Design strategy of yellow thermally activated delayed fluorescent dendrimers and their highly efficient non-doped solution-processed OLEDs with low driving voltage

We designed and synthesized two dendrimers TA-Cz and TA-3Cz with TADF characteristics by using non-conjugated aliphatic chains carbazole/tricarbazole as peripheral dendrons. Both dendrimers possess excellent thermal and morphological stabilities. Introduced the phenyl bridge to increase the distance of the emission core TA between donor (D) and acceptor (A) is a promising route to simultaneously achieve small singlet–triplet energy splitting (ΔE_ST) and enhanced PL quantum yields (PLQYs). Furthermore, non-conjugated aliphatic chains carbazole/tricarbazole dendrons were conveniently introduced to the TADF core, which can effective encapsulate the emission core to restrain the concentration quenching effect and make the fluorescence of the core independent. By utilizing TA-3Cz emitter as the non-doped solution-processed emissive layers, the resulting yellow OLED achieved low driving voltage of 2.4 V and superior external quantum efficiency of 11.8%. Thus, our results here provide a facile strategy to obtain highly efficient non-doped solution-processed OLEDs by employing the reasonable molecular design of the TADF core and the utilization of flexible alkyl chain.     

Aggregation-induced emission type thermally activated delayed fluorescent materials for high efficiency in non-doped organic light-emitting diodes

Aggregation-induced emission (AIE) type thermally activated delayed fluorescent (TADF) emitters were developed by asymmetric substitution of donor moieties to a diphenylsulfone acceptor. The AIE properties of the TADF emitters increased the quantum efficiency of the non-doped TADF devices and asymmetric substitution was more effective than symmetric substitution to enhance the quantum efficiency of the non-doped devices.     

Simple-structure organic light emitting diodes: Exploring the use of thermally activated delayed fluorescence host and guest materials

We investigate the dependence of the performance of non-doped blue light emitting devices with thermally activated delayed fluorescence (TADF) material bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) emission layer on hole and electron transport layers as well as emission layer thickness and study the underlying device physics. On this basis, efficient green and orange devices using DMAC-DPS as host material and TADF material (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) or 2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,4-dicyanobenzene (4CzTPN-Ph) as emitting dopant are reported. In addition, white devices using single DMAC-DPS: 4CzTPN-Ph emission layer show the maximum external quantum efficiency of 13.4%, maximum power efficiency of 38.3 lm W⁻¹ and current-insensitive Commission Internationale de I'Eclairage (CIE) coordinates of (0.29, 0.39). Compared to the approach of combining TADF host and fluorescent dopant, the present devices enable the utilization of all excitons for light emission and the adoption of broad dopant concentration without significantly affecting device efficiency, which is important for the realization of the desired colour purity for display applications, while maintaining the advantages of simple-structure and low-cost.     

1,8-Naphthalimide-based hybrids for efficient red thermally activated delayed fluorescence organic light-emitting diodes

The connection of an electron deficient 1,8-naphthalimide acceptor moiety with a phenoxazine or phenothiazine donor moiety afforded 6-(10H-phenoxazin-10-yl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (PXZ-NAI) and 6-(10H-phenothiazin-10-yl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (PTZ-NAI) as two novel red thermally activated delayed fluorescence (TADF) emitters. The two emitters exhibited distinct TADF characteristics with small energy gaps between the lowest singlet and triplet excited states, which originated from the well-separated highest occupied molecular orbital and lowest unoccupied molecular orbital distributions. The optimized TADF organic light-emitting diodes based on PXZ-NAI and PTZ-NAI offered maximum external quantum efficiencies (EQEs) of 13.0% and 11.4% with maximum power efficiencies of 14.8 and 9.8 lm/W, respectively. Both devices emitted red electroluminescence spectra with peaks at 624 and 632 nm, and Commission Internationale de L'Eclairage coordinates of (0.610, 0.388) and (0.630, 0.368), respectively. These findings demonstrate that the combination of 1,8-naphthalimide with phenoxazine or phenothiazine could be an effective pathway for developing red TADF emitters.     

Novel beryllium complex as the non-doped emitter for highly efficient deep-blue organic light-emitting diode

Efficient blue organic light-emitting diodes have been developed based on one novel fluorescent beryllium complex bis(2-(2-hydroxyphenyl)-4-methyl-pyridine)beryllium (Be(4-mpp)₂). The simple double-layer device based on Be(4-mpp)₂ as the EML as well as the ETL not only shows pure and stable blue emission with the CIE coordinates of (0.14, 0.09), but also presents very high EL efficiency in terms of both the peak values (5.4% for EQE and 4.2 lm W⁻¹ for PE) and the EQE value remaining ?4.0% in very wide brightness range (10–10,000 cd m⁻²) that indicates very good operational stability. They are the highest EL efficiencies ever reported for such saturated and stable OLED (CIE: x < 0.15, y < 0.10) to the best of our knowledge.     

