for renewable energy applications metal chalcogenide ... from diverse perspectives in a series of...

Provides a highly focused comprehensive overview of advanced cutting-edge research on metal chalcogenide nanostructures for photovoltaics, hydrogen

production, thermoelectric, and other renewable energy applications

Meeting impending energy requirements by an ecologically benign approach entails scientific innovations to proficiently produce, store, transfer, and utilize enormous amounts of energy. The critical requirement to attain this goal demands cost-effective materials that convert maximum energy from the sun and other renewable means. Metal chalcogenide semiconductor nanostructures presents the most important class of nanomaterial that provides highly efficient transport of electrons and excitons, and is regarded as the most promising building block for nanoscale renewable energy nanodevices and nanosystems.

The objective of Metal Chalcogenide Nanostructures for Renewable Energy Applications is to illuminate the essential and underlying science related to semiconductor metal chalcogenide nanostructures fabrication for potential renewable energy applications. It is an illustrative snapshot of the latest developments from diverse perspectives in a series of chapters based on synthesis, properties, characterization, and applications of metal sulfide, selenide, and telluride nanostructures from distinguished expert researchers. Subjects included in the book:

An overview of the current status of metal chalcogenide nanostructures• State of-the-art methods for chalcogenide nanostructures fabrication• Growth mechanism and surface functionalization of chalcogenide nanostructures• Structural and optical properties of chalcogenide nanostructures• Formation of multiple metal chalcogenide nanocomposites (hetrostructures)• Doping of metal chalcogenide nanostructures • Synthesis of metal chalcogenide 0,1,2 and 3-dimensional nanostructures• Applications of metal chalcogenide in high performance renewable energy • conversion devices (photovoltaics, hydrogen production thermoelectric, etc).

ReadershipScientists, researchers, and engineers in advanced renewable energy materials research; industrial sectors intending to produce and producing advanced metal chalcogenide nanosheets (inorganic graphene) for photovoltaics; and PhD, master’s degree, and upper-level undergraduate-level courses on materials synthesis techniques and applications in renewable energy applications.

Dr. Ahsanuhaq Qurashi is an Assistant Professor at the Center of Excellence in Nanotechnology (CENT) and the Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia. He has published more than 45 SCI articles in high impact journals in the areas of materials science, chemistry, physics, applied sciences and nanotechnology, as well as more than 50 conference proceedings, six book chapters and review articles.

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Cover Design: Russell RichardsonCover Illustration: Couretsy of Dreamstime.com

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Qurashi

Metal Chalcogenide Semiconductor Nanostructures

and Their Applications in Renewable Energy

Scrivener Publishing100 Cummings Center, Suite 541J

Beverly, MA 01915-6106

Publishers at ScrivenerMartin Scrivener([email protected])

Phillip Carmical ([email protected])

Metal Chalcogenide Semiconductor

Nanostructures and Their Applications in

Renewable Energy

Edited by

Ahsanulhaq Qurashi

Copyright © 2015 by Scrivener Publishing LLC. All rights reserved.

Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts.Published simultaneously in Canada.

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Cover design by Russell Richardson

Library of Congr ess Cataloging-in-Publication Data:

ISBN 978-1-118-23791-5

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

Contents

Preface xi

Part 1: RENEWABLE ENERGY CONVERSION SYSTEMS

1 Introduction: An Overview of Metal Chalcogenide Nanostructures for Renewable Energy Applications 3Ahsanulhaq Qurashi1.1 Introduction 31.2 Metal Chalcogenide Nanostructures 71.3 Growth of Metal Chalcogenide Nanostructures 81.4 Applications of Metal Chalcogenide Nanostructures 161.5 Summary and Future Perspective 18References 18

2 Renewable Energy and Materials 23Muhammad Asif2.1 Global Energy Scenario 232.2 Role of Renewable Energy in Sustainable

Energy Future 252.3 Importance of Materials Role in

Renewable Energy 27References 30

3 Sustainable Feed Stock and Energy Futures 33H. Idriss3.1 Introduction 333.2 Discussion 34

3.2.1 Nuclear Technology 353.2.2 Solar Energy 363.2.3 Hydrogen by Water Splitting 38

References 41

vi Contents

Part 2: SYNTHESIS OF METAL CHALCOGENIDE NANOSTRUCTURES

4 Metal-Selenide Nanostructures: Growth and Properties 45Ramin Yousefi4.1 Introduction 454.2 Growth and Properties of Different Groups of

Metal-Selenide Nanostructures 484.2.1 Metal Selenides from II–VI Semiconductors 484.2.2 ZnSe 484.2.3 CdSe 544.2.4 HgSe 57

4.3 Metal Selenides from III–VI Semiconductors 574.3.1 In2Se3 58

4.4 Metal Selenides from IV–VI Semiconductors 614.4.1 SnSe 614.4.1 PbSe 62

4.5 Metal Selenides from V–VI Semiconductors 664.5.1 Sb2Se3 664.5.2 Bi2Se3 68

4.6 Metal Selenides from Transition Metal (TM) 694.6.1 Copper Selenide (CuSe, Cu3Se2) 704.6.2 Iron Selenide (FeSe2, FeSe) 714.6.3 MoSe2 724.6.3 WSe2 74

4.7 Ternary Metal-Selenide Compounds 754.7.1 CuInSe2 (Copper Indium Diselenide) 754.7.2 CdSSe 764.7.3 CdZnSe 77

4.8 Summary and Future Outlook 78Acknowledgment 79References 79

5 Growth Mechanism and Surface Functionalization of Metal Chalcogenides Nanostructures 83Muhammad Nawaz Tahir, Jugal Kishore Sahoo, Faegheh Hoshyargar, Wolfgang Tremel5.1 Introduction 84

