Mounggon Kim

Design Strategy for 25% External Quantum Efficiency in Green and Blue Thermally Activated Delayed Fluorescent Devices

Carbazole- and triazine-derived thermally activated delayed fluorescent (TADF) emitters, with three donor units and an even distribution of the highest occupied molecular orbital, achieve high external quantum efficiencies of above 25% in blue and green TADF devices.

Room-Temperature-Phosphorescence-Based Dissolved Oxygen Detection by Core-Shell Polymer Nanoparticles Containing Metal-Free Organic Phosphors

The highly sensitive optical detection of oxygen including dissolved oxygen (DO) is of great interest in various applications. We devised a novel room-temperature-phosphorescence (RTP)-based oxygen detection platform by constructing core-shell nanoparticles with water-soluble polymethyloxazoline shells and oxygen-permeable polystyrene cores crosslinked with metal-free purely organic phosphors. The resulting nanoparticles show a very high sensitivity for DO with a limit of detection (LOD) of 60 nm and can be readily used for oxygen quantification in aqueous environments as well as the gaseous phase.

Triplet exciton recycling of a phosphorescent emitter by an up-conversion process using a delayed fluorescence type low triplet energy host material

A low triplet energy host material with thermally activated delayed fluorescence character was applied as the host material of a blue phosphorescent emitter and was compared with a high triplet energy host material. The low triplet energy host material had triplet energy lower than the triplet energy of the triplet emitter and singlet energy higher than the triplet energy of the triplet emitter. The low triplet energy host material performed as well as a high triplet energy host material in terms of quantum efficiency by recycling triplet excitons of the phosphorescent emitter via an up-conversion process, but performed better than high triplet energy host materials in terms of driving voltage. Therefore, the thermally activated delayed fluorescence type low triplet energy host material increased the power efficiency of the devices. An energy transfer process from the low triplet energy host to the triplet emitter through the conversion of triplet excitons into singlet excitons was proposed and confirmed as the mechanism of efficient light emission.

Molecular design approach of increasing the triplet energy of host materials using pyrrole as a core structure

We developed and synthesized 9,9′-(1-phenyl-1H-pyrrole-2,5-diyl)bis(9H-carbazole) (PPyCz2) as a hole transport type high triplet energy host material for blue phosphorescent organic light emitting diodes. We approached the molecular design by inserting a pyrrole moiety to prevent extension of conjugation of molecule and achieved a high triplet energy of 2.99 eV in the PPyCz2 host. Furthermore, the PPyCz2 host showed a high external quantum efficiency of 14.8% in blue phosphorescent organic light emitting diodes.