Sang Kyu Jeon

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.

In-Situ Formed Type I Nanocrystalline Perovskite Film for Highly Efficient Light-Emitting Diode

Excellent color purity with a tunable band gap renders organic-inorganic halide perovskite highly capable of performing as light-emitting diodes (LEDs). Perovskite nanocrystals show a photoluminescence quantum yield exceeding 90%, which, however, decreases to lower than 20% upon formation of a thin film. The limited photoluminescence quantum yield of a perovskite thin film has been a formidable obstacle for development of highly efficient perovskite LEDs. Here, we report a method for highly luminescent MAPbBr3 (MA = CH3NH3) nanocrystals formed in situ in a thin film based on nonstoichiometric adduct and solvent-vacuum drying approaches. Excess MABr with respect to PbBr2 in precursor solution plays a critical role in inhibiting crystal growth of MAPbBr3, thereby forming nanocrystals and creating type I band alignment with core MAPbBr3 by embedding MAPbBr3 nanocrystals in the unreacted wider band gap MABr. A solvent-vacuum drying process was developed to preserve nanocrystals in the film, which realizes a fast photoluminescence lifetime of 3.9 ns along with negligible trapping processes. Based on a highly luminescent nanocrystalline MAPbBr3 thin film, a highly efficient green LED with a maximum external quantum efficiency of 8.21% and a current efficiency of 34.46 cd/A was demonstrated.

Long lifetime blue phosphorescent organic light-emitting diodes with an exciton blocking layer

An acridine derived compound, 9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine (PCZAC), was newly designed as a hole transport type high triplet energy material for application as a hole transport type exciton blocking layer of blue phosphorescent organic light-emitting diodes. The PCZAC compound provided a high triplet energy of 2.99 eV and a high glass transition temperature of 101 °C for high efficiency and long lifetime. PCZAC showed an improved quantum efficiency and more than 8 time improvement of the lifetime of blue phosphorescent organic light-emitting diodes.

Highly Efficient Soluble Blue Delayed Fluorescent and Hyperfluorescent Organic Light-Emitting Diodes by Host Engineering

Solution-processed high-efficiency fluorescent organic light-emitting diodes with an external quantum efficiency over 18% were developed by engineering a host material and device structure designed for solution process. A high triplet energy host material designed for the solution process, (oxybis(3-(tert-butyl)-6,1-phenylene))bis(diphenylphosphine oxide) (DPOBBPE), worked efficiently as the host of blue fluorescent devices because of good solubility, high photoluminescence quantum yield, and good film properties. The DPOBBPE host enabled a high external quantum efficiency of 18.8% in the fluorescent organic light-emitting diodes by the solution process. Moreover, 25.8% external quantum efficiency in the soluble blue thermally activated delayed fluorescent devices was also realized. The 25.8% external quantum efficiency of the DPOBBPE delayed fluorescent device and 18.8% external quantum efficiency of the fluorescent device are the highest efficiency values achieved in the solution-processed blue fluorescent organic light-emitting diodes. Moreover, the solution-processed fluorescent device showed an improved blue color coordinate of (0.14, 0.20) compared to (0.17, 0.31) of the delayed fluorescent device.

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.

Blue-shifted emission color and high quantum efficiency in solution-processed blue thermally activated delayed fluorescence organic light-emitting diodes using an intermolecular interaction suppressing host decorated with blocking groups

A high triplet energy material with the ability to suppress intermolecular interactions was synthesized as a host for a blue thermally activated delayed fluorescence (TADF) emitter for blue-shifted emission color and high quantum efficiency in solution-processed TADF organic light-emitting diodes. The host material, (5-(tert-butyl)-2-(4-(tert-butyl)phenoxy)phenyl)diphenylphosphine oxide (POBBPE), was designed to have a phenoxyphenyl core structure for high triplet energy, a diphenylphosphine oxide group for electron transport, and two tert-butyl units to prevent intermolecular interaction. Solution-processed blue TADF devices were developed using the POBBPE host and 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzCN) as the emitter, and a high quantum efficiency of 23.3% was achieved. In particular, the color coordinates of the solution-processed POBBPE:5CzCN devices were (0.16, 0.23), which were significantly blue-shifted compared to the color coordinates of (0.17, 0.31) from a previous 5CzCN device fabricated using a well-known bis[2-(diphenylphosphino)phenyl]ether oxide host.