Dual‐Site Defect Passivation and δ ‐Phase Engineering for High‐Efficiency and Stable Perovskite Solar Cells

ABSTRACT Organic–inorganic perovskite solar cells are limited by surface defects and instability. Conventional ammonium‐salt passivation often forms insulating 2D phases that hinder charge transport. Here, we report ethylammonium trifluoroacetate (EATFA) as a bifunctional surface treatment for FAPbI 3 films. EA + and TFA − synergistically passivate A‑site and halide‑site defects, respectively, while inducing an extraction‐efficient δ ‑EA x FA 1‐ x PbI 3 surface layer instead of a 2D phase. This dual‑site δ ‑phase engineering suppresses non‑radiative recombination, optimizes energy‑level alignment, and improves charge extraction. As a result, EATFA‑treated devices achieve a power conversion efficiency of 25.54%, with a high V oc of 1.17 V and FF of 84.00%, along with enhanced stability, retaining >90% efficiency over 960 h under continuous illumination. This work provides a coherent passivation strategy that concurrently addresses efficiency and stability challenges in perovskite photovoltaics.