Thermally Activated Delayed Fluorescence Organic Light-emitting Diodes Research Articles

Benzonitrile/pyridylbenzoimidazole hybrid electron-transport material for efficient phosphorescence and TADF OLEDs

A new electron-transport material (ETM) of iTPyBI-CN is developed through non-catalytic C-N coupling reaction from 2,4,6-trifluorobenzonitrile and 2-pyridyl-1H-benzo[d]imidazole. Compared to the commercial ETM of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI), the donor–acceptor type iTPyBI-CN exhibits a more twisted geometry for comparable triplet energy, higher LUMO and slightly deeper HOMO levels. In both phosphorescence and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs), devices based on iTPyBI-CN as ETM demonstrates better electroluminescence (EL) efficiencies than TPBI, with maximum external quantum efficiency (EQE) of 20.2, 22.0, 16.3 and 16.1 % versus 16.4, 18.9, 11.0 and 3.0 % for various phosphorescent and TADF OLEDs.

Highly reduced efficiency roll-off with 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-containing unit on bipolar host for improved OLEDs device lifespan

The electrical, thermal, and optical properties of the host material used in the light-emitting layer of a thermally activated delayed fluorescence organic light-emitting diode (TADF-OLED) directly affect its efficiency, lifetime, and color. In this study, a new host material was designed, synthesized, and then applied to a green TADF-OLED device to evaluate its performance as a host. The host material was designed in a bipolar configuration, ensuring the balanced introduction and transport of electrons and holes. This balance was achieved by incorporating a carbazole unit to facilitate hole injection and transport. Moreover, a 5,9-dioxa-13b-boranaphtho [3,2,1-de]anthracene (DOBNA) unit containing a boron atom was introduced for electron injection and transport. These units were linked to ortho terphenyl spacers. The DOBNA unit improved the electron transport properties, the thermal stability of molecule, and anionic bond dissociation energy. Consequently, the device evaluated using the DOBNA-introduced material as a host for 4CzIPN, a TADF dopant, enhanced the performance by ∼ four times from LT80@1000 nit to 15.1 h compared to the unipolar host material comprising carbazole and ortho-terphenyl.

Excitonic Degradation Mechanisms in Phosphorescent and Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diodes

AbstractIn this study, the degradation mechanisms of triplet exciton harvesting organic light‐emitting diodes (OLEDs), namely, phosphorescent OLEDs and thermally activated delayed fluorescence (TADF) OLEDs, are investigated. Two common green emitters, fac‐tris(2‐phenylpyridine) iridium (III) (Ir(ppy)3) and 1,2,3,5‐tetrakis(carbazol‐9‐yl)‐4,6‐dicyanobenzene (4CzIPN), are doped in an exciplex‐forming cohost, and their degradation processes are comprehensively evaluated using various analytical approaches. Triplet–triplet‐annihilation induces the formation of defects, such as charge traps and exciton quenchers, triggering luminance loss during the device aging process of Ir(ppy)3‐based phosphorescent OLEDs. Electron trapping‐induced triplet‐polaron annihilation and narrow emission zone severely impair the device stability of 4CzIPN‐based TADF OLEDs, despite limited material degradation and charge trap formation.

Rational Molecular Design Strategy for Host Materials in Thermally Activated Delayed Fluorescence-OLEDs Suitable for Solution Processing.

Herein, a novel core molecule for V-shaped host molecules was synthesized, wherein two carbazoles were directly linked to cyclohexane. Cy-mCP and Cy-mCBP hosts were also successfully prepared for solution-processable thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). The Cy-mCP and Cy-mCBP molecules contained a cyclohexane linker directly linked to two small molecular hosts (mCP and mCBP), exhibiting twice the molecular weight while maintaining the basic properties of a single host molecule with improved film-forming ability and solubility in organic solvents. These host materials showed superior thermal stability and high glass transition temperatures compared to lower molecular weight hosts. Green TADF-OLEDs were prepared using the two host materials and 2,4,5,6-tetra(3,6-di-tert-butylcarbazol-9-yl)-1,3-dicyanobenzene (t4CzIPN) emitter, achieving device efficiencies similar to that of a low-molecular-weight host. However, after the incorporation of a V-shaped host, superior characteristics were observed in terms of the thermal stability and operational stability of the device. The synthesis of V-shaped molecules by directly linking two carbazoles to a cyclohexane linker is promising for the development of different hosts for solution-processable OLEDs.

