Observation of Reverse Intersystem‐Crossing From the Upper‐Level Triplet to Lowest Singlet Excitons (T2→S1) in Tetra(t‐butyl)rubrene‐Based OLEDs for Enhanced Light‐Emission

Abstract 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 intersystem‐crossing (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 intersystem‐crossing (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.