Abstract <p>This study presents an optimized architecture for Cu₂ZnSnS₄ (copper zinc tin sulfide, CZTS)-based solar cells, comprising a ZnO/Nb₂O₅/CZTS/FeSi₂/PTAA/Ni layer structure. Using SCAPS-1D (Solar Cell Capacitance Simulator in One Dimension), the thickness of the CZTS absorber was reduced from the conventional 2 µm to an ultra-thin 0.5 µm without compromising device efficiency. The incorporation of Nb₂O₅ (niobium oxide) as a buffer layer, FeSi₂ (iron disilicide) as a secondary absorber, and PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) as a back surface field (BSF) layer offers an effective strategy to achieve high efficiency while minimizing material consumption in thin-film solar cells. Three device configurations were evaluated to identify the optimal design. For the best configuration, key characteristics such as external quantum efficiency (EQE), energy band alignment, and generation-recombination profiles were analyzed. The influence of ZnO (zinc oxide) and PTAA layer thicknesses, defect densities (<i>N</i>_t) in CZTS and FeSi₂, acceptor (<i>N</i>_A) and donor (<i>N</i>_D) densities, operating temperature, and the CZTS bandgap was systematically investigated. The optimized structure, featuring 0.05 µm ZnO and 0.04 µm PTAA layers, defect densities of 10¹² cm^–3 for CZTS and 10¹¹ cm^–3 for FeSi₂, acceptor densities of 10²⁰ cm^–3 for CZTS and FeSi₂ and 10¹⁹ cm^–3 for PTAA, donor concentrations of 10¹⁹ cm^–3 for ZnO and 10²⁰ cm^–3 for Nb₂O₅, an operating temperature of 310 K, and a CZTS bandgap of 1.32 eV, achieved a simulated efficiency of 35.46%. This ultra-thin CZTS-based solar cell establishes a new benchmark for efficiency, combining reduced material use with superior performance. These results demonstrate the potential of this approach for advancing environmentally friendly and cost-effective solar energy technologies.</p>