5.1.2 Structure of Layered Transition Metal Chalcogenides (LTMCs) 87

Contents vii

5.2 Synthetic Methods for Layered Metal Chalcogenides 895.2.1 Laser Ablation 895.2.2 Arc Discharge 905.2.3 Microwave-Induced Plasma 905.2.4 Electron Beam Irradiation 905.2.5 Spray Pyrolysis 915.2.6 Sulfidization with H2S 915.2.7 Hydrothermal 915.2.8 Metal Organic Chemical Vapor Deposition

(MOCVD) Technique 915.2.9 Vapor–Liquid–Solid (VLS) Growth 945.2.10 Oxide-to-Sulfide Conversion 955.2.11 Hot-Injection Solution Synthesis 985.2.12 Liquid Exfoliation 99

5.3 Surface Functionalization of Layered Metal Dichalcogenide Nanostructures 1025.3.1 Surface Functionalization Based on

Polymeric Ligands 1025.3.2 Surface Functionalization Based on

Pearson Hardness 1075.3.3 Surface Functionalization of Metal

Chalcogenides by Silane 1105.4 Applications of Inorganic Nanotubes and Fullerenes 110

5.4.1 Energy 111References 113

6 Optical and Structural Properties of Metal Chalcogenide Semiconductor Nanostructures 123Ihsan-ul-Haq Toor and Shafique Khan6.1 Optical Properties of Metal Chalcogenides Semiconductor

Nanostructures 1246.1.2 Metal Chalcogenide Nanocrystals 126

6.2 Structural Properties and Defects of Metal Chalcogenide Semiconductor Nanostructures 133

References 142

7 Structural and Optical Properties of CdS Nanostructures 147Y. Al-Douri7.1 Introduction 1477.2 Nanomaterials 150

viii Contents

7.3 II–VI Semiconductors 1527.4 Sol-Gel Process 1557.5 Structural and Surface Characterization of

Nanostructured CdS 1567.6 Optical Properties 1597.7 Conclusion 161Acknowledgments 162References 162

Part 3: APPLICATIONS OF METAL CHALCOGENIDES NANOSTRUCTURES

8 Metal Sulfide Photocatalysts for Hydrogen Generation by Water Splitting under Illumination of Solar Light 167Zhonghai Zhang8.1 Introduction 1678.2 Photocatalytic Water Splitting on Single Metal Sulfide 169

8.2.1 CdS 1698.2.2 ZnS 1708.2.2 SnS2 172

8.3 Photocatalytic Water Splitting on Multi-metal Sulfide 1738.3.1 ZnIn2S4 1738.3.2 CuS/ZnS 1758.3.4 CuGa3S5 1768.3.5 CdS–MoS2 1778.3.6 NiS–CdS 1788.3.7 Mn–Cd–S 1798.3.8 PbS/CdS 1808.3.9 AGa2In3S8 (A = Cu or Ag) 180

8.4 Metal Sulfides Solid-Solution Photocatalysts 1808.5 Summary and Future Outlook 184References 184

9 Metal Chalcogenide Hierarchical Nanostructures for Energy Conversion Devices 189Ramin Yousefi, Farid Jamali-Sheini, Ali Khorsand Zak9.1 Introduction 190

9.1.1 Why Metal Chalcogenide Semiconductors Matter for Energy Conversion 191

9.2 Main Characteristics of Cd-Chalcogenide Nanocrystals (CdE; E = S, Se, Te) 192

Contents ix

9.3 Different Methods to Grow Cd-Chalcogenide Nanocrystals 1929.3.1 Thermal Evaporation Method to Grow

Cd-Chalcogenide Nanocrystals 1929.3.2 Chemical Bath Deposition Method to Grow

Cd-Chalcogenide Nanocrystals 2059.3.3 Electrochemical Deposition Method to Grow

Cd-Chalcogenide Nanocrystals 2109.3.4 Pulsed Laser Deposition (PLD) Method to Grow

Cd-Chalcogenide Nanocrystals 2129.4 Solar Energy Conversion 212

9.4.1 Modeling of Solar Energy Conversion 2139.4.2 Semiconductor Solar Cells 2169.4.3 Hierarchical Branching Nanostructures as

Better Solar Energy Harvesting 2189.5 Cd-Chalcogenide Nanocrystals as Solar Energy

Conversion 2199.5.1 CdS Nanostructures Solar Cells 2199.5.2 CdSe Nanostructures Solar Cells 2239.5.3 CdTe Nanostructures Solar Cells 226

9.6 Summary and Future Outlook 230References 230

10 Metal Chalcogenide Quantum Dots for Hybrid Solar Cell Applications 233Mir Waqas Alam and Ahsanulhaq Qurashi10.1 Introduction 23310.2 Chemical Synthesis of Quantum Dots 235

10.2.1 Single-Step Synthesis of Highly Luminescent Quantum Dots 235

10.2.2 Electrochemical Deposition Method 23510.2.3 Chemical Aerosol Flow Method 23610.2.4 Chemical Bath Deposition (CBD) 237

10.3 Quantum Dots Solar cell 23810.4 Summary and Future Prospects 243References 243

11 Solar Cell Application of Metal Chalcogenide Semiconductor Nanostructures 247Hongjun Wu11.1 Introduction 24711.2 Chalcogenide-Based Thin-Film Solar Cells 248

x Contents

11.3 CdTe-Based Solar Cells 24911.4 Cu(In,Ga)(S,Se)2 (CIGS)-Based Solar Cells 25111.5 Metal Chalcogenides-Based Quantum-Dots-Sensitized

Solar Cells (QDSSCs) 25311.6 Hybrid Metal Chalcogenides Nanostructure-Conductive

Polymer Composite Solar Cells 25711.7 Conclusions 261References 262

12 Chalcogenide-Based Nanodevices for Renewable Energy 269Y. Al-Douri12.1 Introduction 26912.2 Renewable Energy 27212.3 Nanodevices 27412.4 Density Functional Theory 27712.5 Analytical Studies 27812.6 Conclusion 284References 285

