Near-infrared thermally activated delayed fluorescent dendrimers for the efficient non-doped solution-processed organic light-emitting diodes

Two near-infrared (NIR) dendrimers with TADF characteristics are reported to develop the non-doped solution-processed OLED for the first time. The rigid ring end-capped aliphatic chain dendrons are introduced to improve the dissolvability and film-forming ability. The dendrimers possess excellent thermal and morphological stabilities. Simultaneously, the dendrimers exhibit self-host feature that the peripheral carbazole/tricarbazole dendrons can encapsulate the core to prevent concentration quenching. Employing MPPA-3Cz as the emitter, the non-doped solution-processed device exhibits a maximum external quantum efficiency (EQE) of 0.254% with a peak wavelength at 715 nm, which is comparable to the most-efficient solution-processed NIR FOLEDs with similar electroluminescent spectra. Moreover, the device shows negligible efficiency roll-off at high current density. Our results indicate that the design of long-wavelength TADF dendrimers will be a promising strategy for the efficient non-doped solution-processed NIR OLEDs.     

Molecular engineering of “A-D-D-A” dual-emitting-cores emitters with thermally activated delayed fluorescence and aggregation-induced emission characteristics

Endowing thermally activated delayed fluorescence (TADF) emitter with aggregation-induced emission (AIE) peculiarity is of great significance for realizing more promising commercial applications. Herein, two new dual-emitting-cores emitters with a structure of acceptor-donor-donor-acceptor (A-D-D-A), namely 2DBT-BZ-2Cz and 2DFT-BZ-2Cz, were designed and synthesized to explore their luminescence trait. The emitters, adopting dual carbazole as donor segments and dual phenyl ketone in peripheral skeleton as electron acceptor units, were featured with small singlet (S₁)–triplet (T₁) splitting energy (ΔE_ST) of 0.02 eV and 0.01 eV. The efficient thermally activated delayed fluorescence (TADF) characteristics and aggregation-induced emission property make them suitable for nondoped OLED devices. The solution-processed green OLEDs based on 2DBT-BZ-2Cz demonstrated greater device performance with current efficiency of 20.7 cd A⁻¹ and maximum luminescence of 10,000 cd m⁻². This work thus provides the direction to explore luminogens of dual-emitting-cores with TADF and AIE features as promising candidates in solid state lighting.     

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.     

Highly Efficient Green‐Emitting Phosphorescent Iridium Dendrimers Based on Carbazole Dendrons

Green‐emitting iridium dendrimers with rigid hole‐transporting carbazole dendrons are designed, synthesized, and investigated. With second‐generation dendrons, the photoluminescence quantum yield of the dendrimers is up to 87 % in solution and 45 % in a film. High‐quality films of the dendrimers are fabricated by spin‐coating, producing highly efficient, non‐doped electrophosphorescent organic light‐emitting diodes (OLEDs). With a device structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/neat dendrimer/1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene/LiF/Al, a maximum external quantum efficiency (EQE) of 10.3 % and a maximum luminous efficiency of 34.7 cd A^–1 are realized. By doping the dendrimers into a carbazole‐based host, the maximum EQE can be further increased to 16.6 %. The integration of rigid hole‐transporting dendrons and phosphorescent complexes provides a new route to design highly efficient solution‐processable dendrimers for OLED applications.     

Constructing High-Efficiency Orange-Red Thermally Activated Delayed Fluorescence Polymers by Excited State Energy Levels Regulation via Backbone Engineering

Thermally activated delayed fluorescence (TADF) materials have attracted extensive attention because of their 100% theoretical exciton utilization. Solution-processable orange-red TADF polymers are one of indispensable participants. Herein, a series of orange-red TADF polymers with dibenzothiophene (DBT) and carbazole (Cz) units as joint backbones are synthesized. Their performance can be successfully optimized by regulating the connection positions of DBT units through backbone engineering. It is found that the pNAI37 series with DBT units embedded in the polymeric backbones at the 3, 7 sites display a better performance than those connected at the 2, 8 sites. The optimal polymer, pNAI3705, exhibits a better excited state nature, leading to the photoluminescence quantum yield of 60%. Consequently, pNAI3705 based organic light-emitting diodes reach a maximum external quantum efficiency of 20.16%, and maintain 10.61% at 500 cd m⁻², which is in first tier among orange-red polymers. These results unambiguously suggest the potential application of the combined DBT and Cz backbones in TADF polymers. This design strategy may provide a versatile approach for optimizing the properties of TADF polymers through backbone engineering.     

