Bipolar Host Materials Research Articles

Bipolar host materials comprising carbazole, pyridine and triazole moieties for efficient and stable phosphorescent OLEDs

Achieving an excellent device performance with slow roll-off efficiency is still a crucial requirement for phosphorescent OLEDs in displays and white lighting applications. Three bipolar host materials comprised of carbazole, pyridine and triazole units, namely o-CzTPy, p-CzTPy, and 3-CzTPy were synthesized and developed. They are designed by selecting the pyridine and triazole groups together as an electron-transporting unit, and varying the different linking modes of the functional groups to optimize performance. These materials possessed high triplet energies (2.66 eV–2.69 eV), and good bipolar charge transporting abilities. FIrpic and Ir(ppy)3 based blue and green PhOLEDs incorporating these bipolar hosts show low turn-on voltage, high efficiencies and slow efficiency roll-off. Among them, a maximum efficiency of 22.25% (41.98 cd A−1) was achieved for o-CzTPy based blue device. More importantly, the o-CzTPy based green device achieved the desirable high efficiency of 29.08% (96.98 cd A−1), which is among the highest values for PhOLEDs with dopant Ir(ppy)3 ever reported in public scientific literatures. These excellent results demonstrated that these bipolar hosts possess high practical value for application in commercial PhOLEDs.

Combined experimental and density functional theory studies on novel 9‐(4/3/2‐cyanophenyl)‐9H‐carbazole‐3‐carbonitrile compounds for organic electronics

AbstractWe have synthesized a series of novel hybrid molecules 9‐(2‐cyanophenyl)‐9H‐carbazole‐3‐carbonitrile (o‐CNCbzCN), 9‐(3‐cyanophenyl)‐9H‐carbazole‐3‐carbonitrile (m‐CNCbzCN) and 9‐(4‐cyanophenyl)‐9H‐carbazole‐3‐carbonitrile (p‐CNCbzCN), comprising electron‐donating carbazole and electron‐accepting nitrile groups. Three positional isomers were synthesized with a view to tune photophysical and electrochemical properties of the hybrids. The photophysical study displayed absorption maxima in the range of 281–340 nm and 277–298 nm whereas emission maxima in the range of 349–366 nm and 366–369 nm in toluene and dimethylformamide (DMF), respectively. These molecules demonstrated suitable frontier molecular orbital (FMO) energy levels and ensure good thermal and morphological stability. Among these synthesized molecules, m‐CNCbzCN showed very high decomposition temperature (Td = 341°C) whereas p‐CNCbzCN exhibited good glass transition (Tg = 182°C) as well as melting temperature (Tm = 236°C), indicating its significant stability and potential utility as a bipolar host material for efficient phosphorescent organic light‐emitting diodes (PhOLEDs).

Highly Efficient Simple-Structure Sky-Blue Organic Light-Emitting Diode Using a Bicarbazole/Cyanopyridine Bipolar Host.

Up to now, the most efficient blue phosphorescent organic light-emitting diode (PhOLED) was achieved with a maximum external quantum efficiency (ηext) of 34.1% by using an exciplex cohost. It still remains a challenge to obtain such high efficiencies using a single-host matrix. In this work, a highly efficient sky-blue PhOLED is successfully fabricated using a newly developed bipolar host material, namely 5-(2-(9H-[3,9'-bicarbazol]-9-yl)phenyl)nicotinonitrile (o-PyCNBCz), which realizes a ηext of 29.4% at a practical luminance of 100 cd m-2 and a maximum ηext of 34.6% (at 23 cd m-2). The present device is characterized by simple configuration with a single host and single emitting layer. o-PyCNBCz also reveals high efficiency of 28.2% (94.8 cd A-1) when used as the host for green PhOLED. Under identical conditions, o-PyCNBCz always outperforms than its isomer 3-PyCNBCz (5-(9-phenyl-9H-[3,9'-bicarbazol]-6-yl)nicotinonitrile) in terms of more balanced charge transportation, higher photoluminescent quantum yields of over 90%, and higher horizontal orientation ratio of the emitting dipole for the host-dopant films, which finally lead to its superior performance in PhOLEDs. It is observed that all these merits of o-PyCNBCz benefit from its ortho-linking style of carbazole (p-type unit) and cyanopyridine (n-type unit) on the phenylene bridge and the resultant molecular conformation.

