Triarylamine-Pyridine-Carbonitriles for Organic Light-Emitting Devices with EQE Nearly 40

A theoretical investigation of benzothiadiazole derivatives for high efficiency OLEDs

It is of great importance and worthy of efforts to give a clear structure-property relationship and microscopic mechanism of fluorescence emitters with high quantum yield. In this work, we perform a detailed computational investigation to give an explanation to the high efficiency of a fluorescence emitter XBTD-NPh based TADF sensitized fluorescence (TSF) OLEDs, and construct a symmetry structure DSBNA-BTD. Theoretical calculations show that XBTD-NPh is a long-time phosphorescent material at 77 K and TADF is attributed to the RISC of T1 to S1 state. For DSBNA-BTD, excitons arrived at T1 state comes to a large rate of nonradiatively path to the ground state, meaning it is may not be an efficient TADF molecule. For both molecules, the fast IC between T2 and T1 state results in that the hot exciton channel T1-Tn-S1 makes no contribution to the TADF.

High-performance near-infrared OLEDs maximized at 925 nm and 1022 nm through interfacial energy transfer

Using a transfer printing technique, we imprint a layer of a designated near-infrared fluorescent dye BTP-eC9 onto a thin layer of Pt(II) complex, both of which are capable of self-assembly. Before integration, the Pt(II) complex layer gives intense deep-red phosphorescence maximized at ~740 nm, while the BTP-eC9 layer shows fluorescence at > 900 nm. Organic light emitting diodes fabricated under the imprinted bilayer architecture harvest most of Pt(II) complex phosphorescence, which undergoes triplet-to-singlet energy transfer to the BTP-eC9 dye, resulting in high-intensity hyperfluorescence at > 900 nm. As a result, devices achieve 925 nm emission with external quantum efficiencies of 2.24% (1.94 ± 0.18%) and maximum radiance of 39.97 W sr-1 m-2. Comprehensive morphology, spectroscopy and device analyses support the mechanism of interfacial energy transfer, which also is proved successful for BTPV-eC9 dye (1022 nm), making bright and far-reaching the prospective of hyperfluorescent OLEDs in the near-infrared region.

Indolo[3,2-a]carbazoles as Engineered Materials for Optoelectronic Applications: Synthesis, Structural Insights, and Computational Screening

The biological and medicinal importance of indolocarbazoles has been known for the past several decades. However, in recent times, these compounds have been emerging as potential candidates for optoelectronic applications, although several challenges are associated with their synthesis. We report here a Pd(II)-catalyzed process for the synthesis of indolo[3,2-a]carbazoles. The reaction proceeded under neat conditions and in the presence of aqueous nonmetallic oxidant TBHP, and the products were purified directly after the completion of the reaction. Also, the possibility of employing the present method for reaction with gram-scale feed was investigated. A detailed single-crystal analysis of several indolo[3,2-a]carbazoles revealed how the molecular arrangement can be tuned by altering the functionalization. Finally, the developed molecules were screened computationally to assess their potential for possible use as hole transport materials (HTMs) for organic light-emitting diodes (OLEDs).

Robust luminogens as cutting-edge tools for efficient light emission in recent decades

Blue luminogens play a vital role in white lighting and potential metal-free fluorescent materials and their high-lying excited states contribute to harvesting triplet excitons in devices. However, in TADF-OLEDs (ΔEST < 0.1 eV), although T1 excitons transfer to S1via RISC with 100% IQE, the longer lifetime of blue TADF suffers from efficiency roll-off (RO). In this case, hybridized local and charge transfer (HLCT) materials have attracted significant interest in lighting owing to their 100% hot exciton harvesting and enhanced efficiency. Both academics and industrialists widely use the HLCT strategy to improve the efficiency of fluorescent organic light-emitting diodes (FOLEDs) by harvesting dark triplet excitons through the RISC process. Aggregation-induced emissive materials (AIEgens) possess tight packing in the aggregation state, and twisted AIEgens with HLCT behaviour have a shortened conjugation length, inducing blue emission and making them suitable candidates for OLED applications. TTA-OLEDs are used in commercial BOLEDs because of their moderate efficiency and reasonable operation lifetime. In this review, we discuss the devices based on TTA fluorophores, TADF fluorophores, HLCT fluorophores, AIEgens and HLCT-sensitized fluorophores (HLCT-SF), which break through the statistical limitations.

Phosphorescent Platinum(II) Complexes with a Spiro-fused Xanthene Unit: Synthesis and Photophysical Properties

Novel platinum(II) complexes, derived from the spiro[fluorene-9,9'-xanthene] (SFX) motif, were synthesized and combined with different auxiliary ligands such as acetylacetonate (acac), bis(2,4,6-trimethylphenyl)propane-1,3-dionate (mesacac) and dihydrobis(3,5-dimethylpyrazole-1-yl) borate. The final products were obtained in yields of up to 36 % and characterized by NMR, X-ray and combustion analysis. These complexes have structured green-blue emission spectra with Commission Internationale de l'Éclairage (CIExy) coordinates of (0.21;0.46). Excellent photoluminescence quantum yields (PLQYs) ranging from 87 %-91 % were found. The emission lifetimes vary from 33 μs to 43 μs. Calculations on the B3LYP/6-311++G** level of theory reveal, that the nature of the emissive state is dependent on the positional regioisomerism of the SFX motif. The 2-SFX complexes demonstrate ligand-centered (3LC) emission, while the 2'-SFX regioisomer with the mesacac ligand shows a strong 3MLCT character.

