Showing papers on "OLED published in 2022"

Efficient selenium-integrated TADF OLEDs with reduced roll-off

Abstract: Organic light emitters based on multiresonance-induced thermally activated delayed fluorescent materials have great potential for realizing efficient, narrowband organic light-emitting diodes (OLEDs). However, at high brightness operation, efficiency roll-off attributed to the slow reverse intersystem crossing (RISC) process hinders the use of multiresonance-induced thermally activated delayed fluorescent materials in practical applications. Here we report a heavy-atom incorporating emitter, BNSeSe, which is based on a selenium-integrated boron–nitrogen skeleton and exhibits 100% photoluminescence quantum yield and a high RISC rate (kRISC) of 2.0 × 106 s−1. The corresponding green OLEDs exhibit excellent external quantum efficiencies of up to 36.8% and ultra-low roll-off character at high brightnesses (with very small roll-off values of 2.8% and 14.9% at 1,000 cd m−2 and 10,000 cd m−2, respectively). Furthermore, the outstanding capability to harvest triplet excitons also enables BNSeSe to be a superior sensitizer for a hyperfluorescence OLED, which shows state-of-the-art performance with a high excellent external quantum efficiency of 40.5%, power efficiency beyond 200 lm W−1, and luminance close to 20,0000 cd m−2. Green OLEDs based on BNSeSe offer high operational efficiency and reduced efficiency roll-off.

Achieving Ultimate Narrowband and Ultrapure Blue Organic Light‐Emitting Diodes Based on Polycyclo‐Heteraborin Multi‐Resonance Delayed‐Fluorescence Emitters

Abstract: To achieve an ultimate wide color gamut for ultrahigh-definition displays, there is great demand for the development of organic light-emitting diodes (OLEDs) enabling monochromatic, ultrapure blue electroluminescence (EL). Herein, high-efficiency and ultrapure blue OLEDs based on polycyclo-heteraborin multi-resonance thermally activated delayed fluorescence (MR-TADF) materials, BOBO-Z, BOBS-Z, and BSBS-Z, are reported. The key to the design of the present luminophores is the exquisite combination and interplay of multiple boron, nitrogen, oxygen, and sulfur heteroatoms embedded in a fused polycyclic π-system. Comprehensive photophysical and computational investigations of this family of MR-TADF materials reveal that the systematic implementation of chalcogen (oxygen and sulfur) atoms can finely modulate the emission color while maintaining a narrow bandwidth, as well as the spin-flipping rates between the excited singlet and triplet states. Consequently, OLEDs based on BOBO-Z, BOBS-Z, and BSBS-Z demonstrate narrowband and ultrapure blue EL emission, with peaks at 445-463 nm and full width at half maxima of 18-23 nm, leading to Commission Internationale de l'Éclairage-y coordinates in the range of 0.04-0.08. Particularly, for OLEDs incorporating sulfur-doped BOBS-Z and BSBS-Z, notably high maximum external EL quantum efficiencies of 26.9% and 26.8%, respectively, and small efficiency roll-offs are achieved concurrently.

High‐Performance Narrowband Pure‐Red OLEDs with External Quantum Efficiencies up to 36.1% and Ultralow Efficiency Roll‐Off

Abstract: High‐color‐purity blue and green organic light‐emitting diodes (OLEDs) have been resolved thanks to the development of B/N‐based polycyclic multiple resonance (MR) emitters. However, due to the derivatization limit of B/N polycyclic structures, the design of red MR emitters remains challenging. Herein, a series of novel red MR emitters is reported by para‐positioning N–π–N, O–π–O, B–π–B pairs onto a benzene ring to construct an MR central core. These emitters can be facilely and modularly synthesized, allowing for easy fine‐tuning of emission spectra by peripheral groups. Moreover, these red MR emitters display excellent photophysical properties such as near‐unity photoluminescence quantum yield (PLQY), fast radiative decay rate (kr) up to 7.4 × 107 s−1, and most importantly, narrowband emission with full‐width at half‐maximum (FWHM) of 32 nm. Incorporating these MR emitters, pure red OLEDs sensitized by phosphor realize state‐of‐the‐art device performances with external quantum efficiency (EQE) exceeding 36%, ultralow efficiency roll‐off (EQE remains as high as 25.1% at the brightness of 50 000 cd m−2), ultrahigh brightness over 130 000 cd m−2, together with good device lifetime.

Narrowband Emissive Thermally Activated Delayed Fluorescence Materials

Abstract: Organic thermally activated delayed fluorescence (TADF) materials have attracted significant research interest in the field of organic electronics because of their inherent advantage of 100% exciton utilization capability in organic light‐emitting diodes (OLEDs) without the use of noble metals. However, despite their high internal electroluminescence quantum efficiencies approaching unity, broad emission spectra with sizable full width at half maxima (FWHM; 60–100 nm) present a critical issue that must be solved for their application in ultrahigh‐definition OLED displays. Recently, a new paradigm of TADF materials featuring the multiple resonance (MR) effect based on heteroatom‐doped polycyclic aromatic frameworks, referred to as MR‐TADF materials, has emerged and garnered considerable research interest owing to their remarkable features of efficient narrowband emissions with extremely small FWHMs (≤30 nm). Currently, MR‐TADF materials occupy a prominent position in the cutting‐edge research on organic light‐emitting materials from both chemical and physical perspectives. This review article focuses on recent progress in narrowband emissive MR‐TADF systems from the perspective of molecular design, photophysical properties, and electroluminescence performance in OLEDs. The current status and future prospects of this advanced material technology are discussed comprehensively.

Extending the π-Skeleton of Multi-Resonance TADF Materials towards High-Efficiency Narrowband Deep-Blue Emission.

Abstract: Multi-resonance TADF (MR-TADF) emitters are promising for high resolution OLEDs, but concurrent optimization of excited-state dynamics and color purity remains a tough challenge. Herein, three deep-blue MR-TADF compounds ( BN1 - BN3 ) featuring gradually enlarged ring-fused structure and increased rigidity are accessed by lithium-free borylation in high yields from the same precursor, which all possess CIE y below 0.08. Structure-property investigation demonstrates a strategic implementation of improving oscillator strength ( f osc ) and accelerating reverse intersystem crossing (RISC) process by extending the π-skeleton, where BN3 realizes a maximum external quantum efficiency (EQE) of 37.6% and reduced roll-off, representing the best efficiency reported for deep-blue TADF OLEDs. The internal regulation of efficiency and color purity of these compounds validate the general effectiveness to achieve advanced deep-blue narrowband emitters with higher-order boron/nitrogen-based MR-motifs.

