Highly Efficient Phosphorescent Organic Light-Emitting Diodes Using Benzo[4',5']imidazo[2',1':2,3]imidazo[4,5,1-jk]carbazole as a Rigid and Effective Electron Acceptor in Bipolar Host

A novel bipolar host architecture was investigated to improve the external quantum efficiency (EQE) of green phosphorescent organic light-emitting diodes (PhOLEDs). The host was developed by incorporating carbazole as a hole-transport unit and fused rigid benzo[4',5']imidazo[2',1':2,3]imidazo[4, 5, 1-jk]carbazole (BzICz) as a new electron transport unit. The primary goal of the BzICz-based host design was to achieve a high triplet energy and bipolar charge transport characteristics. The green PhOLEDs fabricated using the new BzICz and carbazole-based host demonstrated a high EQE of 26.6% due to their high triplet energy and good bipolar charge transporting characteristics.

Novel V-Shaped Bipolar Host Materials for Solution-Processed Thermally Activated Delayed Fluorescence OLEDs

Three V-shaped host molecules with a cyclohexane linker were successfully synthesized for thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). The unipolar host molecules, BBCzC and BTDC, contained two 9-phenyl-9H-3,9'-bicarbazole (PBCz) moieties and two 2,12-di-tert-butyl-7-phenyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (PDBNA) moieties, respectively. BCzTC, a bipolar host molecule, consisted of a donor unit, PBCz, and an acceptor unit, PDBNA, connected by a cyclohexane linker. Three host molecules showed good solubility in various organic solvents, making them suitable for solution processing. Among the solution-processed green TADF-OLEDs using three host molecules and a green TADF emitter, the one with BCzTC showed the highest external quantum efficiency of up to 30% with a high power efficiency of 71 lm W^-1 and a current efficiency of 102 cd A^-1. Compared with BBCzC and BTDC, BCzTC exhibited a relatively high photoluminescence quantum yield (PLQY), an excellent balance in hole and electron transport properties in the emitting layer, and more efficient energy transfer to the emitter, giving such an excellent device performance.

Universal Bipolar Host Materials for Blue, Green, and Red Phosphorescent OLEDs with Excellent Efficiencies and Small-Efficiency Roll-Off

Host materials are indispensable for the fabrication of organic light-emitting diodes (OLEDs) with phosphorescent emitters, but high-quality host materials that can efficiently and simultaneously function in blue, green, and red phosphorescent OLEDs (PHOLEDs) are much rare. In this work, four bipolar materials are developed using carbazole and 9,9-dimethyl-9,10-dihydroacridine as hole-transporting groups, pyridine as electron-transporting groups, and biphenyl and m-methylbiphenyl as π-spacers. The crystal and electronic structures indicate that these materials have highly twisted conformations, which endow them with aggregation-induced emission features, intramolecular charge transfer processes, wide energy band gaps, and high triplet energies. The carrier transport ability and energy transfer property analyses show that these materials are able to achieve balanced hole and electron transports and can serve as bipolar host materials for PHOLEDs. A series of monochromatic PHOLEDs with different phosphorescent dopants, including blue-emissive FIrpic, green-emissive Ir(ppy)₂(acac), and red-emissive Ir(piq)₂(acac), are fabricated by employing these four host materials. The green PHOLEDs can provide an impressive luminance of up to 230 200 cd m^-2. Based on an identical host material, excellent external quantum efficiencies as high as 25.12, 24.73, and 19.71%, as well as minor efficiency roll-off, are attained for blue, green, and red PHOLEDs, respectively, clearly demonstrating the promising applications as universal bipolar host materials in PHOLEDs with monochromatic light and white light.

Enhancing the Kinetic Stability and Lifetime of Organic Light-Emitting Diodes based on Bipolar Hosts by using Spiroconjugation

We examined how to enhance the lifetime of organic light-emitting diodes (OLEDs) based on bipolar host molecules ET-HT, where ET and HT refer to electron- and hole-transporting units, respectively, by analyzing their thermodynamic and kinetic stabilities. Our DFT calculations reveal that the thermodynamic stability of ET-HT is determined by that of its anion, which is difficult to improve by chemical modifications of ET and HT. The kinetic stability of ET-HT can be enhanced by the spiroconjugation between ET and HT, which occurs when their π-frameworks are extended and have an orthogonal arrangement. Green OLED devices were fabricated by using ET-HTs with and without spiroconjugation, to find that the device with spiroconjugation has a lifetime that is approximately 6 times longer than the one without spiroconjugation.

