${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ (CZTS) is a promising nontoxic and widely available solar absorber. In a recent experimental study of CZTS with the Zn(O, S) buffer layer, the best-performing cell has a Zn/Sn ratio of 0.98 before annealing, different from the conventional tin-poor condition [L. Huang, et al., Sol. RRL 4, 2000010 (2020)]. A common belief is that a normal Sn content favors the formation of a ${\mathrm{Sn}}_{Zn}$ antisite, which is detrimental as it produces a deep gap state. Experimentally, it is challenging to investigate the interface electronic environment with different defect profiles. We constructed a CZTS(112)-ZnS(111) interface and simulated a low concentration of oxygen by introducing it as a dopant in pure ZnS near the interface. We discovered that the ${\mathrm{O}}_{\mathrm{S}}$ isovalent center strongly couples with the ${\mathrm{Sn}}_{Zn}$ antisite near the interface and forms various stable codoping configurations with clean gaps. As a result, a growth condition with stoichiometric Sn concentration can be restored.

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• Defects
• Doping effects
• First-principles calculations
• Growth
• Doped semiconductors
• Semiconductor compounds
• Density functional theory