Fullerene‐Mediated Iodine Immobilization at Buried Interfaces in Sn–Pb Perovskite Solar Cells

Narrow‐bandgap perovskite solar cells (PSCs) are promising absorbers for tandem photovoltaics; however, their performance remains limited by interfacial recombination and instability, particularly Sn 2+ oxidation and halide‐related degradation. Although fullerene derivatives have been explored as charge‐selective interlayers, their mechanistic role in regulating buried‐interface chemistry remains unclear. Herein, we demonstrate that indene‐C 60 bisadduct (ICBA) functions as an electronic and chemical regulator in inverted (p‐i‐n) PSCs. When introduced via precursor additive engineering, ICBA preferentially accumulates at the buried perovskite/HTL interface, where its ambipolar, weakly electron‐selective character suppresses undesired electron extraction while electronically modifying the buried perovskite surface to preserve hole extraction. More importantly, spectroscopic and morphological analyses reveal that ICBA interacts with iodine species and mitigates Sn 2+ oxidation during thermal annealing. This dual interfacial regulation reduces trap formation and energetic disorder, enhances photoluminescence and carrier lifetimes, and improves stability. Devices incorporating ICBA achieve a peak power conversion efficiency of 22.7% and retain over 93% of their efficiency after 2496 h of dark storage, compared with 81% for reference devices. These findings establish a chemically informed buried‐interface stabilisation strategy for improving the performance and durability of lead‐reduced perovskite photovoltaics.