Achieving 37.1% Green Electroluminescent Efficiency and 0.09 eV Full Width at Half Maximum Based on a Ternary Boron-Oxygen-Nitrogen Embedded Polycyclic Aromatic System.

Abstract: Herein, a ternary boron-oxygen-nitrogen embedded polycyclic aromatic hydrocarbon with multiple resonance thermally activated delayed fluorescence (MR-TADF), namely DBNO, is developed by adopting the para boron-π-boron and para oxygen-π-oxygen strategy. The designed molecule presents a vivid green emission with high photoluminescence quantum yield (96%) and extremely narrow full width at half maximum (FWHM) of 19 nm/0.09 eV, which surpasses all previously reported green TADF emitters to date. In addition, the long molecular structure along the transition dipole moment direction endows it with a high horizontal emitting dipole ratio of 96%. The organic light-emitting diode (OLED) based on DBNO reveals a narrowband green emission with a peak at 504 nm and a FWHM of 24 nm/0.12 eV. Particularly, a significantly improved device performance is achieved by the TADF-sensitization ( hyperfluorescence ) mechanism, presenting a FWHM of 27 nm and maximum external quantum efficiency of 37.1%.

Fusion of Multi-Resonance Fragment with Conventional Polycyclic Aromatic Hydrocarbon for Nearly BT2020 Green Emission.

Abstract: Herein, we report a general strategy for achieving ultra-pure green emissions by suppressing the shoulder peaks in the emission spectra of conventional polycyclic aromatic hydrocarbons (PAHs). Through precise synthetic fusion of multi-resonance (MR) fragments with conventional PAH, extended π-conjugation lengths, increased molecular rigidity, and reduced vibrational frequency could be simultaneously realized. The proof-of-concept emitters exhibited ultra-pure green emissions with dominant peaks at ca. 521 nm, photoluminescence quantum yields that are greater than 99%, a small full-width-at-half-maximum of 23 nm, and CIE coordinates of (0.16, 0.77). The bottom-emitting organic light-emitting diode (OLED) exhibited a record-high CIE y value of 0.74 and a high maximum external quantum efficiency of 30.5%. The top-emitting OLED not only achieved a BT.2020 green color (CIE: 0.17, 0.78) for the first time but also showed superior performance among all green OLED devices, with a current efficiency of 220 cd A -1 .

Toward a BT.2020 green emitter through a combined multiple resonance effect and multi-lock strategy

Abstract: Color-saturated green-emitting molecules with high Commission Internationale de L'Eclairage (CIE) y values have great potential applications for displays and imaging. Here, we linked the outer phenyl groups in multiple-resonance (MR)-type blue-emitting B (boron)-N (nitrogen) molecules through bonding and spiro-carbon bridges, resulting in rigid green emitters with thermally activated delayed fluorescence. The MR effect and multiple interlocking strategy greatly suppressed the high-frequency vibrations in the molecules, which emit green light with a full-width at half-maximum of 14 nm and a CIE y value of 0.77 in cyclohexane. These were the purest green molecules with quantum efficiency and color purity that were comparable with current best quantum dots. Doping these emitters into a traditional green-emitting phosphorescence organic light-emitting diode (OLED) endowed the device with a Broadcast Service Television 2020 color-gamut, 50% improved external quantum efficiency, and an extremely high luminescence of 5.1 × 105 cd/m2, making it the greenest and brightest OLED ever reported.

Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

Abstract: Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.

Ultrafast Triplet-Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens.

Abstract: Narrowband emissive organoboron emitters featuring the multi-resonance (MR) effect have now become a critical material component for constructing high-performance organic light-emitting diodes (OLEDs) with pure emission colors. These MR organoboron emitters are capable of exhibiting high-efficiency narrowband thermally activated delayed fluorescence (TADF) by allowing triplet-to-singlet reverse intersystem crossing (RISC). However, RISC involving spin-flip exciton upconversion is generally the rate-limiting step in the overall TADF; hence, a deeper understanding and precise control of the RISC dynamics are ongoing crucial challenges. Here, we introduce the first MR organoboron emitter ( CzBSe ) doped with a selenium atom, demonstrating a record-high RISC rate exceeding 10 8 s -1 , which is even higher than its fluorescence radiation rate. Furthermore, the spin-flip upconversion process in CzBSe can be accelerated by factors of ~20000 and ~800, compared to those of its oxygen- and sulfur-doped homologs ( CzBO and CzBS ), respectively. Unlike CzBO and CzBS , the photophysical rate-limiting step in CzBSe is no longer RISC, but the fluorescence radiation process; this behavior is completely different from the conventional time-delaying TADF limited by the slow RISC. Benefitting from its ultrafast exciton spin conversion ability, OLEDs incorporating CzBSe achieved a maximum external electroluminescence quantum efficiency as high as 23.9%, accompanied by MR-induced blue narrowband emission and significantly alleviated efficiency roll-off features.

Progress in the development of the display performance of AR, VR, QLED and OLED devices in recent years

Abstract: The remarkable progress of virtual reality, augmented reality, quantum dot light-emitting diode, and organic light-emitting diode as next-generation displays has overcome the leadership of the liquid crystal display during the last two years. This paper discusses the key technological advancements and performance of these new-generation display devices.

Highly Efficient Asymmetric Multiple Resonance Thermally Activated Delayed Fluorescence Emitter with EQE of 32.8% and Extremely Low Efficiency Roll-Off.

Abstract: Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters show great potentials for high color purity organic light-emitting diodes (OLEDs). However, the simultaneous realization of high photoluminescence quantum yield (PLQY) and high reverse intersystem crossing rate (kRISC) is still a formidable challenge. Herein, a novel asymmetric MR-TADF emitter (2Cz-PTZ-BN) is designed that fully inherits the high PLQY and large kRISC values of the properly selected parent molecules. The resonating extended π-skeleton with peripheral protection can achieve high PLQY of 96% and fast kRISC of above 1.0×105 s-1, and boost the performance of corresponding pure green device with an outstanding external quantum efficiency (EQE) up to 32.8% without utilizing any sensitizing hosts. Remarkably, the device sufficiently maintains high EQE of exceeding 23% at a high luminance of 1000 cd m-2, representing the highest value for reported green MR-TADF materials at the same luminescence.

3D-printed flexible organic light-emitting diode displays

Abstract: OLED arrays were fully 3D-printed under ambient conditions to produce a flexible information display technology.

