Probing charge carrier and triplet dynamics in TADF-based OLEDs using transient electroluminescence studies

Abstract

Thermally activated delayed fluorescence (TADF) systems exhibit high emissive yield due to efficient back-conversion of nonemissive triplet states to emissive singlet states via reverse intersystem crossing (RISC). In this paper, both the charge carrier and triplet exciton dynamics are explored using transient electroluminescence (TrEL) measurements in the TADF molecule, 2,3,4,6-Tetra(9H-carbazol-9-yl)-5-fluorobenzonitrile (4CzFCN)-based devices. The analysis of the rising edge of the TrEL pulse indicates that the carriers follow multiple trapping, de-trapping, and exciton recombination dynamics. The trailing edge of the TrEL pulse provides insight into the monomolecular and bimolecular exciton dynamics. These studies along with a kinetic model reveal triplet harvesting processes in a 4CzFCN molecule via both RISC and triplet–triplet annihilation (TTA). Furthermore, at high temperatures, the analysis suggests that TADF processes are dominant with negligible contribution from TTA. The presence of bimolecular triplet processes acts as bottlenecks for accessing higher efficiencies in TADF organic light emitting diodes.