Management of Host–Guest Triplet Exciton Distribution for Stable, High‐Efficiency, Low Roll‐Off Solution‐Processed Blue Organic Light‐Emitting Diodes by Employing Triplet‐Energy‐Mediated Hosts

AbstractHigh‐quality hosts are indispensable for simultaneously realizing stable, high efficiency, and low roll‐off blue solution‐processed organic light‐emitting diodes (OLEDs). Herein, three solution processable bipolar hosts with successively reduced triplet energies approaching the T1 state of thermally activated delayed fluorescence (TADF) emitter are developed and evaluated for high‐performance blue OLED devices. The smaller T1 energy gap between host and guest allows the quenching of long‐lived triplet excitons to reduce exciton concentration inside the device, and thus suppresses singlet‐triplet and triplet‐triplet annihilations. Triplet‐energy‐mediated hosts with high enough T1 and better charge balance in device facilitate high exciton utilization efficiency and uniform triplet exciton distribution among host and TADF guest. Benefited from these synergetic factors, a high maximum external quantum efficiency (EQEmax) of 20.8%, long operational lifetime (T50 of 398.3 h @ 500 cd m−2), and negligible efficiency roll‐off (EQE of 20.1% @ 1000 cd m−2) are achieved for bluish‐green TADF OLEDs. Additionally introducing a narrowband emission multiple‐resonance TADF material as terminal emitter to accelerate exciton dynamic and improve exciton utilization, a higher EQEmax of 23.1%, suppressed roll‐off and extended lifetime of 456.3 h are achieved for the sky‐blue sensitized OLEDs at the same brightness.

Power Efficiency Enhancement of Organic Light-Emitting Diodes Due to the Favorable Horizontal Orientation of a Naphthyridine-Based Thermally Activated Delayed Fluorescence Luminophore.

Four emitters based on the naphthyridine acceptor moiety and various donor units exhibiting thermally activated delayed fluorescence (TADF) were designed and synthesized. The emitters exhibited excellent TADF properties with a small ΔE ST and a high photoluminescence quantum yield. A green TADF organic light-emitting diode based on 10-(4-(1,8-naphthyridin-2-yl)phenyl)-10H-phenothiazine exhibited a maximum external quantum efficiency of 16.4% with Commission Internationale de L'éclairage coordinates of (0.368, 0.569) as well as a high current and power efficiency of 58.6 cd/A and 57.1 lm/W, respectively. The supreme power efficiency is a record-high value among the reported values of devices with naphthyridine-based emitters. This results from its high photoluminescence quantum yield, efficient TADF, and horizontal molecular orientation. The molecular orientations of the films of the host and the host doped with the naphthyridine emitter were explored by angle-dependent photoluminescence and grazing-incidence small-angle X-ray scattering (GIWAXS). The orientation order parameters (ΘADPL) were found to be 0.37, 0.45, 0.62, and 0.74 for the naphthyridine dopants with dimethylacridan, carbazole, phenoxazine, and phenothiazine donor moieties, respectively. These results were also proven by GIWAXS measurement. The derivative of naphthyridine and phenothiazine was shown to be more flexible to align with the host and to show the favorable horizontal molecular orientation and crystalline domain size, benefiting the outcoupling efficiency and contributing to the device efficiency.

A dual-locked triarylamine donor enables high-performance deep-red/NIR thermally activated delayed fluorescence organic light-emitting diodes.

