Negligible Efficiency Roll-off Research Articles

Highly Efficient, Low-Roll-Off, Red, "Hot Exciton" Organic Light-Emitting Diodes Promoted by the Heavy-Atom Effect of Selenium.

It is important to attain red hot exciton materials applicable in highly efficient organic light-emitting diodes with low-efficiency roll-off, but their development is restricted by the energy gap law. Herein, the sulfur atom was replaced by a heavier selenium atom based on benzothiadiazole to obtain a new benzoselenadiazole acceptor with a heavy atom effect and stronger electron-withdrawing ability. Two novel red hot exciton materials named BSe-DtBuTPA and BSe-2PhCz-d24 were designed and synthesized based on the benzoselenadiazole unit. Benefiting from the heavy-atom effect of selenium and the small ΔES1T2, both emitters exhibited ultrafast high-lying reverse intersystem crossing rate constants (7.00 × 107 and 1.17 × 107 s-1). The devices based on BSe-DtBuTPA and BSe-2PhCz-d24 demonstrated maximum external quantum efficiencies of 4.81 and 7.15% with emission peaks at 653 and 596 nm, respectively. The device based on deep-red BSe-DtBuTPA exhibited negligible efficiency roll-off of 18.5% at 10000 cd/m2.

Geometrically isomeric [Ir(C^N)(C’^N’)(N’’^N’’’)]-tris-heteroleptic complexes versus [Ir(C^N)2(N’’^N’’’)]-bis-heteroleptic complex: Towards High-performance near-infrared (NIR) polymer light-emitting diodes

Despite efficient NIR-OLEDs/PLEDs fabricated from [Ir(C^N)2(O^O)]/[Ir(C^N)2(N^O)]-bis-heteroleptic and few [Ir(C^N)(C’^N’)(O^O)]/[Ir(C^N)(C’^N’)(N^O)]-tris-heteroleptic iridium(III)-complexes, the problems to dissociation of N^O/O^O-ancillary units upon acid-hydrolysis or pyrolysis, together with geometrically isomeric complexity, have to be faced. Herein, a molecular design strategy to C1-symmetrical [[Ir(C^N)(C’^N’)(N’’^N’’’)]-tris-heteroleptic Ir(III)-complex (HC^N = Hiqbt, HC’^N’ = Hppy and HN’’^N’’’ = Hftrpz) is developed. The improved stability of [Ir(iqbt)(ppy)(ftrpz)] (2) is arisen from the strong HN’’^N’’’-ligand-field strength as compared to the N^O/O^O-ancillary counterparts. Meanwhile, owing to the Hftrpz-induced asymmetry, two geometrical isomers (2a and 2b) of [Ir(iqbt)(ppy)(ftrpz)] are first isolated. Noticeably, using the two isomeric NIR-phosphors [Ir(iqbt)(ppy)(ftrpz)]-a/b (2a and 2b) as the dopants, their high-performance NIR-PLEDs than that with the [Ir(iqbt)2(ftrpz)] (1) counterpart, are realized. Especially the NIR-PLED-2b, exhibits both an attractively high efficiency (ηEQEmax = 5.94 %; λem = 692 nm) and almost negligible (< 5 %) efficiency roll-off. And thus, this finding engenders [Ir(C^N)(C’^N’)(N’’^N’’’)]-tris-heteroleptic Ir(III)-complexes a new platform to the realization of high-performance, low-cost, large-area and scalable NIR-PLEDs.

New Phenanthro[9,10-d]oxazole-Based Fluorophores with Hybridized Local and Charge-Transfer Characteristics for Highly Efficient Blue Non-Doped OLEDs with Negligible Efficiency Roll-Off.

