Synergistic Regulation of Ionic Defects Enabled by NdCl₃ for Efficient and Stable Wide-Bandgap Perovskite Solar Cells

Wide-bandgap mixed-halide perovskites are attractive top-cell absorbers for tandem photovoltaics owing to their high-voltage potential and complementary spectral response. However, ionic defects generated during solution processing promote nonradiative recombination, ion migration, etc. Here, NdCl₃ is introduced into Cs_0.17FA_0.83PbI_1.8Br_1.2 perovskites to synergistically regulate ionic defects. Structural analyses suggest that Nd³⁺ preferentially incorporates into interstitial sites, where its strong electrostatic interaction with neighboring halide ions may contribute to restricting their migration. The reduced defect density observed upon NdCl₃ modification points to effective defect passivation as a primary outcome. Meanwhile, Cl^- compensates for halide vacancies, improves the local coordination environment, and suppresses trap-assisted nonradiative recombination. This synergistic regulation promotes grain growth, reduces defect density, facilitates carrier transport, and improves interfacial energy-level alignment. Consequently, the NdCl₃-modified device achieves the highest power conversion efficiency of 19.87%. Moreover, the unencapsulated device retains 92.5% of its initial efficiency after 1680 h of ambient shelf storage (~25 °C, ~20% RH). These results demonstrate that the cooperative regulation of cation-associated ion migration and anion vacancies provides an effective route toward efficient and stable wide-bandgap perovskite photovoltaics.