High Triplet Energy Host Materials Research Articles

High triplet energy host material with a 1,3,5-oxadiazine core from a one-step interrupted Fischer indolization

Energy-efficient and deep-blue organic light-emitting diode (OLED) with long operating stability remains a key challenge to enable a disruptive change in OLED display and lighting technology. Part of the challenge is associated with a very narrow choice of the robust host materials having over 3 eV triplet energy level to facilitate efficient deep-blue emission and deliver excellent performance in the OLED device. Here we show the molecular design of new 1,3,5-oxadiazines (NON)-host materials with high triplet energy over 3.2 eV, enabling deep-blue OLED devices with a peak external quantum efficiency of 21%. A series of NON-host materials are prepared by the condensation of substituted arylhydrazines and cyclohexylcarbaldehyde in a 2:3 ratio. This straightforward “one-pot” procedure enables the formation of indoline-containing derivatives with three fused heterocyclic rings and two stereogenic centres. All materials emit UV-fluorescence in the range of 315–338 nm while possessing highly desirable characteristics for application in deep-blue OLED devices: good thermal stability, a wide energy gap (3.9 eV), a high triplet energy level of (3.3 eV), and excellent volatility during sublimation.

High triplet hexahydroacridine derivatives as a host prevent exciton diffusion to adjacent layers in solution processed OLEDs

One important key to improve OLEDs technology is the development and synthesis of high triplet energy host materials, which play a crucial role in improving the efficiency and lifetime. The present approach shows that it is possible to control the properties of the host materials by carefully selecting the units. Therefore, a hexahydroacridine derivative was chosen to increase the ET value due to lower conjugation. In this study, three hosts with high triplet energy (>3 eV) were designed and investigated based on hexahydroacridine (ACD) as a constant unit and branches of triphenylamine (TPA), pyrene and pyridine derivatives as different groups. Density functional theory (DFT) calculations showed the agreement of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) values with the experimental results, with the highest HOMO from DFT calculation at 5.95 eV and cyclic voltammetry (CV) at 6.09 eV for the ACD-PYRIDINE. The photophysical properties were fully discussed and revealed the fluorescence mechanism of the hosts, so that with the addition of 9-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]-N3, N3, N6, N6-tetraphenyl-9H-carbazole-3,6-diamine (DACT-II), thermally activated delayed fluorescence (TADF) was achieved in the emitter layer with different concentrations of the hosts of 50, 70 and 90 wt%. The promising synthesized hosts were used for the fabrication of green TADF OLEDs. The fabricated OLED based on 90 % wt. ACD-TPA had CIE coordinates X = 0.26385 and Y = 0.55236, with turn on voltage 3.5 V, as well as current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) of 40 cd A−1, 26 lm.W−1 and 13 %, respectively. Furthermore, the extracted brightness (52508 cd m−2) exceeded the values from previous studies based on acridine as an emitter layer in doped OLEDs.

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.

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.

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.

CN-carbazole modified diphenylsilane-type high triplet energy hosts for blue phosphorescent organic light-emitting diodes

Abstract Diphenylsilane-derived high triplet energy host materials were developed by coupling diphenylsilane with CN-modified carbazoles. Two host materials with two CN-modified carbazoles or one CN-modified carbazole were synthesized, and their photophysical and device behavior were investigated. The host materials delivered a high triplet energy over 3.00 eV, a high glass transition temperature over 130 °C, ambipolar charge transport, and high external quantum efficiency over 20% as hosts of deep blue-emitting Ir emitters.

Carbazole-dibenzothiophene core as a building block of host materials for blue phosphorescent organic light-emitting diodes

A high triplet energy core structure, carbazole-dibenzothiophene, was designed and synthesized as a building block of high triplet energy host materials for blue phosphorescent organic light-emitting diodes. The carbazole and dibenzothiophene were interconnected via 3- position of carbazole and 2- position of dibenzothiophene not to sacrifice the high triplet energy the two units. Three compounds with phenyl, dibenzothiophene and 9-phenylcarbazole functional groups as side groups of the carbazole-dibenzothiophene main structure worked as hosts providing high triplet energy over 2.80 eV and the host with a dibenzothiophene substituent achieved high external quantum efficiency over 20% in blue phosphorescent organic light-emitting diodes.

