Interface-engineered CZTS solar cells with Cd-free buffers and MoS2 back contacts: a TCAD route toward enhanced photovoltaic performance

This work presents a comprehensive numerical investigation of Cu 2 ZnSnS 4 (CZTS) thin-film solar cells in conventional substrate and tandem configurations, with particular emphasis on absorber optimization, buffer layer engineering, and MoS 2 interfacial effects. Device simulations were carried out using AMPS-1D and Silvaco-Atlas TCAD to systematically analyze the influence of absorber thickness, acceptor concentration, buffer layer properties, and MoS 2 back-contact layers on photovoltaic performance. For the substrate configuration, an optimal efficiency of 13.86% was achieved with a CZTS thickness of 3 μm and acceptor concentration of 3 × 10 16 cm −3 , yielding Voc = 0.98 V, Jsc = 17.13 mA/cm 2 , and FF = 82.4%. Optimization of the CdS buffer layer revealed that thinner layers (~ 50 nm) combined with donor concentrations around 5 × 10 16 cm −3 significantly enhance fill factor and conversion efficiency by minimizing parasitic absorption and series resistance. Environmentally friendly Cd-free buffer layers (In 2 S 3 , ZnS, ZnS 0.5 O 0.5 , SnS 2 , and MoS 2 ) were also evaluated, with In 2 S 3 and ZnS 0.5 O 0.5 demonstrating performance comparable to or exceeding conventional CdS due to favorable band alignment and reduced interface recombination. Furthermore, the incorporation of a thin MoS 2 interfacial layer at the back contact increased the short-circuit current density from 12.9 to 15.11 mA/cm 2 by improving carrier collection and suppressing recombination, although excessive MoS 2 thickness was found to degrade device performance through increased series resistance. These results provide clear design guidelines for improving CZTS solar cells and highlight the strong potential of Cd-free buffer layers and interface engineering toward sustainable, earth-abundant photovoltaic technologies.