Full Confinement of High-Lying Triplet States to Achieve High-Level Reverse Intersystem Crossing in Rubrene: A Strategy for Obtaining the Record-High EQE of 16.1% with Low Efficiency Roll-Off

A high-level reverse intersystem crossing (HL-RISC, T-2 -> S-1 -> S-0+h nu) process has recently been discovered as a promising route for achieving highly efficient organic light-emitting diodes (OLEDs), but the prerequisites for the occurrence of HL-RISC in rubrene is still vague and the reported external quantum efficiencies (EQEs) of rubrene-doped OLEDs are typically limited to several percent. Herein, using the fingerprint magneto-electroluminescence tools, it is found that the energy confinement of high-lying triplet states (T-2,T- rub) is of great importance for the achievement of the HL-RISC process. Namely, when the triplet energies of hosts satisfy the criterion ofE(T-1,T- host) >= E(T-2,T- rub), the high-level Dexter energy transfer channel (T-1,T- host -> T-2,T- rub) can facilitate the occurrence of HL-RISC (T-2,T- rub -> S-1,S- rub) in rubrene. Most importantly, through selecting an exciplex with a high triplet energy as the co-host for rubrene dopant so as to simultaneously utilize the HL-RISC of the dopant and the RISC of the host, a record high EQE up to 16.1% is achieved and no obvious efficiency roll-off is observed at high luminance due to the absence of triplet-charge annihilation. Accordingly, this work not only deepens the physical understanding of this amazing HL-RISC channel, but also provides a new direction for designing a series of highly efficient OLEDs.