High Triplet Energy Host Research Articles

Asymmetric Decoration of a meta-Linked Bitriazine-Based Host for Highly Efficient and Stable Blue Phosphorescent Organic Light-Emitting Diodes

High triplet energy hosts for blue phosphorescent organic light-emitting diodes were developed by decorating a meta-linked bitriazine core with carbazole and tetraphenylsilyl functional groups. A symmetric host with two carbazole units as the two triazine units of the core and an asymmetric host with one carbazole unit and one tetraphenylsilyl unit as the two triazine units were prepared. The triplet energy of these two hosts was 2.97 eV, suitable for triplet exciton harvesting of blue phosphors. Comparing the two host designs, the asymmetric decoration of the two triazine units with carbazole and tetraphenylsilyl units was superior to the symmetric decoration of the two triazine units with two carbazoles in terms of high external quantum efficiency (EQE) and long-term device stability. A high EQE of 19.7% and a long device lifetime of 2093.6 h at 100 cd m-2 were achieved using the asymmetrical host.

A novel benzo[4,5]furo[3,2-d]pyrimidine-based host as a n-type host for blue phosphorescent organic light-emitting diodes

An electron-transport-type host was synthesized using a benzo[4,5]furo[3,2-d]pyrimidine (BFP) core to develop a high triplet energy host. The host was designed to have both the tetraphenylsilyl blocking group and the hole-transport-type carbazole group. In the device application, the host was used as an electron-transport-type host mixed with a hole-transport-type 3,3†-di(9H-carbazol-9-yl)-1,1†-biphenyl (mCBP) host in the blue phosphorescent organic light-emitting diodes (PhO-LEDs) doped with a [[5-(1,1-dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]bis-[[6-(1,1-dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]iridium (Ir(cb)3) blue phosphor. As a result, the mCBP:CzBFPmSi mixed host showed high external quantum efficiency over 20% at 1000 cd m−2 and low efficiency roll-off in the blue PhOLEDs.

Effects of Substitution Position of Carbazole-Dibenzofuran Based High Triplet Energy Hosts to Device Stability of Blue Phosphorescent Organic Light-Emitting Diodes.

High triplet energy hosts were developed through the modification of the substitution position of carbazole units. Two carbazole-dibenzofuran-derived compounds, 9,9′-(dibenzo[b,d]furan-2,6-diyl)bis(9H-carbazole) (26CzDBF) and 4,6-di(9H-carbazol-9-yl)dibenzo[b,d]furan (46CzDBF), were synthesized for achieving high triplet energy hosts. In comparison with the reported hole transport type host, 2,8-di(9H-carbazol-9-yl)dibenzo[b,d]furan (28CzDBF), 26CzDBF and 46CzDBF maintained high triplet energy over 2.95 eV. The device performances of the hosts were evaluated with electron transport type host, 2-phenyl-4, 6-bis(3-(triphenylsilyl)phenyl)-1,3,5-triazine (mSiTrz), to comprise a mixed host system. The deep blue phosphorescent device of 26CzDBF:mSiTrz with [[5-(1,1-dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]bis[[6-(1,1-dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]iridium (Ir(cb)3) dopant exhibited high external quantum efficiency of 22.9% with a color coordinate of (0.14, 0.16) and device lifetime of 1400 h at 100 cd m−2. The device lifetime was extended by 75% compared to the device lifetime of 28CzDBF:mSiTrz (800 h). These results demonstrated that the asymmetric and symmetric substitution of carbazole can make differences in the device performance of the carbazole- and dibenzofuran- derived hosts.

27‐2: Invited Paper: Host Strategy for High‐Efficiency and Long‐Lifetime Blue Organic Light‐Emitting Diodes

Recently, high efficiency and long lifetime blue organic light‐emitting diodes are being actively studied for commercialization. The blue technologies include phosphorescent devices, thermally activated delayed fluorescence devices and hyperfluorescence devices. They all require high triplet energy host to fully utilize the triplet excitons of the emitting materials. Therefore, it is very important to develop the high triplet energy hosts for the blue organic light‐emitting diodes. In this paper, we present a strategy to develop the high triplet energy hosts for high efficiency and long device lifetime.

