Bipolar Host Research Articles

Universal polymeric bipolar hosts for highly efficient solution-processable blue and green thermally activated delayed fluorescence OLEDs

Two novel polymeric host materials, P(NmCP) and P(mCP), were synthesized, and high-performing sky-blue and green thermally activated delayed fluorescence organic light-emitting diodes were achieved.

Tert-Butyl-substituted bicarbazole as a bipolar host material for efficient green and yellow PhOLEDs

We designed a novel bipolar host material with a high ET and thermal stability for multi-colour PhOLEDs.

Pyrimidine-based bipolar host materials for high efficiency solution processed green thermally activated delayed fluorescence OLEDs

In this study, three bipolar host materials (i.e.Py2Cz, Py2BFCz, and Py2ICz) for solution-processable green-emitting TADF-OLEDs were synthesized by coupling 2,4-diphenylpyrimidine to mCP and modifying the carbazole-donating units in the mCP structure.

Balancing charge-transporting characteristics in bipolar host materials

Bipolar host materials with balanced charge transporting property were synthesized using a solvent-less green reaction method. The characteristics of these synthesized bipolar host materials were examined by thermal and spectroscopic analysis. The energy levels of these materials were estimated from cyclic voltammograms and absorption spectra. The optimized molecular geometries and spatial distributions were obtained from molecular simulations. Moreover, the current density vs. voltage features of single-carrier devices were investigated to assess the bipolar transport characteristics of the host materials. As the length of the alkyl chain increased, the electron-transporting capability and hole-transporting ability exhibited optimum values at the octyl chain attachment. The current efficiency, power efficiency and quantum efficiency values of white organic light-emitting diodes prepared by blending blue and yellow iridium phosphors with these bipolar hosts were high with decreasing alkyl chain length. The result was remarkably similar to the trend of current density characteristics of single-carrier devices. The power efficiency of the octyl chain attached bipolar host was approximately three times higher than that of typical blended hole and electron transporting materials. This enhanced efficiency was attributed to the well-balanced charge transfer by the bipolar host material inside an emissive layer. Moreover, the morphology of the device fabricated with a blend of charge transport materials changed due to deterioration, whereas that of the device fabricated with bipolar host materials did not change after 12 h of operation at 9 V.

Bipyridine‐Containing Host Materials for High Performance Yellow Thermally Activated Delayed Fluorescence‐Based Organic Light Emitting Diodes with Very Low Efficiency Roll‐Off

AbstractTwo bipolar host materials 3‐CBPy and 4‐mCBPy are reported. These hosts are structural analogs of the common host materials CBP and mCBP wherein the phenyl rings have been replaced with pyridines. The two materials possess deep highest occupied molecular orbital (HOMO) and shallow lowest unoccupied molecular orbital (LUMO) levels along with sufficiently high energy S1 and T1 states that make them suitable hosts for yellow emitters in electroluminescent devices. Yellow‐emitting thermally activated delayed fluorescence organic light‐emitting diodes are fabricated using 2,4,6‐tris (4‐(10H‐phenoxazin‐10‐yl)phenyl)‐1,3,5‐triazine (tri‐PXZ‐TRZ) as the dopant emitter with either 3‐CBPy or 4‐mCBPy employed as the host. Their device performance is compared to analogous devices using CBP and mCBP as host materials. The pyridine‐containing host devices show markedly improved external quantum efficiencies (EQE) and decreased roll‐off. The 7 wt% tri‐PXZ‐TRZ‐doped device exhibits very low turn‐on voltage (2.5 V for both 3‐CBPy and 4‐mCBPy) along with maximum external quantum efficiencies (EQEmax) reaching 15.6% (for 3‐CBPy) and 19.4% (for 4‐mCBPy). The device using 4‐mCBPy also exhibits very low efficiency roll‐off with an EQE of 16.0% at a luminance of 10 000 cd m−2.