Highly Efficient Soluble Blue Delayed Fluorescent and Hyperfluorescent Organic Light-Emitting Diodes by Host Engineering.

Solution-processed high-efficiency fluorescent organic light-emitting diodes with an external quantum efficiency over 18% were developed by engineering a host material and device structure designed for solution process. A high triplet energy host material designed for the solution process, (oxybis(3-(tert-butyl)-6,1-phenylene))bis(diphenylphosphine oxide) (DPOBBPE), worked efficiently as the host of blue fluorescent devices because of good solubility, high photoluminescence quantum yield, and good film properties. The DPOBBPE host enabled a high external quantum efficiency of 18.8% in the fluorescent organic light-emitting diodes by the solution process. Moreover, 25.8% external quantum efficiency in the soluble blue thermally activated delayed fluorescent devices was also realized. The 25.8% external quantum efficiency of the DPOBBPE delayed fluorescent device and 18.8% external quantum efficiency of the fluorescent device are the highest efficiency values achieved in the solution-processed blue fluorescent organic light-emitting diodes. Moreover, the solution-processed fluorescent device showed an improved blue color coordinate of (0.14, 0.20) compared to (0.17, 0.31) of the delayed fluorescent device.

Lifetime enhancement of blue thermally activated delayed fluorescent devices by separated carrier channels using dibenzofuran-triazine type hosts

Lifetime enhancement of blue thermally activated delayed fluorescent (TADF) devices was achieved using a carrier separating host structure which was developed using dibenzofuran-triazine type electron-transporting type host materials. Two host materials built on a chemical platform of dibenzofuran-triazine were synthesized as high triplet energy host materials for a blue TADF emitter and were mixed with a common hole-transporting type host. The separation of the carrier channel in the emitting layer lead to voltage reduction, external quantum efficiency increase, and lifetime extension of blue TADF devices.

CN-Modified Host Materials for Improved Efficiency and Lifetime in Blue Phosphorescent and Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes.

CN-modified host materials, 9-(2-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile (o-CzCN) and 9-(3-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-carbazole-3-carbonitrile (m-CzCN), which can improve the external quantum efficiency and lifetime of both blue phosphorescent and thermally activated delayed fluorescent (TADF) emitters were developed. A molecular design approach to stabilize the molecular structure and reduce the energy gap produced two high triplet energy host materials of o-CzCN and m-CzCN compatible with the phosphorescent and TADF emitters. The new host materials lowered operation voltage, increased quantum efficiency, and elongated lifetime of both phosphorescent and TADF devices.

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.

Chemical Bond Stabilization and Exciton Management by CN Modified Host Material for Improved Efficiency and Lifetime in Blue Phosphorescent Organic Light‐Emitting Diodes

A high triplet energy host material for improved efficiency and elongated lifetime is developed by modifying carbazole linked terphenyl with a CN substituent. The CN modification strengthens electron injection and transport of the host material, which reduces the driving voltage, improved quantum efficiency, and power efficiency of blue phosphorescent organic light‐emitting diodes. Furthermore, the CN modification also extends the lifetime of the blue device due to the chemical bond stabilizing effect of the CN moiety in radical, anion, and cation states. Therefore, the molecular design modifying the backbone structure with CN is an efficient approach for high efficiency and long lifetime.

Triplet exciton recycling of a phosphorescent emitter by an up-conversion process using a delayed fluorescence type low triplet energy host material

A low triplet energy host material with thermally activated delayed fluorescence character was applied as the host material of a blue phosphorescent emitter and was compared with a high triplet energy host material.

Design, synthesis, characterization and application of a novel electron-deficient moiety 1,5-diazacarbazole in high triplet energy host materials

A novel electron deficient, high triplet energy moiety 1,5-diazacarbazole (NCz) was designed, synthesized and firstly introduced for the construction of phosphorescent host materials.

