Deep-blue Thermally Activated Delayed Fluorescence Research Articles

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.

An Ultraviolet Thermally Activated Delayed Fluorescence OLED with Total External Quantum Efficiency over 9.

Owing to the difficulty in acquiring compounds with combined high energy bandgaps and lower-lying intramolecular charge-transfer excited states, the development of ultraviolet (UV) thermally activated delayed fluorescence (TADF) materials is quite challenging. Herein, through interlocking of the diphenylsulfone (PS) acceptor unit of a reported deep-blue TADF emitter (CZ-PS) by a dimethylmethylene bridge, CZ-MPS, a UV-emissive TADF compound bearing a shallower LUMO energy level and a more rigid structure than those of CZ-PS is achieved. This represents the first example of a UV-emissive TADF compound. Organic light-emitting diode (OLED) using CZ-MPS as the guest material can emit efficient UV light with emission maximum of 389nm and maximum total external quantum efficiency (EQEmax ) of 9.3%. Note that this EQEmax value is twice as high as the current record EQEmax (4.6%) for UV-OLEDs. This finding may shed light on the molecular design strategy for high-performance UV-OLED materials.

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.

Achieving efficient deep-blue TADF in carbazole-pyrimidine compounds

An approach for achieving deep blue thermally activated delayed fluorescence (TADF) is presented. A simple carbazole-pyrimidine compound Cbz-PYR with near-UV emission and simultaneous room-temperature phosphorescence (RTP) was selectively modified by lowering the singlet state energy, simultaneously preserving high triplet energy. The modified compound tCbz-mPYRs was shown to be efficient TADF emitter with 0.5 solid-state emission yield and peak wavelength of 428 nm. When used in OLED device, tCbz-mPYRs based OLED showed electroluminescence with 8.7% external quantum efficiency (EQE) and Commission Internationale de l’Eclairage (CIE) coordinates of (0.16; 0.12). • Deep-blue TADF was achieved in carbazole-pyrimidine compounds. • The bandgap was controlled by the expedient manipulation of molecular structure. • EQE of OLED device peaked at 8.7% with CIE colour coordinates of (0.16; 0.12).

An effective strategy for simply varying relative position of two carbazole groups in the thermally activated delayed fluorescence emitters to achieve deep-blue emission.

The development of efficient deep-blue thermally activated delayed fluorescence (TADF) materials is especially important for organic light-emitting devices as displays and lighting sources. However, finding suitable deep-blue TADF emitters is still challenging. Based on an experimentally reported blue-light TADF emitter DCZ-TTR, two new molecules (DCZ1-TTR and DCZ2-TTR) have been designed to investigate the impact of the change of relative position in two carbazole groups on their TADF properties. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the Marcus rate theory have been performed. It is found that the absorption and emission spectra simulated using the BMK functional can reproduce the available experimental data very well. The fluorescence emissions of DCZ1-TTR and DCZ2-TTR are predicted to show clear blue-shifting in cyclohexane with respect to their analogue DCZ-TTR. Especially, the emission wavelength of DCZ2-TTR is calculated to be 435nm, in the deep-blue light range. According to the Marcus rate theory, the rates of reverse intersystem crossing of DCZ1-TTR and DCZ2-TTR are estimated to be two orders of magnitude larger than that of DCZ-TTR, which is more favorable for the occurrence of delayed fluorescence. This strongly suggests that our newly designed two molecules DCZ1-TTR and DCZ2-TTR can be also expected to be potential blue-light or even deep-blue-light TADF emitters. This may be an effective strategy for realizing deep-blue emission by simply varying relative position of two carbazole groups in the TADF molecules. To our best knowledge, this is a novel finding, which may be useful in preparing highly efficient deep-blue TADF-OLED materials.

Highly Twisted Donor-Acceptor Boron Emitter and High Triplet Host Material for Highly Efficient Blue Thermally Activated Delayed Fluorescent Device.

New highly efficient thermally activated delayed fluorescence (TADF) dopant materials (PXB-DI and PXB-mIC) for blue organic light-emitting diodes are reported. These materials were designed by combining highly conjugated rigid ring donor moieties and a boron acceptor with a highly twisted configuration to have high TADF performance and minimized self-quenching properties. In addition, a new high triplet energy and hole transport-type host material, 5-(5-(2,4,6-triiso-propylphenyl)pyridin-2-yl)-5 H-benzo[ d]benzo[4,5]imidazo[1,2- a]imidazole (PPBI), is also reported. This host represents deeper blue color owing to keeping the original spectra of emitters. A fabricated blue TADF device with PXB-mIC in the PPBI host exhibited maximum external quantum efficiency (EQE) of 12.5% with a CIE of (0.15, 0.08), which is close to that of the National Television System Committee blue color. The blue TADF device performances of the PPBI host was compared with the electron transport-type 2,8-bis(diphenylphosphine oxide)dibenzofuran (DBFPO) host. The blue TADF device with PXB-DI in the DBFPO host exhibited a maximum EQE of 37.4% in the sky blue region. This study demonstrates that our molecular design concept of new emitters and host is beneficial for future high-efficiency deep-blue TADF devices.

