Effect of Cu deficiency on the optical properties and electronic structure of CuIn1−xGaxSe2

Abstract Spectroscopic ellipsometry measurements of CuInSe 2 (CIS) and CuIn 1− x Ga x Se 2 (CIGS) over a range of Cu compositions reveal that there are important differences in electronic and optical properties between α -phase CIS/CIGS and Cu-poor CIS/CIGS. We find a reduction in the imaginary part of the dielectric function ϵ 2 in the spectral region, 1–3 eV. This reduction can be explained in terms of the Cu-3 d density of states. An increase in band gap is found for Cu-poor CIS and CIGS due to the reduction in repulsive interaction between Cu-3 d and Se-4 p states. We also characterize the dielectric functions of polycrystalline thin-film α -phase CuIn 1− x Ga x Se 2 ( x =0.18 and 0.36) to determine their optical properties and compare them with similar compositions of bulk polycrystalline CuIn 1− x Ga x Se 2 . The experimental results have important implications for understanding the functioning of polycrystalline optoelectronic devices.