Electrophosphorescent Devices Research Articles

Blue Phosphorescence and Hyperluminescence Generated from Imidazo[4,5-b]pyridin-2-ylidene-Based Iridium(III) Phosphors.

Four isomeric, homoleptic iridium(III) metal complexes bearing 5‐(trifluoromethyl)imidazo[4,5‐b]pyridin‐2‐ylidene and 6‐(trifluoromethyl)imidazo[4,5‐b]pyridin‐2‐ylidene‐based cyclometalating chelates are successfully synthesized. The meridional isomers can be converted to facial isomers through acid induced isomerization. The m‐isomers display a relatively broadened and red‐shifted emission, while f‐isomers exhibit narrowed blue emission band, together with higher photoluminescent quantum yields and reduced radiative lifetime relative to the mer‐counterparts. Maximum external quantum efficiencies of 13.5% and 22.8% are achieved for the electrophosphorescent devices based on f‐tpb1 and m‐tpb1 as dopant emitter together with CIE coordinates of (0.15, 0.23) and (0.22, 0.45), respectively. By using f‐tpb1 as the sensitizing phosphor and t‐DABNA as thermally activated delayed fluorescence (TADF) terminal emitter, hyperluminescent OLEDs are successfully fabricated, giving high efficiency of 29.6%, full width at half maximum (FWHM) of 30 nm, and CIE coordinates of (0.13, 0.11), confirming the efficient Förster resonance energy transfer (FRET) process.

Pyridinyl-Carbazole Fragments Containing Host Materials for Efficient Green and Blue Phosphorescent OLEDs.

Pyridinyl-carbazole fragments containing low molar mass compounds as host derivatives H1 and H2 were synthesized, investigated, and used for the preparation of electro-phosphorescent organic light-emitting devices (PhOLEDs). The materials demonstrated high stability against thermal decomposition with the decomposition temperatures of 361–386 °C and were suitable for the preparation of thin amorphous and homogeneous layers with very high values of glass transition temperatures of 127–139 °C. It was determined that triplet energy values of the derivatives are, correspondingly, 2.82 eV for the derivative H1 and 2.81 eV for the host H2. The new derivatives were tested as hosts of emitting layers in blue, as well as in green phosphorescent OLEDs. The blue device with 15 wt.% of the iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) emitter doping ratio in host material H2 exhibited the best overall characteristics with a power efficiency of 24.9 lm/W, a current efficiency of 23.9 cd/A, and high value of 10.3% of external quantum efficiency at 100 cd/m2. The most efficient green PhOLED with 10 wt% of Ir(ppy)3 {tris(2-phenylpyridine)iridium(III)} in the H2 host showed a power efficiency of 34.1 lm/W, current efficiency of 33.9 cd/A, and a high value of 9.4% for external quantum efficiency at a high brightness of 1000 cd/m2, which is required for lighting applications. These characteristics were obtained in non-optimized PhOLEDs under an ordinary laboratory atmosphere and could be improved in the optimization process. The results demonstrate that some of the new host materials are very promising components for the development of efficient phosphorescent devices.

Does Through‐Space Charge Transfer in Bipolar Hosts Affect the Efficiency of Blue OLEDs?

AbstractAdditional energy loss pathways which occur in blue electrophosphorescent devices via through‐space charge transfer (TSCT) states of bipolar hosts are identified for the first time in this study. Blue phosphorescent organic light‐emitting diodes (PhOLEDs) with maximum external quantum efficiency from 22.7 to 30.5% are fabricated using new bipolar derivatives of benzimidazole, linked through phenyl spacer with different number of tert‐butyl substituted carbazoles, as hosts. Applicability of the newly synthesized compounds as hosts for blue PhOLEDs is justified by their high triplet levels (2.94–2.98 eV), bipolar charge‐transporting properties with charge mobilities up to 2.34 × 10−3 cm2 V−1 s−1 at electric field of 2.7 × 105 V cm−1, and high ionization potentials (5.63–5.81 eV). Weak microsecond‐lived emission, which is attributed to TSCT, is detected for the developed bipolar hosts. The different efficiencies of the fabricated PhOLEDs are partly related to energy losses via TSCT states, the presence of which is detected for the developed donor–acceptor hosts in both solution and solid‐state. To prevent energy losses through TSCT in PhOLEDs, the excited state energy of emitter has to be lower than triplet levels of bipolar host. It also has to be lower than the TSCT energy level of bipolar host for the fabrication highly efficient PhOLEDs.