13 Metal Tellurides Nanostructures for Thermoelectric Applications 289Salman B. Inayat13.1 Introduction 29013.2 Thermoelectric Microdevice Fabricated by a MEMS-Like

Electrochemical Process 29013.3 Bi2Te3-Based Flexible Micro Thermoelectric

Generator 29213.4 High-Thermoelectric Performance of Nanostructured

Bismuth Antimony Telluride Bulk Alloys 29313.5 Nano-manufactured Thermoelectric Glass Windows

for Energy Efficient Building Technologies 29413.6 Conclusion 296References 297

Index 299

xi

Preface

Meeting impending energy requirements by an ecologically benign approach entails scientific innovations to proficiently produce, store, trans-fer, and utilize enormous amounts of energy. The critical requirement to attain this goal demands to develop cost-effective materials by imparting novel intriguing features to convert maximum energy from sun and other renewable means.

Metal chalcogenide semiconductor nanostructures present the most important class of nanomaterial that provides highly anisotropic diverse morphologies, described by the efficient transport of electrons and exci-tons, and has been regarded as the most promising building block for nanoscale renewable energy nanodevices and nanosystems. The growth, characterization, and applications of nanostructures entreat various disci-plines of science and engineering. The objective of this book is to illuminate the essentials, underlying science related to semiconductor metal chalco-genide nanostructures fabrication for potential renewable energy applica-tions. The effect is an illustrative snapshot of the latest developments from diverse perspectives in a series of chapters based on synthesis, properties, characterization, and applications of metal sulfide, selenide, and telluride nanostructures from distinguished betrothed researchers.

This book contents are divided into three main sections. Chapters 1–3 present an overview of increasing greenhouse emissions,

recent research and substantial progress reported in the literature, covering formation of 0, 1, 2, and 3 dimensional metal sulfide, selenide, and telluride nanostructures. The application of chalcogenide materials for renewable energy conversion, which includes photovoltaics, hydrogen production, thermoelectrics, fuel cell, supercapacitors, and lithium-ion batteries and their future projections are covered in Chapter 1. The potential impact of materials for alternative energy conversion systems and various important renewable energy alternatives is anticipated in Chapters 2 and 3.

xii Preface

Chapters 4–7 are devoted to comprehensive synthesis of metal chalco-genide (sulfide, selenide, and telluride) nanostructures including inorganic graphenes (layered structures) by various important methods, their char-acterization and growth mechanism for the formation of enthralling mor-phologies, and various important protocols for surface functionalization of chalcogenides to improve the processability in technological applica-tions are included in Chapters 4 and 5. The potential to engineer semi-conductor nanostructures properties during and after fabrication presents an exciting realms and extensive prospect to simply improve the perfor-mance of renewable energy conversion systems. Chapters 6 and 7 provide detailed account of structural and optical properties of semiconductor chalcogenides.

Chapters 8–13 are typically covering applications of metal chalco-genides nanostructures in diverse renewable energy conversion devices. Chapter 8 presents updated works metal sulfide nanostructures for solar-driven hydrogen production through water splitting. Chapter 9 gives brief account on hierarchical chalcogenide nanostructures, their properties and applications in energy conversion devices. Chapters 10 and 11 are based on metal chalcogenides in photovoltaic applications. Chapter 12 focuses on theoretical work including indirect band gap calculations results and density functional theory. Chapter 13 focuses on metal telluride nano-structures for thermoelectric devices operating around room temperature.

Ahsanulhaq QurashiDhahran, Saudi Arabia

August 2014

Ahsanulhaq Qurashi (ed.) Metal Chalcogenide Semiconductor Nanostructures and Their Applications in Renewable Energy, (269–288) 2015 © Scrivener Publishing LLC

269

12

Chalcogenide-Based Nanodevices for Renewable Energy

Y. Al-Douri

Institute of Nano Electronic Engineering, University Malaysia Perlis, Malaysia

AbstractThe indirect energy gap (Γ–X) of the chalcogenide compounds of metal chalco-genide (CdS and CdTe) is calculated using density functional theory (DFT) of the full potential-linearized augmented plane wave (FP-LAPW) method as imple-mented in WIEN2K code. The Engel–Vosko generalized gradient approximation (EV-GGA) formalism is used to optimize the corresponding potential for ener-getic transition and optical properties calculations of chalcogenide compounds as a function of quantum dot diameter and is used to test the validity of our model of quantum dot potential for improving the solar cell efficiency.

Keywords: Chalcogenide, quantum dot, optical properties

12.1 Introduction

Since II–VI semiconductor compounds have a large optical gap, the feasi-bility of green–blue optoelectronic devices and solar cells has been demon-strated [1]. A wide range of electronic properties of binary compounds has been predicted using theoretical calculations. Different attempts [2] have been made using different models. Experimental and theoretical results comparison may allow testing different competing theoretical approxima-tions. It is advantageous to use the computational method based on total energy calculations to study the phase transition from the coordinated

*Corresponding author: [email protected]

270 Metal Chalcogenide Semiconductor Nanostructures

number Nc = 4–6 fold [3]. Third-generation approaches to photovoltaics (PVs) aim to decrease costs and significantly increasing efficiencies but maintaining the economic and environmental cost advantages of thin-film deposition techniques [4]. There are several approaches to achieve such multiple energy threshold devices [4,5]; tandem or multicolor cells, con-centrator systems, intermediate-level cells, multiple carrier excitation, up/down conversion, and hot carrier cells [6].

