Surface-induced phase engineering and defect passivation of perovskite nanograins for efficient red light-emitting diodes.

Organic-inorganic hybrid lead halide perovskites are potential candidates for next-generation light-emitting diodes (LEDs) in terms of tunable emission wavelengths, high electroluminescence efficiency, and excellent color purity. However, the device performance is still limited by severe non-radiative recombination losses and operational instability due to a high degree of defect states on the perovskite surface. Here, an effective surface engineering method is developed via the assistance of guanidinium iodide (GAI), which allows the formation of surface-2D heterophased perovskite nanograins and surface defect passivation due to the bonding with undercoordinated halide ions. Efficient and stable red-emission LEDs are realized with the improved optoelectronic properties of GAI-modified perovskite nanograins by suppressing the trap-mediated non-radiative recombination loss. The champion device with a high color purity at 692 nm achieves an external quantum efficiency of 17.1%, which is 2.3 times that of the control device. Furthermore, the operational stability is highly improved, showing a half-lifetime of 563 min at an initial luminance of 1000 cd m. The proposed GAI-assisted surface engineering is a promising approach for defect passivation and phase engineering in perovskite films to achieve high-performance perovskite LEDs.