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

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Introduction / project objectives
Polycrystalline thin film Cu(In,Ga)Se2 (CIGS) solar cells are important because of very high efficiency,
long term stable performance, and their potential for low cost generation of solar electricity. The
National Renewable Energy Laboratory, USA has reported a world record efficiency of 19.5% [1] for
the CIGS solar cells grown on glass substrates and several groups including ETHZ have achieved
efficiencies exceeding 16% on glass substrates.
The objectives of the work packages, we are involved, include:
� Development of high efficiency lightweight and flexible CIGS solar cells on polymer foils and
improvement in processes for highest record efficiencies.
� Development of alternative processes towards simple and low cost manufacturing of high
efficiency flexible solar cells.
� Development of critical cell components of CIGS tandem solar cells for next generation of more
efficient solar cells.
� Development of a new spray technology incorporating ultrasonic fine and condensed mist forming
for alternative buffer layers
In the first reporting, we described, how the ETH group succeeded in developing 14.1% efficiency
solar cell. The CIGS layers were grown by a vacuum evaporation process on static substrates with
good reproducibility. The development of ultrasonic spray pyrolysis (USP) equipment and the first
results were also described in details in the first report and work is going on.
This year the research focus was placed on the development of InxSy alternative buffer layer
deposition process by a physical vapour deposition (PVD) method and continuation of the work on
CIGS solar cells on transparent conducting oxides (TCO) for tandem solar cell applications.
Solar cells with of CBD-CdS buffer layer are considered as a standard although other materials grown
by CBD have been employed in CIGS solar cells. However, there are some limiting issues associated
with the use of CdS as buffer layer and CBD technique. CdS has an optical band gap (Eg) of 2.48 eV
which causes some optical absorption losses in the shorter wavelength region of the solar spectrum.
Moreover, Cd-free CIGS solar cells are often desired for some environmental reasons. In addition,
CBD is a non-vacuum process which makes it incompatible to vacuum based in-line production of the
solar modules on industrial scale.
Indium sulfide has a great potential as an alternative buffer layer to conventional CdS buffer. The
optical band gap of In2S3 thin film strongly depends on the method of preparation and can vary from
2.0-3.0 eV [3]. Various groups have prepared In2S3 films by different methods like modulated flux
deposition [4], atomic layer epitaxy [5], atomic layer deposition (ALD) [6], reactive evaporation [7], and
thermal evaporation [3]. Out of these, In2S3 buffer layers deposited by ALD produced a record
efficiency of 16.4% on a small area of 0.1 cm 2 [8]. However, ALD is a slow, batch-to-batch process,
and expensive to upscale for large area module production, while the physical vapor deposition (PVD)
methods hold their high industrial relevance. A cell made of PVD-In2S3 has resulted in maximum
efficiency of 14.8% [9]. The cell efficiency depends strongly on the method of evaporation, source
material and post-deposition annealing treatments [3]. For an objective to develop flexible CIGS solar
cells with alternative buffer layers we have been investigating the deposition of In2S3 by a PVD
method, and especially the evaporated source materials and In2S3 layers on soda lime glass (SLG)
substrates. The In2S3 buffer layers were used to develop CIGS solar cell and the effects of vacuum
annealing treatments on the photovoltaic properties have been investigated.
Work and results
PVD grown In2S3 buffer layer
For the development of the solar cells CIGS layers were grown on Mo/SLG by coevaporation of
constituent elements with 3-stage process. The Indium sulfide buffer layer was deposited on
CIGS/Mo/SLG substrates in a high vacuum evaporation system by using In2S3 compound as a source
material. Two different types of In2S3 powders, powder-A and -B, obtained from different suppliers,
were used. The powder-A having finer particle (size ~2 µm) and narrow distribution in particle size was
dark red in color. On the other hand, the powder-B (light red color) was found to be coarser (size 5-
10 µm) than powder-A with a moderate distribution in particle size. Thereafter, a window layer of
ATHLET, D. Brémaud, ETH Zurich
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