Small-Molecule Emitters with High Quantum Efficiency: Mechanisms, Structures, and Applications in OLED Devices

Organic emitters play a vital role in determining the overall performance of organic light emitting diode (OLED) devices. Traditional fluorescent emitters can only achieve external quantum efficiency (EQE) of 5%, far below expectation; therefore many efforts have been spent on increasing the EQE of OLEDs. Phosphorescence, thermally activated delayed fluorescence, triplet-triplet annihilation, and hybridized local and charge transfer are the most widely applied approaches to harvest the 75% triplet excitons for luminescence. As for selecting or designing suitable emitters for practical applications, it is strongly demanded to have an overall view about emitters of high exciton utilizing efficiency (EUE) from molecule level, i.e., the four common approaches mentioned above and some latest ones of the doublet, singlet fission, triplet-polar annihilation, and rotationally accessed spin state inversion, and also from the aggregated state such as aggregation-induced emission. In this review, the current progress of highly efficient emitters is presented, covering the chemical structures, the high-EUE mechanisms in molecule level and aggregated state, and their applications in OLED devices. This review hopefully will illustrate highly efficient electroluminescent materials and their mechanisms, but more importantly, provide helpful information on how to design or select suitable emitters for specific OLED devices.