Acridine modified dibenzothiophene derivatives as high triplet energy host materials for blue phosphorescent organic light-emitting diodes

Acridine modified dibenzothiophene derivatives were synthesized as host materials of imidazole ligand based Ir emitters for blue phosphorescent devices. The shallow energy levels of the acridine based host materials by the strong electron donating acridine moiety were well matched with the energy levels of Ir blue triplet emitter derived from imidazole. Additionally, strong electron transporting character of the acridine moiety could increase hole density in the emitting layer and balance holes and electrons. The energy level matching between the host and dopant materials and increased hole density in the emitting layer provided a very high external quantum efficiency of 27.8% in the blue phosphorescent organic light-emitting diodes.

Carbazole-carboline core as a backbone structure of high triplet energy host materials

A carbazole-carboline core was developed as a backbone structure of high triplet energy host materials and three derivatives of the carbazole-carboline core were synthesized. The carbazole and carboline units were linked via the 3-position of the carbazole and the 6-position of the carboline, and the carbazole-carboline core was modified with cyanophenyl, phenyl and 9-carbazolylphenyl moieties to control the charge transport properties of the host materials. Triplet energy of the carbazole-carboline core derived host materials were above 2.85 eV irrespective of the substituent of the carbazole-carboline core. The new materials were tested as the host materials for blue phosphorescent organic light-emitting diodes and the quantum efficiency of the blue devices reached 23.4%.

Pyridoindole modified carbazole compounds as high triplet energy host materials of imidazole derived blue triplet emitters for high quantum efficiency

Carbazole compounds modified with a pyridoindole moiety were examined as thermally stable high triplet energy host materials for tris[1-(2,4-diisopropyldibenzo[b,d]furan-3-yl)-2-phenylimidazole] (Ir(dbi)3) based blue phosphorescent organic light-emitting diodes. A well-known carbazole compound, N,N′-dicarbazolyl-3,5-benzene, was substituted with one or two pyridoindole moieties to develop the thermally stable host materials for Ir(dbi)3 blue triplet emitters. Remarkably high glass transition temperature of 196°C and thermal decomposition temperature of 486°C in addition to high triplet energy of 2.89eV were achieved by the pyridoindole modification. The pyridoindole modified carbazole compounds also delivered high quantum efficiency of 25.4% in the blue phosphorescent devices by doping Ir(dbi)3.

Phenothiazine dioxide based high triplet energy host materials for blue phosphorescent organic light-emitting diodes

Donor–acceptor type high triplet energy host materials were derived from a phenothiazine dioxide based electron transport moiety and a carbazole based hole transport moiety.

Tetraphenylsilane group containing carbazoles as high triplet energy host materials for solution-processable PhOLEDs

A series of solution processable, wide band-gap host materials composed of carbazole and tetraphenylsilane groups were designed and synthesized. Their thermal, electrochemical, and photophysical properties were fully investigated. The introduction of bulky tetraphenylsilane and \textit{tert}-butyl groups around the carbazole led to high glass transition temperatures (T$_{g})$ between 120 and 204 $^{\circ}$C. The triplet energies (E$_{T})$ of the synthesized materials were examined by low temperature (77 K) photoluminescence studies and determined as E$_{T}>$ 2.6 for all compounds. Phosphorescent organic light-emitting devices with the ITO/PEDOT:PSS/EML/TPBi/Cs$_{2}$CO$_{3}$/Al device structure were fabricated by using synthesized materials as the host and two kinds of phosphorescent emitters, FIrpic and Ir(ppy)$_{3}$, as the guests. The highest luminous and power efficiency values obtained by using FIrpic were 3.6 cd A$^{-1}$ and 1.48 lm W$^{-1}$, respectively, with commission International de I'Eclairage (CIE) coordinates of (0.17, 0.36), whereas these values were 7.8 cd A$^{-1}$ and 2.9 lm W$^{-1}$ for the device structure when Ir(ppy)$_{3}$ was used as the guest [CIE (0.28, 0.62)].