Sterically Crowded Donor-Rich Imidazole Systems as Hole Transport Materials for Solution-Processed OLEDs

Imidazole, being an interesting dinitrogenic five-membered heterocyclic core, has been widely explored during the last several decades for developing various fascinating materials. Among the different domains where imidazole-based materials find wide applications, the area of optoelectronics has seen an overwhelming growth of functional imidazole derivatives developed through remarkable design and synthesis strategies. The present work reports a design approach for integrating bulky donor units at the four terminals of an imidazole core, leading to the development of sterically populated imidazole-based molecular platforms with interesting structural features. Rationally chosen starting substrates led to the incorporation of a bulky donor at the four terminals of the imidazole core. In addition, homo- and cofunctional molecular systems were synthesized through a suitable combination of initial ingredients. Our approach was extended to develop a series of four molecular systems, i.e., Cz3PhI, Cz4I, Cz3PzI, and TPA3CzI, containing carbazole, phenothiazine, and triphenylamine as known efficient donors at the periphery. Given their interesting structural features, three sterically crowded molecules (Cz4I, Cz3PzI, and TPA3CzI) were screened by using DFT and TD-DFT calculations to investigate their potential as hole transport materials (HTMs) for optoelectronic devices. The theoretical studies on several aspects including hole reorganization and exciton binding energies, ionization potential, etc., revealed their potential as possible candidates for the hole transport layer of OLEDs. Single-crystal analysis of Cz3PhI and Cz3PzI established interesting structural features including twisted geometries, which may help attain high triplet energy. Finally, the importance of theoretical predictions was established by fabricating two solution-process green phosphorescent OLED devices using TPA3CzI and Cz3PzI as HTMs. The fabricated devices exhibited good EQE/PE and CE of ∼15%/56 lm/W/58 cd/A and ∼13%/47 lm/W/50 cd/A, respectively, at 100 cd/m2.

Exploring Robust Delayed Fluorescence Materials via Structural Rigidification for Realizing Organic Light-Emitting Diodes with High Efficiencies and Small Roll-Offs

Thermally activated delayed fluorescence (TADF) materials have been widely studied for the fabrication of high-performance organic light-emitting diodes (OLEDs), but the serious efficiency roll-offs still remain unsolved in most cases. Herein, it is wish to report a series of robust green TADF compounds containing rigid xanthenone acceptor and acridine-based spiro donors. The enhancement in molecular rigidity not only endows the compounds with improved thermal stability but also results in reduced geometric vibrations and thus lowered reorganization energies. These compounds exhibit distinct merits of high thermal stabilities, excellent photoluminescence quantum efficiencies (96%-97%), large horizontal dipole orientation ratios (87.4%-92.1%) and fast TADF rates (1.4-1.5 × 106 s-1 ). The OLEDs using them as emitters furnish superb electroluminescence performances with outstanding external quantum efficiencies (ηext s) of up to 37.4% and very small efficiency roll-offs. Moreover, highly efficient hyperfluorescence OLEDs are obtained by using them as sensitizers for the green mutilresonance TADF emitter BN2, delivering excellent ηext s of up to 34.2% and improved color purity. These results disclose the high potential of these TADF compounds as emitters and sensitizers for OLEDs.

Cyano Decoration of π-Bridge to Boost Photoluminescence and Electroluminescence Quantum Yields of Triazine/Carbazole Based Blue TADF Emitter

In general, a large donor-acceptor dihedral angle is required to guarantee sufficient frontier molecular orbitals separation for thermally activated delayed fluorescence (TADF) emitters, which is intrinsically unfavorable for the radiative transition. We present a molecular design method favoring both reverse intersystem crossing (RISC) and radiative transitions even at a moderate D-A angle. A blue TADF emitter TrzBuCz-CN was designed with triazine/tert-butylcarbazole as donor/acceptor and cyano (CN) incorporated on the phenylene bridge. In comparison with the methyl decoration in similar way (TrzBuCz-Me), CN decoration reduced the D-A dihedral angle from 70° to 60°, which is intrinsically not favorable for sufficient FMO separation, but unexpectedly reduced the singlet and triplet energy gap (ΔEST ) and thus facilitated TADF feature by pulling down the lowest singlet state energy. While the reduced distorsion instead improved the HOMO-LUMO overlap and boosted the fluorescence quantum yield from 41 % to 94 %. The blue organic light-emitting diode of TrzBuCz-CN exhibited an external quantum efficiency of 13.7 % with emission peak at 466 nm, greatly superior to 6.0 % of TrzBuCz-Me. The result provides a feasible design strategy to facilitate both RISC and radiation processes by CN decoration of the linking bridge of TADF emitters.

