Solid experimental evidence for reverse intersystem crossing from high-lying triplet states: A case study on hot exciton mechanism in OLEDs

Abstract

Hot excitons have been attempted to utilize the triplet excitons in organic light-emitting diodes (OLEDs). Due to the transient and dark nature of high-lying triplet states (Tn, n ≥ 2), the normative methods to characterize the hot exciton mechanism have not been thoroughly developed. Here, a normal technique combining transient photoluminescence and magneto-electroluminescence (MEL) measurements has been proven to visualize the reverse intersystem crossing process from T2 to S1 states in 5,6,11,12-tetraphenylnaphthacene (rubrene) molecules. Rubrene is chosen as a model system since its T1 is far below S1 and T2 is resonant with S1. This hot exciton process opens an additional route, marked as Dexter energy transfer channel (CT3→T2→S1, DET channel), together with the well-known Förster resonance energy transfer channel (CT1→S1) to transfer the host energy to the guest. With proper approximates, the DET channel assisted by the hot excitons process can contribute about 46.6% excitons to rubrene S1 and 83.4% rubrene emission in rubrene-doped devices. These studies set an in situ normative characterizing frame to visualize the hot excitons process in OLEDs.