Highly efficient exciplex organic light-emitting diodes by exciplex dispersion in the thermally activated delayed fluorescence host

In this work, we describe a new approach of improving the external quantum efficiency (EQE) of exciplex organic light-emitting diodes (OLEDs). A new approach of dispersing the exciplex in the thermally activated delayed fluorescence (TADF) host was developed to increase the EQE of the exciplex OLEDs. The new emitting layer structure was made up of a mixture of TADF material and an n-type material which can generate exciplex with the TADF material. The content of the n-type material was maintained below 10 wt % to have a morphology with the exciplex dispersed in the TADF matrix material. The emitting layer structure with the exciplex dispersed in the TADF host suppressed exciton quenching and improved the photoluminescence quantum yield of the exciplex. As a result, the exciplex OLEDs with the n-type material content of 1 wt% showed high EQE of 15.3% compared with EQE of 10.8% of the conventional exciplex OLEDs with the n-type material content of 50%.     

Highly efficient solution-processed small-molecule white organic light-emitting diodes

Solution-processed small-molecule white organic light-emitting diodes (WOLEDs) were fabricated with a co-host of hole-transporter 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TCTA) and electron-transporter 2,7-Bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13). By doping 15 wt% FIrpic or F₃Irpic and 0.5 wt% Ir(MDQ)₂(acac) in to the TCTA/SPPO13 host, highly efficient white OLEDs have been achieved which exhibit nearly identical emission spectra at different luminance. The F₃Irpic and Ir(MDQ)₂(acac)-based WOLED shows maximum efficiencies of 40.9 cd/A, 36.7 lm/W and 16.9%, and even high efficiencies of 30.1 cd/A and 12.3% at the practical luminance of 1000 cd/m², which are among the highest efficiencies of the solution-processed small-molecule WOLEDs. These results demonstrate a convenient way to realize solution-processed WOLEDs with high efficiency and high spectral stability through full small-molecule materials system.     

Dimesitylarylborane-based luminescent emitters exhibiting highly-efficient thermally activated delayed fluorescence for organic light-emitting diodes

Triarylboranes are widely used as luminescent molecules. However, there are few reports focused on thermally activated delayed fluorescence (TADF) characteristics. In this study, new donor-acceptor triarylboranes exhibiting TADF characteristics are designed, synthesized, and applied to organic light-emitting diodes (OLEDs) as an emitter. Electro-accepting dimesitylphenylborane connected with carbazole (7), 9,9-dimethylacridane (8), and phenoxazine (9) as electron-donating units are synthesized in only two steps with good yield. Compounds 8 and 9 exhibit light blue and green TADF with good PL quantum yields of 89 and 87% in toluene, respectively. On the other hand, compound 7 shows normal deep blue fluorescence without TADF characteristics. Density functional theory and time-dependent density functional theory studies reveal that high electron-donating ability of donor unit and large dihedral angles between cross-linking phenylene and donor units are attributed to spatially separate the HOMO and LUMO, which results in lowering the energy gap between lowest singlet (S₁) and triplet (T₁) excited states and accelerating reverse intersystem crossing of excitons from T₁ to S₁ states. OLEDs using compounds 8 and 9 as emitters exhibit light blue and green emission with very good external quantum efficiencies of 16.0 and 17.3%, respectively.     

Fluorene derivatives for highly efficient non-doped single-layer blue organic light-emitting diodes

Diphenylamino- and triazole-endcapped fluorene derivatives which show a wide energy band gap, a high fluorescence quantum yield and high stability have been synthesized and characterized. Single-layer electroluminescent devices of these fluorene derivatives exhibited efficient deep blue to greenish blue emission at low driving voltage. The single-layer OLED of PhN-OF(1)-TAZ shows a maximum current efficiency of 1.54 cd/A at 20 mA cm⁻² with external quantum efficiency (EQE) of 2.0% and CIE coordinates of (0.153, 0.088) in deep blue region, while the single-layer device of oligothienylfluorene PhN-OFOT-TAZ shows a maximum brightness of 7524 cd/m² and a maximum current efficiency of 2.9 cd/A with CIE coordinates of (0.20, 0.40) in greenish blue.     

