Copper Zinc Tin Sulfide–Based Thin–Film Solar Cells

Copper Zinc Tin Sulfide (CZTS) –based thin films are produced by the thin layer deposition of photovoltaic materials onto a substrate. These semiconducting compounds have low–toxicity and are considered the most promising and cost–efficient technology for producing energy from solar cells. CZTS–based devices are made from Earth–abundant materials and are therefore more sustainable, cheaper to produce than commercialized devices consisting of cadmium telluride and copper indium gallium selenide and, increasingly, efficient enough to be used for large–scale deployment.

This book is divided into four parts. Part I comprises an introduction to CZTS thin films and a discussion of their market challenges. Part II continues with a description of the physics and chemistry of quaternary
chalcogenide semiconductors. Part III covers the synthesis and applications of CZTS for solar cell technology, and Part IV investigates device physics.

This book draws on state–of–the–art research and contains contributions from scientists directly involved in improving the efficiency of current devices. It is essential reading for researchers and advanced students working in thin–film technology, materials science, and in the photovoltaics industry.

Preface ix

List of Contributors xi

Part I Introduction 1

1 An Overview of CZTS–Based Thin–Film Solar Cells 3
Kentaro Ito

1.1 Introduction 3

1.2 The Photovoltaic Effect 4

1.3 In Pursuit of an Optimal Semiconductor for Photovoltaics 22

1.4 Conclusions 36

Acknowledgements 37

References 37

2 Market Challenges for CZTS–Based Thin–Film Solar Cells 43
Arnulf Jäger–Waldau

2.1 Introduction 43

2.2 Compound Thin–Film Technologies and Manufacturing 45

2.3 Challenges for CZTS Solar Cells in the Market 49

2.4 Conclusion 51

References 51

Part II The Physics and Chemistry of Quaternary Chalcogenide Semiconductors 53

3 Crystallographic Aspects of Cu2ZnSnS4 (CZTS) 55
Susan Schorr

3.1 Introduction: What Defines a Crystal Structure? 55

3.2 The Crystal Structure of CZTS 57

3.3 Point Defects in CZTS and the Role of Stoichiometry 68

3.4 Differentiation between Intergrown Kesterite– and Stannite–Type Phases: A Simulational Approach 71

3.5 Summary 72

References 73

4 Electronic Structure and Optical Properties from First–Principles Modeling 75
Clas Persson, Rongzhen Chen, Hanyue Zhao, Mukesh Kumar and Dan Huang

