig. 6a). However, for the PbS/CdS QD-based LSC, the PL spectra remain unchanged along the optical path as long as 5 cm due to the large Stocks shift, which suppresses the reabsorption (Fig. 6b). Eventually, the large-

Fig. 6 Normalized absorption and PL spectra measured at different optical paths for the samples of (a) the large-sized PbS QDs and (b) the small-sized PbS/CdS core/shell QDs. Optical efficiency of PbS/CdS QD-based LSC coupled with Si diode as a function of (c) side length or (d) G factor. (e) Photograph of a QD-polymer-based LSC (dimension: 10 cm × 1.5 cm × 0.2 cm) comprising PbS/CdS QDs in ambient light. Scale bar is 1 cm. (Ref. [58]. Copyright 2016, Wiley-VCH)

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

297

Fig. 7 (a) Absorption and PL spectra of PbS/CdS in solution and a polymer matrix. (b) Optical/quantum efficiency of PbS/CdS QD-embedded LSCs with different G factors. (c) Photograph of an LSC device (scale bar: 1 cm). (d) Scheme of the experimental setup for exciting and taking the photograph of an illuminating LSC device (region of q angle is the escape cone of emitted light). (e) Photograph of an illuminating LSC, captured by a NIR camera with a 780-nm long-pass filter. (Ref. [90]. Copyright 2017, The Royal Society of Chemistry)

area semitransparent PbS/CdS QD-based LSC (10 × 1.5 × 0.2 cm3 ) was prepared (Fig. 6e). The ηopt shows an exponential decrease with increasing side length or geometric factor (G) of the LSC (Fig. 6c, d). The semitransparent PbS/CdS QDbased LSC with G of 10 shows the ηopt of 6.1%, which is over 15 times higher than that of PbS QD-based LSC (G = 4.2) with similar concentration of QDs (~20 μM). Similarly, Tan et al. [90] fabricated NIR LSCs using ultrasmall PbS/CdS core/shell QDs with a large Stokes shift (0.36 eV) and good stability (no absorption and PL spectra change before and after QDs transfer into the polymer matrix) (Fig. 7). The LSC can absorb the sunlight and reemit photons in the 700–1100-nm wavelength range, matching well with the absorption range of the Si PV cell (Fig. 7a). The ηopt

298

G. Liu et al.

of the core/shell PbS/CdS QD-based LSC with QD concentration of 24 μM is ~4% at G factor of 10 and decreases to ~1.2% at G factor of 50 (10 cm in length). The optical efficiency decreases with the increasing length of the LSC mainly due to the small overlap between the absorption and emission spectra, which leads to a reabsorption phenomenon (Fig. 7b).

3.2 Visible CdSe/CdS Core/Shell QD-Based LSCs Benefiting from the structured engineered optical properties in visible core/thickshell CdSe/CdS QDs, they have been widely used as high-quality emitters for large-scale LSCs [91–94]. Meinardi et al. [60] used CdSe/CdS QDs to prepare LSCs. Similar to PbS/CdS QDs, the Stokes shift of CdSe/CdS QDs can be also enhanced by increasing the shell thickness. The thicker the shell layer, the smaller the spectral overlap and the larger the Stokes shift. The intensities of both PL and 835-nm scattered light exhibit the same distance (d)-dependence, confirming that emission losses due to reabsorption are virtually nonexistent. Monte Carlo simulation shows a 100-fold increase of emitted photons propagated to the edges of a core/shell giant QD-based LSC, due to the reduced reabsorption and increased QY, compared to the LSC comprising core-only QDs. The prepared LSC (21.5 × 1.3 × 0.5 cm3 ) shows a remarkable quantum efficiency (QE, defined as the ratio between the number of photons collected by the photodiode and the total number of photons absorbed by the LSC) of 10.2% and ηopt of ~1% with negligible reabsorption losses, which achieved an essential requirement for applications as PV windows. Coropceanu and Bawendi fabricated the LSCs using core/thick-shell CdSe/CdS QDs with very high QY (86%) [59], and the QDs were embedded in a polymer matrix to achieve highly transparent LSCs (Fig. 8a). The EQE spectrum exhibits an excellent agreement with the absorbance of the LSC, suggesting the minimal energy dependence of the QY (Fig. 8b). The LSC exhibits optical efficiency as high as 48% at 400 nm. With the wavelength increases, the optical efficiency of the LSCs decreases, mainly due to the reduction of absorption at longer wavelengths (Fig. 8b, c). Compared to Meinardi’s device, the significantly higher efficiency of CdSe/CdS QD-based LSCs by Coropceanu and Bawendi is mainly due to the smaller device area (2 × 2 × 0.2 cm2 vs. 21.5 × 1.3 × 0.5 cm3 ) and the higher QY (86% vs. 40%), which leads to less reabsorption energy losses. In the simplest planner LSC architecture, the emitted photons propagated in random directions which implies that a number of photons strike the interfaces within an incident angle smaller than the critical cone, leading to the escape and scattering losses. Several adjustments have been exploited to reduce the escape loss in order to achieve higher efficiency, such as the use of diffuse or specular mirror [92–95]. Bronstein et al. [16] developed a strategy to couple a photonic mirror with CdSe/CdS QD-based LSC to form an optical cavity where emitted photons were trapped (Fig. 9). In the design, a tuned wavelength-selective mirror transmits blue light and reflects red emissions that trap the PL inside the cavity, increasing

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

299

Fig. 8 (a) CdSe/CdS QD-based LSCs in ambient light (left) and under UV illumination (right) with the edges clear (top) and blocked by carbon paint (bottom). (b) External quantum efficiency (EQE, defined as the ratio of the number of emitted photons and the total number of incident photons) spectra of the LSC prototype. (c) Optical efficiency of the LSC. (d) Distribution of outcomes for photons incident on the LSC from the Monte Carlo simulation. (Ref. [59]. Copyright 2014, American Chemical Society)

the intensity of emitted light inside the cavity (Fig. 9a). Subsequently, they used the core/thick-shell CdSe/CdS QDs with QY of 68% to prepare a microcell-LSC integrated with a photonic mirror and a trench-shaped diffuse trench reflector, which increases the collection efficiency of red photons, mitigates the escape and scattering losses, and improves the power output of red photons (Fig. 9b, c). Connell et al. [94] also used core/thick-shell CdSe/CdS QDs as fluorophore to prepare LSCs with reflectors. They explored the design of the spectrally selective top mirror on the performance of the LSCs and found that the mirrors designed to trap emitted light result in higher performance than mirrors designed for sunlight transmission. When the QY is higher, the concentration of the fluorophores is lower, the lateral size is larger, and the overlap between the absorption and emission spectra is lower. The integration of mirrors and LSCs can trap both the incident and emitted light into the LSCs and enhance the optical efficiency of the LSCs due to the increasing photons collected by the PV cells. However, the application of LSCs in BIPVs is limited because of the lack of transparency.

300

G. Liu et al.

Fig. 9 (a) Graphic showing a typical transmission electron micrograph and schematic of giant CdSe/CdS QDs, incorporated into a traditional LSC (open top) and the luminescent concentrator cavity (with mirror). The black rectangle is a PV cell. (b) J-V characteristics of the LSC device with and without the photonic mirror. (c) Experimental and simulated photon concentration ratios at different optical densities of QD, with a G of 61. (Ref. [16]. Copyright 2015, American Chemical Society)

Another LSC architecture is the multiple LSCs, including tandem structure and sandwich structure [21, 23–24, 33, 45, 96–99]. In the tandem configuration, each layer absorbs the solar spectrum selectively to improve the optical efficiency of LSCs attributed to the increased absorption efficiency of the devices [21, 97]. Recently, Liu et al. [98] fabricated the tandem LSC based on carbon dots (C-dots)

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

301

and core/thick-shell CdSe/CdS QDs to improve the optical efficiency of the LSCs (Fig. 10a). First, they synthesized C-dots and CdSe/CdS QDs by a hydrothermal method and SILAR approach, respectively, with high QY (40% for C-dots and 50% for CdSe/CdS QDs). To experimentally investigate the optical efficiency of the LSCs, they fabricated tandem LSC (10 × 10 cm2 ) comprising C-dots (top layer) and CdSe/CdS QDs (bottom layer). The C-dot-based LSC was prepared by spin-coating a C-dot/polyvinylpyrrolidone (PVP) polymer on a glass substrate, and the CdSe/CdS QD-based LSC was prepared by embedding the giant QDs in a poly(lauryl methacrylate-co-ethylene glycol dimethacrylate) (PLMA-co-EGDM) polymer matrix, respectively. No significant changes in the absorption and PL peak position were observed in the investigated QDs in solution and embedded in the polymer (Fig. 10b). The tandem device shows a ηopt of 1.4% under simulated one sun illumination (100 mW/cm2 ), which is improved 16% of the optical efficiency compared to the single-layer CdSe/CdS QD-based LSCs. The enhancement of the optical efficiency for the tandem structure is mainly due to the increased absorption of sunlight by both C-dots and QDs and the decreased escape loss, as the escaped emission from the top layer can be partially reabsorbed by the bottom layer. In addition, the tandem structure largely enhances the photostability of the CdSe/CdS QD-based LSC due to the protection of the C-dot layer. After 70 h of continuous UV illumination, the PL intensity of the CdSe/CdS QD-based LSC with the C-dot protective layer maintains 75% of its initial value compared to 43% of the LSC without the C-dot layer (Fig. 10c). Although the tandem LSCs mentioned above can improve the optical efficiency and photostability of the device, the long-term stability of the LSC under surrounding environments (light, moisture, etc.) still needs to increase due to the sensitivity of the QDs. Liu et al. [45] further fabricated the sandwich structured (or laminated) LSC. CdSe/CdS QD/polymer film was laminated between two sheets of glass (Fig. 10d). The structure can isolate the QDs from the ambient environments so as to improve the long-term stability of the device. After 3 months, the efficiency of the sandwich structure LSC maintains 85% of its initial efficiency value, while for the single-layer device, it drops more than 25%. The PL intensity of the sandwich structure LSC increases compared to that of the single-film LSC, which demonstrates the enhancement of optical efficiency of sandwich structure LSC (Fig. 10e). Compared to the single-layer LSCs, the improvement of optical efficiency for sandwich structure LSCs is mainly attributed to the lower G factor, less photon escape, and reduced reabsorption loss due to the propagation of emitted light in the glass (Fig. 10f). The optimized optical efficiency of the large-scale sandwich structure LSCs (10 × 10 cm2 ) based on CdSe/CdS QDs is 2.95%, increasing 75% compared to the single-layer LSCs. The sandwich structure LSCs provide a promising pathway to achieve power generation in different application places, such as BIPVs and plastic greenhouses.