Panchal et al. [7] had reviewed research works on silicon quantum dots (Si-QDs) embedded in the silicon nitride (SiNx) dielectric matrix films with different fabrication techniques and different characteristics. They discussed the advantages of SiNx as a dielectric compared to silicon diox-ide (SiO2) for Si-QDs from a device point of view, and summarized the fab-rication techniques along with different optimized deposition conditions. Theoretically, Aguilera et al. [8] have been presented absorption proper-ties enhancement for two CuGaS2-based intermediate band materials for high-efficiency, lower-cost PV devices. They showed, using density func-tional theory (DFT), the effect of this intermediate band on the optical properties of the derived alloys, highlighted the significant enhancement of the absorption coefficient observed in the most intense range of the solar emission, and studied the reflectance and transmittance properties of the materials in order to understand the effect of the thickness of the sample on the optical properties. Tuan et al. [9] have devoted to improve the efficiencies of dye-sensitized solar cell (DSSC) by both materials and electrical approach. Their TD-DFT-based procedure made it possible to get insights into the geometrical and electronic structures of the dyes and to unravel the structural modifications optimizing the properties of catechol-based DSSC. Udipi et al. [10] presented semiclassical simulation results for the potential energy profile and electron density distribution in 200 nm silicon quantum dot. For the solution of the continuity equa-tion, the efficient difference approximations, proposed by Scharfetter and Gummel [11], extended to three dimensions. In essence, they followed the two-dimensional approach due to Selberherr et al. [12] extend two to three dimensions.

Liu et al. [13] synthesized the center hollow ZnO and TiO2 nanotubes arrays by chemical etching ZnO nanorods and sol-gel process assisted by ZnO nanorods templates, respectively. Furthermore, as an application of the ZnO and TiO2 nanotubes, they successfully fabricated and character-ized the DSSCs and the cell performance, respectively. They got an effi-ciency of DSSCs based on ZnO (1.2%) and TiO2 (2.1%) nanotubes. While, Das and Sokol [14] have analyzed the nanostructured zinc oxide (ZnO) at low temperature. Also, they performed the structural characterization,

Chalcogenide-Based Nanodevices for Renewable Energy 271

size and distribution of synthesized ZnO particles using X-ray diffrac-tion (XRD) and neutron scattering technique, and fabricated the hybrid polymer–metal oxide bulk heterojunction solar cell by blending ZnO and regioregular poly-(3-hexylthiophene) (P3HT) through solution process and flow coating on a flexible substrate. They have concluded that the decrease in the photoluminescence (PL) emission intensity of more than 79% for ZnO:P3HT composites film indicates high charge generation effi-ciency. The cell shows Voc and Isc of 0.33 V and 6.5 mA/cm2, respectively. The performance and stability of the cell were investigated using UV illu-mination of white light. Finally, Badescu and Badescu [15] have analyzed a system of improving solar cell efficiencies by up-conversion of sub-band-gap light to increase solar cell efficiency. The system involves add-ing to the cell a so-called up-converter, which is a device able to convert the low-energy (sub-energy band gap) incident solar photons into pho-tons of higher energy. Their main novelty consists of taking into account appropriately the refractive index of solar cell and converter materials. They concluded as follows: (1) The maximum solar energy conversion efficiency increases in case of the cell and rear converter (C–RC) system as compared to the efficiency of a solar cell operating alone, especially at higher values of the concentration ratio; (2) The solar energy conversion efficiency of the C–RC system increases by increasing both the cell and the up-converter refractive indices; (3) The energy conversion efficiency does not increase by adding a front up-converter to the cell, whatever the value of the concentration ratio is.

The investigation of further materials research is interesting when one tries to gain some information about the diameter dependence of the com-pounds; especially, it is proved with some of the materials [16]. It seems more fundamental to relate the diameter dependence behavior to the bonds between the nearest atoms. By controlling the evolution with diam-eter dependence of the compound, it could attempt to link the effect of quantum dot diameter to the quantum dot potential. In this context, we have used this procedure for testing the validity of our model [17] of quan-tum dot’s (QD) potential. This model is based on the indirect energy band gaps (Γ–X) using empirical pseudopotential method (EPM). The obtained energy band gaps are used to calculate the quantum dot potential and to predict materials for QDs. The aim of this chapter is to verify this model [17] for calculating the diameter dependence on QD’s potential for dot diameters down to 55 and 65 nm for CdS and CdTe, respectively, using the full potential-linearized augmented plane wave (FP-LAPW), to investigate the optical properties of refractive index and optical dielectric constant using specific models for both compounds and to review current status of


renewable energy, nanodevices, in addition to display the importance of using DFT.

12.2 Renewable Energy

Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable (natu-rally replenished). About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from hydroelectricity. New renewables (small hydro, modern biomass, wind, solar, geothermal, and biofuels) accounted for another 3% and are growing very rapidly [18]. The share of renewables in electricity generation is around 19%, with 16% of global elec-tricity coming from hydroelectricity and 3% from new renewables [18].

Since 2004, PVs regard the fastest growing energy source. At the end of 2011, the PV capacity worldwide was 67,000 MW, and PV power stations are popular in Germany and Italy [19]. Solar thermal power stations oper-ate in the USA and Spain, and the largest of these is the 354 MW SEGS power plant in the Mojave Desert [20]. The world’s largest geothermal power installation is the Geysers in California, with a rated capacity of 750 MW. Brazil has one of the largest renewable energy programs in the world, involving production of ethanol fuel from sugarcane, and ethanol now pro-vides 18% of the country’s automotive fuel [21]. Ethanol fuel is also widely available in the USA.

While many renewable energy projects are large scale, renewable tech-nologies are also suited to rural and remote areas, where energy is often crucial in human development [22]. As of 2011, small solar PV systems provide electricity to a few million households, and micro-hydro config-ured into mini-grids serves many more. Over 44 million households use biogas made in household-scale digesters for lighting cooking and more than 166 million households rely on a new generation of more efficient biomass cookstoves [23]. United Nations’ Secretary-General Ban Ki-moon has said that renewable energy has the ability to lift the poorest nations to new levels of prosperity [24].

