THIN-FILM SILICON
SOLAR CELLS
Arvind Shah, Editor
The main authors of Thin-Film Silicon Solar Cells are Christophe Ballif, Wolfhard Beyer, Friedhelm Finger, Horst Schade, Arvind Shah, and Nicolas Wyrsch, with additional contributions by Jean-Eric Bouree, Corinne Droz, Luc Feitknecht, Daniel Oppizzi,
Martin Python, Julian Randall, Ricardo Rüther, Michael Stückelberger, and Reto Tscharner.
E P F L P r e s s
1 INTRODUCTION 1 1.1 A strong market growth from 1999 to 2008 1
1.2 A technology coming to maturity: crystalline silicon 2 1.3 High-efficiency crystalline silicon solar cells 4 1.4 The silicon feed-stock issue: a trigger for thin-film deployment . . . . 5
1.5 Thin-film silicon: a unique thin-film technology with a "long"
history 8 1.6 Amorphous silicon, microcrystalline silicon and "micromorph"
devices 9 1.7 Synergy with the display sector and emergence of a large
PV sector 11 1.8 Perspectives and challenges for thin-film silicon technology 13
1.9 References 15 2 BASIC PROPERTIES OF HYDROGENATED AMORPHOUS
SILICON (a-Si:H) 17 2.1 Introduction 17
2.1.1 Structure of amorphous silicon 18 2.1.2 "Free" and "trapped" carriers (electrons and holes);
mobility gap 22
2.2 Gap states 24 2.2.1 Bandtail states 24
2.2.2 Midgap states: dangling bonds 27 2.2.3 Light-induced degradation (Staebler-Wronski effect) 30
2.3 Optical absorption: optical gap and sub-bandgap absorption 35
2.3.1 Absorption coefficient plot 36 2.3.2 Link between density of states and absorption
coefficient 38 2.3.3 Exponential density of states in bandtails and Urbach
energy in plot of absorption coefficient 40 2.3.4 Determination of the optical gap 41 2.3.5 Relationship between sub-bandgap absorption and
defect density 43 2.3.6 Measurement of sub-bandgap absorption 44
2.4 Transport, conductivity and recombination 47
2.4.2 Measurement of conductivity in a co-planar
configuration 48 2.4.3 Dark conductivity adark 49
2.4.4 Recombination 53 2.4.5 Photoconductivity 58 2.5 Doping of amorphous silicon layers 61
2.6 Hydrogen in a-Si:H 64 2.6.1 Introduction 64 2.6.2 Hydrogen incorporation 65
2.6.3 Hydrogen dilution during deposition 67 2.6.4 Hydrogen effusion and hydrogen surface desorption . . . . 67
2.6.5 Hydrogen diffusion 70 2.6.6 Hydrogen solubility effects 70 2.6.7 Hydrogen effects on optoelectronic properties 73
2.6.8 Effect of hydrogen incorporation on the bandgap
ofa-Si:H 73 2.6.9 Stability of dangling bond passivation 74
2.6.10 Hydrogen and material microstracture 74 2.6.11 Role of hydrogen in light-induced degradation 74 2.7 Amorphous silicon-germanium and silcon-carbon Alloys 76
2.7.1 Introduction 76 2.7.2 Fabrication 77 2.7.3 Structure of a-Si:Ge:H and a-Si:C:H alloys 79
2.7.4 Hydrogen incorporation, effusion, surface desorption
and diffusion 80 2.7.5 Microstructural effects (voids) 82
2.7.6 Dangling bonds, density of defect states 83 2.7.7 Hydrogen stability versus alloy composition 84
2.7.8 Doping effects 84 2.7.9 Light-induced degradation 84
2.7.10 Optical absorption 84 2.7.11 Electronic transport properties 85
2.7.12 Slope of the valence bandtail; Urbach energy 86 2.7.13 Strategies for obtaining good quality alloys 87
2.8 Conclusions 87 2.9 References 89
3 BASIC PROPERTIES OF HYDROGENATED
MICROCRYSTALLINE SILICON 97
3.1 History 97 3.2 Structural properties of ux;-Si:H 101
3.2.1 Structure 101 3.2.2 Defects and gap states 107
3.2.3 Hydrogen, defect passivation, impurities and doping . . . . 113
3.2.4 Schematic picture for the structure of ue-Si:H 118 3.2.5 Relationships between structural and other properties
of ue-Si:H material 122
3.3 Optical properties 124 3.4 Electronic properties and transport 126
3.5 Metastability - instability 131
