Red Organic Light-emitting Diodes Research Articles

High-efficiency and stable red to near-infrared organic light-emitting diodes using dinuclear platinum(ii) complexes

High-performance red and NIR organic light-emitting diodes (OLEDs) have been achieved using highly robust and phosphorescent diplatinum(ii) complexes featuring strong intramolecular Pt–Pt and π–π interactions.

Multifaceted Sulfone-Carbazole-Based D-A-D Materials: A Blue Fluorescent Emitter as a Host for Phosphorescent OLEDs and Triplet-Triplet Annihilation Up-Conversion Electroluminescence.

Electroluminescence (EL) from the singlet-excited (S1) state is the ideal choice for stable, high-performing deep-blue organic light-emitting diodes (OLEDs) owing to the advantages of an adequately short radiative lifetime, improved device durability, and low cost, which are the most important criteria for their commercialization. Herein, we present the design and synthesis of three donor-acceptor-donor (D-A-D)-configured deep-blue fluorescent materials (denoted as TC-1, TC-2, and TC-3) composed of a thioxanthone or diphenyl sulfonyl acceptor and phenyl carbazolyl donor. These systems exhibit strong deep-blue photoluminescence (422-432 nm) in solutions and redshifted emission (472-486 nm) in thin films. The solid-state photoluminescence quantum yield (PLQY) was estimated to be 78 and 94% for TC-2 and TC-3, respectively. TC-2 and TC-3 possess good molecular packing and large molecular cross-sectional areas, which not only improves the PLQY but enhances the triplet-triplet annihilation up-conversion (TTAUC) efficiency of fluorescent emitters. Furthermore, both compounds were applied as an acceptor for confirming their TTAUC property using bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) (Ir(MDQ)2acac) as the sensitizer. Non-doped OLEDs based on TC-2 and TC-3 exhibit blue EL in the 461-476 nm range. In particular, TC-3 exhibits a maximum external quantum efficiency (EQEmax) of 5.1%, and its EL maximum is 476 nm. In addition, the three emitters were employed as hosts in red OLEDs using bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2acac) as the phosphorescent dopant. The red phosphorescent OLEDs based on TC-1, TC-2, and TC-3 achieve excellent EQEmax values of 21.6, 22.9, and 21.9%, respectively, and peak luminance efficiencies of 12.0, 14.0, and 12.3 cd A-1. These results highlight these fluorophores' versatility and promising prospects in practical OLED applications.

Structure-property investigation of two red TADF isomers with different D-A conjugation for superior exciton utilization

In this text, by linking a phenoxazine donor (D) into different positions of the 11,12-diphenyldipyrido[3,2-a:2′,3′-c]phenazine acceptor (A), Two novel red thermally activated delayed fluorescence (TADF) isomers, PPZBP and BPZPP, were designed and synthesized. The different connection modes bring different steric hindrance between D and A segments, which lead to tunable conjugation of the whole molecule, resulting in the change of locally excited triplet (3LE) state. Beneficial from the suitable conjugation, PPZBP shows better energy level alignments than BPZPP for superior triplet up conversion. PPZBP-based red organic light-emitting diode realizes a higher EQE of 22.0% than that (14.8%) of the BPZPP-based one. This work not only demonstrates the importance of controlling D-A conjugations on the 3LE states and the resultant influences on RISC rates, it also gives a deep insight into preferred design strategies for high-efficiency red TADF emitters.

