Defect Passivation and Charge Transport Enhancement in High‐Voltage Cu 2 ZnSn(S,Se) 4 Solar Cells via MnS Interfacial Layer

ABSTRACT The large open‐circuit voltage ( V OC ) deficit remains a central bottleneck in Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells, originating from the coupled effects of uncontrolled MoSe 2 growth at the rear contact and defect‐mediated non‐radiative recombination in the absorber. Here, we report a defect‐selective back‐contact engineering strategy via a thermally oxidized MnS interlayer that simultaneously regulates interfacial reaction kinetics and defect energetics. The MnS interlayer suppresses excessive MoSe 2 formation and reduces the valence‐band offset from 0.32 to 0.10 eV, thereby promoting hole‐selective transport. Meanwhile, the junction quality is substantially improved, as evidenced by an expanded depletion width (236 to 286 nm), a reduced interfacial defect density (1.31 × 10 15 to 4.60 × 10 14 cm −3 ), and prolonged carrier lifetimes (1.20 to 2.48 and 99 to 208 µs, respectively). First‐principles calculations further reveal that Mn incorporation reconstructs defect formation energetics by suppressing deep Sn Zn antisites while favoring shallow acceptor‐type defects, thus mitigating Shockley–Read–Hall recombination and strengthening p‐type transport. Consequently, a V OC of 550.7 mV and an efficiency of 14.35% are achieved, representing the highest performance reported to date for Mn‐modified CZTSSe solar cells.