Formation of CuInSe2 films from metal sulfide and selenide precursor nanocrystals by gas-phase selenization, an in-situ XRD study

In this work phase pure CuInSe2 thin flms were obtained by selenization of ternary CuInSe2 and CuInS2 nanocrystals and mixtures of binary nanocrystals such as CuS, In2S3, Cu2Se and In2Se3. The temperature of the selenium source was kept at 400 °C during selenization. Monitoring the process using in-situ x-ray diffraction, the effect of selenization on the phase formation and grain growth in the precursor film was investigated. Whereas CuInSe2 and CuInS2 nanocrystals exhibit little grain growth, we found that mixtures of binary nanocrystals can show significant sintering depending on the reaction conditions. For the mixture of CuS and In2S3 nanocrystals, the crystallinity and the morphology of the obtained fims strongly depends on the Cu/In ratio, with a Cu excess strongly promoting grain growth. With mixtures of Cu2Se and In2Se3 nanocrystals the selenium partial pressure plays a crucial role. Selenium evaporation from the mixed compounds results in CuInSe2 films composed of relatively small crystallites. Higher selenium partial pressures however resulted in improved sintering. Incomplete propagation of the selenization reaction through the layer was observed though, only leading to a well sintered CuInSe2 top layer above a fine grained bottom layer.