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
2.4.1 Transport model ................................. 47
2.4.2 Measurement of conductivity in a co-planar
configuration ................................... 48
2.4.3 Dark conductivity odark ......................... 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 microstructure ............ 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 ofa-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 μc-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
μc-Si:H ........................................ 118
3.2.5 Relationships between structural and other
properties of μc-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 "piw-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 "Pn-type" or "classical" diode: dark
characteristics ....................................... 154
4.3 The "Pn-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: "pin-TYPE" SOLAR CELLS ............................ 176
4.5 Introduction to "pin-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 Pf and nf ............................. 183
4.5.4 Trapped charge carriers pt and nt in
bandtails ...................................... 186
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 i-layer by
trapped carriers: Concept ...................... 189
4.6.2 Deformation of electric field in i-layer 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 i-layer: Concept .................... 196
4.7.3 Field deformation by charged dangling bonds
within the i-layer: numerical simulation for
an amorphous silicon solar cell with di -
300 nm ......................................... 198
4.7.4 Field deformation within the i-layer: summary
of situation for different i-layer
thicknesses .................................... 198
4.8 Recombination and Collection .......................... 201
4.8.1 p/i and i/n 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 Jsc 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 η 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
finding tandem Jsc, Voc, FF ..................... 240
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/μc-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 μc-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
6.2.3 Filament materials ............................. 307
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 p-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 applications ............................ 387
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 μc-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
INDEX ......................................................... 425
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