Kyoung Soo Yook

Organic Materials for Deep Blue Phosphorescent Organic Light‐Emitting Diodes

Recently, great progress has been made in the device performance of deep blue phosphorescent organic light-emitting diodes (PHOLEDs) by developing high triplet energy charge-transport materials, high triplet energy host and deep blue emitting phosphorescent dopant materials. A high quantum efficiency of over 25% and a high power efficiency of over 15 lm/W have already been achieved at 1000 cd m(-2) in the deep blue PHOLEDs with a y color coordinate less than 0.20. In this work, recent developments in organic materials for high efficiency deep blue PHOLEDs are reviewed and a future strategy for the development of high efficiency deep blue PHOLEDs is proposed.

Small Molecule Host Materials for Solution Processed Phosphorescent Organic Light-Emitting Diodes

Solution processed phosphorescent organic light-emitting diodes (OLEDs) have been actively developed due to merits of high quantum efficiency of phosphorescent materials and simple fabrication processes of solution processed OLEDs. The device performances of the solution processed phosphorescent OLEDs have been greatly improved in the last 10 years and the progress of the device performances was made by the development of small molecule host materials for solution processes. A hybrid host of polymer and small molecules, a single small molecule host and a mixed host of small molecule hosts have effectively enhanced the quantum efficiency of the solution processed phosphorescent OLEDs. Therefore, this paper reviews recent developments in small molecule host materials for solution processed phosphorescent OLEDs and provides future directions for the development of the small molecule host materials.

High‐Efficiency Deep‐Blue‐Phosphorescent Organic Light‐Emitting Diodes Using a Phosphine Oxide and a Phosphine Sulfide High‐Triplet‐Energy Host Material with Bipolar Charge‐Transport Properties

2010 WILEY-VCH Verlag Gmb Blue-phosphorescent organic light-emitting diodes (PHOLEDs) have been developed for more than 10 years towards use in active-matrix-type organic light-emitting diodes. There has been much improvement in quantum efficiency, lifetime, and color purity although the device performances of the blue PHOLEDs are not yet good enough for practical applications. Most research into blue PHOLEDs was focused on the development of new host and dopant materials. The best-known host material in the blue PHOLEDs is N,N-dicarbazolyl-3,5-benzene (mCP). It has good hole-transport properties due to a carbazole unit in the backbone structure and a wide triplet bandgap of 2.90 eV for efficient energy transfer. However, its electron injection and transport properties are poor because of the high energy of the lowest unoccupied molecular orbital (LUMO) of 2.4 eV. Silicone-based wide-triplet-bandgap host materials were also developed and tetraaryl-based silane materials have been used as host materials in blue PHOLEDs. However, the energy of the highest occupied molecular orbital (HOMO) of the silane-based host materials is around 7.0 eV, which is not suitable for hole injection. Therefore, it was difficult to balance holes and electrons in the light-emitting layer. To overcome the poor hole injection in the silane-based host materials, silane compounds with a carbazole moiety in the molecular structure were evaluated as triplet host materials in blue PHOLEDs. However, the carbazole-based host materials showed strong hole-transport properties and bipolar transport behavior was not observed. In addition, phosphine oxide-type host materials were synthesized, but only sky-blue PHOLEDs were reported due to the low triplet energy. Our group also reported phosphine oxide-type host materials with a carbazole moiety in the backbone structure and high efficiency could be obtained. Although several classes of host materials have been synthesized, no host materials could show a theoretical maximum quantum efficiency in the deep-blue PHOLED with Commission International De L’Eclairage (CIE) color coordinate (xþ y) values below 0.30. In this work, we synthesized bipolar-type high-triplet-energy host materials with a carbazole core structure. Phosphine oxide (PPO21) and phosphine sulfide (PPS21) host materials with the carbazole core structure were synthesized and evaluated as host materials in the deep-blue PHOLEDs. A theoretical maximum quantum efficiency over 19% with a deep-blue CIE coordinate of (0.14,0.16) was demonstrated in the deep-blue PHOLEDs using the high-triplet-energy host materials for the first time. The host materials synthesized in this work have a 9-phenylcarbazole core structure with two phosphine oxide or phosphine sulfide units. One diphenylphosphine oxide or sulfide unit was attached to the 3-position of the carbazole unit to control the HOMO level and the charge transport properties. The other diphenylphosphine oxide or sulfide unit was connected to the phenyl group of the 9-phenylcarbazole to manage the electron-transport properties. The host materials were synthesized by the coupling reaction of the chlorodiphenylphosphine with 3-bromo-9-(4-bromophenyl)carbazole using n-butyllithium followed by oxidation and sulfonation (Scheme 1). The product was purified by a column chromatography and it was confirmed with H NMR spectroscopy, differential scanning calorimetry (DSC), high performance liquid chromatography (HPLC) andmass spectrometry (MS). The purity of the host materials was over 99% from HPLC. Physical properties of the host materials are summarized in Table 1.. The two high-triplet-energy host materials showed high glass-transition temperature (Tg) above 110 8C due to the two rigid diphenylphosphine oxide or sulfide groups. The significantly higher Tg of the PPS21 compared to PPO21 is due to large atom size of the sulfur. The two diphenylphosphine oxide or sulfide units also stabilized the amorphous morphology of the host materials, and a smooth surface roughness less than 1 nm was obtained from the evaporated film. The surface morphology of the evaporated films was kept stable even after thermal treatment at 80 8C for 1 h because of the rigid molecular structure and corresponding high Tg. The HOMO and LUMO levels of the host materials were measured using cyclic voltametry (CV), and they are summarized in Table 1. TheHOMO/LUMO levels of the PPO21 and PPS21 are mainly determined by the carbazole backbone structure, and the diphenylphosphine oxide or sulfide groups shift the HOMO/ LUMO levels through the control of the electron density in the carbazole core. TheHOMO level of the PPO21 was 6.25 eV, which corresponds to a change of 0.37 eV compared with that of the phenylcarbazole moiety without any substituent, 5.88 eV. The LUMO level (2.68 eV) of the PPO21 was also shifted by 0.30 eV by the diphenylphosphine oxide unit. The electron withdrawing

High efficiency in a solution-processed thermally activated delayed-fluorescence device using a delayed-fluorescence emitting material with improved solubility.

