Morphological, Structural, and Optical Properties of Single-Phase Cu(In,Ga)Se2 Thin Films from the Selenization of Thermally Evaporated InSe/Cu/GaSe Precursors
The relatively small band gap values ( ？eV) of CuInSe2 thin films limit the conversion efficiencies of completed CuInSe2/CdS/ZnO solar cell devices. In the case of traditional two-stage growth techniques, limited success has been achieved to homogeneously increase the band gap by substituting indium with gallium. In this study, thermal evaporation of InSe/Cu/Gase precursors was exposed to an elemental Se vapour under defined conditions. This technique produced large-grained, single-phase Cu(In,Ga)Se2 thin films with a high degree of in-depth compositional uniformity. The selenization temperature, ramp time, reaction period, and the effusion cell temperature with respect to the Cu(In,Ga)Se2 films were optimized in this study. The homogeneous incorporation of Ga into CuInSe2 led to a systematic shift in the lattice spacing parameters and band gap of the absorber films. Under optimized conditions, gallium in cooperation resulted only in a marginal decrease in the grain size, X-ray diffraction studies confirmed single-phase Cu(In,Ga)Se2 material, and X-ray photoluminescence spectroscopy in-depth profiling revealed a uniform distribution of the elements through the entire depth of the alloy. From these studies optimum selenization conditions were determined for the deposition of homogeneous Cu(In,Ga)Se2 thin films with optimum band gap values between 1.01 and 1.21？eV. 1. Introduction CuInSe2 (CIS) based thin film module technology is the candidate with best chances to compete with crystalline silicon. CuInSe2 has a band gap of about 1.0？eV, which limits the conversion efficiency of complete CuInSe2/CdS/ZnO devices. In order to increase the conversion efficiency of devices, it is necessary to increase the band gap value of the absorber films. This can be achieved by systematically substituting some indium with a group III element, such as gallium and/or selenium, with another group VI element such as sulphur. The substitution of In with Ga and/or Se with S results in the shrinkage of the lattice parameters and thus an increase in the band gap [1]. The conversion efficiencies of polycrystalline thin film solar cells based on Cu(In,Ga)Se2 (CIGS) have already reached values above 19% at the laboratory scale [2]. The absorber films of these high efficiency devices are produced using a single-stage technique in which all the elements (Cu, In, Ga, and Se) are coevaporated from individual sources. This technique allows a controlled introduction of Ga into the structure and hence formation of single-phase material. In general, two-stage processing of thin
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