302

G. Liu et al.

Fig. 10 (a) Photograph (left) and schematic diagram (right) of the tandem LSC comprising Cdots and CdSe/CdS QDs. (b) Absorption and PL spectra of the C-dots (top) and CdSe/CdS QDs (bottom) in solution and LSC, respectively. (c) PL spectra of the CdSe/CdS QD-based LSCs before and after 70 h of UV illumination with or without the C-dot protective layer (top) and the absorption and PL spectra of C-dot-based LSC before and after 70 h of illumination under ambient conditions (bottom) [98]. Copyright 2018, The Royal Society of Chemistry. (d) Photograph (left and right) and schematic diagram (middle) of the sandwich structure LSC based on CdSe/CdS QDs. (e) Schematic diagram of PL spectra of single and sandwich structure LSCs. (f) Experimental and calculated optical efficiency of different LSCs based on CdSe/CdS QDs. “G” and “F” represent the glass and QD film, respectively. (Ref. [45]. Copyright 2019, Elsevier Ltd.)

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

303

3.3 Eco-Friendly InP/ZnO Core/Shell QD-Based LSCs The above mentioned core/shell QDs in LSCs usually contain toxic heavy metal elements, such as Pb and Cd, which is not eco-friendly and limits their practical use. Heavy metal free QDs including InP/ZnO, CuInS2 /ZnS, AgInS2 /ZnS, etc. are more suitable for optoelectronic devices due to their eco-friendly and superior optical properties [25, 33, 48]. InP QDs are one of the typical binary eco-friendly QDs with a bulk bandgap of 1.35 eV, which could tune the emission wavelength from the visible to the NIR region by changing their size [100]. Formation of a core/shell structure can largely improve the QY and photostability of InP QDs. The QY exceeds 70% after coating the InP core with appropriate shell materials, such as InP/ZnS, InP/ZnSeS, InP/GaP/ZnS, etc. [101–103] Thus, they are very promising materials in optoelectronic applications. Karatum et al. [104] synthesized type II InP/ZnO QDs by adjusting the shell coverage for optimum in-film QE. They found that even though InP/2ZnO (i.e., InP core surrounded by 2 monolayers of ZnO shell) achieves lower efficiency loss due to large Stokes shift, InP/1ZnO (i.e., InP core surrounded by 1 monolayer of ZnO shell) shows higher in-film efficiency because of higher QY. This study presents an opportunity for the ability of engineering nontoxic QDs for minimizing solid-state efficiency loss by forming shells owing to type II band alignment. Sadeghi et al. used binary nontoxic InP/ZnO core/shell QDs to fabricate LSCs [25]. They investigated the shell thickness effect in order to optimize the Stokes shift of the core/shell QDs. The emission peak of the QDs redshifts with increasing the ZnO shell thickness due to the type II band alignment, in which the electron tends to delocalize toward the ZnO shell, while the hole was confined in the core (Fig. 11a). As a result, with the shell thickness increased, the spectral overlap between the absorption and PL emission decreased, indicating the reduction of reabsorption loss. Figure 11b shows the schematic and photographs of the InP/ZnO QD-based LSC. The LSC under ambient light exhibits a good transparency which could appropriately reduce the incident sunlight and transmit it to the inside of the rooms. Under UV light, the PL emitted by the QDs was especially visible in the edges. Moreover, the LSC shows great structural flexibility which indicates that the QD-polymer slabs can be useful to be integrated on curved surfaces and to capture the sunlight all day. Figure 11c shows the dependence of the output optical intensity of the InP/2ZnO and InP/5ZnO (i.e., InP core surrounded by 5 monolayers of ZnO shell) QDbased LSCs versus the illuminated area, which could illustrate the effect of the Stokes shift on the LSC performance. The InP/2ZnO-based LSC shows a nearexponential behavior for the output optical intensity as the coverage area increased due to the reabsorption loss of InP/2ZnO, while the InP/5ZnO-based LSC exhibits a linear response of optical intensity with the increasing illuminated area due to the negligible reabsorption loss (Fig. 11c). Furthermore, thanks to the low reabsorption loss in the LSC based on InP/5ZnO QDs, the optical output decay was mainly due to the scattering loss (Fig. 11d). Figure 11e shows the simulated optical efficiency of the LSCs with a different QE and G factor. The simulated optical efficiency of

304

G. Liu et al.

Fig. 11 (a) Absorption and PL spectra of the InP/ZnO QDs with increasing shell layers (0, 2, 5 monolayers) in hexane. (b) Schematic and photographs of InP/ZnO QD-LSC under ambient light (middle) and UV irradiation (right). (c) Dependence of the optical output intensity by varying the illuminated area. (d) Optical output intensity of polydimethylsiloxane (PDMS) and InP/5ZnO QDLSC. (e) Simulations of optical efficiency for the QE levels of QDs as 16.2, 30, 60, and 100%. (Ref. [25]. Copyright 2018, American Chemical Society)

LSCs is improved with higher QE of QDs and lower G factor. The optical efficiency of the InP/5ZnO QD-based LSC with QD concentration of 0.05 wt.% reached 1.4% at G factor of 5 and decreased to 0.225% at G factor of 30.

3.4 Cu-Based Ternary or Quaternary Eco-Friendly Core/Shell QD-Based LSCs Ternary eco-friendly core/shell QDs are emerging alternatives for heavy metalbased toxic QDs due to their environmental-friendly, broad light absorption and tunable emission by the size effect, composition ratio, and the surface ion situations [33, 48, 77, 105]. Li et al. [106] fabricated the heavy metal free CuInS2 /ZnS QDbased LSC with an optical efficiency of 26.5% (LSC size: 2.2 × 2.2 × 0.3 cm3 ). The one-pot method was used to synthesize the core/shell QDs with a QY of 81% and large Stokes shift of 150 nm. The as-prepared CuInS2 /ZnS QDs are able to convert short-wavelength light (90%) and NIR-emitting CuInS2 /ZnS QDs as emitters to fabricate the laminated glass LSCs. The absorption and emission spectra of the CuInS2 /ZnS QDs show a very large Stokes shift (>550 meV) with band edge at ~2.0 eV and emission peak at ~1.44 eV (862 nm) (Fig. 12c). The PL emission spectrum is well matched with the peak EQE of c-Si solar cells (850– 950 nm) to achieve high PCE. The large Stokes shift has practical implications for the high-efficiency LSCs as the QDs have little reabsorption and thus minimal intrinsic optical losses. By incorporating the high QY NIR QDs into the polymer interlayer between two sheets of glass, a laminated LSC with high optical efficiency was achieved (Fig. 12d). The I-V curves and respective QE curves of the LSC with a mirrored background and a black background show that the LSC with a mirror substrate exhibits better performance than that with a black background (Fig. 12e, f). The as-fabricated laminated LSC based on NIR CuInS2 /ZnS QDs reaches an optical efficiency of 8.1% and PCE of 2.94% and 2.18% (with and without a reflective substrate) for a 10 × 10 cm2 device. Due to its high efficiency and high capability of integration into buildings, the laminated LSCs have the potential to realize netzero power consumption for BIPVs. Beside the binary and ternary semiconductors, the ternary alloyed heavy metal free QDs (CuInSex S2-x , Zn and Al co-doped CuInS2 ) have also been used as emitters to fabricate LSCs [46, 77]. Meinardi et al. [77] used NIR-emitting CuInSex S2-x /ZnS (CISeS/ZnS) QDs as high-quality fluorophores to fabricate LSCs (12 × 12 × 0.3 cm3 ). Both the absorption and PL emission spectra of the QDs in the polymer are identical to those in the solution, which reveals that the spectral and dynamical properties of CISeS/ZnS QDs are unaffected by the radical polymerization procedure. The core/shell QDs exhibit a large Stokes shift (up to 530 nm), but the overlap between the absorption and PL emission spectra is still clear, suggesting a partial energy loss due to reabsorption. Due to the scattering at optical imperfections, the PL intensity decreased with increasing distance from the edge. The normalized PL spectra slightly changed with varying distance, indicating that the reabsorption loss is very small. The PL intensity drops with increasing distance, due to the combined effect of light escaping from the waveguide and reabsorption by the QDs and the polymer matrix [77]. The QD-based LSC usually only comprises pure QDs without other impurities. Liu et al. [107] fabricated CuInS2 /ZnS QD-based LSC with SiO2 particles as the scattering center to realize highly efficient scattering-enhanced LSC devices. The schematic and photographs for scattering-enhanced LSC are shown in Fig. 13e. SiO2 particles could change the direction of incident sunlight, which would increase the probability of light captured and absorbed by QDs. Moreover, the emission light reemitted from QDs will also be redirected by the SiO2 particles, which will be more likely guided to the PV cells installed at the edge of LSCs. Since SiO2 particles induced scattering effect, they achieved a PCE of 4.2%, which showed

