Exploring the Interplay of Bulk and Surface Carrier Dynamics in Additive‐Engineered Halide Perovskites

Abstract Additive engineering has emerged as a strategy to enhance the performance and stability of perovskite solar cells. This study explores the effects of three (2,3,4,5,6‐pentafluorophenyl)alkylammonium iodide salts that differ in the length of the linking alkyl group (1, 2, or 3 methylene units) on the photophysical properties of methylammonium lead triiodide (MAPI) films. The bulk carrier dynamics measured using femtosecond transient absorption spectroscopy (TAS) reveal that the ammonium salt with a single methylene unit used at a concentration of 0.32 mol% led to longer carrier lifetimes as compared to MAPI, indicating that the additive delays carrier recombination. Femtosecond transient reflection spectroscopy (TRS) was used to probe the carrier dynamics on the film surface, as some fluorinated additives concentrate near the surface of the perovskite layer. The TRS measurements showed that at the optimum concentration, the additive with the single methylene unit reduced the prompt recombination as compared to the longer chain containing salts, thus indicating it also acts as a surface passivating agent. These results provide insight into how the alkyl chain length in ammonium salt additives containing perfluorinated phenyl moieties impacts the passivation of the perovskite film and device efficiency.