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

Spectroscopic ellipsometry measurements of CuInSe₂ (CIS) and CuIn_1−xGa_xSe₂ (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 ?₂ in the spectral region, 1-3 eV. This reduction can be explained in terms of the Cu-3d 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-3d and Se-4p states. We also characterize the dielectric functions of polycrystalline thin-film α-phase CuIn_1−xGa_xSe₂ (x=0.18 and 0.36) to determine their optical properties and compare them with similar compositions of bulk polycrystalline CuIn_1−xGa_xSe₂. The experimental results have important implications for understanding the functioning of polycrystalline optoelectronic devices.