Linking First-Principles Insights with Device Performance: GA ⁺ -Stabilized, Co ²⁺ -Doped Tin Halide Perovskites for High-Efficiency Photovoltaics

We present a targeted investigation of GA + /Co 2+ dual-site engineering in tin halide perovskites, GA(Sn 1– x Co x )I 2–2 x Cl 1+2 x, establishing its quantitative impact on structural stabilization and electronic optimization in lead-free photovoltaics. GA + incorporation strengthens the perovskite framework through A-site steric stabilization and effective suppression of Sn oxidation pathways, while Co 2+ alloying modifies the B-site potential landscape to yield tunable direct bandgaps of 1.26–1.40 eV, enhanced dielectric screening (ε r ≈ 10), and a high absorption coefficient (α > 10 4 cm –1 ). This dual-site strategy reduces deep-level trap formation and mitigates ionic migration, outperforming conventional Sn-based systems in both thermodynamic robustness and carrier recombination kinetics. Device simulations of a TiO 2 /perovskite/Cu 2 O stack predict a theoretical efficiency of 19.61% with improved V oc, reduced interfacial recombination resistance, and a built-in potential of ∼1.19 V, consistent with strengthened internal electrostatics induced by GA + /Co 2+ synergy. These results demonstrate that coordinated A- and B-site modification provides a mechanistically coherent route to stabilize Sn-based perovskites while achieving competitive, lead-free photovoltaic performance.