Despite more than 20 years of research, the root cause of the impractically short lifetimes of blue phosphorescent organic light-emitting diodes (PHOLEDs) has remained unclear. To overcome this, the authors investigate how the electrical properties of the emission layer (EML) of blue PHOLEDs affect degradation of the devices. It is found that a large density of dopant carriers is the dominant factor triggering triplet-polaron annihilation (TPA), which is a major defect-generation and hence lifetime-reduction mechanism. In order to reduce the generation of the TPA-induced defects to ensure long device lifetimes, the dopant carrier density should be minimized by suppressing the spontaneous charge transfer from the host to the dopant initially and by supplying sufficient charges with opposite polarity into the EML. However, there exists another critical factor that offsets the low overall density of defects against device lifetimes-that is, the non-uniform distribution of defects leading to intense exciton quenching. These two degradation factors are predetermined, and hence can be controlled, by the charge mobilities of the PHOLED EML. Given these considerations, it is demonstrated that the long-lifetime blue PHOLEDs can be realized.

A molecular structural approach is applied by introducing substituent groups (X) to explore the structure–property correlation of thermally activated delayed fluorescence (TADF) mechanism and develop blue TADF materials. D–A–X emitters show blue emissions from 446 to 487 nm and exhibit high rate constants of reverse intersystem crossing (krISC) from 0.76 × 106 to 2.13 × 106 s−1. Organic light emitting diodes (OLEDs) based on D–A–X emitters exhibit efficient external quantum efficiency from 17.2% to 23.9%. Furthermore, the theoretical analysis of spin–flip transitions between states of various nature reveals that the highest rISC rates can be achieved by the increase of charge‐transfer (CT) strength and enhancement of direct transition between triplet (3CT) and singlet (1CT) charge transfer states. Rotational tolerance of dihedral angle, low energy gap, and low reorganization energy between the 3CT and 1CT states provides fast rISC even when triplet states of different (LE) nature have much higher energy not to enable the three‐level interaction. By both experimental and theoretical methods, the investigations reveal that for the design of efficient TADF‐OLED emitters, the enhancement of the 3CT–1CT transition is as much important as that of 3LE–1CT.

A novel P precursor with controllable reactivity and new metal complex precursors soluble in ODE are created and used for blue-emitting In1−xGaxP@ZnS QDs (PLQY: 65%; FWHM: 46 nm). The III–V-CQD based blue-QLED of the best EQE until now is reported.