Microstructure and Optical Properties of Sprayed γ-CuI Thin Films for CuInS2 Solar Cells

γ-CuI thin films were prepared by a spraying method using acetonitrile as a solvent, CuI and iodine as reagents. The influences of substrate temperature on the structure, topography, and optical properties of CuI films were investigated. Scanning electron microscope (SEM) photos revealed that the shape and grain size of CuI grains were related to substrate temperature. X-ray diffraction results showed that substrate temperature affected the crystalline quality of CuI films. When the substrate temperature was 110°C, CuI thin films showed γ-phase zinkblende structure with (111) preferred orientation. The dimension of the globular CuI crystallite was approximately 35 nm, the energy band gap was 2.97 eV, the maximum transmittance was 87.3% in the part of the visible region, and the open circuit voltage was close to 380 mV. This opened a route for a cadmium-free buffer layer for CuInS2 solar cells.