306

G. Liu et al.

Fig. 12 (a) The relationships of AM 1.5 G spectrum, c-Si PV cell-responsive spectrum, and the absorption as well as emission spectra of CuInS2 /ZnS QDs. (b) Photographs of the LSC prototype (22 mm × 22 mm × 3 mm) non-containing and containing CuInS2 /ZnS QDs under UV light (365 nm) and different schematic of LSC-PV devices [106]. Copyright 2015, Springer Nature. (c) Absorption and PL spectra for NIR-emitting CuInS2 /ZnS QDs in toluene. (d) Diagram of the laminated glass LSC design. (e) National Renewable Energy Laboratory (NREL)-certified I–V curves measured under AM1.5 illumination in a solar simulator for LSC with an absorbing black background (dashed line) and a mirrored background behind the window (solid line), simulating a low-e coating. (f) Normalized QE measured and averaged from four locations on the device for both a black and mirrored background. The inset shows a photo of the LSC device. (Ref. [33]. Copyright 2018, American Chemical Society)

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

307

Fig. 13 (a) Schematic for scattering-enhanced LSC. (b) The photographs of pure PMMA light guide bulk and scattering-enhanced LSC under daylight (left) and scattering-enhanced LSC contained SiO2 (5 μm) under UV (365 nm) light (right). (Ref. [107])

an improvement of 60.3% compared with the pure QD-based LSC without SiO2 particles, while the presence of a SiO2 nanoparticle will affect the transparency of the LSCs.

4 Conclusions and Perspectives In this chapter, we introduce various types of core/shell QDs and their application in LSCs. In the LSCs, the energy loss is mainly due to the reabsorption induced by the overlap between the absorption and emission of the fluorescent materials and their QY which is typically lower than 100%. In the core/shell QDs, by tuning the core size and shell thickness, one can tune the band alignment, further tuning the optical

308

G. Liu et al.

properties of QDs. Although high-quality fluorophores (e.g., CdSe/CdS, PbS/CdS QDs) have been used to fabricate high-efficiency LSCs with excellent stability, these QDs contain toxic Cd and Pb. Compared to heavy metal QDs, heavy-metal-free QDs become more promising candidates for potential applications in LSCs due to their low toxicity. The binary and ternary core/shell eco-friendly QDs (e.g., InP/ZnO, CuInS2 /ZnS QDs) have been used for LSCs benefitting from their size-dependent optical properties. Beside the tremendous effort in exploring the structure, properties, and applications of core/shell QDs in LSCs, there are still several critical limitations and challenges that need to be resolved: (i) The optical properties of the core/shell QDs still need to be optimized. And the effective synthetic approaches for core/shell QDs with large Stokes shift, high QY, and long-term stability should be developed. (ii) Although the eco-friendly core/shell QDs are nontoxic materials, the organic precursors are still hazardous. Eco-friendly core/shell QDs with green synthesis process need to be developed. (iii) The optical efficiency of the large-scale LSCs is still low, which is difficult to realize further commercialization. (iv) The size of the QD-based LSCs is still small and should be enlarged while simultaneously maintaining their efficiency. The LSCs can be applied in buildings as part of the roofs, facades, or windows, to form net-zero energy buildings. The LSCs can be also used in solar noise barriers which serve as electricity-generating device and noise barriers. Besides the application in BIPVs, the LSCs can be used as solar concentrated chemical photoreactors as well [18, 108]. Specially, the LSCs can convert the sunlight into selected emission light to match the active spectrum range of a photocatalyst or photosensitizer. To date, the challenges related to the applications of LSCs are the cost, durability and efficiency. In addition, the replacement of toxic materials with green and earth-abundant materials for fabrication of LSCs is important to address environmental sustainability. With further optimizations of both the fluorescent materials and devices’ structure, highly efficient, long-term stable and large-area LSCs based on core/shell QDs need to be achieved in order to realize a net-zero power consumption of future buildings. Acknowledgments H. Zhao acknowledges the start funding support from Qingdao University and the funding from the Natural Science Foundation of Shandong Province (ZR2018MB001).

References 1. Zhang, S., Wu, S., Chen, W., Zhu, H., Xiong, Z., Yang, Z., Chen, C., Chen, R., Han, L., Chen, W.: Solvent engineering for efficient inverted perovskite solar cells based on inorganic CsPbI2 Br light absorber. Mater Today Energy. 8, 125–133 (2018) 2. Zhao, H., Rosei, F.: Colloidal quantum dots for solar technologies. Chem. 3(2), 229–258 (2017) 3. Giustino, F., Snaith, H.J.: Toward lead-free perovskite solar cells. ACS Energy Lett. 1(6), 1233–1240 (2016)

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

309

4. Song, S., Kang, G., Pyeon, L., Lim, C., Lee, G.-Y., Park, T., Choi, J.: Systematically optimized bilayered electron transport layer for highly efficient planar perovskite solar cells (η = 21.1%). ACS Energy Lett. 2, 2667–2673 (2017) 5. Wu, L., Chen, S.Y., Fan, F.J., Zhuang, T.T., Dai, C.M., Yu, S.H.: Polytypic nanocrystals of Cu-based ternary chalcogenides: colloidal synthesis and photoelectrochemical properties. J Am Chem Soc. 138(17), 5576–5584 (2016) 6. Feng, H.P., Tang, L., Zeng, G.M., Zhou, Y., Deng, Y.C., Ren, X., Song, B., Liang, C., Wei, M.Y., Yu, J.F.: Core-shell nanomaterials: applications in energy storage and conversion. Adv Colloid Interface Sci. 267, 26–46 (2019) 7. Chen, D., Wang, A., Buntine, M.A., Jia, G.: Recent advances in zinc-containing colloidal semiconductor nanocrystals for optoelectronic and energy conversion applications. ChemElectroChem. 6(18), 4709–4724 (2019) 8. NRE, Best research-cell efficiency chart, plotted from 1976 to the present. 20191106. https:// www.nrel.gov/pv/cell-efficiency.html 9. Green, M.A.: Third generation photovoltaics: advanced solar energy conversion. Springer, Berlin (2006) 10. Wehrspohn, R.B., Gombert, A., Gombert, A., Heile, I., Wüllner, J., Gerstmaier, T., van Riesen, S., Gerster, E., Röttger, M., Lerchenmüller, H.: Recent progress in concentrator photovoltaics. Photonic Solar Energ Syst III. 7725, 772508 (2010) 11. Kirkpatrick, D., Eisenstadt, E., Haspert, A.: DARPA pushes for 50% efficient photovoltaics to power soldiers’ small tools. SPIE-The International Society for Optical Engineering (2006) 12. Weber, W.H., Lambe, J.: Luminescent greenhouse collector for solar radiation. Appl Optics. 15(10), 2299–2300 (1976) 13. Currie, M.J., Mapel, J.K., Heidel, T.D., Goffri, S., Baldo, M.A.: High-efficiency organic solar concentrators for photovoltaics. Science. 321(5886), 226–228 (2008) 14. Desmet, L., Ras, A.J.M., de Boer, D.K.G., Debije, M.G.: Monocrystalline silicon photovoltaic luminescent solar concentrator with 4.2% power conversion efficiency. Opt Lett. 37, 3087– 3089 (2012) 15. Corrado, C., Leow, S.W., Osborn, M., Chan, E., Balaban, B., Carter, S.A.: Optimization of gain and energy conversion efficiency using front-facing photovoltaic cell luminescent solar concentrator design. Sol Energ Mat Sol C. 111, 74–81 (2013) 16. Bronstein, N.D., Yao, Y., Xu, L., O’Brien, E., Powers, A.S., Ferry, V.E., Alivisatos, A.P., Nuzzo, R.G.: Quantum dot luminescent concentrator cavity exhibiting 30-fold concentration. ACS Photonics. 2(11), 1576–1583 (2015) 17. Klimov, V.I., Baker, T.A., Lim, J., Velizhanin, K.A., McDaniel, H.: Quality factor of luminescent solar concentrators and practical concentration limits attainable with semiconductor quantum dots. ACS Photonics. 3(6), 1138–1148 (2016) 18. Cambie, D., Zhao, F., Hessel, V., Debije, M.G., Noel, T.: A leaf-inspired luminescent solar concentrator for energy-efficient continuous-flow photochemistry. Angew Chem Int Ed Engl. 56(4), 1050–1054 (2017) 19. Brennan, L.J., Purcell-Milton, F., McKenna, B., Watson Trystan, M., Gun’ko, Y.K., Evans, R.C.: Large area quantum dot luminescent solar concentrators for use with dye-sensitised solar cells. J Mater Chem A. 6(6), 2671–2680 (2018) 20. Talite, M.J., Huang, H.Y., Wu, Y.H., Sena, P.G., Cai, K.B., Lin, T.N., Shen, J.L., Chou, W.C., Yuan, C.T.: Greener luminescent solar concentrators with high loading contents based on in situ cross-linked carbon nanodots for enhancing solar energy harvesting and resisting concentration-induced quenching. ACS Appl Mater Interfaces. 10(40), 34184–34192 (2018) 21. Wu, K., Li, H., Klimov, V.I.: Tandem luminescent solar concentrators based on engineered quantum dots. Nat Photonics. 12(2), 105–110 (2018) 22. AbouElhamd, A.R., Al-Sallal, K.A., Hassan, A.: Review of core/shell quantum dots technology integrated into building’s glazing. Energies. 12(6), 1058 (2019) 23. Sadeghi, S., Melikov, R., Bahmani Jalali, H., Karatum, O., Srivastava, S.B., Conkar, D., FiratKaralar, E.N., Nizamoglu, S.: Ecofriendly and efficient luminescent solar concentrators based on fluorescent proteins. ACS Appl Mater Interfaces. 11(9), 8710–8716 (2019)

