Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Collaborations
IMT is active in a collaboration on a national level with the EPFL for advanced transmission electron
microscopy (TEM) sample preparation, supporting the textured heterojunction solar cell improvement.
In addition, international “informal” collaboration are conducted with the German Hahn-Meitner-Institut
(HMI), the Japanese National Institute of Advanced Industrial Science and Technology (AIST) and the
US-American National Renewable Energy Laboratory (NREL).
Evaluation 2007 and Outlook 2008
THIFIC was successfully launched in mid-2007. From the beginning of the project, we pursued the
amorphous/crystalline silicon interface recombination modeling for fast heterojunction solar cell single
process step analysis and improvement. Textured wafers can now be cleaned and therefore the same
kind of lifetime measurement assisted layer development is performed. Up to now, a textured silicon
heterojunction solar cell with an open-circuit voltage of 700 mV is achieved. Despite the current gain
from the textures geometrical light-trapping, this cell’s efficiency remains, with 17%, lower than the
best one on flat silicon (19%) because of its low fill factor. Fig. 7 illustrates that this fill factor loss is
partly caused by the texture based surface increase on same sized cells as compared to flat cells. For
a flat small cell doubling the surface leads to a relative fill factor loss of more than 10% because of
series resistance due to the lacking front metal grid. Worth recognizing positively too in Fig. 7 is that
the open-circuit voltage is even higher when quadrupling the textured solar cells size. Therefore the
most urgent step to take at present in our silicon heterojunction solar cell processing is the development
of a front contact metallization. Actually three different metallization schemes are under test.
Shortly they will reach the application phase to solar cells and will then permit us to validate our first
results also on larger solar cells.
Fig. 7: Series resistance losses in small sized textured cells without front metal grid.
References
[1] S. Olibet, E. Vallat-Sauvain, C. Ballif: Effect of light induced degradation on passivating properties of a-Si:H layers
deposited on crystalline Si, Proceedings of the 21st EU-PVSEC, Dresden, Germany, p.1366, 2006.
[2] L. Fesquet, S. Olibet, E. Vallat-Sauvain, A. Shah, C. Ballif: High quality surface passivation and heterojunction fabrication
by VHF-PECVD deposition of amorphous silicon on crystalline Si: Theory and experiments, to be published
in the Proceedings of the 22th EU-PVSEC, Milano, Italy, 2007.
[3] S. Olibet, E. Vallat-Sauvain and C. Ballif: Model for a-Si:H/c-Si interface recombination based on the amphoteric
nature of silicon dangling bonds, Physical Review B 76, 035326, 2007.
[4] S. Olibet, E. Vallat-Sauvain, C. Ballif, L. Fesquet: Recombination through amphoteric states at the amorphous/crystalline
silicon interface: modelling and experiment, Proceedings of NUMOS-workshop (Numerical modelling
of thin film solar cells), Gent, Belgium, p. 141, 2007.
[5] S. Olibet, E. Vallat-Sauvain, C. Ballif, L. Korte, L. Fesquet: Silicon Solar Cell Passivation using Heterostructures,
Proceedings of the 17 th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes, Vail, Colorado
USA, p. 130, 2007.
[6] S. Olibet, E. Vallat-Sauvain, C. Ballif, L. Korte, L. Fesquet: Heterojunction solar cell efficiency improvement on various
c-Si substrates by interface recombination modelling, To be presented at the PVSEC-17, Fukuoka, Japan, 2007.
THIFIC, S. Olibet, IMT Neuchâtel
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