A crystallographic description of experimentally identified formation reactions of Cu(In,Ga)Se2

This work describes solid-state reactions for the formation of the chalcopyrite compounds CuInSe2, CuGaSe2 and Cu(In,Ga)Se2 on atomic scale. The most important chalcopyrite formation reactions which were identified by the authors by real-time in situ X-ray diffraction in preceding experiments are (A) CuSe+InSe→CuInSe2 (B) Cu2Se+2 InSe+Se→2 CuInSe2 and (C) Cu2Se+In2Se3→2 CuInSe2. During the selenistaion of a metallic precursor containing gallium a separate fourth reaction occurs: (D) Cu2Se+Ga2Se3→2 CuGaSe2. The quaternary compound is finally formed by interdiffusion of CuInSe2 with CuGaSe2 (E). These five reactions differ in their activation energy and reaction speed. We explain these differences qualitatively by analysing the involved crystal structures for each reaction. It turns out that all reactions involved in the formation of Cu(In,Ga)Se2 are promoted by epitaxial relations, which facilitates the formation of polycrystalline thin films at temperatures much below those necessary for single crystal growth. Recommendations for the growth of larger grains of Cu(In,Ga)Se2 containing fewer defects are given. - Graphical abstract: The epitaxial connection of the crystal structures of β-Cu2Se (bottom) and β-In2Se3 (top) is the initiating step of one possible solid-state reaction for the formation of α-CuInSe2