Ambipolar Host Research Articles

Constructing Highly Efficient Circularly Polarized Multiple Resonance Thermally Activated Delayed Fluorescence Materials with Intrinsically Helical Chirality.

Advanced circularly polarized multiple resonance thermally activated delayed fluorescence (CP-MR-TADF) materials synergize the advantages of circularly polarized luminescence (CPL), narrowband emission and TADF characteristic, which can be fabricated into highly efficient circularly polarized organic light-emitting diodes (CP-OLEDs) with high color purity, directly facing the urgent market strategic demand of ultra-high-definition and 3D displays. In this contribution, based on edge topology molecular engineering (ETME) strategy, a pair of high-performance CP-MR-TADF enantiomers, (P and M)-BN-Py, has been developed, which merges the intrinsically helical chirality into MR framework. The optimized CP-OLEDs with emitters (P and M)-BN-Py and the newly developed ambipolar transport host PhCbBCz exhibit pure green emission with sharp peaks of 532nm, full-width at half-maximums (FWHMs) of 37nm, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.29, 0.68). Importantly, they achieve remarkable maximum external quantum efficiencies (EQEs) of 30.6% and 29.2%, and clear circularly polarized electroluminescence (CPEL) signals with electroluminescence dissymmetry factors (gEL s) of -4.37 × 10-4 and +4.35 × 10-4 for (P)-BN-Py and (M)-BN-Py, respectively. This article is protected by copyright. All rights reserved.

Ambipolar hosts for blue TADF OLEDs: Assessment of the device performance and lifetime

Designing host materials suitable for thermally activated delayed fluorescence (TADF) based OLEDs emitting in the blue spectral range imposes great challenge, since several important factors including high compound stability, high triplet energy and balanced charge carrier mobility must be fulfilled. In attempt to address the issue, herein, we employed ambipolar hosts (ATRZ and tert-butyl modified ATRZtBu) composed of acridine donors and triazine acceptor linked in a bridgeless configuration and exhibiting triplet energies above 3 eV. The performance of blue TADF OLEDs containing the ambipolar hosts was thoroughly assessed and compared with that featuring benchmark host mCBP. A special emphasis was placed on the device operational lifetime, which was found to be at least twice as long for ATRZ device compared to the reference mCBP device. The enhanced device stability was attributed to broadened/shifted recombination zone from EML/HBL interface towards the center of EML resulting in a suppressed degradation due to the reduced triplet exciton concentration.

Carbazole-substituted benzobisoxazoles: near-UV fluorescent emitters and ambipolar hosts for organic light-emitting diodes

The design of organic light emitting diode (OLED) materials with the potential for exhibiting thermally-activated delayed fluorescence (TADF) is reported.

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Ambipolar Self-Host Functionalization Accelerates Blue Multi-Resonance Thermally Activated Delayed Fluorescence with Internal Quantum Efficiency of 100.

Emerging multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters can combine 100% exciton harvesting and high color purity for their organic light-emitting diodes (OLED). However, the highly planar configurations of MR molecules lead to intermolecular-interaction-induced quenching. A feasible way is integrating host segments into MR molecules, namely a "self-host" strategy, but without involving additional charge transfer and/or vibrational components to excited states. Herein, an ambipolar self-host featured MR emitter, tCBNDADPO, is demonstrated, whose ambipolar host segment (DADPO) significantly and comprehensively improves the TADF properties, especially greatly accelerated singlet radiative rate constant of 2.11 × 108 s-1 and exponentially reduced nonradiative rate constants. Consequently, at the same time as preserving narrowband blue emission with an FWHM of ≈28nm at a high doping concentration of 30%, tCBNDADPO reveals state-of-the-art photoluminescence and electroluminescence quantum efficiencies of 99% and 30%, respectively. The corresponding 100% internal quantum efficiency of tCBNDADPO supported by an ultrasimple trilayer and heavily doped device demonstrates the feasibility of the ambipolar self-host strategy for constructing practically applicable MR materials.

