Thoughts on the microstructure of polycrystalline thin film CuInSe2 and its impact on material and device performance

Abstract

The microstructure and morphology of polycrystalline thin film CuInSe₂ were studied extensively in the compositional range 17–32 at.% Cu. The grain size varied with substrate temperature, copper content, and in variable ways with substrate type, and ranged in size from 0.1 to 5.0 μm. The morphology of copper-rich films appeared additionally to depend on the resident nucleation and growth of the Cu_2−δSe binary compound. A microstructural model of polycrystalline thin film CuInSe₂ is presented and suggests that the intergranular microstructure is dominated by the compositional and substrate temperature dependence of Cu_2−δSe precipitation at grain boundaries and free surfaces. The intragranular microstructure of the near-stoichiometric grain is a phase-separated mixture of ordered chalcopyrite and disordered sphalerite, with $\text{Cu}{}_{\mathrm{x}}\text{Se}\text{(x=0.5, 1.0, 1.5, 2.0)}$ minority phase inclusions. Off-stoichiometric copper-poor film compositions additionally contain isolated grains of the chalcopyrite-variant ordered-vacancy compound CuIn₂Se_3.5. The potential ramifications of the microstructure on the device performance include a reduction in the photo-active volume, carrier transport across phase boundaries, and dependence of transport parameters on the crystallite size.