Using the fingerprint magneto-electroluminescence trace, we observe a fascinating high-level reverse intersystem crossing (HL-RISC) in rubrene-doped organic light-emitting diodes (OLEDs). This HL-RISC is achieved from high-lying triplet states (T2,rub) transferred from host triplet states by the Dexter energy transfer to the lowest singlet states (S1,rub) in rubrene. Although HL-RISC decreases with bias current, it increases with lowering temperature. This is contrary to the temperature-dependent RISC from conventional thermally activated delayed fluorescence, because HL-RISC is an exothermic process instead. Moreover, owing to the competition of exciton energy transfer with direct charge trap, HL-RISC changes nonmonotonically with the dopant concentration and increases luminous efficiency to a maximum at 10% of rubrene, which is about ten times greater than that from the pure-rubrene device. Additionally, the HL-RISC process is not observed in bare rubrene-doped films because of the absence of T2,rub. Our findings pave the way for designing highly efficient orange fluorescent OLEDs.

Room-temperature observation for reverse intersystem crossing (RISC) from triplet to singlet charge-transfer states (CT 3 → CT 1 ) and clarification of its physical mechanisms are the key requirements for designing highly efficient exciplex-based organic light-emitting diodes (OLEDs). Herein, balanced and unbalanced exciplex-based OLEDs were fabricated by employing different holeinjection layers, and RISC of CT states was directly observed via analyzing magnetoconductance (MC) and magneto-electroluminescence (MEL) traces of the balanced device at room temperature. Specifically, current-dependent MC traces of the balanced device always present B-mediated RISC features, whereas those from the unbalanced one depict the superposition of B-mediated intersystem crossing (ISC) and the dissociation of CT 3 by excessive charge carriers. Simultaneously, MEL curves of the balanced device display the conversion from ISC to RISC with lowering bias current, but those from the unbalanced one always show ISC under all of bias currents. Moreover, although all of current-dependent magneto-efficiency (Mη) traces exhibit ISC, Mη values are ∼2 times lower in the balanced device than the unbalanced one. These rich changes of magnetic-field responses demonstrate that balanced carrier injection can facilitate the occurrence of RISC by reducing the dissociation of CT 3 . Expectedly, the current efficiency of electroluminescence from the balanced device is increased by ∼2.2 times, which originates from the improvement of delayed luminescence because of the enhanced RISC. Accordingly, this work not only clarifies the prerequisite for observing RISC of CT states but also provides strategies for designing high-efficiency exciplex-based OLEDs.

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We fabricated quantum-well organic light-emitting diodes (QW-OLEDs) based on thermally activated delayed fluorescence (TADF) and measured their magnetic field effects curves over various magnetic field ranges.

An in situ heating method was used to investigate the effect of high temperature on the magneto-electroluminescence (MEL) of rubrene-based organic light-emitting diodes.