An integrated DFT + U and device simulation design of doped CsGeCl₃ for high efficiency flexible solar absorbers

The urgent need for non-toxic, high-performance perovskites is a critical bottleneck in the advancement of sustainable solar energy. This work introduces a self-consistent pathway that directly links first-principles quantum mechanics with device-level performance simulation. Using this integrated approach, we investigate Cesium Germanium Chloride (CsGeCl₃) and its enhancement via Manganese (Mn) and Iron (Fe) doping. Our Density Functional Theory with Hubard-U correction (DFT + U) calculations, which supply all critical parameters for the Solar Cell Capacitance Simulator (SCAPS-1D), show that while pristine CsGeCl₃ has a wide bandgap of 3.44 eV and a low simulated efficiency of 5%, strategic doping offers a transformative enhancement. Fe-doping, in particular, engineers an optimal bandgap of 1.21 eV by creating a functional intermediate band, which significantly boosts sub-bandgap photon absorption. Consequently, SCAPS-1D simulations predict a remarkable power conversion efficiency (PCE) of 31.6% for the Fe-doped structure (CsGe_0.875Fe_0.125Cl₃). Furthermore, our analysis confirms that doping improves the mechanical ductility and stability, indicating high suitability for flexible solar applications.