Green Phosphorescent Organic Light-emitting Diodes Research Articles

Colour stability of Blue–Green and white phosphorescent organic light-emitting diode employing a 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole host

An ambipolar compound, 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole, is employed as a universal single host for efficient sky-blue (B), green (G), orange–red (R), BG, and white (W) phosphorescent organic light-emitting diodes (PhOLEDs) with an external quantum efficiency (ηEQE) of more than 20%. By integrating B and G emitters into the emitting layer (EML), an efficient BG PhOLED exhibits a maximum current efficiency (ηC) of 81.7 cd/A, power efficiency (ηP) of 73.2 lm/W, and ηEQE of 23.1%. Another BG PhOLED exhibits excellent colour stability with a slight shift (Δx of 0.0006, Δy of 0.0014) in CIE1931 colour coordinates while its luminance varies from 3.5 to 38360 cd/m2. Furthermore, a 1 nm R EML is employed as a probe inserted at three distinct positions to investigate the main recombination zone; the main recombination zone is detected near the interface between B/G EML. At the same time, a W spectrum combines individual R, G, B emission peaks. Besides, a W PhOLED simply comprises of a B and R EML, exhibiting good colour stability (Δx of 0.0004, Δy of 0.0023 between 4 and 12 V) and high efficiency (a maximum ηC, ηP, ηEQE of 58.4 cd/A, 50.7 lm/W, 21.9%). In addition, transient electroluminance signals described green emitters have less trapped charges and a longer exciton lifetime which benefit the device lifetime.

Triphenyl phosphine oxide and carbazole-based polymer host materials for green phosphorescent organic light-emitting diodes

Four novel polymers, poly(3,6-9-decyl-carbazole-alt-1,3-benzene) (PB13CZ), poly(3,6-9-decyl-carbazole-alt-bis(4-phenyl) (phenyl) phosphine oxide) (PTPPO38CZ), poly(3,6-9-decyl-carbazole-alt-2,4-phenyl(diphenyl) phosphine oxide) (PTPPO13CZ) and poly(3,6-9-decyl-carbazole-alt-bis(3-phenyl) (phenyl) phosphine oxide) (PTTPO27CZ) were synthesized, and their thermal, photophysical properties and device applications were further investigated to correlate the chemical structures with the photoelectric performance of bipolar host materials for phosphorescent organic light emitting diodes. All of them show high thermal stability as revealed by their high glass transition temperatures and thermal decomposition temperatures at 5% weight loss. These polymers have wide band gaps and relatively high triplet energy levels. As a result, the spin coating method was used to prepare the green phosphorescent organic light emitting diodes with polymers PTPPO38CZ, PTPPO13CZ and PTTPO27CZ as the typical host materials. The green device of polymer PTPPO38CZ as host material shows electroluminescent performance with maximum current efficiency of 2.16 cd·A−1, maximum external quantum efficiency of 0.7%, maximum brightness of 1475 cd·m−2 and reduced efficiency roll-off of 7.14% at 600 cd·m−2, which are much better than those of the same devices hosted by polymers PTTPO27CZ and PTPPO13CZ.

Green phosphorescent organic light-emitting devices based on different electron transport layers combining with fluorescent sub-monolayer

We report a small molecule host of 4,4(-N,N)-dicarbazole-biphenyl (CBP) doped with 8% tris(2-phenylpyridine) iridium (Irppy3) for use in efficient green phosphorescent organic light-emitting devices (PHOLEDs) combined with different electron transport layers of Alq and BAlq. The PHOLEDs exhibit maximum current efficiency and power efficiency of 19.8 cd/A and 6.21 lm/W, respectively. The high performance of such PHOLEDs is attributed to the better electron mobile ability of BAlq and sub-monolayer quinacridone (QAD) as carrier trapping layer and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, which can reduce the triplet-triplet annihilation and improve the efficiency of the device.

Electron-Rich 4-Substituted Spirobifluorenes: Toward a New Family of High Triplet Energy Host Materials for High-Efficiency Green and Sky Blue Phosphorescent OLEDs.

