Materials For Organic Electroluminescent Devices Research Articles

(Invited) SAE-MEMS Technology for Electrostatic Vibrational Energy Harvesters

Energy harvesting [1] is expected to be a self-powering technology for small wireless sensor nodes in the next generation Internet-of-Things (IoT). In particular, small vibrational energy harvesters (VEHs) based on micro-electro-mechanical systems (MEMS) technology, which are battery-free and can generate electricity even at night or in the dark, are being investigated worldwide for next-generation power sources for small wireless IoT nodes [2,3]. Electrostatic MEMS VEHs, which use semi-permanent charge-holding dielectrics called "electrets," are useful for power generation using environmental vibrations because they have a higher output power density at lower frequencies than other methods [4]. Further miniaturization, higher performance, and higher productivity will be required to use electret-type MEMS VEHs as power sources for compact wireless IoT nodes and other devices. Although the integration of MEMS VEHs with power management circuits enables miniaturization, higher performance, and improved productivity, the monolithic integration of electret-type MEMS VEHs and electronic circuits is an unexplored area in conventional technology. Conventional electrostatic MEMS VEHs involve corona discharge, electron beam irradiation, X-ray irradiation, or high-temperature treatment as electret charge processing, which imposes restrictions on the design and manufacturing of MEMS structures and the integration of MEMS and electronic circuits. Recently, self-assembled electrets (SAEs), which are electrets that do not require any charging process, have been developed using materials for organic electroluminescent devices [5]. The surface charge density of SAEs is equivalent to those of previously reported electrets for VEHs, and thus SAE is expected to improve the performance of VEH and simplify the manufacturing process. However, there have been no reports of MEMS devices using SAEs.Our proposed method is the first technology to form SAEs inside MEMS structures using only a vacuum evaporation process at room temperature, and we have succeeded in actual vibration power generation [6]. This technology can be integrated into existing semiconductor processes because the electret formation requires only room-temperature deposition, making it possible to monolithically integrate electret-type MEMS VEHs and electronic circuits on the same substrate. In addition, since the SAE surface potential is proportional to the film thickness, the amount of electricity generated can be further increased by increasing the thickness of the SAE film. Therefore, this technology is expected to accelerate the miniaturization, performance, and productivity improvement of MEMS environmental vibration power generation devices. Akinaga, “Recent advances and future prospects in energy harvesting technologies,” Jpn. J. Appl. Phys. 59 (2020) 110201.D. Mitcheson, E. M. Yeatman, G. K. Rao, A. S. Holmes, and T. C. Green, “Energy Harvesting from Human and Machine Motion for Wireless Electronic Devices” Proc. IEEE 96 (2008) 1457–1486.Suzuki, “Recent progress in MEMS electret generator for energy harvesting,” IEEJ Trans. Elec. Electron. Eng. 6 (2011) 101-111.Toshiyoshi, S. Ju, H. Honma, C.-H. Ji, and H. Fujita, “MEMS vibrational energy harvesters,” Sci. Technol. Adv. Mater. 20 (2019) 124-143.Tanaka, N. Matsuura, and H. Ishii, “Self-Assembled Electret for Vibration-Based Power Generator,” Sci. Rep. 10 (2020) 6648.Yamane, H. Kayaguchi, K. Kawashima, H. Ishii, and Y. Tanaka, “MEMS post-processed self-assembled electret for vibratory energy harvesters,” Appl. Phys. Lett. 119 (2021) 254102.

Investigation on potential applications of thermal activated delayed fluorescence materials in organic electroluminescent devices

In the past few years, organic battery light emitting devices have attracted a lot of attention, showing great application potential. Fluorescence material, phosphorescence material and thermal activated delayed fluorescence (TADF) material, have attracted more and more attention. However, phosphorescent material is limited by the useage of a heavy metal, Ir. So people want to find a more efficient and environmental-friendly material:TADF. TADF polymer can be used as a new type of organic photoelectric functional materials due to its high exciton utilization. This work focuses on the working principle, advantages, applications of TADF materials and the imagination for future applications. OLED emission of different light colors is also worthy of attention, the differences and efficiency of the three colors have sparked discussion as well. On this basis, starting from the luminescence principle and device structure of TADF, the advantages and disadvantages of TADF materials are summarized. Finally, the solution to “aggregation-caused quenching” problem (ACQ) is found, so as to focus on the application of technology and envisage the future application direction.

