Strategies to improve the external quantum efficiency of perovskite light-emitting diode

Metal halide perovskite (MHP) has the potential to become the core luminescent material for the next generation of display and lighting equipment due to its excellent photoelectric properties and solution processible. Its unique crystal structure and flexible elemental composition enable it to have adjustable high-color-purity ultra-wide color gamut and bipolar ultra-fast mobility. As early as the 1990s, researchers attempted to prepare perovskite light-emitting diode (PeLED) using layered MHP as the light-emitting material. However, due to the immature membrane forming method and device structure, the device can only light up normally at 110 K. In 2014, Snaith et al first realized PeLED devices that were normally lit at room temperature. After nearly 5 years' development. the external quantum efficiency (EQE) of PeLED has been significantly improved, from 0.76% to 20.7% for near-infrared devices and from 0.1% to 20.31% for green devices. At the same time, the highest EQEs for sky-blue devices and blue devices are 12.1% and 9.5%, respectively. In this paper. the factors limiting device EQE are analyzed in principle. The limitation of device EQE can be considered from four factors: Transition selection factor chi, charge balance factor eta(r), optical coupling factor eta(out) and photoluminescence quantum yield phi(PL). The photoluminescence quantum yield of perovskite layer is the key factor to determine device EQE, while the charge balance factor and the light coupling factor are the important factors to further improve device EQE. The transition selective factor of perovskite is generally considered to be 100%, which means that both triplet and singlet states of perovskite materials are involved in luminescence. The charge balance factor mainly depends on the choice of electron transport material and hole transport material in the device structure, while the optical coupling factor depends on the morphology of the device film and the external light emission structure in the device. Last but not the least, the photoluminescence quantum yield depends on the type of perovskite material, film quality, and defect passivation. This paper summarizes the strategies reported in the literature to improve EQE of devices from two aspects of "improving the photoluminescence quantum yield of perovskite luminescence layer" and "adjusting device structure to enhance the charge balance factor and the light coupling factor". In order to obtain the devices with high photoluminescence quantum yield, we first need to prepare high quality perovskite film. On this basis, researchers try to reduce the grain size to reduce the quenching of the excited state at the interface between the transport layers and the perovskite emitting layer. In addition, dimensionality control is a special method to reduce grain size. and has both size effect and funneling effect to change the dominant behavior of radiation recombination in the perovskite emitting layer, from free carriers dominant to exciton dominant. Finally, defect passivation is of great significance to further improve the photoluminescence quantum yield. In addition, this paper also points out the core issues that need to be addressed urgently and the future direction of this region. Blue PeLED that meets the BT2020 standard and the improvement of device lifetime will become the core topics in the future. The development of lead-free perovskite luminescent materials is the only way for PeLED to be commercialized, and will become one of the directions of researchers' efforts. And exploring high-efficiency white emitting devices under high brightness will become another core topic of this region.