Electron Transport Type Host Material Research Articles

A Design Strategy for Multiple Resonance‐Induced Pure Violet Thermally Activated Delayed Fluorescence Emitters with a Narrow Emission Band

AbstractThis study proposes a novel approach to develop highly efficient, narrow‐emitting violet materials based on boron and oxygen polycyclic aromatic hydrocarbon multiple resonance structure. Herein, B‐2OCz is developed by fusing indole with a 5,9‐dioxa‐13bboranaphtho[3,2,1‐de]anthracene (DOBNA) core to enhance its thermally activated delayed fluorescence (TADF) properties and molecular rigidity. On the other hand, the B‐2OCz‐Si is decorated with a bulky tetraphenylsilyl substituent. B‐2OCz‐Si exhibits exceptional features such as violet emission at 397 nm, a very small full width at half maximum of 16 nm, and 82% of photoluminescence quantum yield. The B‐2OCz‐Si devices achieve a high external quantum efficiency of over 15%, violet emission with a peak wavelength of 423 nm, and color coordinates of (0.156, 0.037). Furthermore, the B‐2OCz‐Si is used as an electron transport type host material for phosphorescent organic light‐emitting diodes (PhOLEDs), based on its high triplet energy and TADF properties. As compared to the conventional triazine based host materials, these newly developed DOBNA‐based materials display superior device lifetime performance. All these potential aspects corroborate that this new class of DOBNA‐based materials can work as a promising host material for PhOLEDs and violet‐emitting fluorescent devices.

Thermally activated delayed fluorescence type exciplex host for long lifetime in deep blue phosphorescent organic light-emitting diodes

High performance deep blue phosphorescent organic light-emitting diodes (PhOLEDs) were developed using a thermally activated delayed fluorescence type exciplex host. Three isomeric electron transport type host materials based on a triazine core, tetraphenylsilane and benzonitrile groups were synthesized for high triplet energy and good electron transport property in solid state. They formed thermally activated delayed fluorescence type exciplexes with a carbazole based hole transport type host. Among the three hosts, 3-(4,6-bis(3-(triphenylsilyl)phenyl)-1,3,5-triazin-2-yl)benzonitrile host achieved high external quantum efficiency of 21.0% and deep blue color coordinate of (0.14, 0.18). Moreover, the lifetime of the deep blue PhOLEDs was improved by more than 1.6 times using the new exciplex host compared with the state of the art mixed host.

Benzonitrile and dicyanocarbazole derived electron transport type host materials for improved device lifetime in blue thermally activated delayed fluorescent organic light-emitting diodes

The electron transport host materials derived from dibenzofuran, benzonitrile and dicyanocarbazole units showed high triplet energy above 3.0 eV, high efficiency over 20% and 31 h lifetime up to 75% of initial luminance at 500 cd m−2.

Development of solution-processable blue/hybrid-white OLEDs based on thermally activated delayed fluorescence

In this work, hybrid-WOLEDs are fabricated by employing single emitting layers (S-EMLs), which consist of 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (DTPDDA) as a blue thermally activated delayed fluorescent (TADF) emitter and bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)iridium(III) (Ir(MDQ)2acac) as a phosphorescent red emitter. This architecture employs EMLs of a blue exciplex-forming co-host, which are hole transport type host material was 1,3-bis(N-carbazolyl) benzene (mCP) and the electron transport type host material was diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1). The resulting hybrid-WOLEDs showed maximum external quantum efficiency (EQE) of 6.16% and current efficiency (CE) of 11.58cd/A with Commission Internationale de L’Eclairage coordinates of (0.32, 0.33).

High triplet energy exciplex hosts for deep blue phosphorescent organic light-emitting diodes

High triplet energy exciplex hosts for deep blue phosphorescent organic light-emitting diodes were developed by synthesizing a high triplet energy hole transport type host material designed for exciplex formation with a high triplet energy electron transport type host material derived from a diphenyltriazine.

