Optimizing CZTSSe solar cell performance through engineered TiO₂ bandgap: quantitative insights from SCAPS-1D device simulation

Recently, the kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell has attracted significant research attention as a promising solution to address the energy crisis, owing to its earth-abundant elements and tunable bandgap. However, the conventional CdS buffer layer commonly used in these devices still faces challenges including parasitic light absorption and potential environmental toxicity. This study investigates titanium dioxide (TiO2) as a cadmium-free buffer layer for CZTSSe solar cells through SCAPS-1D numerical simulation. The results demonstrate that TiO2 (Eg = 3.4 eV), with its wider bandgap compared to conventional CdS, enables superior photon transmission to the absorber layer, yielding a 15-20% enhancement in short-circuit current density (JSC). The interface engineering analysis reveals that higher activation energy at the TiO2/CZTSSe heterojunction effectively suppresses non-radiative recombination, leading to a remarkable open-circuit voltage (VOC) improvement. Optimal device performance is achieved at donor concentrations >1×10¹⁷ cm⁻³, delivering a record conversion efficiency of 19.00% - representing a 47.86% increase over CdS-based reference devices. These findings establish TiO2 as a promising alternative buffer material and provide fundamental guidelines for developing eco-friendly CZTSSe photovoltaics.