Optical Properties and Electronic Structures of (4CuInSe2)y(CuIn5Se8)1-y

Spectroscopic ellipsometric measurements of thin polycrystalline ${(4\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2})}_{y}{(\mathrm{Cu}{\mathrm{In}}_{5}{\mathrm{Se}}_{8})}_{1\ensuremath{-}y}$ films reveal that there are important differences in optical properties and electronic structures between $\ensuremath{\alpha}$-phase $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$ and Cu-poor $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$. We report the optical functions of thin-film polycrystalline ${(4\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2})}_{y}{(\mathrm{Cu}{\mathrm{In}}_{5}{\mathrm{Se}}_{8})}_{1\ensuremath{-}y}$ 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 $1--3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for Cu-poor $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$. This reduction can be explained in terms of density of the Cu $3d$ states in $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$. Cu-poor $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$ samples show an increase in band gap due to reduced $p\text{\ensuremath{-}}d$ interaction. We find that the reduction in the amplitudes of ${E}_{0}(A,B,C)$ transitions at the $\ensuremath{\Gamma}$ 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 ${\mathrm{Cu}}_{8}{\mathrm{In}}_{8}{\mathrm{Se}}_{16}$ $(\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2})$, ${\mathrm{Cu}}_{5}{\mathrm{In}}_{9}{\mathrm{Se}}_{16}$ ($\mathrm{Cu}{\mathrm{In}}_{3}{\mathrm{Se}}_{5}$-like), and ${\mathrm{Cu}}_{2}{\mathrm{In}}_{10}{\mathrm{Se}}_{16}$ $(\mathrm{Cu}{\mathrm{In}}_{5}{\mathrm{Se}}_{8})$ are in good agreement with those of thin-film polycrystalline $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$ samples with 24.1, 15.6, and $9.1\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Cu, respectively.