Constructing Organic Electroluminescent Material with Very High Color Purity and Efficiency Based on Polycyclization of Multiple Resonance Parent Core.

Abstract: Multiple resonance thermally activated delayed fluorescence (MR-TADF) compounds have set off an upsurge of research because of their tremendous application prospect in the field of wide color gamut display. Herein, we propose a novel MR-TADF molecular construction paradigm based on polycyclization of multiple resonance parent core, and construct a representative multiple resonance polycyclic aromatic hydrocarbon (MR-PAH) based on para-alignment boron and nitrogen atoms into six-membered ring (p-BNR). Through the retrosynthesis analysis, a concise synthesis strategy with wide applicability has been proposed, encompassing programmed sequential boron esterification, Suzuki coupling and Scholl oxidative coupling. The target model molecule BN-TP shows green fluorescence with emission peak at 523 nm and narrow full-width at half-maximum (FWHM) of 34 nm. The organic light-emitting diode (OLED) employing BN-TP as an emitter exhibits ultrapure green emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.70), and achieves maximum external quantum efficiency (EQE) of 35.1%.

Sulfone‐Incorporated Multi‐Resonance TADF Emitter for High‐Performance Narrowband Blue OLEDs with EQE of 32%

Abstract: The blue multi‐resonance thermally activated delayed fluorescence materials, simultaneously realizing narrow full‐width at half‐maximum, high external quantum efficiency (EQE), and low efficiency roll‐off, remains a formidable challenge. Herein, three novel emitters, namely PTZBN1, PTZBN2, and PTZBN3, are designed by gradual peripheral modification in boron/nitrogen (B/N) embedded polycyclic skeleton, which exhibit progressively hypsochromic‐shifted emission from 490 nm (PTZBN1) to 468 nm (PTZBN3) with photoluminescence quantum yields up to 98%. In particular, the incorporation of sulfone unit in the boron/nitrogen (B/N) embedded polycyclic skeleton provides a simple but effective tactic for narrowband blue emission. The organic light‐emitting diodes based on PTZBN2 achieve one of the‐state‐of‐the‐art EQEs of 34.8% with electroluminescence (EL) peak at 478 nm. Impressively, PTZBN3‐based device exhibits not only a high maximum EQE of 32.0% with EL peak at 468 nm, but also low efficiency roll‐off.

Deep‐Blue OLEDs with Rec.2020 Blue Gamut Compliance and EQE Over 22% Achieved by Conformation Engineering

Abstract: To achieve high‐efficiency deep‐blue electroluminescence satisfying Rec.2020 standard blue gamut, two thermally activated delayed fluorescent (TADF) emitters are developed: 5‐(2,12‐di‐tert‐butyl‐5,9‐dioxa‐13b‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐10,10‐diphenyl‐5,10‐dihydrodibenzo[b,e][1,4]azasiline (TDBA‐PAS) and 10‐(2,12‐di‐tert‐butyl‐5,9‐dioxa‐13b‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐9,9‐diphenyl‐9,10‐dihydroacridine (TDBA‐DPAC). Inheriting from their parented organoboron multi‐resonance core, both emitters show very promising deep‐blue emissions with relatively narrow full width at half‐maximum (FWHM, ≈50 nm in solution), high photoluminescence quantum yield (up to 92.3%), and short emission lifetime (≤2.49 µs) with fast reverse intersystem crossing (>106 s−1) in doped films. More importantly, replacing the spiro‐centered sp3 C atom (TDBA‐DPAC) with the larger‐radius sp3 Si atom (TDBA‐PAS), enhanced conformational heterogeneities in bulky‐group‐shielded TADF molecules are observed in solution, doped film, and device. Consequently, OLEDs based on TDBA‐PAS retain high maximum external quantum efficiencies ≈20% with suppressed efficiency roll‐off and color index close to Rec.2020 blue gamut over a wide doping range of 10–50 wt%. This study highlights a new strategy to restrain spectral broadening and redshifting and efficiency roll‐off in the design of deep‐blue TADF emitters.

Chiral Multi-Resonance TADF Emitters Exhibiting Narrowband Circularly Polarized Electroluminescence with EQE of 37.2.

Abstract: Highly efficient circularly polarized luminescence (CPL) emitters with narrowband emission remain a formidable challenge for circularly polarized OLEDs (CP-OLEDs). Here, a promising strategy for developing chiral emitters concurrently featuring multi-resonance TADF (MR-TADF) and circularly polarized electroluminescence (CPEL) is demonstrated by the integration of molecular rigidity, central chirality and MR effect. A pair of chiral green emitters denoted as ( R )- BN-MeIAc and ( S )- BN-MeIAc is designed. Benefited by the rigid and quasi-planar MR-framework, the enantiomers not only display mirror-image CPL spectra, but also exhibit TADF properties with high photoluminescence quantum yield of 96%, narrow FWHM of 30 nm, and high horizontal dipole orientation of 90% in doped film. Consequently, the enantiomer-based CP-OLEDs achieved excellent external quantum efficiencies of 37.2% with very low efficiency roll-off, representing the highest device efficiency of all the reported CP-OLEDs.

Space-Confined Donor-Acceptor Strategy Enables Fast Spin-Flip of Multiple Resonance Emitters for Suppressing Efficiency Roll-Off.

Abstract: Multi-resonance boron-nitrogen-containing thermally activated delayed fluorescence (MR-TADF) emitters have experienced great success in assembling narrowband organic light-emitting diodes (OLEDs). However, the slow reverse intersystem crossing rate (kRISC) of MR-emitters (103-105 s-1) that will lead to severe device efficiency roll-off has received extensive attention and remains a challenging issue. Herein, we put forward a "space-confined donor-acceptor (SCDA)" strategy to accelerate RISC process. The introduction of SCDA units onto the MR-skeleton induces intermediate triplet states, which leads to a multichannel RISC process and thus increases kRISC. As illustrated examples, efficient MR-emitters have been developed with a sub-microsecond delayed lifetime and a high kRISC of 2.13 × 106 s-1, which enables to assemble high-performance OLEDs with a maximum external quantum efficiency (EQEmax) as high as 32.5% and an alleviated efficiency roll-off (EQE1000: 22.9%).

