Realizing High Brightness Quasi‐2D Perovskite Light‐Emitting Diodes with Reduced Efficiency Roll‐Off via Multifunctional Interface Engineering

Abstract Quasi‐2D perovskites have recently flourished in the field of luminescence due to the quantum‐confinement effect and the efficient energy transfer between different n phases resulting in exceptional optical properties. However, owing to the lower conductivity and poor charge injection, quasi‐2D perovskite light‐emitting diodes (PeLEDs) typically suffer from low brightness and high‐efficiency roll‐off at high current densities compared to 3D perovskite‐based PeLEDs, which is undoubtedly one of the most critical issues in this field. In this work, quasi‐2D PeLEDs with high brightness, reduced trap density, and low‐efficiency roll‐off are successfully demonstrated by introducing a thin layer of conductive phosphine oxide at the perovskite/electron transport layer interface. The results surprisingly show that this additional layer does not improve the energy transfer between multiple quasi‐2D phases in the perovskite film, but purely improves the electronic properties of the perovskite interface. On the one hand, it passivates the surface defects of the perovskite film; on the other hand, it promotes electron injection and prevents hole leakage across this interface. As a result, the modified quasi‐2D pure Cs‐based device shows a maximum brightness of > 70,000 cd m⁻² (twice that of the control device), a maximum external quantum efficiency (EQE) of > 10% and a much lower efficiency roll‐off at high bias voltages.