Resonance hosts for high efficiency solution-processed blue and white electrophosphorescent devices

The exploration of high-performance solution-processible host materials for blue and white electrophosphorescent devices is a key and fundamental challenge in the ongoing development of organic semiconductors. Herein, two solution-processible resonance host materials with self-adaptive characteristics are delicately designed and constructed. Because of the dynamic tautomerization upon resonance variation, these smart hosts show self-adaptive and selectively enhanced charge carrier flux at high triplet energy levels. Conferred by the resonance molecules, solution-processed blue and white devices exhibit excellent maximum current efficiencies (CEs) of 29.8 and 57.3 cd A−1, and external quantum efficiencies (EQEs) up to 14.5% and 23.5%, respectively. Our works highlight the significant progress of the solution-processed phosphorescent organic light-emitting diodes (PhOLEDs) using resonance host molecules, potentially furnishing a leap forward in constructing advanced organic semiconductors for next-generation optoelectronic devices.

Novel self-host heteroleptic green iridium dendrimers based on carbazole dendrons for solution-processable non-doped phosphorescent organic light-emitting diodes

Several novel green-emitting iridium dendrimers has been successfully designed and synthesized with carbazole as the dendron, which is covalently attached to the emissive bis[2-(2-dibenzothienyl)pyridine]picolate iridium(III) [(DBTPy)2Irpic] core through a nonconjugated link to form an efficient self-host system for nondoped electrophosphorescent device applications. All the iridium dendrimers display green emission, possess good thermal stability and high photoluminescence quantum yields. As well as, non-doped solution-processed OLEDs were successfully fabricated based on the resulting Ir-1, Ir-2 and Ir-3 as green-emitting phosphors. By increasing the density of carbazole dendrons at the edge of the emissive core, luminescence self-quenching caused by the intermolecular interactions in the solid state can be significantly reduced. Therefore, the OLEDs based on Ir-3 show the best performances with a maximal luminous efficiency of 0.24 cd/A and a maximum brightness of 2520 cd/m2. This work will shed light on the development of novel self-host phosphorescent iridium dendrimers for solution processable nondoped electrophosphorescent devices.

Isomer dependent molecular packing and carrier mobility of N-phenylcarbazole–phenanthro[9,10-d]imidazole based materials as hosts for efficient electrophosphorescence devices

A phenanthro[9,10-d]imidazole derivative with enhanced carrier mobility was developed for high brightness and low efficiency roll-off phosphorescent organic light-emitting diodes.

Solution processible triphenylphosphine-oxide-cored dendritic hosts featuring thermally activated delayed fluorescence for power-efficient blue electrophosphorescent devices

A series of phosphine oxide based dendritic hosts have been developed. Compare with conventional dendritic host H2, the new hosts not only maintain high T1 but also have a small ΔEST and bring higher device performance.

Improvement on the performance of electrophosphorescent organic light-emitting device employing n-doped bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimi-dine as electron-transporting layer

Abstract The effects of using Cesium azide as an n-dopant in Bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimi-dine on the performances of green phosphorescent organic light-emitting device had been studied in detail for the first time. By optimizing the device structure, phosphorescent organic light-emitting device having a maximum current efficiency of 76.4 cd/A and a maximum external quantum efficiency of 21.7% was realized. In particular, the maximum power efficiency and the power efficiency at 100 cd/m2 were as high as 99.9lm/W and 99.4lm/W, respectively, which were better than the parameter on power efficiency of most reported Ir (ppy)3-OLEDs without out-coupling enhancement. Origins of the improvement on device characteristics in the n-doped device were systematically investigated by experimental and theoretical methods and the role of Cesium azide in enhancing the electron conductivity of the n-doped electron-transporting layer was discovered by X-ray photoelectron spectroscopy measurement.

Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers

Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.

