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

Eidgenössisches Departement für
Umwelt, Verkehr, Energie und Kommunikation UVEK
Bundesamt für Energie BFE
HIGH EFFICIENCY THIN-FILM PASSIVATED
SILICON SOLAR CELLS AND MODULES
THIFIC: THIN FILM ON CRYSTALLINE SI
Annual Report 2008
Author and Co-Authors S. De Wolf, S. Olibet, J. Damon-Lacoste, L. Fesquet, C. Ballif
Institution / Company Institute of Microtechnology (IMT) / University of Neuchâtel
Address Rue A.L. Breguet 2, 2000 Neuchâtel
Telephone, E-mail, Homepage +41 (0) 32 718 33 78, stefaan.dewolf@epfl.ch, www.unine.ch/pv
Project- / Contract Number Axpo Naturstrom-fonds 0703
Duration of the Project (from – to) 01.06.2007 - 31.05.2011
Date 08.12.2008
ABSTRACT
THIFIC (Thin film on crystalline Si) is a project sponsored by the Axpo Naturstrom-Fonds. It aims at
developing a new kind of ultra-high efficiency (20-22%) solar cells by depositing very thin film silicon
layers (typically 5-10 nanometers of amorphous and/or microcrystalline silicon) on top of silicon wafers.
These so-called “silicon heterojunction cells” can be fully processed at low temperature (typically
below 200°C) and may make use of thin wafers (down to 100 µm), thereby saving Si material. The
cells will be integrated into innovative light weight modules with efficiencies reaching up to 20%.
These concepts should open the path towards a really competitive production of solar electricity
based on high efficiency crystalline Si products. This wafer based approach is complementary to the
fabrication of solar modules based exclusively on thin films, which should allow the fabrication of
lower efficiency modules at lower cost/Wp.
At the Institute of Microtechnology of the University of Neuchâtel, an activity was started in 2005 in
the field silicon heterojunctions. On small area devices (4.5x4.5mm 2 ), an open-circuit voltage higher
than 700 millivolt and an efficiency of 19% were achieved on flat monocrystalline n-type wafers. The
launch of this Axpo Naturstrom-Fonds project was based on these results, which were achieved in a
short time. Meanwhile, based on single process step analysis, a better physical understanding of
interface phenomena occurring in silicon heterojunction solar cells has been obtained. A crucial issue
is that the a-Si:H / c-Si interface should be atomically sharp. The precise crystal orientation of the
wafers plays in this a role, and hence also the surface texture (which allows better light trapping).
Previously, well cleaned alkaline textured crystalline silicon surfaces could be obtained. Now we
achieved crucial insight in how to avoid epitaxial growth on such surfaces, allowing the fabrication of
high quality a-Si:H/c-Si textured interfaces. By this, we have demonstrated an open-circuit voltage of
700 millivolt, also on textured surfaces.
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