Unusual Optical, Electric, and Magnetic Behaviors of OLEDs due to Exothermic/Endothermic Dexter-Energy-Transfer and Fusion Channels of Hot/Cold Triplet Excitons

Hot-exciton-based organic light-emitting diodes (HE-OLEDs) have aroused more attention due to their advantages of low cost, high efficiency, and negligible efficiency roll-off. However, their physical behaviors need further investigation because of the evolution diversity of excited states existing in HE-OLEDs. Herein, we employ the two frequently used hosts tris(8-hydroxyquinoline)aluminum (Alq3) and 4,4′-N,N′-dicarbazolebiphenyl (CBP) and the hot-exciton emissive guests rubrene (Rb) and its derivative 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (TBRb) to make four HE-OLEDs and use well-known fingerprint magneto-electroluminescence (MEL) curves to probe their microscopic dynamic processes. Interestingly, we find abundant optical, electric, and magnetic behaviors of HE-OLEDs due to exothermic and endothermic Dexter-energy-transfer (DET) and triplet fusion (TF) channels of hot/cold excitons. Specifically, for the case of different bias currents at room temperature, both the low-field MEL curves of Rb- and TBRb-doped Alq3-based OLEDs show a normal intersystem crossing (ISC) of polaron pairs, but those of Rb- and TBRb-doped CBP-based OLEDs present a conversion from a high-level reverse ISC (HL-RISC, S1 ← T2) to an ISC and a normal HL-RISC, respectively, which weakens with an elevated bias current. Moreover, both the high-field MEL curves of Rb- and TBRb-doped Alq3-based (CBP-based) OLEDs show a normal T1F (T2F) of cold (hot) triplet excitons, which strengthens with elevated bias currents. For the case of constant bias currents at variable temperatures, both the low-field MEL curves of Rb- and TBRb-doped Alq3-based OLEDs show an abnormal ISC, which rises with a reducing temperature, but those of Rb- and TBRb-doped CBP-based OLEDs depict a conversion from an ISC to a HL-RISC and a normal HL-RISC, respectively, which intensifies with a decreasing temperature. In addition, the high-field MEL curves of Rb- and TBRb-doped Alq3-based OLEDs separately show a normal and an abnormal T1F, but those of both of Rb- and TBRb-doped CBP-based OLEDs exhibit a normal T2F, which strengthens with a reducing temperature. Furthermore, the quantum efficiency of Rb- and TBRb-doped Alq3-based OLEDs separately show nonmonotonically and monotonically decreased tendencies, but both Rb- and TBRb-doped CBP-based OLEDs show monotonically increased tendencies with a decreasing temperature. Surprisingly, all of the above enriched physical behaviors can be reasonably interpreted within the frames of excited state dynamics that DET from host cold T1,Alq3 (T1,CBP) to guest hot T2 is an endothermic (exothermic) process and T1F in Rb (TBRb) is an exothermic (endothermic) process, but T2F in both Rb and TBRb is exothermic.