New concepts for front side metallization of industrial silicon solar cells [Elektronische Ressource] / vorgelegt von Ansgar Mette
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New concepts for front side metallization of industrial silicon solar cells [Elektronische Ressource] / vorgelegt von Ansgar Mette

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New Concepts for Front Side Metallization of Industrial Silicon Solar Cells Dissertation zur Erlangung des Doktorgrades der Fakultät für Angewandte Wissenschaften der Albert-Ludwigs-Universität Freiburg im Breisgau vorgelegt von Ansgar Mette Fraunhofer-Institut für Solare Energiesysteme Freiburg im Breisgau 2007 Dekan: Prof. Dr. Bernhard Nebel Hauptreferent: PD Dr. Volker Wittwer Koreferent: Prof. Dr. Jürgen Wilde Datum der Prüfung: 26. November 2007 To my parents, with love and gratitude. Table of contents INTRODUCTION..........................................................................................1 Thesis motivation ...................................................................................................... 1 Thesis outline ............................................................................................................ 3 1 CRYSTALLINE SILICON SOLAR CELLS ............................................5 1.1 Structure and operation principle.................................................................... 5 1.1.1 Current-Voltage curve.............................................................................. 6 1.1.2 Physical and technological efficiency limitations ................................... 9 1.

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Published 01 January 2007
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New Concepts for
Front Side Metallization of
Industrial Silicon Solar Cells


Dissertation zur Erlangung des Doktorgrades

der Fakultät für Angewandte Wissenschaften

der Albert-Ludwigs-Universität Freiburg im Breisgau



vorgelegt von

Ansgar Mette



Fraunhofer-Institut für Solare Energiesysteme
Freiburg im Breisgau

2007












Dekan: Prof. Dr. Bernhard Nebel
Hauptreferent: PD Dr. Volker Wittwer
Koreferent: Prof. Dr. Jürgen Wilde
Datum der Prüfung: 26. November 2007


























To my parents, with love and gratitude.
























