Observation of Reverse Intersystem-Crossing From the Upper-Level Triplet to Lowest Singlet Excitons (T_2→S_1) in Tetra(t-butyl)rubrene-Based OLEDs for Enhanced Light-Emission

摘要

High external quantum efficiency (EQE) up to 25% has recently beenreported from tetra(t-butyl)rubrene (TBRb)-based organic light-emittingdiodes (OLEDs), but its physical origin is still vague. Herein, using thefeatured responses of the evolution processes of electron-hole pairs toan external magnetic field, an unreported high-level reverse intersystemcrossing(HL-RISC) from upper-level triplet to lowest singlet excitons (T2→S1)is observed when T_2 is well confined in the active layer of pure TBRb. ThisHL-RISC channel becomes stronger with lowering operational temperaturesbecause it is not an endothermic process. Due to the larger separationdistance 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 thebehaviors of excited states in pure rubrene (Rb) with negligible ISC andstrong SF. More importantly, HL-RISC is stronger in TBRb than in Rb-dopedsystems, which is consistent with the higher EQE frequently reported fromTBRb-doped OLEDs. Thus, this work deepens the physical understanding ofmicroscopic processes in typical organic multi-functional semiconductors ofTBRb or Rb and paves the way for fabricating further high-efficiency yellowOLEDs.