Real-time investigations on the formation of Cu(In,Ga)(S,Se)2 while annealing Cu-In-Ga precursors with different sulphur-selenium mixtures

To enhance the conversion efficiency of thin film photovoltaics, chalcopyrite based absorber materials are usually substituted in the cation and anion lattice to yield an absorber with a graded bandgap composed of mixed pentanary crystals Cu(In,Ga)(Se,S)(2). Applying in-situ investigations during the crystallisation of the chalcopyrite is a prerequisite to understand the formation of inhomogeneities and elemental gradients caused by the growth process. The variation of anions yields a combined crystallisation path due to metal sulphoselenides with chalcogen exchange reactions upon heating, i.e. copper sulphoselenides act as a chalcogen buffer and afford substitution of S with Se during annealing. The extended chalcogen substitutions occur due to the complete solid solution of S and Se in Cu(S,Se) and Cu2-x(S,Se). On the contrary, different reaction paths are found for the cations. Although the metals show a good intermixture within the intermetallic alloys featuring In-Ga substitutions, no formation of a mixed or ternary (In,Ga)-chalcogenide is observed. This can be attributed to a narrow solid solution range of the (In,Ga)-chalcogenides. As a consequence the preceding formation of an In-rich and the delayed crystallisation of a Ga-rich chalcopyrite cause a vertical elemental gradient with Ga-accumulation near the back electrode. An interdiffusion of the In-and Ga-rich chalcopyrites is observed for increasing annealing time and temperature. The presented study is focused on quantification of the phase evolution, variation of lattice parameter and diffusion of elements as obtained by Rietveld refinements. (C) 2012 Elsevier B.V. All rights reserved.