Control of Charge Transport in Iridium(III) Complex‐Cored Carbazole Dendrimers by Generation and Structural Modification

Here, the charge transporting properties of a family of highly phosphorescent iridium(III) complex‐cored carbazole dendrimers designed to have improved charge transport by incorporating carbazole units into the dendrons are studied. Firstly, the effect of the dendrimer generation and the role of dendron for materials with one dendron per ligand of the core are considered. It is shown, in contrast to previously reported light‐emitting dendrimers, that in this case the carbazolyl‐based dendrons have an active role in charge transport. Next, the effect on the charge transport of attaching two dendrons per ligand to the dendrimer core is explored. In this latter case, for the so called “double dendron” material a highly non‐dispersive charge transport behavior is observed, together with a time‐of‐flight mobility of the order of 10^?3 cm² V^?1 s^?1. Furthermore the lowest energetic disorder parameter (σ) ever reported for a solution‐processed conjugated organic material is found, σ < 20 meV.     

Effect of dendron generation on properties of self-host heteroleptic green light-emitting iridium dendrimers

A series of self-host heteroleptic green light-emitting iridium (Ir) dendrimers G1 and G2 have been synthesized under mild conditions with high yields, and their photophysical, electrochemical and electroluminescent properties are investigated in detail. Compared with the model compound G0, both G1 and G2 exhibit similar photophysical and electrochemical properties, indicating that the incorporation of carbazole dendrons via a flexible non-conjugated spacer can retain the independence of the emissive Ir core. However, the device performance gradually increases with the increasing dendron generation due to the reduced intermolecular interactions. As a result, a peak luminous efficiency of 17.2 cd/A has been obtained for the G2-based non-doped device, which is about 6 times that of G0. Further dispersing the dendrimer G2 into a host matrix, the efficiency can be improved to 29.2 cd/A.     

Copper(I) Complex as Sensitizer Enables High-Performance Organic Light-Emitting Diodes with Very Low Efficiency Roll-Off

Organic light-emitting diodes (OLEDs) utilizing purely organic thermally activated delayed fluorescence (TADF) sensitizers have recently achieved high efficiencies and narrow-band emissions. However, these devices still face intractable challenges of severe efficiency roll-off at practical luminance and finite operational lifetime. Herein, a carbene-Cu(I)-amide complex, (MAC*)Cu(Cz), is demonstrated as a TADF sensitizer for both fluorescent and TADF OLEDs. The (MAC*)Cu(Cz)-sensitized fluorescent OLED not only achieves a high external quantum efficiency (EQE) of 14.6% with an extremely low efficiency roll-off of 12% at the high luminance of 10 000 nits, but also delivers a 15 times longer operational lifetime than that of the non-sensitized reference device. More importantly, utilizing the (MAC*)Cu(Cz) sensitizer in the multi-resonance (MR) TADF OLED results in a record-high EQE of 26.5% together with a full-width at half maximum of 46 nm and an emission peak at 566 nm. This value is the state-of-the-art efficiency for yellow-emitting MR-TADF OLEDs. The photophysical analysis proved that the fast reverse intersystem crossing process of (MAC*)Cu(Cz) is the key factor to suppress triplet exciton involved quenching at high luminance. This finding firstly demonstrates the use of Cu(I) complex as an efficient TADF sensitizer and paves the way for practical applications of TADF sensitized OLEDs.     

Green phosphorescent iridium dendrimers containing dendronized benzoimidazole-based ligands for OLEDs

A series of novel non-conjugated functionalized benzoimidazole-based dendrimers containing peripheral benzyl ether type dendrons have been synthesized and characterized. These compounds undergo cyclometalation with iridium trichloride to form iridium(III) complexes. The emission wavelengths of these dendrimers are in the range from 510 to 530 nm, and the photoluminescence quantum yields (PLQYs) in the range from 0.45 to 0.80. Dendrimers (Gn)₂Ir(acac) and (Gn)₃Ir exhibit a reversible one-electron oxidation wave at ∼0.55 V and ∼0.37 V (vs. Ag/AgNO₃), respectively. With a device configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid)/4,4′-bis(N-carbarzolyl)biphenyl:(G2)₃Ir 20 wt% dopant/1,3,5-tris(2-N-phenyl-benzoimidazolyl)benzene/LiF/Al has a maximum external quantum efficiency (EQE) of 17.6% and a maximum current efficiency of 61.5 cd/A.     