4.1 Introduction 75

4.2 Computational Background 77

4.3 Crystal Structure 80

4.4 Electronic Structure 82

4.5 Optical Properties 97

4.6 Summary 101

Acknowledgements 102

References 102

5 Kesterites: Equilibria and Secondary Phase Identification 107
Dominik M. Berg and Phillip J. Dale

5.1 Introduction 107

5.2 Chemistry of the Kesterite Reaction 108

5.3 Phase Identification 116

Acknowledgements 128

References 128

6 Growth of CZTS Single Crystals 133
Akira Nagaoka and Kenji Yoshino

6.1 Introduction 133

6.2 Growth Process 134

6.3 Properties of CZTS Single Crystals 141

6.4 Conclusion 145

Acknowledgements 146

References 146

7 Physical Properties: Compiled Experimental Data 149
Sadao Adachi

7.1 Introduction 149

7.2 Structural Properties 150

7.3 Thermal Properties 152

7.4 Mechanical and Lattice Dynamic Properties 157

7.5 Electronic Energy–Band Structure 162

7.6 Optical Properties 169

7.7 Carrier Transport Properties 170

References 176

Part III Synthesis of Thin Films and Their Application to Solar Cells 181

8 Sulfurization of Physical Vapor–Deposited Precursor Layers 183
Hironori Katagiri

8.1 Introduction 183

8.2 First CZTS Thin–Film Solar Cells 184

8.3 ZnS as Zn–Source in Precursor 184

8.4 Influence of Absorber Thickness 187

8.5 New Sulfurization System 188

8.6 Influence of Morphology 189

8.7 Co–Sputtering System with Annealing Chamber 190

8.8 Active Composition 191

8.9 CZTS Compound Target 192

8.10 Conclusions 201

References 201

9 Reactive Sputtering of CZTS 203
Charlotte Platzer–Björkman, Tove Ericson, Jonathan Scragg and Tomas Kubart

9.1 Introduction 203

9.2 The Reactive Sputtering Process 205

9.3 Properties of Sputtered Precursors 206

9.4 Annealing of Sputtered Precursors 214

9.5 Device Performance 215

9.6 Summary 217

References 217

10 Coevaporation of CZTS Films and Solar Cells 221
Thomas Unold, Justus Just and Hans–Werner Schock

10.1 Introduction 221

10.2 Basic Principles 221

10.3 Process Variations 227

Acknowledgements 236

References 236

11 Synthesis of CZTSSe Thin Films from Nanocrystal Inks 239
Charles J. Hages and Rakesh Agrawal

11.1 Introduction 239

11.2 Nanocrystal Synthesis 241

11.3 Nanocrystal Characterization 249

11.4 Sintering 251

11.5 Conclusion 264

References 264

12 CZTS Thin Films Prepared by a Non–Vacuum Process 271
Kunihiko Tanaka

12.1 Introduction 271

12.2 Sol–Gel Sulfurization Method 272

12.3 Preparation of CZTS Thin Films by Sol–Gel Sulfurization Method 274

12.4 Chemical Composition Dependence 279

12.5 H2S Concentration Dependence 282

12.6 CZTS Solar Cell Prepared by Non–vacuum Processes 284

References 285

13 Growth of CZTS–Based Monograins and Their Application to Membrane Solar Cells 289
Enn Mellikov, Mare Altosaar, Marit Kauk–Kuusik, Kristi Timmo, Dieter Meissner, Maarja Grossberg, Jüri Krustok and Olga Volobujeva

13.1 Introduction 289

13.2 Monograin Powder Growths, Basics of the Process 291

13.3 Influence of Chemical Etching on the Surface Composition of Monograins 295

13.4 Thermal Treatment of CZTS–Based Monograins 298

13.5 Optoelectronic Properties of CZTS–Based Monograins and Polycrystals 300

13.6 Conclusion 306

References 306

Part IV Device Physics of Thin–Film Solar Cells 311

14 The Role of Grain Boundaries in CZTS–Based Thin–Film Solar Cells 313
Joel B. Li and Bruce M. Clemens

14.1 Introduction 313

14.2 CIGSe and CdTe Solar Cells 314

14.3 CZTS–Based Thin–Film Solar Cells 318

14.4 Conclusion 327

References 328

15 CZTS–Based Thin–Film Solar Cells Prepared via Coevaporation 335
Byungha Shin, Talia Gershon and Supratik Guha

15.1 Introduction 335

15.2 Preparation of CZTS and CZTSe Absorbers 337

15.3 Fundamental Properties of Coevaporated CZTS and CZTSe Absorbers 338

15.4 Device Characteristics of Full–Sulfide CZTS Thin–Film Solar Cells 348

15.5 Device Characteristics of Full–Selenide CZTSe Thin–Film Solar Cells 354

15.6 Summary 358

References 358

16 Loss Mechanisms in Kesterite Solar Cells 363
Alex Redinger and Susanne Siebentritt

16.1 Introduction 363

16.2 Current State–of–the–Art CZTS–Based Thin–Film Solar Cells 364

16.3 Dominant Recombination Path 366

16.4 Band–Gap Variations 372

16.5 Series Resistance and its Relation to Voc Losses 376

16.6 Conclusion 381

Acknowledgements 382

References 382

17 Device Characteristics of Hydrazine–Processed CZTSSe 387
Oki Gunawan, Tayfun Gokmen and David B. Mitzi

17.1 Introduction 387

17.2 Device Characteristics 389

17.3 Summary 406

Acknowledgements 407

References 408

Subject Index 413

The semiconducting compound (CZTS) is made up earth–abundant, low–cost and non–toxic elements, which make it an ideal candidate to replace Cu (In,Ga) Se2 (CIGS) and CdTe solar cells which face material scarcity and toxicity issues.   (Materials and Corrosion, 1 October 2014)