Chan Seok Oh

Molecular design of host materials for high power efficiency in blue phosphorescent organic light-emitting diodes doped with an imidazole ligand based triplet emitter

A molecular design strategy to facilitate electron injection and to reduce driving voltage was proposed to reach high power efficiency in blue phosphorescent organic light-emitting diodes (PHOLEDs) doped with a phenylimidazole ligand based triplet emitter. The host materials were designed to keep the hole transport properties and triplet energy of a parent molecule. One or two CN units were attached to 3- or 3,6-positions of carbazole of 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) to manage electron transport properties of the parent mCBP host. The CN unit donated electron accepting character to the mCBP host and increased electron current density in the device, which improved power efficiency of the blue PHOLEDs from 41.8 lm W−1 to 57.1 lm W−1.

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 of thermally activated delayed fluorescent emitters for blue-shifted emission by methoxy substitution

The molecular design method of thermally activated delayed fluorescent emitters for blue-shifted emission was developed by modifying a phenyl linker using a methoxy substituent. One methoxy or two methoxy groups were introduced into the phenyl linker connecting a diphenyltriazine acceptor and a dimethylacridine donor to manage the emission spectrum. Substitution of one methoxy group shifted the emission color to a short wavelength while preserving the external quantum efficiency of the pristine material without the methoxy substituent. A high external quantum efficiency of over 20% was obtained while blue-shifting the emission wavelength by 12 nm. However, the substitution of two methoxy groups decreased the quantum efficiency of the devices although the emission color was shifted to a short wavelength.

Rational molecular design overcoming the long delayed fluorescence lifetime and serious efficiency roll-off in blue thermally activated delayed fluorescent devices

Blue thermally activated delayed fluorescent (TADF) devices with short excited-state lifetime, high reverse intersystem crossing rate, and low-efficiency roll-off were developed by managing the molecular structure of donor-acceptor-type blue emitters. Three isomers of blue TADF emitters with a diphenyltriazine acceptor and three carbazole donors were synthesized. The position of the donor moieties in the phenyl linker connecting the donor and acceptor moieties was controlled to devise compounds with a short delayed fluorescence lifetime. A blue TADF emitter with three carbazole donors at 2-, 3-, and 4- positions of a phenyl linker shortened the excited state lifetime to 4.1 μs, showed a high external quantum efficiency of 20.4 %, and low efficiency roll-off of less than 10 % at 1000 cd m-2 . Therefore, a molecular design distorting the donors by aligning them in a consecutive way is useful to resolve the issues of long delayed fluorescence lifetime and efficiency roll-off of blue TADF devices.