Unraveling the Important Role of High‐Lying Triplet–Lowest Excited Singlet Transitions in Achieving Highly Efficient Deep‐Blue AIE‐Based OLEDs

Aggregation-induced emission (AIE) materials are attractive for achieving highly efficient nondoped organic light-emitting diodes (OLEDs) owing to their strong luminescence in the solid state. However, the electroluminescence efficiency of most AIE-based OLEDs remains low owing to the waste of triplet excitons. Here, using theoretical calculations, photophysical dynamics, and magnetoluminescence measurements, the spin conversion process is demonstrated between the high-lying triplet state (T_n ) and the lowest excited singlet state (S₁ ) in AIE materials. Moreover, the relative positions of T_n (n < 4) and S₁ are shown to have a significant impact on the spin-conversion efficiency, thus influencing the harvesting of triplet excitons and the device efficiency. Finally, by selecting an upconversion material with an appropriate energy level for further utilizing the triplet excitons, a deep-blue fluorescent OLED with CIE coordinates of (0.15, 0.08), a maximum external quantum efficiency of 10.2%, low efficiency roll-off, and a high brightness of 16817 cd m^-2 is developed. This is one of the most efficient deep-blue OLEDs based on AIE materials reported so far. These findings also provide new insights into the design of more efficient AIE molecules and corresponding OLEDs by managing high-lying triplet excitons.