Unusual Optical,\nElectric, and Magnetic Behaviors\nof OLEDs due to Exothermic/Endothermic Dexter-Energy-Transfer and\nFusion Channels of Hot/Cold Triplet Excitons

Hot-exciton-based organic light-emitting diodes (HE-OLEDs)\nhave\naroused more attention due to their advantages of low cost, high efficiency,\nand negligible efficiency roll-off. However, their physical behaviors\nneed further investigation because of the evolution diversity of excited\nstates existing in HE-OLEDs. Herein, we employ the two frequently\nused hosts tris(8-hydroxyquinoline)aluminum (Alq<sub>3</sub>) and\n4,4′-<i>N</i>,<i>N</i>′-dicarbazolebiphenyl\n(CBP) and the hot-exciton emissive guests rubrene (Rb) and its derivative\n2,8-di-<i>tert</i>-butyl-5,11-bis(4-<i>tert</i>-butylphenyl)-6,12-diphenyltetracene (TBRb) to make four HE-OLEDs\nand use well-known fingerprint magneto-electroluminescence (MEL) curves\nto probe their microscopic dynamic processes. Interestingly, we find\nabundant optical, electric, and magnetic behaviors of HE-OLEDs due\nto exothermic and endothermic Dexter-energy-transfer (DET) and triplet\nfusion (TF) channels of hot/cold excitons. Specifically, for the case\nof different bias currents at room temperature, both the low-field\nMEL curves of Rb- and TBRb-doped Alq<sub>3</sub>-based OLEDs show\na normal intersystem crossing (ISC) of polaron pairs, but those of\nRb- and TBRb-doped CBP-based OLEDs present a conversion from a high-level\nreverse ISC (HL-RISC, S<sub>1</sub> ← T<sub>2</sub>) to an\nISC and a normal HL-RISC, respectively, which weakens with an elevated\nbias current. Moreover, both the high-field MEL curves of Rb- and\nTBRb-doped Alq<sub>3</sub>-based (CBP-based) OLEDs show a normal T<sub>1</sub>F (T<sub>2</sub>F) of cold (hot) triplet excitons, which strengthens\nwith elevated bias currents. For the case of constant bias currents\nat variable temperatures, both the low-field MEL curves of Rb- and\nTBRb-doped Alq<sub>3</sub>-based OLEDs show an abnormal ISC, which\nrises with a reducing temperature, but those of Rb- and TBRb-doped\nCBP-based OLEDs depict a conversion from an ISC to a HL-RISC and a\nnormal HL-RISC, respectively, which intensifies with a decreasing\ntemperature. In addition, the high-field MEL curves of Rb- and TBRb-doped\nAlq<sub>3</sub>-based OLEDs separately show a normal and an abnormal\nT<sub>1</sub>F, but those of both of Rb- and TBRb-doped CBP-based\nOLEDs exhibit a normal T<sub>2</sub>F, which strengthens with a reducing\ntemperature. Furthermore, the quantum efficiency of Rb- and TBRb-doped\nAlq<sub>3</sub>-based OLEDs separately show nonmonotonically and monotonically\ndecreased tendencies, but both Rb- and TBRb-doped CBP-based OLEDs\nshow monotonically increased tendencies with a decreasing temperature.\nSurprisingly, all of the above enriched physical behaviors can be\nreasonably interpreted within the frames of excited state dynamics\nthat DET from host cold T<sub>1,Alq3</sub> (T<sub>1,CBP</sub>) to\nguest hot T<sub>2</sub> is an endothermic (exothermic) process and\nT<sub>1</sub>F in Rb (TBRb) is an exothermic (endothermic) process,\nbut T<sub>2</sub>F in both Rb and TBRb is exothermic.