Deep-red Organic Light-emitting Diodes Research Articles

Cyano-substituted Bis((benzothiophen-2-yl)pyridine) (acetylacetonate) iridium complexes for efficient and stable deep red organic light-emitting diodes emitting at 673 nm

This study explores the development of cyano-substituted bis((benzothiophen-2-yl)pyridine) (acetylacetonate) iridium complexes, specifically Ir(btpCN)2(acac), for use in efficient and stable deep red organic light-emitting diodes (OLEDs) emitting at 673 nm. The new emitter, Ir(btpCN)2(acac), was designed to achieve red-shifted emission through strategic cyano substitution at the meta-position of pyridine moiety of btp ligand, leveraging the favorable overlap between its emission spectrum and the absorption spectrum of the exciplex host composed of BCzPh and CN-T2T. The OLED devices employing Ir(btpCN)2(acac) as the emitter exhibited a peak external quantum efficiency (EQE) of 10.2 % and an emission wavelength of 673 nm. Significantly, these devices demonstrated superior operational stability, with a lifetime (LT50) of 190.8 h at an initial luminance of 200 cd m−2, which is among the highest reported for deep-red OLEDs in the literature. This remarkable stability is achieved without compromising the device performance, making Ir(btpCN)2(acac) a highly promising candidate for commercial applications. In addition, the straightforward synthesis process of Ir(btpCN)2(acac) further enhances its potential for widespread use. Overall, our findings highlight the potential of cyano-substituted Ir complexes for creating efficient, stable, and commercially viable deep-red OLEDs. The balanced performance of Ir(btpCN)2(acac) in terms of efficiency, stability, and ease of synthesis marks a significant advancement in the development of OLED technology suitable for phototherapy and other applications requiring reliable deep-red light sources.

Multiple Enol-Keto Isomerization and Excited-State Unidirectional Intramolecular Proton Transfer Generate Intense, Narrowband Red OLEDs.

A novel series of excited-state intramolecular proton transfer (ESIPT) emitters, namely, DPNA, DPNA-F, and DPNA-tBu, endowed with dual intramolecular hydrogen bonds, were designed and synthesized. In the condensed phase, DPNAs exhibit unmatched absorption and emission spectral features, where the minor 0-0 absorption peak becomes a major one in the emission. Detailed spectroscopic and dynamic approaches conclude fast ground-state equilibrium among enol-enol (EE), enol-keto (EK), and keto-keto (KK) isomers. The equilibrium ratio can be fine-tuned by varying the substitutions in DPNAs. Independent of isomers and excitation wavelength, ultrafast ESIPT takes place for all DPNAs, giving solely KK tautomer emission maximized at >650 nm. The spectral temporal evolution of ESIPT was resolved by a state-of-the-art technique, namely, the transient grating photoluminescence (TGPL), where the rate of EK* → KK* is measured to be (157 fs)-1 for DPNA-tBu, while a stepwise process is resolved for EE* → EK* → KK*, with a rate of EE* → EK* of (72 fs)-1. For all DPNAs, the KK tautomer emission shows a narrowband emission with high photoluminescence quantum yields (PLQY, ∼62% for DPNA in toluene) in the red, offering advantages to fabricate deep-red organic light-emitting diodes (OLED). The resulting OLEDs give high external quantum efficiency with a spectral full width at half-maximum (FWHM) as narrow as ∼40 nm centered at 666-670 nm for DPNAs, fully satisfying the BT. 2020 standard. The unique ESIPT properties and highly intense tautomer emission with a small fwhm thus establish a benchmark for reaching red narrowband organic electroluminescence.

High-efficiency tandem red organic light-emitting diodes adopting exciplex hosts

Abstract Deep-red organic light-emitting diodes (OLEDs) exhibit significant potential for applications in infrared medical treatment and infrared imaging. However, the OLED performance drops significantly with increasing emission wavelength due to the energy gap law. To overcome this challenge, an efficient interfacial exciplex host, NPB/TPBi, has been developed. Simulation results indicate that the interfacial exciplex host demonstrates superior performance, compared with the bulk exciplex host and single host. To further boost the performance of red OLEDs, the interfacial exciplex host was incorporated into tandem devices, achieving an external quantum efficiency of up to 27.3%. This work offers important insights for the fabrication of efficient red OLEDs.

Thermally activated delayed fluorescence emitters showing wide-range near-infrared piezochromism and their use in deep-red OLEDs.

