Theoretical Limits on the Stability of Single-Phase Kesterite-Cu2ZnSnS4

The single-phase stability of Cu₂ZnSnS₄ (CZTS), after an intrinsic defect was incorporated in it, has been examined here for the first time based on ab initio calculations. The stability analysis of such a non-stoichiometric-defect incorporated CZTS shows that the single-phase formation is unlikely at thermodynamic equilibrium conditions. In addition, the effective growth condition of CZTS is determined and quantified for all the elements (Cu-poor, Zn-rich, Sn-poor, and S-rich) to extract maximum photovoltaic efficiency from CZTS. These conditions promote (i) spontaneous formation of Cu vacancy (V_Cu), which might benefit p-type conduction, and (ii) the co-existence of ZnS while suppressing other harmful defects and secondary phases. Further, the results presented here explain the unavailability of single-phase CZTS to date.