Efficient Thermally Evaporated Formamidinium‐Based Ruddlesden–Popper Perovskite Light‐Emitting Diodes

ABSTRACT Vacuum thermal evaporation offers advantages such as large‐area uniformity and facile perovskite LED, quasi‐2D perovskite, thermal evaporation pixelation, making it a scalable and industry‐compatible route for fabricating perovskite light‐emitting diodes (PeLEDs). However, due to poor exciton confinement and energy management, the efficiency of thermally evaporated PeLEDs has remained inferior to that of solution‐processed devices. Here, we present a formamidinium‐based Ruddlesden–Popper (RP) type thermally evaporated perovskites FA 2 (FA 0.3 Cs 0.7 ) n‐1 Pb n Br 3n+1 . We further achieve well‐controlled phase distribution and uniform film morphology by adjusting the FA precursor ratio during deposition. Subsequent defect passivation using triphenylphosphine oxide (TPPO) enhanced the film quality, yielding a high photoluminescence quantum yield (PLQY) of 86.1%. Moreover, unlike bulky organic cations such as phenethylammonium (PEA + ), FA + exhibits smaller spatial spacing, which facilitates carrier injection and rapid energy transfer. As a result, we demonstrate all‐evaporated PeLEDs with an external quantum efficiency (EQE) of 19.6%. This work highlights the critical role of small interlayer cations in constructing efficient quasi‐2D architectures under solvent‐free conditions, offering a universal strategy for high‐performance and manufacturable perovskite optoelectronics.