3.6 Alloys 132 3.7 Summary 134 3.8 References 135 4 THEORY OF SOLAR CELL DEVICES (SEMI-CONDUCTOR
DIODES) 145 PART I: INTRODUCTION AND "/jm-TYPE" DIODES 145
4.1 Conversion of light into electrical carriers by a
semi-conductor diode 145 4.1.1 First step: generation of electron-hole pairs 145
4.1.2 Second step: separation of electrons and holes 152 4.2 The "ри-type" or "classical" diode: dark characteristics 154 4.3 The "pra-type" or "classical" diode: Properties under
illumination 158 4.3.1 Photo-generation and superposition principle
(ideal case) 158 4.3.2 Limitations of a "real" diode (under illumination) 160
4.3.3 Maximum power point (MPP) and fill factor (FF) of a
solar cell 163 4.3.4 Basic solar cell parameters Jsc, Voc, FF 164
4.4 Limits on solar cell efficiency 169 4.4.1 Limits at standard test conditions (STC) 169
4.4.2 Variation in light intensity 171 4.4.3 Variation in operating temperature 172 4.4.4 Variation in the spectrum of the incoming light 175
4 THEORY OF SOLAR CELL DEVICES (SEMI-CONDUCTOR
DIODES) 176 PART II: >«-TYPE" SOLAR CELLS 176
4.5 Introduction to "рш-type" solar cells 176 4.5.1 Basic structure and properties 176 4.5.2 Formation of the internal electric field 179
4.5.3 Carrier profiles in the intrinsic layer: free carriers
pfandn{ 183
4.6 Effect of trapped charge in valence and conduction bandtails
on electric field and carrier transport 189 4.6.1 Deformation of electric field in Mayer by trapped
carriers: Concept 189 4.6.2 Deformation of electric field in Mayer by trapped
carriers: numerical simulations for amorphous silicon. . . 190 4.6.3 Mobilities in amorphous and microcrystalline silicon . . . 192
4.7 Dangling bonds and their role in field deformation 193
4.7.1 Dangling bond charge states 193 4.7.2 Field deformation by charged dangling bonds
within the Mayer: Concept 196 4.7.3 Field deformation by charged dangling bonds within
the Mayer: numerical simulation for an amorphous
silicon solar cell with dx = 300 nm 198
4.7.4 Field deformation within the Mayer: summary of
situation for different Mayer thicknesses 198
4.8 Recombination and Collection 201 4.8.1 pli and iln interfaces 201 4.8.2 Recombination 203 4.8.3 Collection and drift lengths 204
4.9 Electrical description of the pin-solar cell 205 4.9.1 Equivalent circuit and extended "superposition
principle" 205 4.9.2 Shunts 210 4.9.3 Variable illumination measurements (VIM) 211
4.9.4 Reverse saturation current J0 and open circuit
voltage Voc 213
4.9.5 Fill factor in pin-type thin-film silicon solar cells 214 4.9.6 Limits for the short-circuit current /sc in pin-type
thin-film silicon solar cells 215 4.10 Light-induced degradation or "Staebler-Wronski effect"
in thin-film silicon solar cells 216 4.11 Spectral response, light trapping and efficiency limits 218
4.11.1 Spectral response (SR) and external quantum
efficiency (EQE) measurements 218 4.11.2 Light trapping in thin-film silicon solar cells 221
4.11.3 Limits for the efficiency r\ in pin-type thin-film
silicon solar cells 225 4.12 Summary and conclusions 229
4.12 References 231 5 TANDEM AND MULTI-JUNCTION SOLAR CELLS 237
5.1 Introduction, general concept 237 5.2 Principle of the two-terminal tandem cell 240
5.2.1 Construction of basic J-V diagram: Rules for
5.2.2 Recombination (tunnel) junction 242 5.2.3 Efficiency limits for tandems 243 5.3 Practical problems of two-terminal tandem cells 246
5.3.1 Light trapping 246 5.3.2 Efficiency variation due to changes in the solar
spectrum 248 5.3.3 Temperature coefficients 248
5.3.4 Pinholes and Shunts 249
5.3.5 Cracks 250 5.4 Typical tandem and multi-junction cells 251
5.4.1 Amorphous tandem cells a-Si:H/a-Si:H 251 5.4.2 Triple-junction amorphous cells with germanium 252