Climate change concerns, coupled with high oil prices, peak oil, and increasing government support, are driving increasing renewable energy legislation, incentives, and commercialization [25]. New government spending, regulation, and policies helped the industry weather the global financial crisis better than many other sectors [26]. According to a 2011


projection by the International Energy Agency, solar power generators may produce most of the world’s electricity within 50 years, dramatically reduc-ing the emissions of greenhouse gases that harm the environment [27].

Ferekides et al. [28] have continued their research on polycrystalline thin-film CdTe to be a leading material for the development of cost-effective and reliable PVs. Thin-film CdTe solar cells and modules are typically heterojunctions with CdS being the n-type partner, or window layer. The preferred configuration for CdTe solar cells is the superstrate structure. The cadmium chloride heat treatment, the back contact forma-tion process, and the utilization of resistive buffer layers in tandem with a thin cadmium sulfide window layer are important areas of research in thin-film CdTe solar cells. They reviewed work on CdTe thin-film solar cells sponsored by the National Renewable Energy Laboratory (NREL). Results for a vapor chloride heat treatment with high-throughput charac-teristics, a dry back contact process, and a comparative study of resistive buffer layers and their effect on the performance of CdTe solar cells are presented.

Dhere et al. [29] have studied one avenue to enhance CdTe cell per-formance to improve the optical transmission of the transparent con-ductive oxide (TCO)/window layer stack. They examined soda lime float glass coated with an Al-doped ZnO layer and a buffer layer. The possible advantages of using a ZnO-based TCO include reduced surface roughness, improved transparency, and an integrated buffer layer that can be opti-mized for use in a CdTe PV device. Also, they modified device processing to address the chemical and thermal differences between the ZnO-based TCO stack produced by Saint-Gobain and the TCOs previously used at the NREL. These process modifications produced ~8% efficiency for devices without a buffer layer. Incorporation of buffer layers has already produced devices with ~11% and >12% efficiency for CdTe deposition temperatures of 570° and 500°C, respectively.

Recently, Mohamed et al. [30], have created nanofabrication tech-niques for many different types of advanced nanosized semiconductors. Photocatalytic materials used to degrade organic and inorganic pollut-ants now include, in addition to TiO2, ZnO, Fe2O3, WO3, MoS2, and CdS. Nanoparticles’ unique properties, e.g., surface to volume ratio and quan-tum effects, continue to improve and expand photocatalysis’ role in areas like environmental remediation, odor control, sterilization, and renewable energy. Controlling semiconductor size, shape, composition, and micro-structure promises to benefit future research and applications in these fields.


12.3 Nanodevices

Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materi-als, devices, or other structures with at least one dimension sized from 1 to 100 nm. Quantum mechanical effects are important at this quantum-realm scale. Nanotechnology is considered a key technology for the future. Consequently, various governments have invested billions of dollars in its future. The USA has invested 3.7 billion dollars through its National Nanotechnology Initiative followed by Japan with 750 million and the European Union 1.2 billion [31].

Nanotechnology is very diverse, ranging from extensions of conven-tional device physics to completely new approaches based upon molecu-lar self-assembly, from developing new materials with dimensions on the nanoscale to direct control of matter on the atomic scale. Nanotechnology entails the application of fields of science as diverse as surface science, organic chemistry, molecular biology, semiconductor physics, microfab-rication, etc.

Nanotechnology may be able to create many new materials and devices with a vast range of applications, such as in medicine, electronics, bioma-terials, and energy production. On the other hand, nanotechnology raises many of the same issues as any new technology, including concerns about the toxicity and environmental impact of nanomaterials [32] and their potential effects on global economics, as well as speculation about various doomsday scenarios. These concerns have led to a debate among advocacy groups and governments on whether special regulation of nanotechnology is warranted.

One-dimensional (1D) ZnO nanostructures have widely been studied by Li et al. [33] not only because of their rich morphologies produced by various methods, but also because of their wide applications in optics, elec-tronics, piezoelectronics, sensing, etc. Particularly, as an environmental-friendly material, 1D ZnO nanostructures have intensively been studied for clean and sustainable solar energy devices. They presented a compre-hensive overview of the progress made in the different types of 1D ZnO nanostructure solar cells. Herein, the synthetic methods are not in the main focus and are summarized in the form of tables, rather we mainly emphasize the most exciting applications of 1D ZnO nanostructured solar cells, such as (2D and 3D) dye- and quantum dot-sensitized, bulk hetero-junctions, p–n and Schottky junctions, and integrated devices.

The research of nanoscale process engineering (NPE) is based on the interdisciplinary nature of nanoscale science and technology. It mainly


deals with transformation of materials and energy into nanostructured materials and nanodevices, and synergizes the multidisciplinary conver-gence between materials science and technology, biotechnology, and infor-mation technology. The core technologies of NPE concern all aspects of nanodevice construction and operation, such as manufacture of nanoma-terials “by design,” concepts and design of nanoarchitectures, and man-ufacture and control of customizable nanodevices. Two main targets of NPE at present are focused on nanoscale manufacture and concept design of nanodevices [34]. The research progress of nanoscale manufacturing processes focused on creating nanostructures and assembling them into nanosystems and larger-scale architectures has built the interdiscipline of NPE. The concepts and design of smart, multi-functional, environmentally compatible and customizable nanodevice prototypes built from the nano-structured systems of nanocrystalline, nanoporous, and microemulsion systems are most challenging tasks of NPE. The development of NPE may be also impelling led us to consider the curriculum and educational reform of chemical engineering in universities.