Introduction of a Chiral Biphenanthrene-Diol Unit to Achieve Circularly Polarized Thermally Activated Delayed Fluorescence

Circularly polarized luminescence (CPL) materials are promising candidates for future display technology. However, such highly efficient emitters suffer from the issues of difficult chiral separation and low photoluminescence quantum yield (PLQY). In this work, the chiral 4,4'-biphenanthrene-3,3'-diol (BIPOL) unit was introduced into a thermally activated delayed fluorescence (TADF) framework for the first time. We presented two series of enantiomers, R/S-o-DCzBPNCN and R/S-p-DCzBPNCN, and the synthesis of enantiopure BIPOL can be prepared via normal column chromatography. Notably, o-DCzBPNCN showed narrow singlet-triplet gap of 0.05 eV, efficient TADF, and high PLQYs of 82 % in doped films. In addition, R/S-o-DCzBPNCN exhibited high luminescence dissymmetry factor (gPL ) values of -1.94×10-2 /+1.91×10-2 in doped films. The strategy of BIPOL introduction offers a new approach to organic emitters with stereospecific synthesis, TADF, and chiroptical properties.

Adjusting the Electron-Withdrawing Ability of Acceptors in Thermally Activated Delayed Fluorescence Conjugated Polymers for High-Performance OLEDs

Constructing high-performance solution-processed organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) conjugated polymers remains a challenging issue. The electron-withdrawing ability of acceptors in TADF units significantly affects the TADF properties of the conjugated polymers. Herein, we have designed three TADF conjugated polymers, in which phenoxazine donors and anthracen-9(10H)-one acceptors are incorporated into the polymeric backbones and side chains, respectively, and the carbazole derivative is copolymerized as the host. By incorporating different heteroatoms, such as nitrogen, oxygen, or sulfur, with slightly different electronegativities into anthracen-9(10H)-one, the effect of the electron-withdrawing ability of the acceptor on the performance of conjugated TADF polymer-based OLEDs is thus systematically studied. It is found that the introduction of a nitrogen atom can enhance the spin-orbital coupling and RISC process due to the modulated energy levels and nature of the excited states. As a result, the solution-processed OLEDs based on the prepared polymer p-PXZ-XN display an excellent comprehensive performance with an EQEmax of 17.6%, a low turn-on voltage of 2.8 V, and a maximum brightness of 14750 cd m-2. Notably, the efficiency roll-off is quite low, maintaining 15.1% at 1000 cd m-2, 12.1% at 3000 cd m-2, and 6.1% at 10000 cd m-2, which ranks in the first tier among the reported TADF conjugated polymers. This work provides a guideline for constructing high-efficiency TADF polymers.

Neoteric Advances in Oxygen Bridged Triaryl Boron-based Delayed Fluorescent Materials for Organic Light Emitting Diodes

Since their first demonstration, thermally activated delayed fluorescence (TADF) materials have been emerged as the most promising emitters because of their promising applications in optoelectronics, typified by organic light-emitting diodes (OLEDs). In which, the rigid oxygen bridged boron acceptor-featured (DOBNA) emitters have gained tremendous impetus for OLEDs, which is ascribed to their excellent external quantum efficiency (EQE). However, these materials often displayed severe efficiency roll-off and poor operational stability. Therefore, there needs to be a comprehensive understanding of the aspect of the molecular design and structure-property relationship. To the best of our knowledge, there is no detailed review on the structure-function outlook of DOBNA-based emitters emphasizing the effect of the nature of donor units, their number density, and substitution pattern on the physicochemical properties, excited state dynamics and OLED performance were reported. To fill this gap, herein we presented the recent advancements in DOBNA-based acceptor featured TADF materials by classifying them into several subgroups based on the molecular design i. e. donor-acceptor (D-A), D-A-D, A-D-A, and multi-resonant TADF (MR-TADF) emitters. The detailed design concepts, along with their respective physicochemical and OLED performances were summarized. Finally, the prospective of this class of materials in forthcoming OLED displays is also discussed.

Design and Synthesis of Red Through-Space Charge Transfer Thermally Activated Delayed Fluorescence Emitters with Donor/Acceptor/Donor Stacking

Red through-space charge transfer thermally activated delayed fluorescence (TSCT TADF) materials named SAF36DCPP and SAF27DCPP with sandwiched structures were synthesized. Single crystals indicated that the intramolecular C-H···π interactions play a vital role in rigidifying the sandwiched structure, which results in a fluorescence yield of 63% for SAF36DCPP compared to 40% for SAF27DCPP. Organic light-emitting diodes with SAF36DCPP as the emitter realized a maximum external quantum efficiency of 16.12%.

Rational Design of Highly Efficient Orange-Red/Red Thermally Activated Delayed Fluorescence Emitters with Submicrosecond Emission Lifetimes

The development of orange-red/red thermally activated delayed fluorescence (TADF) materials with both high emission efficiencies and short lifetimes is highly desirable for electroluminescence (EL) applications, but remains a formidable challenge owing to the strict molecular design principles. Herein, two new orange-red/red TADF emitters, namely AC-PCNCF3 and TAC-PCNCF3, composed of pyridine-3,5-dicarbonitrile-derived electron-acceptor (PCNCF3) and acridine electron-donors (AC/TAC) are developed. These emitters in doped films exhibit excellent photophysical properties, including high photoluminescence quantum yields of up to 0.91, tiny singlet-triplet energy gaps of 0.01 eV, and ultrashort TADF lifetimes of less than 1 µs. The TADF-organic light-emitting diodes employing the AC-PCNCF3 as emitter achieve orange-red and red EL with high external quantum efficiencies of up to 25.0% and nearly 20% at doping concentrations of 5 and 40 wt%, respectively, both accompanied by well-suppressed efficiency roll-offs. This work provides an efficient molecular design strategy for developing high-performance red TADF materials.

