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. - Highlights: • Different types of colloidal nanocrystals were used as precursors to obtain CuInSe₂ films by gas-phase selenization. • In-situ XRD was used to study the effect of selenization on the phase formation and grain growth in the precursor films. • For a mixture of binary metal sulfides the crystallinity and the morphology strongly depend on the Cu/In ratio. • Higher selenium partial pressures result in improved sintering for a mixture of binary metal selenides.