To review the present status and possible future developments of quan-tum dot infrared photodetectors (QDIPs), it summarizes the fundamental properties of QDIPs. Next, an emphasis is put on their potential develop-ments. Investigations of the performance of QDIPs as compared to other types of infrared photodetectors are presented by Martyniuk and Rogalski [35]. A model is based on fundamental performance limitations enabling a direct comparison between different infrared material technologies. It is assumed that the performance is due to thermal generation in the active detector’s region. In comparative studies, the HgCdTe photodiodes, quan-tum well infrared photodetectors (QWIPs), type-II superlattice photodi-odes, Schottky barrier photoemissive detectors, doped silicon detectors, and high-temperature superconductor detectors are considered by them.

Theoretical predictions indicate that only type-II superlattice photo-diodes and QDIPs are expected to compete with HgCdTe photodiodes. QDIPs theoretically have several advantages compared with QWIPs including the normal incidence response, lower dark current, higher oper-ating temperature, and higher responsivity and detectivity. Comparison of QDIP performance with HgCdTe detectors gives clear evidence that the QDIP is suitable for high operation temperature. It can be expected that improvement in technology and design of QDIP detectors will make it possible to achieve both high sensitivity and fast response useful for prac-tical application at room temperature [35].

Silicon oxide films containing CdS quantum dots have been deposited by Schuler et al. [36] on glass substrates by a sol-gel dip-coating process.


Hereby the CdS nanocrystals are grown during the thermal annealing step following the dip-coating procedure. Total hemispherical transmittance and reflectance measurements were carried out by means of a spectropho-tometer coupled to an integrating sphere. For CdS-rich films, an absorp-tion edge at photon energies in the vicinity of the band gap value of bulk CdS is observed. For lower CdS concentrations, the absorption edge shifts to higher photon energies, as expected for increasing quantum confine-ment. The samples show visible PL which is concentrated by total internal reflection and emitted at the edges of the substrate. The edge emission has been characterized by angle-dependent photoluminescent (PL) spectros-copy. Information on the lateral energy transport within the sample can be extracted from spectra obtained under spatial variation of the spot of excitation. Advantages of the proposed concept of quantum dot containing coatings on glass panes for photoluminescent solar concentrators are the high potential for low-cost fabrication on the large scale and the suitability for architectural integration [36].

Han et al. [37] have archived heterojunctions of CdS nanowire (CdSNW) and carbon nanotube (CNT) in the nanochannels of anodic aluminum oxide (AAO) templates via sequentially electrodepositing CdSNWs and chemical vapor depositing CNTs. Transport measurements reveal that Ohmic-like behavior has been achieved, which may result from a very low energy barrier in the junction of CdSNW/CNT. Furthermore, three-seg-ment heterostructures of CNT/CdSNW/CNT have also been obtained by adding a procedure of selectively etching part of the deposited CdSNWs before chemical vapor depositing CNTs. The approach could be exploited to build nanodevices and functional networks consisting of well-intercon-nected two- or three-segment nanoheterostructures.

Kong et al. [38] have researched Au/CdS heterostructure nanocrys-tals with a flower-like shape through an Au-nanorod-induced hydro-thermal method. The Au/CdS nanoflowers possessed the average size of about 350 nm while the nanorods constructing the nanoflowers had the average diameter, length, and aspect ratio of approximately 50 nm, 100 nm, and 2, respectively. A preliminary experiment model to reveal the Au/CdS growth mechanism was also put forward. The route devised here should be perhaps extendable to fabricate other Au/semiconductor heterostructured nanomaterials, and the Au/CdS nanoflowers may have potential applications in nanodevices, biolabels, and clinical detection and diagnosis.

Recently, Zillner et al. [39] have demonstrated fast and well-controlled electrophoretic deposition of CdTe and CdSe nanoparticle (CdTe-np and CdSe-np) layers and nanoparticle layer systems from an exhaustible source.


They [39] have proposed an approach to be suited for practical realization of engineering materials with different band gaps for various promising appli-cations such as fabrication of nanodevices. The formation of a charge selec-tive contact across the CdTe-np/CdSe-np heterojunction was investigated by surface photovoltage methods and evidence of the separation of charge carriers at a CdTe-/CdSe-np heterojunction in between was demonstrated.

12.4 Density Functional Theory

DFT is a quantum mechanical modeling method used in physics and chemistry to investigate the electronic structure (principally the ground state) of many-body systems, in particular atoms, molecules, and the con-densed phases. With this theory, the properties of a many-electron system can be determined using functionals, i.e., functions of another function, which in this case is the spatially dependent electron density. DFT is among the most popular and versatile methods available in condensed-matter physics, computational physics, and computational chemistry.

DFT has been very popular for calculations in solid-state physics since the 1970s. However, DFT was not considered accurate enough for calculations in quantum chemistry until the 1990s, when the approximations used in the theory were greatly refined to better model the exchange and correlation interactions. In many cases, the results of DFT calculations for solid-state systems agree quite satisfactorily with experimental data. Computational costs are relatively low when compared to traditional methods, such as the Hartree–Fock theory and its descendants based on the complex many-elec-tron wavefunction.

Despite recent improvements, there are still difficulties in using DFT to properly describe intermolecular interactions, especially van der Waals forces (dispersion), charge transfer excitations, transition states, global potential energy surfaces, and some other strongly correlated systems, and in calculations of the band gap in semiconductors. Its incomplete treatment of dispersion can adversely affect the accuracy of DFT (at least when used alone and uncorrected) in the treatment of systems which are dominated by dispersion (e.g., interacting noble gas atoms) [40] or where dispersion competes significantly with other effects (e.g., in biomolecules) [41]. The development of new DFT methods designed to overcome this problem, by alterations to the functional [42] or by the inclusion of additive terms [43–45], is a current research topic.

