Interface-engineered HTL-free mixed-cation perovskite solar cells with ZnO/BaSnO3 bilayer electron transport architecture: A SCAPS-1D investigation toward near-ideal photovoltaic performance

Hole transport layer-free (HTL-free) perovskite solar cells (PSCs) have emerged as promising candidates for next-generation photovoltaics due to their simplified architecture, reduced fabrication cost, and enhanced operational stability. In this study, SCAPS-1D simulations were utilized to investigate and optimize mixed-cation (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 HTL-free PSCs employing single and bilayer electron transport layers (ETLs). Seven device architectures based on TiO 2 , ZnO, BaSnO 3 , and their bilayer combinations were systematically investigated. Among the investigated structures, the FTO/ZnO/BaSnO 3 /(FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 /Au prototype configuration exhibited the best photovoltaic performance due to favorable band alignment, enhanced carrier selectivity, and minimized interfacial recombination. After optimization of absorber thickness, carrier concentration, defect density, and back-contact work function, the optimized device (FTO/ZnO/BaSnO 3 /(FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 /Pt) achieved an enhanced PCE of 29.10%, with an open-circuit voltage of 1.34 V, short-circuit current density of 24.30 mA/cm 2 , and fill factor of 89.11%. Sensitivity analysis further revealed strong tolerance to ETL and interface defects, highlighting the robustness of the proposed architecture. The results establish ZnO/BaSnO 3 bilayer ETLs as an effective strategy for realizing high-efficiency, stable, and cost-effective HTL-free perovskite photovoltaics.