Spiro Compounds for Organic Light-Emitting Diodes

ConspectusThe development of cutting-edge optoelectronic devices is rapidly changing science and technology. The organic light-emitting diodes (OLEDs), as one of the most applicable techniques of organic electronics, are experiencing the process of materials’ upgrade. Among all the organic units to construct OLED materials, the spiro structure and its derived compounds have been widely studied due to the good thermal stability and unique orthogonally rigid structure. It had shown great potential in first generation fluorescent OLEDs (FOLEDs) and second generation phosphorescent OLEDs (PhOLEDs). Recently, it has achieved outstanding performance in third generation thermally activated delayed fluorescence (TADF) OLEDs, which is attributed to its orthogonal structure caused by the spiro atom; thus the spiro compound can innately separate the hole and electron which is beneficial for minimizing the singlet–triplet splitting energy (ΔEST). The most classic aromatic spiro unit is 9,9′-spriobifluorene (SBF), which was first reported in 1930. The study of aromatic spiro compounds is closely related to the development of organic semiconductors; now, the structure–property relationship of spiro compounds is getting clearer and attracting wide attention from the material community.In this Account, we briefly summarize the efforts on the functionalization and application of spiro compounds in OLEDs. The functionalization of spiro compounds in our group mainly focuses on the development of electron-rich spiro backbone and the adjustment of different substitution positions. In this regard, we have systematically studied the electron-rich spiro backbones which could greatly facilitate the carrier injection/transport and the new derivation positions (C1, C3, and C4 at fluorene moiety) can markedly affect the energy levels of triplet excitons, which is crucial for host materials in OLEDs, especially for the blue ones. The two main methods, i.e., backbone modification and position engineering, can cooperate to construct a rigid scaffold to realize spatial donor/acceptor intramolecular interaction, which confines the donor/acceptor distance. Afterward, the application of spiro compounds in OLEDs is elucidated from FOLEDs to PhOLEDs, then to TADF OLEDs. The spiro[acridine-9,9′-fluorene] (SAF) unit is our first example to enhance the performance of blue and violet fluorescent emitters, which have reached the theoretical upper limit of FOLEDs. For PhOLEDs, the spiro-type compounds are mainly used as host materials. The substitution positions of the SBF unit are thus thoroughly investigated to afford host materials with high triplet energy. And the SAF unit is modified to form ambipolar host materials to balance hole/electron injection for better white PhOLEDs. During this research, we understood the importance of the orthogonal arrangement in the spiro structure for ambipolar material design. Finally, TADF materials based on the spiro compounds are also presented. The spiro materials used in TADF are usually performed as an individual donor or acceptor before our contribution. Our tendency is to utilize the through-space charge transfer, i.e., charge transfer occurring between the spatial-close donor and acceptor, to construct efficient TADF materials, which paves a new avenue for the future study of spiro compounds.

Enhanced performance of white organic light-emitting devices based on ambipolar white organic single crystals

Organic single crystals are highly promising for applications in optoelectronic devices because of their higher mobility and thermal stability than amorphous thin films. Although white organic single crystals have been fabricated by the double-doped method and applied to realize white organic light-emitting devices (WOLEDs), the unbalanced carrier transport properties of the unipolar crystals severely limit the device performance. Here, ambipolar white organic single crystals are obtained by using mixed p- and n-type molecules as an ambipolar host for the red and green dopants. The white crystal with balanced carrier transport and balanced blue, green, and red emission intensity was applied to the single-crystal WOLEDs. The highest brightness of 1956 cd m−2 and the current efficiency of 1.31 cd A−1 are achieved, which are the best performance of the single-crystal WOLEDs reported to date. A high color rendering index is obtained, which varies between 82 and 87 with increasing driving current. It is expectable that this strategy would support the practical applications of organic single crystal-based OLEDs.

High-triplet-energy Bipolar Host Materials Based on Phosphine Oxide Derivatives for Efficient Sky-blue Thermally Activated Delayed Fluorescence Organic Light-emitting Diodes with Reduced Roll-off

We designed and synthesized two new ambipolar host materials, namely CzPO and Cz3PO. Combining CzPO and Cz3PO with CzTRZ2 as the emitter resulted in improved maximum external quantum efficiencies o...

Excited-state engineering of universal ambipolar hosts for highly efficient blue phosphorescence and thermally activated delayed fluorescence organic light-emitting diodes

The development of high-performance universal host materials for both blue electrophosphorescence and thermally activated delayed fluorescence (TADF) is one of the main challenges for organic light-emitting diode (OLED) technology. In this work, based on four quaternary ambipolar hosts with insulated donor-acceptor (D-A) structures, the influence of the excited-state characteristics for these hosts on their device performance was systematically investigated. It was showed that on the premise of the similar electrical properties and excited-state energy levels for positive host-dopant energy transfer (ET), the weak exciton capture ability, embodied as low exciton binding energy (Eb), for the hosts could facilitate the Dexter ET to the dopant through charge migration. Since the charge-transfer -featured excited states and small Eb of blue TADF emitters, in contrast to the blue phosphorescence (PH) counterparts, the influence of the host Eb on the device efficiencies of blue TADF dyes was dramatically enlarged, reflecting the higher sensitivity of the latter to host characteristics. In consequence, 9CzFDBFDPO with the smallest Eb endowed its blue TADF and PH devices with the state-of-the-art performance, e.g. EQE beyond 20% and EQE roll-off within 20% at 500 cd m−2. This work showed that based on the systematically optimized excited-state characteristics, the ambipolar hosts can be competent in both high-efficiency blue PH and TADF OLEDs, which is crucial for their commercial applications.