Highly Efficient Solution-Processed Thermally Activated Delayed Fluorescence Bluish-Green and Hybrid White Organic Light-Emitting Diodes Using Novel Bipolar Host Materials.

Two pyridine-containing bipolar host materials with high triplet energy, 9,10-dihydro-9,9-dimethyl-10-(3-(6-(3-(9,9-dimethylacridin-10(9H)-yl)phenyl)pyridin-2-yl)phenyl acridin (DDMACPy) and N-(3-(6-(3-(diphenyl amino)phenyl)pyridin-2-yl)phenyl)-N-phenylbenzenamine (DTPAPy), are synthesized from the modification of the commonly adapted host material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (DCzPPy). The highest occupied molecular orbital levels of DDMACPy (5.50 eV) and DTPAPy (5.60 eV) are found to be shallower than that of DCzPPy (5.90 eV) that leads to the improvement in hole injection from the hole transport layer PEDOT:PSS (WF = 5.10 eV). These host materials are used in the emitting layer of bluish-green organic light-emitting diode (OLED) with the thermally activated delayed fluorescence (TADF) emitter, 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine, as the guest. The DDMACPy-based device shows the highest performance among them, with the maximum external quantum efficiency (EQEmax), current efficiency (CEmax), and power efficiency (PEmax) of 21.0%, 53.1 cd A-1, and 44.0 lm W-1 at CIE (0.17, 0.42), respectively. By further doping with the red-emitting phosphor iridium(III) bis(2-phenylquinoline)(2,2,6,6-tetramethylheptane-3,5-ionate) [Ir(dpm)PQ2] and yellow-emitting phosphor iridium(III) bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C20)acetylacetonate (PO-01-TB) emitters into the bluish-green emitting layer, a TADF-phosphor hybrid white OLED (T-P WOLED) is obtained with excellent EQEmax, CEmax, and PEmax of 17.4%, 48.7 cd A-1, and 44.5 lm W-1 at CIE (0.35, 0.44), respectively. Moreover, both the bluish-green and WOLED show a low efficiency roll-off with external quantum efficiencies at the brightness of 1000 cd m-2 (EQE1000) of 18.7 and 16.2%, respectively, which are the highest performance records among the solution-processed TADF bluish-green and T-P WOLEDs.

High-Efficiency Solution-Processed Organic Light-Emitting Diodes with Tetradentate Platinum(II) Emitters.

The realization of high-efficiency solution-processed organic light-emitting diodes (OLEDs) using phosphorescent tetradentate Pt(II) emitters and bipolar organic hosts is demonstrated in this work. To investigate the effect of organic host on the platinum dopant, the performances of solution-processed Pt-OLEDs with various combinations between four tetradentate Pt(II) emitters, including two newly developed tetra-Pt-S2 and tetra-Pt-S3 and three bipolar organic hosts m-TPAPy, o-TPAPy, and o-CzPy, have been analyzed and compared. Among the tetradentate Pt(II) complexes studied in this work, tetra-Pt-S3 exhibited the best electroluminescent performance attributable to its bulky molecular scaffold structure, high emission quantum yield, and good solubility in common organic solvents. High external quantum efficiencies (EQEs) of up to 22.4% were achieved in the solution-processed OLED with tetra-Pt-S3 emitter and m-TPAPy host at the dopant concentration of 4 wt %. At a high luminance of 1000 cd m-2, the EQE of this device decreased slightly to 21.0%.

New bipolar host materials using Phenanthro[9,10-d]oxazole moiety for highly efficient red phosphorescence

Six new bipolar host materials including phenanthro[9,10-d]oxazole as an electron withdrawing unit and three different amine groups as electron donating units have been designed and synthesized for use in red phosphorescent organic light-emitting diodes (PhOLEDs). The synthesized materials showed PLmax at 397–455 nm and high thermal properties above the decomposition temperature (Td) of 320 °C. In addition, all six materials showed distinct oxidation-redox peaks as bipolar materials. These six new bipolar host materials are suitable for use in red PhOLEDs due to their triplet energy level (e.g., 2.2–2.5 eV). As a result, when the six new materials were used as host materials for red PhOLEDs, 24.0–29.4 cd/A maximum current efficiency, 16.5–19.3% external quantum efficiency, CIE (0.64, 0.35), and 610 nm ELmax were achieved.