Novel benzothiadiazine 1,1-dioxide based bipolar host materials for efficient red phosphorescent organic light emitting diodes

Abstract In this work, three novel bipolar host materials TPA-SA, 3CBZ-SA and 4CBZ-SA have been designed and synthesized by incorporating triphenyl amine and carbazole as donor and benzothiadiazine 1,1-dioxide as an acceptor. These molecules exhibit moderately high triplet energies and bipolar carrier transport characteristics (ambipolarity) which is useful for the exothermic energy transfer to the dopants and also for the balanced carrier injection/transport in the emissive layers. These materials exhibited good performances in PhOLEDs and furnished external quantum efficiency in the range of 10.0–15.0%. Notably, a red phosphorescent device using TPA-SA as the host doped with Ir(pq)2(acac) exhibited a maximum EQE, power efficiency and current efficiency of 15.0%, 16.0 lm/W, and 25.3 cd A−1, respectively.

Carbazole/triphenylamine-cyanobenzimidazole hybrid bipolar host materials for green phosphorescent organic light-emitting diodes

Abstract With a view to attain balanced charge flux for higher device performance of PhOLEDs, we have used carbazole/triphenyl amine as hole transporting moiety and cyano along with benzimidazole as electron transporting core in 3-Cbz-ImdCN, 4-Cbz-ImdCN and TPA-ImdCN. Their thermal, photophysical and electrochemical properties have been evaluated to shed light on structure-property-performance relationship. Good performances have been exhibited by these bipolar host materials in green PhOLEDs with maximum external quantum efficiencies were observed in the range of 5.3–11.5% using Ir(ppy)3 emitter. Further, 3-Cbz-ImdCN hosted orange and red PhOLEDs with the Ir(MDQ)2acac and Ir(piq)2acac emitters revealed the external quantum efficiencies of 5.1% and 6.3%, respectively. In all the devices pure emission was observed from dopants only which clearly implies that the devices possess effective energy transfer from the host to the guest.

A theoretical design of bipolar host materials for blue phosphorescent OLED

A series of host molecules have been designed and characterized for use in phosphorescent organic light-emitting diode devices. The parent host molecule was modified by adding nitrogen-containing group at different positions. The first triplet excited (T1) and first singlet excited (S1) states energies, frontier orbital energy levels, reorganization energies and injection barriers of the molecules designed were calculated in comparison to those of the reference host, emitter, hole and electron transport materials. Interestingly, our results suggest that addition of nitrogen group into the dibenzothiophene by a N–C linkage increases the triplet energy separation and decreases the injection barriers making them suitable for use as blue phosphorescent materials.

Strategy to improve the efficiency of solution-processed phosphorescent organic light-emitting devices by modified TADF host with tert-butyl carbazole

A new bipolar host material t3Cz-SO was designed and synthesized by introducing tert-butyl carbazole unit to the peripheral of parent TADF units. The calculated triplet energy and ΔEST of t3Cz-SO are 2.85 and 0.11 eV, respectively. Moreover, t3Cz-SO exhibited excellent film-forming ability and morphological stability, which are suitable for the fabrication of solution-processed OLEDs. The FIrpic-based device host with t3Cz-SO achieved low turn-on voltage of 3.5 V and high power efficiency of 16.5 lm W−1, which is almost ten times higher than the unmodified parent TADF molecule. This study shows that the introduction of tert-butyl carbazole unit to the periphery of small molecular TADF emitter is an effective method for the development of efficient electroluminescent materials.

Enabling high efficiency and long lifetime of pure blue phosphorescent organic light emitting diodes by simple cyano modified carbazole-based host

Herein, we report two simple and efficient high triplet energy bipolar host materials named 2'-(9H-carbazol-9-yl)-[1,1′-biphenyl]-2-carbonitrile (CzBPCN) and 9-(2′-cyano-[1,1′-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile (CNCzBPCN) by connecting carbazole type donor and cyano acceptor on 2 and 2′ positions of a biphenyl linker, respectively. The effect of cyano substitution on carbazole building block on photophysical and electroluminescence properties was unveiled. Both the compounds revealed high triplet energy (ET) of above 3.0 eV attributed to the interrupted interchromophoric electronic interactions between the donor and acceptor units. The computational studies and carrier transport analysis proved that the compounds secured good bipolar charge transporting feature. The applicability of these materials as hosts for blue phosphorescent organic light emitting diodes (PhOLEDs) was tested. Interestingly, the CNCzBPCN hosted device exhibited superior performance with three fold improved external quantum efficiency (EQEmax) of 20.2% over the CzBPCN hosted device which showed EQEmax of 7.1%. Importantly, the CNCzBPCN hosted device demonstrated 11 fold extended operational lifetime of 121 h up to 50% of its intimal luminance (L0 = 200 cd/m2) compared to CzBPCN. We believe that this work demonstrates the cost-effective and efficient high triplet energy bipolar host materials for simultaneous achievement of high EQE over 20% and long operational lifetime of blue PhOLEDs.