Organic small molecules exhibiting both thermally activated delayed fluorescence (TADF) and wide-ranging piezochromism (Δλ > 150 nm) in the near-infrared region have rarely been reported in the literature. We present three emitters MeTPA-BQ, tBuTPA-BQ and TPPA-BQ based on a hybrid acceptor, benzo[g]quinoxaline-5,10-dione, that emit via TADF, having photoluminescence quantum yields, ΦPL, of 39-42% at photoluminescence (PL) maxima, λPL, of 625-670 nm in 2 wt% doped films in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP). Despite their similar chemical structures, the PL properties in the crystalline states of MeTPA-BQ (λem = 735 nm, ΦPL = 2%) and tBuTPA-BQ (λem = 657 nm, ΦPL = 11%) are significantly different. Further, compounds tBuTPA-BQ and TPPA-BQ showed a significant PL shift of ∼98 and ∼165 nm upon grinding of the crystalline samples, respectively. Deep-red organic light-emitting diodes with MeTPA-BQ and tBuTPA-BQ were also fabricated, which showed maximum external quantum efficiencies, EQEmax, of 10.1% (λEL = 650 nm) and 8.5% (λEL = 670 nm), respectively.

Deep-Red Organic Light-Emitting Diodes with Increased External Quantum Efficiency and Extended Operational Lifetime by Managing the Composition of Mixed Cohosts

Deep-Red Organic Light-Emitting Diodes with Increased External Quantum Efficiency and Extended Operational Lifetime by Managing the Composition of Mixed Cohosts

Simultaneously optimizing radiative decay and up-conversion of triphenylamine-based thermally activated delayed fluorescence emitters to achieve efficient deep-red organic light-emitting diodes

Triphenylamine (TPA) has been commonly used as the electron donor to construct red and near-infrared thermally activated delayed fluorescence (TADF) emitters as the strong intramolecular charge transfer with electron acceptors to induce rapid radiative decay to compete with the small energy gap. For TPA-derivates, even stronger conjugation, accelerated up-conversion and enhanced rigidity are highly desired to further overcome the serious non-radiative decay and promote exciton utilization. Targeting at optimizing TPA-based emitters, herein, three new molecules were developed. Among them, 4,4′,4′'-(pyrazino[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline-3,6,11-triyl)tris(N,N-diphenylaniline) (TTPA-DCPPm) were designed by incorporating intramolecular hydrogen bonding and quasi-degenerate molecular orbitals. The rigidity is enhanced and radiative decay as well as reverse intersystem crossing (RISC) are promoted. Accordingly, TTPA-DCPPm demonstrates an order of magnitude smaller non-radiative decay rate, a 2.6 times larger fluorescence decay rate and a 1.8 times faster RISC rate compared with the control group, which supports its OLED with a maximum external efficiency of 26.5% with an emission peak at 640 nm. This work paves the way for developing efficient TPA-based deep-red emitters.

An efficient solution-processable hybridized local and charge-transfer (HLCT)-based deep-red fluorescent emitter for simple structured non-doped OLED

The development of highly efficient deep-red fluorescent emitters is still a challenging task due to their intrinsic low bandgap characteristics resulting in low fluorescence emission in solid-state. Moreover, most of efficient deep-red organic light-emitting diodes (OLEDs) have complex device structures and are fabricated by high-cost vacuum deposition techniques. Herein, a novel solution-processable hybridized local and charge-transfer (HLCT) deep-red fluorescent molecule, TPA-NZF, is designed and synthesized as an emitter for low-cost and simple-structured OLEDs. TPA-NZF comprises a strong electron-deficient naphthothiadiazole (NZ) functionalized with triphenylamine (TPA) as a donor and dioctylfluorene as a solubilizing moiety. The photophysical results and theoretical calculations reveal a strong donor-acceptor feature and deep-red fluorescent emission with a solid-state photoluminescence quantum yield as high as 44%, decent hole-transporting mobility, good film morphology, and thermal and electrochemical stabilities. TPA-NZF is successfully fabricated as a solution-processed non-doped emitter in simple structured OLEDs. The device shows excellent electroluminescence (EL) performance (maximum external quantum efficiency (EQEmax) of 3.59%) and a deep-red emission with CIE coordinates of (0.66, 0.33).

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).

