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Extensive research efforts have been made over the last few years to proffer solution to the high recombination rate and stability issues associated with Sn-based perovskite solar cells. In line with this, we modeled cesium tin iodide (CsSnI3)–based perovskite solar cell (PSC) with titanium (IV) oxide (TiO2) and copper thiocyanate (CuSCN) as the electron and hole transport materials respectively by employing one-dimensional solar cell capacitance simulator (1D-SCAPS). For the CsSnI3-based PSC, n-i-p planar configuration was employed and previously published relevant data were used for the simulation. The results obtained for the initially modeled PSC compared well with similar devices in the literature. Based on the established relationship between charge carrier lifetime and power conversion efficiency (PCE) of a solar cell, we varied defect density in the cesium tin iodide (CsSnI3) from 1013 to 1017 cm−3 and studied its influence on the performance parameters of the modeled CsSnI3-based PSC. We find that the diffusion length and lifetime of charge carriers are significantly increased with decreasing defect density of the CsSnI3 absorber. In this study, we report that it is possible to significantly reduce the dominated Shockley–Read–Hall (SRH) high recombination rate occurring in CsSnI3 perovskite layer even at a low defect density of 1013 cm−3 by using a combination of SnCl2 and Br− as additive and dopant respectively for CsSnI3 enrichment. This strategy ensured a reduced concentration of Sn4+ vacancy (VSn) in the CsSnI3 absorber and an improved carrier lifetime beyond 0.05 ns as obtained for the initially modeled CsSnI3-based PSC by a magnitude of the order of 103 while the diffusion length was improved from 1.1 µm by a magnitude of the order of 102 for the enriched CsSnI3-based PSC. For the optimized CsSnI3-based PSC, we recorded an open-circuit voltage (VOC) of magnitude 1.289 V, short-circuit current density (JSC) value 32.60 mA∙cm−2, fill factor (FF) 83.56%, and PCE 35.12%. [ABSTRACT FROM AUTHOR]