This textbook results from lecture courses I have been giving to students who are interested in principles of solar energy conversion and in solar cell materials. Students listening to my courses usually study in the fields of engineering, materials science or natural science at universities around the world. I have had, and I continue to have, the great pleasure to teach not only in Europe but also in Asia, Africa and Latin America. A broad understanding of materials and combinations of materials for photovoltaic solar energy conversion helps to implement renewable energy worldwide. The textbook gives a comprehensive introduction to materials concepts for solar cells including basic principles and materials specific concepts.
The success in the development and application of solar cells is closely related to countless improvements of materials and to the development of new materials for solar cells. Quality criteria of solar cell materials are consequences of basic principles of solar cells. Materials and technological concepts follow from the ways in which different photovoltaic absorbers can be realized. Materials concepts of solar cells are based (i) on the growth of large crystals (wafer-based technology), (ii) on the growth of sophisticated layer systems on substrate crystals (epitaxy-based technology), (iii) on the deposition of absorber layers on foreign substrates (thin-film pho- tovoltaics) and (iv) on the combination of very different materials on a nanometer scale (nano-composite solar cells). Interfaces between materials for charge separation, electric contacts to external leads and passivation are considered for all kinds of solar cells.
I would like to express my appreciation to all of my students, but especially those from the Free University Berlin for their interest and questions, to my former teacher Fred Koch for inspiration, to Günther Materials Concepts for Solar Cells Seliger for the opportunity to teach at GPE (Global Production Engineering for Solar Technology) at the Technical University Berlin, to friends at the Kasetsart University in Bangkok and other universities around the world for supporting my teaching projects, to Martha Lux-Steiner at the Helmholtz Centre Berlin for giving me the opportunity to teach alongside my scientific work, to Bernhard Reinhold for discussing aspects of this textbook, to Brian Edlefsen Lasch for his critical reading of the manuscript, and to Catharina Weijman from Imperial College Press for excellent collaboration.

Preface vii

Symbols and Abbrevations xix

Part I. Basics of Solar Cells and Materials Demands 1

1. Basic Characteristics and Characterization of Solar Cells 3

      1.1 Solar Radiation and Two Fundamental Functions of a Solar Cell 3

      1.2 Basic Characteristics of a Solar Cell 8

      1.3 The Ideal Solar Cell under Illumination 10

      1.3.1 Diode equation of the ideal solar cell 10

      1.3.2 Open-circuit voltage and diode saturation current 12

      1.3.3 Dependence of the fill factor on the short-circuit current density 14

      1.3.4 Dependence of the efficiency of an ideal solar cell on the light intensity 14

      1.4 Real Solar Cells: Consideration of Series and Parallel Resistances 16

      1.4.1 Resistive losses and tolerable series and parallel resistances 16

      1.4.2 Influence of series and parallel resistances on the basic characteristics of a solar cell 18

      1.4.3 Influence of series and parallel resistances on the intensity dependence of the basic characteristics of a solar cell 20