UV-ozone treated Ti3C2Tx-MXene nanosheets as hole injection layer for organic light-emitting diodes

In this work, we successfully synthesized Ti3C2Tx-MXene nanosheets with a lateral dimension of 1 ∼ 2 μm by using wet-chemical etchant and sonication-assisted liquid-phase exfoliation method. After UV-ozone treatment, the work function for Ti3C2Tx-MXene nanosheets was adjusted to 5.14 eV well matching to indium tin oxide (ITO) anode (4.80 eV). Using Ti3C2Tx-MXene nanosheets as hole injection layer (HIL), Ir(ppy)3-based green organic light-emitting diodes (OLEDs) were fabricated, and the resulting devices exhibit a low turn-on voltage of 3.0 V and the high current efficiency, power efficiency and external quantum efficiency of 72.79 cd/A, 54.44 lm/W and 20.34 %, which are obviously better than these of 67.38 cd/A, 50.38 lm/W and 18.65 % for PEDOT:PSS-based reference device. Further, FIrpic- and Ir(pq)2acac-based blue and red OLEDs with Ti3C2Tx-MXene nanosheets HIL were demonstrated, which also show the significantly improved device efficiency of 44.56 cd/A, 32.44 lm/W and 22.07 % for blue device and 39.17 cd/A, 38.54 lm/W and 21.90 % for red device than these for corresponding PEDOT:PSS-based devices, proving the universal applicability of Ti3C2Tx-MXene nanosheets as HIL in OLEDs. The higher performance for Ti3C2Tx-MXene nanosheets-based devices can be attributed to the appropriate work function, high transmittance, high carrier mobility and good film forming property for Ti3C2Tx-MXene nanosheets. This is the first report about the MXene using as HIL in OLEDs, which opens up a new way to develop high-performance holes injection materials for solution-processed OLEDs.

Multiplying the efficiency of red thermally activated delayed fluorescence emitter by introducing intramolecular hydrogen bond

As one of the three primary colors, red organic light-emitting diodes (OLEDs) are indispensable in practical applications. However, red emitters are generally subject to severe non-radiative exciton loss due to their narrow energy gap. In this work, three new thermally activated delayed fluorescence (TADF) emitters were developed, namely 4-(acenaphtho[1,2-b]quinoxalin-9-yl)-N,N-diphenylaniline (TPA-AP), 4-(acenaphtho[1,2-b]pyrido[2,3-e]pyrazin-10-yl)-N,N-diphenylaniline (TPA-APy), and 4-(acenaphtho[1,2-b]pyrazino[2,3-e]pyrazin-9-yl)-N,N-diphenylaniline (TPA-APm), employing a series of finely modified acenaphtho[1,2-b]quinoxaline (AP) derivatives as acceptor units. Among three TADF emitters, an intramolecular hydrogen bond is formed between the donor (D) and acceptor (A) units in TPA-APm. Consequently, the overlap of the frontier molecular orbitals (FMOs) of TPA-APm can increase appropriately, and the fluorescence radiative rate (kF) of TPA-APm is nearly twofold than that of TPA-AP and TPA-APy. Furthermore, the non-radiative decay rate (knr) of TPA-APm is less than that of TPA-AP and TPA-APy by an order of magnitude, which is attributed to the improved molecular rigidity caused by intramolecular hydrogen bond. As a result, TPA-APm-based OLEDs achieved a multiplied external quantum efficiency (EQE) of 21.1% with the electroluminescence peak at 590 nm, comparing to only 7.0% and 11.5% for the TPA-AP-based and TPA-APy-based devices, respectively. These results demonstrate appropriate intramolecular hydrogen bond can suppress the influence of non-radiative decay by simultaneously enhancing molecular rigidity and facilitating the fluorescence process, and have great potential in the design of efficient red TADF emitters.

Reverse intersystem crossing accelerating assistant dopant for high efficiency and long lifetime in red hyperfluorescence organic light-emitting diodes

