Boosting Efficiency of Fully Thermally Evaporated PeLEDs via Benzylphosphonic Acid‐Induced In Situ Crystallization Control and Defect Passivation

ABSTRACT Fully thermally evaporated perovskite light‐emitting diodes (PeLEDs) represent a promising pathway toward scalable and commercial optoelectronic applications. However, rapid crystallization and excessive precursor reactivity during deposition often result in uncontrolled grain growth and high defect densities, limiting device performance. In this work, we propose an in situ crystallization regulation strategy by incorporating benzylphosphonic acid (BPA) into thermally evaporated CsMAPbBr 3 films. The phosphonic acid group in BPA coordinates with Pb 2+ ions during film growth, effectively slowing down the crystallization rate, reducing the average grain size from ∼200 to ∼40 nm. Moreover, BPA enables in situ passivation of vacancy‐related non‐radiative recombination centers, enhancing the radiative recombination efficiency. As a result, the fully thermally evaporated PeLEDs achieve a maximum external quantum efficiency (EQE max ) of 12.68% and a peak luminance of 62,104 cd m −2 . This work provides a simple yet effective approach to simultaneously regulate crystallization and passivate defects of the thermally evaporated perovskite.