Structures, vibrational spectra and HOMO-LUMO analysis of 3-Mercapto-4-methyl-4H-1, 2, 4-triazole: an organic electroluminescent materials

3-Mercapto-4-methyl-4H-1, 2, 4-triazole (MMTZ) is an Electron-transporting materials for organic electroluminescent devices. FT-Infrared and FT-Raman spectra of MMTZ were measured in the range 4000-400 cm−1 and 3700-100 cm−1, respectively. MMTZ exists in thiol and thione tautomeric forms. In order to make sure which one was the stable structure, the calculation of Gibbs free energy was calculated based on density functional theory (DFT) using B3LYP/6-311++G(d, p) basis set. The result showed that the thione form was the most stable tautomer. Vibrational assignments of MMTZ were made on the basis of potential energy distribution (PED). The calculated HOMO and LUMO energies showed that charge transfer occured within the molecule.

Synthesis, characterization, and electroluminescent properties of monodisperse oligofluorenes as emissive materials for organic electroluminescent devices

Abstract A series of new monodisperse oligofluorenes mainly based on 9,9-bis(2-methylbutyl)fluorene units whose rigid core has been extended with 1-(trifluoromethyl)benzene termini ( F1, F2, F3 ) or hydrogen termini ( 8, 12 ) are designed and synthesized. The effects of conjugated length and end-capper groups on thermal, optical and electrochemical properties are studied. The oligofluorenes exhibit a narrower bandgap, higher HOMO, lower LUMO and red-shift on absorption/emission spectra with increasing number of fluorene units. Based on the comparing data of the compounds which have the same number of fluorene units, oligomers terminaled with electron-withdrawing group have a higher T g , narrower bandgap and red-shifts on absorption/emission spectra. Furthermore, in order to investigate the possibility of using these molecules as emitters in organic light-emitting diodes (OLEDs), solution-processed OLEDs with structure of ITO/PEDOT:PSS/PVK:oligomers/CsF/Al are fabricated. All devices based on the oligomers emit deep-blue light peaked at ca. 392–420 nm. Among these devices, the device based on hydrogen-terminated compound ( 12 ) displays the best performance with a high maximum luminance of 670 cd/m 2 , a low turn-on voltage of 4.20 V, and a maximum luminance efficiency of 0.38 cd/A.

Synthesis and properties of new luminescent hole transporting materials of triarylamine with dehydroabietic acid methyl ester moieties

A series of triarylamines based on dehydroabietic acid methyl ester moieties (6a–h) were synthesized for possible application as hole transporting materials for organic electroluminescent devices. The target compounds were characterized by elemental analysis, FT-IR, NMR, and mass spectrometry. Their optical, electrochemical, and thermal properties were investigated using UV–vis, PL spectroscopy, cyclic voltammetry (CV), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), respectively. CV measurements show that the compounds present suitable HOMO values (in a range of −4.63 to −5.11 eV) for hole injection, which is confirmed by theoretical calculations. All compounds were thermally stable. Organic light-emitting diode devices having 6f, 6g, and 6h as a hole transporting layer showed better performance of maximum brightness, turn-on voltage, and maximum luminous efficiency than a comparable device NPB. These compounds could be excellent candidates for applications in OLED devices.

Synthesis and properties of oligofluorene-thiophenes as emissive materials for organic electroluminescent devices: color-tuning from deep blue to orange

A series of oligofluorene-thiophenes end-capped with 3,6-di-tert-butylcarbazole and pyrene were designed and synthesized for application as color tunable emissive materials for organic electroluminescent devices. They were characterized by 1H NMR, 13C NMR, FT-IR, UV–vis, PL spectroscopy, and mass spectrometry. Theoretical calculations revealed that the carbazole moiety attached to the end of the molecule is nearly perpendicular to oligofluorene-thiophene-pyrene plane and π-electrons in the ground state delocalize over the entire molecule. Their optical, thermal, and electrochemical properties could be tuned by varying the number of thiophene units in the molecule. All were electrochemically and thermally stable molecules. OLED devices of these materials emitted brightly in various colors from deep blue to orange. Particularly, deep blue (CIE coordinates of 0.16, 0.14) and green (CIE coordinates of 0.27, 0.61) devices showed high color quality close to the NTSC standards with high luminance efficiencies of 1.14 and 11.15 cd/A, respectively.

Synthesis, Properties and Applications of Biphenyl Functionalized 9,9‐Bis(4‐diphenylaminophenyl)fluorenes as Bifunctional Materials for Organic Electroluminescent Devices

