X-ray fluorescence investigation of the Ga distribution in Cu(In,Ga)Se2 thin films

The efficiencies of Cu(In,Ga)Se-2/CdS/ZnO solar cell devices in which the absorbers are produced by classical two-step processes are significantly lower that those in which coevaporated absorbers are used. A significant problem related to two-step growth processes is the reported segregation of Ga towards the Mo back contact, resulting in separate CuInSe2 and CuGaSe2 phases. Furthermore, it is often reported that material losses (especially In and Ga) occur during high-temperature selenization of metallic precursors. In this study, X-ray fluorescence (XRF) analysis was used to study the diffusion behaviour of the chalcopyrite elements in single-stage and two-stage processed Cu(In,Ga)Se-2 thin films. This relatively simple characterization technique proved to be very reliable in determining the degree of selenium incorporation, possible material losses and the in-depth compositional uniformity of samples at different stages of processing. This information is especially important in the case of two-stage growth processes, involving high-temperature selenization steps of metallic precursors. Device quality Cu(In,Ga)Se, thin films were prepared by a relatively simple and reproducible two-step growth process in which all the metals were evaporated from one single crucible in a selenium-containing environment. The precursors were finally treated in an H2Se/Ar atmosphere to produce fully reacted films. XRF measurement indicated no loss of In or Ga during this final selenization step, but a significant degree of element diffusion which depended on the reaction temperature. It was also possible to produce Cu(In,Ga)Se-2 thin films with an appreciable amount of Ga in the near-surface region without separated CuInSe2 and CuGaSe2 phases. (C) 2000 Elsevier Science B.V. All rights reserved.