Stabilizing Halide Distribution in Mixed Halide Perovskites through Diammonium-Based Passivation

Mixed halide perovskite solar cells (PSCs) are promising for high-efficiency tandem photovoltaic architectures, but their performance is hindered by light-induced halide segregation and defect-mediated nonradiative recombination. Here, we introduce 1,3-diaminopropane dihydroiodide (PDADI) as a multifunctional surface passivation material to simultaneously suppress halide migration and reduce defect densities in FA_0.83Cs_0.17Pb-(I_0.6Br_0.4)₃ wide bandgap (∼1.78 eV) perovskite films. Our investigations reveal that PDADI-treated films exhibit a substantial reduction in light-induced phase segregation, with the photoluminescence (PL) peak shift suppressed under continuous illumination, an effect further confirmed by UV-vis absorption spectroscopy. Fourier-transform infrared analysis shows that PDADI's terminal -NH₃ ⁺ groups engage in strong bidentate coordination with undercoordinated Pb²⁺ ions, while its iodide counterions help compensate halide vacancies. This dual interaction stabilizes the perovskite lattice, reducing the trap density as evidenced by space-charge-limited current measurements. Consequently, trap-assisted nonradiative recombination is reduced, resulting in enhanced PL intensity and longer carrier lifetimes. PSCs incorporating PDADI passivation achieve a notable improvement in power conversion efficiency, increasing from 14.11% to a champion value of 16.54%, driven by an increase in open-circuit voltage from 1.135 to 1.243 V, and a fill factor improvement from 68.8% to 72.7%. Additionally, J-V hysteresis is significantly reduced from 9.6% to 3.6%, indicating improved charge extraction and suppressed ion migration. These results highlight the potential of PDADI as an effective molecular passivator for enhancing the performance and stability of wide bandgap mixed halide PSCs, advancing their applicability in tandem solar cell technologies.