NIR TADF emitters and OLEDs: challenges, progress, and perspectives

Abstract: Near-infrared (NIR) light-emitting materials show excellent potential applications in the fields of military technology, bioimaging, optical communication, organic light-emitting diodes (OLEDs), etc. Recently, thermally activated delayed fluorescence (TADF) emitters have made historic developments in the field of OLEDs. These metal-free materials are more attractive because of efficient reverse intersystem crossing processes which result in promising high efficiencies in OLEDs. However, the development of NIR TADF emitters has progressed at a relatively slower pace which could be ascribed to the difficult promotion of external quantum efficiencies. Thus, increasing attention has been paid to NIR TADF emitters. In this review, the recent progress of NIR TADF emitters has been summarized along with their molecular design strategies and photophysical properties, as well as electroluminescence performance data of their OLEDs, respectively.

High Efficiency of over 25% and Long Device Lifetime of over 500 h at 1000 nit in Blue Fluorescent Organic Light‐Emitting Diodes

Abstract: In this study, a multiple resonance (MR) type blue emitter is synthesized, characterized, and evaluated for highly efficient and stable blue fluorescent organic light‐emitting diodes (OLEDs). The MR blue fluorescent emitter has a di‐tert‐butyl benzene substituent in the MR core structure to minimize quenching mechanisms by intermolecular interaction. The emitter shows a high photoluminescence quantum yield and small full width at half maximum of 22 nm, which realize high external quantum efficiency (EQE) of 11.4% in the single unit OLED and device lifetime up to 95% of the initial luminance (LT95) of 208 h at 1000 cd m−2 and over 10 000 h at 100 cd m−2. The optimized tandem device of the new blue emitter achieves high EQE over 25% and extremely long LT95 of over 500 h at 1000 cd m−2 and 30 000 h at 100 cd m−2. The lifetime of this work is one of the best data of blue OLED lifetime reported in the literature.

Modular, n-Doped Concave PAHs for High-Performance OLEDs with Tunable Emission Mechanisms.

Abstract: Although bowl-shaped N-pyrrolic polycyclic aromatic hydrocarbons (PAHs) can achieve excellent electron-donating ability, their application for optoelectronics is hampered by typically low photoluminescence quantum yields (PLQYs). To address this issue, we report the synthesis and characterization of a series of curved and fully conjugated nitrogen-doped PAHs. Through structural modifications to the electron-accepting moiety, we are able to switch the mechanism of luminescence between TADF and RTP, and to tune the overall PLQY in the range from 9% to 86%. As a proof of concept, we constructed solid-state OLED devices which has not been explored so far in the context of concave N-doped systems being TADF/RTP emitters. The best-performing dye, possessing peripheral trifluoromethyl group adjacent to phenazine acceptor, exhibits yellow to orange emission with a maximum external EL quantum efficiency (EQE) of 12%, which is the highest EQE in an curved D-A embedded N-PAH to date.

Modular Nitrogen‐Doped Concave Polycyclic Aromatic Hydrocarbons for High‐Performance Organic Light‐Emitting Diodes with Tunable Emission Mechanisms

Abstract: Abstract Although bowl‐shaped N‐pyrrolic polycyclic aromatic hydrocarbons (PAHs) can achieve excellent electron‐donating ability, their application for optoelectronics is hampered by typically low photoluminescence quantum yields (PLQYs). To address this issue, we report the synthesis and characterization of a series of curved and fully conjugated nitrogen‐doped PAHs. Through structural modifications to the electron‐accepting moiety, we are able to switch the mechanism of luminescence between thermally activated delayed fluorescence (TADF) and room‐temperature phosphorescence (RTP), and to tune the overall PLQY in the range from 9 % to 86 %. As a proof of concept, we constructed solid‐state organic light‐emitting diode (OLED) devices, which has not been explored to date in the context of concave N‐doped systems being TADF/RTP emitters. The best‐performing dye, possessing a peripheral trifluoromethyl group adjacent to the phenazine acceptor, exhibits yellow to orange emission with a maximum external quantum efficiency (EQE) of 12 %, which is the highest EQE in a curved D‐A embedded N‐PAH to date.

Fused π‐Extended Multiple‐Resonance Induced Thermally Activated Delayed Fluorescence Materials for High‐Efficiency and Narrowband OLEDs with Low Efficiency Roll‐Off

Abstract: The simultaneous achievement of multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) materials with strong narrowband emission and efficient reverse intersystem crossing (RISC) process can further promote the advancement of organic light‐emitting diodes (OLEDs). Herein, a new strategy is proposed to achieve two π‐extended MR‐TADF emitters (NBO and NBNP) peaking at 487 and 500 nm via fusing conjugated high‐triplet‐energy units (carbazole, dibenzofuran) into boron‐nitrogen (B/N) framework, aiming to increase charge transfer delocalization of the B/N skeleton and minimize singlet‐triplet energy gap (∆EST). This strategy endows the two emitters with full width at half maximum of 27 and 29 nm, and high photoluminescence efficiencies above 90% in doped films, respectively. Additionally, considerable rate constants of RISC are obtained due to the small ∆EST (0.12 and 0.09 eV) and large spin‐orbital coupling values. Consequently, the OLEDs based on NBO and NBNP show the maximum external electroluminescence quantum efficiency of up to 26.1% and 28.0%, respectively, accompanied by low‐efficiency roll‐off. These results provide a feasible design strategy to construct efficient MR‐TADF materials for OLEDs with suppressed efficiency roll‐off.

Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

Abstract: Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes Xiaojie Chen, Dongyu Ma, Tiantian Liu, Zhu Chen, Zhan Yang, Juan Zhao, Zhiyong Yang, Yi Zhang and Zhenguo Chi Xiaojie Chen PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Dongyu Ma PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Tiantian Liu PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Zhu Chen PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Zhan Yang PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Juan Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Zhiyong Yang PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Yi Zhang PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author and Zhenguo Chi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-Performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100900 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Hybridized local and charge-transfer (HLCT) excited-state fluorophores, which enable full exciton utilization through a reverse intersystem crossing from high-lying triplet states to singlet state, have attracted increasing attention toward organic light-emitting diodes (OLEDs) application. Herein, we report three D–π–A–π–D-type isomers o-2CzBT, m-2CzBT, and p-2CzBT by adjusting the donor (D) units from ortho-, meta-, to para-substituted positions with the acceptor (A) core unit, respectively. The HLCT properties of the three compounds are evidently confirmed by theoretical calculations, solvatochromic behaviors, and transient decay lifetimes analyses. As the substituted position changes from the ortho-, meta-, and para-positions, the reduced steric hindrance brings about decreased torsional angle between D and A moieties, resulting in increased oscillator strength. Accordingly, the para-substituted p-2CzBT is endowed with a more locally excited component that accounts for faster radiative decay, leading to a higher fluorescent efficiency than that of o-2CzBT and m-2CzBT. As expected, p-2CzBT enables its nondoped and doped OLEDs with higher external quantum efficiencies (EQEs) of 12.3% and 15.0%, respectively, which are among the state-of-the-art efficiencies of HLCT-based OLEDs. Moreover, o-2CzBT and m-2CzBT are also utilized as host materials for high-performance OLEDs, thus extending the application of HLCT materials. Download figure Download PowerPoint Introduction Organic light-emitting diodes (OLEDs) have attracted widespread interest because of their promising applications in the fields of flat-panel display, solid-state lighting, and so on. To promote practical applications of OLEDs, great efforts have been paid to the development of high-efficiency luminescent materials, which are of vital importance to device performance. As known, the first-generation conventional fluorescent emitters have an upper limit of internal quantum efficiency (IQE) of 25% and fail to utilize triplet excitons which account for 75%. Although second-generation phosphorescent emitters can achieve an IQE of 100%, they suffer from high cost due to the incorporation of noble metal, as well as shortage of efficient blue phosphorescent emitters.1,2 In this regard, third-generation pure organic emitters including thermally activated delayed fluorescence (TADF)3–7 and hybridized local and charge-transfer (HLCT)8–11 excited-state emitters, which render a 100% exciton utilization efficiency while avoiding noble metal, have been considered as potential candidates. Among these, TADF emitters have been widely explored, as their intrinsic long exciton lifetimes tend to cause serious efficiency roll-off when the devices are driven under high currents.12,13 In comparison, the much short exciton lifetimes of HLCT emitters benefit to suppress exciton annihilation and thus improve efficiency roll-off.14,15 Given the fact that the HLCT concept was proposed a few years ago, to date the development of HLCT emitters still lags behind TADF emitters.16–18 In particular, device performances of most HLCT-based OLEDs remain restricted as their external quantum efficiencies (EQEs) are generally <10%,10,15,19–23 leaving much room for efficiency improvement. Seen in this light, there is an urgent need for simple and effective strategies to develop high-efficiency HLCT emitters and related OLEDs. With respect to the design of HLCT molecules, a key point is to improve their photoluminescence quantum yield (PLQY). The adjustments of donor (D) and acceptor (A) moieties have pivotal roles in PLQY of HLCT materials through the regulation of locally excited (LE) and charge-transfer (CT) components. For instance, the weaker D can help to increase the LE component and decrease the CT component,24 leading to HLCT materials with high PLQYs.25 In further consideration that the torsional angle between D and A moieties can also exert an influence on the PLQY of organic molecules,26–28 this is generally adopted in twisted D–A-type TADF molecules by introducing large steric hindrance to reinforce CT characteristics.5,29 Inspired by this design principle for TADF materials, exploring HLCT materials with high PLQYs is feasible through the adjustment of LE and CT components by virtue of tuning the torsional angle between D and A moieties. With decreasing steric hindrance in HLCT molecules, the LE component can be increased as the torsional angle decreases, and consequently high PLQYs can be realized. In light of this, we designed three D–π–A–π–D-type positional isomers o-2CzBT, m-2CzBT, and p-2CzBT (Figure 1), wherein the D (carbazole) units were ortho-, meta-, and para-substituted with the A (benzothiadiazole) core unit, respectively. Herein, benzothiadiazole is a universally recognized A in the HLCT system as it possesses high luminous efficiency, a large lowest triplet excited state (T1)–high-energy triplet state (T2) energy difference, but the small lowest singlet excited state (S1)–T2 energy gap, which conforms the molecular principles of HLCT materials.10 For these HLCT materials, as the substituted position between D and A units varies, the modified steric hindrance brings about a change in the torsional angle. As expected, the reduced steric hindrance endows p-2CzBT with a smaller torsional angle, leading to higher LE component accounting for high PLQY, and, accordingly, performance doped OLEDs with an EQE of 15.0% are achieved. Figure 1 | Design strategy and molecular structures of o-2CzBT, m-2CzBT, and p-2CzBT. Download figure Download PowerPoint Experimental Methods Compounds o-2CzBT, m-2CzBT, and p-2CzBT were synthesized by the Suzuki coupling reaction of 4,7-dibromobenzo[c]-[1,2,5]-thiadiazole and different boronic acid or borate ester with carbazole unit ( Supporting Information Scheme S1). The three compounds were characterized by proton (1H) nuclear magnetic resonance (NMR), carbon (13C) NMR, electron impact-mass spectrometry (EIMS), and high-resolution mass spectrometry (HRMS) ( Supporting Information Figures S1–S12). We noted that p-2CzBT was previously reported with attention paid to spectroscopic and electrochemical properties,30 whereas the luminescence mechanism and OLED application deserve further studies. Results and Discussion Theoretical calculations Density functional theory (DFT) calculations of o-2CzBT, m-2CzBT, and p-2CzBT were performed at the B3LYP/6-31G(d) level. DFT-optimized molecular geometry is displayed in Figure 2a, and it can be seen that the torsional angles between D and A (θ1 and θ2) are obviously altered by changing the linking positions of the carbazole Ds, while θ1 and θ2 gradually decrease from o-2CzBT and m-2CzBT to p-2CzBT due to decreased steric hindrance. With respect to the orbital distributions of the three molecules, Figure 2b shows that the lowest unoccupied molecular orbitals (LUMOs) are mainly located on the benzothiadiazole A, and the highest occupied molecular orbitals (HOMOs) are distributed on carbazole Ds, while the HOMO of p-2CzBT is also partially delocalized on the benzothiadiazole, which facilitates HOMO and LUMO overlaps, indicating extended π-conjugation as a result of decreased torsional angles. Then, to evaluate the excited states and transition characters of the excited singlet state (S1) and triplet states (T1, T2), natural transition orbitals (NTOs) calculations were carried out based on time-dependent DFT (TD-DFT) by Multiwfn software.31 As shown in Figure 2c and Supporting Information Figures S13 and S14, for S0 → S1 transition of the three