This article presents an enormously effective Cu 2 ZnSnS 4 (CZTS)-based n-ZnS/p-CZTS/p + -WSe 2 thin film solar cell. The device has been studied by varying the thickness, doping concentration and defect density of each layer utilizing SCAPS-1D simulation software. The power conversion efficiency (PCE) for n-ZnS/p-CZTS single heterojunction is 14.06% with the J SC = 20.26 mA cm −2 , V OC = 0.88 V and FF = 78.59%, respectively. This PCE is elevated to 27.31% with the J SC = 33.72 mA cm −2 , V OC = 0.97 V and FF = 83.75%, respectively due to insertion of WSe 2 back surface field (BSF) layer in the same structure. The significant improvement of PCE mainly depends on short circuit current which is resulted due to WSe 2 layer that absorbs sub-band gap photons through tail-statesassisted (TSA) photon upconversion method. These entire results demonstrate the potential of WSe 2 as BSF layer in CZTS-based thin film solar cells.