A Feasible Strategy for a Highly Efficient Thermally Activated Delayed Fluorescence Emitter Over 900 nm Based on Phenalenone Derivatives

Near-infrared (NIR) organic light-emitting diodes (OLEDs) suffer from the low external electroluminescence (EL) quantum efficiency (EQE), which is a critical obstacle for potential applications. Herein, 1-oxo-1-phenalene-2,3-dicarbonitrile (OPDC) is employed as an electron-withdrawing aromatic ring, and by incorporating with triphenylamine (TPA) and biphenylphenylamine (BBPA) donors, two novel NIR emitters with thermally activated delayed fluorescence (TADF) characteristics, namely OPDC-DTPA and OPDC-DBBPA, are first developed and compared in parallel. Intense NIR emission peaks at 962 and 1003 nm are observed in their pure films, respectively. Contributed by the local excited (LE) characteristics in the triplet (T1 ) state in synergy with the charge transfer (CT) characteristics for the singlet (S1 ) state to activate TADF emission, the solution processable doped NIR OLEDs based on OPDC-DTPA and OPDC-DBBPA yield EL peaks at 834 and 906 nm, accompanied with maximum EQEs of 0.457 and 0.103 %, respectively, representing the state-of-the-art EL performances in the TADF emitter-based NIR-OLEDs in the similar EL emission regions so far. This work manifests a simple and effective strategy for the development of NIR TADF emitters with long wavelength and efficiency synchronously.

Modifications of Pyridine-3,5-dicarbonitrile Acceptor for Highly Efficient Green-to-Red Organic Light-Emitting Diodes

The strategy of acceptor modification is a powerful technique for tuning the emission color of thermally activated delayed fluorescence (TADF) emitters. In this study, we have successfully designed and synthesized three TADF emitters with donor-acceptor (D-A) structures using a 4-(diphenylamino)-2,6-dimethylphenyl (TPAm) donor and various pyridine-3,5-dicarbonitrile (PC) acceptor units. As a result, three compounds named TPAmbPPC, TPAm2NPC, and TPAmCPPC exhibited greenish-yellow to orange-red emissions with high photoluminescent quantum yields (76-100%) in thin films. Remarkably, a greenish-yellow device based on TPAmbPPC and TPAm2NPC showed a high maximum external quantum efficiency (EQE_max) of 39.1 and 39.0%, respectively. Furthermore, benefiting from the suitable steric hindrance between the acceptor and donor, the nondoped organic light-emitting diodes (OLEDs) based on TPAmbPPC demonstrated an exceptional EQE_max of 21.6%, indicating its promising potential as an efficient emitter for the application of OLED applications. Furthermore, orange-red OLED devices based on TPAmCPPC exhibited a high EQE_max of 26.2%, a CE of 50.1 cd A^-1, and a PE of 52.4 lm W^-1.

Realizing efficient blue and deep-blue delayed fluorescence materials with record-beating electroluminescence efficiencies of 43.4

As promising luminescent materials for organic light-emitting diodes (OLEDs), thermally activated delayed fluorescence materials are booming vigorously in recent years, but robust blue ones still remain challenging. Herein, we report three highly efficient blue and deep-blue delayed fluorescence materials comprised of a weak electron acceptor chromeno[3,2-c]carbazol-8(5H)-one with a rigid polycyclic structure and a weak electron donor spiro[acridine-9,9'-xanthene]. They hold distinguished merits of excellent photoluminescence quantum yields (99%), ultrahigh horizontal transition dipole ratios (93.6%), and fast radiative transition and reverse intersystem crossing, which furnish superb blue and deep-blue electroluminescence with Commission Internationale de I'Eclairage coordinates (CIE_x,y) of (0.14, 0.18) and (0.14, 0.15) and record-beating external quantum efficiencies (η_exts) of 43.4% and 41.3%, respectively. Their efficiency roll-offs are successfully reduced by suppressing triplet-triplet and singlet-singlet annihilations. Moreover, high-performance deep-blue and green hyperfluorescence OLEDs are achieved by utilizing these materials as sensitizers for multi-resonance delayed fluorescence dopants, providing state-of-the-art η_exts of 32.5% (CIE_x,y = 0.14, 0.10) and 37.6% (CIE_x,y = 0.32, 0.64), respectively, as well as greatly advanced operational lifetimes. These splendid results can surely inspire the development of blue and deep-blue luminescent materials and devices.

Multiresonant TADF materials: triggering the reverse intersystem crossing to alleviate the efficiency roll-off in OLEDs