A highly efficient deep-red hyperfluorescence device achieving long device lifetime was fabricated by adopting a newly designed 2,5-bis(12H-benzo[4,5]thieno[2,3-a]carbazole-12-yl)-3,6-di(9H-carbazol-9-yl)terephthalonitrile (12BTCzTPN) thermally activated delayed fluorescence (TADF) assistant dopant. The 12BTCzTPN was designed via donor modification of 2,3,5,6-tetra(9H-carbazol-9-yl)terephthalonitrile (4CzTPN) to increase reverse intersystem crossing (RISC) rate for fast up-conversion. The carbazole donor moiety was replaced with 12H-benzo[4,5]thieno[2,3-a]carbazole (12BTCz) donor, which induced a small singlet–triplet energy gap and large spin–orbit coupling via the heavy atom effect of sulfur, resulting in 4.5-times faster RISC rate than the 4CzTPN TADF molecule. The addition of 12BTCzTPN as an assistant dopant to a deep-red hyperfluorescence device resulted in a device with a high external quantum efficiency of 19.9% and a long device lifetime of 1312 h at 3000 cd/m2, which is up to 90% of the initial luminance.

Vacuum deposited WO3/Al/Al:Ag anode for efficient red organic light-emitting diodes

Vacuum deposited multilayered electrode with ultra-smooth surface morphology and suitable work function for efficient electron or hole injection at the interface between the organic and metal layers is attracting more attentions recently. Following the trend, a simplified structure of WO 3 /Al/Al:Ag anode with the sheet resistance of 1.93 Ω/sq, work function of ∼5.11 eV, and surface roughness of 0.72 nm was fabricated to develop a red organic light-emitting diode (OLED) with the external quantum efficiency (EQE) of ∼16.18% at 1000 cd m −2 , which is much higher than that of an red OLED based on standard ITO electrode (EQE ∼ 12.32%). In addition, the red OLED with our proposed ITO-free anode exhibit a bright emission with luminance of >10,000 cd m −2 at 5 V as well as could be fabricated on a large active area of 4 cm 2 without any breakdown issue. This is attributed to the ultra-smooth multilayer with a suitable workfunction reducing the contact barrier energy in the interface of organic and metal layer. Impedance spectroscopy, transient electroluminescence, and optical simulation were carried out to understand the device physics and carrier recombination behaviors. In the present work, we have developed the ITO-free, red, and exciplex organic light-emitting diodes (OLEDs) demonstrating remarkably high EL efficiency and low turn-on voltage. The simplified structure of WO 3 /Al/Al:Ag anode with the sheet resistance of 1.93 Ω/sq, work function of ∼5.11 eV, and surface roughness of 0.72 nm was fabricated to develop a red organic light-emitting diode (OLED) with the external quantum efficiency (EQE) of ∼16.18% at 1000 cd/m 2 . • The ITO-free red phosphorescent organic light-emitting diodes (OLEDs) with a metal-alloy electrode was demonstrated. • The WO 3 /Al/Al:Ag anode with low resistance of 1.93 Ω/sq, work function of 5.11 eV, and smooth surface roughness of 0.72 nm. • The ITO-free OLEDs exhibit a bright emission with luminance of >10,000 cd m −2 at 5 V. • The external quantum efficiency (EQE) of ITO-free OLEDs achieved 16.18% at 1000 cd m −2 .

Rational design of CN substituted dibenzo[a,c]phenazine acceptor for color tuning of thermally activated delayed fluorescent emitters