Benzofurodibenzofuran as a universal chemical platform of highly efficient sky-blue thermally activated delayed fluorescence emitters and hosts

A benzofuran based chemical platform, benzofurodibenzofuran, was designed and synthesized as a core structure to develop hosts for phosphorescent organic light-emitting diodes and thermally activated delayed fluorescence (TADF) emitters. The high triplet energy and rigid molecular structure of the benzofurodibenzofuran unit enabled the universal application of it as the building unit of the sky-blue hosts and emitters. The sky-blue TADF emitters built on the benzofurodibenzofuran platform showed high external quantum efficiency, narrow emission spectrum and blue-shifted emission color compared with the conventional phenyl based emitters. Moreover, the high triplet energy hosts obtained from benzofurodibenzofuran efficiently harvested the triplet excitons of the sky-blue phosphors and TADF emitters. Therefore, the benzofurodibenzofuran can work as the basic platform of designing host and emitters for high efficiency sky-blue organic light-emitting diodes.

CN engineered electron transport type hosts for high efficiency and extended lifetime in blue thermally activated delayed fluorescent organic light-emitting diodes

The 4DBF-BNCZ and 4DBF-PCZ with CN substituent are developed as the electron transport and high triplet energy hosts for blue TADF application. The 4DBF-BNCZ demonstrates a high EQE of 20% and device lifetime of 32.2 h up to 80%.

Lowest unoccupied molecular orbital managing function of CN-substituted dibenzofuran in high triplet energy hosts for blue thermally-activated delayed fluorescence organic light-emitting diodes

The effect of lowest unoccupied molecular orbital management of high triplet energy electron transport type hosts in organic light-emitting diodes was investigated by synthesizing three hosts derived from CN-modified dibenzofuran.

Low efficiency roll-off blue TADF OLEDs employing a novel acridine–pyrimidine based high triplet energy host

A novel pyrimidine-based host material with a triplet energy of 3.07 eV was synthesized. Sky blue and blue OLEDs were fabricated, obtaining high external quantum efficiency and extremely low efficiency roll-off.

CN decoration of dibenzofuran modified biphenyl for high triplet energy host for blue phosphorescent organic light-emitting diodes

Abstract A strongly electron deficient and high triplet energy host for blue emitters was developed by decorating a dibenzofuran modified biphenyl backbone structure with multiple CN units. Two hosts, 6,6′-bis(6-cyanodibenzo[b,d]furan-4-yl)-[1,1′-biphenyl]-3,3′-dicarbonitrile(CNDBF1) and 2,2′-bis(6-cyanodibenzo[b,d]furan-4-yl)-[1,1′-biphenyl]-4,4′-dicarbonitrile(CNDBF2), were derived from the CN decoration strategy for application in blue organic light-emitting diodes requiring high triplet energy host. They showed high triplet energy above 2.79 eV and acted as the electron transport type host based on the strong electron deficiency. The mixture of the CNDBF1 and CNDBF2 hosts with a hole transport type 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl host performed as the exciplex host of a blue phosphor and accomplished high external quantum efficiency of 22.7% in the blue phosphorescent organic light-emitting diodes.

High Triplet Energy Hosts for Blue Organic Light‐Emitting Diodes

AbstractBlue organic light‐emitting diodes (OLEDs) have been a bottleneck for OLEDs lighting and flexible displays. Improving the operational lifetimes and simultaneously decreasing efficiency roll‐off while maintaining high quantum efficiency is currently a challenge in the scientific community and industry. Optimizing the fabrication process of devices and developing stable and efficient luminescence and transporting materials and developing host materials with high triplet energy is an effective way to overcome this obstacle. On the one hand, the host material can disperse the blue emitters to reduce the possibility of exciton annihilation. On the other hand, it can adjust the carrier transport, exciton formation, and energy transfer in the device. In recent years, many efforts have been undertaken for the design, synthesis, and applications of the novel host. A systematic summary and comments on the recent advances of high triplet energy hosts for blue OLEDs are provided here, which specifically include bipolar transport hosts, single thermally activated delayed fluorescence (TADF) hosts, TADF assistant hosts, exciplex hosts, exciplex free type mixed hosts, and electroplex hosts. Moreover, future prospects for host for high performance blue OLEDs are also proposed.