compounds, the partial separation and partial overlap of hole and particle distributions demonstrate hybrid features of LE and CT, verifying a distinct HLCT character of the excited states. As known that, the LE state is more efficiently radiative than the CT state as it is beneficial for promoting PLQY.32 In comparison, more LE components are observed in para-linked p-2CzBT for S0 → S1 excitation, which can be quantitatively evidenced by a larger hole–particle overlap integral (〈ΨH|ΨP〉, Supporting Information Table S1).33,34 Meanwhile, as the torsional angle decreases, the oscillator strengths of the S1 → S0 transition (fS) increase, which are 0.0702, 0.2940, and 0.7970 for o-2CzBT, m-2CzBT, and p-2CzBT, respectively. These results indicate that the change of the linking positions of the carbazole Ds can afford the excited state (S1) of para-linked p-2CzBT with more LE components, leading to faster radiative decay and a higher PLQY.35 Figure 2 | (a) Optimized geometry structures, and (b) HOMO and LUMO distributions of o-2CzBT, m-2CzBT, and p-2CzBT. (c) NTOs of p-2CzBT. (d) Energy level diagrams of the first 10 singlet and triplet excited states of o-2CzBT, m-2CzBT, and p-2CzBT. Download figure Download PowerPoint In a further set of experiment, the energy levels of the first 10 singlet and triplet excited states of the three compounds investigated by TD-DFT are plotted in Figure 2d. It was found that all three compounds present low energy levels of T1 and clearly large energy gaps between T2 and T1 (ΔET2–T1), which can greatly suppress interconversion (IC) decay from T2 to T1 as the rate of IC is in an inverse relation with ΔET2–T1.36 In contrast, the three compounds exhibit small energy gaps between T2 and S1, which can trigger spin-flip at high-lying excited states and boost the high-lying reverse intersystem crossing (hRISC) process occurring from T2 to S1, according to the Fermi golden rule.37–39 Moreover, the three compounds exhibit large spin-orbital coupling (SOC) constants between T2 and S1 states (〈S1|ⒽSO|T2〉, Supporting Information Table S1), which are in favor of fast hRISC.40,41 As a result, due to a much larger rate of hRISC than the rate of IC, the three compounds can make efficient utilization of triplet excitons for radiative emission through hRISC process, corresponding to the “hot exciton” mechanism.42,43 It is worth noting that the large energy gap between T1 and S1 prevents the occurrence of RISC process from T1 to S1, excluding the existence of the TADF process.44–46 Photophysical properties The UV–vis absorption spectra and photoluminescence (PL) spectra of o-2CzBT, m-2CzBT, and p-2CzBT neat films are displayed in Figure 3a and summarized in Table 1. In the UV–vis absorption spectra of the three compounds, broad and featureless absorption bands around 350–470 nm assigned to intramolecular CT (ICT) absorption can be observed, along with a red-shift of the absorption band from o-2CzBT and m-2CzBT to p-2CzBT. As seen from the PL spectra of o-2CzBT, m-2CzBT, and p-2CzBT, the PL emission peaks are located at 504, 516, and 563 nm, respectively, showing spectral red-shift by changing the carbazole units from ortho- and meta-positions to para-substituted positions. The red-shifted emission is mainly associated with the relative planarization of p-2CzBT when compared with o-2CzBT and m-2CzBT with twisted molecular conformations, which interrupt intramolecular π-conjugation.19,47,48 The PLQYs of o-2CzBT, m-2CzBT, and p-2CzBT neat films are measured to be 16.9%, 26.6%, and 64.4%, respectively, and the highly increased PLQY in p-2CzBT is benefited from the enhanced LE components and is consistent with its larger oscillator strength.25,49 Then, transient PL decay curves of the three compounds were recorded to investigate their fluorescent behaviors. As plotted in Figure 3b, o-2CzBT, m-2CzBT, and p-2CzBT neat films exhibit short excited lifetimes without delay, the nanosecond-scaled lifetimes are fitted to be 10.7, 11.8, and 5.3 ns, respectively, and temperature-dependent transient decay curves ( Supporting Information Figure S15) show the absence of long lifetimes, helping to rule out the possibility of TADF in the compounds.25,50 Besides, no phosphorescence was observed from the three compounds at 77 K, which may be caused by low ratios of T1 excitons in the HLCT compounds. Thereafter, the radiative transition rates (kr) of o-2CzBT, m-2CzBT, and p-2CzBT are estimated to be 1.58 × 107, 2.25 × 107, and 1.22 × 108 s−1, respectively, and the increasing trend of kr agrees well with that of their fS.51 These results affirm the important influence of the linking position on the compounds. Moreover, photophysical properties of doped films by dispersing the compounds into a common host 4,4′-bis(9H-carbazol-9-yl)biphenyl (CBP) were examined ( Supporting Information Figures S16 and S17), and a bathochromic shift of PL emission as well as nanosecond-scaled lifetimes are also observed in the doped films. Besides, the PLQYs of o-2CzBT, m-2CzBT, and p-2CzBT-doped films were collected to be 38.5%, 61.6%, and 98.5%, respectively, which are improved compared with that of the corresponding nondoped films. Figure 3 | (a) UV–vis absorption spectra (inset: neat films under UV-light illumination) and PL spectra, and (b) transient PL decay curves of o-2CzBT, m-2CzBT, and p-2CzBT neat films. (c) Linear fitting of the Stokes shift (νa-νf) vs solvent polarity (f) of o-2CzBT, m-2CzBT, and p-2CzBT. Download figure Download PowerPoint Table 1 | Photophysical, Thermal, and Electrochemical Properties of o-2CzBT, m-2CzBT, and p-2CzBT Compound λUV (nm)a λPL (nm)b τ (ns)c PLQY (%)d kr (s−1)e Td (°C)f Tg (°C)g HOMO (eV)h LUMO (eV)i Nondoped/Doped o-2CzBT 361 504 10.7 16.9/38.5 1.58 × 107 341 110 5.2 2.4 m-2CzBT 390 516 11.8 26.6/61.6 2.25 × 107 454 127 5.2 2.5 p-2CzBT 423 563 5.3 64.4/98.5 1.22 × 108 441 / 5.6 3.1 aAbsorption peak in films. bPL emission peak in films. cLifetime in films. dPL quantum yield. ekr = PLQY/τ. fThermal decomposition temperature corresponding to 5% weight loss. gGlass-transition temperature. hHOMO levels were calculated from the oxidation onset potentials in CV curves. iLUMO levels estimated by the empirical equation ELUMO = EHOMO + Eg. The solvatochromic effects of the three compounds were analyzed in solvents with different polarities from hexane to acetone, as illustrated in Supporting Information Figure S18. It was found that the PL spectral peaks of the three compounds gradually red-shifted with increased solvent polarity. Consequently, the Stokes shift (νa-νf) versus the solvent polarity (f) is fitted (Figure 3c) according to the Lippert–Mataga relation.52 As seen, o-2CzBT, m-2CzBT, and p-2CzBT present two independent slopes, and the lower slope in low-polarity solvents is ascribed to the LE state while the larger slope in high-polarity solvents is attributed to the CT state, which is a typical behavior of HLCT materials.32,53–55 Therefore, based on the aforementioned theoretical calculations (electronic structures and properties of excited states) and photophysical studies (nanosecond-scaled lifetimes and solvatochromic effects), the HLCT feature is verified for o-2CzBT, m-2CzBT, and p-2CzBT through the coupling and intercrossing between the LE and CT states.56 Thermal and electrochemical properties The thermal properties of the three compounds were examined with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) ( Supporting Information Figure S19). On the one hand, the decomposition temperatures (Td: corresponding to 5% weight loss) for o-2CzBT, m-2CzBT, and p-2CzBT are found to be 341, 454, and 441 °C, respectively. On the other hand, the glass-transition temperatures (Tg) of o-2CzBT and m-2CzBT were found to be 110 and 127 °C, respectively, while there was no Tg observed for p-2CzBT before its melt temperature (Tm) of 334 °C. These results suggest that all compounds exhibit high thermal stability beneficial for practical applications in OLEDs. Furthermore, cyclic voltammetry (CV) measurements were carried out ( Supporting Information Figure S20) to investigate energy levels of the compounds. The HOMO levels of the o-2CzBT, m-2CzBT, and p-2CzBT were shown to be 5.2, 5.2, and 5.6 eV, respectively, and their LUMO levels were calculated to be 2.4, 2.5, and 3.1 eV with the band gaps (Eg) evaluated from corresponding absorption spectra. Electroluminescent properties Device characterizations of OLEDs To examine the electroluminescent (EL) properties of