Band alignment and defect engineering in lead-free MASnI 3 /chalcogenide perovskite double-absorber solar cells: A comparative SCAPS-1D study

Lead-free perovskite solar cells have attracted increasing attention as potential candidates for sustainable photovoltaic applications. In this work, a fully lead-free double-absorber solar cell is proposed by integrating MASnI₃ as the primary absorber with four selenide-based chalcogenide perovskites (CPs), namely BaHfSe₃, CaHfSe₃, BaZrSe₃, and CaZrSe₃, as complementary secondary absorbers. The FTO/Zn₂SnO₄/MASnI₃/CP/Cu₂O/Au architecture is systematically investigated using SCAPS-1D through band-alignment, thickness, resistive-loss, doping, bulk-defect, and interface-defect analyses. The results reveal distinct photovoltaic responses between the Hf and Zr-based CPs, highlighting the important influence of interfacial band alignment. The thickness-optimized MASnI₃/CaHfSe₃ and MASnI₃/BaHfSe₃ configurations yield simulated power conversion efficiency (PCE) of 27.42% and 27.36%, respectively, compared with 25.47% for the thickness-optimized MASnI₃ single-absorber reference, whereas the Zr-based configurations exhibit lower simulated efficiencies. Non-ideal parameter analyses further demonstrate that device performance is strongly influenced by resistive losses and defect-assisted recombination. For the investigated Hf-based configurations, the bulk-defect density of MASnI₃ has a greater influence than that of the secondary CP absorber, while the MASnI₃/CP heterojunction is identified as the most defect-sensitive interface. Overall, the results identify CaHfSe₃ and BaHfSe₃ as the more promising secondary absorbers and demonstrate that double-absorber performance is governed by absorber selection, interfacial band alignment, and defect-assisted recombination. These findings provide a theoretical basis for further experimental investigation of fully lead-free MASnI₃/selenide-CP double-absorber solar cells.