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.

A Silane-Based Bipolar Host with High Triplet Energy for High Efficiency Deep-Blue Phosphorescent OLEDs with Improved Device Lifetime.

A high triplet energy host is developed using a silane moiety, 9-(4-(triphenylsilyl)dibenzo[b,d]furan-2-yl)-9H-carbazole (SiDBFCz), is designed through extensive density functional theory (DFT) calculations to obtain appropriate hole and electron injection barriers. The chemical hardness and the charge transport characteristics are comprehensively investigated to realize a bipolar host with high triplet energy over 2.9 eV for deep blue phosphorescent organic light-emitting diodes (PHOLEDs). The synthesized SiDBFCz clearly exhibits the bipolar characteristics especially with emitter molecules doped. An external quantum efficiency over 19 % without any microcavity optimization is achieved thanks to the good charge balance in the SiDBFCz PHOLED. The device lifetime of the SiDBFCz PHOLED is improved more than 1000 %, compared to the unipolar control devices at an initial luminance of 500 cd m-2 . The dramatic enhancement of the operational stability of the deep blue PHOLED is also thoroughly investigated in terms of electrochemical stability of host molecules in charged or excited states. The results clearly indicate that the device lifetime is strongly correlated with the bond dissociation energy and the activation energy for the bond dissociation reaction in triplet excited state.

Novel Positive Polaron Stabilizing n-Type Host for High Efficiency and Long Lifetime in Blue Phosphorescent Organic Light-Emitting Diodes.

Four electron transport type hosts withstanding the positive polaron stress were synthesized using the benzo[4,5]thieno[3,2-d]pyrimidine core to develop high triplet energy hosts. Four benzo[4,5]thieno[3,2-d]pyrimidine-derived hosts, 4-(9H-carbazol-9-yl)-2-(3-(triphenylsilyl)phenyl)benzo[4,5]thieno[3,2-d]pyrimidine (CzBTPmSi), 2,4-di(9H-carbazol-9-yl)benzo[4,5]thieno[3,2-d]pyrimidine (2CzBTP), 2-(9H-carbazol-9-yl)-4-(3-(triphenylsilyl)phenyl)benzo[4,5]thieno[3,2-d]pyrimidine (mSiBTPCz), and 2,4-bis(3-(triphenylsilyl)phenyl)benzo[4,5]thieno[3,2-d]pyrimidine (2mSiBTP), were designed to have either the tetraphenylsilyl blocking group or the hole transport type carbazole group. The CzBTPmSi and mSiBTPCz were prepared to study the effect of substitution positions of tetraphenylsilyl and carbazole on the device performances, and the 2CzBTP and 2mSiBTP were synthesized as reference materials. In the device application, the four hosts were used as electron transport type hosts mixed with a hole transport type 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) host in the mixed host. Among the four mixed hosts, the mCBP/CzBTPmSi mixed host showed an external quantum efficiency of 23.9% and a device lifetime over 4000 h at 100 cd m-2 in the blue phosphorescent organic light-emitting diodes.

Rigid Oxygen‐Bridged Boron‐Based Blue Thermally Activated Delayed Fluorescence Emitter for Organic Light‐Emitting Diode: Approach towards Satisfying High Efficiency and Long Lifetime Together

