Microcharacterization of CuInSe 2 Grown by Coevaporation and Selenization
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MICROCHARACTERIZATION OF CuInSe2 GROWN BY COEVAPORATION AND SELENIZATION
M. H. Bode, M.M. AI-Jassim, J. Tuttle, and D. Albin National Renewable Energy Laboratory Golden, Colorado 80401 ABSTRACT Thin films of CuInSe2, grown by coevaporation or by selenization of a Cu-In precursor, were analyzed in a scanning transmission electron microscope. While the coevaporated film shows clear evidence of second phases (Cu2Se) around the individual grains, no second phases could be found in the selenized material. Structural characterization also showed the presence of two ordered phases in the coevaporated films, the ordered-vacancy compound CuIn2Se3.5, and a CuPtordered phase of CuInSe2. INTRODUCTION CuInSe2, hereafter referred to as CIS, exhibits some unique physical properties, which make this semiconductor a very promising candidate for low-cost solar cells. First, its high absorption coefficient and the direct band structure allow the efficient absorption of solar photons in films as thin as 2 gm. The bandgap (= 1 eV) is comparable to the band gap of silicon. Second, the components of CIS, Cu, In, and Se are abundant and cause less concern for environmental problems than, for example, the components of CdTe, another material being developed for lowcost solar cells. Third, even though the exact mechanism is at best unclear at the moment, grain boundaries in CIS don't seem to have adverse affects on the performance of polycrystalline thin-film solar cells. They actually seem to improve the photovoltaic properties of thin films over that of single crystal CIS. Polycrystalline solar cells with efficiencies in the 15% rangel,2, 3 have been reported, while singlecrystal devices so far have not gone beyond 5% efficiency. Efficiencies above 18%, or even 20%, have been projected for optimum materials and process parameters. This, however, is a formidable task due to the complex chemo-physical behavior of CIS during growth. 4 In this paper, we report on the structural and chemical aspects of thin films of CIS, grown either by an evaporation technique or by an alternative method, the selenization of CuInprecursor films. EXPERIMENTAL The CIS films used for this investigation were grown by two different methods. Method A consists of a conventional physical vapor deposition (PVD) technique, slightly modified to promote grain growth. 5 Films grown by this method are then subjected to a second treatment step to adjust the overall composition of the film. 6 Method B employs the more recently developed technique of selenization. Here, elemental Cu and In are deposited on glass and then subjected to a treatment in a Se atmosphere to convert the Cu-In alloy into CIS. In contrast to the usual selenization in H2 Se, these films were treated in a pure Se atmosphere, which is advantageous from environmental and safety points of view, as elemental Se is much less toxic than H 2 Se. Mat. Res. Soc. Symp. Proc. Vol. 283. @1993 Materials Research Society
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Substrate material for solar cells is usually glass, coated with a thin film of molybdenum
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