Spatial inhomogeneities and defect structures in CIGS and CIS materials: An ab-initio based Monte Carlo study

The chalcopyrite semiconductors CuIn_1-xGa_xSe₂ (CIGS) and CuInSe₂ (CIS) are excellent materials for high efficiency and low cost thin-film solar cells. This is due to the effective absorption of the solar spectrum and the inherent resilience to defects and composition fluctuations. Although the CIGS and CIS material in solar cells is highly inhomogeneous and exhibits a lot of different defects, the cell efficiencies are exceptionally high. If single crystalline absorbers are used, efficiencies are lower. Therefore, studying spatial inhomogeneities and defect structures is of great importance for understanding what supports and what diminishes the efficiency and robustness of the cells. This article describes Monte Carlo (MC) simulations based on ab initio density functional theory (DFT) that are used to investigate spatial inhomogeneities, disorder phenomena and stoichiometries in CIGS and CIS materials. For CIGS systems the temperature-dependent spatial In-Ga distribution has been studied. The simulations 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 contributes to the comparably low efficiency of Ga-rich CIGS solar cells. For the CIS material, Cu-poor defect structures have been investigated. The simulations show that CuIn₅Se₈ undergoes a discontinuous order-disorder phase transition. Grand-canonical MC simulations provide a map in which various stoichiometries occur, depending on the chemical potentials of Cu and In. In the CIS film production process based on chemical vapor deposition, the chemical potentials can be adjusted by varying the partial vapor pressures.