• Rational color tuning of thermally activated delayed fluorescence emitters by molecular engineering. • CN substituent management of dibenzophenazine acceptor to manage the light-emitting properties. • High external quantum efficiency from yellow to deep red organic light-emitting diodes. Thermally activated delayed fluorescent (TADF) emitters with emission wavelength beyond 600 nm and high external quantum efficiency (EQE) above 20 % are rarely reported because of the inherent limitation of the energy gap law. Rigid and planar dibenzo[ a,c ]phenazine (BP) core was proven to be a useful acceptor to develop highly efficient red TADF emitters by minimizing non-radiative loss mechanism. In this work, by varying the number and substitution position of donor/acceptor unit attached to the BP acceptor, four long wavelength TADF emitters (3-AcBPmCN, 3-AcBPdCN, 3-AcBPtCN and 11-ACBPdCN) were developed to control the emission wavelength from yellow (552 nm) to deep-red (637 nm) region. A directly attached 9,9-dimethyl-9,10-dihydroacridine (Ac) donor and newly developed 3-CNBP, 11,12-CNBP, 3,11,12-CNBP and 3,6-CNBP acceptor units were utilized in the synthesis of TADF emitters of 3-AcBPmCN, 3-AcBPdCN, 3-AcBPtCN and 11-AcBPdCN respectively. With the increasing number of cyano units on BP acceptor, decreased HOMO-LUMO energy gap and singlet–triplet energy gap (Δ E ST ) were reported for three TADF emitters of 3-AcBPmCN, 3-AcBPdCN and 3-AcBPtCN. Further, it was demonstrated that in between the two donor-substitution positions of BP acceptor (3 and 11), 11-position substituted 11-AcBPdCN TADF emitter exhibited higher EQE (23.1 %) than the 3-position substituted 3-AcBPdCN TADF emitter (17.3 %) with the similar color coordinate (0.53, 0.46).

Efficient yellow and red thermally activated delayed fluorescence materials based on a quinoxaline-derived electron-acceptor

Highly efficient yellow and red organic light emitting-diodes are realized by employing thermally activated delayed fluorescence emitters based on a new quinoxaline-derived electron-acceptor.

A reverse intersystem crossing managing assistant dopant for high external quantum efficiency red organic light-emitting diodes

Assistant dopants with reduced Dexter energy transfer rates were designed by replacing the donor moiety of 2,3,5,6-tetra(9H-carbazol-9-yl)terephthalonitrile (4CzTPN) with 5H-benzo[4,5]thieno[3,2-c]carbazole (BTCz).

Modulating up-conversion and non-radiative deactivation to achieve efficient red thermally activated delayed fluorescence emitters

Improving up-conversion and suppressing non-radiative inactivation by employing the novel dibenzothioxanthene acceptor and employing a bulky donor is demonstrated as a new perspective toward highly efficient red organic light-emitting diodes.

Precise modulation of the triplet state distribution for high-efficiency non-doped standard saturated red OLEDs

AIE-active tetraphenylethylene precisely regulates triplet state energy level distribution to induce spontaneous upper reversed intersystem crossing to produce a desirable standard saturated red organic light-emitting diode.

High-performance solution-processed red hyperfluorescent OLEDs based on cibalackrot

Solution-processed hyperfluorescent red organic light-emitting diodes with external quantum efficiencies of 15.3% was achieved by using cibalackrot as the fluorescent emitter, where attaching tert-butyl groups to the assistant host significantly contributes to device performance.

Rigid Bridge-Confined Double-Decker Platinum(II) Complexes Towards High-Performance Red and Near-Infrared Electroluminescence.

A molecular design to high-performance red and near-infrared (NIR) organic light-emitting diodes (OLEDs) emitters remains demanding. Herein a series of dinuclear platinum(II) complexes featuring strong intramolecular Pt⋅⋅⋅Pt and π-π interactions has been developed by using N-deprotonated α-carboline as a bridging ligand. The complexes in doped thin films exhibit efficient red to NIR emission from short-lived (τ=0.9-2.1 μs) triplet metal-metal-to-ligand charge transfer (3 MMLCT) excited states. Red OLEDs demonstrate high maximum external quantum efficiencies (EQEs) of up to 23.3 % among the best PtII -complex-doped devices. The maximum EQE of 15.0 % and radiance of 285 W sr-1 m-2 for NIR OLEDs (λEL =725 nm) are unprecedented for devices based on discrete molecular emitters. Both red and NIR devices show very small efficiency roll-off at high brightness. Appealing operational lifetimes have also been revealed for the devices. This work sheds light on the potential of intramolecular metallophilicity for long-wavelength molecular emitters and electroluminescence.

Highly Efficient Heteroleptic Cerium(III) Complexes with a Substituted Pyrazole Ancillary Ligand and Their Application in Blue Organic Light-Emitting Diodes.