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

High-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 (EQE_max) 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 EQE_max 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.     

Highly efficient near-infrared thermally activated delayed fluorescence material based on a spirobifluorene decorated donor

The development of red/near-infrared (NIR) thermally activated delayed fluorescence (TADF) emitters are relatively lagging due to the spin statistics and energy gap law. Herein, we designed and synthesized a new NIR TADF emitter, 3-(4-(9,9′-spirobi[fluorene]-3-yl(phenyl)amino)phenyl)acenaphtho[1,2-b]pyrazine-8,9-dicarboni-trile (SDPA-APDC), by incorporating a spiro-type electron-donating moiety N,N-diphenyl-9,9′-spirobi[fluorene]-2-amine (SDPA) to an electron-withdrawing unit acenaphtho[1,2-b]pyrazine-8,9-dicarbonitrile (APDC). The photophysical, electrochemical and thermal properties of SDPA-APDC have been systematically explored. Consequently, the emitter was found high photoluminescence quantum yield (PLQY), narrow bandgap, small singlet-triplet energy gap (ΔE_ST) and excellent thermal stability. Furthermore, SDPA-APDC was developed for electroluminescence devices. The doped devices of SDPA-APDC achieved a red emission peak at 696 nm with a maximum external quantum efficiency (EQE) of 10.75%. And the non-doped device exhibited a NIR emission peak at 782 nm with a maximum EQE of 2.55%     

Charge‐Transfer Exciton Manipulation Based on Hydrogen Bond for Efficient White Thermally Activated Delayed Fluorescence

Despite the success of thermally activated delayed fluorescence (TADF) emitters in monochromatic organic light‐emitting diodes (OLED), only few efficient full‐TADF white OLEDs (WOLED) are reported because of the challenge in rational exciton allocation between blue and other color emitters. Herein, it is demonstrated that the appropriate exciton delocalization in blue TADF matrixes can simultaneously support the sufficient blue emission and the energy loss–free charge and exciton transfer to yellow TADF emitters. Through introducing steric hindrance–modulated intermolecular hydrogen bond networks, a fluorinated carbazole‐phosphine oxide hybrid realizes the balance of exciton localization and delocalization, giving rise to state‐of‐the‐art external quantum efficiency beyond 20% from its simple trilayer full‐TADF WOLEDs, accompanied by excellent spectral stability. The correlation between the efficiencies of the blue TADF matrixes and their intermolecular interactions reveals that the exciton delocalization is crucial for the exciton allocation optimization in multicomponent emission systems.     

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.     

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.     

Novel D-D′-A structure thermally activated delayed fluorescence emitters realizing over 20% external quantum efficiencies in both evaporation- and solution-processed organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) emitters have only realized limited performance in solution-processed organic light-emitting diodes (OLEDs) comparing to in evaporation-processed OLEDs. To address this issue, a novel D-D′-A structure, where A is the electron-accepting group, D is the primary electron-donating group and D′ is the secondary electron-donating group, was proposed to develop efficient solution-processable TADF emitters in this work. As the intermediate D′ spacer weakens the direct intramolecular interaction between D and A groups, D-D′-A structure molecules simultaneously possess intramolecular and intermolecular charge-transfer transition channels, suppressing the aggregation-caused quenching induced by solution process. Accordingly, a novel TADF emitter 2-(3,6-bis(9,9-dimethylacridin-10(9H)-yl)-9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (DMAC-Cz-TTR) was designed and synthesized. In the optimized evaporation- and solution-processed OLEDs, DMAC-Cz-TTR successfully realized similar maximum external quantum efficiencies (EQEs) of 21.1% and 20.6%, respectively. To the best of our knowledge, this is the first TADF emitter realizing nearly equal performance in both evaporation- and solution-processed OLEDs with over 20% EQEs. The outstanding performance of DMAC-Cz-TTR successfully demonstrates the feasibility of the D-D′-A structure to develop efficient solution-processable TADF emitters.     

