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.

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 (T2, 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(T1, host) ≥ E(T2, rub), the high‐level Dexter energy transfer channel (T1, host → T2, rub) can facilitate the occurrence of HL‐RISC (T2, rub → S1, 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.

Over twelve-fold enhancement of external quantum efficiency (EQE) is observed in 9,10-bis(4-(9H-carbazol-9-yl)-2,6-dimethylphenyl)-9,10-diboraanthracene (CzDBA)-based organic light-emitting diodes (OLEDs) with an interlayer between the hole-transporting layer (HTL) and the emission layer, where the CzDBA emitter is a typically donor–acceptor–donor (D–A–D)-type thermally activated delayed fluorescence material. Analyses of the fingerprint magneto-electroluminescence traces indicate that the interlayer ensures the charge balance of the emission layer in devices, avoiding triplet-charge annihilation and contributing to the enhancement of EQE. Additionally, experimental results also show that intersystem crossing (ISC) and reverse ISC (RISC) processes coexist in the device with an interlayer. Notably, ISC boosts with increasing bias currents and working temperatures, respectively, exhibiting abnormal current and normal temperature dependences. This abnormal phenomenon is caused by the weakened RISC between charge-transfer states of CzDBA molecules at large bias currents. More interestingly, as bias currents increase, ISC in the device without an interlayer first exhibits normal current dependences and then turns into an abnormal one, which may attribute to the competitive effects of exciplex at the HTL/CzDBA interface and excited states of CzDBA molecules. Our findings not only unravel the underlying mechanisms in D–A–D-type molecules but also provide ideas for designing highly efficient devices.

No abstract

Charge-transporting layers with different triplet energies can efficiently tune the conversion from intersystem crossing to high-level reverse intersystem crossing processes in rubrene-doped OLEDs.