Insights into Intrinsic Defects and the Incorporation of Na and K in the Cu2ZnSnSe4 Thin-Film Solar Cell Material from Hybrid-Functional Calculations

We have performed density functional theory calculations using the HSE06 hybrid functional to investigate the energetics, atomic, and electronic structure of intrinsic defects as well as Na and K impurities in the kesterite structure of the Cu2ZnSnSe4 (CZTSe) solar cell material. We found that both Na and K atoms prefer to be incorporated into this material as substitutional defects in the Cu sublattice. At this site highly stable (Na-Na), (K-K), and (Na-K) dumbbells can form. While Na interstitial defects are stable in CZTSe, the formation of K interstitial defects is unlikely. In general, the calculated formation energies for Na-related defects are always lower compared to their K-related counterparts. On the basis of thermodynamic charge transition level calculations, we can conclude that the external defects are harmless except Na-sn and K-sn. These defects induce gap states that might be detrimental for the device performance.