310

G. Liu et al.

24. Sol, J., Dehm, V., Hecht, R., Wurthner, F., Schenning, A., Debije, M.G.: Temperatureresponsive luminescent solar concentrators: tuning energy transfer in a liquid crystalline matrix. Angew Chem Int Ed Engl. 57(4), 1030–1033 (2018) 25. Sadeghi, S., Bahmani Jalali, H., Melikov, R., Ganesh Kumar, B., Mohammadi Aria, M., Ow-Yang, C.W., Nizamoglu, S.: Stokes-shift-engineered indium phosphide quantum dots for efficient luminescent solar concentrators. ACS Appl Mater Interfaces. 10(15), 12975–12982 (2018) 26. Luo, X., Ding, T., Liu, X., Liu, Y., Wu, K.: Quantum cutting luminescent solar concentrators using ytterbium doped perovskite nanocrystals. Nano Lett. 19, 338–341 (2018) 27. Sharma, M., Gungor, K., Yeltik, A., Olutas, M., Guzelturk, B., Kelestemur, Y., Erdem, T., Delikanli, S., McBride, J.R., Demir, H.V.: Near-unity emitting copper-doped colloidal semiconductor quantum wells for luminescent solar concentrators. Adv Mater. 29(30), 1700821 (2017) 28. Mateen, F., Oh, H., Jung, W., Lee, S.Y., Kikuchi, H., Hong, S.-K.: Polymer dispersed liquid crystal device with integrated luminescent solar concentrator. Liq Cryst. 45(4), 498–506 (2017) 29. Gutierrez, G.D., Coropceanu, I., Bawendi, M.G., Swager, T.M.: A low reabsorbing luminescent solar concentrator employing pi-conjugated polymers. Adv Mater. 28(3), 497–501 (2016) 30. Zhang, J., Wang, M., Zhang, Y., He, H., Xie, W., Yang, M., Ding, J., Bao, J., Sun, S., Gao, C.: Optimization of large-size glass laminated luminescent solar concentrators. Sol Energy. 117, 260–267 (2015) 31. Zhao, H., Benetti, D., Jin, L., Zhou, Y., Rosei, F., Vomiero, A.: Absorption enhancement in “giant” core/alloyed-shell quantum dots for luminescent solar concentrator. Small. 12(38), 5354–5365 (2016) 32. Meinardi, F., Bruni, F., Brovelli, S.: Luminescent solar concentrators for building-integrated photovoltaics. Nat Rev Mater. 2(12), 17072 (2017) 33. Bergren, M.R., Makarov, N.S., Ramasamy, K., Jackson, A., Guglielmetti, R., McDaniel, H.: High-performance CuInS2 quantum dot laminated glass luminescent solar concentrators for windows. ACS Energy Lett. 3(3), 520–525 (2018) 34. You, Y., Tong, X., Wang, W., Sun, J., Yu, P., Ji, H., Niu, X., Wang, Z.M.: Eco-friendly colloidal quantum dot-based luminescent solar concentrators. Adv Sci. 6(9), 1801967 (2019) 35. Mazzaro, R., Gradone, A., Angeloni, S., Morselli, G., Cozzi, P.G., Romano, F., Vomiero, A., Ceroni, P.: Hybrid silicon nanocrystals for color-neutral and transparent luminescent solar concentrators. ACS Photonics. 6(9), 2303–2311 (2019) 36. Zhou, Y., Zhao, H., Ma, D., Rosei, F.: Harnessing the properties of colloidal quantum dots in luminescent solar concentrators. Chem Soc Rev. 47(15), 5866–5890 (2018) 37. Moraitis, P., Schropp, R.E.I., van Sark, W.G.J.H.M.: Nanoparticles for luminescent solar concentrators – a review. Opt Mater. 84, 636–645 (2018) 38. Reinders, A.H.M.E., de la Grée, G. D., Papadopoulos, A., Rosemann, A., Debije, M. G., Cox, M., Krumer, Z.: Leaf roof – designing luminescent solar concentrating PV roof tiles. 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 3447–3451 (2016) 39. van Sark, W., Moraitis, P., Aalberts, C., Drent, M., Grasso, T., L’Ortije, Y., Visschers, M., Westra, M., Plas, R., Planje, W.: The “electric mondrian” as a luminescent solar concentrator demonstrator case study. Sol RRL. 1, 1600015 (2017) 40. Debije, M.G., Tzikas, C., Rajkumar, V.A., de Jong, M.M.: The solar noise barrier project: 2. The effect of street art on performance of a large scale luminescent solar concentrator prototype. Renew Energy. 113, 1288–1292 (2017) 41. Kanellis, M., de Jong, M.M., Slooff, L., Debije, M.G.: The solar noise barrier project: 1. Effect of incident light orientation on the performance of a large-scale luminescent solar concentrator noise barrier. Renew Energy. 103, 647–652 (2017) 42. Batchelder, J.S., Zewail, A.H., Cole, T.: Luminescent solar concentrators 1- theory of operation and techniques for performance. Appl Optics. 18(18), 3090–3110 (1979)

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

311

43. Wilton, S.R., Fetterman, M.R., Low, J.J., You, G., Jiang, Z., Xu, J.: Monte Carlo study of PbSe quantum dots as the fluorescent material in luminescent solar concentrators. Opt Express. 22(1), A35–A43 (2014) 44. Zhou, Y., Benetti, D., Tong, X., Jin, L., Wang, Z.M., Ma, D., Zhao, H., Rosei, F.: Colloidal carbon dots based highly stable luminescent solar concentrators. Nano Energy. 44, 378–387 (2018) 45. Liu, G., Mazzaro, R., Wang, Y., Zhao, H., Vomiero, A.: High efficiency sandwich structure luminescent solar concentrators based on colloidal quantum dots. Nano Energy. 60, 119–126 (2019) 46. Zhu, M., Li, Y., Tian, S., Xie, Y., Zhao, X., Gong, X.: Deep-red emitting zinc and aluminium co-doped copper indium sulfide quantum dots for luminescent solar concentrators. J Colloid Interface Sci. 534, 509–517 (2019) 47. Li, H., Wu, K., Lim, J., Song, H.-J., Klimov, V.I.: Doctor-blade deposition of quantum dots onto standard window glass for low-loss large-area luminescent solar concentrators. Nat Energy. 1, 16157 (2016) 48. Chen, W., Li, J., Liu, P., Liu, H., Xia, J., Li, S., Wang, D., Wu, D., Lu, W., Sun, X.W., Wang, K.: Heavy metal free nanocrystals with near infrared emission applying in luminescent solar concentrator. Solar RRL. 1(6), 1700041 (2017) 49. Zhao, H., Sun, R., Wang, Z., Fu, K., Hu, X., Zhang, Y.: Zero-dimensional perovskite nanocrystals for efficient luminescent solar concentrators. Adv Funct Mater. 29(30), 1902262 (2019) 50. Mateen, F., Ali, M., Oh, H., Hong, S.-K.: Nitrogen-doped carbon quantum dot based luminescent solar concentrator coupled with polymer dispersed liquid crystal device for smart management of solar spectrum. Sol Energy. 178, 48–55 (2019) 51. Khan, A.H., Pinchetti, V., Tanghe, I., Dang, Z., Martín-García, B., Hens, Z., Van Thourhout, D., Geiregat, P., Brovelli, S., Moreels, I.: Tunable and efficient red to near-infrared photoluminescence by synergistic exploitation of core and surface silver doping of CdSe nanoplatelets. Chem Mater. 31(4), 1450–1459 (2019) 52. El-Bashir, S.M.: Coumarin-doped PC/CdSSe/ZnS nanocomposite films: a reduced selfabsorption effect for luminescent solar concentrators. J Lumin. 206, 426–431 (2019) 53. Rowan, B.C., Wilson, L.R., Richards, B.S.: Advanced material concepts for luminescent solar concentrators. IEEE J Sel Top Quant Electronics. 14(5), 1312–1322 (2008) 54. Liang, H., Zeng, Z., Li, Z., Xu, J., Chen, B., Zhao, H., Zhang, Q., Ming, H.: Fabrication and amplification of rhodamine B-doped step-index polymer optical fiber. J Appl Polym Sci. 93(2), 681–685 (2004) 55. Dienel, T., Bauer, C., Dolamic, I., Brühwiler, D.: Spectral-based analysis of thin film luminescent solar concentrators. Sol Energy. 84(8), 1366–1369 (2010) 56. Slooff, L.H., Bende, E.E., Burgers, A.R., Budel, T., Pravettoni, M., Kenny, R.P., Dunlop, E.D., Büchtemann, A.: A luminescent solar concentrator with 7.1% power conversion efficiency. Phys Status Solidi – R. 2(6), 257–259 (2008) 57. Goetzberger, A., Greubel, W.: Solar energy conversion with fluorescent collectors. Appl Phys. 14, 123–139 (1977) 58. Zhou, Y., Benetti, D., Fan, Z., Zhao, H., Ma, D., Govorov, A.O., Vomiero, A., Rosei, F.: Near infrared, highly efficient luminescent solar concentrators. Adv Energy Mater. 6(11), 1501913 (2016) 59. Coropceanu, I., Bawendi, M.G.: Core/shell quantum dot based luminescent solar concentrators with reduced reabsorption and enhanced efficiency. Nano Lett. 14(7), 4097–4101 (2014) 60. Meinardi, F., Colombo, A., Velizhanin, K.A., Simonutti, R., Lorenzon, M., Beverina, L., Viswanatha, R., Klimov, V.I., Brovelli, S.: Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA matrix. Nat Photonics. 8(5), 392–399 (2014) 61. Tytus, M., Krasnyj, J., Jacak, W., Chuchmala, A., Donderowicz, W., Jacak, L.: Differences between photoluminescence spectra of type-I and type-II quantum dots. J Phys Conf Ser. 104, 012011 (2008)