The calculations of energetic transitions were carried out using the FP-LAPW method as implemented in WIEN2K code [46]. In the


FP-LAPW method, the unit cell of zinc-blende structure is partitioned into non-overlapping muffin-tin spheres around the atomic sites and an interstitial region. Among these two types of regions different basis sets are used, the Kohn–Sham equation which is based on the DFT [47,48] is solved in a self-consistent scheme. The exchange correlation potential was treated using the generalized gradient approximation (GGA) [49] in which the orbital of Cd (4d105s2), S (3s23p4), and Te (4d105s25p4) are treated as valence electrons for the total energy calculations. Moreover, the Engel and Vosko’s (EV-GGA) formalism [50] is used for electronic and optical prop-erties calculations.

The crystal structure of these compounds is zinc blende with two atoms per unit cell, the full space group is 216 (F-43m), which includes 24 sym-metry operations and excludes inversion symmetry. In the calculation, 537 plane waves have been used for the expansion of the charge density and the potential in the interstitial region and lattice harmonics up to l = 8 for the expansion inside the muffin-tin spheres. The muffin-tin radii were assumed to be 2.0 atomic units (a.u.) for Cd, S, and Te. The dependence of the total energy on the number of k points in the irreducible wedge of the first Brillouin zone (BZ) has been explored within the linearized tet-rahedron scheme [49] by performing the calculation for 10 k points and extrapolating to an infinite number of k points. A satisfactory degree of convergence was achieved by considering a number of FP-LAPW basis functions up to RMTKmax = 8 (where RMT is the average radius of the muf-fin-tin spheres and Kmax is the maximum value of the wave vector K = k + G). This corresponds, at the equilibrium lattice constant, to about 217 basis functions. In order to keep the same degree of convergence for all the studied lattice constants, we kept the values of the sphere radii and of Kmax constant over the whole range of lattice spacing. We also mention that the integrations in reciprocal space were performed using the special points method. A mesh of 4 × 4 × 4 which represents 100 k points in the first BZ was used. This corresponds to 10 special k points in the irreducible wedge for the zinc-blende structure. The ab-initio calculation of the valence and conduction band energy eigenvalues has been performed at 111 points in the 1/48-th of the irreducible BZ.

12.5 Analytical Studies

A chalcogenide is a chemical compound consisting of at least one chalco-gen ion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term is


more commonly reserved for sulfides, selenides, and tellurides, rather than oxides. An optical processing chip using a chalcogenide as a photodetector has been developed by the University of Sydney with potential to speed up links between optical fiber networks and computers [51].

Normally, the covalent semiconductors are four-fold coordinated. The reason that the density of structure is so low is because the nearest neigh-bors of atoms are bound by overlapping hybridized orbitals, which are the well-known sp3 hybrids with tetrahedral shape. Hence, it is possible to tune the energy band gaps using dot diameter. The calculated values of the direct (Γ→Γ) and the indirect (Γ→X) and (Γ→L) energy band gaps within EVGGA of the investigated CdS and CdTe at different dot diameters are listed in table 12.1 along with the experimental data [52,54] and other theoretical calculations [53,55]. Our calculated value of the (Γ→Γ) energy band gap is slightly underestimated compared to the experimental data. This could be attributed to our use of the EVGGA approximation. Due to these values, CdS and CdTe have been classified as direct energy band gap semiconductor. Because of their use in infrared light generation and detec-tion, the energy band gap variations of dot diameters represent an impor-tant property to study. As mentioned in table 12.1, the energy band gaps correlate inversely with the dot diameters and confirmed by figure 12.1.

The energy band gaps between the valence band maximum (VBM) at the Γ point and the conduction band minimum (CBM) at the X point are computed on the basis of the FP-LAPW. By means of our recent model [17], the quantum dot potential has evaluated, according to the formula:

QD gP Eba

= −. . .ΓΧ 10 3 l, (12.1)

where b a/ is constant (in eV–1) [see table 4 in Ref. 17], gE ΓΧ is the energy gap along Γ–X (in eV), and l is an appropriate parameter for group IV (l = 6), III–V (l = 4), and II–VI (l = 2) semiconductors (in V).

A correlation between QD’s diameter and pressure effect changes is stated. If quantum dot diameter is changed, the strong sp3 covalent bonding that characterizes the covalent structure is affected. From our view point, this discrepancy at diameter dependence is an immediate consequence of the difference in the corresponding quantum dot potential. In table 12.2, the calculated quantum dot potential at quantum diameter dependence is computed.

The critical dot diameter is the value that separates the decrease and the increase of the QD’s potential. The diameter dependence correlates with transition pressure (Pt) that is important to be computed from difference


in molar free energies of compounds. The Gibbs free-energy difference, ∆Gt between compounds which has the tetrahedral coordination at diam-eter dependence is nearly given by ∆G=∆H–T∆S (in kJ.mol–1) where H is enthalpy, T is temperature, and S is entropy. Most of energies are larger for smaller bond lengths. Changing the QD’s potential with dot diameter is confirmed by the change of the energy gaps at principal points (Γ–Γ, Γ–X, and Γ–L) as shown in Table 12.1. The QD’s potential varies inversely with quantum diameter (Table 12.2) and confirmed by Figure 12.2. The rela-tion shape is linear for CdS and nonlinear for CdTe. As a consequence,

56.0 56.5 57.0 57.5 58.0 58.5

2.0

2.5

3.0

3.5

4.0

4.5

Ener

gy g

ap (e

V)

Dot diameter (nm)

Γ-XΓ-L

Γ-Γ

(a)

64.8 65.0 65.2 65.4 65.6 65.8 66.0 66.2

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

Ener

gy g

ap (e

V)

Dot diameter (nm)

(b)

Γ-XΓ-L

Γ-Γ

Figure 12.1 Calculated energy band gaps direct (Γ→Γ), and indirect (Γ→X) and (Γ→L) of CdS (a) and CdTe (b) as a function of QD’s diameter.


fluctuations of the QD’s potential appear. Our calculated QD’s potential values are in accordance with other data [10]. It is mentioned that the variation of the QD’s potential is an indication of the electron tunnels the quantum dot.