High-triplet-energy Bipolar Host Materials Based on Phosphine Oxide Derivatives for Efficient Sky-blue Thermally Activated Delayed Fluorescence Organic Light-emitting Diodes with Reduced Roll-off

We designed and synthesized two new ambipolar host materials, namely CzPO and Cz3PO. Combining CzPO and Cz3PO with CzTRZ2 as the emitter resulted in improved maximum external quantum efficiencies o...

Phthalonitrile-based bipolar host for efficient green to red phosphorescent and TADF OLEDs

A bipolar host material named 4-(9H-carbazol-9-yl)phthalonitrile (CzPN) was developed by combining carbazole and phthalonitrile as donor and acceptor units, respectively. The introduction of the phthalonitrile unit realized a relatively small energy gap of 3.00 eV as well as a deep lowest unoccupied molecular orbital level of −2.72 eV. The characterization of carrier-only devices demonstrated the bipolar charge transporting property of CzPN. With a triplet energy of 2.70 eV, CzPN was investigated as host in green, yellow, and red phosphorescent organic light-emitting diodes (PhOLEDs) and green thermally activated delayed fluorescence (TADF) devices. A low turn-on voltage of 2.8 V was achieved by all the four diodes. Both PhOLEDs and TADF devices exhibited high external quantum efficiency values over 20% with relatively small efficiency roll-offs. These findings demonstrate that CzPN is an excellent host material for both phosphorescent and TADF emitters, and phthalonitrile is a promising unit to develop universal host materials.

High Triplet Energy Phosphine Sulfide Host Materials with Selectively Modulated Electrical Performance for Blue Electrophosphorescence

Bipolar host materials consisting of electron-donating (D) and -accepting (A) moieties are significant advances in broadening the recombination zone to suppress the quenching effects of emissive ph...

Methoxycarbazolyl-disubstituted dibenzofuranes as holes- and electrons-transporting hosts for phosphorescent and TADF-based OLEDs

In the search of universal host materials for organic light emitting diodes a new series of bipolar host materials containing methoxy-substituted carbazoles as the electron-donating and dibenzofuran as an electron-accepting units were designed and synthesized. Different linking topologies and number of methoxy groups attached to carbazolyl moiety were used to understand the impact of the strength of the donor moiety on the thermal, optical, photophysical, electrochemical and electroluminescent properties. The synthesized compounds exhibited relatively high thermal stability with 5% weight loss temperatures exceeding 378 °C and formed molecular glasses with high glass-transition temperatures ranging from 120 to 148 °C. High triplet energy values of 2.86–2.96 eV were estimated for dilute THF solutions at 77K. Hole and electron drift mobilities estimated using time-of-flight technique in solid layers approached 10−4 cm2V−1s−1 at high electric fields exceeding 3.6 × 105 V cm−1. The synthesized methoxy-carbazole based compounds were tested as hosts in electrophosphorescent and TADF organic light-emitting diodes reaching luminance of 53000 cd m−2 and external quantum efficiency of 12.5%, in the best case.

Ultrahigh-performance blue organic light-emitting diodes based on SiO2 coated Ag nanocubes and its working mechanism

Abstract Ag nanocubes coated with SiO2 shell were synthesized and incorporated into the emission layer of blue organic light-emitting diodes to improve the device performance. The thickness of SiO2 shell was changed to adjust the distance between Ag NCs and excitons to investigate the coupling of the localized surface plasmon resonance (LSPR) of Ag NCs and the radiation of excitons. In addition, the host materials with different carrier transporting properties are used to study the correlation between the LSPR enhanced exciton radiation and the exciton recombination zone in emission layer. Research findings demonstrate the thickness of SiO2 shell and the exciton recombination zone together affect the coupling of LSPR and exciton radiation. Using the Ag NCs coated with 15 nm SiO2 shell and the bipolar host material of 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy), the blue OLED achieved an very high improvement in the current efficiency. The maximum peak current efficiency of the blue OLED with Ag NCs reached to 97 cd/A, being nearly 4 times of that of the device without Ag NCs.

