Controllable SnS‐Vapor‐Assisted Selenization to Enhance Carrier Transport for Highly Efficient Kesterite Solar Cells

Abstract The charge loss within polycrystalline Cu ₂ ZnSn(S, Se) ₄ (CZTSSe) absorbers remains a key bottleneck restricting the performance of this emerging thin‐film solar cell. Herein, a SnS‐vapor‐assisted selenization strategy is proposed to mitigate defects and suppress charge loss in CZTSSe films by tuning the annealing temperature and incorporation of SnS. Notably, SnS does not contribute directly to grain growth, but it plays a crucial role in suppressing Sn volatilization from the film surface. In addition, the thermodynamic equilibrium between SnS and SnS ₂ regulates the sulfur partial pressure by mediating S transport during annealing. An initial high‐temperature annealing step promotes Se‐for‐S substitution, effectively suppressing S enrichment and its adverse effects on grain nucleation and growth kinetics. Subsequently, a lower‐temperature annealing step sustains sulfur partial pressure through thermodynamic equilibrium of SnS, facilitating defect passivation and interface bandgap tuning. These collective effects have significantly facilitated carrier transport within the device, leading to a notable improvement in the performance of kesterite solar cells. Specifically, a power conversion efficiency exceeding 15% is attained, ranking among the highest values reported for CZTSSe solar cells via vapor‐assisted selenization strategy.