X-ray fluorescence investigation of the Ga distribution in Cu(In,Ga)Se-2 thin films

The efficiencies of Cu(In,Ga)Se-2/CdS/ZnO solar cell devices in which the a bsorbers are produced by classical two-step processes are significantly low er that those in which coevaporated absorbers are used. A significant probl em related to two-step growth processes is the reported segregation of Ga t owards the Mo back contact, resulting in separate CuInSe2 and CuGaSe2 phase s. Furthermore, it is often reported that material losses (especially In an d Ga) occur during high-temperature selenization of metallic precursors. In this study, X-ray fluorescence (XRF) analysis was used to study the diffus ion behaviour of the chalcopyrite elements in single-stage and two-stage pr ocessed Cu(In,Ga)Se-2 thin films. This relatively simple characterization t echnique proved to be very reliable in determining the degree of selenium i ncorporation, possible material losses and the in-depth compositional unifo rmity of samples at different stages of processing. This information is esp ecially important in the case of two-stage growth processes, involving high -temperature selenization steps of metallic precursors. Device quality Cu(I n,Ga)Se, thin films were prepared by a relatively simple and reproducible t wo-step growth process in which all the metals were evaporated from one sin gle crucible in a selenium-containing environment. The precursors were fina lly treated in an H2Se/Ar atmosphere to produce fully reacted films. XRF me asurement indicated no loss of In or Ga during this final selenization step , but a significant degree of element diffusion which depended on the react ion temperature. It was also possible to produce Cu(In,Ga)Se-2 thin films w ith an appreciable amount of Ga in the near-surface region without separate d CuInSe2 and CuGaSe2 phases. (C) 2000 Elsevier Science B.V. All rights res erved.