Deep-red thermally activated delayed fluorescence emitters based on a phenanthroline-containing planar acceptor

Organic luminescent materials possessing thermally activated delayed fluorescence (TADF) feature have aroused the tremendous attention. In this work, two novel TADF emitters, 10-(acenaphtho [1′,2':5,6]pyrazino [2,3-f] [1,10]phenanthrolin-12-yl)-10H-phenoxazine (APPT-PXZ) and 7-(acenaphtho [1′,2':5,6]pyrazino [2,3-f] [1,10]phenanthrolin-12-yl)-7H-benzo [c]phenoxazine (APPT-BPXZ), are developed via combining the strong electron-donating and electron-withdrawing units. Both target compounds exhibit orthogonal configurations to decrease singlet-triplet splitting energy (ΔEST) for the remarkable TADF property. Furthermore, APPT-PXZ and APPT-BPXZ-based devices realize the maximum external quantum efficiencies of 7.2% and 2.0% with the electroluminescence peaks at 640 nm and 665 nm, respectively. This work facilitates the development of the deep-red organic light-emitting diodes.

The recombination zone adjusted by the gradient doping of TPA-DCPP for efficient and stable deep red organic light emitting diodes.

Deep-red organic light-emitting diodes (DR-OLEDs) or near-infrared organic light-emitting diodes (NIR-OLEDs) have a wide range of applications in real life, such as special light sources for plant growth in agriculture, optical communications, infrared imaging, infrared medical imaging and other fields. However, the device performance of DR-OLEDs is still far behind that of red, green and blue OLEDs. In addition to the well-known energy gap law, the location of the recombination region also has a significant impact on the device performance. If the recombination area is too close to the cathode, the electrons in the electron transport layer will easily cause exciton quenching. In this study, for the first time, we adopted a quantum well-like structure by changing the host (CBP) and guest (TPA-DCPP) thicknesses as the light-emitting layer to manage the position of the recombination zone, and then improved the carrier injection and transportation as well as increased the exciton recombination rate. Furthermore, we introduced a hole trap layer to reduce the current density and suppress the recombination zone movement; finally, we prepared high-brightness and high-efficiency DR-OLEDs based on the TADF material with a wavelength of 674 nm, a maximum brightness of 1151 cd m−2 and a maximum EQE of 4.4%.

Highly-efficient solution-processed deep-red organic light-emitting diodes based on heteroleptic Ir(III) complexes with effective heterocyclic Schiff base as ancillary ligand

Abstract Three new deep-red heteroleptic phosphorescent iridium(III) complexes Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3), comprising cyclometalated ligand 1-pheynylisoquionoloine(piq) and heterocyclic Schiff base ancillary ligands 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol(L1), 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol (L2), and 4-(4-methoxyphenyl)imino-methyl-3-methyl-1-phenyl-1H-pyrazol-5-ol (L3) have been designed, synthesized, and characterized. All the compounds emit deep red emission with λmax values in the spectral range of 602–620 nm, high quantum yield 0.44 to 0.52 and short excited state lifetime τ (0.51–0.55 μs) due to dominant strong field ligands, resulting an efficient triplet metal-ligand charge transfer (3MLCT) excited state. Time dependent density functional theory (TD-DFT) calculations and electrochemical measurements of the compounds strongly support their genuine deep red phosphorescent emission. The combination of ancillary and cyclometalated ligands significantly influence the molecular orbitals of Ir(III) complex, leading to clearly distinct electron density distributions of the LUMO and HOMO. The compounds show good thermal stability and quantum yield, these characteristics making them an ideal candidate to exploit in phosphorescent organic light emitting diodes (PhOLEDs). Highly-efficient PhOLEDs were developed by using Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3) in solution process as deep red emitters and device composed of Ir(piq)2(L3) exhibited an excellent external quantum efficiency of 14.9% and current efficiency of 10.8 cd/A with the stable CIE coordinates of (0.67,0.33).

Highly efficient deep-red organic light-emitting diodes using exciplex-forming co-hosts and thermally activated delayed fluorescence sensitizers with extended lifetime

High-efficiency deep-red phosphorescent OLEDs with a narrow FWHM based on exciplex-forming co-hosts and TADF sensitizers realized an EQE of 21.5%.

Highly Efficient Solution-Processed Deep-Red Organic Light-Emitting Diodes Based on an Exciplex Host Composed of a Hole Transporter and a Bipolar Host.