      1.5 Characterization of Solar Cells 24

      1.5.1 characteristics and ideality factor 24

      1.5.2 Temperature-dependent diode saturation current and activation energy 25

      1.5.3 Measurement of I–V characteristics with loads 27

      1.5.4 Measurement of the energy conversion efficiency with a pyranometer 32

      1.5.5 Measurement of the solar energy conversion efficiency with a sun simulator 34

      1.5.6 Spectral dependence of the quantum efficiency 36

      1.6 Summary 39

      1.7 Tasks 42

2. Photocurrent Generation and the Origin of Photovoltage 44

      2.1 Energy Gap of Photovoltaic Absorbers 44

      2.1.1 Light absorption and band gap of photovoltaic absorbers 44

      2.1.2 Photon flux and maximum photocurrent of solar cells 47

      2.1.3 The ultimate efficiency of solar cells 50

      2.2 Photocurrent Limitation by Reflectivity and Transmission of Light 51

      2.2.1 Reflectivity and antireflection coatings 51

      2.2.2 Absorption coefficient and transmission losses 53

      2.2.3 Increase of the optical path by light trapping 57

      2.3 Free Charge Carriers in Ideal Semiconductors 61

      2.3.1 Densities of free charge carriers and Fermi-energy 61

      2.3.2 Thermal equilibrium and density of intrinsic charge carriers 65

      2.3.3 Doping, majority and minority equilibrium charge carriers 67

      2.3.4 Fermi-level splitting for photo-generated charge carriers 72

      2.4 Summary 75

      2.5 Tasks 78

3. Influence of Recombination on the Minimum Lifetime 80

      3.1 Minimum Lifetime 80

      3.1.1 Decay of photocurrent, recombination rate and lifetime 80

      3.1.2 Diffusion length and lifetime 82

      3.1.3 Minimum lifetime condition for photovoltaic absorbers 83

      3.2 Radiative Recombination 85

      3.3 Auger Recombination 87

      3.4 Shockley–Read–Hall Recombination 90

      3.4.1 Elementary processes at a trap state 90

      3.4.2 Emission and capture rates of electrons and holes 92

      3.4.3 Minimum Shockley–Read–Hall recombination lifetimes 93

      3.4.4 Shockley–Read–Hall recombination rate 94

      3.4.5 Strategies for minimizing Shockley–Read–Hall recombination 96

      3.5 Surface Recombination 99

      3.5.1 Surface recombination velocity 99

      3.5.2 Surface recombination lifetime 101

      3.5.3 Strategies for minimizing surface recombination 102

      3.6 Summary 105

      3.7 Tasks 107

4. Charge Separation Across pn-junctions 109 109

      4.1 Concept of the Ideal Charge-Selective Contact 110

      4.2 The Ideal Semi-Infinite pn-junction in Thermal Equilibrium 111

      4.2.1 Formation of a pn-junction 111

      4.2.2 Space charge regions of a pn-junction 113

      4.2.3 Diffusion potential of a pn-junction 118

      4.3 Collection of Photo-Generated Charge Carriers at a pn-junction 121

      4.3.1 pn-junction as an ideal charge-selective electron and hole contact 121

      4.3.2 Charge carriers photo-generated in a space charge region 123

      4.3.3 Charge carriers photo-generated in a neutral region 123

      4.4 Diode Saturation Current Density of a pn-junction 127

      4.4.1 Fermi-level splitting and densities of minority charge carriers 127

      4.4.2 Diode saturation current density of a semi-infinite pn-junction 129

      4.4.3 Shockley–Read–Hall recombination in a pn-junction 132

      4.4.4 The two-diode model of an illuminated pn-junction 134

      4.4.5 Consideration of surface recombination 137

      4.4.6 The way to very high energy conversion efficiencies of solar cells 140

      4.5 Summary 143

      4.6 Tasks 145

5. Ohmic Contacts for Solar Cells 147

      5.1 Concept of Ideal Ohmic Contacts 147

      5.2 Ohmic Metal–Semiconductor Contacts 151

      5.2.1 Work function, electron affinity and ionization energy 151

      5.2.2 Formation of metal–semiconductor contacts 152

      5.2.3 Fermi-level pinning at metal–semiconductor contacts 156

      5.3 Tunneling Ohmic Contacts 161

      5.3.1 Tunneling at metal–semiconductor contacts 161

      5.3.2 Transmission probability of a tunneling barrier 162