Table of contents

INTRODUCTION..........................................................................................1
Thesis motivation ...................................................................................................... 1
Thesis outline ............................................................................................................ 3
1 CRYSTALLINE SILICON SOLAR CELLS ............................................5
1.1 Structure and operation principle.................................................................... 5
1.1.1 Current-Voltage curve.............................................................................. 6
1.1.2 Physical and technological efficiency limitations ................................... 9
1.2 Loss mechanisms due to front side metallization pattern............................. 13
1.2.1 Optical losses ......................................................................................... 14
1.2.2 Electrical losses...................................................................................... 15
1.2.3 Grid optimization for maximum power point........................................ 19
1.3 Metal-semiconductor contact........................................................................ 20
2 CELL PROCESSING AND METALLIZATION TECHNOLOGIES ......26
2.1 Processing of silicon solar cells .................................................................... 26
2.1.1 Screen-printed solar cell......................................................................... 26
2.1.2 PERL and LFC solar cell ....................................................................... 28
2.1.3 Comparison of an evaporated with a screen-printed front..................... 29
2.2 Metallization technologies............................................................................ 30
2.2.1 Thick- and thin-film metallization technologies.................................... 30
2.2.2 Screen-printing....................................................................................... 32
2.2.3 Stencil-printing....................................................................................... 36
2.2.4 Pad-printing............................................................................................ 37
2.2.5 Ink-jet printing ....................................................................................... 39
2.2.6 Dispensing.............................................................................................. 41
2.2.7 Photolithographical definition and evaporation..................................... 42
2.2.8 Laser micro-sintering ............................................................................. 43 ii Table of contents
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2.2.9 Nickel plating ......................................................................................... 43
2.2.10 Thickening of metal contacts by means of plating ................................ 45
2.3 Two-layer concept......................................................................................... 45
3 GRID DESIGN OF THE TWO-LAYER CONTACT STRUCTURE.......47
3.1 Calculation parameters and assumptions...................................................... 47
3.2 Solar cell with two busbars ........................................................................... 50
3.2.1 Assuming a cell with a sheet resistance of 55 Ω/sq............................... 50
3.2.2 Variation of the emitter sheet resistance................................................ 51
3.2.3 Influence of the contact resistivity ......................................................... 52
3.2.4 Considering irradiation reflection from the contact............................... 54
3.3 Solar cell with alternative busbar geometries ............................................... 55
3.3.1 Three busbars ......................................................................................... 56
3.3.2 Mesh of wires......................................................................................... 57
3.4 Losses caused by busbar and tab resistance.................................................. 59
3.4.1 Busbar losses.......................................................................................... 59
3.4.2 Tab losses ............................................................................................... 59
3.5 Chapter summary .......................................................................................... 60
4 SERIES RESISTANCE DETERMINATION FROM IV-CURVES.........62
4.1 Series resistance determination methods ...................................................... 62
4.1.1 Fitting of the two-diode equation to a dark IV-curve............................. 62
4.1.2 Comparison of the dark with the one-sun IV-curve............................... 63
4.1.3 Comparison of the Suns-V with the one-sun IV-curve ........................ 64 oc
4.1.4 Comparison of at least two IV-curves measured at different irradiation
intensities ............................................................................................... 64
4.1.5 Comparison of j and V values with the one-sun IV-curve................. 66 sc oc
4.1.6 Integration of the area under an IV-curve .............................................. 66
4.2 Correlation between fill factor and series resistance .................................... 67 Table of contents iii
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4.3 Experimental comparison of determination methods................................... 68
4.4 Influence of non-ideal IV-curves on r determination .................................. 71 s
4.5 Detailed discussion of presented methods.................................................... 72
5 EVAPORATION OF DIFFERENT CONTACT METALS .....................74
5.1 Solar cell processing ..................................................................................... 74
5.2 IV-Parameters of processed solar cells ......................................................... 75
5.2.1 Different metals evaporated on inverted pyramid front......................... 75
5.2.2 Different metals evaporated on a random pyramid front....................... 76
5.3 Discussion of the different metals ................................................................ 77
5.3.1 Aluminum .............................................................................................. 78
5.3.2 Palladium................................................................................................ 79
5.3.3 Silver ...................................................................................................... 80
5.3.4 Titanium-Palladium ............................................................................... 80
5.3.5 Titanium ................................................................................................. 82
5.3.6 Nickel ..................................................................................................... 82
5.3.7 Chromium .............................................................................................. 83
5.4 Chapter summary .......................................................................................... 84
6 SCREEN-PRINTING OF HOTMELT SILVER PASTE.........................85
6.1 Comparison of hotmelt and conventional paste............................................ 85
6.1.1 Composition ........................................................................................... 85
6.1.2 Rheology ................................................................................................ 87
6.1.3 Thermogravimetrical analysis................................................................ 89