AbstractFour new bifunctional materials, namely BPTF, CBPTF, CMBPTF and BPVTF, having 9,9‐bis(4‐diphenylaminophenyl)fluorene as a molecular platform and biphenyl derivatives as end‐capping substituents were synthesized and characterized. Their optimized structures revealed that both triphenylamine moieties at the C‐9 position of the fluorene ring generated a bulky molecular structure. These molecules showed strong blue emission in both solution and in the solid state, and were thermally stable amorphous materials with glass transition temperatures up to 207 °C. The abilities of these materials to act as blue‐light‐emitting materials for blue OLEDs and hole‐transporting materials for green OLEDs in terms of device performance and thermal properties were superior to commonly used N,N′‐diphenyl‐N,N′‐bis(1‐naphthyl)‐(1,1′‐biphenyl)‐4,4′‐diamine (NPB). Efficient, and bright nondoped deep‐blue and Alq3‐based green OLEDs with maximum luminance efficiencies and CIE coordinates of 2.48 cd A–1 and x = 0.15 and y = 0.07, and 4.40 cd A–1 and x = 0.28 and y = 0.52 were achieved, respectively, with BPTF having two biphenyl substituents as active layers. Both devices showed low turn‐on voltages of 3.1 and 2.8 V, respectively. Notably, the color purities of these deep‐blue and green devices were close to the National Television Standards Committee blue (CIE coordinates of x = 0.14 and y = 0.08) and green (CIE coordinates of x = 0.26 and y = 0.65) standards.

Electron‐transporting semi‐rigid polyester‐imides made up of a tetracarboxdiimide of p‐terphenyl analogue of 1,3,4‐oxadiazole: Preparation, and thermal, optical, and electrochemical properties

AbstractElectron‐transporting semi‐rigid polyester‐imides composed of a tetracarboxdiimide (2,5‐di(3,4‐dicarboxphthalimido)‐1,3,4‐oxadiazole; DDPOD) of p‐terphenyl analogue of 1,3,4‐oxadiazole were successfully prepared by the melt polycondensation of a monomer, a dialcohol derivative of DDPOD, with four diesters in the presence of zinc acetate as a catalyst, and their thermal, UV–vis absorption, photoluminescent (PL) and electrochemical properties were examined. The structures of the resulting polymers were characterized by Fourier transform infrared and 1H NMR spectroscopies and elemental analyses. Differential scanning calorimetry measurements, X‐ray diffraction analyses and polarizing microscope observations suggested that the polymers prepared from aliphatic diesters formed highly ordered thermotropic liquid crystalline smectic phases. The UV–vis and PL spectra in the solutions and in the films displayed that the obtained polymers showed absorption and bluish violet‐light emissions based on the DDPOD moiety. Cyclic voltammetry measurements revealed that these polymers had more stable HOMO levels and were more sensitive to oxidation than polymers comprising electron‐accepting 2,5‐diphenyl‐1,3,4‐oxadiazole in the backbones. The DDPOD unit brought the extension of differences between electron‐injection and hole‐injection barriers in the polymers. It was found that the polymers containing the DDPOD unit in the main chains in this study not only showed bluish violet light‐emitting properties but also behaved as electron‐transporting materials for organic electroluminescent devices. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Carbazole dendronised triphenylamines as solution processed high Tg amorphous hole-transporting materials for organic electroluminescent devices

Carbazole dendrimers up to 4th generation were synthesized. They showed significantly high T(g), amorphous and stable electrochemical properties, and great potential as solution processed hole-transporting materials for OLEDs. Alq3-based green devices exhibited high luminance efficiency and CIE coordinates of 4.45 cd A(-1) and (0.29, 0.53), respectively.

Synthesis and Properties of Diphenylquinoxaline as Electron-Transporting Materials

Bis-(2,3-diphenylquinoxaline) (DDPQL) has been synthesized by condensation reaction with 3,3'-diaminobenzidine and benzil. The thermal and electrical properties of DDPQL were measured as well. Organic electroluminescent device using DDPQL as electron-transporting layer was fabricated. The structure of the organic electroluminescent device is ITO/α-NPD/ Alq/DDPQL/LiF/Al. The maximum brightness of the electroluminescent device at voltage of 13.5V is 12,100cd/m², the maximum current efficiency at voltage of 13.0V is 4.97cd/A, the external quantum efficiency is 1.92%, and the lowest drive-voltage is 3.5V. This indicates that the compounds are useful as electron-transporting materials in organic electroluminescent devices.

Luminescence of Zn2L2- and PVC-based organic molecular compositions under photo- and electroexcitation

