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 Σ3 (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. The intrinsic grain boundaries of Cu₂ZnSnSe₄ introduce deep gap states and act as Shockley-Read-Hall recombination centers. The segregation of defects such as Zn_Sn, Na⁺_i, and O_Se eliminates the gaps states and produces the band bending around grain boundaries. The efficiencies of Cu₂ZnSnSe₄-based solar cell are expected to be further enhanced by proper treatment at grain boundaries.