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.

Bicarbazole/nitrogen heterocycle based bipolar host materials for efficient green phosphorescent organic light-emitting diodes

In this paper, three novel bipolar host materials, 9,9′-Diphenyl-6-(4-pyridinyl)-3,3′-bicarbazole(BCz4Py), 9,9′-Diphenyl-6-(3-pyridinyl)-3,3′-bicarbazole(BCz3Py) and 9,9′-Diphenyl-6-(5-pyrimidinyl)-3,3′-bicarbazole(BCz5Pm), were synthesized by incorporation of bicarbazole as donor with pyridine/pyridimide based N-hexahydroxy nitric heterocycle as acceptor units. The photophysical, thermal and electrochemical properties of these three materials were fully investigated. These compounds all exhibited superior thermal stability with high glass-transition temperature (Tg > 130 °C) and decomposition temperature (Td > 400 °C). Appropriate HOMO (highest occupied molecular orbitial) -LUMO (lowest unoccupied molecular orbital) levels and triplet energy (2.74–2.75 eV) of these compounds make them suitable hosts for green iridium pohosphor Ir(ppy)3 in phosphorescent organic light emitting devices (PhOLEDs). Among them, the BCz3Py hosted device exhibited outstanding performance with maximum external quantum efficiency (EQE) of 14.9%, maximum current efficiency (CE) of 49.9 cd/A, and maximum power efficiency (PE) of 29.1 lm/W, suggesting that BCz3Py is a promising bipolar material for high-efficiency green PhOLEDs.

Solution-Processed Highly Efficient Bluish-Green Thermally Activated Delayed Fluorescence Emitter Bearing an Asymmetric Oxadiazole-Difluoroboron Double Acceptor.

Difluoroboron (BF2)-containing dyes have attracted great interest owing to their exceptionally high luminescence efficiency and good electron-withdrawing properties. However, only a few reports on difluoroboron-based thermally activated delayed fluorescence (TADF) have been addressed. In this contribution, a novel BF2-containing TADF molecule of BFOXD, which contains two acceptor fragments of oxadiazole (OXD) and BF2 and one donor unit of 9,9-dimethylacridine, was synthesized and characterized. For comparison, the precursor of OHOXD bearing one acceptor unit was also investigated. Both molecules clearly show TADF characteristics with sky-blue emission in solution and film state. Additionally, OHOXD undergoes excited-state intramolecular proton transfer-coupled intramolecular charge transfer processes. Using 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) as the host, the organic light-emitting diodes fabricated via a solution process show maximum external quantum efficiency (EQE) of 2.98 and 13.8% for OHOXD- and BFOXD-based devices, respectively. While the bipolar TADF host of 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) is utilized instead of CzSi, the OHOXD- and BFOXD-based devices exhibit better performances with the maximum EQEs of 12.1 and 20.1%, respectively, which render the most efficient and the bluest emission ever reported for the BF2-based TADF molecules. This research demonstrates that introduction of one more acceptor unit into the TADF molecule could have a positive effect on emission efficiency, which opens a new way to design high-efficiency TADF molecules.

