Engineering Grain Boundaries in Cu2ZnSnSe4 for Better Cell Performance: A First-Principle Study

Through first-principle density functional theory (DFT) calculations, the atomic structure and electronic properties of intrinsic and passivated Sigma 3 (114) grain boundaries (GBs) in Cu2ZnSnSe4 (CZTSe) are studied. Intrinsic GBs in CZTSe create localized deep states within the band gap and thus act as Shockley-Read-Hall recombination centers, which are detrimental to cell performance. Defects, such as Zn-Sn (Zn atoms on Sn sites), Na-i(+) (interstitial Na ions), and O-Se (O atoms on Se sites), prefer to segregate into GBs in CZTSe. The segregation of these defects at GBs exhibit two beneficial effects: 1) eliminating the deep gap states via wrong bonds breaking or weakening at GBs, making GBs electrically benign; and 2) creating hole barriers and electron sinkers, promoting effective charge separation at GBs. The results suggest a unique chemical approach for engineering GBs in CZTSe to achieve improved cell performance.