New Benzimidazole-Based Bipolar Hosts: Highly Efficient Phosphorescent and Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes Employing the Same Device Structure

The rapid development of the thermally activated delayed fluorescence (TADF) emitters makes it necessary to produce new versatile host materials for both phosphorescent and TADF emitters. Three new bipolar host materials, 9,9'-(2'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (o-mCPBI), 9,9'-(3'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (m-mCPBI), and 9,9'-(4'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (p-mCPBI), are designed and synthesized by integrating mCP with benzimidazole moiety via the ortho-, meta-, and para-positions of N-phenyl. The influence of different linking modes on the thermal, photophysical, electrochemical, and charge transport properties of the compounds is studied. By employing the same device structure except the emitting layer, the device performances of the blue, green, yellow, red, white phosphorescent and blue, green TADF organic light-emitting diodes (OLEDs) based on the three hosts are investigated. Among these three hosts, o-mCPBI exhibits the best device performance with external quantum efficiencies of over 20% for phosphorescent OLEDs and enhanced efficiencies for TADF devices. All these devices show relatively low efficiency roll-offs at high brightness. The versatility of the benzimidazole-based bipolar host o-mCPBI, such as an extremely high triplet energy level, suitable molecular orbital energy levels, improved thermal and charge transport properties, and excellent device performances for various colors, makes it a universal host material for highly efficient both phosphorescent and TADF OLEDs.

Universal Host Materials for High-Efficiency Phosphorescent and Delayed-Fluorescence OLEDs

A series of bipolar hosts, namely, 5-(2-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (o-CzDPz), 5-(3-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (m-CzDPz), 5-(9-phenyl-9H-carbazol-3-yl)-1,3-dipyrazolbenzene (3-CzDPz), and 5-(3,5-di(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (mCPDPz), are developed for phosphorescent and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). They are designed by selecting pyrazole as n-type unit and carbazole as p-type one. The triplet energy (E(T)), the frontier molecular orbital level, and charge transporting abilities, are adjusted by varying the molar ratio of pyrazole to carbazole and the linking mode between them. They have high E(T) values of 2.76-3.02 eV. Their electroluminescence performance is evaluated by fabricating both phosphorescent and TADF devices with blue or green emitters. The m-CzDPz hosted blue phosphorescent OLEDs achieves high efficiency of 48.3 cd A(-1) (26.8%), the 3-CzDPz hosted green phosphorescent device exhibits 91.2 cd A(-1) (29.0%). The blue and green TADF devices with 3-CzDPz host also reach high efficiencies of 26.2 cd A(-1) (15.8%) and 41.1 cd A(-1) (13.3%), respectively. The excellent performance of all these OLEDs verifies that these pyrazole-based bipolar compounds are capable of being universal host materials for OLED application. The influence of molar ratio of n-type unit to p-type one and the molecular conformation of these hosts on their device performance is discussed and interpreted.

Multi-3,3'-Bicarbazole-Substituted Arylsilane Host Materials with Balanced Charge Transport for Highly Efficient Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes

A series of 3,3'-bicarbazole (mCP)-functionalized tetraphenylsilane derivatives (SimCPx), including bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2), tris(3,5-di(9H-carbazol-9-yl)phenyl)methylsilane (SimCP3-CH3), tris(3,5-di(9H-carbazol-9-yl)phenyl)phenylsilane (SimCP3-Ph), and tetrakis(3,5-di(9H-carbazol-9-yl)phenyl)silane (SimCP4), serving as bipolar blue hosts for bis[2-(4,6-difluorophenyl)pyridyl-N,C2']iridium(III) (FIrpic), have been synthesized by incorporating different ratios of mCP subunits into a central silicon atom. All of the SimCPx derivatives have wide bandgaps and high triplet energies because of the indirect linkage by silicon between each mCP subunit. The good solubility and high thermal and morphological stability of SimCPx are beneficial for forming amorphous and homogeneous films through solution processing. Density functional theory simulations manifest the better bipolar characteristics for SimCPx using three and four mCP units rather than the represented bipolar host SimCP2. As a result, SimCP4 presents the best electron-transporting ability for charge balance. Consequently, the lowest driving voltage of 4.8 eV, and the favorable maximum efficiencies of 14.2% for external quantum efficiency (28.4 cd A(-1), 13.5 lm W(-1)), are achieved by solution-processed, SimCP4-based blue phosphorescent organic light-emitting diodes as the highest performance among SimCPx, in which 32% improved device efficiencies compared to that of SimCP2 are obtained. It is inspiring to develop efficient bipolar hosts for blue phosphors by just incorporating monopolar carbazole into arylsilanes in two steps.

