Copper barium tin sulfide (CBTS) is a direct band gap earth abundant, non-toxic and quaternary semiconductor compound. It is used as absorber because of its direct band gap of 1.9 eV. A numerical guide is proposed for CBTS-based photovoltaic cell to enhance the efficiency of experimentally designed device with introducing Cu2O as back surface field (BSF) layer by means of numerical modeling. Device optimization was performed in SCAPS-1D software under 1.5 AM illumination spectrum. After introducing BSF layer and optimized physical parameters, promising result was achieved with PCE of 9.72%, V-oc of 0.81 V, J(sc) of 15.73 mA/cm(2) and FF of 78.23%. The promising outcomes of this work will give a guideline for the feasible production of high-efficiency inorganic CBTS-based photovoltaic cells.