Optical properties and electronic structures of (4CuInSe2)y(CuIn5Se8)1-y

Spectroscopic ellipsometric measurements of thin polycrystalline (4CuInSe(2))(y)(CuIn5Se8)(1-y) films reveal that there are important differences in optical properties and electronic structures between alpha-phase CuInSe2 and Cu-poor CuInSe2. We report the optical functions of thin-film polycrystalline (4CuInSe(2))(y)(CuIn5Se8)(1-y) and describe how they change depending on the degree of Cu deficiency. We find a reduction in the absorption strength in the spectral region of 1-3 eV for Cu-poor CuInSe2. This reduction can be explained in terms of density of the Cu 3d states in CuInSe2. Cu-poor CuInSe2 samples show an increase in band gap due to reduced p-d interaction. We find that the reduction in the amplitudes of E-0(A,B,C) transitions at the Gamma point and E-1(A,B) transitions at the N point are due to Cu deficiency. Local density approximation calculations with a modeled on-site self-interaction correction of the absorption coefficients of Cu8In8Se16 (CuInSe2), Cu5In9Se16 (CuIn3Se5-like), and Cu2In10Se16 (CuIn5Se8) are in good agreement with those of thin-film polycrystalline CuInSe2 samples with 24.1, 15.6, and 9.1 at. % Cu, respectively.