The hunt for narrow-band emissive pure organic molecules capable of harvesting both singlet and triplet excitons for light emission has garnered enormous attention to promote the advancement of organic light-emitting diodes (OLEDs). Over the past decade, organic thermally activated delayed fluorescence (TADF) materials based on donor (D)/acceptor (A) combinations have been researched for OLEDs in wide color gamut (RGB) regions. However, due to the strong intramolecular charge-transfer (CT) state, they exhibit broad emission with full-width-at-half maximum (FWHM) > 70 nm, which deviates from being detrimental to achieving high color purity for future high-end display electronics such as high-definition TVs and ultra-high-definition TVs (UHDTVs). Recently, the new development in the sub-class of TADF emitters called multi-resonant TADF (MR-TADF) emitters based on boron/nitrogen atoms has attracted much interest in ultra-high definition OLEDs. Consequently, MR-TADF emitters are appeal to their potentiality as promising candidates in fabricating the high-efficient OLEDs due to their numerous advantages such as high photoluminescence quantum yield (PLQY), unprecedented color purity, and narrow bandwidth (FWHM ≤ 40 nm). Until now many MR-TADF materials have been developed for ultra-gamut regions with different design concepts. However, most MR-TADF-OLEDs showed ruthless external quantum efficiency (EQE) roll-off characteristics at high brightness. Such EQE roll-off characteristics were derived mainly from the low reverse intersystem crossing (k_RISC) rate values. This feature article primarily focuses on the design strategies to improve k_RISC for MR-TADF materials with some supportive strategies including extending charge delocalization, heavy atom introduction, multi-donor/acceptor utilization, and a hyperfluorescence system approach. Furthermore, the outlook and prospects for future developments in MR-TADF skeletons are described.

Solution-Processed Pure Red TADF Organic Light-Emitting Diodes With High External Quantum Efficiency and Saturated Red Emission Color

In spite of recent research progress in red thermally activated delayed fluorescence (TADF) emitters, highly efficient solution-processable pure red TADF emitters are rarely reported. Most of the red TADF emitters reported to date are designed using a rigid acceptor unit which renders them insoluble and unsuitable for solution-processed organic light-emitting diodes (OLEDs). To resolve this issue, a novel TADF emitter, 6,7-bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-5,8-dicarbonitrile (tBuTPA-CNQx) is designed and synthesized. The highly twisted donor-acceptor architecture and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital distribution lead to a very small singlet-triplet energy gap of 0.07 eV, high photoluminescence quantum yield of 92%, and short delayed fluorescence lifetime of 52.4 µs. The peripheral t-butyl phenyl decorated quinoxaline acceptor unit and t-butyl protected triphenylamine donor unit are proven to be useful building blocks to improve solubility and minimize the intermolecular interaction. The solution-processed OLED based on tBuTPA-CNQx achieves a high external quantum efficiency (EQE) of 16.7% with a pure red emission peak at 662 nm, which is one of the highest EQE values reported till date in the solution-processed pure red TADF OLEDs. Additionally, vacuum-processable OLED based on tBuTPA-CNQx exhibits a high EQE of 22.2% and negligible efficiency roll-off.

Conformational isomeric thermally activated delayed fluorescence (TADF) emitters: mechanism, applications, and perspectives

Thermally activated delayed fluorescence (TADF) materials have received enormous attention and the mechanism behind them has been investigated in depth. It has been found that some donor-acceptor (D-A) type TADF emitters could obviously exhibit dual stable conformations in the ground states and their distributions significantly affect the physical properties and device performances. Therefore, professional analysis and a summary of the relationship between molecular structures and performances are very important. In this review, we first summarize the mechanism and properties of TADF emitters with conformational isomerism. We also classify their recent progress according to their different applications, and provide an outlook on their perspectives.

Carbonyl-Containing Thermally Activated Delayed Fluorescence Emitters for Narrow-Band Electroluminescence

Carbonyl-containing derivatives show enduring vitality in the field of thermally activated delayed fluorescence (TADF) materials; they can realize high device efficiency by using both singlet and triplet excitons for electroluminescence. Recently, a system based on fused ketone/amine exhibited huge potential for constructing multi-resonance TADF (MR-TADF) emitters, which exhibit higher narrow-band emission than conventional TADF emitters with twisted donor-acceptor (D-A) structure. Herein, we summarize current research progress in both traditional and MR-type ketone derivatives with TADF characteristics for introducing the molecular design strategy of maintaining high device efficiency while keeping narrow-band emission profile. We hope this review can inspire the emergence of more high-performance narrow-band materials.

Carbonyl (CO)/N-based thermally activated delayed fluorescent materials with high efficiency and fast reverse intersystem crossing rate: a theoretical design and study

Two novel highly efficient organic TADF materials based on MR structures have been theoretically designed and investigated.

Precise Functionalization of a Multiple-Resonance Framework: Constructing Narrowband Organic Electroluminescent Materials with External Quantum Efficiency over 40

It is of important strategic significance to develop high-efficiency narrowband organic electroluminescent materials that can be employed to fabricate ultrahigh-definition displays with wide color gamut. This topic implies a great challenge to molecular design and synthesis, especially for the development of universality, diversity, scalability, and robustness of molecular architectonics. In this work, a synthetic methodology is demonstrated for functionalizing brominated BN-containing multiple-resonance (MR) frameworks with multifarious functional groups, such as donors, acceptors, and moieties without obvious push-pull electron properties. The m-DPAcP-BNCz-based organic light-emitting diode (OLED) exhibits green emission with a full-width at half-maximum (FWHM) of 28 nm and a maximum external quantum efficiency (EQE) of 40.6%. The outstanding performance of m-DPAcP-BNCz is attributed to the perfect integration of the inherent advantages of the MR framework and the donor-acceptor configuration, which can not only achieve bathochromic shift and narrowband emission, but also obtain high photoluminescence (PL) quantum yield (Φ_PL ) and horizontal emitting dipole orientation ratio (Θ_// ). This straightforward and efficient approach provides insightful guidance for the construction and enrichment of more high-efficiency narrowband emitters.