It is vital to develop highly efficient non-doped blue organic light-emitting diodes (OLEDs) with high color purity and low-efficiency roll-off for applications in display and lighting. Herein, two blue D-A fluorophores TPA-PO and TPA-DPO are designed and synthesized, in which phenanthro[9,10-d]oxazole (PO) acts as the acceptor and triphenylamine as the donor. TPA-PO and TPA-DPO display good thermal stability and efficient luminescence efficiency in neat film. Results based on photophysical property and theoretical calculation demonstrate that TPA-PO and TPA-DPO possess the hybridized local and charge-transfer (HLCT) feature, which can utilize the triplet exciton to achieve highly efficient electroluminance (EL). The non-doped OLEDs with TPA-PO/TPA-DPO as pure emissive layer show the uniform EL emission peak at 468 nm, corresponding to CIE coordinates of (0.168, 0.187) and (0.167, 0.167), respectively. The TPA-DPO-based non-doped OLEDs provide the maximum external quantum efficiency (EQE) of 7.99 % and high exciton utility efficiency of 48.4 %~72.6 %. Moreover, the TPA-DPO-based device exhibits low-efficiency roll-off, still maintaining the EQE of 6.03 % at the high luminance of 5000 cd m-2. Those findings state clearly that PO is a promising building block of blue fluorophore with a potential HLCT feature to be applied in non-doped OLEDs.

Anthracene-triphenylamine materials with high luminance and negligible efficiency roll-off for organic light-emitting diodes

Two novel anthracene-triphenylamine emitters, TPA-tBuANT and TPA-PyrANT, were synthesized efficiently by the coupling reactions of anthracene and triphenylamine derivatives. They emit blue-green light in the diluted solutions, having emission peaks at 488 and 510 nm, and exhibit high photoluminescence fluorescence quantum efficiency (PLQY) of 53% and 80%, respectively. Their PLQY were 19% and 49% in solid states at 470 and 487 nm, respectively. Employing TPA-tBuANT as an emitter, the maximum external quantum efficiencies (EQEs) were up to 4.1% in the non-doped devices of organic light-emitting diodes (OLEDs) and 5.2% in the doped devices. The maximum EQEs of the devices based on TPA-PyrANT were 4.5% for non-doped devices and 5.6% for the doped devices. The devices all exhibit negligible efficiency roll-off.

A high-efficiency blue multiple resonance emitter with enhanced horizontal emitting dipole orientation based on indolocarbazole

An indolocarbazole substituent is introduced during the multiple resonance emitter fabrication with a related P-type electroluminescence device exhibited high maximum efficiency of 11.5% and a negligible efficiency roll-off.

Full-Color Sterically Shielded Boron Difluoride Emitters with Efficient and Ultrapure Electroluminescence via Sensitized Fluorescence.

Emitters with narrowband emissions are essential to improve the color purity of organic light-emitting diodes (OLEDs). Boron difluoride (BF) derivatives have preliminarily exhibited small full width at half-maximum (FWHM) values in electroluminescent devices, which, however, still face formidable challenges in recycling triplet excitons and realizing full-color emissions covering the whole visible spectra. Here, a systematic molecular engineering on the aza-fused aromatic emitting core and peripheral substitutions is made, affording a family of full-color BF emitters spanning from blue (461 nm) to red (635 nm), with high photoluminescence quantum yields of >90% and a small FWHM of 0.12 eV. The device architectures are delicately manipulated to form effective thermally activated sensitizing emissions, first affording the highest maximum external quantum efficiency of >20% for BF-based OLEDs with negligible efficiency roll-off.

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 0.07eV, high photoluminescence quantum yield of 92%, and short delayed fluorescence lifetime of 52.4µs. 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.