We report herein a detailed structure-properties relationship study of the first examples of electron-rich 4-substituted spirobifluorenes for organic electronic applications, namely, 4-phenyl-N-carbazole-spirobifluorene (4-PhCz-SBF) and 4-(3,4,5-trimethoxyphenyl)-spirobifluorene (4-Ph(OMe)3-SBF). The incorporation of the electron-rich moieties in the ortho position of the biphenyl linkage (position C4) induces unique properties, very different from those previously described in the literature for this family of semiconductors. Both dyes can be readily synthesized, possess high triplet energies and excellent thermal stability, and their HOMO energy levels are highly increased compared to those of other 4-substituted SBFs. We also provide in this work the first rationalization of the peculiar fluorescence of 4-substituted SBFs. Finally, the present dyes have been successfully incorporated as host in green and blue phosphorescent organic light-emitting diodes with high performance either for the green (EQE of 20.2%) or the blue color (EQE of 9.6%). These performances are, to the best of our knowledge, among the highest reported to date for 4-substituted SBF derivatives.

Elimination of Plasmon Losses and Enhanced Light Extraction of Top-Emitting Organic Light-Emitting Devices Using a Reflective Subelectrode Grid

We demonstrate high-efficiency light extraction for top-emitting organic light-emitting devices (OLEDs) comprising a transparent conductive oxide on the surface of a nondiffractive, reflecting metal-coated scattering grid located beneath the organic active region. The grid scatters light trapped in waveguide modes without changing the device electrical properties or causing significant plasmonic losses. This results in an increase in external quantum efficiency for green phosphorescent organic light-emitting devices from 20 ± 1% to 30 ± 2%, for structures without and with the reflecting grid. Adding a low refractive index capping layer reduces the spectral angular dependence characteristic of top-emitting OLEDs. The improvement in light extraction by substrate modification allows for optimization of the optical design without necessitating changes in the design or structure of the OLEDs themselves.

Highly efficient green, blue, and white phosphorescent inverted organic light-emitting diodes by improving charge injection and balance

Highly efficient green, blue, and white phosphorescent inverted organic light-emitting diodes were demonstrated by improving charge injection and balance.

Synthesis and characterization of green-emitting Ir(iii) complexes based on a functionalized benzimidazole ligand

Two new Ir(iii) complexes containing a functionalized benzimidazole ligand (L) were successfully synthesized for use in green phosphorescent organic light-emitting diodes.

Stable green phosphorescence organic light-emitting diodes with low efficiency roll-off using a novel bipolar thermally activated delayed fluorescence material as host.

A novel bipolar hosting material, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), was designed, synthesized, and characterized for green phosphorescent organic light-emitting diodes (PhOLEDs). The DPDDC exhibits excellent hole and electron transport properties, superior thermal stability, a high glass-transition temperature and a small singlet-triplet energy gap for efficient reverse intersystem crossing from triplet to singlet, reducing the triplet density of the host for PhOLEDs. The electrophosphorescence properties of the devices using DPDDC as the host and three green phosphorescent iridium(iii) complexes, bis(2-(4-tolyl)pyridinato-N,C2')iridium(iii) acetylacetonate, bis(2-phenylpyridine)iridium(iii) acetylacetonate, and bis(4-methyl-2,5-diphenylpyridine)iridium(iii) acetylacetonate [(mdppy)2Iracac] as the emitter were investigated. The green PhOLED with 5 wt% (mdppy)2Iracac presents an excellent performance, including a high power efficiency of 92.3 lm W-1, high external quantum efficiency of 23.6%, current efficiency roll-off as low as 5.5% at 5000 cd m-2 and a twentyfold lifetime improvement (time to 90% of the 5000 cd m-2 initial luminance) over the reference electrophosphorescent device.

Highly efficient triazine/carbazole-based host material for green phosphorescent organic light-emitting diodes with low efficiency roll-off

Two novel triazin/carbazole-based host materials were designed and synthesized, which demonstrated outstanding EL performance with maximum CE, PE and EQE of 69.3 cd A−1, 54.2 lm W−1 and 21.9%, respectively.

Pyridine linked fluorene hybrid bipolar host for blue, green, and orange phosphorescent organic light-emitting diodes toward solution processing

Balanced charge transportation is achieved in a series of bipolar host materials based on a pyridine hybrid fluorene host containing a triphenylamine or carbazole active group.

