In this paper, we presented a numerical study of a CdS/Sb2Se3 mono junction solar cell (SC) using the SC Capacitive Simulator (SCAPS-1D). We validated an experimental work using a variety of Sb2Se3 experimental parameters, and the results showed excellent agreement between numerical and experimental J-V curves, yielding a PCE of 7.54% .To continue, we analyzed the impact of Sb2Se3 thin layer thickness, charge carrier concentration, bulk defect density, and interface defect (CdS/Sb2Se3) on solar cell characteristics. With the optimum Sb2Se3 layer thickness of 1.2 m, carrier concentration of 10 15 cm-3 , bulk defect of 10 13 cm-3 , and CdS/Sb2Se3 interface defect densities of 10 10 cm-2 , we were able to attain an efficiency of 16.62%, Jsc = 35.38 mA/cm 2 , Voc = 0.66 V, and FF = 70.33%. Finally, we investigated the insertion effect of n-GaAs (ETL) and P +-CuO HTL (BSF) on Sb2Se3 solar cell efficiency. The novel ITO/n-CdS/n-GaAs/p-Sb2Se3/p +-CuO HTL/Au heterostructure achieved a huge efficiency of 19.60%.