Mitigating Residual FAI for Air‐Stable FAPbI 3 LEDs

ABSTRACT Formamidinium lead iodide (FAPbI 3 ) perovskite light‐emitting diodes (PeLEDs), showing record external quantum efficiencies (EQEs), are typically fabricated in an n‐i‐p structure through the deposition of a precursor solution rich in formamidinium iodide (FAI) on top of ZnO/ITO substrates. The ZnO electron transport layer is known to interact with the precursor solution during the fabrication, which can help to remove most of the excess FAI to ensure good stoichiometry and crystallinity as well as promote the formation of films with strong emission. However, residual FAI often remains on the perovskite surface, where it compromises device efficiency and stability. To mitigate this, we introduce here a thiol‐functionalized tertiary ammonium halide, 2‐diethylaminoethanethiol hydrochloride (DEAET), on top of FAPbI 3 . Beyond effectively regulating excess FAI, DEAET also acts as a passivation agent through synergistic hydrogen‐bonding and electrostatic interactions. This dual functionality suppresses non‐radiative pathways and promotes radiative recombination. The resulting DEAET‐passivated LEDs exhibit increased EQE and an impressive 15‐days air stability without encapsulation. The encouraging findings of this study lay the foundation for the utilization of thiol‐based salts as efficient agents for interface engineering in FAPbI 3 perovskite, that opens new possibilities for air‐stable perovskite light‐emitting diodes.