Improved stability in organic light‐emitting devices by mixing ambipolar and wide energy gap hosts

AbstractWe demonstrate improved stability in phosphorescent organic light‐emitting devices (OLEDs) by incorporating a wide energy gap host material into an ambipolar emissive layer. Unlike conventional mixed‐host OLEDs that combine hole‐ and electron‐transporting hosts, charge transport in this device occurs primarily along the ambipolar host and the emitter, while the wide energy gap host serves to modify the charge injection and transport characteristics of the emissive layer. This approach allows both the width and position of the exciton recombination zone to be tuned without introducing exciplex states. Whereas overall device stability improves with increasing recombination zone width in conventional mixed‐host OLEDs, mixing in this system reduces the recombination zone extent yet still increases device lifetime. By decoupling luminance losses into the photostability of the emitter and the exciton formation efficiency, we show that this enhancement arises from a trade‐off between bulk and interfacial degradation. The addition of the wide energy gap host moves the recombination zone away from the interface between the hole‐transport layer and the emissive layer, sacrificing a modest increase in bulk degradation to substantially reduce interfacial degradation. We find that the lifetime can be improved by 50% by balancing these competing degradation pathways.

Enhanced near-infrared electroluminescence from a neodymium complex in organic light-emitting diodes with a solution-processed exciplex host

We report enhanced near-infrared (NIR) electroluminescence from a Nd3+-complex with thenoyltrifluoroacetone and 1,10-phenanthroline ligands. The NIR-emitting complex was blended into an exciplex-forming co-host system comprising 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene as the electron transport material and 4,4′,4″-tris(carbazol-9-yl)triphenylamine as the hole transport material in solution-processed small molecule organic light-emitting diodes (OLEDs). This binary ambipolar host system favors direct charge trapping and exciton formation on the Nd3+-complex molecules. Efficient energy transfer from the singlet and triplet exciplexes formed between the host molecules to the Nd3+ ions contributes to the enhanced luminescence efficiency. The photoluminescence quantum yield of this blend is 1.2%, and the optimized OLED shows a maximum electroluminescence external quantum efficiency of 0.034%. The device also exhibits a low efficiency roll-off of only 12% over a current density range of 100 mA/cm2, due to the reduced triplet-polaron annihilation.

Dipole Moment in the Excited State: An Important Property for TADF Hosts

In this issue of Chem, Xu and co-workers report highly efficient blue organic light-emitting diodes with small-efficiency roll-off by reducing the dipole moment of the host materials in the excited state. Their simple but effective molecular design introduces a σ-bridge into ambipolar host materials as a separator of π-conjugation.

Dipole-Dipole Interaction Management for Efficient Blue Thermally Activated Delayed Fluorescence Diodes

Summary Thermally activated delayed fluorescence (TADF) diodes have emerged in recent years. However, blue devices still suffer from low luminance and serious efficiency roll-off, partially because of the utilization of unipolar host materials, which can suppress interaction-induced quenching at the cost of carrier flux unbalance. Herein, we demonstrate that the strong host excited-state dipole field can significantly worsen exciton quenching of blue TADF dopants through host-dopant dipole-dipole interactions. To integrate low excited-state polarity and ambipolar characteristics, we developed a donor-σ-acceptor host with dramatically reduced excited-state dipole moments by one order of magnitude to only ∼2 Debye, which gave rise to state-of-the-art external quantum efficiency beyond 20% from its devices, as well as record-low roll-off. Comparison between the device efficiencies of the hosts and various excited-state polarities revealed that the suppression of dipole-dipole interaction-induced quenching is the primary determinant of device performance for ambipolar blue TADF hosts.

