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

elow 1 at%. A dependency on temperature cannot be observed in this case. Apart from the expected
In, S and Cl no other elements were detected in these layers. In all these layers 40±1 at% of indium
and 60±1 at% of sulfur were measured.
The concentrations of the different elements detected by ERDA measurements confirm the RBS
results. In all samples Cl, O, N, and C impurities were found, however the impurity concentration
decreases with increasing substrate temperature and degree of sulfur excess in the solution. Only the
layers sprayed with S deficiency have significant chlorine impurities.
Solar cell characteristics
CIGS solar cells with sprayed In2S3 buffer layer typically show very low short circuit current density Jsc
in the range of 15 mA/cm 2 and poor fill factor FF. We attribute the low current to a high defect density
in the buffer layer and at the interfaces. In order to improve these parameters, a chemical surface
treatment was applied after deposition of the indium sulfide buffer. The In2S3/CIGS/Mo/SLG layer
stack was immersed in an aqueous solution containing cadmium acetate, ammonia and thiourea. After
the treatment, the sample was rinsed with high purity water and finished to a solar cell device.
A comparison of the J-V characteristics of CIGS solar cells with treated and untreated USP-In2S3 layer,
as shown in Figure 2.4, suggests that the cell performance after chemical surface treatment improves
(increase in Jsc and fill factor) significantly to a value comparable to the CBD-CdS reference cell.
Figure 2.4: J-V characteristics of solar cells with sprayed In2S3 buffer layer (a); sprayed In2S3 buffer
layer and surface chemical treatment of the buffer layer after deposition (b); and CdS buffer layer
deposited by CBD (c).
Summary - Ultrasonic spray pyrolysis
Continuous polycrystalline In2S3 layers with low chlorine impurity content can be prepared by
ultrasonic spray pyrolysis if sulfur is provided in stoichiometric concentration or in excess in the
precursor solution. Increasing the deposition temperature or the absolute indium salt concentration in
the solution increases the film growth rate. Sulfur deficiency in the precursor solution yields
amorphous compact and smooth films but high chlorine contamination. The variation of sulfur
deficiency in the precursor solution is a tool for buffer layer band gap engineering by introducing
chlorine. Furthermore reflection losses can be reduced with this parameter. Sulfur excess variation
mainly influences the layer growth and absorber surface by adding more sulfur. Best cell efficiency of
12.4 % was obtained with a Cu(In,Ga)(S,Se)2 absorber and USP-In2S3 buffer layer sprayed with sulfur
excess.
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ATHLET - Advanced Thin Film Technologies for Cost Effective Photovoltaics, A. N. Tiwari, ETH Zürich
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