Blue Electrophosphorescent Devices Research Articles

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.

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.

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.

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.

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.

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.

Highly Efficient Deep‐Blue Electrophosphorescent Pt(II) Compounds with Non‐Distorted Flat Geometry: Tetradentate versus Macrocyclic Chelate Ligands

A new class of deep blue electrophosphorescent Pt(II) emitters have been designed and synthesized. This new class of deep blue Pt(II) emitters employ tetradentate and macrocyclic chelate chromophores to constrain the Pt(II) molecules in a non‐distorted flat geometry in both the ground state and the excited state. The new deep blue emitters do not produce excimer emission, with emission quantum efficiency as high as 95% in 10% doped PMMA (poly(methyl methacrylate) films, and excellent UV stability, compared to the corresponding bidentate Pt(II) emitters. The macrocyclic tetradentate chelate Pt(II) compounds are the first examples of fully sterically constrained deep blue Pt(II) emitters that do not display structural distortion and have a higher thermal stability and a higher emission quantum efficiency than the corresponding non‐macrocyclic tetradentate Pt(II) analogues. A computational study supports that the macrocylic Pt(II) compounds are structurally more stable than the tetradentate Pt(II) molecules. Bright and efficient deep blue electrophosphorescent devices using a macrocyclic Pt(II) emitter have been successfully fabricated with a maximum brightness of 10 680 cd m−2, maximum external quantum efficiency of 15.4% (at 490 cd m−2), and Commission Internationale de L'Eclairage (1931) coordinates (x + y) of less than or near 0.30, respectively.

A Bulky Pyridinyl?uorene/Triphenylamine Hybrid Used as Host Material for Heavily‐Doped Blue Electrophosphorescent Devices

AbstractIn this work, a novel molecule pyridinyl?uorene/triphenylamine hybrid (TPyFTPA) with bulky steric hindrance effects has been synthesized successfully by substituting 9‐(pyridine‐2‐yl)‐fluoren‐9‐yl with triphenylamine (TPA) via Friedel‐Crafts reaction, which possesses good thermal stability and triplet energy (ET) of 420°C with 5% weight loss and 2.86 eV, respectively. Moreover, the bulky steric hindrance material shows high stable morphology by heating to 200°C without finding melting phenomena and crystallization that is demonstrated by differential scanning calorimetry (DSC) curve. The bulky pyridinyl?uorene end‐capped TPA has been used as host material for blue phosphorescent organic light‐emitting diodes (PhOLEDs) with maximum external quantum efficiencies (EQEs) of 2.7%, 3.7%, and 3.5%, at the doping ratios of 10%, 30%, and 40%, respectively. The performances of TPyFTPA‐based blue PhOLEDs own wide concentration ranging from 10% to 40%, which indicates the bulky TPyFTPA might be a potential candidate for inexpensive products with simplifying process for the applications in full‐color display and solid state lighting.

Influence of the D/A ratio of 1,3,5-triphenylbenzene based starburst host materials on blue electrophosphorescent devices: a comparative study

A series of starburst host materials, composed of a central 1,3,5-triphenylbenzene (TPB) skeleton with different peripheral modifications were purposefully designed and synthesized by tuning the ratio of p-type carbazole units and n-type diphenylphosphine oxide (DPPO) units from 0 : 3 to 3 : 0. Their optical, electrochemical and thermal properties were investigated systematically. By gradually tuning the D : A ratio of the peripheral groups, the desired ambipolar characteristics were achieved by DCzPOPB and CzDPOPB. As a result, the blue phosphorescent organic light-emitting diode (PhOLED) fabricated with the hosts doped with FIrpic achieved a low turn-on voltage of 3.3 V, a maximum current efficiency of 32.2 cd A−1, a maximum external quantum efficiency of 16.5%, a maximum luminescence of 22 440 cd m−2, as well as a low efficiency roll-off. Compared to the unipolar analogues, the device performances were significantly improved. In addition, the systematic study of the varied ratios of DPPO and carbazole units revealed the structure–property relationships of these starburst molecules, and a new avenue is proposed for the design of bipolar hosts for blue PhOLEDs.

Tuning the energy gap and charge balance property of bipolar host by molecular modification: Efficient blue electrophosphorescence devices based on solution-process

Three bipolar hosts composed of electron-accepting diphenylphosphine oxide and electron-donating carbazole/triphenylamine have been synthesized and characterized. With structural topology modification, the particular physical properties of the materials can be subtly optimized, such as the thermal stability, singlet–triplet energy gap and charge balance ability. Both DFT calculation and experiment results demonstrate that the introduced triphenylamine can effective minimize the HOMO–LUMO energy gap, while the carbazole units can prevent the excited energy loss and keep high triplet energy (ET=3.0eV) due to the enhanced molecular rigidity. As a result, solution-processed blue PHOLEDs exhibited a high current efficiency of 25.2cdA−1 and a power efficiency of 11.5lmW−1, which implies that the unique molecular modulation is very cost-effective and competitive for the device performance improving.

Bis(phosphine oxide)/triphenylamine based material for solution-processed blue electrofluorescent and green electrophosphorescent devices

A novel multifunctional material TPA-BPhPO has been designed and synthesized to explore its use as the blue emitter and the host material for solution-processed blue electrofluorescent and green electrophosphorescent devices.

