Blue Thermally Activated Delayed Fluorescence Research Articles

Three‐ and Four‐Coordinate, Boron‐Based, Thermally Activated Delayed Fluorescent Emitters

AbstractRecent developments in purely organic‐material‐based thermally activated delayed fluorescence (TADF) emitters are helping to make them suitable for commercial applications in the near future. In spite of their high external quantum efficiencies, the broad emission spectra and short device lifetimes are the main barriers to using TADF emitters. Among the classes of materials, boron‐embedded polycyclic aromatic compounds have shown potential as TADF emitters that overcome the technical issues. In particular, highly efficient boron‐based TADF emitters with the advantage of a narrowband emission spectrum have been reported recently relying on the concept of a multi‐resonance effect. Hence, boron TADF emitters have the potential to become next‐generation emitters. Based on the future scope of the boron‐based TADF emitters (especially in development of blue TADF emitters), three‐ and four‐coordinate boron TADF emitters are reviewed. Additionally, future prospects of boron‐based TADF emitters are discussed to resolve the issues of low efficiency and short lifetimes of blue emitters.

Host engineering based on multiple phosphorylation for efficient blue and white TADF organic light-emitting diodes

It is challenging to develop host materials for efficient blue and white thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLED), since optoelectronic properties of host molecules should be accurately optimized to suppress triplet quenching. In order to investigate the influence of peripheral groups on host performance, we construct three phosphorylated carbazole hosts with 1–3 diphenylphsophine oxide (DPPO) substituents at 1,8 positions of carbazole and para position of N-phenyl, respectively named tBCzPSPO, tBCzPDPO and tBCzPTPO. It shows that DPPO groups at 1 and 8 positions change the electronic states and frontier molecular orbitals, due to the electron-withdrawing effect of PO, while, DPPO at para-position of N-phenyl only provides additional steric hindrance. Furthermore, DPPO groups are involved in frontier molecular orbitals for carrier transport assistance, but excluded from the first triplet (T1) state. Therefore, tri-phosphorylated tBCzPTPO is superior in triplet protection, giving rise to high photoluminescence quantum yields more than 80%. The single-emissive-layer blue and white TADF OLEDs based on tBCzPTPO further achieved the state-of-the-art external quantum efficiencies beyond 20% and power efficiencies up to 60 lm W−1.

Rigidity and Polymerization Amplified Red Thermally Activated Delayed Fluorescence Polymers for Constructing Red and Single‐Emissive‐Layer White OLEDs

AbstractPolymerization could be a feasible method to overcome the rigid structure induced self‐quenching effect in conventional thermally activated delayed fluorescence (TADF) emitters. Despite steady progress in TADF polymer research, developing an efficient red TADF polymer still remains a great challenge because of the large non‐radiative internal conversion rate governed by the energy gap law. Herein, a novel strategy for constructing a red TADF conjugated polymer is presented by means of embedding quinoxaline‐6,7‐dicarbonitrile (QC) as an acceptor into a polycarbazole (PCz) backbone and attaching donor 9,10‐dihydroacridine (A) as a pendant. The obtained polymers PCzAQCx with the appropriate molar content of the AQC unit (x≥ 0.5) exhibit efficient TADF features with a dominant emissive peak at 627−661 nm and a photoluminescence quantum yield of up to 76% in neat film. The non‐doped electroluminescent devices with the polymers produce red emissions with a maximum external quantum efficiency (EQE) of up to 12.5% and the emission peak at 620 nm, which represents state‐of‐the‐art performance for solution‐processed devices based on red TADF polymers. Furthermore, combined with a blue TADF emitter, the bright white devices with tunable spectra cover the whole visible–near infrared range from 400 to 900 nm and a record‐high EQE of up to 22.4% is achievable.

Through‐Space Charge‐Transfer Polynorbornenes with Fixed and Controllable Spatial Alignment of Donor and Acceptor for High‐Efficiency Blue Thermally Activated Delayed Fluorescence

AbstractThrough‐space charge transfer polynorbornenes with fixed and controllable spatial alignment of donor and acceptor in edge‐to‐face/face‐to‐face stacking patterns are developed for achieving high‐efficiency blue thermally activated delayed fluorescence (TADF). The alignment is realized by using the cis, exo‐configuration of norbornene to confine donor and acceptor in close proximity, and utilizing orthogonal and dendritic structures of donors to provide either perpendicular or parallel stacking motif relative to acceptors. Compared to edge‐to‐face counterparts, polynorbornenes with face‐to‐face aligned donor and acceptor exhibit much larger oscillator strength and higher photoluminescence quantum yield. The resulting polymers exhibit deep blue (422 nm) to sky blue (482 nm) emission and TADF effect with reverse intersystem crossing rates of 0.4–5.9×106 s−1, giving the maximum external quantum efficiency of 18.8 % for non‐doped blue organic light‐emitting diodes by solution process.