Achieving high-performance non-doped sky-blue OLEDs with negligible efficiency roll-off by combining AIE, HLCT and mechanoluminescence features

The hot exciton materials with high-lying reverse intersystem crossing provide great potential for the realization of efficient, low-cost and stable organic light-emitting diodes (OLEDs). However, it is highly desired but still challenging to achieve high-performance non-doped blue OLEDs with hot exciton luminogens. In this work, a new hybridized local and charge transfer (HLCT) luminogen 2-(4-(2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-phenanthro[9,10-d]imidazole-1-yl)phenyl)phenanthro[9,10-d]oxazole (abbreviated as TPE-PI-PO) with the incorporation of tetraphenylethylene is obtained for constructing the high-performance non-doped blue OLEDs. TPE-PI-PO possesses the multifunctional features of aggregation-induced emission (AIE), HLCT and mechanoluminescence with high photoluminescence quantum yield (58%) in the neat film. The non-doped OLEDs based on TPE-PI-PO exhibits sky-blue emission, high luminance of 13,020 cd m−2 and the maximum efficiencies of 11.62 cd A−1, 5.08% with the negligible efficiency roll-off as low as 1.6% at 1000 cd m−2. The electroluminescent performance is highly competitive with the best reported non-doped blue fluorescent OLEDs based on AIE luminogens. These results can shed light on a feasible design principle to develop high-quality HLCT-type luminogens for high-performance non-doped blue OLEDs.

Conformational Engineering of Two-Coordinate Gold(I) Complexes: Regulation of Excited-State Dynamics for Efficient Delayed Fluorescence.

Carbene-Au-amide (CMA) type complexes, in which the amide and carbene ligands act as an electron donor (D) and acceptor (A), respectively, can exhibit strong delayed fluorescence (DF) from a ligand to ligand charge transfer (LLCT) excited state. Although the coplanar donor-acceptor (D-A) conformation has been suggested to be a crucial factor favoring radiative decay of the charge-transfer excited state, the geometric structural factor underpinning the excited-state mechanism of CMA complexes remains an open question. We herein develop a new class of carbene-Au-carbazolate complexes by introducing large aromatic substituents onto the carbazolate ligand, the presence of which are conceived to restrict the rotation of the Au-N bond and thus confine a twisted D-A conformation in both ground and excited states. A highly twisted D-A orientation is found for the complexes in their crystal structures. Photophysical studies reveal that the twisted conformation induces a decrease in the gap (ΔEST) between the lowest singlet excited state (S1) and the triplet manifold (T1) and thus a faster reverse intersystem crossing (RISC) from T1 to S1 at the expense of oscillator strength for an S1 radiative transition. In comparison with the coplanar analogue, the twisted complexes exhibit comparable or improved DF with quantum yields of up to 94% and short emission lifetimes down to sub-microseconds. The tuning of excited-state dynamics has been well interpreted by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, which unveil much faster RISC rates for twisted complexes. Solution-processed organic light-emitting diodes (OLEDs) based on the new CMA complexes show promising performances with almost negligible efficiency rolloff at a brightness of 1000 cd m-2. This work implies that neither a coplanar ground-state D-A conformation nor a dynamic rotation of the M-N bond is the key to the realization of efficient DF for CMA complexes.

Highly efficient non-doped deep-blue OLED with NTSC CIEy and negligible efficiency roll-off based on emitter possessing hydrogen bond and hybridized excited state

The efficient deep-blue organic emitters with satisfying National Television System Committee (NTSC) standard Commission Internationale de L'Eclairage (CIE) coordinate of (0.14, 0.08) are indispensable component for full-color organic light-emitting diodes (OLEDs) display. In this work, we designed and synthesized a novel deep-blue material TPIN with donor‒π‒acceptor (D‒π‒A) structure by utilizing triphenylamine (TPA) as donor, 1,4,5-triphenyl-1H-imidazole (PIM) as acceptor and pyridyl as π-bridge. The pyridyl is expected to endow TPIN with multiple intra-/intermolecular hydrogen bands (H-bands) interactions which could inhibit molecular vibration and affect the radiative transition, aiming to obtain efficient deep-blue OLED with high color purity and negligible efficiency roll-off. In addition, the photophysical experiments and theoretical calculations were implemented to verify the unique hybridized excited state of TPIN. And X-ray crystallographic analysis of TPIN single-crystal indicates that there exist many C–H⋯N and C–H···π interactions in the stacking structure. The non-doped device based on TPIN exhibited good deep-blue electroluminescence (EL) performance with emission peak at 443 nm, corresponding to a CIE coordinate of (0.151, 0.080), and a maximum external quantum efficiency (EQEmax) of 3.71% with negligible efficiency roll-off (EQE = 3.70% @ 1000 cd m−2).

