Objective  In recent years, metal halide perovskite light-emitting materials have attracted great attention for their application in metal halide perovskite light-emitting diodes (PeLEDs) due to their outstanding optoelectronic properties, and are considered as the next generation of light-emitting sources in the field of display and lighting. Exciton utilization is one of the key factors affecting the efficiency of PeLEDs. Various methods have been employed to confine excitons in the perovskite light-emitting layer and recycle the energy of excitons to improve the utilization rate of excitons. This article will review the attempts made to improve the optoelectronic properties of green and blue PeLEDs by utilizing traditional fluorescent materials, phosphorescent materials, and thermally activated delayed fluorescent materials. It will also briefly introduce the principle of exciton confinement, as well as the energy transfer mechanism of different types of light-emitting materials introduced into green and blue PeLEDs and the physical mechanism of improving the optoelectronic properties.

  Methods  Films are fabricated using the methods of spincoating and vacuum thermal evaporation deposition. All the perovskites films are obtained by spincoating method. Various types of organic luminescent materials are introduced into the perovskite emissive layer as additives or inserted between the perovskite emissive layer/transport layer as sensitizers for exciton recycling, or through multiple coating to create multi-quantum well structures. These materials are brought into the PeLEDs through additive-assisted methods, device interface engineering, and structural optimization methods.

  Results and Discussions   It has been demonstrated that the introduction of traditional fluorescent emitters with a larger bandgap than that of the perovskite can better recycle singlet excitons. The incorporation of organic phosphorescent materials and different types of TADF materials, which have internal quantum efficiencies near 100% and energies of both singlet and triplet excitons that are significantly higher than the bandgap of the perovskites, can better recycle and utilize both singlet and triplet excitons of the perovskites. This leads to a potential internal quantum efficiency value of 100% for PeLEDs. Compared to traditional TADF materials, new TADF materials with a "core"-"shell" structure (such as Cz-3CzCN and Cz-4CzCN) and semiconductor TADF polymers with a "TADF core"-"shell" structure (such as P-Cz5CzCN) can not only passivate the defects in the perovskites film but also effectively suppress "exiton"-"exiton" quenching due to direct contact between the TADF emission cores. This further improves the utilization of excitons, greatly enhancing the efficiency and stability of green- and blue-emitting PeLEDs.

  Conclusions  This article reviews the work made by the groups of Gao Chunhong, Ban Xinxin and Wang Zhaokui in the fields of exciton confinement and exciton recycling in the past five years. These approaches mentioned above have been demonstrated in PeLEDs based on 3D perovskite emissive films (CsPbBr₃) and quasi-2D perovskite emissive films (PEA₂Cs_n-1Pb_nBr_3n+1, p-F-PEA₂Cs_n-1Pb_nBr_3n+1). These methods can also be extended to various types of light-emitting devices to achieve efficient and stable PeLEDs, providing a feasible strategy for the commercialization of PeLEDs.