Simulation and optimization of FA0.75MA0.25Sn0.95Ge0.05I3/FA0.75Cs0.25SnI3 lead-free mixed cation Sn-based heterojunction perovskite solar cells with various ETLs and HTLs using SCAPS-1D

This simulation study analyzes and optimizes lead-free mixed cation Sn-based heterojunction perovskite solar cells (HPSCs) with dual (FA 0.75 MA 0.25 Sn 0.95 Ge 0.05 I 3 /FA 0.75 Cs 0.25 SnI 3 ) absorber layers to maximize the overall power conversion efficiency (PCE) using the one-dimensional solar cell capacitance simulator (SCAPS-1D). The primary goal of this work is to examine the effects of different absorber layer thicknesses, defect densities, series and shunt resistances, operating temperatures and back contact materials on the solar cell performance. The study compares five different electron transport layers (ETLs-TiO₂, WS 2 , C 60 , ZnOS and PCBM) and six different hole transport layers (HTLs-CuSCN, Spiro-OMeTAD, CuI, P3HT and GO) to choose the best high-performance device. Finally, directional light response, device performance under right and left side illumination are compared. At optimal levels, the ETL ZnOS achieved 26.53 % efficiency, surpassing other ETLs such as TiO₂ (26.48 %), WS₂ (25.86 %), C 60 (24.55 %) and PCBM (21.22 %). The HTL with CuI achieved 26.58 % efficiency, surpassing CuSCN (26.48 %), Spiro-OMeTAD (26.19 %), P3HT (26.55 %), GO (25.93 %) and PEDOT:PSS (26.57 %). Based on the simulation results, the optimized FTO/ZnOS/FA 0.75 MA 0.25 Sn 0.95 Ge 0.05 I 3 /FA 0.75 Cs 0.25 SnI 3 /CuI/Ni structure achieves a high PCE, lowers recombination losses and enhances electron and hole transport under right side illumination. This device with optimized parameters showed a PCE of 32.39 %, a short circuit current density (J SC ) of 35.31 mA/cm 2 , an open circuit voltage (V OC ) of 1.05 V and a fill factor (FF) of 87.68 % at 300 K. The optimized structure of the FA 0.75 MA 0.25 Sn 0.95 Ge 0.05 I 3 /FA 0.75 Cs 0.25 SnI 3 based lead-free mixed cation Sn-based heterojunction shows great promise for producing high-efficiency PSCs by significantly lowering carrier recombination losses. • This work examines the effects of different absorber layer thicknesses, defect densities, series and shunt resistances, operating temperatures and back contact materials on the solar cell performance. • The study compares five different electron transport layers (ETLs-TiO₂, WS 2 , C 60 , ZnOS and PCBM) and six different hole transport layers (HTLs-CuSCN, Spiro-OMeTAD, CuI, P3HT and GO) to choose the best high-performance device. • This device with optimized parameters showed a PCE of 32.39 %, a short circuit current density (J SC ) of 35.31 mA/cm 2 , an open circuit voltage (V OC ) of 1.05 V and a fill factor (FF) of 87.68 % at 300 K.