CN-substituted ortho-terphenyl core based high triplet energy bipolar host materials for stable and efficient blue TADF devices

Two novel, CN-substituted ortho-terphenyl (OTP) core based bipolar type host materials were designed and synthesized for application in blue thermally activated delayed fluorescence (TADF) devices.

A solution-processable wholly-aromatic bipolar host material for highly efficient blue electroluminescent devices

Highly efficient blue electroluminescent devices were realized by using a solution-processable wholly-aromatic bipolar host material prepared through an insulating C(sp3) bridge linking strategy.

Low-driving-voltage sky-blue phosphorescent organic light-emitting diodes with bicarbazole-bipyridine bipolar host materials

Two bipolar host containing bipyridine and bicarbazole exhibited rapid and balanced charge transportation, which resulting the extremely low turn-on voltages of 2.3 and 2.4 V, respectively, and the high efficiency of 23.7% and 52.6 lm W−1.

Highly distorted bipolar host material based on benzimidazole and indole derivative for efficient green and red solution-processed PhOLEDs

A novel bipolar host material 2-(2′-(5-(dibenzo[b,d]furan-4-yl)-1H-indol-1-yl)-[1,1′-biphenyl]-3-yl)-1-phenyl-1H-benzo[d]imidazole (DY1) based on electron-donating indole and electron-withdrawing benzimidazole was designed and synthesized for solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). The highly distorted conformation derived from the orthogonally connected donor and acceptor on the 1,3′-biphenyl bridge endowed DY1 with high thermal stability (Tg 122 °C) and a high triplet energy of 2.69 eV. Solution-processed PhOLEDs fabricated with DY1 host achieved excellent performance of 47.05 cd/A, 34.0 lm/W and 14.27% for green PhOLED, and 32.95 cd/A, 17.0 lm/W and 16.57% for the red PhOLED, respectively. This result revealed the structural advantage of the host material in high efficiency and low efficiency roll-off in PhOLEDs.

High Triplet Energy Host Materials for Blue TADF OLEDs-A Tool Box Approach.

The synthesis of stable blue TADF emitters and the corresponding matrix materials is one of the biggest challenges in the development of novel OLED materials. We present six bipolar host materials based on triazine as an acceptor and two types of donors, namely, carbazole, and acridine. Using a tool box approach, the chemical structure of the materials is changed in a systematic way. Both the carbazole and acridine donor are connected to the triazine acceptor via a para- or a meta-linked phenyl ring or are linked directly to each other. The photophysics of the materials has been investigated in detail by absorption-, fluorescence-, and phosphorescence spectroscopy in solution. In addition, a number of DFT calculations have been made which result in a deeper understanding of the photophysics. The presence of a phenyl bridge between donor and acceptor cores leads to a considerable decrease of the triplet energy due to extension of the overlap electron and hole orbitals over the triazine-phenyl core of the molecule. This decrease is more pronounced for the para-phenylene than for the meta-phenylene linker. Only direct connection of the donor group to the triazine core provides a high energy of the triplet state of 2.97 eV for the carbazole derivative CTRZ and 3.07 eV for the acridine ATRZ. This is a major requirement for the use of the materials as a host for blue TADF emitters.

