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

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Measurements of solar cells performance
The CdS buffer layer and ZnO top contact layer were deposited on the fabricated CIGS layers to obtain
solar cells. Eventhough the organic solution approach gives more reliable and reproducible incorporation
of copper, the performance of the solar cells could not be improved yet. Solar cells made by
the organic approach resulted in efficiencies of up to 3.5%, whereas the cells processed from the water
solutions exhibited efficiencies of up to 4.1 % under simulated AM1.5 illumination. This may be due
to the non-optimal interface region between the CIGS and buffer layer or due to the incorporation of
unwanted impurities, which leads to the degraded electrical properties of the CIGS absorber. Further
investigations are needed.
Evaluation 2008 and outlook
We have investigated a simple, low-cost method for incorporating Cu into indium gallium selenide
layers. When these layers were immersed in a ethylene glycole solution containing Cu + ions, it was
found that Cu was incorporated into the layers as Cu-Se compounds with no increase in the thickness
of the layers. The principal research findings and achievements are summarized as follows:
� The ion-exchange process in organic medium is advantageous over the aqua solution technique,
which was investigated in 2007, because no corrosion of the metal back contact is observed,
and it is possible to achieve sufficient Cu incorporation in thicker (In,Ga)2Se3 layers of 1.5-2 microns.
� There is a preferential exchange of Cu for In rather than Ga, possibly due to differences in enthalpy
of formation, ionic radii and solubility. This preferential exchange results in the existence
of the Ga gradient in the final CIGS layers.
� Controlling the Se supply during the annealing treatment is the key to controlling the morphology
of the CIGS layers. Delaying the supply of Se until the substrate temperature reaches �400°C
leeds to smoother CIGS layers by preventing the formation of Se-rich copper selenides during
substrate heating.
� Eventhough the organic solution approach gives more reliable and reproducible incorporation of
copper, the performance of the solar cells could not be improved yet. Solar cells made by the organic
approach resulted in efficiencies of up to 3.5%, whereas the cells processed from the water
solutions exhibited efficiencies of up to 4.1 % under simulated AM1.5 illumination. This may
be due to the non-optimal interface region between the CIGS and buffer layer or due to the incorporation
of unwanted impurities, which leads to the degraded electrical properties of the CIGS
absorber.
A patent has been formulated together with a specialised patent attorney and was filed (PCT application
number PCT/EP1007/059422). The response of the patent agency has been received, indicating
that most of the proposed ideas are novel indeed although a further administrative processing is required
to finalize the patent filling.
References
[1] AM. Gabor, JR. Tuttle, DS. Albin et al. High Efficiency CuInxGa1-xSe2 Solar-Cells made from (Inx,Ga1-x)2Se3 precursor
films, Applied Physics Letters 65 (2), 198-200, 1994.
[2] C.J. Hibberd, K. Ernits, M. Kaelin, U. Müller, A. N. Tiwari, Chemical Oncorporation of Copper into Indium Selenide
Thin-films for processing of CuInSe2 Solar Cells, Prog. Photovolt: Res. Appl. 16, 585-593 (2008).
[3] T. B. Massalski, J. L. Murray, L. H. Bennett, H. Baker, Binary alloy phase diagrams Vol. 1 (1986) Ohio.
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Thin Film CIGS Solar Cells with a Novel Low Cost Process, A. N. Tiwari, ETH Zürich