Deep-red/near-infrared (DR/NIR) organic light-emitting diodes (OLEDs) have attracted a great deal of attention due to their widespread application fields, such as night-vision devices, optical communication, and information-secured displays. However, most DR/NIR OLEDs show low electroluminescence efficiencies, hampering their applications. Herein, we constructed a high-performance DR/NIR thermally activated delayed fluorescence (TADF) emitter based on an advanced dual-locked triarylamine donor (D) unit. Promisingly, such a novel D segment brings numerous advantages: a larger stereoscopic architecture, an enhanced electron-donating ability, and a stiffer molecular structure. In view of these features, the newly developed emitter DCN-DSP shows redshifted emission, a narrowed ΔEST, an enhanced PLQY value and aggregation-induced emission (AIE) properties, which allows for effectively alleviating concentration quenching compared to the control compound using a conventional triarylamine derivative as D units. The DCN-DSP-based OLEDs with modulated doping concentrations exhibit champion EQEs of 36.2% at 660 nm, 26.1% at 676 nm and 21.3% at 716 nm, which are record-high efficiencies among all TADF OLEDs in the similar emission ranges. This work realizes the efficiency breakthrough of DR/NIR TADF OLEDs, and such a promising molecular design approach may inspire even better DR/NIR TADF emitters in the future.

Novel benzonitrile-based AIE host with high triplet energy for highly efficient solution-processed blue TADF OLEDs

Developing high-performance host materials for blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is an effective way to improve emission efficiency through overcoming intermolecular interactions. In this work, three novel AIE-active materials with twisted donor-acceptor structures (3phCN-Cz, 3phCN-ICz and 3phCN-tCzICz) are designed and synthesized by modifying the donor unit. Encouragingly, all the molecules exhibited high triplet energies (>2.80 eV) to prevent exciton returning and aggregation‐induced emission (AIE) property for suppressing exciton quenching. Furthermore, systematic studies demonstrated that 3phCN-ICz and 3phCN-tCzICz with larger steric resistance units can not only possess good thermal and electrochemical stability but also exhibit excellent inhibition ability of intermolecular interactions, which are suitable for constructing high-efficiency solution-processed blue TADF devices. Ultimately, the 4TCzBN was employed as the blue TADF emitter, and the solution-processed OLEDs based on 3phCN-ICz achieved the maximum external quantum efficiency (EQEmax), the maximum current efficiency (CEmax), and the lowest turn-on voltage of 22.04%, 41.88 cd A−1, and 3.5 V, respectively. This work demonstrates the potential application of AIE materials with high triplet energy for solution-processed blue TADF OLEDs.

Recent Progress in Blue Thermally Activated Delayed Fluorescence Emitters and Their Applications in OLEDs: Beyond Pure Organic Molecules with Twist D-π-A Structures.

Thermally activated delayed fluorescence (TADF) materials, which can harvest all excitons and emit light without the use of noble metals, are an appealing class of functional materials emerging as next-generation organic electroluminescent materials. Triplet excitons can be upconverted to the singlet state with the aid of ambient thermal energy under the reverse inter-system crossing owing to the small singlet-triplet splitting energy (ΔEST). This results from a specific molecular design consisting of minimal overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, due to the spatial separation of the electron-donating and electron-releasing part. When a well-designed device structure is applied, high-performance blue-emitting TADF organic light-emitting diodes can be realized with an appropriate molecular design. Unlike the previous literature that has reviewed general blue-emitting TADF materials, in this paper, we focus on materials other than pure organic molecules with twist D-π-A structures, including multi-resonance TADF, through-space charge transfer TADF, and metal-TADF materials. Cutting-edge molecules with extremely small and even negative ΔEST values are also introduced as candidates for next-generation TADF materials. In addition, OLED structures used to exploit the merits of the abovementioned TADF emitters are also described in this review.