o-2CzBT, m-2CzBT, and p-2CzBT, doped OLEDs were constructed with a configuration as: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (30 nm)/m-bis(N-carbazolyl)benzene (mCP) (20 nm)/emitting layer (EML) (30 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) (40 nm)/LiF (1 nm)/Al (100 nm), as plotted in Figure 4a, wherein PEDOT:PSS and LiF act as charge-carrier injection layers, mCP and TPBi serve as charge-carrier transporting layers, and EML is CBP:10 wt % o-2CzBT, CBP:10 wt % m-2CzBT, or CBP:10 wt % p-2CzBT. The device characteristics are demonstrated in Figures 4b and 4c and summarized in Table 2. It is noted that the luminance of the three doped devices increased less than linearly as the current density gradually increased ( Supporting Information Figure S21), especially at higher current density. Therefore, the contribution of triplet–triplet annihilation (TTA) upconversion to high efficiencies can be excluded, because in TTA-mechanism-based OLEDs the luminance is inclined to increase more than linearly with increasing current density, considering the second-order TTA process.57,58 These results further support the HLCT property of the molecules that accounts for high EQEs achieved from their devices. As shown from the EQE–current density curves, o-2CzBT and m-2CzBT-based devices achieve a maximum EQE of 5.3% and 8.4%, respectively. These EQE values are higher than that obtained from traditional fluorescent OLEDs, benefiting from their HLCT property. Remarkably, the p-2CzBT-based device exhibits a maximum EQE of 15.0%, which is amongst the state-of-the-art efficiencies of HLCT-based OLEDs reported so far. In comparison with o-2CzBT and m-2CzBT-based devices, p-2CzBT-based device demonstrates low efficiency roll-off. To explore the efficiency roll-off behaviors, single carrier devices were constructed ( Supporting Information Figure S22), and the results confirm much more balanced electron and hole transporting abilities of p-2CzBT than that of o-2CzBT and m-2CzBT. Moreover, the EQE curves are also fitted in the presence of either TTA or singlet–triplet annihilation (STA), as plotted in Supporting Information Figure S23. It is found that, the TTA or STA fitted curves could not satisfactorily model the experimental data due to considerable deviations exist between them, implying TTA or STA involved exciton quenching should not play a critical role on the efficiency roll-off. Therefore, these results certify that the low efficiency roll-off in p-2CzBT-based device is mainly related to well-balanced charge carriers, while the charge-carrier unbalance causes efficiency roll-off in o-2CzBT and m-2CzBT-based devices.59,60 As seen from the EL spectra of the devices (inset of Figure 4c), when the substitution position changes from ortho- or meta-, to para-, the EL spectral peaks of o-2CzBT, m-2CzBT, and p-2CzBT are gradually red-shifted from 488 and 490, to 508 nm, respectively, which is consistent with the observations from their PL spectra. In addition, nondoped OLEDs by adopting o-2CzBT, m-2CzBT, and p-2CzBT neat films as EMLs were also fabricated with a similar device structure as that of the doped ones. In comparison with their corresponding doped OLEDs, the nondoped o-2CzBT, m-2CzBT, and p-2CzBT devices present red-shifted EL emission with spectral peaks at 503, 508, and 545 nm, respectively (inset of Figure 4d). The EQE–current density curves of the nondoped OLEDs (Figure 4d) demonstrate that due to a much higher PLQY of the p-2CzBT neat film than that of o-2CzBT and m-2CzBT, the p-2CzBT-based nondoped device realizes a higher EQE of 12.3% ( Supporting Information Figure S24), which is rarely reported among HLCT-OLEDs based on a nondoping technique ( Supporting Information Table S2). Figure 4 | (a) Device configuration, (b) current density–voltage–luminance curves, and (c) EQE–current density curves of doped OLEDs (inset: EL spectra). (d) EQE–current density curves of nondoped OLEDs (inset: EL spectra and device photographs). Download figure Download PowerPoint Table 2 | EL Characteristics of OLEDs Based on o-2CzBT, m-2CzBT, and p-2CzBT Device Lmax (cd m−2) CE (cd A−1)a,b PE (lm W−1)a,b EQE (%)a,b λEL (nm) o-2CzBT Nondoped 2682 10.4/2.2 5.9/1.1 4.2/0.9 503 Doped 3505 11.0/2.4 6.3/1.0 5.3/1.2 488 m-2CzBT Nondoped 4310 20.6/8.6 10.0/4.0 6.9/2.8 508 Doped 3884 18.4/5.3 9.6/2.4 8.4/2.6 490 p-2CzBT Nondoped 43,386 40.9/30.9 23.3/17.5 12.3/9.3 545 Doped 21,577 44.5/40.0 20.0/17.0 15.0/13.2 508 o-2CzBT:TPABHO 11,677 18.2/8.9 10.4/4.7 9.3/4.5 600 m-2CzBT:TPABHO 8314 13.0/5.2 7.4/2.5 7.6/2.8 604 Note:Lmax, maximum luminance; CE, current efficiency; PE, power efficiency; λEL, EL peak wavelength. aMaximum efficiency. bEfficiency at a luminance of 1000 cd m−2. Device characterization of HLCT-host OLED It is speculated that o-2CzBT and m-2CzBT have potential as host materials for orange and red organic emitters, considering their good charge-carrier transporting abilities with greenish-blue emission. In this regard, 4-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)benzaldehyde (TPABCHO) (inset of Figure 5a), which is an orange HLCT emitter we previously reported,16 was chosen as a dopant. There is considerable overlap between the UV–vis absorption spectrum of TPABCHO and the PL spectra of o-2CzBT and m-2CzBT ( Supporting Information Figure S25), facilitating efficient Förster resonance energy transfer (FRET) from o-2CzBT/m-2CzBT to TPABCHO. Thereby, TPABCHO-based OLEDs were fabricated (Figure 5). It was found that o-2CzBT and m-2CzBT enable TPABCHO devices with maximum EQEs of 9.3% and 7.6%, respectively, which are higher than the EQE (5.0%) obtained from a nondoped TPABCHO device,16 confirming the important role of o-2CzBT/m-2CzBT hosts. In addition, these two TPABCHO devices present the same EL spectra with only one emission peak around 600 nm stemmed from TPABCHO (inset of Figure 5b), inferring complete energy transfer from host to dopant based on the FRET process ( Supporting Information Figure S26). These results demonstrate it is an effective strategy to achieve high-efficiency OLEDs by employing HLCT host and HLCT dopant materials, which are likely to guarantee high exciton utilization efficiency and thereby excellent device performances. Figure 5 | (a) Current density–voltage–luminance curves (inset: chemical structure of TPABCHO) and (b) EQE–current density curves of TPABCHO-based OLEDs (inset: EL spectra). Download figure Download PowerPoint Conclusions Three D–π–A–π–D fluorophores, o-2CzBT, m-2CzBT, and p-2CzBT, through regulation of D and A torsional angles were designed with HLCT properties, as evidenced by detailed theoretical and photophysical studies. When the D (carbazole) units were ortho-, meta-, and para-substituted with the A (benzothiadiazole) core unit in o-2CzBT, m-2CzBT, and p-2CzBT, respectively, the steric hindrance was gradually reduced and brought about a decrease in torsional angle between D and A units, accounting for the reinforced LE component and oscillator strength that contributed to high PLQYs, along with red-shifted PL emissions. Accordingly, the para-substituted p-2CzBT with a smaller torsional angle was endowed with a higher PLQY than o-2CzBT and m-2CzBT. Importantly, the p-2CzBT-based OLEDs presented the best device performances, such as a high EQE of 12.3% in nondoped device and an excellent EQE of 15.0% in doped device, which are among the state-of-the-art efficiencies of HLCT-based OLEDs reported so far. Furthermore, o-2CzBT and m-2CzBT were successfully adopted as the host materials for an orange HLCT dopant, resulting in orange HLCT OLEDs with high efficiency. This work sheds new light on molecular design principles for high-efficiency HLCT materials, helping to promote its future developments to catch up with the achievements afforded by TADF materials. Supporting Information Supporting Information is available and includes general methods, OLEDs fabrication and characterization, materials synthesis, supplementary figures, and supporting tables. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the National Natural Science Foundation of China (NSFC: nos. 51733010, 51973239, and 52073316), and the Guangdong Science and Technology Plan (nos. 2015B090913003 and 2015B090915003). References 1. Qin W.; Yang Z.; Jiang Y.; Lam J. W. Y.; Liang G.; Kwok H. S.; Tang B. Z.Construction of Efficient Deep Blue Aggregation-Induced Emission Luminogen from Triphenylethene for Nondoped Organic Light-Emitting Diodes.Chem. Mater.2015, 27,

