Investigation of SnS thin film as a BSF layer in Cu2ZnSnS4-based solar cells: a strategy to improve efficiency using SCAPS-1D

Photovoltaic devices based on the quaternary material Copper-Zinc-Tin-Sulfide (Cu2ZnSnS4) commonly called CZTS have shown promising conversion efficiency in the last decade. However, this conversion efficiency still remains below expectations due to several factors including the high recombination rate at the CZTS/Mo rear contact. Indeed, this high recombination process at the rear contact is due, on the one hand, to the appearance of a MoS2 layer caused by the reaction between sulfur (S) from the CZTS and Mo, and, on the other hand, to the presence of voids in the lower part of the CZTS absorber, caused by the volatilization of sulfur (S) from the CZTS absorber for high sulfurization temperatures. In this study, a numerical simulation of the architecture of a ZnO:Al/i–ZnO/n–CdS/p-CZTS/SnS/Mo photovoltaic device was carried out to investigate the effect of the Tin Sulfide (SnS) as a Back Surface Field (BSF) layer on the overall performance of CZTS-based solar cells. Thus, using Solar Cell Capacitance Simulator in one dimension (SCAPS-1D), the thickness, bandgap energy and the Acceptor concentration (NA) of the p-SnS layer were varied to observe their effects on the performance of the CZTS solar cell. In addition, the band alignments at the Mo/SnS/CZTS interface were analyzed to elucidate the impact of this layer on the transport properties of photogenerated charges. The results show that the insertion of a 30 nm thin SnS BSF layer, with a band gap of 1.30 eV and doping greater than 1016 cm−3 is ideal for boosting the efficiency of the CZTS cell from 7.0% to 13.33%. This improvement in device performance proves that the SnS BSF layer limits carrier recombination processes at the CZTS/Mo back contact. After optimization, the design of a photovoltaic device with a ultrathin layer of 600 nm CZTS absorber with an efficiency of 11.17% (with VOC=970 mV, JSC=15.65 mA/cm2 and FF=74.2%) is also possible in order to avoid material waste, reduce manufacturing costs and consequently reduce the price of CZTS solar cells to make them more competitive in the photovoltaic market.