AbstractThermally activated delayed fluorescence (TADF) materials have emerged as an efficient emitter for achieving high efficiency of blue organic light emitting diodes (OLEDs). However, it is challenging to satisfy both high device efficiency and long operational lifetime together. Here, highly efficient and electrochemically stable blue TADF emitter, 5‐(5,9‐dioxa‐13b‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐10,15‐diphenyl‐10,15‐dihydro‐5H‐diindolo[3,2‐a:3′,2′‐c]carbazole (DBA‐DI) is designed and synthesized for high efficiency and long lifetime OLED. This emitter exhibits high photoluminescence quantum yield of 95.3%, small single‐triplet energy gap of 0.03 eV, short delayed exciton lifetime of 1.25 µs, and high bond dissociation energy (BDE). Also, phosphine oxide free high triplet energy host systems (single and mixed) and exciton blocking layer materials are analyzed using molecular and optical simulations to find an efficient host system with high BDE and suitable emission zone for high efficiency and stable OLEDs. The fabricated OLED with DBA‐DI and high triplet host exhibited a maximum external quantum efficiency (EQE) of 28.1% with blue CIE color coordinates of (0.16, 0.39) and long operational lifetime (LT50) of 329 h at the initial luminance of 1000 cd m−2. Furthermore, the mixed host‐based TADF device showed a slightly lower EQE of 26.4% and almost two times longer lifetime (LT50: 540 h) than the single host device.

Solution processed deep blue thermally activated delayed fluorescent emitters using a solubility enhancing acceptor

A blue solution type organic light-emitting diode was developed by designing a thermally activated delayed fluorescent emitter with tert-butylcarbazole donors and a methoxy substituted phenyltriazine acceptor. The methoxy substituted diphenyltriazine acceptor was adopted for both solubility through solubilizing effect of the methoxy unit and blue shifted emission color through electron donating character weakening the acceptor character of diphenyltriazine. The doping of the soluble blue emitter in the high triplet energy host provided high external quantum efficiency of 16.1% and blue color coordinate of (0.16, 0.20). The methoxy substitution in addition to the tert-butyl substitution enabled the high external quantum efficiency and deep blue color coordinate in the solution processed thermally activated delayed fluorescent devices.

Molecular Engineering of Cyano‐Substituted Carbazole‐Based Host Materials for Simultaneous Achievement of High Efficiency and Long Lifetime in Blue Phosphorescent Organic Light‐Emitting Diodes

AbstractA series of isomeric high triplet energy host materials named 1CNCzBN, 2CNCzBN, 3CNCzBN, and 4CNCzBN containing a cyanocarbazole and a benzonitrile for blue phosphorescent organic light‐emitting diodes (PhOLEDs) is presented. The effect of the substitution position of the cyano unit in the carbazole core on physicochemical and electroluminescence characteristics is described. All the compounds show high triplet energy (ET) of above 2.85 eV and the 3CNCzBN compound containing the cyano unit at C3 position of carbazole exhibits superior ET of 3.04 eV. Except for 1CNCzBN, all the compounds reveal good electron‐transporting properties, evidenced by the single carrier device analysis. Furthermore, the applicability of these compounds as electron‐transport‐type host materials for blue PhOLEDs is tested by employing a blue phosphor and the blue PhOLEDs exhibit high EQE over 20%. Among the compounds, the 3CNCzBN‐based device exhibits excellent performance with maximum external quantum efficiency (EQE) of 25.6% and current efficiency (CE) of 52.1 cd A−1. Importantly, all the compounds reveal excellent operational stability with long lifetime and the 3CNCzBN device displays superior lifetime of 20 h up to 70% of its initial luminance (L0 = 1000 cd m−2). It is believed that this study will provide fundamental insights to develop highly efficient cyano featured carbazole‐based host materials for blue PhOLEDs with superior EQE and long lifetime.

Simultaneous Achievement of High Efficiency and Long Lifetime in Deep Blue Phosphorescent Organic Light‐Emitting Diodes

AbstractHigh triplet energy exciplex hosts are developed using new electron transport type hosts with triphenylsilyl blocking groups to obtain high solid state triplet energy and exciplex triplet energy. Two triphenyltriazine derived hosts, 2‐phenyl‐4,6‐bis(4‐(triphenylsilyl)phenyl)‐1,3,5‐triazine (pSiTrz) and 2‐phenyl‐4,6‐bis(3‐(triphenylsilyl)phenyl)‐1,3,5‐triazine (mSiTrz), are synthesized as the high triplet energy hosts with electron transport properties. The two hosts show high triplet energy over 2.90 eV both in solution and solid film, and afford high triplet energy over 2.90 eV in the mixed hosts with a hole transport type 2,2′‐di(9H‐carbazol‐9‐yl)‐1,1′‐biphenyl (oCBP) host. The doping of a deep blue phosphorescent emitter in the mixed host oCBP:mSiTrz achieves high external quantum efficiency over 20% with a color coordinate of (0.14, 0.16) and device lifetime of longer than 1000 h at 100 cd m‐2. This is the first demonstration of simultaneous achievement over 20% external quantum efficiency (EQE) and 1000 h lifetime in the deep blue organic light‐emitting diodes with a x+y color coordinate value below 0.30.

