Degradation mechanisms and lifetime extending strategy of phosphorescent and thermally activated delayed-fluorescence organic light-emitting diodes

Phosphorescence and thermally activated delayed-fluorescence (TADF) devices have been developed as highly efficient organic light-emitting diodes (OLEDs). However, they suffer from short device lifetimes despite having high external quantum efficiencies. Therefore, it is essential to overcome lifetime hurdles in phosphorescent and TADF OLEDs based on a deep understanding of their degradation mechanisms. In this work, detailed degradation processes of phosphorescent and TADF OLEDs are compared from the viewpoint of material and device degradation. In the case of phosphors, stabilization of the chemical bond between the heavy metal and the ligand is key to chemical stability. To chemically stabilize TADF emitters, increasing bond dissociation energy between nitrogen of donor and the p-linker or acceptor is of importance. Considering the device degradation mechanisms, triplet–triplet and triplet–polaron annihilation are critical to device lifetime in both phosphorescent and TADF OLEDs, with singlet–triplet annihilation further degrading TADF OLED device lifetimes. Several device approaches have been used to address degradation processes, and device lifetimes of phosphorescent and TADF OLEDs have recently been dramatically improved. Potential solutions and future prospects for high-efficiency and long-lifetime OLEDs are proposed in this review.