Thin-film photovoltaics research has expanded owing to low manufacturing costs and efficient material use, especially with chalcogenide absorbers such as Cu(In,Ga)\(\textrm{Se}_{2}\) (CIGS), \(\textrm{CuInS}_{2}\) (CIS), CdTe, and \(\textrm{Cu}_{2}\textrm{ZnSnS}_{4}\) (CZTS). However, concerns regarding cadmium toxicity, the high cost of indium and gallium, and efficiency constraints in CZTS have motivated the exploration of alternative materials. Copper Barium Tin Sulphide (\(\textrm{Cu}_{2}\textrm{BaSnS}_{4}\), CBTS) is a promising low-cost chalcogenide absorber for thin-film solar cells (TFSCs); though systematic studies on optimizing its device architecture–especially buffer-layer (BL) compatibility remain largely unexplored. Here, SCAPS-1D numerical simulations are carried out to investigate the effect of various sulfur (S)-based BLs (ZnS, CdS, SnS, \(\textrm{ZrS}_{2}\), \(\textrm{MoS}_{2}\), \(\textrm{Ag}_{2}\textrm{S}\), \(\textrm{In}_{2}\textrm{S}_{3}\), and \(\textrm{WS}_{2}\)) on the Pt/CBTS/variable BLs/ZnO/ITO device under AM 1.5G illumination at 300 K. Device performance was evaluated in terms of open circuit voltage (\(V_{oc}\)), short circuit current (\(J_{sc}\)), fill factor (FF), and photoconversion efficiency (PCE) by defining key material parameters such as thickness, band gap, doping concentration, and carrier mobility. Among the BLs studied, ZnS exhibited the highest efficiency of 19.94%. The influence of temperature (300–600 K) revealed performance degradation due to increased saturation current, while impedance spectroscopy analyses elucidate carrier dynamics. Further optimization of layer thicknesses, doping, and defect densities leads to a maximum simulated efficiency of 21.5% for a Pt/CBTS/ZnS/ZnO/ITO device, demonstrating the potential of ZnS as an effective Cd-free BL for high-performance CBTS thin-film SCs.