Effect of a broad recombination zone with a triple-emitting layer on the efficiency of blue phosphorescent organic light-emitting diodes

The device characteristics of blue phosphorescent organic lightemitting diodes (PHOLEDs) with a broad recombination region within emitting layers (EMLs) were investigated by changing the combination and the composition of the host materials. Six types of devices were fabricated with the novel host material 9-(4-(triphenylsilyl)phenyl)-9H-carbazole, hole transport-type host material N,N′-dicarbazolyl-3,5-benzene, and electron transporttype host material 2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole] as diverse EML structures. Balanced chargecarrier injection and a distributed recombination zone within EMLs were achieved through a triple-emitting layer (T-EML). The properties of a device with a T-EML using a stepwise structure without any mixed host system were found to be superior to the other PHOLEDs. This can be explained in terms of improved charge balance and triplet-exciton confinement within the broad recombination region. Open image in new window

Improved efficiency of red phosphorescent organic light-emitting diodes with combination of heterojunction and mixed host structure.

We report an improvement of efficiency in red phosphorescent organic light-emitting diodes (PHOLEDs) based on a combination of heterojunction (HJ) structure and mixed host (MH) system using a phosphorescent red emitter: bis(2-phenylquinolinato)-acetylacetonate iridium III [Ir(pq)2(acac)] doped in 4,4,N,N'-dicarbazolebiphenyl (CBP) of hole transport type host material and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) of electron transport type host material. This combination device resulted in an effective electron and hole balance and distribution of the recombination zone. Therefore, highly efficient red PHOLEDs with maximum luminous efficiency and external quantum efficiency of 21.93 cd/A and 14.09% were achieved. Moreover, the combination device showed a power efficiency of 9.51 lm/W, which is higher than 7.61 lm/W in the control device at a luminance of 1000 cd/m2.

Enhanced efficiency of blue phosphorescent organic light-emitting diode base on triple-emitting layer with mixed host system.

We report an improvement of efficiency in blue phosphorescent organic light-emitting diodes (PHOLEDs) based on triple-emitting layer (T-EML) with mixed host (MH) system using a phosphorescent blue emitter: iridium(III)bis[(4,6-di-fluoropheny)-pyridinato-N,C2]picolinate (Flrpic) doped in N,N'-dicarbazolyl-3,5-benzene (mCP) of hole transport-type host material and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) of electron transport-type host material. This T-EML device resulted in both an effective electron and hole balance and efficient distribution of the recombination zone. As a result, the property of T-EML device which demonstrated a maximum luminous efficiency of 24.45 cd/A and a external quantum efficiency of 10.9%. It also showed a high maximum power efficiency of 13.82 lm/W, which is approximately two times higher than that of the control device.

High efficiency blue phosphorescent organic light-emitting diodes using 2-(1H-pyrazol-1-yl)pyridin-3-ol ligand based Be compound

An organometallic complex based on Be and pyridinepyrazole type ligand, bis(2-(1H-pyrazol-1-yl)pyridin-3-olate) beryllium (BePyPy), was synthesized as an electron transport type host material for blue phosphorescent organic light-emitting diodes. High triplet energy of 2.79 eV was obtained and the highest occupied molecular orbital/lowest unoccupied molecular orbital of BePyPy was −6.26 eV/−2.71 eV. The BePyPy host was effective as the host material for blue phosphorescent organic diodes and a high quantum efficiency of 19.7% was achieved. In addition, a mixed host device with the BePyPy host showed high quantum efficiency of 22.0% in the blue device.