Theoretical Investigations of the Spiro‐Annulated Triphenylamine/ N‐Phenylcarbazole‐Based Ambipolar Host Materials for OLEDs

AbstractA series of spiro‐annulated triphenylamine/N‐phenylcarbazole derivatives as ambipolar host molecules have been designed by linking different electron acceptors (i. e., diphenylphosphine, phenylsulfone, triphenylsilane and diphenylborane). To reveal their electronic structures, optoelectronic properties and structure‐property relationships of the host materials, a throughout investigation was implied using density functional theory (DFT) and time‐dependent density functional theory (TDDFT) calculations. Results show that the triplet energy (ET), HOMO and LUMO energy levels, as well as some other properties i. e. ionization potentials (IPs), electron affinities (EAs), reorganization energies (λ) can be tuned by chemical modifications of different electron acceptors. The host‐guest models have also been built to investigate the match of the hosts and the reference guests. Comprehensively considering these indexes, the potential performance of 10‐phenyl‐2′‐(phenylsulfonyl)‐10H‐spiro[acridine‐9, 9′‐fluorene] (SOSTF) is a big leap in comparison with experimental host material diphenyl(10‐phenyl‐10H‐spiro[acridine‐9, 9′‐fluoren]‐2′‐yl)phosphine oxide (POSTF). It has been proved that a spiro‐annulated structure can be modified in constructing high ET host material for blue phosphorescent organic light‐emitting diodes.

Modulation of ambipolar charge transport characteristics in side-chain polystyrenes as host materials for solution processed OLEDs

A series of side-chain polystyrenes was developed as ambipolar hosts for solution processed organic light emitting diodes (OLEDs). The series was derived from the hole-only transport host molecule 1,3-Bis(N-carbazolyl)benzene (mCP). Electron transport ability was incorporated into the host polymers by the introduction of electron-poor heterocycles (pyridine or triazine) and extending delocalization of the lowest unoccupied molecular orbital (LUMO). The materials were tested in Ir-based green OLED devices with all organic layers processed from solution. Devices with the polymer combining triazine and carbazole on its side-chain exhibited a low luminance on-set voltage of 3.0 V and a current efficacy of 28.9 cd/A, which was more than 10 times higher than for devices with the mCP-based polymer (1.6 cd/A). The increase in performance is most likely due to an improvement of charge balance in the emissive layer, showing that our ambipolar polymers are good candidates for further wet-process optoelectronic applications.

Orthogonally Substituted Benzimidazole-Carbazole Benzene As Universal Hosts for Phosphorescent Organic Light-Emitting Diodes.

The novel ambipolar hosts of o-CbzBz and o-DiCbzBz contain carbazole and benzimidazole through an ortho-connection. The orthogonal conformations cause the triplet state to be confined at the carbazole units to secure efficient energy transfer. The phosphorescent organic light-emitting diodes (PhOLEDs) show a high current efficiency, power efficiency, and low efficiency roll-off. o-DiCbzBz can be used as a host for sky-blue, green, and orange-red PhOLEDs, giving 57.5, 78.4, and 60.3 cd/A, respectively.

Green and red phosphorescent organic light-emitting diodes with ambipolar hosts based on phenothiazine and carbazole moieties: photoelectrical properties, morphology and efficiency

The two low-molar mass compounds based on phenothiazine and carbazole moieties were synthesized and investigated as the ambipolar hosts for the green and red phosphorescent organic light emitting diodes.

Quantum chemical design of carbazole- and pyridoindole-based ambipolar host materials for blue phosphorescent OLEDs

Structure–property relationship of carbazole- and pyridoindole-type host materials for blue PhOLEDs using quantum chemical calculations.

Computational design of high efficiency nonplanar tri-s-triazine-based ambipolar host materials for phosphorescent blue emitters.

A series of nonplanar tri-s-triazine-based molecules were designed, and their optical, electronic, and charge transport properties as ambipolar host materials for blue electrophosphorescence emitters were explored by density functional theory. The influence of the linkage between tri-s-triazine and carbazole, diphenylamine and triphenylamine, as well as the influence of a series of electron-donating and electron-withdrawing substituents on triplet energy, energy level matching and charge transport of the designed molecules was discussed in detail. Our results reveal that the molecules under investigation can serve as host materials for blue electrophosphorescence emitters. We also predicted the mobility of designed molecules with better performance in the P1[combining macron] space group. Based on the investigated results, we proposed a rational way for the design of host materials for OLEDs, and also expanded the application field of tri-s-triazine.

Rational design of carbazolyl and aryl phosphine oxide (APO) based ambipolar host materials for blue electrophosphorescence: a density functional theory study

Carbazolyl and Aryl Phosphine Oxide (APO) based ambipolar host materials.