Highly Twisted Donor-Acceptor Boron Emitter and High Triplet Host Material for Highly Efficient Blue Thermally Activated Delayed Fluorescent Device.

New highly efficient thermally activated delayed fluorescence (TADF) dopant materials (PXB-DI and PXB-mIC) for blue organic light-emitting diodes are reported. These materials were designed by combining highly conjugated rigid ring donor moieties and a boron acceptor with a highly twisted configuration to have high TADF performance and minimized self-quenching properties. In addition, a new high triplet energy and hole transport-type host material, 5-(5-(2,4,6-triiso-propylphenyl)pyridin-2-yl)-5 H-benzo[ d]benzo[4,5]imidazo[1,2- a]imidazole (PPBI), is also reported. This host represents deeper blue color owing to keeping the original spectra of emitters. A fabricated blue TADF device with PXB-mIC in the PPBI host exhibited maximum external quantum efficiency (EQE) of 12.5% with a CIE of (0.15, 0.08), which is close to that of the National Television System Committee blue color. The blue TADF device performances of the PPBI host was compared with the electron transport-type 2,8-bis(diphenylphosphine oxide)dibenzofuran (DBFPO) host. The blue TADF device with PXB-DI in the DBFPO host exhibited a maximum EQE of 37.4% in the sky blue region. This study demonstrates that our molecular design concept of new emitters and host is beneficial for future high-efficiency deep-blue TADF devices.

Less Is More: Dilution Enhances Optical and Electrical Performance of a TADF Exciplex.

A surprising yet highly practical approach to improve the performance of a TADF exciplex blend is reported. Using the TSBPA donor and PO-T2T acceptor to form an exciplex, we are able to blue shift the emission, increase PLQY from 58 to 80%, and increase the device EQE from 14.8 to 19.2% by simply diluting the exciplex with an inert high triplet energy host material—here either UGH-3 or DPEPO. These effects are explained in terms of an increasing donor–acceptor distance and associated charge separation, while different behaviors observed in the different hosts are attributed to different energy barriers to electron transfer through the host. We expect that the observed performance-enhancing effects of dilution will be general to different exciplex blends and host materials and offer a new way to optimize the electrical properties of exciplex emission layers with narrow blue emission.

Design of High‐Efficiency and Long‐Lifetime White Organic Light‐Emitting Diodes by Selective Management of Singlet and Triplet Excitons Using a Triplet Exciton Manager

AbstractA novel device design extending the lifetime of white organic light‐emitting diodes (WOLEDs), comprising a thermally activated delayed fluorescent (TADF) blue emitter and a yellow phosphorescent emitter, is developed using a triplet exciton manager (TEM), which selectively quenches the triplet exciton of the blue TADF emitter while harvesting the yellow phosphorescent emission. The TEM host is designed to have a lower triplet energy than the blue TADF emitter and a higher triplet energy than the yellow phosphorescent emitter. The singlet excitons of the TEM host are used to harvest only the singlet excitons of the blue TADF emitter by energy transfer, while the triplet excitons of the TEM host are utilized to activate the phosphorescent emission of the yellow emitter. The singlet and triplet excitons of the TEM host are thus selectively used to harness the blue singlet emission and yellow triplet emission, respectively. The triplet excitons of the blue TADF emitter can thus be excluded from the emission process, which improves the lifetime of the TEM‐based WOLEDs by five times while achieving similar external quantum efficiency as that of the conventional WOLED with a common high triplet energy host activating the triplet excitons of the blue TADF emitter.