High-Efficiency, Non-doped, Pure-Blue Fluorescent Organic Light-Emitting Diodes via Molecular Tuning Regulation of Hot Exciton Excited States.

Tremendous efforts have been made on researching triplet-triplet annihilation (TTA) and thermally activated delayed fluorescence (TADF) materials for realizing high-efficiency blue organic light-emitting diodes (OLEDs) through utilizing triplet exciton conversion to the lowest singlet excited state (S1) from the lowest triplet excited state (T1). However, hot exciton conversion from the upper triplet energy level state (Tn, n > 1) to the lowest singlet excited state (S1) is an increasingly promising method for realizing pure-blue non-doped OLEDs with performances comparable to those of TTA and TADF materials. Herein, two pure-blue fluorescent emitters of donor (D)-π-acceptor (A) type, PIAnCz and PIAnPO, were designed and synthesized. The excited-state characteristics of PIAnCz and PIAnPO, confirmed by theoretical calculations and photophysical experiments, demonstrated these materials' hot exciton properties. Based on PIAnCz and PIAnPO as emission layer materials, the fabricated non-doped devices exhibited pure-blue emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.12) and (0.16, 0.15), maximum luminescences of 10,484 and 15,485 cd m-2, and maximum external quantum efficiencies (EQEs) of 10.9 and 8.3%. Besides, at a luminescence of 1000 cd m-2, the EQEs of PIAnPO-based devices can still be high at 7.7%, and the negligible efficiency roll-off was 6.0%. The device performance of both materials demonstrates their outstanding potential for commercial application.

Stimuli-Responsive Aggregation-Induced Delayed Fluorescence Emitters Featuring the Asymmetric D-A Structure with a Novel Diarylketone Acceptor Toward Efficient OLEDs with Negligible Efficiency Roll-Off.

Multifunctional luminescent materials with aggregation-induced delayed fluorescence (AIDF) are capable of suppressing concentration-caused emission quenching and exciton annihilation when used as organic light-emitting diode (OLED) emitters. In this contribution, three stimuli-responsive AIDF luminogens, pipd-BZ-PXZ, pipd-BZ-PTZ, and pipd-BZ-DMAC, featuring a D-A asymmetric framework based on a fused N-heterocycle diarylketone acceptor (imid-azo[1,2-a]pyridin-2-yl(phenyl)methanone pipd) are designed and synthesized. Interestingly, pipd-BZ-PTZ forms two different kinds of crystals (G-crystal and O-crystal) with distinct intermolecular interactions between pipd moieties. The G-crystal with a looser packing mode presents significant morphology-dependent stimuli-responsive behavior with a shifted emission wavelength of 56 nm. Generated by a strong intramolecular charge transfer effect, pipd-BZ-PXZ and pipd-BZ-PTZ exhibit orange to red emission in solution and neat films. Both nondoped and doped devices are fabricated for comparison. Nondoped devices present moderate performance with external quantum efficiencies and current efficiency that reach 7.04% and 19.86 cd A-1, respectively, and the corresponding efficiency roll off at 1000 cd m-2 is as small as 2.3%, which is among the best records of AIDF-OLEDs with an emission wavelength over 570 nm. Doped devices show better performance with corresponding efficiencies of up to 55.41 cd A-1 and 15.77%.