Electroluminescence enhancement in blue phosphorescent organic light-emitting diodes based on different hosts

Blue phosphorescent organic light-emitting diodes (OLEDs) are fabricated by utilizing the hole transport-type host material of 1,3-bis(carbazol-9-yl)benzene (MCP) combined with the electron transport-type host material of 1,3-bis (triphenylsilyl) benzene (UGH3) with the ratios of 1:0, 8:2 and 6:4, and doping with blue phosphorescent dopant of bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic). The device with an optimum concentration proportion of MCP:UGH3 of 8:2 exhibits the maximum current efficiency of 19.18 cd/A at luminance of 35.71 cd/m2 with maintaining Commission Internationale de L’Eclairage (CIE) coordinates of (0.1481, 0.2695), which is enhanced by 35.7% compared with that of 1:0 with (0.1498, 0.2738). The improvements are attributed to the effective carrier injection and transport in emitting layer (EML) because of mixed host materials. In addition, electron and exciton are confined in the EML, and 4,4′,4″-Tris(carbazol-9-yl)-triphenylamine (TCTA) and Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) have the high lowest unoccupied molecular orbital (LUMO) energy level and triplet exiton energy.

Novel electron-type host material for unilateral homogeneous phosphorescent organic light-emitting diodes with low efficiency roll-off

Two novel simple electron-transport (ET) type host materials, 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)benzene (MDBIP) and 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)pyridine (MDBIPy) have been designed and synthesized. The two compounds exhibit high decomposition temperatures (Td: 444 °C for MDBIP and 450 °C for MDBIPy) and a stable amorphous glassy state (Tg: 108 °C for MDBIP and 110 °C for MDBIPy). In the typical device ITO/MoO3 (10 nm)/4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB, 80 nm)/TCTA (5 nm)/Host: Ir(ppy)3 (9 wt%, 20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), when MDBIPy was used as host material, the device B showed a maximum efficiency of 21.1%, 75.8 cd A−1 and 83.6 lm W−1. When MDBIPy was utilized as both the electron transporting and host layer, the unilateral homogeneous device C exhibited a maximum efficiency of 20.6%, 74.2 cd A−1 and 71.8 lm W−1. More interestingly, device C showed low efficiency roll-off relative to device B. When the brightness of the device is over 1000 cd m−2, the current efficiency of device C is higher than device B. At a brightness of 5000 cd m−2, the current efficiency of device C is only roll off 5.6%. These results demonstrate that utilizing electron-transport type host materials to fabricate unilateral homogeneous PhOLEDs is a promising way to simplify the device configuration and optimize the performance of OLED devices.

High efficiency deep blue phosphorescent organic light-emitting diodes using a tetraphenylsilane based phosphine oxide host material

High efficiency deep blue phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated using a tetraphenylsilane based phosphine oxide derivative, bis(4-(diphenylphosphoryl)phenyl)diphenylsilane (BDDS), as the electron transport type host material. The BDDS was effective as the host material for deep blue PHOLED and a mixed host of the BDDS with hole transport type host material further improved the quantum efficiency of deep blue PHOLEDs. A maximum quantum efficiency of 18.1% and a quantum efficiency of 12.2% at 1000cd/m2 with a color coordinate of (0.137, 0.191) were achieved.

Red phosphorescent organic light-emitting diodes using pyridine based electron transport type triplet host materials

Pyridine based electron transport type host materials were developed and their device performances were investigated according to doping concentration. The pyridine substituent was combined with a spirofluorenebenzofluorene core unit and a high quantum efficiency of 13.3% was achieved in red phosphorescent organic light-emitting diodes at a low doping concentration of 2%. A simple red device without any electron transport layer could be fabricated and a simple device without any electron transport layer showed better power efficiency than the standard device with an electron transport layer.

Simplified white phosphorescent organic light-emitting diodes without any charge transport layer

High efficiency simple white phosphorescent organic light-emitting diodes without any charge transport layer were developed using an electron transport type high triplet energy host material. A red emitting layer/interlayer/blue emitting layer stack structure was inserted between an anode and a cathode. The electron transport type host material was used as the host for the blue emitting layer and it was mixed with a hole transport type host material in the red emitting layer. A high current efficiency of 16.2 cd/A and a maximum power efficiency of 16.5 lm/W could be obtained from the simple white device.