The refractive index n is a very important physical parameter related to the microscopic atomic interactions. From theoretical view point, there are basically two different approaches of viewing this subject: firstly, the refrac-tive index will be related to the density and local polarizability, and secondly, the refractive index will be closely related to the energy band structure of the material, through the dielectric constant. [56]. Consequently, many attempts have been made in order to relate the refractive index and the energy gap Eg through simple relationships [57–62]. However, these rela-tions of n are independent of temperature and incident photon energy. Here, the various relations between n and Eg will be reviewed. Ravindra et al. [62] had been presented a linear form of n as a function of Eg:

n Eg= +a b , (12.2)

Table 12.1 The calculated principal energy gaps for CdS and CdTe (in eV) at different QD’s diameters (in nm) compared to other experimental data and theoretical results.

QD’s diameter Eg (Γ–Γ) Eg (Γ–X) Eg (Γ–L)

Bulk55.7556.1756.3757.7858.1758.56

Bulk66.3666.1766.0265.8665.5965.33

2.359, 2.42a, 2.361b

2.3522.2922.2622.0511.9911.993

1.368, 1.8c, 1.8d

1.1331.163

1.16391.2081.2531.283

CdS4.6264.5844.4324.3583.8713.7413.619

CdTe3.2413.05

3.0673.081

3.08193.0963.111

3.4323.2613.1633.1142.7882.6992.613

2.2882.2672.2442.2292.244

2.25962.2745

aRef. [52] expt., bRef. [53] theor., cRef. [54] expt., dRef. [55] theor.


where a = 4 048. and b = −0 62. eV–1. Herve and Vandamme [63] proposed an empirical relation as follows:

n AE Bg

= ++

1

2

, (12.3)

where A = 13.6 eV and B = 3.4 eV. For group II–IV semiconductors, Ghosh et al. [64] have published an empirical relationship based on the band structure and quantum dielectric considerations of Penn [65] and Van Vechten [66]:

n AE Bg

221− =

+( ), (12.4)

where A = 8.2Eg + 134, B = 0.225Eg + 2.25, and (Eg + B) refers to an appro-priate average energy gap of the material. Thus, using these three models the variation of n with dot diameter has been calculated. The results are displayed in Figure 12.3. The calculated refractive indices and the dielectric optical constants of the end-point compounds are investigated and listed in Table 12.3.

Table 12.2 The calculated quantum dot potential for CdS and CdTe (in mV) compared to other value at different QD’s diameters (in nm).

QD’s diameter PQD cal. PQD [10]

55.7556.1756.3757.7858.1758.56

66.3666.1766.0265.8665.5965.33

CdS0.8740.8450.8310.7380.7130.690

CdTe0.5820.5850.58850.5880.5900.5937

≤ 1

≤ 1


64.8 65.0 65.2 65.4 65.6 65.8 66.0 66.2

0.582

0.584

0.586

0.588

0.590

0.592

0.594

Qua

ntum

dot

pot

enti

al (m

V)

Dot diameter (nm)

(b)

56.0 56.5 57.0 57.5 58.0 58.5

0.70

0.75

0.80

0.85

0.90

Dot diameter (nm)

(a)

Qua

ntum

dot

pot

enti

al (m

V)

Figure 12.2 QD’s diameter dependence of the quantum dot potential for CdS (a) and (b) CdTe.

This is verified by the calculation of the optical dielectric constant ε∞ which depends on the refractive index. Note that e∞ = n2 [67]. It is clear that the calculated n using the model of Herve and Vandamme [63] is in accordance with the experimental value and due to reflectiv-ity parameter is important in enhancing the photo conversion for solar cells. Again, a linear dependence of the CdS and CdTe properties on the dot diameter is observed and that the refractive index for small diam-eter dependence tends to shift toward the blue–green. It means a high-absorption and low-reflection spectrum may be attributed to increase solar cells efficiency.


12.6 Conclusion

A review of II–VI compounds is presented. Explanations and previous studies of renewable energy and nanodevices are given sufficiently. DFT with theoretical details is shown advantageously. The FP-LAPW method provides a good way to calculate the electronic properties, confirm its validity, investigate optical properties of low-reflectivity value for II–IV compounds, and prove that 55.75 and 65.33 nm dot diameters for CdS and CdTe, respectively, are more suitable for solar cells applications, expecting new trends for other compounds and new realization for quan-tum dots.

56.0 56.5 57.0 57.5 58.0 58.52.55

2.60

2.65

2.70

2.75

2.80

2.85

Refr

acti

ve in

dex

(n)

Dot diameter (nm)

Ravindra et al.Herve & VandammeGhosh et al.

(a)

64.8 65.0 65.2 65.4 65.6 65.8 66.0 66.2

3.0

3.1

3.2

3.3

3.4

3.5

Refr

acti

ve in

dex

(n)

Dot diameter (nm)

Ravindra et al.Herve & VandammeGhosh et al.

(b)

Figure 12.3 QD’s diameter dependence of the refractive index (n) for CdS (a) and CdTe (b).


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Table 12.3 Calculated refractive indices for CdS and CdTe at diameter depen-dence using the Ravindra et al. [62], Herve and Vandamme [63], and Ghosh et al. [64] models corresponding to optical dielectric constant.

QD’s diameter n ε∞

55.7556.1756.3757.7858.1758.56

66.3666.1766.0265.8665.5965.33

CdS2.589a, 2.567b, 2.611c, 2.38*

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10.575a, 9.431b, 10.89c

aRef. [62], bRef. [63], cRef. [64], *Ref. [62] expt.


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