The influence of dipyridylamine-carbazole based bipolar host materials for green PHOLEDs

The series of new bipolar host materials with dipyridylamine and carbazole moieties connected to the triazine core unit were synthesized for green phosphorescent organic light-emitting diodes (PHOLEDs). Especially, we controlled the hole transporting behaviors by attaching different connectivity of carbazole moieties attached to the triazine core in new synthetic host materials. From this approach, we found that triazine derivatives with dipyridylamine-carbazole based substituents exhibited moderately high glass temperature above 139 °C as well as relatively high triplet energy (T1 > 2.78 eV). With those bipolar host materials, we prepared green PHOLEDs. And the new synthetic dipyridylamine-carbazole based triazine derivative, [4,6-bis(9-phenyl-9H-carbazol-3-yl)-N,N-di(pyridin-2-yl)-1,3,5-triazin-2-amine (3-BCTPy)] showed relati-vely high device efficiencies, up to 70.6 cd/A and 18.9% (external quantum efficiency, EQE) when we utilized it as a host materials and bis(2-phenylpyridine)(acetyl-acetonato) iridium (III) [Ir(ppy)2(acac)] as a dopant.

One-shot triphenylamine/phenylketone hybrid as a bipolar host material for efficient red phosphorescent organic light-emitting diodes

Abstract A new bipolar host material, 2,2′,2′′-tris(diphenylketone)amine (N-DPK) was developed in a one-shot by Ullmann reaction, and characterized by spectroscopic techniques and spatial arrangement was ascertained by single X-ray crystal structure analysis. The compound is possessing C3-symmetry with three diphenylketone units connected to the central nitrogen atom. In this organic architecture, the electron-withdrawing units, phenylketone are incorporated to ortho-position of triphenylamine, electron-donating unit to generate rigidity, which is responsible to reduce the gap between single and triplet energy levels. The material possessed triplet energy (ET) at 2.53 eV, indicating that it could be used as a host material for red phosphorescent organic light-emitting diodes. The OLED device was fabricated with bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) (Ir(MDQ)2acac) and N-DPK, as guest and host, respectively, and 10 wt% doping concentration exhibited good device performance with maximum external quantum efficiency (EQE) of 16.1%.

P‐185: Highly Efficient and Long Lifetime Bipolar Host Material for Red Phosphorescent Organic Light‐Emitting Diodes Using Benzocarbazole and Diphenyltriazine Derivatives

Red phosphorescent organic light‐emitting diodes (phOLEDs) can achieve 100% internal quantum efficiency by harvesting both singlet and triplet excitons through spin orbit coupling. To improve the external quantum efficiency (EQE) and device lifetime, many molecular designs of bipolar hosts with appropriate energy band gap and triplet energy were studied in doping system phOLEDs. Two kinds of bipolar hosts were synthesized by combining benzocarbazole hole type donor and diphenyltriazine electron type acceptor to achieve relatively low triplet energy for red phOLEDs. Two hosts with the benzocarbazole and diphenyltriazine linked through para and meta positions of the phenyl unit proved a triplet energy of 2.52 eV and bipolar carrier transport properties. The band gap of the emitters between highest energy occupied molecular orbital (HOMO) and lowest energy occupied molecular orbital (LUMO) is 2.89 eV and 2.83 eV, while the EQE was 21.7% and 20.7% respectively, as a result of doping with red emitter. It was confirmed that life time was 30 times longer than that of general 4,4′‐bis(n‐carbazolyl)‐1,1′‐biphenyl (CBP) host at 5000 cd/m2.

Benzocarbazole and diphenyltriazine derived hosts for red phosphorescent organic light-emitting diodes

A bipolar host design for red phosphorescent organic light-emitting diodes (PhOLEDs) was explored as an objective to improve the external quantum efficiency and device lifetime. Main direction of the host design was to secure low triplet energy, bipolar charge transport properties, and stability under positive and negative polarons. The objective of the host development was attained by a host design featured by low triplet energy benzocarbazole hole transport unit and diphenyltriazine electron transport unit. Two hosts with the benzocarbazole and diphenyltriazine linked through para or meta positions of the phenyl unit demonstrated a triplet energy of 2.52 eV and bipolar carrier transport properties. Based on the benzocarbazole and diphenyltriazine derived hosts, high external quantum efficiency over 23% and more than 30 times extension of the device lifetime were achieved in the red PhOLEDs.