•A Sb2Se3 based heterojunction solar cell is stimulated with the SCAPS software.•The tin sulfide (SnS) layer was applied as a back surface field (BSF) layer.•Severe open-circuit voltage (VOC) loss in the device was recovered with the BSF layer.•The device efficiency of 17.75% was achieved at an ultrathin absorber layer thickness of 700 nm. Recently, Sb2Se3 based solar cells have shown severe open-circuit voltage (VOC) loss that results in low device efficiency. In the present work, a model of Sb2Se3 solar cell with tin sulfide (SnS) as a back surface field layer (BSF) layer was designed and investigated via a solar cell capacitance simulator (SCAPS)-1D simulation software. The influence of the ultrathin BSF layer was investigated as a function of Sb2Se3 absorber thickness, and the corresponding device performance was analyzed. Apart from this, the effect of high BSF layer thickness, doping concentration, interface defect density, and resistance were also examined. Applying the BSF layer decreased the valence band offset value, which eases the hole transport at the back interface. At the same time, the high electric field region generated at the back interface creates a barrier for the minority carriers. Synergistically, in the presence of the BSF layer, the carrier recombination current densities were found to be almost negligible. As a result, the introduction of the 10 nm thin BSF layer significantly improved the device's VOC and power conversion efficiency (PCE) from 0.416 to 0.603 V and 9.61 to 17.75%, respectively for the absorber layer thickness of 700 nm. The present simulation study performed with the full earth-abundant element-based low-cost materials may assist the photovoltaic community in combating the VOC loss and further improving the PCE of Sb2Se3 based solar cells.