Through-Space Charge-Transfer Polynorbornenes with Fixed and Controllable Spatial Alignment of Donor and Acceptor for High-Efficiency Blue Thermally Activated Delayed Fluorescence.

Through-space charge transfer polynorbornenes with fixed and controllable spatial alignment of donor and acceptor in edge-to-face/face-to-face stacking patterns are developed for achieving high-efficiency blue thermally activated delayed fluorescence (TADF). The alignment is realized by using the cis, exo-configuration of norbornene to confine donor and acceptor in close proximity, and utilizing orthogonal and dendritic structures of donors to provide either perpendicular or parallel stacking motif relative to acceptors. Compared to edge-to-face counterparts, polynorbornenes with face-to-face aligned donor and acceptor exhibit much larger oscillator strength and higher photoluminescence quantum yield. The resulting polymers exhibit deep blue (422 nm) to sky blue (482 nm) emission and TADF effect with reverse intersystem crossing rates of 0.4-5.9×106 s-1 , giving the maximum external quantum efficiency of 18.8 % for non-doped blue organic light-emitting diodes by solution process.

Thermally Assisted Fluorescent Polymers: Polycyclic Aromatic Materials for High Color Purity and White-Light Emission.

Thermally activated delayed fluorescence (TADF) sensitization of fluorescence is a promising strategy to improve the color purity and operational lifetime of conventional TADF organic light-emitting diodes (OLEDs). Here, we propose a new design strategy for TADF-sensitized fluorescence based on acrylic polymers with a pendant energy-harvesting host, a TADF sensitizer, and fluorescent emitter monomers. Fluorescent emitters were rationally designed from a series of homologous polycyclic aromatic amines, resulting in efficient and color-pure polymeric fluorophores capable of harvesting both singlet and triplet excitons. Macromolecular analogues of blue, green, and yellow fourth-generation OLED emissive layers were prepared in a facile manner by Cu(0) reversible deactivation radical polymerization, with emission quantum yields up to 0.83 in air and narrow emission bands with full width at half-maximum as low as 57 nm. White-light emission can easily be achieved by enforcing incomplete energy transfer between a deep blue TADF sensitizer and yellow fluorophore to yield a single white-emissive polymer with CIE coordinates (0.33, 0.39) and quantum yield 0.77. Energy transfer to the fluorescent emitters occurs at rates of 1-4 × 108 s-1, significantly faster than deactivation caused by internal conversion or intersystem crossing. Rapid energy transfer facilitates high triplet exciton utilization and efficient sensitized emission, even when TADF emitters with a low quantum yield are used as photosensitizers. Our results indicate that a broad library of untapped polymers exhibiting efficient TADF-sensitized fluorescence should be readily accessible from known TADF materials, including many monomers previously thought unsuitable for use in OLEDs.

Asymmetrically phosphorylated carbazole host for highly efficient blue and white thermally activated delayed fluorescence diodes

Host development is crucial and challenging for thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLED), which requires accurate modulation of host-dopant energy transfer and host/dopant-dopant quenching. Here, we demonstrate that phosphine oxide (PO) groups can be used to simultaneously facilitate carrier transport, positive energy transfer and quenching suppression. Two diphenylphosphine oxide (DPPO) groups at carbazole and N-phenyl of 3,6-di-tert-butyl-N-phenylcarbazole induce the spatially different steric hindrance and inductive effect of DPPOs in tBCzPPOSPO host. The asymmetric configuration with two bulky DPPOs at the same side of tBCzPPOSPO results in the lamellar structure and head-to-tail molecular packing in each layer of its single crystal. tBCzPPOSPO reveals the first singlet (S1) and triplet energy levels (T1) of 3.42 and 2.98 eV, providing the positive energy transfer to blue TADF emitter. Intermolecular interactions are mitigated by steric hindrance of tert-butyls and DPPO groups for quenching suppression. A corner of tBCzPPOSPO is uncovered, rendering electron and hole mobility by level of 10−6 cm2 V−1 s−1. Therefore, based on conventional blue and yellow TADF emitters, tBCzPPOSPO endowed its single-emissive-layer blue and white OLEDs with the state-of-the-art performance, including maximum external quantum efficiencies more than 20% and a top-rank power efficiency of 63.5 lm W−1.

