Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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on the front side. All the layers are amorphous, except the p layer, which is microcrystalline. Then ITO
was deposited on front and back side, and a thick Aluminum layer was sputtered on the back side.
The small cell efficiency was about 15%, with a Voc of 690 mV.
Using the same cell structure, screen printed cell exhibited much lower efficiency, due to higher series
resistance, and optical losses due to the metallic grid. The high series resistance is mainly due to the
intrinsic conductivity of non optimized silver paste. Contact resistance exhibits a very low value
(1.10 -3 Ohm.cm 2 ), and is thus not taken into account. However, after a plating step, the fill factor
greatly improves to the value obtained on small cells, increasing at the same time the cell efficiency.
Using these two steps, one can say that there are now reduced electrical losses due to metallization,
but optical losses (10%) coming from the optical shadowing by the grid remain relatively important.
Collaborations
IMT is active in 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” collaborations have been 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). IMT is since 2008 also
taking part in the European project Hetsi, connecting the major European Institutes and Universities
working on heterojunction devices.
Evaluation 2008 and Outlook 2009
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. Crucial parameters have now been identified to fabricate
successfully devices on textured wafers. For this, at the end of 2008, an industrial compatible
wetbench is being installed in completely new cleanroom facilities, dedicated for heterojunction devices
studies. The same facilities will host a new multi-chamber PECVD system for further device development.
In addition, larger area devices can now be fabricated as well (up to 5x5 cm 2 ). For this,
different metallization schemes are under test. In early 2009, a state-of-the-art screen-printer will be
installed as well as IMT.
References
[1] 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.
[2] S. Olibet, C. Monachon, A. Hessler-Wyser, E. Vallat-Sauvain, L. Fesquet, J. Damon-Lacoste, S. De Wolf, and C. Ballif,
Textured silicon heterojunction solar cells with over 700 mv open-circuit voltage studied by transmission electron
microscopy , , to be published in the Proceedings of the 23th EU-PVSEC, Valencia, Spain, 2008.
[3] S. De Wolf and M. Kondo, Abruptness of a-Si:H / c-Si interface revealed by carrier lifetime measurements, Appl.
Phys. Lett. 90, 042111 (2007).
[4] S. De Wolf, S. Olibet, and C. Ballif, Stretched-exponential a-Si:H / c-Si interface recombination decay, Appl. Phys.
Lett. 93, 032101 (2008).
THIFIC, De Wolf, IMT Neuchâtel
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