High-performance red organic light-emitting devices based on an exciplex system with thermally activated delayed fluorescence characteristic

To realize high-performance red organic light-emitting diodes (OLEDs), a novel exciplex system Tris-PCz:B4PyPPM with thermally activated delayed fluorescence (TADF) characteristic was developed as the host of both red fluorescent and phosphorescent emitters. Due to the blue-green PL spectrum and suitable triplet energy level of 2.53 eV, the exciplex system matches the requirements of both red fluorescent and phosphorescent dopants. With an optimized mixing molar ratio of 5:5, Tris-PCz:B4PyPPM exciplex system shows outstanding host performance in both the red fluorescent and phosphorescent devices. The red fluorescent device based on DCJTB exhibits a low turn-on voltage of 2.3 V and an extremely high maximum external quantum efficiency (EQE) of 9.3%. And the red phosphorescent device with the Ir(MDQ)₂acac as the emitter realizes a maximum EQE of 20.3%. These remarkable results prove the feasibility of TADF exciplex systems simultaneously as the hosts of both high-performance fluorescent and phosphorescent red devices.     

High efficiency fluorescent white organic light-emitting diodes having a yellow fluorescent emitter sensitized by a blue thermally activated delayed fluorescent emitter

Fluorescent white organic light-emitting diodes having a blue thermally activated delayed fluorescent emitter and a yellow fluorescent emitter was developed by co-doping the blue and yellow emitters in a single emitting layer. The blue delayed fluorescent device showed high quantum efficiency of 22.6% at a very high doping concentration of 50% and the white devices exhibited a high quantum efficiency of 15.5% even though a fluorescent yellow emitter was doped in the blue thermally activated delayed fluorescent emitting layer. Minimized charge trapping and Dexter energy transfer by low yellow doping concentration of 0.05% as well as efficient Förster energy transfer could develop the high efficiency fluorescent white organic light-emitting diodes.     

Efficient non-doped monochrome and white phosphorescent organic light-emitting diodes based on ultrathin emissive layers

Efficient red, orange, green and blue monochrome phosphorescent organic light-emitting diodes (OLEDs) with simplified structure were fabricated based on ultrathin emissive layers. The maximum efficiencies of red, orange, green and blue OLEDs are 19.3 cd/A (17.3 lm/W), 45.7 cd/A (43.2 lm/W), 46.3 cd/A (41.6 lm/W) and 11.9 cd/A (9.2 lm/W). Moreover, efficient and color stable white OLEDs based on two complementary colors of orange/blue, three colors of red/orange/blue, and four colors of red/orange/green/blue were demonstrated. The two colors, three colors and four colors white OLEDs have maximum efficiencies of 30.9 cd/A (27.7 lm/W), 30.3 cd/A (27.2 lm/W) and 28.9 cd/A (26.0 lm/W), respectively. And we also discussed the emission mechanism of the designed monochrome and white devices.     

Color stable and highly efficient hybrid white organic light-emitting devices using heavily doped thermally activated delayed fluorescence and ultrathin non-doped phosphorescence layers

Blue/orange complementary fluorescence/phosphorescence hybrid white organic light-emitting devices with excellent color stability and high efficiency have been fabricated, which are based on an easily fabricated multiple emissive layer (EML) configuration with an ultrathin non-doped orange phosphorescence EML selectively inserted between heavily doped blue thermally activated delayed fluorescence (TADF) EMLs. Through systematic investigation and improvement on luminance-dependent color shift and efficiency deterioration, a slight Commission Internationale de 1′Eclairage coordinates shift of (0.008, 0.003) at a practical luminance range from 1000 to 10000 cd/m², a maximum power efficiency of 45.8 lm/W, a maximum external quantum efficiency (EQE) of 15.7% and an EQE above 12% at 1000 cd/m² have been achieved. The heavily doped blue TADF emitters which act as the main charge transport channels and recombination sites in the host with high-lying lowest triplet excited state, take advantage of the bipolar transport ability to broaden the major charge recombination region, which alleviates triplet energy loss. The selectively inserted ultrathin non-doped orange EML makes its emission mechanism dominated by Förster energy transfer, which is effective to keep color stable under different drive voltages.     

Achieving high-performance phosphorescent organic light-emitting diodes using thermally activated delayed fluorescence with low concentration

We fabricated phosphorescent organic light-emitting diodes (PhOLEDs) using thermally activated delayed fluorescence (TADF) material 10,10''-(4,4''-sulfonylbis(4,1-phenylene)) bis(9,9-dimethyl-9,10-dihydroacridine) (DMAC-DPS) with low concentration, which showed better performance compared with 1,3-bis(carbazole-9-yl) benzene (mCP) based devices. When the concentration of DMAC-DPS was 1wt%, the driving voltage of the device was only 3.3 V at 1 000 cd/m2, and the efficiency and lifetime of the device were effectively improved compared with those of mCP based devices. The result indicated that DMAC-DPS could effectively improve the performance of phosphorescent devices. We believe that the better device performance can be attributed to the optimization of the energy transfer process in the emitter layer and lifetime of triplet excitons by DMAC-DPS. The study may provide a simple and effective strategy to achieve high-performance OLEDs.     

Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence

We study external quantum efficiency (η_EQE) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet–triplet annihilation and triplet–triplet annihilation dominate η_EQE roll-off because of the relatively long 2CzPN triplet lifetime of 273 μs. This work yields a method to relax the exciton quenching process in TADF based OLEDs.     

Vacuum-free lamination of low work function cathode for efficient solution-processed organic light-emitting diodes

The low work function cathode of blade-coated organic light-emitting diode is transferred from a soft polydimethylsiloxane (PDMS) mold by lamination without vacuum. The cathode is a bilayer of polyethylene glycol (PEG) (<10 nm) and Al (100 nm). A sacrificial layer of polystyrene with low Mw 1500 and melting point of 120 °C is inserted between the cathode and PDMS for the subsequent mold removal at 150 °C by melting polystyrene. Current efficiency of 3 cd/A (1.1%) and luminance of 2500 cd/m² are achieved for green polyfluorene fluorescent emitter. 25 cd/A (8.2%) and 3200 cd/m² are achieved for green phosphorescent tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)₃) emitter in polymer blend host. The efficiency is about 70% of the devices with thermally evaporated cathode. The turn-on voltage is about 5 V higher.     

Bright and efficient inverted organic light-emitting diodes with improved solution-processed electron-transport interlayers

Highly efficient inverted organic light-emitting diodes (iOLEDs) are reported by including in the structure a surface modifier, polyethylenimine-ethoxylated (PEIE), to decrease the cathode work function and a hole blocking layer, 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) to increase the efficiency of the device. The two compounds have been processed in a single step, by using a mixture PEIE:TPBi spun from the same solution. It is demonstrated by time-of-flight secondary-ion mass spectrometry (TOF-SIMS) that a bilayer is formed and same performances as the separately processed materials are obtained. This technic enables to reach high luminances (40 000 cd m⁻²) and high current efficiencies (13 cd/A) using the conjugated Super Yellow (SY) polymer as the emissive layer while reducing the number of processing steps.     

Interlayer free hybrid white organic light-emitting diodes with red/blue phosphorescent emitters and a green thermally activated delayed fluorescent emitter

Two different hybrid white organic light-emitting diodes (WOLEDs) with red/blue phosphorescent emitters and a green thermally activated delayed fluorescent (TADF) emitter were designed to develop high efficiency hybrid WOLEDs. One hybrid WOLED (type I) had a device structure with a hybrid emitting layer of green TADF and red phosphorescent emitters stacked on a blue phosphorescent emitting layer and the other hybrid WOLED (type II) had a device architecture with the green TADF emitting layer stacked on a red and blue phosphorescent emitting layer. Efficient energy transfer from the green TADF emitter to the red phosphorescent emitter was observed and balanced white emission could be obtained by optimizing the device structure of the hybrid WOLEDs. A quantum efficiency of 16.2% with a color coordinate of (0.45,0.47) and a quantum efficiency of 18.0% with a color coordinate of (0.37,0.47) were achieved in the type I and type II hybrid WOLEDs, respectively.     

Near infrared electroluminescence from Nd(TTA)3phen in solution-processed small molecule organic light-emitting diodes

We report on the near infrared electroluminescence properties of a Nd³⁺ complex with thenoyltrifluoroacetone and 1,10-phenantroline ligands in solution-processed organic light-emitting diodes. Spin-coated blends containing a 1,3-bis(9-carbazolyl)benzene host doped with the Nd³⁺ complex were found to exhibit a photoluminescence quantum yield of about 0.5%, regardless of the doping concentration level. Electroluminescent devices based on these small molecule blends showed the characteristic emission of Nd³⁺ at 890, 1060 and 1330 nm with an external quantum efficiency as high as 0.022%. These improved performances were mainly attributed to direct charge trapping and exciton formation on the near infrared emitter. Importantly, the efficiency roll-off at high current densities due to triplet-triplet exciton annihilation in the device containing 20 wt% of the complex was lower than what is typically observed in lanthanide complex-based electroluminescent devices. This is presumably due to the high triplet energy of the host material, which prevents guest-to-host energy-back transfer and thus host-guest triplet-triplet exciton annihilation.     

Thermally activated delayed-fluorescence organic light-emitting diodes based on exciplex emitter with high efficiency and low roll-off

Highly efficient thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) based on exciplex are demonstrated in a blended system with commercially available 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC) and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T). By well adjusting the ratio between these two materials, the optimized device shows a low turn-on voltage of 2.4 V and a high external quantum efficiency (EQE) of 11.6%. More importantly, the device retains an EQE of 9.4% even at a high luminescence of 1000 cd/m². The low efficiency roll-off is attributed to the small singlet-triplet splitting and the short of the delayed fluorescence lifetime. Both EQE and efficiency roll-off are ones of the best performance among the reported TADF OLEDs based on exciplex.