With the aim to achieve highly efficient deep-red emission, we introduced an exciplex forming cohost, 4,4',4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA): 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) (1:1). Due to the efficient triplet up-conversion processes upon the exciplex forming cohost, excellent performances of the devices were achieved with deep-red emission. Using the heteroleptic iridium complexes as the guest dopants, the solution-processed deep-red phosphorescent organic light-emitting diodes (PhOLEDs) with the iridium(III) bis(6-(4-(tert-butyl)phenyl)phenanthridine)acetylacetonate [(TP-BQ)2Ir(acac)]-based phosphorescent emitter exhibited an electroluminescent peak at 656 nm and a maximum external quantum efficiency (EQE) of 11.9%, which is 6.6 times that of the device based on the guest emitter doped in the polymer-based cohost. The unique exciplex with a typical hole transporter and a bipolar material is ideal and universal for hosting the red PhOLEDs and tremendously improves the device performances.

Highly Efficient Long-Wavelength Thermally Activated Delayed Fluorescence OLEDs Based on Dicyanopyrazino Phenanthrene Derivatives.

Highly efficient long-wavelength thermally activated delayed fluorescence (TADF) materials are developed using 2,3-dicyanopyrazino phenanthrene (DCPP) as the electron acceptor (A), and carbazole (Cz), diphenylamine (DPA), or 9,9-dimethyl-9,10-dihydroacridine (DMAC) as the electron donor (D). Because of the large, rigid π-conjugated structure and strong electron-withdrawing capability of DCPP, TADF molecules with emitting colors ranging from yellow to deep-red are realized with different electron-donating groups and π-conjugation length. The connecting modes between donor and acceptor, that is, with or without the phenyl ring as π-bridge, are also investigated to study the π-bridge effect on the thermal, photophysical, electrochemical, and electroluminescent properties. Yellow, orange, red, and deep-red organic light-emitting diodes (OLEDs) based on DCPP derivatives exhibit high efficiencies of 47.6 cd A-1 (14.8%), 34.5 cd A-1 (16.9%), 12.8 cd A-1 (10.1%), and 13.2 cd A-1 (15.1%), with Commission Internationale de L'Eclairage (CIE) coordinates of (0.44, 0.54), (0.53, 0.46), (0.60, 0.40), and (0.64, 0.36), respectively, which are among the best values for long-wavelength TADF OLEDs.

Deep-red organic light-emitting diodes with stable electroluminescent spectra based on zinc complex host material

Efficient deep-red organic light-emitting diodes with 13.1% EQE, CIE coordinates of (0.70, 0.30) and stable electroluminescent spectra.

Up to 100% Formation Ratio of Doublet Exciton in Deep-Red Organic Light-Emitting Diodes Based on Neutral π-Radical.

In a neutral π-radical-based organic light-emitting diode (OLED), although the emission comes from the doublet excitons and their transition to the ground state is spin-allowed, the upper limit of internal quantum efficiency (IQE) is not clear, 50% or 100%? In this work, the deep-red OLEDs based on a neutral π-radical were fabricated. Up to 100% doublet exciton formation ratio was obtained through rational designing device structure and host-guest doping system. This indicates the IQE of neutral π-radical-based OLEDs will reach 100% if the nonradiative pathways of radicals can be suppressed. The maximum external quantum efficiency of the optimized device is as high as 4.3%, which is among the highest values of deep-red/near-infrared OLEDs with nonphosphorescent materials as emitters. Our results also indicate that using partially reduced radical mixture as emitter may be a way to solve aggregation-caused quenching in radical-based OLEDs.

Efficient deep-red organic light-emitting diodes based on blending MEH-PPV with a small molecule compound

Efficient deep-red organic light-emitting diodes (OLEDs) were investigated based on the blend of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with 4,7-bis(5-(7-(9H-carbazol-9-yl)-9,9-dioctyl-9H-fluoren-2-yl)thiophen-2-yl)benzo[c] [1,2,5] thiadiazole (compound 1). By optimizing the blend ratio, the turn-on voltage of the devices was significantly reduced from 4.9 V to 2.4 V. A highest external quantum efficiency of 2.56% was achieved at a blend ratio of 95:5 (wt) for compound 1: MEH-PPV. The CIE coordinate was measured to be (0.70, 0.30), with the luminescence peak at around 680 nm. Based on experimental observations the improvement mechanism is described.