Highly efficient nondoped blue organic light-emitting diodes with high brightness and negligible efficiency roll-off based on anthracene-triazine derivatives

A nondoped device based on the PIAnTAZ emitter shows blue electroluminescence with a maximum EQE of 7.96%, a maximum luminance of 58 675 cd m−2 and negligible efficiency roll-offs.

Tetraphenylethylene-substituted phenothiazine-based AIEgens for non-doped deep-blue organic light-emitting diodes with negligible efficiency roll-off

Deep-blue organic light-emitting diodes (OLEDs) with low efficiency roll-off and high color purity are critical for the tetraphenylethylene-based AIEgens derivative. In this work, the novel blue emitter, 3-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-10-(4-(1,2,2-triphenylvinyl)phenyl)-10H-phenothiazine (TPEPPI), was designed and synthesized to develop a new material possessing aggregation-induced emission (AIE) characteristics and deep-blue emission. The TPEPPI hardly emits fluorescence in tetrahydrofuran (THF), the luminescence intensity increased by 5.5 times when the water content reaches 95%, showing strong fluorescence in aggregated state. Three different non-doped device structures were fabricated, all of the OLEDs showed deep-blue emission (∼460 nm). The optimized device Ⅲ exhibited an emission peak at 467 nm, a maximum current efficiency of 4.25 cd A−1, a maximum power efficiency of 3.35 lm W−1, a maximum luminance of 16750 cd m−2, a maximum external quantum efficiency of 2.36%, and the essentially negligible efficiency roll-off of 3.3%, which is the first observation of deep-blue emission using a tetraphenylethylene-substituted phenothiazine-based AIEgens derivative in OLEDs.

High-Performance Non-doped OLEDs with Nearly 100 % Exciton Use and Negligible Efficiency Roll-Off.

Non-doped organic light-emitting diodes (OLEDs) possess merits of higher stability and easier fabrication than doped devices. However, luminescent materials with high exciton use are generally unsuitable for non-doped OLEDs because of severe emission quenching and exciton annihilation in neat films. Herein, we wish to report a novel molecular design of integrating aggregation-induced delayed fluorescence (AIDF) moiety within host materials to explore efficient luminogens for non-doped OLEDs. By grafting 4-(phenoxazin-10-yl)benzoyl to common host materials, we develop a series of new luminescent materials with prominent AIDF property. Their neat films fluoresce strongly and can fully harvest both singlet and triplet excitons with suppressed exciton annihilation. Non-doped OLEDs of these AIDF luminogens exhibit excellent luminance (ca. 100000 cd m-2 ), outstanding external quantum efficiencies (21.4-22.6 %), negligible efficiency roll-off and improved operational stability. To the best of our knowledge, these are the most efficient non-doped OLEDs reported so far. This convenient and versatile molecular design is of high significance for the advance of non-doped OLEDs.

Novel spironaphthalenone-based host materials for efficient red phosphorescent and thermally activated delayed fluorescent OLEDs

Abstract Two spiro-D configured host materials DCPSO and DAPSO bearing electron-withdrawing group spironaphthalenone and electron-donating group phenylcarbazole or triphenylamine were designed and synthesized. The spironaphthalenone moiety was embedded in the host skeleton such that the host materials have twisted conformation and separated molecular frontier orbitals. The triplet energy of the DCPSO and DAPSO were estimated to be 2.60 and 2.45 eV, respectively, rendering them suitable host materials for red organic light-emitting diodes (OLEDs). The single-carrier transporting devices revealed that DCPSO is a bipolar material with more balanced hole and electron flux in the emitting layer than that of DAPSO. Employing DCPSO as host, high external quantum efficiencies of 16.6% and 13.3% as well as negligible efficiency roll-off were achieved for red phosphorescent and red TADF OLEDs.

