Tricomponent exciplex-based organic light-emitting diodes (TE-OLEDs) with high- and low-energy exciplexes have recently gained much attentions because of their already-reported high external quantum efficiency (EQE) and their exciplex-to-exciplex energy transfer (EE-ET) is considered as an important factor to influence the device performance. However, few works provide evidence to prove this EE-ET process due to the lack of the absorption band of exciplexes. Herein, the EE-ET channel from a high-energy exciplex donor (EXED) to another low-energy exciplex acceptor (EXEA) in a TE-OLED is demonstrated by probing magnetic field effects (MFEs) on the electrical and optical properties of devices including magneto-conductance (MC), magneto-efficiency (Mη), and magneto-electroluminescence (MEL), because this EE-ET can influence the evolution channels of spin-pair states in the TE-OLED which could be visualized by these featured MFE traces. Specifically, all the MC, Mη, and MEL curves of the single EXED-based OLED depict the normal bias-current (I)-dependent intersystem crossing (ISC) from singlet to triplet polaron pairs, which weakens with increasing I. Moreover, the Mη and MEL traces of the single EXEA-based OLED, respectively, present the abnormal and normal I-dependent ISC, while its MC curves show the conversion from reverse ISC (RISC) of exciplexes to ISC of polaron pairs with increasing I. However, both MEL and Mη traces of the TE-OLED with simultaneous EXED and EXEA show the abnormal I-dependent RISC from triplet to singlet exciplexes, which enhances with increasing I, while its MC traces display normal I-dependent RISC. These RISC behaviors have seldom been observed previously in the literature, which are induced by the EE-ET process from EXED to EXEA that facilitate the RISC channel of EXEA via increasing the quantity of triplet exciplex states. Moreover, higher EQE is obtained in the TE-OLED with this EE-ET channel from EXED to EXEA than the single EXEA-based OLED. Thus, these MFE measurements provide new strategies for recognizing the EE-ET process in OLEDs based on multiple exciplex emitter systems and pave the way for designing superior performance exciplex-based OLEDs.

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High external quantum efficiency (EQE) up to 25% has recently been reported from tetra(t-butyl)rubrene (TBRb)-based organic light-emitting diodes (OLEDs), but its physical origin is still vague. Herein, using the featured responses of the evolution processes of electron-hole pairs to an external magnetic field, an unreported high-level reverse intersystemcrossing (HL-RISC) from upper-level triplet to lowest singlet excitons (T 2 →S 1 ) is observed when T 2 is well confined in the active layer of pure TBRb. This HL-RISC channel becomes stronger with lowering operational temperatures because it is not an endothermic process. Due to the larger separation distance of TBRb molecules with four tert-butyl groups, the intersystemcrossing (ISC) process of polaron pairs is stronger than the singlet fission (SF) process existing in pure TBRb, which is markedly different from the behaviors of excited states in pure rubrene (Rb) with negligible ISC and strong SF. More importantly, HL-RISC is stronger in TBRb than in Rb-doped systems, which is consistent with the higher EQE frequently reported from TBRb-doped OLEDs. Thus, this work deepens the physical understanding of microscopic processes in typical organic multi-functional semiconductors of TBRb or Rb and paves the way for fabricating further high-efficiency yellow OLEDs.

The molecular aggregation effect on the photophysical properties of many organic semiconductors has been well studied, but this behavior of rubrene molecules is still vague in rubrene-doped organic light-emitting diodes. Surprisingly, via recording temperature-dependent electroluminescence and photoluminescence spectra, an intriguing H-type aggregation of rubrene molecules is realized from rubrene-doped devices and films only when 4,4′,4″-tris­(N-3-methyl­phenyl-N-phenyl-amino)­triphenyl­amine (m-MTDATA) or 4,4′,4″-tris­(N-(2-naphthyl)-N-phenyl-amino)­triphenyl­amine (2TNATA) is selected as the host; otherwise, the J-type aggregation of rubrene molecules is solely observed. We propose that this H-type aggregation is caused by the increased steric hindrance between adjacent rubrene molecules because the unique molecular structures of 12 freely rotating chemical bonds in m-MTDATA or 2TNATA are suppressed in rubrene-doped solid-state films. More importantly, we find that the H-type aggregation of rubrene molecules induces a nearly 3-fold increase in the fluorescence of rubrene-doped devices at 20 K. The increased fluorescence can be mainly attributed to the intensified triplet–triplet annihilation (TTA) process because the parallel alignment of transition dipole moments in H-aggregation can facilitate the TTA process via enlarging the TTA reaction cross-section. Thus, our findings deepen the understanding of molecular interactions in aggregation systems and pave the way for achieving highly efficient devices operated especially in a low-temperature environment such as outer space.

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