Charge-transfer versus energy-transfer in quasi-2D perovskite light-emitting diodes

"Ruddlesden-Popper" perovskites with multiple quantum wells (MQWs) layered structure are extremely attractive for light-emitting diodes (LEDs) due to their effective energy confinement and high device stability properties. Two important processes of energy transfer (ET) and charge transfer (CT) are responsible for the effective energy confinement in MQWs. At present stage, the ET mechanism has been widely accepted as the main origin of high efficiency in MQWs. However, the high dielectric constant (epsilon) of hybrid perovskites should significantly limit the ET process, because the ET rate is inversely proportional to the epsilon. Therefore, the detailed contribution of CT mechanism, especially under electrical excitation, should be carefully considered. Herein, we report a high quality quasi-2D perovskite film of PA(2)(CsPbBr3)(n-1)PbBr4 MQWs through a simple solution process, with highly-dense and smooth morphology, and a high photoluminescence (PL) quantum efficiency. Then, the PL and transient PL spectra were measured under electrical injection in a hole-only and electron-only devices, respectively, to verify the CT mechanism. Furthermore, the photo-excitation intensity dependent PL spectra reveal that the CT mechanism is more important than ET mechanism for the electroluminescence of PA(2)(CsPbBr3)(n-1)PbBr4 perovskite LEDs (PeLEDs), resulting in a pure cyan emission at similar to 505 nm and maximal external quantum efficiency up to 3.6%, representing one of the best results in reported blue PeLEDs.