Deep-blue thermally activated delayed fluorescence emitter with a very high fluorescence rate

Conventional thermally activated delayed fluorescence (TADF) molecules achieve small energy differences between the lowest singlet and triplet excited states (ΔE_ST) by enhancing the intramolecular charge transfer, which inevitably leads to a wide emission spectrum and low fluorescence rate. Here, we prepared a deep blue TADF molecule via a small ΔE_ST pyridine-phenol fluoroboron complex as the acceptor. The small ΔE_ST is maintained when carbazole donors are attached to the 4-position of the phenyl rings in the fluoroboron complex. Benefiting from the strong electron coupling between the donor (D) and acceptor (A) moieties, the compound Cz-4-BF exhibits a high fluorescence rate of 4.8 × 10⁸ s⁻¹ and a small D-A dihedral angle change in the excited state. Consequently, a photoluminescence (PL) quantum yield of nearly 100% and a PL spectrum with full-width at half-maximum (FWHM) < 60 nm were obtained in solution and low-concentration doped films. A TADF-sensitized fluorescence (TSF) device containing Cz-4-BF achieves an external quantum efficiency of 21%, which is higher than the devices employing classical fluorescent emitters and multiple resonance-type TADF emitters. The Cz-4-BF-based TSF device shows significantly improved color coordinates of (0.14, 0.10) versus a control device without Cz-4-BF.     

High‐Performance Dibenzoheteraborin‐Based Thermally Activated Delayed Fluorescence Emitters: Molecular Architectonics for Concurrently Achieving Narrowband Emission and Efficient Triplet–Singlet Spin Conversion

Thermally activated delayed fluorescence (TADF) materials, which enable the full harvesting of singlet and triplet excited states for light emission, are expected as the third‐generation emitters for organic light‐emitting diodes (OLEDs), superseding the conventional fluorescence and phosphorescence materials. High photoluminescence quantum yield (Φ_PL), narrow‐band emission (or high color purity), and short delayed fluorescence lifetime are all strongly desired for practical applications. However, to date, no rational design strategy of TADF emitters is established to fulfill these requirements. Here, an epoch‐making design strategy is proposed for producing high‐performance TADF emitters that concurrently exhibiting high Φ_PL values close to 100%, narrow emission bandwidths, and short emission lifetimes of ≈1 µs, with a fast reverse intersystem crossing rate of over 10⁶ s^?1. A new family of TADF emitters based on dibenzoheteraborins is introduced, which enable both doped and non‐doped TADF‐OLEDs to achieve markedly high external electroluminescence quantum efficiencies, exceeding 20%, and negligible efficiency roll‐offs at a practical high luminance. Systematic photophysical and theoretical investigations and device evaluations for these dibenzoheteraborin‐based TADF emitters are reported here.     

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.     

High-Performance Solution-Processed Nondoped Circularly Polarized OLEDs with Chiral Triptycene Scaffold-Based TADF Emitters Realizing Over 20% External Quantum Efficiency

Recently, circularly polarized organic light-emitting diodes (CP-OLEDs) fabricated with thermally activated delayed fluorescence (TADF) emitters are developed rapidly. However, most devices are fabricated by vacuum deposition technology, and developing efficient solution-processed CP-OLEDs, especially nondoped devices, is still a challenge. Herein, a pair of triptycene-based enantiomers, (S,S)-/(R,R)-TpAc-TRZ, are synthesized. The novel chiral triptycene scaffold of enantiomers avoids their intermolecular π–π stacking, which is conducive to their aggregation-induced emission characteristics and high photoluminescence quantum yield of 85% in the solid state. Moreover, the triptycene-based enantiomers exhibit efficient TADF activities with a small singlet-triplet energy gap (ΔE_ST) of 0.03 eV and delayed fluorescence lifetime of 1.1 µs, as well as intense circularly polarized luminescence with dissymmetry factors (|g_PL|) of about 1.9 × 10⁻³. The solution-processed nondoped CP-OLEDs based on (S,S)-/(R,R)-TpAc-TRZ not only display obvious circularly polarized electroluminescence signals with g_EL values of +1.5 × 10⁻³ and −2.0 × 10⁻³, respectively, but also achieve high efficiencies with external quantum, current, and power efficiency up to 25.5%, 88.6 cd A⁻¹, and 95.9 lm W⁻¹, respectively.