Cationic fluorination of ammonium salts boosts defect passivation for efficient and stable inverted perovskite solar cells

Abstract

Defect-induced non-radiative recombination and insufficient long-term stability hinder the commercial viability of organic-inorganic lead halide perovskites. Here we compare ethylammonium iodide (EAI) with its fluorinated analog 2,2,2-trifluoroethylammonium iodide (TFEAI) as additives in methylammonium lead triiodide (MAPbI3) perovskite solar cells. Combining density functional theory calculations with experiments, we show that the trifluoroethyl group strengthens electrostatic interactions with the perovskite lattice, forming stronger coordination bonds with undercoordinated Pb2+ ions at lead vacancies (VPb) and thereby reducing the trap-state density, as quantified by space-charge-limited current measurements, from 4.61 × 1016 cm−3 in the control to 3.47 × 1016 cm−3 in TFEAI-treated films. TFEAI-treated films consequently exhibit prolonged carrier lifetime and suppressed non-radiative recombination, along with improved grain morphology, optimized energy level alignment, and enhanced hydrophobicity. These advantages enable TFEAI-modified devices to achieve a champion power conversion efficiency of 18.0%, outperforming control devices (16.8%). After 300 h of ambient aging, the fluorinated device retains 79.7% of its initial efficiency versus only 47.8% for the control. This work establishes that cationic fluorination of ammonium salt passivators provides a rational molecular design route to simultaneously boost efficiency and stability in perovskite photovoltaics.