Deep-Blue Thermally Activated Delayed Fluorescence Polymers for Nondoped Solution-Processed Organic Light-Emitting Diodes

The exploitation of deep-blue polymeric emitters is of great importance for the application of solution-processed organic light-emitting diodes (OLEDs) in full color display. The highly efficient deep-blue thermally activated delayed fluorescence (TADF) polymers are rarely reported up to now. Herein, we developed a series of deep-blue TADF polymers to fabricate highly efficient nondoped solution-processed OLEDs. By incorporating appropriate host with high triplet energy and deep-blue emitter with high fluorescent efficiency, the polymers are endowed with distinct TADF features. Using these deep-blue polymeric emitters, the nondoped single polymer based OLEDs achieve a maximum external quantum efficiency of 5.3% with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.09), which represents the state-of-the-art device performance for the TADF-based deep-blue polymer light-emitting diodes (PLEDs).

Achieving Deep-Blue Thermally Activated Delayed Fluorescence in Nondoped Organic Light-Emitting Diodes through a Spiro-Blocking Strategy.

A deep-blue thermally activated delayed fluorescence (TADF) emitter TXADO-spiro-DMACF has been reported for nondoped organic light-emitting diodes (OLEDs) by integrating an appropriate blocking unit with the donor (D)–acceptor (A)–donor (D)-type TADF emitter via a spiro linkage. Benefiting from the characteristic perpendicular arrangement, the intermolecular interactions are expected to be weakened to some degree. As a result, TXADO-spiro-DMACF shows a very small bathochromic shift of 8 nm associated with a narrowed full width at half maximum of 54 nm on going from solution to the film. The corresponding nondoped device successfully achieves a bright deep-blue emission, revealing Commission Internationale de l’Eclairage coordinates of (0.16, 0.09) and a peak external quantum efficiency of 5.3% (5.3 cd/A, 5.9 lm/W). The results clearly indicate that spiro-blocking is a promising strategy to develop deep-blue TADF emitters capable of nondoped OLEDs.

The electron inductive effect of CF3 on penta-carbazole containing blue emitters: Trade-off between color purity and luminescent efficiency in TADF OLEDs

A blue thermally activated delayed fluorescence (TADF) emitter 4'-(trifluoromethyl)-[1,1'-biphenyl]-2,3,4,5,6-pentayl-pentakis(9H-carbazole) 5CzDPhCF3 is designed by involving the non-conjugated CF3 moiety as electron-withdrawing unit, which dominates the acceptor properties by the electron inductive effect instead of other widely reported conjugated effects in TADF materials. Inserting a simple phenyl bridge into the light blue TADF emitter of (6-(trifluoromethyl)benzene-1,2,3,4,5-pentayl)pentakis(9H-carbazole) (5CzPhCF3) to dilute the electron-withdrawing capability and reduce D-A interactions, significantly hypsochromicly shifted emission by 48 and 64 nm is observed in 5CzDPhCF3 with deep blue emission peak at 435 and 431 nm in CH2Cl2 and film, respectively. 5CzDPhCF3 exhibits transient lifetime of 6.9 ns and 0.14 μs, comparative HOMO of ∼ -5.55 eV, whereas higher LUMO of −2.25 eV than −2.56 eV of the control 5CzPhCF3. The triplet energy and singlet-triplet energy gap (ΔEST) for 5CzDPhCF3 is 2.95 and 0.23 eV respectively, higher than 2.80 and 0.02 eV of 5CzPhCF3. And the photoluminescence quantum yield measured for 5CzDPhCF3 and 5CzPhCF3 is 22/27% and 24/43% in toluene at ambient/N2 atmosphere, respectively. Due to the larger ΔEST for less efficient reverse intersystem crossing in 5CzDPhCF3 for TADF emission, a trade-off between the color purity and electroluminescent (EL) efficiency can be found. A maximum external quantum efficiency of 2.0 and 9.5% is achieved in the vacuum-deposited TADF devices based on 5CzDPhCF3 and 5CzPhCF3 emitter, respectively. And the phenyl-bridged 5CzDPhCF3 containing device displays pure deep blue EL emission with satisfactory CIE of (0.157, 0.066), while 5CzPhCF3 only shows light blue emission with CIE of (0.175, 0.327). To the best of our knowledge, the CIE of 5CzDPhCF3 is among the only few examples on deep blue TADF emission with CIE(y) values < 0.07.