Stable Tetradentate Gold(III)-TADF Emitters with Close to Unity Quantum Yield and Radiative Decay Rate Constant of up to 2 × 106 s-1 : High-Efficiency Green OLEDs with Operational Lifetime (LT90 ) Longer than 1800 h at 1000 cd m-2

High maximum external quantum efficiency (EQE_max ), small efficiency roll-offs, and long operational lifetime at practical luminances are three crucial parameters for commercialization of organic light-emitting diodes (OLEDs). To simultaneously achieve these goals, it is desirable to have the radiative decay rate constant (k_r ) as large as possible, which, for a thermally activated delayed fluorescent (TADF) emitter, requires both a large S₁ →S₀ radiative decay rate constant (k_r ^S ) and a small singlet-triplet energy gap (ΔE_ST ). Here, the design of a class of tetradentate gold(III) TADF complexes for narrowing the ΔE_ST while keeping the k_r ^S large is reported. The as-synthesized complexes display green emission with close to unity emission quantum yields, and k_r approaching 2 × 10⁶ s^-1 in thin films. The vacuum-deposited green OLEDs based on 1 and 4 demonstrate maximum EQEs of up to 24 and 27% with efficiency roll-offs of 5.5 and 2.2% at 1000 cd m^-2 , respectively; the EQEs maintain high at 10 000 cd m^-2 (19% (1) and 24% (4)). A long LT₉₀ device lifetime of 1820 h at 1000 cd m^-2 for complex 1 is achieved, which is one of the longest device lifetimes of TADF-OLEDs reported in the literature.

Dense Local Triplet States and Steric Shielding of a Multi-Resonance TADF Emitter Enable High-Performance Deep-Blue OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules based on boron and nitrogen atoms are emerging as next-generation blue emitters for organic light-emitting diodes (OLEDs) due to their narrow emission spectra and triplet harvesting properties. However, intermolecular aggregation stemming from the planar structure of typical MR-TADF molecules that leads to concentration quenching and broadened spectra limits the utilization of the full potential of MR-TADF emitters. Herein, a deep-blue MR-TADF emitter, pBP-DABNA-Me, is developed to suppress intermolecular interactions effectively. Furthermore, photophysical investigation and theoretical calculations reveal that adding biphenyl moieties to the core body creates dense local triplet states in the vicinity of S₁ and T₁ energetically, letting the emitter harvest excitons efficiently. OLEDs based on pBP-DABNA-Me show a high external quantum efficiency (EQE) of 23.4% and a pure-blue emission with a Commission Internationale de L'Eclairage (CIE) coordinate of (0.132, 0.092), which are maintained even at a high doping concentration of 100 wt%. Furthermore, by incorporating a conventional TADF sensitizer, deep-blue OLEDs with a CIE value of (0.133, 0.109) and an extremely high EQE of 30.1% are realized. These findings provide insight into design strategies for developing efficient deep-blue MR-TADF emitters with fast triplet upconversion and suppressed self-aggregation.

The Role of Balancing Carrier Transport in Realizing an Efficient Orange-Red Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diode

Simultaneously realizing improved carrier mobility and good photoluminescence (PL) efficiency in red thermally activated delayed-fluorescence (TADF) emitters remains challenging but important. Herein, two isomeric orange-red TADF emitters, oPDM and pPDM, with the same basic donor-acceptor backbone but a pyrimidine (Pm) attachment at different positions are designed and synthesized. The two emitters show similarly good PL properties, including narrow singlet-triplet energy offsets (0.11 and 0.15 eV) and high photoluminescence quantum yields (ca. 100 and 88%) in doped films. An orange-red organic light-emitting diode (OLED) employing oPDM as an emitter achieves an almost twice as high maximum external quantum efficiency (28.2%) compared with that of a pPDM-based OLED. More balanced carrier-transporting properties are responsible for their contrasting device performances, and the position effect of the Pm substituent leads to significantly distinct molecular packing behaviors in the aggregate states and different carrier mobilities.

Substituent engineering of the diboron molecular architecture for a nondoped and ultrathin emitting layer

Owing to the high technology maturity of thermally activated delayed fluorescence (TADF) emitter design with a specific molecular shape, extremely high-performance organic light-emitting diodes (OLEDs) have recently been achieved via various doping techniques. Recently, undoped OLEDs have drawn immense attention because of their manufacturing cost reduction and procedure simplification. However, capable materials as host emitters are rare and precious because general fluorophores in high-concentration states suffer from serious aggregation-caused quenching (ACQ) and undergo exciton quenching. In this work, a series of diboron materials, CzDBA, iCzDBA, and tBuCzDBA, is introduced to realize the effect of steric hindrance and the molecular aspect ratio via experimental and theoretical studies. We computed transition electric dipole moment (TEDM) and molecular dynamics (MD) simulations as a proof-of-concept model to investigate the molecular stacking in neat films. It is worth noting that the pure tBuCzDBA film with a high horizontal ratio of 92% is employed to achieve a nondoped OLED with an excellent external quantum efficiency of 26.9%. In addition, we demonstrated the first ultrathin emitting layer (1 nm) TADF device, which exhibited outstanding power efficiency. This molecular design and high-performance devices show the potential of power-saving and economical fabrication for advanced OLEDs.