      5.3.3 Contact resistance of a triangular tunneling barrier 166

      5.3.4 Tunneling resistance at degenerated pn-hetero-junctions 169

      5.3.5 Tunneling via defect states near metal–semiconductor contacts 172

      5.4 Summary 174

      5.5 Tasks 176

6. Maximum Energy Conversion Efficiency of Solar Cells 178

      6.1 Diode Equation of the Ideal Single-Junction Solar Cell 178

      6.1.1 Thermal generation and radiative recombination rate constant 178

      6.1.2 Radiative recombination rate of an illuminated ideal absorber 181

      6.1.3 Diode saturation current of an ideal single-junction solar cell 182

      6.2 Maximum Efficiency of Ideal Single-Junction Solar Cells 184

      6.2.1 The Shockley–Queisser limit of solar energy conversion efficiency 184

      6.2.2 Role of temperature and concentrated sunlight 188

      6.2.3 Thermalization losses 189

      6.3 Multi-Junction Solar Cells 191

      6.3.1 Spectral splitting for reduction of thermalization losses 191

      6.3.2 Current-matching condition and optimum band gaps 193

      6.3.3 Dependence of the efficiency on the number of band gaps 195

      6.4 Alternative Concepts for Increasing the Efficiency 200

      6.4.1 Increased quantum efficiency with higher energetic photons 200

      6.4.2 Up-conversion of lower energetic photons 204

      6.5 Summary 206

      6.6 Tasks 208

Part II. Materials Specific Concepts 211

7. Solar Cells Based on Crystalline Silicon 213

      7.1 Principle Architecture of c-Si Solar Cells and Resistive Losses 214

      7.1.1 Base and emitter of conventional c-Si solar cells 214

      7.1.2 Front-contact finger grid and emitter resistance 216

      7.1.3 Minimization of shading caused by the front-contact finger grid 219

      7.1.4 Resistances at emitter and base contacts 222

      7.1.5 Connection of c-Si solar cells in photovoltaic modules and role of silver 224

      7.2 Homogeneously Doped c-Si Wafers with a Low Density of Defects 225

      7.2.1 Siemens process of very pure polycrystalline silicon 225

      7.2.2 Methods of c-Si crystal growth with liquid–solid interfaces 226

      7.2.3 Homogeneous doping of large silicon crystals 230

      7.2.4 Refinement of silicon crystals by segregation 233

      7.2.5 Wafering of c-Si crystals 235

      7.3 Formation of the Emitter 237

      7.3.1 Diffusion from inexhaustible and exhaustible sources 237

      7.3.2 Laser-assisted doping and ion implantation of the emitter 241

      7.3.3 Amorphous silicon emitter 244

      7.4 Passivation and Structuring of c-Si Surfaces 246

      7.4.1 Cleaning and structuring of silicon surfaces by etching 246

      7.4.2 The c-Si/SiO2 interface 248

      7.4.3 Hydrogen passivation of silicon dangling bonds 251

      7.4.4 Passivation with the c-Si/SiNx:H interface 253

      7.4.5 The c-Si/Al2O3 interface 255

      7.4.6 Back surface field and local ohmic contacts 257

      7.5 Summary 259

      7.6 Tasks 263

8. Solar Cells Based on III–V Semiconductors 265

      8.1 III–V Semiconductor Family 265

      8.1.1 Binary III–V semiconductors 265

      8.1.2 Ternary and quaternary III–V semiconductors 267

      8.1.3 Optical absorption of III–V semiconductors 269

      8.2 Hetero-Junctions of III–V Semiconductors 271

      8.2.1 Type I and type II semiconductor hetero-junctions 271

      8.2.2 Role of interface dipoles and interface states for band offsets 275

      8.2.3 Doped type I and type II semiconductor hetero-junctions 277

      8.3 Epitaxial Growth of III–V Semiconductors 279

      8.3.1 Principle of epitaxy with III–V semiconductors 279

      8.3.2 Molecular beam epitaxy of III–V semiconductors 285

      8.3.3 Metalorganic vapor phase epitaxy of III–V semiconductors 288

      8.3.4 Epitaxial lift-off and exfoliation of III–V semiconductors 290

      8.4 Single- and Multi-Junction Solar Cells of III–V Semiconductors 292

      8.4.1 Design of the pn-junction of solar cells of III–V semiconductors 292

      8.4.2 Architecture and band diagram of single-junction solar cells 293

