Hyperfluorescence-based emission in purely organic materials: Suppression of energy-loss mechanisms via alignment of triplet excited states

Hyperfluorescence has received significant attention as a promising strategy to design organic light-emitting diodes (OLEDs) with high color purity and enhanced stability. In this approach, emitters displaying strong and narrow-band fluorescence are integrated in thin films that contain sensitizers showing efficient thermally activated delayed fluorescence (TADF). To ensure high performance, the energies of the electronic states in the fluorescent emitters must be well-aligned, with respect to those in the TADF molecules, in order to enable a fast rate of Förster singlet-exciton energy transfer from the latter to the former. Here, we performed molecular dynamics simulations and density functional theory calculations to study a series of fluorescent emitters commonly considered in hyperfluorescence OLEDs. For all these emitters, the lowest triplet excited state (T1FE) is found to locate substantially below the lowest singlet excited state (S1FE). However, the second and/or third triplet excited states (T2FE and T3FE) appear at an energy close to that of S1FE thus, while energy loss via triplet-exciton Dexter energy transfer from T1 in TADF molecules to T1FE is negligible, it can become significant due to Dexter transfer to T2FE and/or T3FE. As a result, we propose that fluorescent emitters be designed with a large energy gap between T2FE/T3FE and S1FE, as a promising strategy to suppress any Dexter energy-loss mechanism and develop highly efficient hyperfluorescence-based optoelectronic devices.