Blue thermally activated delayed fluorescence based on tristriazolotriazine core: Synthesis, property and the application for solution-processed OLEDs

Blue emitters with thermally activated delayed fluorescence (TADF) characteristics have attracted much attention from both academic and industrial researchers in organic light-emitting diodes (OLEDs). In this contribution, three blue molecules of TTT-Ph-Cz, TTT-Ph-Ac and TTT-Ph-BAc in which the tristriazolotriazine (TTT) was employed as the electron acceptor and arylamine (carbazole and acridine derivatives) as the electron donor were prepared and characterized. Both the acridine-substituted compounds (TTT-Ph-Ac and TTT-Ph-BAc) displayed the blue TADF emission between 460 and 470 nm in film state. Additionally, the TTT-Ph-BAc showed clear aggregation-induced emission character. The solution processed OLEDs based on these three molecules were fabricated, and the TTT-Ph-Ac based device displayed a maximum external quantum efficiency of ～10% and peak wavelength of 480 nm. This research demonstrates that this novel TTT core can be a promising electron accepting unit for the design of TADF materials.

Weakly Conjugated Phosphine Oxide Hosts for Efficient Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes.

The development of host materials with high first-triplet state (T1) energy and high charge mobility is a key to achieve efficient true-blue organic light-emitting diodes (OLEDs), employing phosphorescence and thermally activated delayed fluorescence (TADF). An ether-bridged double triphenylphosphine oxide (TPPO) compound, bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO), was reported to have a very high T1 energy of 3.3 eV but suffers from poor charge mobility. Here, five bridge-controlled multi-TPPO derivatives were studied through a combination of experiments and theory. We demonstrate that the push-pull electron capability of the bridge group governs the T1 energy and electron mobility of these materials. Replacing the ether bridge by a bis(trifluoromethyl)methylene group can reduce the energy barrier for intramolecular electron exchange and consequently enhance the electron mobility by two orders of magnitude without lowering the T1 energy. A blue TADF OLED employing this bis(trifluoromethyl)methylene-bridged compound achieves the same high external quantum efficiency but a much higher current density compared to the control device employing DPEPO. In contrast, a bridge group with strong electron-withdrawing capability, such as phosphine oxygen or sulfone, lowers the T1 energy of the compound by enhancing the electronic coupling between TPPO subunits and inhibits intermolecular electron transfer by trapping the electron charge around the bridge.

Recent Advancement in Boron-Based Efficient and Pure Blue Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes.

In the last few years, electron-deficient materials have been actively researched for application in organic light-emitting diode (OLED) as dopant and electron-transporting materials. The boron-containing materials are interesting as they give good emissive properties in solid state with an electron-accepting character. Recently, many boron-containing materials are used as emissive materials for thermally activated delayed fluorescence (TADF) OLED applications. In this review, boron acceptor-based push–pull small molecules used for application in blue TADF OLEDs are reviewed, covering their different types of acceptor, molecular design, structure–property relation, material properties, and device properties. Also, the importance of boron acceptors to address the key issue of blue TADF OLEDs is discussed.

Effects of Electron Affinity and Steric Hindrance of the Trifluoromethyl Group on the π-Bridge in Designing Blue Thermally Activated Delayed Fluorescence Emitters.

To explore the correlation of the acceptor electron affinity and the molecular conformation to the thermally activated delayed fluorescence (TADF) feature, a series of d-π-A molecules were designed and synthesized with triazine (Trz) as the acceptor (A) and carbazole (Cz) or tert-butylcarbazole (BuCz) as the donor (D). On the phenylene bridge between D and A, methyl or trifluoromethyl was incorporated close either to D or to A to tune the molecular conformation and the electron-withdrawing ability of acceptor. Both the twist angles and the singlet and triplet energy difference (ΔEST ) were observed strongly dependent on the type and position of the substituent on the π-bridge. Only those molecules with trifluoromethyl locating close to the D side, namely TrzCz-CF3 and TrzBuCz-CF3 , exhibit TADF feature, verifying that both sufficient electron affinity of the A unit and large dihedral angle between D and the π-bridge are necessary to ensure the occurrence of TADF. The blue organic light-emitting diodes fabricated with TrzCz-CF3 and TrzBuCz-CF3 achieved external quantum efficiencies of 9.40 % and 14.22 % with CIE coordinates of (0.19, 0.23) and (0.18, 0.29) respectively. This study provides practical design strategy for blue TADF materials particularly when planar and less crowded group is used as donor.