      8.4.3 Ohmic metal contacts with III–V semiconductors 295

      8.4.4 Tandem solar cells with III–V semiconductors 298

      8.4.5 Triple-junction concentrator solar cells 301

      8.4.6 Quadruple-junction solar cells 303

      8.5 Summary 305

      8.6 Tasks 307

9. Thin-Film Solar Cells 310

      9.1 Concept of Thin-Film Photovoltaics 310

      9.1.1 About the architecture of thin-film solar cells and modules 310

      9.1.2 Sputtering and plasma-enhanced chemical vapor deposition 313

      9.2 Transparent Conducting Oxides 317

      9.2.1 Doping and electron mobility in transparent conducting oxides 317

      9.2.2 Resistance of transparent conducting oxide layers 319

      9.2.3 Optical losses in transparent conducting oxides 321

      9.3 Amorphous and Micro-Crystalline Silicon Solar Cells 325

      9.3.1 Disorder and mobility of charge carriers in amorphous silicon 325

      9.3.2 Optical absorption and optical band gap in amorphous silicon 329

      9.3.3 Doping of amorphous silicon 330

      9.3.4 The pin concept of amorphous silicon solar cells 332

      9.3.5 Staebler–Wronski effect in amorphous silicon solar cells 335

      9.3.6 a-Si:H/µc-Si:H tandem solar cells 337

      9.4 Chalcopyrite and Kesterite Solar Cells 338

      9.4.1 The chalcopyrite and kesterite absorber families 338

      9.4.2 Band gaps and absorption in chalcopyrites and kesterites 339

      9.4.3 Lifetime and mobility for the example of CuIn1−xGaxSe2 341

      9.4.4 Defects, phases and self-compensation 342

      9.4.5 Contacts in chalcopyrite and kesterite solar cells 346

      9.4.6 Multi-junction chalcopyrite solar cells(?!) 350

      9.5 Cadmium Telluride Solar Cells 351

      9.5.1 Properties of CdTe 351

      9.5.2 Contacts in CdTe solar cells 352

      9.5.3 Technological aspects of CdTe solar cells 353

      9.6 Summary 355

      9.7 Tasks 359

10. Nano-Composite Solar Cells 362

      10.1 The Concept of Nano-Composite Materials for Photovoltaics 362

      10.1.1 New quality of properties in nano-composite materials 362

      10.1.2 Photovoltaic absorbers with very short diffusion or drift lengths 364

      10.2 Quantum Dot-Based Nano-Composite Solar Cells 367

      10.2.1 Quantum confinement in semiconductor nano-crystals 367

      10.2.2 Solution-based fabrication of semiconductor quantum dots 372

      10.2.3 Colloidal quantum dot layers as nano-composite absorbers 374

      10.2.4 Charge transport and separation in quantum dot solar cells 375

      10.3 Organic Solar Cells 379

      10.3.1 Organic semiconductors with conjugated π-electron systems 379

      10.3.2 Excitons in organic semiconductors 384

      10.3.3 Donor–acceptor hetero-junction and organic nano-composite absorber 385

      10.3.4 Charge-selective and ohmic contacts in organic solar cells 388

      10.3.5 Optimization routes of organic solar cells 392

      10.4 Dye-Sensitized Solar Cells 393

      10.4.1 Light absorption in dye molecules 393

      10.4.2 Dye sensitization of a sintered network of TiO2 nanoparticles 395

      10.4.3 Local charge separation and transport in dye-sensitized solar cells 398

      10.4.4 Passivation and co-sensitization of dye-sensitized solar cells 402

      10.4.5 About the inherent stability of dye-sensitized solar cells 405

      10.4.6 Solar cells sensitized with methyl-ammonium lead iodide 407

      10.5 Light Concentration by Nano-Photonic Concepts for Solar Cells 410

      10.5.1 Optical confinement for solar cells 410

      10.5.2 Enhancement of optical absorption in very thin absorbers 412

      10.5.3 Nanowire array solar cells 413

      10.6 Summary 416

      10.7 Tasks 419

A. Solutions to Tasks 421

A.1 Solutions to Chapter 1 421

A.2 Solutions to Chapter 2 427

A.3 Solutions to Chapter 3 431

A.4 Solutions to Chapter 4 433

A.5 Solutions to Chapter 5 438

A.6 Solutions to Chapter 6 442

A.7 Solutions to Chapter 7 447

A.8 Solutions to Chapter 8 450

A.9 Solutions to Chapter 9 45

A.10 Solutions to Chapter 10 461

Bibliography 465

Index 491

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