Correlating Exciton Dynamics of Thermally Activated Delayed-Fluorescence Emitters to Efficiency Roll-Off in OLEDs

Physical parameters, such as decay rates and photoluminescence (PL) quantum efficiencies, associated with thermally activated delayed fluorescence (TADF) emitters play a critical role in determining the performance of organic light-emitting diodes (OLEDs). Herein, we investigate the impact of TADF decay rates and PL quantum efficiencies on external quantum efficiency (EQE) roll-off. Our analysis reveals that a high EQE with suppressed efficiency roll-off in TADF OLEDs can be achieved by shortening the delayed lifetime with an increasing delayed contribution or by shortening the prompt lifetime with increasing prompt contribution simultaneously. We further show that TADF compounds with long delayed and/or prompt lifetimes can give rise to multiple spin cycling and subsequently result in significant exciton quenching and efficiency roll-off in OLEDs. These results provide universal selection criteria for TADF emitters in OLEDs to concurrently achieve a high maximum EQE and low-efficiency roll-off.

Effects of polystyrene on morphology and electrical performance of solution-processed thermally activated delayed fluorescent organic light-emitting diodes

A polymer was added to the emissive-layer (EML) solution of thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) to enhance their electrical and optical performances. A blue 4CzFCN TADF emitter and m-CBP host were used with 15 wt% dopant, and 1 wt% polystyrene (PS) was added to this EML solution. The PS enhanced the surface morphology, electroluminescence (EL), and external quantum efficiency (EQE). Energy transfer between the host and the dopant material was verified by analyzing the photoluminescence and time-resolved photoluminescence. After PS treatment, the surface roughness was reduced, according to the root-mean-square value obtained using atomic force microscopy. Finally, we fabricated solution-processed TADF-based OLEDs with an EQE of 4.7% at 7.7 V and a maximum luminance of 3700 cd/m2 at 10 V.

Novel carbazole host materials for solution processed TADF Organic Light Emitting Diodes

Several functional and internal layers have been used to improve the performance of solution-processed Organic Light Emitting Diodes (OLED) devices. In the case of thermally activated delayed fluorescence (TADF) OLEDs, a large number of emitter materials have already been reported, but the design and development of potential host materials are overlooked. Efficient solution processable host materials for TADF OLEDs can reduce the complexity in the device architecture and production cost. In this work, we utilized a series of carbazole-based solution-processable host materials (ME4, ME5, ME9, ME10, and ME13) to fabricate the 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) based green TADF OLEDs. Among them, the ME9 composing device showed the best performance. The resultant device exhibited a maximum power efficacy of 40.0 lm/W, current efficacy of 44.7 cd/A, and external quantum efficiency of 14.1% with a maximum luminance of 16,220 cd/m2. The excellent performance may attribute to the low singlet-triplet energy gap (ΔEST), high photoluminescence quantum yield (PLQY), high thermal stability, and unique porous morphology of the ME9.

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Dipyrido[3,2-a:2′,3′-c]phenazine acceptor based thermally activated delayed fluorescence emitters

A rigid π-conjugated and electron-deficient moiety, dipyrido[3,2-a:2′,3′-c]phenazine (DPPZ), was employed as the electron acceptor for thermally activated delayed fluorescence (TADF) emitters with a donor–acceptor structure. Two TADF emitters, DCz-DPPZ and DDPhCz-DPPZ, were prepared by adopting carbazole derivatives as the donor and DPPZ as the acceptor. Compared to DCz-DPPZ without substituents at the carbazole group, DDPhCz-DPPZ showed a narrower energy gap, a longer emission wavelength, and a higher photoluminescence quantum efficiency. The TADF organic light-emitting diodes based on DDPhCz-DPPZ exhibited a higher quantum efficiency of 12.4% than 4.6% of devices based on DCz-DPPZ. These results demonstrate that DPPZ is a suitable acceptor for constructing D–A type TADF emitters.

Customized Orthogonal Solvent System with Various Hole-Transporting Polymers for Highly Reproducible Solution-Processable Organic Light-Emitting Diodes.