312

G. Liu et al.

62. Kim, S., Fisher, B., Eisler, H.-J., Bawendi, M.: Type-II quantum dots: CdTe/CdSe (core/shell) and CdSe/ZnTe (core/shell) heterostructures. J Am Chem Soc. 125(38), 11466–11467 (2003) 63. Gheshlaghi, N., Pisheh, H.S., Karim, M.R., Malkoc, D., Ünlü, H.: Interfacial strain effect on type-I and type-II core/shell quantum dots. Superlattice Microst. 97, 489–494 (2016) 64. Reiss, P., Protiere, M., Li, L.: Core/shell semiconductor nanocrystals. Small. 5(2), 154–168 (2009) 65. De Geyter, B., Justo, Y., Moreels, I., Lambert, K., Smet, P.F., Van Thourhout, D., Houtepen, A.J., Grodzinska, D., de Mello Donega, C., Meijerink, A., Vanmaekelbergh, D., Hens, Z.: The different nature of band edge absorption and emission in colloidal PbSe/CdSe core/shell quantum dots. ACS Nano. 5(1), 58–66 (2010) 66. Vasudevan, D., Gaddam, R.R., Trinchi, A., Cole, I.: Core-shell quantum dots: properties and applications. J Alloys Compd. 636, 395–404 (2015) 67. Dabbousi, B.O., Rodriguez-Viejo, J., Mikulec, F.V., Heine, J.R., Mattoussi, H., Ober, R., Jensen, K.F., Bawendi, M.G.: (CdSe)ZnS core-shell quantum dots: synthesis and characterization of a size series of highly luminescent nanocrystallites. J Phys Chem B. 101(46), 9463–9475 (1997) 68. Zhao, H., Chaker, M., Ma, D.: Effect of CdS shell thickness on the optical properties of watersoluble, amphiphilic polymer-encapsulated PbS/CdS core/shell quantum dots. J Mater Chem. 21(43), 17483 (2011) 69. Pal, B.N., Ghosh, Y., Brovelli, S., Laocharoensuk, R., Klimov, V.I., Hollingsworth, J.A., Htoon, H.: ‘Giant’ CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: strong influence of shell thickness on light-emitting diode performance. Nano Lett. 12(1), 331–336 (2012) 70. Zhu, J., Wang, S.-N., Li, J.-J., Zhao, J.-W.: The effect of core size on the fluorescence emission properties of CdTe@CdS core@shell quantum dots. J Lumin. 199, 216–224 (2018) 71. Itzhakov, S., Shen, H., Buhbut, S., Lin, H., Oron, D.: Type-II quantum-dot-sensitized solar cell spanning the visible and near-infrared spectrum. J Phys Chem C. 117(43), 22203–22210 (2013) 72. Verma, S., Kaniyankandy, S., Ghosh, H.: Charge separation by indirect bandgap transitions in CdS/ZnSe type-II core/shell quantum dots. J Phys Chem C. 117, 10901–10908 (2013) 73. Park, Y.S., Bae, W.K., Padilha, L.A., Pietryga, J.M., Klimov, V.I.: Effect of the core/shell interface on auger recombination evaluated by single-quantum-dot spectroscopy. Nano Lett. 14(2), 396–402 (2014) 74. Selopal, G.S., Zhao, H., Tong, X., Benetti, D., Navarro-Pardo, F., Zhou, Y., Barba, D., Vidal, F., Wang, Z.M., Rosei, F.: Highly stable colloidal “giant” quantum dots sensitized solar cells. Adv Funct Mater. 27(30), 1701468 (2017) 75. Tong, X., Kong, X.-T., Zhou, Y., Navarro-Pardo, F., Selopal, G.S., Sun, S., Govorov, A.O., Zhao, H., Wang, Z.M., Rosei, F.: Near-infrared, heavy metal-free colloidal “giant” core/shell quantum dots. Adv Energy Mater. 8(2), 1701432 (2018) 76. Brovelli, S., Schaller, R.D., Crooker, S.A., Garcia-Santamaria, F., Chen, Y., Viswanatha, R., Hollingsworth, J.A., Htoon, H., Klimov, V.I.: Nano-engineered electron-hole exchange interaction controls exciton dynamics in core-shell semiconductor nanocrystals. Nat Commun. 2, 280 (2011) 77. Meinardi, F., McDaniel, H., Carulli, F., Colombo, A., Velizhanin, K.A., Makarov, N.S., Simonutti, R., Klimov, V.I., Brovelli, S.: Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots. Nat Nanotechnol. 10(10), 878–885 (2015) 78. Tong, X., Zhou, Y., Jin, L., Basu, K., Adhikari, R., Selopal, G.S., Tong, X., Zhao, H., Sun, S., Vomiero, A., Wang, Z.M., Rosei, F.: Heavy metal-free, near-infrared colloidal quantum dots for efficient photoelectrochemical hydrogen generation. Nano Energy. 31, 441–449 (2017) 79. Erickson, C.S., Bradshaw, L.R., McDowall, S., Gilbertson, J.D., Gamelin, D.R., Patrick, D.L.: Zero-reabsorption doped-nanocrystal luminescent solar concentrators. ACS Nano. 8, 3461– 3467 (2014)

Core/Shell Quantum-Dot-Based Luminescent Solar Concentrators

313

80. Bradshaw, L.R., Knowles, K.E., McDowall, S., Gamelin, D.R.: Nanocrystals for luminescent solar concentrators. Nano Lett. 15(2), 1315–1323 (2015) 81. Zhou, J., Zhu, M., Meng, R., Qin, H., Peng, X.: Ideal CdSe/CdS core/shell nanocrystals enabled by entropic ligands and their core size-, shell thickness-, and ligand-dependent photoluminescence properties. J Am Chem Soc. 139(46), 16556–16567 (2017) 82. Ghosh, Y., Mangum, B.D., Casson, J.L., Williams, D.J., Htoon, H., Hollingsworth, J.A.: New insights into the complexities of shell growth and the strong influence of particle volume in nonblinking “giant” core/shell nanocrystal quantum dots. J Am Chem Soc. 134(23), 9634– 9643 (2012) 83. Michalska, M., Aboulaich, A., Medjahdi, G., Mahiou, R., Jurga, S., Schneider, R.: Amine ligands control of the optical properties and the shape of thermally grown core/shell CuInS2 /ZnS quantum dots. J Alloys Compd. 645, 184–192 (2015) 84. Hanifi, D.A., Bronstein, N.D., Koscher, B.A., Nett, Z., Swabeck, J.K., Takano, K., Schwartzberg, A.M., Maserati, L., Vandewal, K., van de Burgt, Y., Salleo, A., Alivisatos, A.P.: Redefining near-unity luminescence in quantum dots with photothermal threshold quantum yield. Science. 363, 1199–1202 (2019) 85. Zhao, H., Chaker, M., Wu, N., Ma, D.: Towards controlled synthesis and better understanding of highly luminescent PbS/CdS core/shell quantum dots. J Mater Chem. 21(24), 8898 (2011) 86. Huang, B., Yang, H., Zhang, L., Yuan, Y., Cui, Y., Zhang, J.: Effect of surface/interfacial defects on photo-stability of thick-shell CdZnSeS/ZnS quantum dots. Nanoscale. 10(38), 18331–18340 (2018) 87. Yang, X., Zhao, D., Leck, K.S., Tan, S.T., Tang, Y.X., Zhao, J., Demir, H.V., Sun, X.W.: Full visible range covering InP/ZnS nanocrystals with high photometric performance and their application to white quantum dot light-emitting diodes. Adv Mater. 24(30), 4180–4185 (2012) 88. Tong, X., Kong, X.T., Wang, C., Zhou, Y., Navarro-Pardo, F., Barba, D., Ma, D., Sun, S., Govorov, A.O., Zhao, H., Wang, Z.M., Rosei, F.: Optoelectronic properties in near-infrared colloidal heterostructured pyramidal “giant” core/shell quantum dots. Adv Sci (Weinh). 5(8), 1800656 (2018) 89. Navarro-Pardo, F., Zhao, H., Wang, Z.M., Rosei, F.: Structure/property relations in “giant” semiconductor nanocrystals: opportunities in photonics and electronics. Acc Chem Res. 51(3), 609–618 (2018) 90. Tan, L., Zhou, Y., Ren, F., Benetti, D., Yang, F., Zhao, H., Rosei, F., Chaker, M., Ma, D.: Ultrasmall PbS quantum dots: a facile and greener synthetic route and their high performance in luminescent solar concentrators. J Mater Chem A. 5(21), 10250–10260 (2017) 91. Chatten, A.J., Barnham, K.W.J., Buxton, B.F., Ekins-Daukes, N.J., Malik, M.A.: Proc. of 3rd World Conf. on photovoltaic energy conversion. IEEE Osaka. 3, 2657 (2003) 92. Xu, L., Yao, Y., Bronstein, N.D., Li, L., Alivisatos, A.P., Nuzzo, R.G.: Enhanced photon collection in luminescent solar concentrators with distributed bragg reflectors. ACS Photonics. 3, 278–285 (2016) 93. Connell, R., Ferry, V.E.: Integrating photonics with luminescent solar concentrators: optical transport in the presence of photonic mirrors. J Phys Chem C. 120(37), 20991–20997 (2016) 94. Connell, R., Pinnell, C., Ferry, V.E.: Designing spectrally-selective mirrors for use in luminescent solar concentrators. J Opt. 20(2), 024009 (2018) 95. Song, H.J., Jeong, B.G., Lim, J., Lee, D.C., Bae, W.K., Klimov, V.I.: Performance limits of luminescent solar concentrators tested with seed/quantum-well quantum dots in a selectivereflector-based optical cavity. Nano Lett. 18(1), 395–404 (2018) 96. Mateen, F., Oh, H., Jung, W., Binns, M., Hong, S.-K.: Metal nanoparticles based stack structured plasmonic luminescent solar concentrator. Sol Energy. 155, 934–941 (2017) 97. Zhao, H., Benetti, D., Tong, X., Zhang, H., Zhou, Y., Liu, G., Ma, D., Sun, S., Wang, Z.M., Wang, Y., Rosei, F.: Efficient and stable tandem luminescent solar concentrators based on carbon dots and perovskite quantum dots. Nano Energy. 50, 756–765 (2018)