Molecular Design Based on Donor-Weak Donor Scaffold for Blue Thermally-Activated Delayed Fluorescence Designed by Combinatorial DFT Calculations.

Quantum chemical calculations are necessary to develop advanced emitter materials showing thermally-activated delayed fluorescence (TADF) for organic light-emitting diodes (OLEDs). However, calculation costs become problematic when more accurate functionals were used, therefore it is judicious to use a multimethod approach for efficiency. Here we employed combinatorial chemistry in silico to develop the deep blue TADF materials with a new concept of homo-junction design. The homo-junction materials containing TADF candidates designed by calculation were synthesized and analyzed. We found that these materials showed the emission from charge transfer (CT) state, and the clear delayed emission was provided in solid state. Because the homo-junction TADF materials showed three exponential decayed emission in solid state, we employed novel four-state kinetic analysis.

Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff

Aromatic organic deep-blue emitters that exhibit thermally activated delayed fluorescence (TADF) can harvest all excitons in electrically generated singlets and triplets as light emission. However, blue TADF emitters generally have long exciton lifetimes, leading to severe efficiency decrease, i.e., rolloff, at high current density and luminance by exciton annihilations in organic light-emitting diodes (OLEDs). Here, we report a deep-blue TADF emitter employing simple molecular design, in which an activation energy as well as spin–orbit coupling between excited states with different spin multiplicities, were simultaneously controlled. An extremely fast exciton lifetime of 750 ns was realized in a donor–acceptor-type molecular structure without heavy metal elements. An OLED utilizing this TADF emitter displayed deep-blue electroluminescence (EL) with CIE chromaticity coordinates of (0.14, 0.18) and a high maximum EL quantum efficiency of 20.7%. Further, the high maximum efficiency were retained to be 20.2% and 17.4% even at high luminance.

Efficient and Stable Deep-Blue Fluorescent Organic Light-Emitting Diodes Employing a Sensitizer with Fast Triplet Upconversion.

Multiple donor-acceptor-type carbazole-benzonitrile derivatives that exhibit thermally activated delayed fluorescence (TADF) are the state of the art in efficiency and stability in sky-blue organic light-emitting diodes. However, such a motif still suffers from low reverse intersystem crossing rates (kRISC ) with emission peaks <470 nm. Here, a weak acceptor of cyanophenyl is adopted to replace the stronger cyano one to construct blue emitters with multiple donors and acceptors. Both linear donor-π-donor and acceptor-π-acceptor structures are observed to facilitate delocalized excited states for enhanced mixing between charge-transfer and locally excited states. Consequently, a high kRISC of 2.36 × 106 s-1 with an emission peak of 456 nm and a maximum external quantum efficiency of 22.8% is achieved. When utilizing this material to sensitize a blue multiple-resonance TADF emitter, the corresponding device simultaneously realizes a maximum external quantum efficiency of 32.5%, CIEy ≈ 0.12, a full width at half maximum of 29 nm, and a T80 (time to 80% of the initial luminance) of > 60 h at an initial luminance of 1000 cd m-2 .

A perspective on blue TADF materials based on carbazole-benzonitrile derivatives for efficient and stable OLEDs

The advent of materials with thermally activated delayed fluorescence (TADF) has triggered a wave of research to break the mutual exclusion of high efficiency and long lifetime of blue organic light-emitting diodes. Representatively, multiple donor-acceptor-type carbazole-benzonitrile (CzBN) derivatives have realized the longest device lifetimes in the blue region up to now together with high efficiencies among all TADF emitters. This perspective timely summarizes the progress of molecular design and device performances of CzBN derivatives and is concluded by providing suggestions on the future development of such materials and the corresponding devices.

Management of Delayed Fluorophor-Sensitized Exciton Harvesting for Stable and Efficient All-Fluorescent White Organic Light-Emitting Diodes.

