Performance Optimization of Lead-Free Cs₂AgBiI₆ Solar Cells with the LBSO Electron Transport Layer: A SCAPS-1D...

Lead toxicity in conventional halide perovskites has long been a concern for practical photovoltaic deployment, and Cs₂AgBiI₆ double perovskites gained significant attention as promising lead-free alternatives due to their high carrier mobility, suitable optoelectronic characteristics, and relatively good chemical stability. In this work, density functional theory (DFT) calculations were performed to investigate the electronic properties of the Cs₂AgBiI₆ absorber, while SCAPS-1D simulations were independently employed to optimize the photovoltaic performance of the proposed device architecture, using lanthanum-doped barium stannate (LBSO) as the electron transport layer (ETL), which, to our knowledge, has not been previously reported for this absorber. LBSO was compared against CdS and ZnS across thicknesses of 10-100 nm, while six HTL candidates, including CBTS, Cu₂O, CuSCN, CuI, Spiro-MeOTAD, and PEDOT:PSS, were evaluated simultaneously. Interface trap densities at both junctions were swept from 1 × 10⁹ to 1 × 10²⁰ cm^-3, revealing that the HTL-side interface is significantly more recombination-sensitive than the ETL side, which DFT analysis helped explain through electronic structure and defect formation energy calculations. Absorber bulk defect density above ∼10¹³ cm^-3 was found to severely degrade performance, and absorber thickness was optimized between 100 and 1200 nm. The final optimized FTO/LBSO/Cs₂AgBiI₆/Cu₂O/Au structure with achieved J _sc =24.66 mA/cm², V _oc =1.248 V, PCE=27.52%, and FF = 89.42% under AM1.5G illumination, demonstrating the potential of the optimized device architecture for lead-free double perovskite photovoltaics.