Triplet-to-Singlet Exciton Transfer in Hyperfluorescent OLED materials

Abstract

Hyperfluorescent organic light-emitting diodes combine two kinds of dopants to maximize device efficiency: one molecule exhibiting thermally activated delayed fluorescence (TADF) and another molecule with a high fluorescence rate and narrow emission spectrum.The postulated role of a TADF sensitizer is to enable up-conversion of triplet to singlet excitons through the reverse intersystem crossing mechanism, which is followed by a Förster energy transfer to the fluorescent emitter. However, a second mechanism based on the direct triplet-to-singlet exciton transfer between TADF molecules is a priori possible, but its role in hyperfluorescence has not been investigated. Here we employ first-principles electronic-structure and kinetic Monte Carlo simulations to study the hyperfluorescence mechanism in four pairs of TADF/fluorescent emitters. We demonstrate how the tripletto-singlet energy transfer mechanism is, in some cases, the main driver for the quantum efficiency boost observed in hyperfluorescent devices.

Quantifying Exciton Annihilation Effects in Thermally Activated Delayed Fluorescence Materials

“…An alternative explanation for an apparent power dependence of r(ISC) could originate from the presence of triplet-to-singlet FRET mechanisms. [34] However, the possibility of this process diminishes with high rISC rate, as observed for CzDBA, which also contributes to the nearly 100% PLQY of CzDBA. Therefore, we do not expect triplet-to-singlet FRET to play a significant role in CzDBA photophysics.…”