Abstract— This paper investigates the performance of a planar n-i-p perovskite solar cells (PSC) with lead-free perovskite absorber for three different metal oxides serving as the electron transport layer (ETL). A tin (Sn) based PSCs – with i) zinc oxide (ZnO), ii) titanium oxide (TiO2) and iii) tin oxide (SnO2) as the ETL, and spiro-MeOTAD as the hole transport layer (HTL) – are modeled and simulated using a 1-dimensional numerical software (SCAPS 1-D). Thicknesses of both the methylammonium tin iodide (CH3NH3SnI3) absorber and the ETL are varied for the purpose of achieving the optimum power conversion efficiency (PCE). For all metal oxide candidates, thickness of lead-free perovskite absorber layer is varied from 400 nm to 1500 nm. The obtained results show that the optimum recorded PCE is achieved at 900 nm. Moreover, the highest PCE value of 8.10% is observed for 80 nm thickness of SnO2 compared to 8.05% for ZnO and 7.99% for TiO2. Additionally, the results unveil that for a constant HTL thickness of 80 nm and ETL thickness increment up to 300 nm, the PCE is slightly reduced between 0.12% and 0.99% for all ETLs. We believe that this is the first simulation effort that evaluates the effect of transport layer thickness on the performance of lead-free PSC, hoping that the findings will be useful for the research community, particularly for those working in the field of solar cells fabrication and development.