Double boron-embedded multiresonant thermally activated delayed fluorescent materials for organic light-emitting diodes

The subclass of multi resonant thermally activated delayed fluorescent emitters (MR-TADF) containing boron atoms has garnered significant attention in the field of organic light emitting diode (OLED) research. Among boron-based MR-TADF emitters, double boron-embedded MR-TADF (DB-MR-TADF) emitters show excellent electroluminescence performances with high photoluminescence quantum yields, narrow band emission, and beneficially small singlet-triplet energy levels in all the full-color gamut regions. This article reviews recent progress in DB-MR-TADF emitters, with particular attention to molecular design concepts, synthetic routes, optoelectronic properties, and OLED performance, giving future prospects for real-world applications.

All-fluorescence white organic light-emitting diodes with record-beating power efficiencies over 130 lm W‒1 and small roll-offs

Improving power efficiency (PE) and reducing roll-off are of significant importance for the commercialization of white organic light-emitting diodes (WOLEDs) in consideration of energy conservation. Herein, record-beating PE of 130.7 lm W⁻¹ and outstanding external quantum efficiency (EQE) of 31.1% are achieved in all-fluorescence two-color WOLEDs based on a simple sandwich configuration of emitting layer consisting of sky-blue and orange delayed fluorescence materials. By introducing a red fluorescence dopant, all-fluorescence three-color WOLEDs with high color rendering index are constructed based on an interlayer sensitization configuration, furnishing ultrahigh PE of 110.7 lm W⁻¹ and EQE of 30.8%. More importantly, both two-color and three-color WOLEDs maintain excellent PEs at operating luminance with smaller roll-offs than the reported state-of-the-art WOLEDs, and further device optimization realizes outstanding comprehensive performances of low driving voltages, large luminance, high PEs and long operational lifetimes. The underlying mechanisms of the impressive device performances are elucidated by host-tuning effect and electron-trapping effect, providing useful guidance for the development of energy-conserving all-fluorescence WOLEDs.

High power efficiency and low roll-off values are essential to the commercialization of white organic light-emitting diodes. Here, the authors construct all-fluorescence devices with an orange emitting layer sandwiched between two sky-blue emitting layers, achieving figure-of-merit of 130.7 lm/W.

A novel orange-red thermally activated delayed fluorescence emitter with high molecular rigidity and planarity realizing 32.5% external quantum efficiency in organic light-emitting diodes

Simultaneous optimization of photoluminescence quantum yield (Φ_PL) and horizontally oriented dipoles (Θ_‖) is considerably challenging for orange and red thermally activated delayed fluorescence (TADF) emitters, due to the conflicts between enhancing molecular rigidity and improving molecular planarity. Herein, a novel orange-red TADF emitter 10-(dipyrido[3,2-a:2',3'-c]phenazin-11-yl)-10H-spiro[acridine-9,9'-fluorene] (SAF-2NP) was constructed with a donor-acceptor structure. The highly rigid donor and acceptor segments ensure the overall rigidity of the emitter. More importantly, the quasi-coplanar structure between the acceptor and the fluorene moiety in the donor unit enlarges the molecular plane without weakening rigidity. Consequently, SAF-2NP exhibited extremely high Φ_PL and Θ_‖ of 99% and 85%, respectively. The optimal organic light-emitting diode using SAF-2NP as the emitter and 4,4'-di(9H-carbazol-9-yl)-1,1'-biphenyl (CBP) as the host demonstrated an unparalleled external quantum efficiency of 32.5% and a power efficiency of 85.2 lm W^-1 without any extra light extraction structure. This work provides a feasible strategy to establish efficient orange and red TADF emitters with both high rigidity and planarity.

Efficiently increasing the radiative rate of TADF material with metal coordination

Herein, a simple and straightforward method to reduce dramatically the lifetime of a pure organic thermally activated delayed fluorescence (TADF) material VIA metal coordination is demonstrated. We designed a mononuclear silver complex [Ag(PPh₂CH₃)(TCzBN-PyPz)]BF₄ (1) with a new emissive TCzBN-PyPz ligand. Even though the ligand and the metal complex have very similar emissive efficiencies and maximal peaks, over three orders of magnitude shorter lifetime of 0.59 μs for the complex than 2074 μs for ligand were obtained. Compared to other methods, the present protocol seems to be simple and highly effective.

Boron-Based Multi-Resonance TADF Emitter with Suppressed Intermolecular Interaction and Isomer Formation for Efficient Pure Blue OLEDs

Multi-resonance (MR) thermally activated delayed fluorescent (TADF) emitters are highly attractive due to their superior color purity as well as efficient light-harvesting ability from singlets and triplets. However, boron and nitrogen-based MR-TADF emitters suffer from their strong π-π interaction owing to their rigid flat cores. Herein, a boron-based multi-resonance blue TADF emitter with suppressed intermolecular interaction and isomer formation is developed through a simple synthetic process by introducing meta-xylene and meta-phenyphenyl groups to the core. The MR-TADF emitter, mBP-DABNA-Me, shows a narrowband blue emission with a peak at 467 nm, along with full width at half maximum of 28 nm, and photoluminescence quantum yield of 97%. Notably, highly efficient pure blue organic light-emitting diode (OLED) is realized using mBP-DABNA-Me, showing a maximum external quantum efficiency of 24.3% and a stable blue emission with a Commission Internationale de L'Eclairage coordinate of (0.124, 0.140). The color purity of the OLED is maintained at a high doping concentration of over 20%, attributed to the suppressed intermolecular interaction between the MR emitters.

