Doping-induced passivation of the Cu2ZnSnS4/MoS2 back interfaces

The performance of thin-film solar cells is often limited by interfacial defects that act as nonradiative recombination centers. In particular, Cu2ZnSnS4 solar cells suffer from dangling bonds of cations at the Cu2ZnSnS4/MoS2 back interface, which tends to introduce defect states within the bandgap and reduce the short-circuit current density. Here, we demonstrate that oxygen doping by the substitution of sulfur or zinc substitution by the replacement of tin into the Cu2ZnSnS4 absorber can effectively passivate the dangling bonds and, thus, suppress the interfacial recombination by first-principles calculations. This occurs because zinc substitution fully saturates the unsatisfied bonds, while oxygen substitution weakens the orbital coupling, thereby eliminating mid-gap states and restoring a clean band structure. Compared to the oxygen-doped interface, the zinc-substituted interface is energetically more stable to facilitate experimental realization and exhibits self-passivated reconstruction that prevents extrinsic defects and preserves the intrinsic material properties. This work offers a generalizable strategy to improve the electrical performance of diverse interfaces for developing high-efficiency solar cells.