6.2 Characterization of hotmelt printed contacts................................................ 90
6.2.1 Hardware setup....................................................................................... 90
6.2.2 Optical characterization ......................................................................... 91
6.2.3 Electrical characterization...................................................................... 94
6.3 Solar cell processing and results................................................................... 95 iv Table of contents
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6.3.1 Monocrystalline silicon solar cells......................................................... 95
6.3.2 Multicrystalline silicon solar cells ......................................................... 96
6.3.3 Variation of the emitter sheet resistance................................................ 98
6.4 Fine-line printing and high sheet resistance emitter ..................................... 99
6.4.1 Motivation .............................................................................................. 99
6.4.2 Contacting high sheet resistance emitter.............................................. 100
6.4.3 Fine-line screen-printing ...................................................................... 102
6.5 Chapter summary ........................................................................................ 103
7 LIGHT-INDUCED PLATING..............................................................105
7.1 Introduction................................................................................................. 105
7.2 Principle of light-induced plating ............................................................... 107
7.2.1 Characterization of the silver cyanide electroplating bath................... 107
7.2.2 Plating of solar cell contacts ................................................................ 109
7.2.3 Plating without external power supply................................................. 115
7.2.4 Plating of n-type silicon solar cells ...................................................... 117
7.2.5 Plating of non-silver contacts............................................................... 118
7.3 Light-induced plating of screen-printed contacts ....................................... 118
7.3.1 Setup..................................................................................................... 118
7.3.2 Small-area monocrystalline silicon solar cells..................................... 119
7.3.3 Large-area multicrystalline silicon solar cells ..................................... 121
7.3.4 Cyanide-free plating for industrial application.................................... 124
7.4 Direct light-induced plating of silver onto silicon surface ......................... 125
7.5 Chapter summary ........................................................................................ 127
8 HIGH-EFFICIENCY SCREEN-PRINTED & PLATED SOLAR CELL129
8.1 Production sequence for LFC and Al-BSF solar cells................................ 129
8.2 Aluminum back surface field cell structure................................................ 130 Table of contents v
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8.3 Laser-fired contact cell structure ................................................................ 133
8.3.1 Variation of the firing temperature ...................................................... 133
8.3.2 Variation of the emitter doping concentration..................................... 135
8.4 Laser-fired contact versus aluminum back surface field ............................ 137
8.5 Chapter summary ........................................................................................ 140
9 PAD-PRINTING: TECHNOLOGY AND ANALYSIS..........................142
9.1 Printing of hotmelt paste............................................................................. 142
9.2 Pad-printed and plated contacts .................................................................. 145
9.2.1 Solar cell processing ............................................................................ 145
9.2.2 Variation of the grid design and emitter doping concentration ........... 146
9.2.3 Influence of a sintering and second plating step.................................. 147
9.3 Chapter summary ........................................................................................ 150
10 METAL AEROSOL JET PRINTING ...............................................151
10.1 Working principle of the metal aerosol jet printer .................................. 151
10.1.1 Deposition head.................................................................................... 152
10.1.2 Pneumatic and ultrasonic atomizer ...................................................... 153
10.2 Nano-particle inks.................................................................................... 155
10.2.1 Metallic nano-particles......................................................................... 156
10.2.2 Ink components .................................................................................... 158
10.2.3 Stabilization of nano-particle inks ....................................................... 158
10.3 Set-up of the printer at Fraunhofer ISE................................................... 159
10.4 Characterization and optimization of printing process ........................... 161
10.5 Solar cell processing and results ............................................................. 165
10.5.1 First aerosol jet printed solar cells using nano particle inks................ 165
10.5.2 Multicrystalline silicon solar cells ....................................................... 165
10.5.3 Monocrystalline silicon solar cells....................................................... 167 vi Table of contents
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10.6 Chapter summary..................................................................................... 171
11 MICROSTRUCTURE ANALYSIS OF CONTACTS........................172
11.1 Review of current models for screen-printed contact formation............. 172
11.2 Solar cell results of pad-printed and plated contacts............................... 175
11.2.1 Design of experiment ........................................................................... 175
11.2.2 IV-parameters ....................................................................................... 176
11.3 Dependence of contact resistance on contact width and height .............. 178
11.3.1 Contact resistance measurement .......................................................... 178
11.3.2 Optical analysis of the interface layer.................................................. 180
11.4 Influence of a plating step on the contact resistance............................... 184
11.4.1 Electrical measurements....................................................................... 184
11.4.2 Micro-structural investigations ............................................................ 186
11.5 Conclusion: New current paths for plated contacts................................. 192
12 SUMMARY .....................................................................................196
DEUTSCHE ZUSAMMENFASSUNG......................................................199
APPENDIX...............................................................................................202
LIST OF SYMBOLS, ACRONYMS, INDICES AND CONSTANTS.........211
LIST OF PUBLICATIONS .......................................................................216
BIBLIOGRAPHY .....................................................................................218
DANKSAGUNG.......................................................................................230