Theoretical maximum quantum efficiency in red phosphorescent organic light-emitting diodes at a low doping concentration using a spirobenzofluorene type triplet host material

Abstract Red phosphorescent organic light-emitting diodes (PHOLEDs) with a 100% internal quantum efficiency at a low doping concentration were fabricated using a spirobenzofluorene type phosphine oxide as the host material. A spirobenzofluorene type phosphine oxide compound, 2,7-bis(diphenylphosphoryl)spiro[fluorene-7,11′-benzofluorene] (SPPO21), was synthesized as the electron transport type host material for the red phosphorescent organic light-emitting diodes. The device performances of the SPPO21 based red PHOLEDs were optimized at a low doping concentration of 1–2% because of energy transfer, little concentration quenching and charge trapping. A high external quantum efficiency of 20% was achieved for the red PHOLEDs with the SPPO21 host.

Stable efficiency roll-off in blue phosphorescent organic light-emitting diodes by host layer engineering

The device characteristics of blue phosphorescent organic light-emitting diodes (PHOLEDs) with mixed host structure were investigated by changing the combination and the composition of host materials in emissive layer. The distributed recombination zone and balanced charge carrier injection within emissive layer were achieved through mixed host optimization with a hole transport-type and an electron transport-type host materials, therefore the device performances were greatly enhanced, with external quantum and power efficiencies of 21.8% and 53 lm/W. Moreover, mixed host blue PHOLEDs exhibited a extremely low stable efficiency roll-off with quantum efficiencies of 20.3% and 18.6% at a luminance of 1000 and 10,000 cd/m 2.

Improved efficiency in solution processed green phosphorescent organic light-emitting diodes using a double layer emitting structure fabricated by a stamp transfer printing process

A high efficiency soluble green phosphorescent organic light-emitting diode (PHOLED) was developed using a double layer emitting structure fabricated by a stamp transfer printing process. One green emitting layer with a hole transport type host material was coated on a hole transport layer and the other green emitting layer with an electron transport type host material was stamp transfer printed on the green emitting layer. The efficiency of solution processed green PHOLEDs was more than quadrupled using the double layer emitting structure fabricated by the stamp transfer printing method.

1,3,5-Triazine derivatives as new electron transport–type host materials for highly efficient green phosphorescent OLEDs

We have synthesized three star-shaped 1,3,5-triazine derivatives—2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), 2,4,6-tris(triphenyl-3-yl)-1,3,5-triazine (T3T), and 2,4,6-tris(9,9′-spirobifluorene-2-yl)-1,3,5-triazine (TST)—as new electron transport (ET)-type host materials for green phosphorescent organic light-emitting devices. The morphological, thermal, and photophysical properties and the electron mobilities of these ET-type host materials are influenced by the nature of the aryl substituents attached to the triazene core. The meta–meta linkage between the 1,3,5-triazine core and the peripheral aryl moieties in T2T and T3T limited the effective extension of their π conjugation, leading to high triplet energies of 2.80 and 2.69 eV, respectively. Time-of-flight mobility measurements revealed the good electron mobilities for these compounds (each > 10−4 cm2V−1 s−1), following the order T3T > TST > T2T. The device incorporating T2T as the host, doped with (PPy)2Ir(acac) and 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TBPI) as the ET layer, achieved a high external quantum efficiency (ηext) of 17.5% and a power efficiency (ηp) of 59.0 lm W−1. For the same device configuration, the T3T-based device provided values of ηext and ηp of 14.4% and 50.6 lm W−1, respectively; the TST-based device provided values of 5.1% and 12.3 lm W−1, respectively. We ascribe the superior performance of the T2T-based devices to balanced charge recombination; we ascribe the poor efficiencies of the TST-based devices to its relatively low triplet energy (2.54 eV), which did not allow efficient confinement of the triplet excitons on the green phosphorescent emitter (PPy)2Ir(acac).