Compositional Band Engineering of CZTSe 1–x S x for High-Efficiency Lead-Free Cs 2 TiBr 6 Perovskite Solar Cells: Insights from SCAPS-1D Modelling

Lead-free perovskite solar cells are gaining attention as environmentally safer alternatives to lead-based systems. This work numerically investigates a fully inorganic structure of ITO/SnO2/Cs2TiBr6/CZTSe1–xSx/Ag, using SCAPS-1D simulations. A CZTSe1–xSx alloy is introduced as a hole transport layer (HTL) to enable controlled band offset engineering at the Cs2TiBr6/HTL interface, facilitating efficient hole extraction and reducing recombination losses. An optimization strategy was implemented. We found that x=0.4 is the optimal sulfur content. At this composition, the device achieved a power conversion efficiency (PCE) of 9.57%. Thickness optimization increased the efficiency to 14.91%, while reducing Cs2TiBr6 absorber defect density further improved it to 15.22%. Finally, optimizing the back-contact work function to 5.4 eV significantly enhanced the PCE to 17.30%, with open-circuit voltage (VOC) = 0.66 V, short-circuit current density (JSC) = 35.41 mA/cm2 and fill factor (FF) of 73.96%. These results highlight band-offset engineering with alloyed chalcogenides as a promising route for efficient, stable CZTS HTL-based lead-free Cs2TiBr6 solar cells.