High-Performance Circularly Polarized Electroluminescence with Simultaneous Narrowband Emission, High Efficiency, and Large Dissymmetry Factor

Organic light-emitting diodes (OLEDs) that can simultaneously achieve narrowband emission, high efficiency, and circularly polarized luminescence remain a formidable challenge. In this study, a simple strategy is developed to address this challenge. A chiral exciplex-forming co-host is first designed by employing a chiral donor and an achiral acceptor molecule. The chiral exciplex host enables an achiral green multiple-resonance thermally activated delayed fluorescence emitter to achieve high-performance circularly polarized electroluminescence (CP-EL) with a high external quantum efficiency of 33.2%, large electroluminescence dissymmetry factor of 2.8 × 10^-3 , and a small full-width at half-maximum of 42 nm. This work provides a general approach for realizing CP-EL using easily available achiral emitters and can significantly extend the scope of circularly polarized OLEDs.

Narrowband blue emission with insensitivity to the doping concentration from an oxygen-bridged triarylboron-based TADF emitter: nondoped OLEDs with a high external quantum efficiency up to 21.4

Blue thermally activated delayed fluorescence (TADF) emitters that can simultaneously achieve narrowband emission and high efficiency in nondoped organic light-emitting diodes (OLEDs) remain a big challenge. Herein, we successfully design and synthesize two blue TADF emitters by directly incorporating carbazole fragments into an oxygen-bridged triarylboron acceptor. Depending on the linking mode, the two emitters show significantly different photophysical properties. Benefitting from the bulky steric hindrance between the acceptor and terminal pendants, the blue emitter TDBA-Cz exhibited a high photoluminescence quantum yield (PLQY) of 88% in neat films and narrowband emission. The corresponding non-doped blue device exhibited a maximum external quantum efficiency (EQE) of 21.4%, with a full width at half maximum (FWHM) of only 45 nm. This compound is the first blue TADF emitter that can concurrently achieve narrow bandwidth and high electroluminescence (EL) efficiency in nondoped blue TADF-OLEDs.

A donor–acceptor TADF emitter showed narrowband high-efficiency blue emission by fine molecular modulation. The corresponding OLEDs exhibited a maximum EQE of 21.4% and a small FWHM of 45 nm, representing the most efficient nondoped blue TADF-OLEDs.

Quenching-Resistant Multiresonance TADF Emitter Realizes 40% External Quantum Efficiency in Narrowband Electroluminescence at High Doping Level

Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters manifest great potential for organic light-emitting diodes (OLEDs) due to their high exciton-utilization efficiency and narrowband emission. Nonetheless, their tendency toward self-quenching caused by strong interchromophore interactions would induce doping sensitivity and deteriorate the device performances, and effective strategy to construct quenching-resistant emitters without sacrifycing color purity is still to be developed. By segregating the planar MR-TADF skeleton using two bulky carbazolyl units, herein a highly emissive molecule with enhanced quenching resistance is reported. The steric effect largely removes the formation of detrimental excimers/aggregates, and boosts the performance of the corresponding devices with a maximum external quantum efficiency (EQE_max ) up to 40.0% and full width at half maximum (FWHM) of 25 nm, representative of the only example of single OLED that can concurrently achieve narrow bandwidth and high EL efficiency surpassing 40% to date. Even at doping ratio of 30 wt%, the EQE_max is retained to be 33.3% with nearly unchanged emission spectrum. This work provides a viable approach to realize doping-insensitive MR-TADF devices with extreme EL efficiency and color purity for high-end OLED displays.

Theoretically elucidating high photoluminescence performance of dimethylacridan-based blue-color thermally activated delayed fluorescent materials

Based on first-principles methods, we comprehensively quantify the luminous quantum efficiencies and related photophysical process rates of dimethylacridan-based blue-color TADF emitters.

High-Performance Orange-Red Organic Light-Emitting Diodes with External Quantum Efficiencies Reaching 33.5% based on Carbonyl-Containing Delayed Fluorescence Molecules

Developing orange to red purely organic luminescent materials having external quantum efficiencies (ηext s) exceeding 30% is challenging because it generally requires strong intramolecular charge transfer, efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (ΦPL ), and large optical outcoupling efficiency (Φout ) simultaneously. Herein, by introducing benzoyl to dibenzo[a,c]phenazine acceptor, a stronger electron acceptor, dibenzo[a,c]phenazin-11-yl(phenyl)methanone, is created and employed for constructing orange-red delayed fluorescence molecules with various acridine-based electron donors. The incorporation of benzoyl leads to red-shifted photoluminescence with accelerated RISC, reduced delayed lifetimes, and increased ΦPL s, and the adoption of spiro-structured acridine donors promotes horizontal dipole orientation and thus renders high Φout s. Consequently, the state-of-the-art orange-red organic light-emitting diodes are achieved, providing record-high electroluminescence (EL) efficiencies of 33.5%, 95.3 cd A-1 , and 93.5 lm W-1 . By referring the control molecule without benzoyl, it is demonstrated that the presence of benzoyl can exert significant positive effect over improving delayed fluorescence and enhancing EL efficiencies, which can be a feasible design for robust organic luminescent materials.