Recently, various hosts and emitters for solution-processable thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) have been developed. However, a few studies have been conducted on hole transport materials (HTMs) with differentiated solubility characteristics for manufacturing multilayer OLEDs using a solution process. Here, three new hole transport (HT) styrene polymers, PICz, PPBCz, and PTPCz, were synthesized by radical polymerization. Each of the polymers exhibited increases in their highest occupied molecular orbital (HOMO) levels and better hole-transporting abilities than poly(9-vinylcarbazole) (PVK) as a reference HT polymer. Furthermore, the three HT polymers exhibited different solubilities in toluene. Therefore, it was not possible to use a toluene solution to prepare the emitting layer (EML). To overcome this problem, ethyl acetate (EA), in which the three HT polymers are insoluble, was used as an orthogonal solvent to prepare an EML solution. In EA-solution-processed green-emitting TADF-OLEDs, the three HT-polymer-based devices displayed somewhat low turn-on voltages of 2.8 V and high external quantum efficiencies (EQEs) of >23%. These values are superior to those of a device with a PVK-HT layer. In addition, the devices manufactured with the EA solution showed high-performance reproducibility owing to the stable formation of each layer. In this study, we removed the HTM solubility constraint by dramatically changing the solvent for preparing the EML solution and provided an efficient strategy for the fabrication of OLED devices via solution processes in the future.

Efficient Nondoped Pure Red/Near-Infrared TADF OLEDs by Designing and Adjusting Double Quantum Wells Structure

The selection of host materials has a great impact on the performance of doping devices, especially near-infrared (NIR) emitters. In this investigation, four diverse host materials serve as a potential barrier layer (PBL) to confine and balance holes and electrons within the potential well layer (PWL), TPA-DCPP, for fabricating nondoped NIR thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) (NNT-OLEDs) with double quantum wells’ (DQWs) structure. The hole-type host (mCP) forms an optimum interface energy barrier (IEB) and disperses carriers and excitons in each well, which helps to widen the recombination interval of carriers and restrain the quenching of excitons. Finally, OLEDs with pure red emission and a maximum external quantum efficiency (EQEmax) of nearly 15% (with 0.5 nm well width), deep-red emission with an EQEmax of 12% (with 1.0 nm well width), and NIR emission with an EQEmax of 3.3% (with 5.5 nm well width) are achieved with tunable emission peaks within the range of 641–700 nm by adjusting well width, which are significantly superior to those of doped devices based on the same emitter. This investigation demonstrates a simple, feasible, and effective design strategy for achieving efficient fluorescent OLEDs with controllable wavelength, which solves the blue shift problem in traditional NIR devices of host–guest structure.

Intramolecular C−H Activation as an Easy Toolbox to Synthesize Pyridine‐Fused Bipolar Hosts for Blue Organic Light‐Emitting Diodes

AbstractThe development of high‐performance blue organic light‐emitting diodes (OLEDs) remains a challenging task. While exploiting new blue emitters has attracted great interest, developing host materials that considerably determine device performance obviously lags behind. Herein, we present an ease of access to the structurally diverse benzoheteroaromatic‐fused pyridine skeletons by the iridium‐catalyzed intramolecular C−H/C−H coupling reaction, which unlocks an unparalleled opportunity to rapidly assemble a library of pyridine‐fused bipolar host materials. As an illustrated example, the benzo[4,5]thieno[2,3‐b]pyridine skeleton (BTP) has been made to a pseudo‐symmetric host (DCz‐BTP). The merging of a pyridine fragment enables strong intermolecular interactions, leading to satisfactory bipolar transporting properties. Utilizing DCz‐BTP as the host, blue thermally activated delayed fluorescent OLEDs (TADF‐OLEDs) exhibit a low turn‐on voltage of 2.8 V and a high maximum external quantum efficiency (EQEmax) of 29.0 % and blue TADF‐sensitized florescent OLEDs (TSF‐OLEDs) give an EQEmax as high as 20.5 %, revealing the potential of the BTP skeleton for the construction of high‐performance host materials.