Synthesis and properties of wide bandgap polymers based on tetraphenylsilane and their applications as hosts in electrophosphorescent devices

By introducing the electron-withdrawing diphenylphosphine oxide in PFSCPO, its doped OLEDs achieve enhanced current and external quantum efficiencies than the PFSC-hosted device.

High efficiency solution-processed blue electrophosphorescent device with a bipolar host material based on diphenylphosphine oxide unit

A new bipolar host material BCz-BPO for solution-processed blue phosphorescent OLED was designed and synthesized. The device showed a maximum luminance efficiency of 30.1 cd A−1, which was about three times higher compared with that of BCz-PO.

High-performance red electrophosphorescent devices based on all-solution-processed hydrogen-bonded supramolecular material

All-solution-processed, high-performance red phosphorescent OLED developed from hydrogen-bonded supramolecular material.

W-shaped bipolar derivatives of carbazole and oxadiazole with high triplet energies for electroluminescent devices

Four new compounds containing accepting oxadiazole and donating carbazole moieties were designed, synthesized and characterized by different techniques ranging from theoretical calculation to testing in electroluminescent devices. The derivatives show high thermal stability with 5% weigh loss temperature ranging from 359 to 383 °C and glass formation capability with glass transition temperatures being in the range of 107–147 °C. The solutions in non-polar solvents and solid neat films of the compounds or emit light in blue region with ca. 15% of photoluminescence quantum yields for non-doped films. The values of triplet energy (2.97–3.04 eV) of the synthesized oxadiazole and carbazole compounds were found to be higher than the energy of visible region excitons. Well-balanced hole and electron mobilities reaching 1 × 10−4 cm2/V at high electric fields were estimated for the layers of the compounds. Two derivatives of oxadiazole and carbazole were applied as hosts in blue electrophosphorescent devices. The best fabricated device demonstrated pure emission of a phosphorescent emitter and showed external quantum efficiency of 8.8% and brightness of 8700 cd/m2 at applied voltage of 10 V.

Carbazole/phenylpyridine hybrid compound as dual role of efficient host and ligand of iridium complex: Well matching of host-dopant for solution-processed green phosphorescent OLEDs

A new bipolar transport material of 9,9'-(4-(pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (2,4-dCzPPy) is designed and synthesized through a simple catalyst free C-N coupling reaction. The carbazole/phenylpyridine hybrid 2,4-dCzPPy is functionalized as dual role of efficient host material and cyclometalated ligands for green emissive Iridium (III) complex of bis(9,9'-(4-(pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole))(acetylacetonato) Ir(dCzPPy)2(acac). Their thermal, photophysical, electrochemical, and electrolumiescent (EL) properties are fully investigated. The D-π-A structure of bipolar compound of 2,4-dCzPPy suppresses the direct intramolecular interaction between the donor and acceptor subunits to give high triplet energy of 2.70 eV. Besides, solution processable electrophosphorescent devices by using 2,4-dCzPPy/Ir(dCzPPy)2(acac) as host/dopant to form emissive layers at a dopant concentration of 8 wt% show a maximum luminescence of 28622 cd/m2 at 10.3 V and maximum current efficiency of 31.8 cd/A, which is significantly higher than other host/dopant combinations, such as 21 cd/A of 4,4′-bis(9-carbazolyl)biphenyl (CBP):Ir(dCzPPy)2(acac) and 15 cd/A of 2,4-dCzPPy:tris(2-4-tolyl phenylpyridine) iridium (Ir(mppy)3), suggesting the well matching of the new host/dopant system which involves 2,4-dCzPPy both as the host matrix and the ligand of phosphorescent emitter.

A Host Material for Deep-Blue Electrophosphorescence Based on a Cuprous Metal–Organic Framework

Although metal–organic frameworks (MOFs) have promising applications in various fields, efforts to utilize MOFs as emissive layers in electroluminescent devices have not progressed well due to their intrinsic nature. Fortunately, by using fluorene derivatives and cuprous ions, a novel MOF (Cu-P6) has been synthesized for blue electrophosphorescence. The topological structure of Cu-P6 displays a 3-connected uninodal net with a cross-stacking pattern. Such a structure allowed phosphorescent Cu-P6 to exhibit a good heat resistance and luminous efficiency (Φ = 0.62) at crystalline state. By containing both donor and acceptor in the structure, Cu-P6 possesses a balanced electron and hole mobility. Especially, electrical-driven deep-blue emission of a MOF was reported for the first time. Overall, this work may open new perspectives on developing highly efficient electrophosphorescent devices based on MOFs. The utilization of earth-abundant cuprous ions is another attractive feature of Cu-P6.

