Single‐Crystal Perovskite Light‐Emitting Diodes with External Quantum Efficiency of over 8% Enabled by Nonstoichiometric Composition Tuning

Abstract Perovskite single crystals (SCs) have been emerging as promising materials for electroluminescence (EL) device applications owing to their superior optoelectronic properties. However, the device performance of single‐crystal perovskite light‐emitting diodes (SC‐PeLEDs) is limited by the lack of effective defect management and energy level modulation. Here, a nonstoichiometric composition tuning (NCT) strategy for the construction of high‐performance SC‐PeLEDs is reported. The NCT strategy, finely tuning the MABr (MA + = CH 3 NH 3 + ) excess in the nonstoichiometric MAPbBr 3 SCs to enhance the crystal quality, reduce the trap density, and elevate the energy level of the resultant perovskite SC, enables markedly decreased nonradiative recombination and more efficient carrier injection for the devices. In consequence, the optimized SC‐PeLEDs exhibit an ultrahigh peak luminance of 161 900 cd m −2 and a large external quantum efficiency (EQE) of up to 8.1%, representing the most efficient SC‐PeLED reported thus far. This strategy also shows a high universality in enhancing the EL performance of other lead bromide perovskite SC‐PeLEDs, endowing them with luminances and EQEs three times larger than those of the pristine devices. This work opens an avenue for the development of high‐performance EL devices based on perovskite SCs.