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

Abstract

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 T_1,Alq3 (T_1,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.