Dibenzothiophene dioxide-benzofuro carbazole based bipolar host material for yellow and red phosphorescent OLEDs

2-(12H-benzofuro[3,2-a]carbazol-12-yl)dibenzo[b,d]thiophene 5,5-dioxide (DBT-INFUR) is a bipolar host material for yellow and red phosphorescent OLEDs. DBT-INFUR was designed and synthesized between dibenzothiphene acceptor and benzofuro carbazole donor with a yield of 60%. We constructed two different devices, namely a yellow and a red phosphorescent OLEDs, and we compared the device properties with reference 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP). DBT-INFUR based yellow phosphorescent OLED device exhibited maximum current and external quantum efficiencies of 41.07 cd/A and 16.5%, respectively and which is better than that of reference CBP based similar device (27.66 cd A, 10.15%). DBT-INFUR based red phosphorescent OLED (12.44%) revealed higher efficiencies when compared to the reference (7.64%) device. The bipolar DBT-INFUR showed excellent host properties with yellow and red phosphorescent devices.

Multichannel Effect of Triplet Excitons for Highly Efficient Green and Red Phosphorescent OLEDs

AbstractWith the inherent organic–metal system property, phosphorescent organic light‐emitting diodes (PhOLEDs) are capable of exhibiting high performance. However, the existence of unintended triplet–triplet annihilation (TTA), which is mainly originated from high triplet exciton density leading to inferior device performance, is a current predicament. For mitigating the TTA in devices, reported herein is an effective host system with the role of precisely controlling the triplet excitons via the multichannel processes, in which a bipolar host material, a hole‐transporting material, and an electron‐transporting material are mixed to manage the triplet exciton transfer. The mixed‐host with so‐called “triplet excitons harvesting” strategy exhibits two concomitant reverse intersystem crossing (RISC) processes that simultaneously occur in the bipolar host material:electron‐transporting material and the host‐transporting material:electron‐transporting material exciplexes, respectively. Highly efficient green and red PhOLEDs are demonstrated by these multichannel processes, in which phosphorescent emitters bis(2‐phenylpyridine) (acetylacetonate) iridium(III) (Ir(ppy)2(acac)) and bis(2‐methyldibenzo[f,h]‐quinoxaline) (acetylacetonate) iridium(III) (Ir(MDQ)2(acac)) are doped into the mixed‐host, respectively. The green and red PhOLEDs realize nearly 30% of maximum external quantum efficiencies of 29.4% and 29.2%, respectively, and also maintain 29.1% and 28.3% at 1000 cd m−2. These excellent efficiencies and low roll‐offs confirm that the concomitant RISC processes effectively manage and utilize triplet excitons.

Imidazo[1,2-b]pyridazine as Building Blocks for Host Materials for High-Performance Red-Phosphorescent Organic Light-Emitting Devices.

A novel electron-transporting unit, imidazo [1,2-b]pyridazine (IP), was first reported for developing host materials. The IP moiety possesses excellent electron-transporting ability and great thermal stability. Using carbazole as p-type units and IP as n-type units, several bipolar host materials, namely, IP6Cz, IP68Cz, IP36Cz, and IP368Cz, were developed through altering the substitution site of the IP core. Among these four materials, 6-site-substituted IP6Cz and 6,8-site-substituted IP68Cz exhibit the best electroluminescence (EL) performance. IP6Cz- and IP68Cz-based red phosphorescent organic light-emitting diodes using Ir(pq)2acac as the emitter exhibit extremely high EL efficiency with the maximum external quantum efficiency (ηext,max) of 26.9 and 25.2% and an insignificant efficiency roll-off. Moreover, IP6Cz- and IP68Cz-based deep-red devices doped by Ir(piq)2acac also show satisfactory EL performance with a ηext,max of 20.5 and 19.9%, respectively. The influence of different substitution sites of the IP core on the photophysical and electrochemical properties was systematically investigated. This study demonstrates that IP could be a first-rate electron-transporting unit for bipolar materials for red-emitting devices.

New carbazole-based bipolar hosts for efficient green phosphorescent organic light-emitting diodes

