Soon Ok Jeon

External quantum efficiency above 20% in deep blue phosphorescent organic light-emitting diodes.

The development of high-effi ciency blue-light-emitting phosphorescent organic light-emitting diodes (PHOLEDs) is important in order to reduce the power consumption of organic light-emitting diodes (OLEDs) in display and lighting applications. [ 1–10 ] Iridium(III) bis(4,6-(difl uorophenyl)-pyridinatoN , C ′ ) picolinate (FIrpic) has typically been used as the dopant material for blue PHOLEDs, with a theoretical maximum quantum effi ciency of 20% already reported in sky blue PHOLEDs using various host and exciton blocking materials. [ 11–14 ] Although 20% external quantum effi ciency was achieved in sky blue PHOLEDs, it could not be achieved in deep blue PHOLEDs owing to the requirement of high triplet energy of the host and deep blue dopant material. Our group has reported a 19.2% maximum external quantum effi ciency in deep blue PHOLEDs, but the effi ciency could not be further improved. [ 8 ] Moreover, the quantum effi ciency dropped sharply at high luminance, and therefore high quantum effi ciency could not be achieved above 100 cd m − 2 . [ 2 ] In particular, the quantum effi ciencies at 1000 cd m − 2

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

Phosphine oxide derivatives for organic light emitting diodes

Recent developments concerning the use of phosphine oxide derivatives with various functional groups as host and ETMs in phosphorescent organic light-emitting diodes are reviewed here. Their strong electron withdrawing properties have allowed high external quantum efficiencies over 20% to be demonstrated in red, green and blue phosphorescent organic light-emitting diodes.

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 ]

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.

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∕◻.

Comparison of symmetric and asymmetric bipolar type high triplet energy host materials for deep blue phosphorescent organic light-emitting diodes

Symmetric and asymmetric bipolar host materials for deep blue phosphorescent organic light-emitting diodes were developed and the chemical structure of the host materials was correlated with the device performances. The bipolar host with the asymmetric molecular structure was better than the host with symmetric molecular structure in terms of driving voltage and quantum efficiency due to better charge transport properties. A high quantum efficiency of 24.5% and a high power efficiency of 31.0 lm W−1 were achieved in the deep blue device using the asymmetric bipolar host due to balanced charge transport and low driving voltage.

Highly efficient and color stable phosphorescent white light-emitting diodes by using a charge confining emitting layer structure

Quantum efficiency of white phosphorescent organic light-emitting diodes was improved by using a device architecture to confine charges inside an emitting layer. The charge confinement was achieved by stacking two emitting layers with different charge transport properties. An emitting layer with electron transport type host was stacked on an emitting layer with hole transport type host. A maximum quantum efficiency of 18.3% and a current efficiency of 31.5 cd/A were achieved with a color coordinate of (0.38,0.36). In addition, there was little change in the emission spectra from 200 to 10,000 cd/m2.

tert-Butylated spirofluorene derivatives with arylamine groups for highly efficient blue organic light emitting diodes

A series of tert-butylated spirofluorene derivatives incorporating a diphenylaminoaryl-vinyl group was synthesized via the Horner–Wadsworth–Emmons olefination and a Suzuki cross-coupling reaction. To examine the electroluminescent properties of these materials, multilayered OLEDs were fabricated into the following device structure: ITO/DNTPD/NPB/MADN:blue dopant materials 1–14/Alq3/Liq/Al. All devices showed efficient blue emission. In particular, one device exhibited highly efficient sky blue emission with a maximum luminance of 25 100 cd m−2 at 8.5 V, as well as luminous, power and external quantum efficiencies of 9.5 cd A−1, 5.1 lm W−1 and 6.7% at 20 mA cm−2, respectively. The peak wavelength of electroluminescence was 458 and 484 nm with CIEx,y coordinates of (0.14, 0.21) at 8.0 V. In addition, a deep blue device with CIEx,y coordinates of (0.15, 0.15) at 8.0 V showed a luminous efficiency and external quantum efficiency of 3.8 cd A−1 and 3.3% at 20 mA cm−2, respectively.

A phosphine oxide derivative as a universal electron transport material for organic light-emitting diodes

A phosphine oxide based organic material, 2-diphenylphosphine oxide-spiro[fluorene-7,11′-benzofluorene] (SPPO2), was developed as a universal electron transport layer with a thermal stability and stable morphology for organic light-emitting diodes (OLEDs). The SPPO2 was synthesized by the reaction of 2-bromo-spiro[fluorene-7,11′-benzofluorene] with diphenyl phosphine chloride. SPPO2 showed a high glass transition temperature (Tg) of 118 °C and good film-forming properties. The SPPO2 reduced the driving voltage of the OLEDs irrespective of the energy level of the host materials due to efficient electron injection from the SPPO2 to host material. In particular, the SPPO2 reduced the driving voltage of the blue device of a wide bandgap host material by more than 3 V. Therefore, the SPPO2 can be used as a universal electron transport layer for OLEDs.