High quantum efficiency above 18% in a solution-processed thermally activated delayed-fluorescence device is achieved by modifying a common delayed-fluorescence emitter with a tert-butyl substituent.

Fabrication and Efficiency Improvement of Soluble Blue Phosphorescent Organic Light‐Emitting Diodes Using a Multilayer Structure Based on an Alcohol‐Soluble Blue Phosphorescent Emitting Layer

The development of soluble phosphorescent organic lightemitting diodes (PHOLEDs) is important because the low effi ciency of soluble fl uorescent organic light-emitting diodes (OLEDs) can be improved by using soluble PHOLEDs instead. [ 1–3 ] In particular, the effi ciency of soluble blue OLEDs is quite low compared to red and green OLEDs, and the development of high-effi ciency soluble blue PHOLEDs is strongly required. A great deal of research has been focused on enhancing the effi ciency of soluble blue PHOLEDs using various device architectures and emitting materials. [ 4–14 ] The most effi cient method of improving the quantum effi ciency of soluble blue PHOLEDs was to use a mixed host emitting layer with hole and electron type host materials. Typically, poly(N-vinylcarbazole) (PVK) has been used as the hole transport type host material and 1,3-bis[(4-tertbutylphenyl)-1,3,4-oxadiazolyl]phenylene has been applied as the electron transport type host material. [ 6–14 ] A high effi ciency of 22 cd/A can be achieved in a solution-processed blue PHOLED because of the charge balance in the emitting layer. [ 9 ] Fluorinated PVK is better than common PVK, and 27 cd/A is the best effi ciency that has been reported. [ 11 ] PVK can also be blended with a phosphine oxide based electron transport type host material in order to enhance the effi ciency compared to the PVK only device. [ 15 ] Soluble small-molecule host-based blue PHOLEDs have also been developed with a current effi ciency of 12.7 cd/A. However, the effi ciency of solution-processed blue PHOLEDs cannot be further improved because of the limitations of the multilayer structure fabrication even though a crosslinkable hole transport material has been applied in these solutionprocessed PHOLEDs. [ 6 ] A high triplet energy hole transport layer is critical to the quantum effi ciency of blue PHOLEDs, [ 17 ]

Stable efficiency roll-off in phosphorescent organic light-emitting diodes

Origin of efficiency roll-off in phosphorescent organic light-emitting diodes was investigated with triplet mixed host devices and stable devices with little efficiency roll-off was developed. Efficiency roll-off was significant in the device with narrow recombination zone (RZ) and charge leakage out of emitting layer at high luminance was critical to efficiency roll-off. Efficiency roll-off could be reduced in triplet mixed host device with broad RZ and little charge leakage at high driving voltage. Triplet mixed host devices with an exciton blocking layer showed a quantum efficiency over 90% of maximum quantum efficiency at a luminance of 20000cd∕m2.

High efficiency blue phosphorescent organic light emitting diodes using a simple device structure

High efficiency blue phosphorescent organic light emitting diodes have been developed by using a simple device structure. A derivative of spirobifluorene based phosphine oxide was used both as a host and an electron transport layer with an exciton blocking function. A maximum quantum efficiency of 19.2% and a current efficiency of 37.2cd∕A were obtained by using a simple device structure without a hole blocking layer.

Cool and warm hybrid white organic light-emitting diode with blue delayed fluorescent emitter both as blue emitter and triplet host

A hybrid white organic light-emitting diode (WOLED) with an external quantum efficiency above 20% was developed using a new blue thermally activated delayed fluorescent material, 4,6-di(9H-carbazol-9-yl)isophthalonitrile (DCzIPN), both as a blue emitter and a host for a yellow phosphorescent emitter. DCzIPN showed high quantum efficiency of 16.4% as a blue emitter and 24.9% as a host for a yellow phosphorescent emitter. The hybrid WOLEDs with the DCzIPN host based yellow emitting layer sandwiched between DCzIPN emitter based blue emitting layers exhibited high external quantum efficiency of 22.9% with a warm white color coordinate of (0.39, 0.43) and quantum efficiency of 21.0% with a cool white color coordinate of (0.31, 0.33) by managing the thickness of the yellow emitting layer.

Transparent organic light emitting diodes using a multilayer oxide as a low resistance transparent cathode

Transparent organic light emitting diodes were developed by using a thermally evaporable WO3∕Ag∕WO3 (WAW) as a transparent cathode. A thin Ag layer was introduced as an interlayer between the Li doped electron transport layer and the WAW electrode. A high transparency over 80% was obtained and electron injection was greatly improved by using the thin Ag interlayer between the Li doped layer and the WAW electrode. The driving voltage at 1000cd∕m2 was only 4.5V and the sheet resistance of the WAW electrode was as low as 12Ω∕◻.

100% internal quantum efficiency and stable efficiency roll-off in phosphorescent light-emitting diodes using a high triplet energy hole transport material

Highly efficient green phosphorescent organic light-emitting diodes were developed by using high triplet energy hole transport materials. The quantum efficiency of green devices could be improved from 12% to 20% at 1000cd∕m2 by using a phenylcarbazole based hole transport material. In addition, the high quantum efficiency could be stably maintained up to a high luminance of 10000cd∕m2.