Wook Song

Host Engineering for High Quantum Efficiency Blue and White Fluorescent Organic Light‐Emitting Diodes

High quantum efficiency in blue and white fluorescence organic light-emitting diodes is achieved by developing a novel device architecture with fluorescent emitters doped in a thermally activated delayed fluorescent emitter as a host material.

Light emission mechanism of mixed host organic light-emitting diodes

Light emission mechanism of organic light-emitting diodes with a mixed host emitting layer was studied using an exciplex type mixed host and an exciplex free mixed host. Monitoring of the current density and luminance of the two type mixed host devices revealed that the light emission process of the exciplex type mixed host was dominated by energy transfer, while the light emission of the exciplex free mixed host was controlled by charge trapping. Mixed host composition was also critical to the light emission mechanism, and the contribution of the energy transfer process was maximized at 50:50 mixed host composition. Therefore, it was possible to manage the light emission process of the mixed host devices by managing the mixed host composition.

High efficiency blue fluorescent organic light-emitting diodes using a conventional blue fluorescent emitter

High efficiency blue fluorescent organic light-emitting diodes (OLEDs) were developed by doping the conventional 2,5,8,11-tetra-tert-butylperylene (TBPe) blue fluorescent dopant in a mixed emitting layer of a wide bandgap host and a blue thermally activated delayed fluorescent emitter which played the role of a sensitizer in blue fluorescence emission. 10-(4-((4-(9H-Carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) was the blue thermally activated delayed fluorescent emitter of the emitting layer and bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) was the host material of the emitting layer. Doping of TBPe at a doping concentration of 0.1% in an emitting layer of DPEPO:CzAcSF achieved a high quantum efficiency of 18.1% in the blue fluorescent OLEDs, which is the best quantum efficiency reported in blue fluorescent OLEDs.

High-power-efficiency hybrid white organic light-emitting diodes with a single emitting layer doped with blue delayed fluorescent and yellow phosphorescent emitters

High-efficiency hybrid white organic light-emitting diodes (HWOLEDs) with a blue thermally activated delayed fluorescent (TADF) emitter and a yellow phosphorescent emitter doped in a single emitting layer were developed. Exciton harvesting by the blue TADF and yellow phosphorescent emitters rendered both singlet and triplet excitons to contribute to the white emission, which leads to a high quantum efficiency of 22.4% and a power efficiency of 60.3 lm W−1 in the HWOLEDs. In addition, the electroluminescence spectra of the HWOLEDs were kept stable from 100 cd m−2 to 5, 000 cd m−2.

Synthesis of pyrimidine-cored host materials bearing phenylcarbazole for efficient yellow phosphorescent devices: effect of linkage position

Two isomeric host materials were synthesized and their photophysical properties and device performances were investigated. In particular, para-substituted pPDPC exhibited the most intense efficiency regardless of the low triplet energy compared to that of meta-substituted mPDPC. The EQE of the pPDPC device reached as high as 22.3% with a yellow color coordinate of (0.49, 0.50).

High triplet energy exciplex hosts for deep blue phosphorescent organic light-emitting diodes

High triplet energy exciplex hosts for deep blue phosphorescent organic light-emitting diodes were developed by synthesizing a high triplet energy hole transport type host material designed for exciplex formation with a high triplet energy electron transport type host material derived from a diphenyltriazine. The highest occupied molecular orbital level of the hole transport type host materials was precisely controlled to manage the exciplex formation with the electron transport type host. Two exciplex hosts with high triplet energies of 2.94 and 2.95 eV were demonstrated using the synthesized hole transport type hosts and they were effective as the hosts of a deep blue triplet emitter.

A stepwise energy level doping structure for improving the lifetime of phosphorescent organic light-emitting diodes

A stepwise energy level doping structure for improving the lifetime of organic light-emitting diodes was developed by doping two emitters with different energy levels in the same host material as separated emitting layers. A hole-trapping phosphorescent emitter was doped in the emitting layer close to the electron transport layer and an electron-trapping thermally activated delayed fluorescent emitter was doped in the emitting layer close to the hole transport layer to show stepwise energy levels between emitting materials in the same host. The stepwise energy level doping structure improved the lifetime of organic light-emitting diodes through suppressed triplet–triplet annihilation and triplet–polaron annihilation by emission zone control and carrier confinement. Moreover, a high quantum efficiency close to 20% was realized in both green and blue organic light-emitting diodes in addition to the improved lifetime. This is the first work reporting both high quantum efficiency close to 20% and elongated lifetime simultaneously.

Electroplex as a New Concept of Universal Host for Improved Efficiency and Lifetime in Red, Yellow, Green, and Blue Phosphorescent Organic Light-Emitting Diodes

Abstract A new concept of host, electroplex host, is developed for high efficiency and long lifetime phosphorescent organic light‐emitting diodes by mixing two host materials generating an electroplex under an electric field. A carbazole‐type host and a triazine‐type host are selected as the host materials to form the electroplex host. The electroplex host is found to induce light emission through an energy transfer process rather than charge trapping, and universally improves the lifetime of red, yellow, green, and blue phosphorescent organic light‐emitting diodes by more than four times. Furthermore, the electroplex host shows much longer lifetime than a common exciplex host. This is the first demonstration of using the electroplex as the host of high efficiency and long lifetime phosphorescent organic light‐emitting diodes.

Various Applications of Blue Thermally Activated Delayed Fluorescent Emitters

We demonstrate the use of blue thermally activated delayed fluorescent emitters as emitters, sensitizers for fluorescent emission and host materials for fluorescent and phosphorescent organic light-emitting diodes.