Model-Driven Performance Optimization of Lead-Free RbGeI₃ Perovskite Solar Cells Using SCAPS-1D

Driven by the urgent global requirement for sustainable and nontoxic photovoltaic materials, employing a combined DFT and SCAPS-1D computational framework, this study evaluates the electronic structure and photovoltaic performance boundaries, defined here as the efficiency ceiling set by intrinsic material quality at low defect density and the degradation floor imposed by high defect density and elevated operating temperature of RbGeI₃-based solar cell architectures. The computational analysis identifies a maximum achievable efficiency of 31.75% under ambient thermal conditions (300 K), supported by first-principles validation of intrinsic material quality, identifying a critical performance threshold at defect densities near 1 × 10¹⁴ cm^-3. By isolating the specific impact of interface trap states at both the HTL/perovskite and ETL/perovskite junctions, we demonstrate that RbGeI₃ possesses significant intrinsic defect tolerance, maintaining performance stability even as defect levels approach 1 × 10¹⁶ cm^-3. In this regime, key metrics such as the fill factor (FF) and open-circuit voltage (V _oc) remain remarkably resilient against structural imperfections, whereas higher densities trigger enhanced nonradiative recombination that significantly degrades output. Furthermore, a broad-spectrum thermal analysis spanning 300-500 K provides a realistic roadmap for operational stability, showing that the device retains a high efficiency of 23.67% even under extreme heat. The results indicate that performance degradation at elevated temperatures is primarily driven by accelerated carrier recombination and reduced V _oc values. Ultimately, these findings highlight RbGeI₃ as a robust, eco-friendly candidate for the next generation of high-efficiency, lead-free photovoltaics.