White organic light-emitting diodes (WOLEDs) using thermally activated delayed fluorescence (TADF)-based single emissive layer (SEL) have attracted enormous attention because of their simple device structure and full exciton utilization potential for high efficiency. However, WOLEDs made of an all-TADF SEL usually exhibit serious efficiency roll-off and poor color stability due to serious exciton-annihilation and unbalanced radiative decays of different TADF emitters. Herein, a new strategy is proposed to manipulate the TADF-sensitized fluorescence process by combining dual-host systems of high triplet energy with a conventional fluorescent emitter of complementary color. The multiple energy-funneling paths are modulated and short-range Dexter energy transfer is largely suppressed due to the steric effect of peripheral tert-butyl group in the blue TADF sensitizer. The resulting all-fluorescent WOLEDs achieve an unprecedentedly high external quantum efficiency of 21.8% with balanced white emission of Commission Internationale de l'Eclairage coordinate of (0.292, 0.343), accompanied with good color stability, reduced efficiency roll-off, and prolonged operational lifetime. These findings demonstrate the validity of this strategy for precisely allocating the exciton harvesting in SEL WOLEDs.

Exciplex Formation and Electromer Blocking for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs with All-Solution-Processed Organic Layers.

Highly efficient solution-processable emitters are greatly desired to develop low-cost organic light-emitting diodes (OLEDs). The recently developed thermally activated delayed fluorescence (TADF) materials are promising candidates, but blue TADF materials compatible with the all-solution-process have still not been achieved. Here, a series of TADF materials, named X-4CzCN, are developed by introducing the bulky units through an unconjugated linker, which realizes high molecular weight to enhance the solvent resistance ability without disturbing the blue TADF feature. Meanwhile, the peripheral wrapping groups efficiently inhibit the triplet-triplet and triplet-polaron quenching by isolating the energy-transfer and charge-transporting channels. The photophysical measurements indicate that a small variation in peripheral unit will have a noticeable effect on the luminescence efficiency. The enlarged volume of peripheral units will make the electroluminescent spectra blueshift, while enhancing the energy transfer of exciplex and blocking the energy leakage of electromer can facilitate the exciton utilization. As a result, the fully solution-processed blue OLED achieves a CIE of (0.16, 0.27), a low turn on voltage of 2.9 eV, and a high external quantum efficiency of 20.6 %. As far as we known, this is the first report of all-solution-processed TADF OLEDs with blue emission, which exhibits a high efficiency even comparable to the vacuum-deposited devices.

Highly Efficient and Color-Stable Thermally Activated Delayed Fluorescence White Light-Emitting Diodes Featured with Single-Doped Single Emissive Layers.

Despite their merits of environmental friendliness, low cost, and large-scale production, thermally activated delayed fluorescence (TADF) based white organic light-emitting diodes (WOLEDs) for daily lighting applications still face the formidable challenges of structural simplification and controllable exciton allocation. Here, the state-of-the-art full-TADF WOLEDs with features of the single-doped single emissive layers (EMLs) and ultrasimple trilayer structure are demonstrated. The EMLs are binary systems as yellow TADF emitter (4CzTPNBu) doped blue TADF matrix (ptBCzPO2 TPTZ) with the large steric hindrance and mismatched frontier molecular orbital energy levels to effectively restrain excessive blue-to-yellow triplet exciton transfer and host-dopant interaction induced triplet quenching. Simultaneously, Förster resonance energy transfer is utilized to optimize exciton allocation for the balance of blue and yellow emissions, giving rise to the photoluminescence quantum yield beyond 90%. Consequently, these single-doped EMLs endow their cool white, pure white, and warm white diodes with the high-quality and ultrastable white light and the 100% exciton utilization efficiencies through the extremely simple structures, making them competent for the diverse daily lighting applications.

An excited state managing molecular design platform of blue thermally activated delayed fluorescence emitters by π-linker engineering

A molecular design platform of blue thermally activated delayed fluorescence (TADF) emitters to boost the external quantum efficiency and efficiency roll-off of blue TADF organic light-emitting diodes (OLEDs) was developed.

Asymmetric-donor (D2D2′)–acceptor (A) conjugates for simultaneously accessing intrinsic blue-RTP and blue-TADF

Asymmetric-donor–acceptor (D2D2′–A) conjugates for simultaneously accessing blue room-temperature phosphorescence (BRTP) and blue thermally activated delayed fluorescence (BTADF).