Design and Performance Optimization of Lead-Free Cs 2 TiBr 6 /Cs 2 SnI 6 Bilayer Perovskite Solar Cells

Lead-free perovskites have attracted increasing attention as environmentally friendly absorber materials for next-generation solar cells. In this work, a bilayer perovskite solar cell based on Cs 2 TiBr 6 and Cs 2 SnI 6 was designed and simulated using SCAPS-1D, with the structure FTO/ETL/Cs 2 TiBr 6 /Cs 2 SnI 6 /HTL/Au. The Cs 2 TiBr 6 /Cs 2 SnI 6 bilayer configuration not only mitigates lead toxicity but also enhances device stability and light-harvesting capability. The effects of various hole transport layers (HTLs) and electron transport layers (ETLs) were systematically analyzed, highlighting that optimal energy-level alignment between absorber and transport layers is crucial for efficient carrier separation and transport. Furthermore, the influence of absorber thickness, doping concentration, defect density, and back electrode work function on device performance was investigated. Under optimized conditions, the bilayer device achieved a short-circuit current density of 26.72 mA/cm 2 , an open-circuit voltage of 1.43 V, a fill factor of 85.05%, and a maximum power conversion efficiency of 32.51%. These results demonstrate the great potential of Cs 2 TiBr 6 /Cs 2 SnI 6 bilayer perovskites for high-efficiency, lead-free photovoltaic and optoelectronic applications.