Molecular engineering of pyrimidine-containing thermally activated delayed fluorescence emitters for highly efficient deep-blue (CIEy < 0.06) organic light-emitting diodes

The development of efficient and robust, deep-blue, thermally activated delayed fluorescence (TADF) emitters, especially those matching the European Broadcasting Union (EBU) standard with Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.06), is one of the challenging issues in organic light-emitting devices (OLEDs). Through sophisticated molecular engineering, two novel pyrimidine derivatives with D-π-A configuration, 5-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (pBFCz-26DPPM) and 5-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-5H-benzo [4,5]thieno[3,2-c]carbazole (pBTCz-26DPPM) have been designed and synthesized. There are only negligible differences in the molecular configuration and thermal, electrochemical and photophysical properties for the two compounds. However, TADF properties can be tuned by changing the heteroatom in the donor moiety. Compound pBFCz-26DPPM offers a shorter delayed fluorescence lifetime (τd) than that of pBTCz-26DPPM. The optimized device serving pBFCz-26DPPM as dopant presents deep-blue Commission Internationale de l’Eclairage (CIE) coordinates of (0.154, 0.054) and maximum external quantum efficiencies (EQEmax) of 6.20%. Remarkably, no-doped device based on pBFCz--26DPPM delivers CIE coordinates of (0.153, 0.067) and EQEs as high as 5.80%. These device performances are amongst some of those of the best performing deep blue TADF OLEDs with similar color gamut.

Adamantyl Substitution Strategy for Realizing Solution-Processable Thermally Stable Deep-Blue Thermally Activated Delayed Fluorescence Materials.

Highly efficient solution-processable emitters, especially deep-blue emitters, are greatly desired to develop low-cost and low-energy-consumption organic light-emitting diodes (OLEDs). A recently developed class of potentially metal-free emitters, thermally activated delayed fluorescence (TADF) materials, are promising candidates, but solution-processable TADF materials with efficient blue emissions are not well investigated. In this study, first the requirements for the design of efficient deep-blue TADF materials are clarified, on the basis of which, adamantyl-substituted TADF molecules are developed. The substitution not only endows high solubility and excellent thermal stability but also has a critical impact on the molecular orbitals, by pushing up the lowest unoccupied molecular orbital energy and triplet energy of the molecules. In the application to OLEDs, an external quantum efficiency (EQE) of 22.1% with blue emission having Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.19) is realized. A much deeper blue emission with CIE (0.15, 0.13) is also achieved, with an EQE of 11.2%. These efficiencies are the best yet among solution-processed TADF OLEDs of CIE y < 0.20 and y < 0.15, as far as known. This work demonstrates the validity of adamantyl substitution and paves a pathway for straightforward realization of solution-processable efficient deep-blue TADF emitters.

45‐4: Approach for Attaining Short Exciton Lifetime in Thermally Activated Delayed Fluorescence Emitters

We report an approach to achieve short delayed fluorescence exciton lifetime in efficient green and blue thermally activated delayed fluorescence (TADF) emitters. The synthesized TADF emitters show small singlet‐triplet energy splitting, high photoluminescence quantum yields, and proper charge transfer characteristics, resulting in short delayed fluorescence exciton lifetime. The fabricated organic light emitting diodes with green and deep blue TADF emitters display maximum external quantum efficiency of 31.4% and 22.5%, respectively, and significantly low efficiency roll‐off characteristics.

Manipulating the Electronic Excited State Energies of Pyrimidine-Based Thermally Activated Delayed Fluorescence Emitters To Realize Efficient Deep-Blue Emission.

The development of efficient and robust deep-blue emitters is one of the key issues in organic light-emitting devices (OLEDs) for environmentally friendly, large-area displays or general lighting. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention. However, only a handful of examples of deep-blue TADF emitters have been reported to date, and the emitters generally show large efficiency roll-off at practical luminance over several hundreds to thousands of cd m-2, most likely because of the long delayed fluorescent lifetime (τd). To overcome this problem, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τd. We then systematically investigated the relationships among the chemical structure, properties, and device performances. The resultant novel pyrimidine emitters, called Ac-XMHPMs (X = 1, 2, and 3), contain different numbers of bulky methyl substituents at acceptor moieties, increasing the excited singlet (ES) and triplet state (ET) energies. Among them, Ac-3MHPM, with a high ET of 2.95 eV, exhibited a high external quantum efficiency (ηext,max) of 18% and an ηext of 10% at 100 cd m-2 with Commission Internationale de l'Eclairage chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs. Our molecular design strategy provides fundamental guidance to design novel deep-blue TADF emitters.

Pyrimidine-based twisted donor-acceptor delayed fluorescence molecules: a new universal platform for highly efficient blue electroluminescence.