Deep Blue Emitter Based on Tris(triazolo)triazine Moiety with CIEy < 0.08 for Highly Efficient Solution‐Processed Organic Light‐Emitting Diodes Via Molecular Strategy of “Hot Excitons”

Abstract: Realizing high efficiency deep blue emission with a Commission international de I'Eclairage (CIE) coordinate of CIEy < 0.08 is still a big challenge. In this contribution, three molecules, named TTT‐TPA‐R (R = H, OMe, tBu), using tris(triazolo)triazine (TTT) as the acceptor and triphenylamine derivatives (TPA‐R, R = H, OMe, and tBu) as the donor are prepared and characterized. All these emitters show deep/pure blue emission between 420 and 470 nm in the PMMA film, concomitant with the excellent emission efficiency of 80–100%. Both experimental and calculated methods demonstrate that these emitters exhibit a clearly hybridized local and charge‐transfer excited state and can harvest both singlet and triplet excitons via reverse intersystem crossing process from the high‐lying triplet to singlet states. Therefore, the solution processable deep blue organic light‐emitting diodes (OLEDs) achieve a maximum external quantum efficiency (EQEmax) of 10.5% which is the recorded value for the solution processable deep blue OLED based on the “hot exciton” mechanism. Using TTT‐TPA‐H as the host material, solution‐processed phosphorescent OLED based on PO‐01 presents the EQEmax of 20.2%. These results pave a novel avenue for designing highly efficient deep blue emitter in solution processable OLED.

Mesityl-Functionalized Multi-Resonance Organoboron Delayed Fluorescent Frameworks with Wide-range Color Tunability for Narrowband OLEDs.

Abstract: Polycyclo-heteraborin multi-resonance (MR) emitters are promising for high color-purity organic light-emitting diodes (OLEDs). Here, unlike the most common heteroatoms ternary-doped (X/B/N) frameworks, a binary-doped (B/N) skeleton is reported with large energy band for wide-range color tunability. Based on this parent-segment, an "one-pot" catalyst-free borylation method is developed which generates deep-blue to pure green MR emitters from readily available starting materials, with peaks at 426-532 nm and full-width-at-half-maxima of 27-38 nm. Impressively, a maximum external quantum efficiency of nearly 40% is recorded for the corresponding device with Commission Internationale de l´Eclairage coordinates of (0.14, 0.16), representing the state-of-the-art performances. This work presents a new paradigm and synthesis of B/N-doped MR emitters and will motivate the study of other novel frameworks.