Carbazole-dendronized thermally activated delayed fluorescent molecules with small singlet-triplet gaps for solution-processed organic light-emitting diodes

Based on a multi-carbazole encapsulation strategy, two novel solution-processable thermally activated delayed fluorescence (TADF) molecules, named 2,8-bis(2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-9,9-dimethylacridin-10(9H)-yl)dibenzo[b,d]thiophene 5,5-dioxide (1CzAcDBTO) and 2,8-bis(9,9-dimethyl-2,7-bis(3,3″,6,6″-tetra-tert-butyl-9′H-[9,3':6′,9″-tercarbazol]-9′-yl)acridin-10(9H)-yl)dibenzo[b,d]thiophene 5,5-dioxide (2CzAcDBTO), were synthesized. Both compounds exhibit excellent thermal stability and a small energy gap between the lowest singlet and triplet state, i.e., 0.02 eV for 1CzAcDBTO and 0.04 eV for 2CzAcDBTO, respectively. Attributed to the more effective reverse intersystem crossing process, the molecule 2CzAcDBTO as the neat emitter realized better electroluminescent performances. The simplified devices with the neat dendrimers as the emissive layers exhibit significant improvements of the external quantum efficiencies (EQEs) by factors of 19.5 and 22.5, respectively compared with the device incorporating the small molecular core AcDBTO. By blending an additional TADF sensitizer with the dendrimers in a common host, enormous enhancements of efficiency and luminance were realized, e.g., a maximum EQE of 7.9% and a maximum luminance of 16380 cd/m2. Remarkably, the EQE remains as high as 7.4% at the high luminance of 1000 cd/m2, revealing a negligible efficiency roll-off.

Highly Efficient Nondoped OLEDs with Negligible Efficiency Roll-Off Fabricated from Aggregation-Induced Delayed Fluorescence Luminogens.

Purely organic emitters that can efficiently utilize triplet excitons are highly desired to cut the cost of organic light-emitting diodes (OLEDs), but most of them require complicated doping techniques for their fabrication and suffer from severe efficiency roll-off. Herein, we developed novel luminogens with weak emission and negligible delayed fluorescence in solution but strong emission with prominent delayed components upon aggregate formation, giving rise to aggregation-induced delayed fluorescence (AIDF). The concentration-caused emission quenching and exciton annihilation are well-suppressed, which leads to high emission efficiencies and efficient exciton utilization in neat films. Their nondoped OLEDs provide excellent electroluminescence efficiencies of 59.1 cd A-1 , 65.7 lm W-1 , and 18.4 %, and a negligible current efficiency roll-off of 1.2 % at 1000 cd m-2 . Exploring AIDF luminogens for the construction of nondoped OLEDs could be a promising strategy to advance device efficiency and stability.

Novel phosphine oxide-based electron-transporting materials for efficient phosphorescent organic light-emitting diodes

Three electron transporting materials comprising a nitrogen heterocycle and a phosphoryl moiety were utilized for green PhOLEDs, which exhibited decent performances with a peak current efficiency above 80 cd A−1 as well as negligible efficiency roll-off.

Suppression of efficiency roll-off in highly efficient blue phosphorescent organic light-emitting devices using novel iridium phosphors with good electron mobility

High-efficiency blue organic light-emitting diodes were reported by adopting two novel iridium phosphors. Due to phosphoryl moiety in ancillary ligands, both complexes (dfppy)2Ir(ppp) and (dfppy)2Ir(dpp) (dyppy = 2-(2,4-difluorophenyl)pyridine, ppp = phenyl(pyridin-2-yl)phosphinate, dpp = dipyridinylphosphinate) own high electron mobility which can balance the injection and transport of carriers. Furthermore, the double light-emitting layers with TcTa (4,4′,4″-tris(carbazol-9-yl)triphenylamine) and 26DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine) hosts broaden the exciton formation zone and suppress efficiency roll-off. The optimized double light-emitting layers devices exhibited decent performances with peak current efficiency near 50 cd/A and external quantum efficiency above 20% as well as negligible efficiency roll-off.