Deep-blue emitters that can harvest both singlet and triplet excited states to give high electron-to-photon conversion efficiencies are highly desired for applications in full-color displays and white lighting devices based on organic light-emitting diodes (OLEDs). Thermally activated delayed fluorescence (TADF) molecules based on highly twisted donor-acceptor (D-A) configurations are promising emitting dopants for the construction of efficient deep-blue OLEDs. In this study, a simple and versatile D-A system combining acridan-based donors and pyrimidine-based acceptors has been developed as a new platform for high-efficiency deep-blue TADF emitters. The designed pre-twisted acridan-pyrimidine D-A molecules exhibit small singlet-triplet energy splitting and high photoluminescence quantum yields, functioning as efficient deep-blue TADF emitters. The OLEDs utilizing these TADF emitters display bright blue electroluminescence with external quantum efficiencies of up to 20.4%, maximum current efficiencies of 41.7 cd A-1, maximum power efficiencies of 37.2 lm W-1, and color coordinates of (0.16, 0.23). The design strategy featuring such acridan-pyrimidine D-A motifs can offer great prospects for further developing high-performance deep-blue TADF emitters and TADF-OLEDs.

Controlling Singlet-Triplet Energy Splitting for Deep-Blue Thermally Activated Delayed Fluorescence Emitters.

The development of efficient metal-free organic emitters with thermally activated delayed fluorescence (TADF) properties for deep-blue emission is still challenging. A new family of deep-blue TADF emitters based on a donor-acceptor architecture has been developed. The electronic interaction between donor and acceptor plays a key role in the TADF mechanism. Deep-blue OLEDs fabricated with these TADF emitters achieved high external quantum efficiencies over 19.2 % with CIE coordinates of (0.148, 0.098).

Diphenylsulphone derivatives for a blue thermally activated delayed fluorescence

Novel thermally activated delayed fluorescence (TADF) emitters, including diphenylsulphone (DPS) as an electron acceptor and spiro[acridine-9,9′-fluorene] (ACFL) and spiro[acridine-9,9′-xanthene] (ACXA) as electron donors, were designed, and their electronic and optical properties were investigated theoretically for an OLED. We obtain the energies of the first singlet (S1) and first triplet (T1)-excited states of TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the ground state using a dependence on charge transfer amounts for the optimal Hartree–Fock percentage in the exchange–correlation of TD-DFT. The calculated ΔEST values of ACFL-DPS (0.012 eV) and ACXA-DPS (0.014 eV) were small enough to TADF because of the large dihedral angles between the donor and accepter moieties. We show that ACXA-DPS would have a suitable deep blue TADF OLED emitter among the materials with small ΔEST and the emission wavelength of 2.802 eV (442 nm).

Extremely condensing triplet states of DPEPO-type hosts through constitutional isomerization for high-efficiency deep-blue thermally activated delayed fluorescence diodes.

The similarity of thermally activated delayed fluorescence (TADF) dyes and their hosts as pure organic molecules makes hosts predominant in intermolecular interactions and crucial to exciton harvesting and utilization in TADF diodes. DPEPO is the most popular high-energy-gap blue TADF host with steric ortho-substituted diphenylphosphine oxide (DPPO) groups for intermolecular interaction suppression, but suffers from serious efficiency roll-off due to its weak electroactivity. On the contrary, para-substituted DPPO with small steric hindrance is superior in intramolecular electronic coupling. In this work, four constitutional isomers of DPEPO are constructed as diphenylether (DPE) with two diphenylphosphine oxide (DPPO) groups substituted at either the 2 or 4 position, namely 22'DPEPO (viz.DPEPO), 24DPEPO, 24'DPEPO and 44'DPEPO, respectively. On the basis of separation configuration, the steric effect and electroactivity of ortho- and para-substituted DPPOs are successfully integrated in 24'DPEPO, accompanied by remarkably reduced intermolecular interactions due to its unsymmetrical configuration. Compared to its congeners, 24'DPEPO has a rigid structure and locally excited states similar to 22'DPEPO for interaction suppression and improved charge mobility comparable to 44'DPEPO for charge flux balance. Significantly, by virtue of the predominant orientation effect of ortho-DPPO on the T1 location, its T1 state is extremely condensed onto a single phenyl, protected from intermolecular interactions by its remaining five phenyls at its maximum extent. Consequently, 24'DPEPO endowed its DMAC-DPS-based deep-blue devices with state-of-the-art performance, including high color purity with chromaticity coordinates of (0.16, 0.17), external quantum efficiency (EQE) beyond 20% and EQE roll-off as low as 32% at 1000 cd m-2. It is shown that the device performance of 24'DPEPO was far beyond simple integration of those of 22'DPEPO and 44'DPEPO, verifying the significance of host optimization.