High efficiency and low roll-off green OLEDs with simple structure by utilizing thermally activated delayed fluorescence material as the universal host

Abstract We achieved high-efficiency and low-roll-off green fluorescent and phosphorescent organic light-emitting diodes (OLEDs) simultaneously by adopting the thermally activated delayed fluorescence material of bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone as the universal host. At a luminance of 1000 cd/m2, fluorescent OLEDs based on C545T get a current efficiency, power efficiency, and external quantum efficiency (EQE) of 31.8 cd/A, 25.0 lm/W, and 9.26%, respectively. This is almost the highest efficiency based on C545T at the luminance of 1000 cd/m2 to date. On the other hand, phosphorescent OLEDs with Ir(ppy)3 as the emitter realize a maximum current efficiency, power efficiency, and EQE of 64.3 cd/A, 62.4 lm/W, and 18.5%, respectively. More important, the EQE remains 17.8% at the representative luminance of 1000 cd/m2 and the roll-off ratio is just 3.78%. The transient photoluminescence decay measurement demonstrates that the up-conversion of host triplet excitons plays a key role in the high efficiency and low roll-off. More detailed discussions are also given.

Novel spirofluorene/indole/carbazole-based hole transport materials with high triplet energy for efficient green phosphorescent organic light-emitting diodes

Three novel host materials 9-(4-(5-(4-(9-phenyl-9H-fluoren-9-yl)phenyl)-1H-indol-1-yl)phenyl)-9H-carbazole (C1), 9-(4-(9,9-diphenyl-9H-fluoren-4-yl)-1H-indol-1-yl)-9-phenyl-9H-carbazole (C2) and 9-(4-(5-(9,9-diphenyl-9H-fluoren-2-yl)-1H-indol-1-yl)phenyl)-9H-carbazole (C3) based on spirofluorene, indole and carbazole derivatives have been designed and synthesized. Their thermal, photophysical and electrochemical properties were fully investigated. The OLED devices with C1–C3 as host were fabricated to investigate their electroluminescent properties. Compound C3 obtained the best device performances with the maximum current efficiency of 45.2 cd A−1 and the maximum power efficiency of 23.2 lm W−1 for green phosphorescent organic light-emitting diodes.

Bipolar Host Materials with Carbazole and Isoquinoline/Benzothiazole Moieties for Green Phosphorescent Organic Light-Emitting Diodes

Bipolar Host Materials with Carbazole and Isoquinoline/Benzothiazole Moieties for Green Phosphorescent Organic Light-Emitting Diodes

Modulation of the Physicochemical Properties of Donor-Spiro-Acceptor Derivatives through Donor Unit Planarisation: Phenylacridine versus Indoloacridine-New Hosts for Green and Blue Phosphorescent Organic Light-Emitting Diodes (PhOLEDs).

This work reports a detailed structure-property relationship study of a series of efficient host materials based on the donor-spiro-acceptor (D-spiro-A) design for green and sky-blue phosphorescent organic light-emitting diodes (PhOLEDs). The electronic and physical effects of the indoloacridine (IA) fragment connected through a spiro bridge to different acceptor units, namely, fluorene, dioxothioxanthene or diazafluorene moiety, have been investigated in depth. The resulting host materials have been easily synthesised through short, efficient, low-cost, and highly adaptable synthetic routes by using common intermediates. The dyes possess a very high triplet energy (ET ) and tuneable HOMO/LUMO levels, depending on the strength of the donor/acceptor combination. The peculiar electrochemical and optical properties of the IA moiety have been investigated though a fine comparison with their phenylacridine counterparts to study the influence of planarisation. Finally, these molecules have been incorporated as hosts in green and sky-blue PhOLEDs. For the derivative SIA-TXO2 as a host, external quantum efficiencies as high as 23 and 14 % have been obtained for green and sky-blue PhOLEDs, respectively.

