Passivating Contacts for Homojunction Solar Cells Using A-Si:H/C-Si Hetero-Interfaces
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Passivating contacts for homojunction solar cells using a-Si:H/c-Si hetero-interfaces THÈSE NO 6392 (2014) PRÉSENTÉE LE 24 OCTOBRE 2014 À LA FACULTÉ DES SCIENCES ET TECHNIQUES DE L'INGÉNIEUR LABORATOIRE DE PHOTOVOLTAÏQUE ET COUCHES MINCES ÉLECTRONIQUES PROGRAMME DOCTORAL EN SCIENCE ET GÉNIE DES MATÉRIAUX ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES PAR Bénédicte DEMAUREX acceptée sur proposition du jury: Prof. D. Damjanovic, président du jury Prof. C. Ballif, Dr S. De Wolf, directeurs de thèse Prof. N. Grandjean, rapporteur Dr R. A. Sinton, rapporteur Dr J. Szlufcik, rapporteur Suisse 2014 Contents Abstract ..........................................................................................................................vii Résumé ...........................................................................................................................ix Acronyms and abbreviations ..................................................................................................xi Chapter 1 Introduction ....................................................................................................... 1 1.1 General context ........................................................................................................... 1 1.1.1 Electricity production and environment: the advantages of solar energy ....... 1 1.1.2 Cost decrease and competitiveness: the economics of photovoltaics ............. 2 1.1.3 Current global installed capacity and outlook .................................................. 4 1.1.4 The different PV technologies: an efficiency race ............................................. 5 1.2 Motivation ................................................................................................................... 7 1.3 Goal and structure ..................................................................................................... 11 1.4 Contribution to the research field ............................................................................. 12 Chapter 2 Contacts and passivation in solar cells ............................................................... 15 2.1 Introduction ............................................................................................................... 15 2.2 Contacts ..................................................................................................................... 16 2.2.1 Metal – Semiconductor contacts .................................................................... 16 2.2.2 Metal – Insulator – Semiconductor contacts .................................................. 18 2.2.3 Characterizing the contact and dependence on doping ................................. 19 2.3 Recombination losses and passivation ...................................................................... 19 2.3.1 Recombination processes ............................................................................... 20 2.3.2 Characterizing the recombination losses ........................................................ 23 2.3.3 Passivation of interface defects ...................................................................... 25 2.4 Contacts, passivation and solar cells ......................................................................... 28 2.4.1 Basic properties of solar cells .......................................................................... 28 2.4.2 Influence of recombination and contacts on cell characteristics ................... 29 iii Contents 2.4.3 Passivation and contacts: a trade-off? ............................................................ 32 2.5 Passivating contacts ................................................................................................... 34 2.5.1 Nomenclature and general principles of passivating contacts ........................ 34 2.5.2 Literature review of passivating contacts ........................................................ 34 2.6 Contacts investigated in this thesis ............................................................................ 38 Chapter 3 Homojunction formation ................................................................................... 39 3.1 Introduction ................................................................................................................ 39 3.2 Diffusion ..................................................................................................................... 41 3.2.1 CSEM and CMi diffusion tubes ......................................................................... 41 3.2.2 Single-side diffusion ......................................................................................... 43 3.2.3 Conclusion ........................................................................................................ 46 3.3 Ion implantation ......................................................................................................... 46 3.4 Low-temperature plasma-deposited silicon epitaxial films ....................................... 48 3.4.1 Motivation and prior art .................................................................................. 48 3.4.2 Plasma-deposited silicon epitaxial growth and properties ............................. 49 3.4.3 Doped homo-epitaxial layers ........................................................................... 65 3.4.4 Summary .......................................................................................................... 68 3.5 Conclusion .................................................................................................................. 68 Chapter 4 Silicon heterojunction solar cells ....................................................................... 71 4.1 Introduction ................................................................................................................ 71 4.2 Fabrication process of a silicon heterojunction solar cell .......................................... 73 4.3 Contact formation ...................................................................................................... 74 4.3.1 Damage at hydrogenated amorphous / crystalline silicon interfaces by indium tin oxide overlayer sputtering .......................................................................... 74 4.3.2 Atomic layer deposited electrodes as protection against sputter damage .... 82 4.3.3 Summary and outlook ...................................................................................... 94 4.4 Contact characterization ............................................................................................ 95 4.4.1 c-Si/a-Si:H/TCO contact: TCO work function ................................................... 95 4.4.2 a-Si:H/TCO contact: contact resistivity measurements ................................... 99 4.4.3 TCO/metal contact ......................................................................................... 102 4.5 FF analysis in silicon heterojunction and passivating contact structures ................ 103 4.6 Summary and conclusions ........................................................................................ 104 iv Contents Chapter 5 Passivating contact solar cells ......................................................................... 107 5.1 Introduction ............................................................................................................. 107 5.2 Highly doped surfaces for silicon heterojunction solar cells ................................... 109 5.3 Characterization of homo-hetero contacts ............................................................. 111 5.4 Homo-hetero solar cells .......................................................................................... 114 5.4.1 Homo-hetero solar cells using an epitaxially grown emitter ........................ 114 5.4.2 Homo-hetero solar cell design ...................................................................... 118 5.4.3 Homo-hetero solar cells with ion-implanted wafers .................................... 124 5.4.4 Improvement of the homo-hetero solar cell design ..................................... 130 5.4.5 Passivating contacts solar cells with diffused wafers ................................... 132 5.5 Conclusions .............................................................................................................. 134 Chapter 6 Conclusions and perspectives .......................................................................... 137 6.1 Summary .................................................................................................................. 137 6.2 Perspectives ............................................................................................................. 140 Appendix A ....................................................................................................................... 143 Appendix B ....................................................................................................................... 145 Curriculum Vitae ................................................................................................................ 147 List of publications ............................................................................................................. 149 Bibliography ...................................................................................................................... 151 Remerciements/Acknowledgments ...................................................................................