In this work, a novel structured Cu$_2$BaSnS$_4$ (CBTS)/ZnS/Zn(O, S) photovoltaic device is proposed.Anontoxic, earth-abundant and auspicious quaternary semiconductor compound copper barium tin sulphide (Cu$_2$BaSnS$_4$) is used as an absorber layer.We propose a novel Zn(O, S) buffer layer for a high-power conversion efficiency (PCE) of CBTS-based thinfilm photovoltaic cells. Solar cell capacitance simulator software is used for device modelling and simulations are performed under a 1.5 AM illumination spectrum. The proposed device is investigated by means of numerical modelling and optimized the parameters to maximize its efficiency. Promising optimized functional parameters had been achieved from the proposed structurewith back surface field layer with a PCE of 18.18%, a fill factor of 83.45%, a short-circuit current of 16.13 mA cm$^{−2}$ and an open-circuit voltage of 1.3 V. The promising results give an imperative standard for possible manufacturing of highefficiency, eco-friendly inorganic CBTS-based photovoltaic cells.