In this work, two new bipolar host materials 3-(5-(9 H -carbazol-9-yl)-4'-(1-phenyl-1 H -benzo[ d ]imidazole-2-yl)-[1,1′-biphenyl]-2-yl)-9- phenyl-9 H -carbazole (BCZ-PBM) and 3-(5-(9 H -carbazol-9-yl)-3'-(4,6- diphenyl-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-2-yl)-9-phenyl-9 H -carbazole (BCZ-TRZ) were designed and synthesized by introducing benzimidazole and triazine groups as acceptor units with carbazole as the donor unit. These two compounds have excellent thermal stability with a high glass transition temperature (T g > 160 °C) and thermal decomposition temperature (T d > 450 °C). Green phosphorescent organic light-emitting devices (PHOLEDs) D1 and D2 were obtained by doping Ir(ppy) 3 into BCZ-PBM and BCZ-TRZ, in which BCZ-PBM based device D1 revealed better performance with an external quantum efficiency (EQE) of 20.4%, maximum current efficiency (CE) of 69.3 cd/A, and maximum power efficiency (PE) of 54.8 lm/W. Notably, the BCZ-PBM-based device exhibited excellent efficacy stability and a low efficiency roll-off of 1.7% at 1000 cd/m 2 and 4.7% at 5000 cd/m 2 . • Two new bipolar host materials were synthesized by introducing benzimidazole, triazine and carbazole groups. • The compounds showed excellent thermal stability with the high T g and T d . • Maximum EQE of 20.4%, CE of 69.3 cd/A, and PE of 54.8 lm/W of green PHOLED based on DCZ-PBM was demonstrated.

C1-, C2-, and C3-Modified Carbazole Derivatives as Promising Host Materials for Phosphorescent Organic Light-Emitting Diodes.

Herein we unveiled the synthetic approach to C1-, C2-, and C3-modified carbazoles for the first time and designed high-triplet-energy bipolar host materials for green phosphorescent organic light-emitting devices. The compounds revealed excellent performance with a maximum external quantum efficiency as high as 23.6% and extremely low-efficiency roll-off of 2.1% at high brightness of 1000 cd/m2.

High-performance sky-blue phosphorescent organic light-emitting diodes employing wide-bandgap bipolar host materials with thermally activated delayed fluorescence characteristics

Abstract Up to date, several kinds of thermally activated delayed fluorescence (TADF) host molecules have been rapidly developed to fulfill the increasing demand of high-efficiency phosphorescent organic light-emitting diodes (PHOLEDs). Herein, three wide-bandgap TADF materials BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz are newly designed and synthesized as host materials for sky-blue PHOLEDs. By modulation of carbazole-units in the molecular skeleton, we find that the optimized molecules can realize both high triplet energies and balanced carrier transport properties with obvious TADF properties. The sky-blue PHOLEDs with ultralow phosphor doping ratio employing BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz as host materials demonstrate really high performance with alleviated roll-offs and achieve impressive current efficiencies (CEs) of 46.5 cd A−1, 50.0 cd A−1, and 34.5 cd A−1, power efficiencies (PEs) of 40.6 lm W−1, 46.8 lm W−1, and 34.8 lm W−1, and external quantum efficiencies (EQEs) of 22.2%, 24%, and 15.8%, respectively. More encouragingly, even at an applied luminance of 1000 cd m−2, the EQEs could maintain high levels of 21.0%, 21.5% and 12.8%, respectively, which represents the potential utility value of the new family of these molecules in blue PHOLEDs. Our molecular design strategy provides a practical way to construct promising wide-bandgap TADF host materials with an excellent combination of TADF property, high triplet energy, and balanced carrier-transporting ability for high-performance devices.

5H-Benzo[d]Benzo[4,5]Imidazo[2,1-b][1,3]Thiazine as a Novel Electron-Acceptor Cored High Triplet Energy Bipolar Host Material for Efficient Solution-Processable Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes.

Organic entities that can transport electrons are seldom available to develop adequate bipolar host materials applicable for solution-processable thermally activated delayed fluorescence (TADF)-organic light-emitting diodes (OLEDs). Therefore, the introduction of new electron-affine entities that plausibly demonstrate high triplet energy (ET) is of urgent need. In this contribution, we introduced benzimidazo[1,2-a][3,1]benzothiazine (BBIT) as a novel electron-affine entity and developed two new bipolar host materials, CzBBIT and 2CzBBIT. Both host materials exhibit high ET of 3.0 eV, superior thermal robustness with the thermal decomposition temperature of up to 392°C, a glass transition temperature of up to 161°C, and high solubility in common organic solvents. Consequently, the solution-processable OLEDs fabricated using a recognized IAcTr-out as the green TADF emitter doped into CzBBIT as the host, realized a maximum external quantum efficiency (EQE) of 23.3%, while the 2CzBBIT:IAcTr-out blend film-based device displayed an EQE of 18.7%. These outcomes corroborated that this work could shed light on the scientific community on the design of new electron-affine entities to establish the effective use of bipolar host materials toward proficient solution-processable TADF-OLEDs.