  Thin-film solar cells with CuIn$_x$Ga$_{1-x}$Se$_2$ (CIGS) absorber are still
far below their efficiency limit, although lab cells reach already 19.9%. One
important aspect is the homogeneity of the alloy. Large-scale simulations
combining Monte Carlo and density functional calculations show that two phases
coexist in thermal equilibrium below room temperature. Only at higher
temperatures, CIGS becomes more and more a homogeneous alloy. A larger degree
of inhomogeneity for Ga-rich CIGS persists over a wide temperature range, which
may contribute to the low observed efficiency of Ga-rich CIGS solar cells.