314

G. Liu et al.

98. Liu, G., Zhao, H., Diao, F., Ling, Z., Wang, Y.: Stable tandem luminescent solar concentrators based on CdSe/CdS quantum dots and carbon dots. J Mater Chem C. 6(37), 10059–10066 (2018) 99. Needell, D.R., Ilic, O., Bukowsky, C.R., Nett, Z., Xu, L., He, J., Bauser, H., Lee, B.G., Geisz, J.F., Nuzzo, R.G., Alivisatos, A.P., Atwater, H.A.: Design criteria for micro-optical tandem luminescent solar concentrators. IEEE J Photovolt. 8(6), 1560–1567 (2018) 100. Tamang, S., Lincheneau, C., Hermans, Y., Jeong, S., Reiss, P.: Chemistry of InP nanocrystal syntheses. Chem Mater. 28, 2491–2506 (2016) 101. Li, L., Reiss, P.: One-pot synthesis of highly luminescent InP/ZnS nanocrystals without precursor injection. J Am Chem Soc. 130, 11588–11589 (2008) 102. Lim, J., Bae, W.K., Lee, D., Nam, M.K., Jung, J., Lee, C., Char, K., Lee, S.: InP@ZnSeS, core@composition gradient shell quantum dots with enhanced stability. Chem Mater. 23(20), 4459–4463 (2011) 103. Kim, S., Kim, T., Kang, M., Kwak, S.K., Yoo, T.W., Park, L.S., Yang, I., Hwang, S., Lee, J.E., Kim, S.K., Kim, S.W.: Highly luminescent InP/GaP/ZnS nanocrystals and their application to white light-emitting diodes. J Am Chem Soc. 134(8), 3804–3809 (2012) 104. Karatum, O., Jalali, H.B., Sadeghi, S., Melikov, R., Srivastava, S.B., Nizamoglu, S.: Lightemitting devices based on type-II InP/ZnO quantum dots. ACS Photonics. 6(4), 939–946 (2019) 105. Nagamine, G., Nunciaroni, H.B., McDaniel, H., Efros, A.L., de Brito Cruz, C.H., Padilha, L.A.: Evidence of band-edge hole levels inversion in spherical CuInS2 quantum dots. Nano Lett. 18(10), 6353–6359 (2018) 106. Li, C., Chen, W., Wu, D., Quan, D., Zhou, Z., Hao, J., Qin, J., Li, Y., He, Z., Wang, K.: Large stokes shift and high efficiency luminescent solar concentrator incorporated with CuInS2 /ZnS quantum dots. Sci Rep. 5, 17777 (2015) 107. Liu, H., Li, S., Chen, W., Wang, D., Li, C., Wu, D., Hao, J., Zhou, Z., Wang, X., Wang, K.: Scattering enhanced quantum dots based luminescent solar concentrators by silica microparticles. Sol Energ Mater Sol C. 179, 380–385 (2018) 108. Liu, G., Sun, B., Li, H., Wang, Y., Zhao, H.: Integration of photoelectrochemical devices and luminescent solar concentrators based on giant quantum dots for highly stable hydrogen generation. J Mater Chem A. 7, 18529–18537 (2019)

Index

A photon to current conversion efficiency (APCE), 231 Absorption coefficient, 124, 144, 146, 147 Alloy interface, 207–208 Ammonia, 85 Amplified spontaneous emission (ASE), 201 Anode, 224–226 Atomic force microscopic (AFM) images, 91, 92 Auger recombination (AR) process, 198

B Band alignment, 221, 224, 226, 232, 233, 240, 242, 245, 246, 248 Biomass conversion, 260 Biosensing, 21 Building-integrated PVs (BIPVs), 288

C Cadmium telluride (CdTe), 14–15 Carrier dynamics, 231, 232, 235, 236, 238, 242, 244–246, 248 Carrier separation, 229 Carrier transfer, 242 Carrier transport, 229–230 Charge extraction (CE), 231 Charge transfer (CT), 267 Chemical bath deposition (CBD) process, 222 Chemical stability, 202, 226, 227, 235, 245, 258, 273–275, 290 Coffee-ring effect, 215

Conduction band (CB), 259, 291 Conduction band minimum (CBM), 32 Core-multishell, see Ternary chalcogenides Core/shell quantum dot atoms and molecules, 123 cylindrical, 159 density-matrix formalism, 127 electronic structure, 124 energy gap, 124 fabrication technology, 127 high quantum yield, 159 impurity states binding energy, 136 donor impurity, 134 electron binding energy, 137, 138 electron energy levels, 137, 138 electronic properties, 134 electron wave function, 135 linear and nonlinear optical absorption, 134 non-integer quantum number, 136 parameters, 134 probability density distribution, 136–137 radial wave function, 135–136 Schrodinger equation, 135 infrared devices, 128 interband optical absorption, 144–147 Kratzer confinement potential, 125, 126 layered geometry, 125 one particle states boundary conditions, 129 charge carriers, 132 density distribution, 131

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 X. Tong, Z. M. Wang (eds.), Core/Shell Quantum Dots, Lecture Notes in Nanoscale Science and Technology 28, https://doi.org/10.1007/978-3-030-46596-4

315

316 Core/shell quantum dot (cont.) electron-hole interaction, 133 energy spectrum dependence, 130 hyper-geometric function, 134 inner and outer interfaces, 128 mathematical model, 128 optical characteristics, 132 radial distribution, 131 radial equation, 129 radial wave function, 131 Schrodinger equation, 128 spherical, 128, 129 spherical oscillator, 133 spherical symmetry, 129 transcendental equation, 130 Winternitz-Smorodinsky parameters, 132, 133, 134 optical properties, 124 optical transitions, 127 optoelectronic devices, 159 photoluminescence, 124 physical processes, 127 physical properties, 123–124, 128 polymer-coated magnetite core, 127 quadrupole moment, 148–150 quantum nanostructures, 125 rectangular and shifted parabolic confinement potentials, 125, 126 semiconductor nanocrystals, 124 single-particle states, 125 spherical, 127, 159 Stark shift, 127 thin spherical nanolayer, 159 two-electron sates Coulomb interaction, 144 effective radius, 143 electrons, 140 energy spectrum, 139 ground-state energy, 141 Hamiltonian, 140 harmonic approximation, 143 harmonic oscillations, 139 Mathieu equation, 139 oscillation, 143 perturbation energy, 144 perturbation theory, 143 radial direction, 140 radial wave function, 140 Schrodinger equation, 141 spherical QD, 140 spherical surface, 138 spherium model, 139

Index transformations, 142 two-dimensional Schrodinger equation, 140 types, 125 Winternitz-Smorodinsky confinement potential, 125, 126 zero-dimensional systems, 124 Counter electrode (CE), 221, 227 Crystal structure, 45–46 Cylindrical nanolayer electron-electron interaction, 150 electron states, 150 energy spectrum, 151–153 orbital current, 153–156 quantum tunneling, 150 ring-shaped systems, 150 spherical symmetry, 150 spin magnetic moment, 150, 156–158 spin-orbit interaction, 150 wave functions, 151–153

D Density functional theory (DFT), 139 Distributed feedback (DFB), 213

E Electrical generation, 260 Electrochemical impedance spectroscopy (EIS), 231 Electrodes, 227 Electroluminescence (EL), 20 Electrolysis, 260 Electron and hole wave function, 207 Electron diffusion process, 229 Electron–hole separation, 229 Electron injection efficiency, 231 Electron transport layers (ETLs), 19, 224 Electrophoretic deposition (EPD) approach, 274 Emission peak efficiency, 36 Energy-dispersive X-ray spectrometer (EDX), 39 Energy-dispersive X-ray spectroscopy (EDS), 61 Energy gap AF parameter, 173 Bose–Einstein factor, 172 electron charge, 173 electronic subsystem, 171 electron–phonon interaction, 172–174