Above 30% external quantum efficiency in green delayed fluorescent organic light-emitting diodes

Highly efficient green thermally activated delayed fluorescent organic light-emitting diodes with an external quantum efficiency of 31.2% were investigated by using 3-(3-(carbazole-9-yl)phenyl) pyrido[3',2':4,5]furo[2,3-b]pyridine (3CzPFP) derived from carbazole and pyrido[3',2':4,5]furo[2,3-b]pyridine. The host material showed well-matched photoluminescence emission with absorption of the green dopant material, (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) and harvested all excitons of 4CzIPN. The 3CzPFP:4CzIPN film exhibited high photoluminescence quantum yield of 100%, and the green delayed fluorescence device employing the 3CzPFP host showed high maximum quantum efficiency of 31.2 ± 0.5% at 1% doping after optimization of the device structure.

Tuning Charge Balance in Solution-Processable Bipolar Triphenylamine-diazafluorene Host Materials for Phosphorescent Devices

Three bipolar hosts, namely TPA-DAF, TPA-DAF2, and TPA-DAF3, comprising an electron-donating triphenylamine (TPA) group and electron-accepting 4,5-diazafluorene (DAF) units are investigated for phosphorescent organic light-emitting diodes (PhOLEDs). Given the nonplanar structure of the sp(3)-hybridized C9 atom in DAF unit, these molecules have a highly nonplanar configuration, good film-forming property, and high triplet energy (ET) of 2.88-2.89 eV. Among them, TPA-DAF shows more balanced carrier injecting/transporting ability, suitable highest occupied molecular orbital (MO) energy level and higher current density, and therefore TPA-DAF-based devices exhibit the best performances, having an extremely slight efficiency roll-off with current efficiency of 20.0 cd/A at 973 cd/m(2), 19.5 cd/A at 5586 cd/m(2), and 17.6 cd/A at 9310 cd/m(2) for blue PhOLEDs; 23.5 cd/A at 1059 cd/m(2) and 15.3 cd/A at 8850 cd/m(2) for green PhOLEDs; and 12.2 cd/A at 1526 cd/m(2), 10.5 cd/A at 5995 cd/m(2), and 9.2 cd/A at 8882 cd/m(2) for red PhOLEDs, respectively. The results also provide a direct proof for the influence of charge balance on the device performance.

Bipolar host with multielectron transport benzimidazole units for low operating voltage and high power efficiency solution-processed phosphorescent OLEDs

Two soluble bipolar host materials (mCP-BPBI and CP-QPBI), comprising different proportions of hole-transporting carbazole and electron-transporting benzimidazole, were synthesized. Their thermal, physical, and electrochemical properties were characterized. The designated bulky star-shaped structures efficiently suppress the direct intramolecular interaction between the donor and acceptor subunits to give high triplet energies. Through computational studies, varying the ratio of hole- and electron-transporting moieties could significantly change the carrier injection/transporting abilities and charge balance properties of the host materials. Indeed, CP-QPBI with more benzimidazole units shows extremely enhanced current density at the same voltage when compared to mCP-BPBI. The operating voltage of solution-processed phosphorescent light-emitting diodes with CP-QPBI as host were dramatically reduced by ∼3 V compared with the similar devices of mCP-BPBI. At the same time, the power efficiencies were improved for 2-2.5 times at the corresponding voltage. Importantly, both blue and green devices maintain their high efficiencies even at brightness up to 1000 cd m(-2), which clearly demonstrates that the new strategy applied to improve electron-transporting ability and charge-balance property of the solution-processable host material by tuning the ratio of donor and acceptor unit is profitable.

New bipolar host materials for realizing blue phosphorescent organic light-emitting diodes with high efficiency at 1000 cd/m2

New host molecules such as 9-(6-(9H-carbazol-9-yl)pyridin-3-yl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (pPCB2CZ) and 9-(6-(9H-carbazol-9-yl)pyridin-2-yl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (mPCB2CZ) were designed and synthesized for blue phosphorescent organic light-emitting diodes (PhOLEDs). The glass transition temperatures of two host molecules were measured higher than 120 °C, and the identical triplet energies were determined to be 2.92 eV for both molecules. The bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic)-doped mPCB2CZ-based PhOLED exhibited practically useful driving voltage of 4.8 V in a simple organic three layer device configuration which has a smaller number of interfaces in conventional multilayer PhOLEDs. Also, the high quantum efficiency of 23.7% is reported at the practically useful brightness value of 1000 cd/m2.