5.4.3 Micromorph (a-Si:H/ue-Si:H) tandem cells 253 5.4.4 Triple-junctions with microcrystalline silicon 254 5.5 Spectral response (SR) and External Quantum Efficiency
(EQE) measurements 255 5.5.1 General principles 255 5.5.2 Use of "colored" bias light beams for
SR/EQE-measurements on tandems and triple-junction cells . . . . 256 5.5.3 SR/EQE measurements for a-Si:H/a-Si:H tandem
cells 257 5.5.4 Shunt detection in sub-cells by SR/EQE
measurements 258 5.5.5 SR/EQE measurements for triple-junction cells 260
5.5.6 SR/EQE measurements for "micromorph" tandem
cells 260 5.5.7 Necessity for voltage correction (with bias voltage) . . . . 262
5.6 Conclusions 264 5.7 References 266 6 MODULE FABRICATION AND PERFORMANCE 269
6.1 Plasma-enhanced chemical vapor deposition (PECVD) 269
6.1.1 Electrical plasma properties 273 6.1.2 VHF plasma excitation 277 6.1.3 Device-grade material 283 6.1.4 Deposition parameters 286 6.1.5 Deposition rate 287 6.1.6 Deposition regimes for a-Si:H and [xc-Si:H 292
6.1.7 Upscaling 297 6.1.8 Deposition systems 299
6.1.9 Roll-to-roll depositions 300 6.1.10 Novel deposition systems 304 6.2 Hot-wire chemical vapor deposition (HWCVD) 306
6.2.1 Introduction 306 6.2.2 Description of the HWCVD technique 306
6.2.4 Types of materials deposited by HWCVD 307 6.2.5 Mechanisms of the deposition process 308
6.2.6 Filament aging 309 6.2.7 Amorphous and microcrystalline silicon films, and
microcrystalline silicon carbide alloys 309 6.2.8 Silicon nitride and silicon oxynitride films 311
6.3. Doped layers 311 6.3.1 ^-layers 312 6.3.2 Doped microcrystalline layers 314
6.4. Transparent conductive oxides (TCO) as contact materials 316 6.4.1 Glass substrates and specific TCO materials 316
6.4.2 Qualification of TCO materials 317 6.4.3 Surface texture of TCO 319
6.4.4 Cell optics 324 6.4.5 Light management in cells 326
6.4.6 Optical losses 328 6.5 Laser scribing and series connection of cells 331
6.5.1 Cell interconnection scheme 331 6.5.2 Power losses due to the series connection of cells 333
6.6 Module performance 336 6.6.1 Efficiencies 336 6.6.2 Energy yield 338 6.6.3 Partial shading 341 6.6.4 Shunting 346 6.7 Module Finishing 351 6.7.1 Encapsulation 352 6.7.2 Module certification 355 6.7.3 Long-term stability 356 6.8 Conclusions 359 6.9 References 360 7 EXAMPLES OF SOLAR MODULE APPLICATIONS 369
7.1 Building-integrated photovoltaics (BIPV): aspects
and examples 369 7.1.1 PV Facade in Munich (Germany) 371
7.1.2 Alpine roof integrated PV 373 7.1.3 PV Roof at Auvernier, Switzerland
(by Reto Tscharner) 374 7.1.4 PV installation in Brazil 376 7.1.5 Stillwell Avenue Station, New York City 380
7.2 Stand-alone and portable applications 382 7.3 Indoor applications of amorphous silicon solar cells 385
7.3.1 Why is amorphous silicon well suited for indoor
applications? 386 7.3.2 Design guidelines for solar powering of indoor
7.4 Space applications 388 7.4.1 Introduction 388 7.4.2 Satellite power generators and specific
power density 390 7.4.3 Radiation resistance of a-Si:H and other PV
technologies 392 7.4.4 a-Si:H based cells for space 393
7.4.5 Space applications of a-Si:H modules 395
7.5 Conclusions 396 7.6 References 397 8 THIN-FILM ELECTRONICS 401
8.1 Thin-film transistors and display technology 401
8.1.1 Introduction 401 8.1.2 TFTs and flat panel displays 402
8.1.3 TFT configurations and basic characteristics 405
8.1.4 a-Si:H TFT operation 407 8.1.5 ue-Si:H and poly-Si TFT performance and
other issues 413 8.2 Large-area imagers 413
8.2.1 Introduction and device configuration 413
8.2.2 Performance and limitations 415 8.3 Thin-film sensors on CMOS Chips 415
8.3.1 Introduction 415 8.3.2 a-Si:H sensor integration 417
8.3.3 Performance and limitations 418
8.4 Conclusions 420 8.5 References 421