Index exciton–phonon interaction, 172 Huang–Rhys factor, 174 linear model, 172 linear–quadratic relation, 172 phonons, 174 photoluminescence bands, 174 physical interpretation, 172 second-order perturbation theory, 172 semiconductor materials, 171 single-phonon approximation, 172 spectrum, 171 temperature-dependent factors, 174 temperature-independent parameters, 173 thermal dilation, 173 thermal expansion and electron–phonon interaction, 173 Ensemble, 182–183 Exciton dynamics process, 206–208 Exciton–phonon interaction LakeShore Model, 174 linear model, 177 longitudinal acoustic vibrations (LA), 178 nonlinear character, 178 optical density, 175 optical transitions, 176, 178 parameters, 177–178 phonon modes, 178 square-shaped symbols, 176 temperature behavior, 178 temperature dependencies, 174, 175, 177 temperature evolution, 176 temperature-induced change, 176 temperature ranges, 178 External quantum efficiency (EQE), 20, 231

F Flash hot injection, 203–204 Fluorine-doped tin oxide (FTO), 224, 274 Förster resonance energy transfer (FRET), 21 Full width at half maximum (FWHM), 36, 43

G Giant volume, 207 Gibbs free energy, 263 Glutathione, 72

H Half-width, 179–181 Heavy metals, 281–283 Hexadecylamine (HDA), 6

317 High-resolution transmission electron microscope (HRTEM) images, 53, 245, 266 Hole transport layers (HTLs), 19 Homogeneous broadening, 186–187 Hydrogen, 260–261

I Impurity states, 134–138 Incident photon to current conversion efficiency (IPCE), 231, 273 Indium-doped tin oxide (ITO), 224 Inhomogeneous broadening, 186–187 Intensity-modulated photocurrent spectroscopy (IMPS), 231 Interfacial defects, 245 Interfacial layers, 245, 247 Internal quantum efficiency (IQE), 231 Inverted type-I core/shell QDs, 237

L Laser devices, 213 Laser interference ablation (LIA), 214 Light absorption, 228 Light harvester, 226 Light-harvesting efficiency (LHE), 231 Luminescence, 31 Luminescent properties absorption and emission spectra, 70 AIS and AIS/ZnS QDs absorption spectra, 76, 77, 79 aggregation stability, 76 bandgap, 77 broad distribution, 83 CdS LO phonon energy, 85 core/shell AIS/ZnS-GSH QDs, 79 core/shell CIS/ZnS QDs, 81 D-A model, 82 distribution of distances, 83 electron and hole traps, 82 electron-hole distances, 82 electron-hole recombination mechanisms, 75 electron-phonon interaction, 84, 86 high-resolution photoelectron spectra, 79 hot-injection/heating-up methods, 76 Huang-Rhys factor, 84, 85 lattice imperfections, 81 lattice vibrations, 84 mercapto-and amino-groups, 85

318 Luminescent properties (cont.) molar AIS concentration, 78 normalized absorption, 86, 87 optical density, 78 phonon energy, 84 phonon peak, 85 PL characteristics, 80 PL quantum yields (QYs), 80 PL spectra, 76, 86, 87 radiative and non-radiative recombination, 83 radiative recombination, 82, 84 semiconductors, 76 size distribution, 82 size-selection method, 76, 82–83 size-selective precipitation, 81 spectral parameters, 83 spectral QD characteristics, 77 spectrum, 83 STE model, 84 S-to-In ratio, 80 surface defects, 81 surface-to-volume ratio, 85 thermal lattice energy, 83 trapped carriers, 83 2-propanol, 77 vibrational modes, 84 vibrational relaxation, 86 analogy, 68 aqueous synthesis and optical properties absorption, 74 Ag/In ratio, 73 AIS QDs, 72 bandgap, 73 biosensing applications, 71 carboxyl anions, 72 efficient phase transfer and stabilization, 71 FWHM, 75 glutathione, 72 heating-up and hot-injection protocols, 71 heat treatment, 75 lattice reconstruction, 74 mercaptoacids, 73 PL efficiency, 72 PL spectra, 74 spectral parameters, 73 thermal treatment, 73, 75 topographic surface, 73 water-soluble multifunctional ligands, 72 Zn2+ ions, 75 ZnS deposition, 75

Index biomedical labeling, 68 biosensing, 94 broad absorption bands, 68 broadband character, 68 D-A model, 69, 71 distributions and sources, 70 donor and acceptor, 70 electrons and holes, 71 emission of photons, 69 “energy depth,” 71 energy factors, 112 high thermal sensitivity and photostability, 94 lattice defects, 68, 69, 113 light-emitting technologies, 94 measurements, 112 multicolored cell tracking, 68 optical and physicochemical properties, 68 optical properties, 70 optoelectronic systems, 68 physical model, 113 radiative and non-radiative processes, 94 reports of experimental evidence, 70 single-particle PL measurements, 91–93 size-selective precipitation, 112 spectral emission parameters, 95 spectral parameters, 112 temperature (see Temperature) temperature-dependent variation, 71, 113 tunneling mechanism, 69 variability of application-relevant features, 68 vibrational modes, 113 Luminescent solar concentrators (LSCs) bandgap alignment and optical properties, 290 core/shell quantum dots exciton dynamic, 290–292 optical efficiency, 293 optoelectronic applications, 293 shell materials, 291, 293 SILAR approach, 293–294 stability, 294–295 Stokes shift, 292–293 water transfer, 294 cu-based ternary/quaternary eco-friendly core/shell QD-based LSCs, 304–307 eco-friendly InP/ZnO Core/Shell QD-Based LSCs, 303–304 efficiency, 288 fluorescent materials, 290 fluorophores, 288 fossil energy, 288 internal quantum efficiency, 289

Index near-infrared PbS/CdS Core/Shell QD-based LSCs, 295–298 optical efficiency, 289, 290 optical performance, 290 PCE, 288 photons traveling, 289 polymer matrix/emissions, 289 PV devices, 288 QD-based LSC, 288, 289 short-circuit current, 290 solar energy, 288 solar tracking system, 288 visible CdSe/CdS Core/Shell QD-based LSCs BIPVs and plastic greenhouses, 301 diffuse/specular mirror, 298 emitted photons, 298 external quantum efficiency (EQE), 298, 299 J-V characteristics, 300 normalized absorption, 296 optical efficiency, 299, 300 optical properties, 298 photons, 298 PL spectra, 296, 297 polymer matrix, 298 sandwich structure, 300 shell layer, 298 tandem structure, 300–302 transmission electron micrograph, 300 waveguide transparent/semitransparent materials, 288

M Mercaptoacetate (MA) anions, 85 Mercaptoacetic acid (MAA), 69, 71 Mercaptopropionic acid, 71 Mesoporous carbon (MC), 223 Microring, 213 Microsphere resonators, 213

N Nanocrystal absorption atomistic pseudopotential approach, 169 Beer–Lambert law, 168 derivative spectrophotometry (see Nanocrystal absorption) disorder effects, 189–191 electronic structure, 166 half-width, 179–181 hardware/numerical methods, 169

319 homogeneous and inhomogeneous broadening, 186–187 measurements, 167 noise level, 169 optical density, 167, 168 particle-size distribution, 168 QD-1 and QD-2 InP/ZnS, 167 QD size distribution, 185–186 quantum-mechanical approaches, 171 simulation, 187–189 spectral processing technique, 169 spin–orbit splitting, 169 temperature band broadening, 189–191 three-layer structure, 167 transition energy, 169 wavelength, 168 Nanoimprint lithography, 214 Near-infrared (NIR) emissions, 42 Nonrenewable energy resources, 260

O Octadecylamine (ODA), 265 One-dimensional (1D) nanostructures, 224, 226 One-electron current, 150 Open-circuit voltage decay (OCVD), 231 Optical absorption biomarkers, 166 biosensors, 166 chemical elements, 166 luminescent processes, 166 nanocrystal absorption (see Nanocrystal absorption) photoluminescence (PL) spectra, 166 quantum dots (QDs), 165 semiconductor nanocrystals, 165 spatial dimensions, 165–166 transition energy, 166 Optical gain performance, 210–213 Optical microstructures, 213 Optical property, 290, 298, 307–308 Optoelectronic properties, 245

P Photoconversion efficiency (PCE), 220 Photoelectrochemical cell applications, 258 band alignment, 259 band structure, 259 CB and VB, 259 charge carriers, 259

320 Photoelectrochemical cell (cont.) charge dynamics quasi-type II core/shell QDs, 271, 272 semiconductor films, 272–273 type I core/shell QDs, 271, 272 type II core/shell QDs, 271, 272 electronic band structure, 259 energy consumption, 257 fossil fuels, 257 heavy metals, 281–283 hydrogen, 258, 260–261 morphologies, 258 organic ligands, 258 PEC hydrogen generation, 261–263 physical/chemical and optical properties, 258 physical properties, 259 quasi-type II core/shell QDs, 269–271 semiconductor colloidal quantum dots (QDs), 258 semiconductor material, 258 solar energy, 258, 260–261 synthesis of core/shell QDs, 264–266 TiO2 , 258 type I core/shell QDs, 266–267 type II core/shell QDs, 267–269 Photoelectrolysis, 261 Photoexcited electrons, 261 Photoluminescence (PL) analysis average radiative lifetime, 87, 88 bandgap energy, 43 distance-dependent Coulomb interaction, 89 emission rate, 89 fluorescence emission spectrometer, 41 growing multishells, 42 higher intensity, MnS shell, 42 influence of shell concentration, 43, 44 non-radiative recombination, 88, 89 phonons, 90 PL spectra, 42 radiative recombination, 88, 89 self-trapped exciton model, 90 size-selected colloidal AIS/ZnS QDs, 87, 88 spectral parameters, 90 valence band (VB), 43 vibrational relaxation, 90 Photoluminescence quantum yields (PL QYs), 197–198 Photophysical/chemical stability, 235–237 Photovoltage decay (TVD), 231 Photovoltaic characterizations, 230–231 Photovoltaic (PV) devices, 288