Aminoborane-based bipolar host material for blue and white-emitting electrophosphorescence devices

A novel aminoborane-based host material, 9-(dimesitylboryl)-9′-phenyl-9H, 9′H-3,3′-bicarbazole (BCzBMes) was developed for blue and white phosphorescent OLEDs (PHOLEDs). The thermal, photophysical and electrochemical properties were systematically investigated. BCzBMes not only has a high triplet energy but also shows a bipolar behavior. To validate the superior properties of BCzBMes, blue and white PHOLEDs were fabricated using BCzBMes as a bipolar host material. A blue PHOLED containing Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrPic) as a dopant exhibited excellent performance with a maximum external quantum efficiency (EQE) of 16.7%. In particular, the blue PHOLED exhibited an extraordinary low efficiency roll-off of 10.1% at a brightness of 5000 cd/m2. Meanwhile, an all-phosphor near-white device hosted by BCzBMes was also fabricated, and a high EQE of 18.8% was achieved. This excellent performance suggests that BCzBMes is a potential bipolar host material for the PHOLEDs.

Dendron engineering in self-host blue iridium dendrimers towards low-voltage-driving and power-efficient nondoped electrophosphorescent devices.

Dendron engineering in self-host blue Ir dendrimers is reported to develop power-efficient nondoped electrophosphorescent devices for the first time, which can be operated at low voltage close to the theoretical limit (Eg/e: corresponding to the optical bandgap divided by the electron charge). With increasing dendron's HOMO energy levels from B-POCz to B-CzCz and B-CzTA, effective hole injection is favored to promote exciton formation, resulting in a significant reduction of driving voltage and improvement of power efficiency. Consequently, the nondoped device of B-CzTA achieves extremely low driving voltages of 2.7/3.4/4.4 V and record high power efficiencies of 30.3/24.4/16.3 lm W-1 at 1, 100 and 1000 cd m-2, respectively. We believe that this work will pave the way to the design of novel power-efficient self-host blue phosphorescent dendrimers used for energy-saving displays and solid-state lightings.

Solution-processed multilayer green electrophosphorescent devices with self-host iridium dendrimers as the nondoped emitting layer: achieving high efficiency while avoiding redissolution-induced batch-to-batch variation.

Self-host Ir dendrimers have been adopted as the nondoped emitting layer for the successful construction of multilayer green phosphorescent organic light-emitting diodes (PhOLEDs) prepared via layer-by-layer solution processing with orthogonal solvents. Unlike previous doped systems, the risk of small-molecular-phosphor redissolution by alcohols and the resultant serious batch-to-batch variation can be eliminated. Consequently, a record-high external quantum efficiency of 21.2% together with good reproducibility is achieved for the green-emitting Ir dendrimer G2, which displays sufficient alcohol resistance owing to the effective encapsulation from the second generation dendritic wedge. The obtained performance is highly competitive with those of doped devices, while avoiding the unwanted redissolution-induced batch-to-batch variation simultaneously, representing an important development in the solution fabrication of multilayer PhOLEDs based on a nondoped system.

An oligocarbazole-encapsulated heteroleptic red iridium complex for solution-processed nondoped phosphorescent organic light-emitting diodes with over 10% external quantum efficiency

By fully encapsulating the heteroleptic red Ir complex with carbazole dendrons, solution-processed nondoped electrophosphorescent devices reveal over 10% EQE.

Methoxyl modification in furo[3,2-c]pyridine-based iridium complexes towards highly efficient green- and orange-emitting electrophosphorescent devices

Highly efficient green- and orange-emitting PhOLEDs were successfully realized via a simple methoxyl modification in furo[3,2-c]pyridine-based Ir complexes.