Compared with red and green organic light-emitting diodes (OLEDs), blue is the bottleneck that restricts the wide development of OLEDs from being the next-generation technology for displays and lighting. As a new type of emitter, a Ce(III) complex shows many satisfactory advantages, such as a short excited-state lifetime, 100% theoretical exciton utilization efficiency, and tunable emission color. Herein we synthesized three heteroleptic Ce(III) complexes Ce(TpMe2)2(dtfpz), Ce(TpMe2)2(dmpz), and Ce(TpMe2)2(dppz) with the hydrotris(3,5-dimethylpyrazolyl)borate (TpMe2) main ligand and different substituted pyrazole ancillary ligands, namely, 3,5-di(trifluomethyl)pyrazolyl (dtfpz), 3,5-dimethylpyrazolyl (dmpz), and 3,5-diphenylpyrazolyl (dppz), and studied their structures and luminescence properties. All the Ce(III) complexes exhibited a near-unity photoluminescence quantum yield both in solution and as a powder with maximum emission wavelengths in the range of 450-486 nm. The OLED employing Ce(TpMe2)2(dppz) as the emitter showed the best performance, including a turn-on voltage, maximum luminance, and external quantum efficiency of 3.2 V, 29 200 cd m-2, and 12.5%, respectively.

Synthesis and photophysical properties of ternary β-diketonate europium(III) complexes incorporating bipyridine and its derivatives

Two new octa-coordinated ternary europium(III) complexes of the type [Eu(btfa)3(Br2-bpy)] (Eu-1) and [Eu(btfa)3(PhE2-bpy)] (Eu-2) (where btfa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione, Br2-bpy = 5,5′-dibromo-2,2′-bipyridine, PhE2-bpy 5,5′-bis(phenylethynyl)-2,2′-bipyridine) together with a previously reported complex [Eu(btfa)3(bpy)] (Eu-3) have been synthesized. The complexes have been characterized by analytical and spectroscopic methods. The photophysical properties of the complexes have also been analyzed both experimentally and theoretically. The contribution of each ligand to the sensitized Eu(III) photoluminescence (PL) has been analyzed and is discussed. An energy transfer (ET) mechanism is proposed and discussed for the sensitized Eu(III) emission using experimental and theoretical data. The Eu(III) complex incorporating the parent bpy showed impressive performance as a double-emitting layer (EML) red organic light emitting diodes (R-OLEDs).

Completely Hazy and Transparent Films by Embedding Air Gaps for Elimination of Angular Color Shift in Organic Light-Emitting Diodes.

Red, green, and blue top-emission organic light-emitting diodes (RGB TOLEDs) suffer from white color change with viewing angle due to the microcavity effect, called white angular dependence (WAD). Great efforts are devoted by applying various kinds of hazy films, but they suffer from poor mechanical stability and optical transmittance. Herein, we introduce an air-gap-embedded hazy film (AEHF) to solve these problems and suppress WAD in RGB TOLEDs. The AEHF is designed with optical simulation to realize high haze with transparency. By tuning geometries of the air gap inside the polymer, the AEHF realizes high haze of more than 90% in all RGB colors while maintaining high transparency. To experimentally demonstrate the AEHF, the O2 plasma is treated on a polymer film with AgCl as an etching mask to fabricate microstructures with high aspect ratios. Afterward, PDMS is coated on the patterned surface; air gaps develop spontaneously in the valleys between microstructures during the coating process. Using these processes, an air gap with 1.2 μm size and 400 nm period is formed inside the film and ∼100% haze is achieved while maintaining a high transmittance of 88%; these results agree well with rigorous coupled wave analysis results. By utilizing the AEHF into TOLEDs, the WAD can be drastically suppressed by 95.2% compared with that of a device without AEHF.

