Recent advances in metal-free phosphorescent materials for organic light-emitting diodes

Metal-free room temperature phosphorescence (RTP) emitters, owing to the high exciton utilization, have become a class of promising materials for phosphorescent organic light-emitting diodes (PhOLEDs). Notably, the increasingly deeper theoretical understanding of photophysical science helps develop high-performance RTP materials, stimulating the development of PhOLEDs. This review outlines recent advances in RTP emitters for PhOLEDs from the perspective of mechanisms and material systems of the emitting layer, including molecular design principles and detailed optoelectronic behaviors. Based on the knowledge of efficient metal-free PhOLED devices, we summarize a series of reported phosphorescent materials with high phosphorescence quantum efficiency and a short phosphorescent lifetime, which have potential application prospects in this field. Conclusions and outlook are presented to point out the development potential and challenges for further research of metal-free PhOLEDs.