Imidazole Acceptor for Both Vacuum-Processable and Solution-Processable Efficient Blue Thermally Activated Delayed Fluorescence

The members of the imidazole family have been widely used for electron transporting, host, conventional fluorescent, and phosphorescent materials. Although the imidazole core also has great potential as an acceptor segment of deep-blue thermally activated delayed fluorescence (TADF) owing to its high triplet energy, the emission color of imidazole-based TADF organic light-emitting diodes (OLEDs) has so far been limited to blue to green. In this work, four acridan-imidazole systems are theoretically designed aiming for deep- or pure-blue emitters. All four emitters exhibit deep-blue to blue emission owing to the high energy levels of the lowest excited singlet states, exhibiting y coordinates of Commission Internationale de l’Eclairage coordinates between 0.06 and 0.26. The molecule composed of a trifluoromethyl-substituted benzimidazole acceptor in combination with a tetramethyl-9,10-dihydroacridine donor (named MAc-FBI) achieves a high maximum external quantum efficiency (EQEMAX) of 13.7% in its application to vacuum-processed OLEDs. The emitter has high solubility even in ecofriendly nonhalogenated solvents, which motivates us to fabricate solution-processed MAc-FBI-based OLEDs, resulting in an even higher EQEMAX of 16.1%.

New bipolar host materials for high power efficiency green thermally activated delayed fluorescence OLEDs

Four bipolar molecules, namely m-CzPym, p-CzPym, m-CzTrz, and p-CzTrz, with carbazole (Cz) donor and a benzonitrile-substituted pyrimidine (Pym) or triazine (Trz) acceptor core were synthesized and characterized. The electron deficiency of heteroaryl cores together with the substitution pattern of benzonitrile were employed to tune the energy levels as well as the thermally activated delayed fluorescence (TADF) characteristics. The four molecules exhibited TADF behavior with inferior photoluminescent quantum yields (PLQYs) that limit their applications as emitters. These bipolar molecules were employed as TADF host materials for the benchmark TADF emitter 4CzIPN to achieve high-performing green TADF organic light-emitting diodes (OLEDs). Among the molecules, m-CzPym-hosted TADF-OLEDs achieved a maximum external quantum efficiency (EQEmax) of 31.5%, maximum power efficiency (PEmax) of 95.6 lm/W, and maximum current efficiency (CEmax) of 100.2 cd/A. Notably, p-CzPym-hosted TADF-OLEDs also achieved a PEmax of 116.5 lm/W, turn-on voltage of 2.5 V, and impressive low efficiency roll-off performance (>89% of EQEmax at 5000 cd/m2), representing one of the highest efficiencies ever reported in 4CzIPN-doped devices. The high device efficiency can be ascribed to the balanced ambipolar carrier-transporting character of the host materials and high PLQY as well as the outstanding light outcoupling efficiency of the emitting layer.

Aryl-Annulated [3,2-a] Carbazole-Based Deep-Blue Soluble Emitters for High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with CIEy

In this study, we demonstrated two deep-blue TADF emitters, BO-tCzPhICz and BO-tCzDICz, for solution-processable thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). They were synthesized by employing an organoboron acceptor and 9-(3,6-di-tert-butyl-9H-carbazol-9-yl)-5-phenyl-5,12-dihydroindolo[3,2-a]carbazole (tCzPhICz) and 12-(3,6-di-tert-butyl-9H-carbazol-9-yl)-15H-diindolo[2,3-b:1',2',3'-lm]carbazole (tCzDICz) as bulky aryl-annulated [3,2-a] carbazole donors, respectively. Both emitters showed sufficient solubility in organic solvents, narrow deep-blue emission, and small energy difference (ΔEST) between singlet and triplet states, which can be applied to solution-processable deep-blue TADF-OLEDs. Solution-processed OLEDs exploiting these TADF emitters displayed deep-blue electroluminescence with CIEy <0.1, and high external quantum efficiencies of 17.8 and 14.8% were observed for BO-tCzPhICz and BO-tCzDICz, respectively. The emitter bearing bulky ICz-based donating units shows highly promising potential for high-efficiency solution-processable deep-blue TADF-OLEDs.