Enhanced electron affinity and charge balance property of a bipolar material: highly efficient solution-processed deep blue electrofluorescent and green electrophosphorescent devices

The slight move in the ratio of the donor and acceptor group can slightly affect the optoelectronic property as a whole, and consequently modulate the electron affinity, charge balance property and exciton recombination efficiency.

A high triplet energy small molecule based thermally cross-linkable hole-transporting material for solution-processed multilayer blue electrophosphorescent devices

A novel cross-linkable material with high triplet energy was synthesized to explore its use as a hole-transporting layer for efficient solution processed multilayer blue electrophosphorescent devices.

A binary solvent mixture-induced aggregation of a carbazole dendrimer host toward enhancing the performance of solution-processed blue electrophosphorescent devices

The trade-off between charge transport and energy transfer is realized by manipulating the dendrimer host H2 aggregation with binary solvent mixture, along with 25% device efficiency enhancement for FIrpic based blue PhOLED.

Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes

Organic light-emitting diodes are a major driving force of the current information display revolution due to their low power consumption and potentially long operational lifetime. Although electrophosphorescent organic emitters have significantly lower power consumption than fluorescent emitters, the short lifetime of electrophosphorescent blue devices has prevented their application in displays for more than a decade. Here, we demonstrate a novel blue electrophosphorescent device with a graded dopant concentration profile in a broadened emissive layer, leading to a lower exciton density compared with a conventional device. Thus, triplet-polaron annihilation that leads to long-term luminescent degradation is suppressed, resulting in a more than threefold lifetime improvement. When this strategy is applied to a two-unit stacked device, we demonstrate a lifetime of 616±10 h (time to 80% of the 1,000 cd m(-2) initial luminance) with chromaticity coordinates of [0.15, 0.29], representing a tenfold lifetime improvement over a conventional blue electrophosphorescent device.

Solution‐Processable Hosts Constructed by Carbazole/PO Substituted Tetraphenylsilanes for Efficient Blue Electrophosphorescent Devices

Two new solution‐processable wide bandgap materials, bis(4‐((4‐(9‐H‐carbazol‐9‐yl)phenyl)diphenylsilyl)phenyl)(phenyl)phosphine oxide (CS2PO) and bis(4‐((4‐(9‐H‐(3,9′‐bicarbazol)‐9‐yl)phenyl)diphenylsilyl)phenyl)(phenyl)phosphine oxide (DCS2PO), have been developed for blue phosphorescent light‐emitting diodes by coupling an electron‐donating carbazole moiety and an electron‐accepting PO unit together via double‐silicon bridges. Both of them have been characterized as having high glass transition temperatures of 159–199 °C, good solubility in common organic solvent (20 mg mL−1), wide optical gap (3.37–3.55 eV) and high triplet energy levels (2.97–3.04 eV). As compared with their corresponding single‐silicon bridged compounds, this design strategy of extending molecular structure endows CS2PO and DCS2PO with higher thermal stability, better solution processability and more stable film morphology without lowering their triplet energies. As a result, DCS2PO/FIrpic doped blue phosphorescent device fabricated by spin‐coating method shows the best electroluminescent performance with a maximum current efficiency of 26.5 cd A−1, a maximum power efficiency of 8.66 lm W−1, and a maximum external quantum efficiency of 13.6%, which is one of the highest efficiencies among small molecular devices with the same deposition process and device configuration.

Novel tri-carbazole modified fluorene host material for highly efficient solution-processed blue and green electrophosphorescent devices

A series of novel solution-processable small-molecule host materials: 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz comprising a fluorene monomer as the rigid core and tri-carbazole as the periphery have been designed and synthesized, and their optical, electrochemical, and thermal properties have been fully characterized. The host materials exhibit high glass-transition temperatures (231–310 °C) and high triplet energy levels (2.61–2.73 eV). High-quality amorphous thin films can be obtained by spin-coating the host materials from solutions. It is found that the HOMO level of the host materials can be tuned by linking the tri-carbazole unit to the 2,7 positions of the fluorine core, resulting in appropriate HOMO energy levels (−5.36 to −5.23 eV) for improved hole-injection in the device. Solution-processed blue and green electrophosphorescent devices bases on the developed host materials exhibit high efficiencies of 21.2 and 34.8 cd A−1, respectively.

Using an Organic Molecule with Low Triplet Energy as a Host in a Highly Efficient Blue Electrophosphorescent Device

AbstractTo achieve high efficiencies in blue phosphorescent organic light‐emitting diodes (PhOLEDs), the triplet energies (T1) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S1) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6‐difluorophenylpyridinato‐N,C2′)iridium(III)] tetrakis(1‐pyrazolyl)borate (FIr6; T1=2.73 eV). In both the photo‐ and electro‐excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic‐layer device, the maximum current efficiency of 37 cd A−1 and power efficiency of 40 Lm W−1 were achieved for the FIr6‐based blue PhOLEDs.

Using an Organic Molecule with Low Triplet Energy as a Host in a Highly Efficient Blue Electrophosphorescent Device

To achieve high efficiencies in blue phosphorescent organic light-emitting diodes (PhOLEDs), the triplet energies (T1) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S1) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6-difluorophenylpyridinato-N,C(2'))iridium(III)] tetrakis(1-pyrazolyl)borate (FIr6; T1=2.73 eV). In both the photo- and electro-excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic-layer device, the maximum current efficiency of 37 cd A(-1) and power efficiency of 40 Lm W(-1) were achieved for the FIr6-based blue PhOLEDs.