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 triplet-to-singlet energy transfer mechanism is, in some cases, the main driver for the quantum efficiency boost observed in hyperfluorescent devices.

Toward highly efficient hyperfluorescence-based emitters through excited-states alignment using novel optimally tuned range-separated models

“…[101][102][103] Therefore, following previous suggestions in the field, we used the 6-31G(d,p) basis set for the optimal tuning procedures and subsequent computations on the B3LYP/6-31G(d,p) optimized geometries. 31,36,104 Besides the calculations in the gas phase using the models called ''OT-RSHs'', the solvent phase has also been considered in our work. For computations in the solution we have employed the polarizable continuum model (PCM) based on the integral equation formalism [105][106][107][108][109] using the models denominated as ''OT-RSHs-PCM'' in our discussion.…”

“…To this end, we have examined the FRET and DET in some pairs of FE/TADF in the hyperfluorescence mechanism. As illustrative examples, two pairs including the FEs and TADF sensitizers, which in turn have previously been investigated in the original experimental work 19 as well as in the recent theoretical studies in the field, 30,31,36 were considered. They are one pair including TBRb and PXZ-TRZ, and another pair composed of TBRb and PPZ-DPO, where for each pair the former plays the role of FE and the latter is the TADF sensitizer.…”

“…The hyperfluorescencebased devices also have advantages for operation lifetime because of the accelerated RISC reducing the triplet exciton lifetime in the emission process. [23][24][25][26][27][28][29][30][31] However, significant improvements are still required for widespread applications of the devices based on the hyperfluorescence mechanism. In particular, to prevent the generation of nonradiative triplet excitons on the FEs (T FE 1 ), the Dexter energy transfer (DET) 32 between the triplet excited-states of the TADF sensitizers and FEs should be largely excluded.…”