Quinolyl functionalized spiro[fluorene-9,9′-xanthene] host materials with bipolar characteristics for green and red phosphorescent organic light-emitting diodes

Spiro[fluorene-9,9′-xanthene] (SFX) bipolar hosts bearing one, two and three quinolyl substituents, namely SFX-bPy, SFX-DbPy and SFX-TbPy, were designed and synthesized for phosphorescent organic light emitting diodes (PhOLEDs). The successive substitution of quinoline at 2′, 2 and 7′ positions of SFX results in reduced LUMO energy levels while leaving the HOMO energy levels nearly intact. The impact of quinoline substitution in these SFX-based hosts on PhOLED performance was investigated in detail through green and red model devices. For the green emitting devices, the device based on SFX-bPy host showed better performance (23.6 cd A−1, 23.4 lm W−1, 6.3%) due to high triplet energy level (T1) and balanced carriers-transporting ability. In contrast, for the red PhOLED devices, the device hosted by SFX-DbPy displayed higher performance (15.8 cd A−1, 16.0 lm W−1, 9.1%), attributable to the well matched T1 and separated frontier molecular orbitals. This work thus sheds light on the rational design of SFX-based bipolar hosts for more efficient PhOLEDs.

1,2,4-Triazole derivatives as host materials for blue and green phosphorescent organic light-emitting devices

A novel host material, namely 9-(6-(3-phenyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)hexyl)-9H-carbazole (PPHCZ) was designed and synthesized by incorporating 1, 2, 4-triazol as the electron-withdrawing moiety and carbazole as electron-donating moiety using hexyl as bridge. The photophysical, electrochemical, thermal properties and performances in phosphorescent organic light-emitting devices (PhOLED) were thoroughly characterized. PPHCZ with a high triplet energy level (3.01 eV) exhibited an obvious intermolecular interaction in thin film. Green and blue PhOLEDs using fac-tris(2-phenylpyridine)iridium and iridium(III) bis(4,6-difluorophenyl)-pyridinato N,C2′]picolinate as phosphorescent dopants have been fabricated.

38-4: The Interfacial Effect between HTL and EML on the Efficiency of Solution Processed Green Phosphorescent OLEDs

We have fabricated highly efficient green phosphorescent organic light-emitting diodes by solution process. Very interestingly, we found that the device efficiency is affected by interface between hole transport layer (HTL) and emission layer (EML). It was proved through spin cleaning process of cross-linkable HTL after curing which may affect the interface mixing. In addition, we tried to add complement layer to manipulate such effects. As a result, we significantly improved device efficiency (up to 72.7 cd/A) after spin cleaning process of the cross-linkable HTL material.

Promising Operational Stability of Potentially High Power Efficiency Organic Light‐Emitting Diodes Utilizing a Simple and Versatile Electron‐Transport/Hole‐Blocking Layer

An electron-transport/hole blocking material Phen-m-PhDPO (3-(m-diphenyl phosphinyl phenyl)-1,10-phenanthroline) is utilized for red, green phosphorescent and blue fluorescent organic light-emitting diodes to show potentially high power efficiencies and operational stability, despite a low electron mobility of 6.34 × 10−8–1.58 × 10−6 cm2 V−1 s−1 at E = 2–5 × 105 V cm−1.

Versatile p-Type Chemical Doping to Achieve Ideal Flexible Graphene Electrodes.

We report effective solution-processed chemical p-type doping of graphene using trifluoromethanesulfonic acid (CF3 SO3 H, TFMS), that can provide essential requirements to approach an ideal flexible graphene anode for practical applications: i) high optical transmittance, ii) low sheet resistance (70 % decrease), iii) high work function (0.83 eV increase), iv) smooth surface, and iv) air-stability at the same time. The TFMS-doped graphene formed nearly ohmic contact with a conventional organic hole transporting layer, and a green phosphorescent organic light-emitting diode with the TFMS-doped graphene anode showed lower operating voltage, and higher device efficiencies (104.1 cd A(-1) , 80.7 lm W(-1) ) than those with conventional ITO (84.8 cd A(-1) , 73.8 lm W(-1) ).

Chlorinated indium tin oxide electrode by InCl3 aqueous solution for high-performance organic light-emitting diodes

The authors develop a facile and effective method to produce the chlorinated indium tin oxide (Cl-ITO) treated by InCl3 aqueous solution and UV/ozone. The work function of the Cl-ITO achieved by this treatment is as high as 5.69 eV, which is increased by 1.09 eV compared with that of the regular ITO without any treatment. Further investigation proved that the enhancement of the work function is attributed to the formation of In-Cl bonds on the Cl-ITO surface. Green phosphorescent organic light-emitting devices based on the Cl-ITO electrodes exhibit excellent electroluminescence performance, elongating lifetime due to the improvement in hole injection.