High-performance red and white organic light-emitting diodes based on a novel red thermally activated delayed fluorescence emitter in an exciplex matrix

Blue and green organic emitters based on thermally activated delayed fluorescence (TADF) mechanism have made great progress in organic light-emitting diode (OLED) applications, whereas red and white OLEDs with this kind of emitters are lagging behind. In this text, we develop high-performance red and white OLEDs by doping a novel red TADF emitter 10,10'-(11,12-diphenyldibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (DBBPZ-DPXZ) into a blue emissive exciplex matrix. The highly rigid components and twisted donor-acceptor (D-A) structure of DBBPZ-DPXZ can suppress non-radiation decays and guarantee a small ΔEST, which results in effective exciton utilization. Meanwhile, the exciplex matrix can also provide additional reverse intersystem crossing (RISC) channel and enhance the utilization of triplet excitons. And thus, with doping 6 wt% DBBPZ-DPXZ into the exciplex host, red OLED with an electroluminescence peak at 628 nm and a high EQE of 20.8% is achieved. Moreover, by dropping doping concentration of DBBPZ-DPXZ into 0.2 wt%, a white OLED with Commission Internationale de l'Eclairage of (0.40, 0.38) at the luminance of 1000 cd m−2 and EQE of 20.7% is realized.

Orange–red organic light emitting diodes with high efficiency and low efficiency roll-off: boosted by a fused acceptor composed of pyrazine and maleimide

The strategy to develop orange–red thermally activated delayed fluorescent (TADF) materials for high-performance organic light-emitting diodes (OLEDs) with both high maximum external quantum efficiencies (EQEsmax) and low efficiency roll-off, is ongoing pursued yet challenging. Herein, two efficient orange–red TADF emitters, T-DMAC-PPyM and P-DMAC-BPyM, with a new acceptor fused by pyrazine and maleimide, were designed and synthesized. Notably, the highly twisted conformation of P-DMAC-BPyM can inhibit the close π-π stacking, so as to reduce exciton quenching, while the “locked” biphenyl bridge can enhance the molecular rigidity, thus reducing the non-radiative decay associated with intramolecular rotation. Consequently, P-DMAC-BPyM exhibits outstanding electroluminescent performance with a remarkably high EQEmax of 26.0% and an impressively low efficiency roll-off (EQE100 is 21.7%; EQE1000 is 18.3%), which are among the best results for orange–red TADF OLEDs.

Theoretical design study on the origin of the improved phosphorescent efficiency of DPEphos quinoline-substituted derivatives for OLEDs

Various tetrahedral heteroleptic Cu(I) complexes, as red organic light-emitting diodes (OLEDs), show a concern of maximizing the quantum yield (QY) in order to improve the optoelectronic performance. Herein, three experimental [Cu(N^N) (bis [2-(diphenylphosphino)phenyl]ether)]+ (named 1, 2 and 3) were selected as the jumping-off point, following two complexes (named 4 and 5) were successfully designed by introducing bulky electron-donating substituents into N^N ligands continuously. As expected, the QY of designed complexes 4 (0.26) and 5 (0.13) exhibit over twice higher than that of 3 (5.4 × 10−2). This can be attributed to the enhanced electron-donating property of N^N ligand, which accelerated the radiative transition rate (kr) through the apparently elevated energy level of the lowest triplet excited state (T1) and strengthened transition dipole moments, even though the spin-orbit coupling (SOC) effect is weakened. Simultaneously, the tetrahedral geometric distortion could be effectively restrained by the bulky N^N ligands, but the high vibrational freedom of the terminal substituents could also bring in some unfavorable intra-ligand deformation, resulting in an upward of the nonradiative transition rate (knr) at 5 (knr: 0.30 × 105 s−1 for 4; 0.95 × 105 s−1 for 5). Therefore, it's worth noting that the balance of excited state energy level, SOC effect as well as the reorganization energy ought to be elaborately regulated to achieve the optimal QY. This detailed investigation on the microscopic mechanism of these Cu(I) complexes can provide instructive inspiration for experimentalists.