Photovoltaic Performance of Lead‐Free Na ₂ GeBr ₆ Perovskite Investigated via Density Functional Theory and SCAPS‐1D Modeling

Alkali metal–based perovskites offer a promising alternative to toxic lead‐based perovskite solar cells. Although lead‐free perovskites typically provide broader absorption, their stability remains a concern. Vacancy‐ordered perovskites, such as Na 2 GeBr 6 , exhibit enhanced stability and possess an appropriate bandgap for photovoltaic applications. In this study, we integrate density functional theory (DFT) and SCAPS‐1D simulation to explore the photovoltaic potential of Na 2 GeBr 6 . Initially, DFT‐based material characterization is performed to analyze the crystal structure and density of states (DOS), along with the evaluation of the dielectric and optical properties. The calculated spectroscopic limited maximum efficiency (SLME) further validates the suitability of Na 2 GeBr 6 as an absorber material. Subsequently, device simulation is conducted using SCAPS‐1D to examine the influence of absorber parameters, such as thickness, bulk defect density, and intrinsic defect states (Interface defect density between C60 / Na 2 GeBr 6 (IDD1) and Interface defect density between Na 2 GeBr 6 /Me‐4PACz (IDD2)) on the device performance. The optimized device achieves a power conversion efficiency of 13.41%, with a short‐circuit current density ( J SC ) of 19.44 mA/cm 2 , an open‐circuit voltage ( V OC ) of 0.932 V, and a fill factor (FF) of 74.01%. These results highlight the potential of Na 2 GeBr 6 as a stable and efficient lead‐free absorber material for next‐generation perovskite solar cells.