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

Abstract A high‐level reverse intersystem crossing (HL‐RISC, T₂ → S₁ → S₀ + hν) 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, rub}) is of great importance for the achievement of the HL‐RISC process. Namely, when the triplet energies of hosts satisfy the criterion of E(T_{1, host}) ≥ E(T_{2, rub}), the high‐level Dexter energy transfer channel (T_{1, host} → T_{2, rub}) can facilitate the occurrence of HL‐RISC (T_{2, rub} → S_{1, 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.