Solar energy has apparently become the most widespread renewable energy to be utilized as sunlight is almost abundantly available across the globe. Nonetheless, the production of Si solar cells is very costly. Hence, it is important to dig out some nontoxic, environment friendly perovskite materials. Mixed lead halide perovskite as the light-harvesting active layer among organic non-organic solar cells is garnering much attention with PCE (Power Conversion Efficiency) more than 25%, but the toxicity of lead is the main concern. For solving realistic problems with ease, numerical analysis is used as experimenting with different hypothesis becomes convenient. This paper makes use of the Solar cell capacitance simulator software (SCAPS-1D) which is most popular among the research community. SCAPS simulation is performed on non-toxic tin iodide perovskite materials with methylammonium (MA) and formamidinium (FA) as A site materials. Accordingly, the thickness of both the absorber materials is changed to obtain the optimal output characteristics. We have proposed a structure considering the two most important layers, i.e. ETL (Electron Transport Layer) and HTL (Hole Transport Layer) as PCBM (Phenyl-C61-butyric acid methyl ester) and PEDOT:PSS (Polystyrene sulfonate) respectively. With the proposed structure (FTO/PCBM/Perovskite/PEDOT:PSS/Au), we have achieved best PCE of 24.72% with MASNI3 at absorber thickness of 500 nm and 23.20% with FASnI3 at absorber thickness of 1100 nm. This study can thus offer a better understanding for researchers for designing of efficient solar cells.