Optimization of full-spectrum light absorption performance in all-inorganic perovskite solar cells doped with Ge²⁺

Full-spectrum light absorption is crucial for enhancing the power conversion efficiency (PCE) of perovskite solar cells (PSCs). This study explores the impact of Ge²⁺-doped on the ultraviolet–visible–near-infrared (UV–visible–NIR) light absorption of Cs-based all-inorganic perovskites using density functional theory and first-principles calculations. Four Ge²⁺-doped perovskites (CsSn_0.5Ge_0.5I_2.25Cl_0.75, CsSn_0.75Ge_0.25I₂Cl, CsPb_0.25Ge_0.75I_2.75Cl_0.25, and CsSn_0.5Ge_0.5IBr₂) exhibit remarkable absorption from UV to NIR. Notably, CsSn_0.75Ge_0.25I₂Cl demonstrates peak absorption coefficients of 1291315 cm⁻¹ in the visible region and 1734261 cm⁻¹ in the NIR region. Band structure and density of states analyses reveal that Ge²⁺ incorporation narrows bandgaps, thereby enhancing light absorption. Deformation potential calculations indicate significantly improved carrier mobility, with CsSn_0.75Ge_0.25I₂Cl exhibiting electron mobility of 6.68 cm²/Vs and hole mobility of 5.39 cm²/Vs. Device simulations of PSCs (Spiro-OMeTAD/Perovskite/TiO₂/FTO) predict a PCE of nearly 29.23 % (V_OC = 1.16 V, J_SC = 21.33 mA/cm², FF = 82.50 %) under optimized conditions (600 nm absorber thickness and 695.9 W/m² light intensity), representing a 32.5 % improvement in PCE compared to pre-optimization conditions. This study also examines defect density and temperature impacts on Ge²⁺-doped PSCs. As defect density rises from 10¹⁵ to 10¹⁹ cm⁻³, performance metrics degrade significantly due to increased carrier recombination and energy loss. Increasing temperature from 300 to 400 K also reduces PCE for all materials. These findings underscore the importance of full-spectrum absorption engineering for enhancing all-inorganic lead-free PSC performance.