Bulk and Interface-Engineered Zn(O,S)/SnSe Heterojunctions via SCAPS-1D Simulation: Toward 22.46% Efficient Ecofriendly Photovoltaics

Tin selenide (SnSe), as a direct bandgap (1.1–1.3 eV) photovoltaic absorber material, has a theoretical power conversion efficiency comparable to commercial thin-film solar technologies. However, the efficiency of experimentally fabricated SnSe solar cells is significantly lower than the theoretical value, mainly constrained by two major bottlenecks: (1) the bulk defects leading to carrier recombination and (2) the bandgap mismatch at the interface. Additionally, the environmental toxicity of the CdS and its parasitic absorption of short-wavelength light also limit the sustainable development. This study proposes and systematically investigates the all-inorganic structure of FTO/Zn(O,S)/SnSe/Carbon. Replacing traditional CdS with Zn(O,S) can achieve tunable bandgaps and eliminate Cd toxicity. Through SCAPS-1D simulation, the thickness and carrier concentration of each functional layer are optimized, and the influences of bulk defect density, interface defect density, and resistance on device performance are analyzed. Meanwhile, the S content in Zn(O,S) is adjusted, and the sulfur enrichment (S content of 70%) induces the best conduction band offset (ΔEc = 0.05 eV). The resulting optimal device exhibits an efficiency of 22.46%, Voc = 0.90 V, Jsc = 30.06 mA/cm2, and FF = 83.39%. This work establishes a universal paradigm combining nontoxic material selection (Zn(O,S)/carbon), interface thermodynamic control, and defect passivation strategies, paving the way for the commercialization of sustainable thin-film solar cells.