Index Polyethyleneimine (PEI), 85, 95 Power conversion efficiency (PCE), 19, 288

Q Quantum-confined Stark effect (QCSE), 209–210 Quantum dot sensitized solar cells (QDSCs) architecture anode, 224–226 CE, 227 electrodes, 227 light harvester, 226 carrier separation, 229 carrier transport, 229–230 conventional p-n junction solar cells, 228 core/shell QDs band alignment, 232 bandgap tuning, 232 classification, 232–233 effective strategy, 232 non-radiative carrier recombination, 231–232 optoelectronic properties, 231 photogenerated electrons, 232 synthesis, 233–235 device architecture, 221 environmental pollution and global warming, 220 fossil fuels, 220 growing energy demand and environmentrelated issues, 220 history, 221–224 hot electron extraction, 221 light absorption, 228 nanomaterials and device architecture, 220 optoelectronic properties, 221 PCE, 220 photoelectric effect, 220 photoelectrochemical, 220 photothermal, 220 photovoltaic (PV), 220 photovoltaic characterizations, 230–231 photovoltaic performance core and shell interface optimization, 245–248 quasi-type-II core/thick-shell QDs, 242–245 reverse type-I core/shell QDs, 237–238 type-I core/shell QDs, 235–237 type-II core/shell QDs (see Type-II core/shell QDs) recombination, 230 silicon solar cell technology, 220

Index single-photon absorption, 221 solar energy, 220 thin-film technology, 220 third-generation PV devices, 220 Quantum yield, 293–294

R Reactive oxygen species (ROS), 22 Reference electrode, 263 Renewable energy resources, 260 Reverse type-I core/shell QDs, 237–238

S Schrödinger equation, 242 Selected area electron diffraction (SAED) patterns, 38, 54, 61 Self-trapped excitons (STE), 69 Semiconductor core/shell quantum dots (QDs) absorption and emission spectra, 13, 14 applications LEDs, 19–21 solar cells, 18–19 band alignment, 8 biology, applications biosensing, 21 gene and drug delivery, 22 therapy, 22 bulk semiconductor material, 2 CdTe, 14 characterization, 14–17 chemical and photochemical stability, 11 classification, 4 dissolving dimethylcadmium and selenium shot, 8 electron-hole pair, 2 electronic properties, 1, 10 electronic wave function, 9 electron leaves, 2 emission wavelength, 11, 12 epitaxial shell growth, 10 evolution, 11, 12 growth mechanism, 7, 9 hot injection method, 8 injection method, 5–6 lattice strain and defect states, 7 material parameter, 7 monolayers, 8 narrow emission peak, 10 near-infrared spectrum, 11 non-injection method, 6 optical properties, 1, 10 organometallic precursors, 12

321 photobleaching, 10 photoblinking, 10 quantum confinement, 2 semiconductor nanostructured materials, 1, 2 shell thickness, 7 SILAR technique, 9, 10 surface effect, 2–4 synthesis, 5, 14–17 TC-SP, 12 TOP-SILAR method, 8 UV/blue/green, 11 zinc acetate and octanthiol, 8 zinc ethylxanthate, 12 zinc stearate, 12 Semiconductor film, 224 Semiconductor nanocrystal, 165, 166, 190 Semiconductors, see Semiconductor core/shell quantum dots (QDs) Shell layer, 245 Shell thickness, 242 Size distribution, 185–186 Soft lithography, 214 Solar cells, 18–19 Solar-driven PEC cells core/alloyed-shell/shell QDs band structure and carrier transition, 278, 280 device stability, 278 hole transfer capability, 277 hole transport layer, 276 interfacial layer, 277 photoanodes, 278–281 photocurrent density, 278, 280 self-oxidation, 276 stability measurements, 278, 279 core/thin-shell QDs, 274–275 “giant” core/shell QDs (g-QDs), 276 photoanodes, 273 photocurrent density, 274 Solar energy, 260–261 Solar energy conversion devices, 242 Spatial electron–hole overlap, 244 Spatial separation, 242 Spectral characteristics, 144 Stability, 294–295 Static and dynamic disorder, 182–183 Stimulated radiation, 213–216 Stokes shift, 290, 292–293 Successive ionic layer adsorption and reaction (SILAR) method, 8, 202–203, 223, 293–294 Superior photophysical/chemical stability, 245

322 Synthesis, see Semiconductor core/shell quantum dots (QDs) Syringe pump injection, 203

T Temperature AIS and AIS/ZnS QDs, 108–109 bulk and nanometer compounds, 111 “classical”, 111 defect-related PL (DPL), 108 electronic structure, 110 electron-phonon interaction, 108 emission bands, 111 evolution behavior, 183–184 static and dynamic disorder, 182–183 excitonic PL (EPL), 108 heating, 95 luminescent properties, 95 mechanisms/routes, 111 multifunctional ligands, 95 optical phonon decay, 110 PEI, 95 PL band maximum position absorption spectra, 100 behavior, 99 excitonic absorption of CdS-PEI QDs, 101 GSH molecules, 101 heating/cooling, 100 linear manner, 101 macromolecular PEI, 101 measurements, 100 parameters, 100 ratiometric sensor, 102 sensor output, 103 size-selected colloidal AIS QDs, 102 spectral PL parameters, 100 stabilization, 101 surface metal complexes, 102 temperature-dependent variation, 102 thermal dissociation of Cd(II)-PEI, 101 two-channel portable spectrometer, 103 PL decay curves characteristics, 106 dependences, 105 electronic energy transfer, 105 energy donor and acceptor, 105–106 energy transfer, 106 fitting parameters, 104, 106–107 kinetic curves, 103 Kohlrausch-type stretched exponent model, 104

Index metal-chalcogenide QDs, 105 non-radiative energy transfer processes, 104 non-radiative recombination, 104, 105 radiative recombination, 104, 105 single-exponential functions, 104 radiative and non-radiative processes, 107–108 radiative recombination, 110 spectral PL parameters, 108 sustainable semiconductor nanomaterials, 108 temperature-dependent PL evolutions, 108 thermal sensitivity, 95 variations, PL intensity, 96–99 Ternary chalcogenides AgInS2 nanocrystals, 33 binary chalcogenides, 31 Cd2+-Doped Core and Core/Shell AgInS2 Nanocrystals crystal structure analysis, 51–52 elemental compositions, 54 photoluminescence properties, 50–51 surface morphology analysis, 53–54 UV-Vis absorption spectra, 48–50 cluster science, 30 core and shell nanocrystals, 32 core-multishell and alloyed AgInS2/CdS/ZnS shell nanocrystals crystal structure analysis, 59–60 morphological studies, 60–61 photoluminescence studies, 56–58 XPS analysis, 58–59 core-multishell architecture, 32 core-multishell CuInS2/MnS/ZnS nanocrystals absorption properties, 39–41 crystal structure analysis, 45–46 influence of shell concentration, 45–46 morphological analysis, 46–48 photoluminescence analysis (see Photoluminescence analysis) XPS analysis, 43–45 core/shell nanoparticles, 33–34 core/shell type, 30, 31 electronic energy levels, 30 energy levels, 30 epitaxial growth, 32 growth of multishell over core nanocrystals, 34 Mn2+ Ion-Doped Core and Core/Shell CuInS2 Nanocrystals crystal structure analysis, 38 morphological properties, 38–39

Index optical properties, 34–35 photoluminescence properties, 36–37 XPS analysis, 37 properties of nanoclusters, 29 semiconductor materials, 30 shell materials, 30, 31 Tetradecylphosphonic acid (TDPA), 240 Thermal cycling alternate ionic layer adsorption and reaction (TCSILAR) method, 202–203 Thermal cycling coupled single precursor (TC-SP), 12 Thermolysis, 260 Thick-shell core/shell quantum dots AR process, 198 basic characteristics, 199–201 flash hot injection, 203–204 “giant” QDs, 198 monolayers (MLs), 198 nucleation and growth dynamics, 198 optical properties, 198 exciton dynamics process, 206–208 fundamental, 204–206 optical gain performance, 210–213 QCSE, 209–210 stimulated radiation, 213–216 optical stability, 198 photophysical and photochemical stability, 198 semiconductor nanocrystals, 197 SILAR method, 202–203 syringe pump injection, 203 ultrafast nonradiative process, 198 Thioglycolic acid (TGA), 14, 237 Total internal reflection (TIR), 288 Transient absorption (TA), 238, 244 Transient photocurrent decay (TCD), 231

323 Transmission electron microscopy (TEM) images, 91 Trioctylphosphine (TOP), 5, 240 Trioctylphosphine oxide (TOPO), 5, 106 Type-II core/shell QDs band engineering approach, 240, 241 electron injection, 238 electron transport layer, 238, 240 light-harvesting materials, 240 QDSCs and optoelectronic devices, 239 solar cell application, 240 structural and optical characterization, 241 TEM images, 239, 240

U Ultraviolet (UV) light stability, 205

V Valence band (VB), 228, 259, 291 Valence band maximum (VBM), 32

W Wide-bandgap semiconductor, 221

X X-ray diffraction (XRD), 38, 78 X-ray photoelectron spectroscopic (XPS), 37, 78

Z (Zero-phonon) emission line (ZPL), 69