Bifunctional Phosphonium Bromide Modifiers for Efficient Light-Emitting Diodes

All-inorganic CsPbBr<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> perovskites, while promising for light-emitting diodes (LEDs), suffer from rapid, uncontrolled crystallization and slower electrochemical performance compared to quasi-2D, nanocrystalline, and hybrid cation systems. Here, we demonstrate that introducing triphenylphosphine tribromide (TTPB) into CsPbBr<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> perovskite precursors simultaneously controls crystallization kinetics and passivates defects, enabling high-performance PeLEDs. TTPB acts as a nucleation center via halogen bonding with precursors, while the triphenylphosphine groups reduce grain size through steric hindrance. This suppresses pinholes and leakage pathways at grain boundaries while enhancing crystallinity and optical properties. Furthermore, TTPB-perovskite interaction improves the energy-level alignment, facilitating efficient hole injection. Consequently, PeLEDs incorporating TTPB achieved a peak external quantum efficiency (EQE) of 22.2% (14.6% for controls), a maximum brightness of 77,754 cd m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> (2.9× enhancement), along with lower turn-on voltage and high reproducibility.