Defect Passivation and Charge Transport Enhancement in High-Voltage Cu₂ZnSn(S,Se)₄ Solar Cells via MnS Interfacial...

The large open-circuit voltage (V_OC) deficit remains a central bottleneck in Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, originating from the coupled effects of uncontrolled MoSe₂ 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 MoSe₂ 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 × 10¹⁵ to 4.60 × 10¹⁴ 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.