1 Spectral and luminescent properties of Zn2L2 complex {mu-2-bis((3-pyridine-2-il)-5-(2-salicyldeniminophenyl)- 1,2,4-triaurum(2-))dizinc(2+)solvate with ethanol} are investigated in solutions, powder, and thin polyvinylcarbazole (PVC) films. The trends in their change versus the nature, aggregate state, and excitation method are established. Electroluminescence (λmаx = 493 nm) of an ITO/PEDOT/PVC : Zn2L2/LiF/CaMg/Ag device is excited, and its current-voltage and brightness-voltage characteristics are investigated. Nowadays organic electroluminescent devices (OELD) are very popular due to numerous applications, in particular, in flat displays. They represent multilayered sandwich structures capable of converting the electric energy into light. The simplified sandwich structures comprise anode/emitting layer/cathode. Their main element is the material of the light-emitting layer. The potential material of the light-emitting layer of the electroluminescent device must have high photoluminescence intensity, thermal stability, and capability of forming thin film coatings (100 nm). From the viewpoint of further commercial production of electroluminescent devices, the material must have low cost and simple procedure of synthesis. In the last decade, the active search is on for new thin-film electroluminescent materials based on metalloorganic coordination compounds. The combination of metal ions and organic ligands in one molecule and the opportunity of purposeful change of the composition and structure of coordination compounds are favorable to the production of molecular materials for OELD with efficient electroluminescence in a wide range of wavelengths on their basis (1, 2). A special place among organic luminescent materials belonging to different classes is occupied by metal complexes with ligands comprising the C = N azomethine bond and electron-donor oxygen and/or nitrogen atoms. They are distinguished by the ease of production and thermal stability; they are capable to form thin layers and to transport electrons in them. In most cases, thin layers of organic substances of different natures are formed on the anode surface either by the vacuum deposition or centrifuging method. In the latter case, compositions including an electroactive low-molecular organic substance, an electroactive polymer, and a solvent are used. In these emitting layers, the polymeric matrix is a solid solvent for the luminophor. The processes of energy and charge transfer can proceed in the layers. At present, a search for photo- and electrostable luminophor: polymer pairs with high emissivity is urgent. However, it is difficult to develop the OELD with high efficiency and long lifetime combining the two methods of thin-film structure production (centrifuging and thermal vacuum deposition) in one device, since under laboratory conditions, it is often impossible to prevent the interaction of thin-film structures with oxygen and moisture in the

P-Doped p-phenylenediamine-substituted fluorenes for organic electroluminescent devices

Two novel p-phenylenediamine-substituted fluorenes have been designed and synthesized. Their applications as hole injection materials in organic electroluminescent devices were investigated. These materials show a high glass transition temperature and a good hole-transporting ability. It has been demonstrated that the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped p-phenylene-diamine-substituted fluorenes, in which F4-TCNQ acts as p-type dopant, are highly conducting with a good hole-transporting property. The organic light emitting devices (OLEDs) utilizing these F4-TCNQ-doped materials as a hole injection layer were fabricated and investigated. The pure Alq 3-based OLED device shows a current efficiency of 5.2 cd/A at the current density of 20 mA/cm 2 and the operation lifetime is 1500 h with driving voltage increasing only about 0.7 mV/h. The device performance and stability of this hole injection material meet the benchmarks for the commercial requirements for OLED materials.

Revisitation of Cycloheptatriene Derivatives as a Building Block for Various Substituted and Fused 1,6-Methano[10]annulenes and Substituted 4,9-Methanothia[11]annulenes

A synthetic maneuver from 1,6-diacetyl- and 1,6-diformyl-1,3,5-cycloheptatrienes toward various 1,6-methano[10]annulenes, such as diaryl-substituted, cyclobutene-annulated, and thiophene-annulated derivatives and its quinodimethane-type compound, and 4,9-methanothia[11]annulenes is spotlighted based on our recent research efforts. The crystal structure analysis of the annulenes, discussion of mechanistic details of a key cyclization step and application of some annulene derivatives to a material in organic electroluminescent devices are also described. Keywords: cyclobutenes, quinodimethanes, thiophenes, X-ray crystallographic analysis, molecular orbital calculations, organic electroluminescent device

Synthesis of N,N,N′,N′‐Tetrasubstituted 1,3‐Bis(4‐aminophenyl)azulenes and Their Application to a Hole‐Injecting Material in Organic Electroluminescent Devices.

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Synthesis of N, N, N′, N′-tetrasubstituted 1,3-bis(4-aminophenyl)azulenes and their application to a hole-injecting material in organic electroluminescent devices

After a preliminary search of the reaction conditions for the Suzuki–Miyaura cross-coupling of haloazulenes with arylboronic acids, the title compounds were synthesized either by the direct coupling reaction between 1,3-dihaloazulene and the corresponding N, N-disubstituted 4-aminophenylboronic acids or by a two-step sequence involving the cross-coupling with 4-bromophenylboronic acid and subsequent Pd-catalyzed amination. Application of the title diamines to a hole-injecting material in organic electroluminescent devices was carried out to provide their prominent characteristics as a novel durable, non-cyanine and non-polyamine substance without color fade. The diamine derivatives, extended by an ethynyl unit between the azulenyl core and the 4-aminophenyl moiety, were also synthesized and found, unfortunately, unsuitable for vacuum deposition in preparing a multilayer composite.

Study on photoluminescence from tris-(8-hydroxyquinoline)aluminum thin films and influence of light

Tris-(8-hydroxyquinoline)aluminum (Alq3), which is the most widely used material in organic electroluminescent devices, has been synthesized. Alq3 thin films have been deposited on glass and silicon substrates. The influence of light exposure on the optical properties of Alq3 thin films has been studied. It is confirmed that the photoluminescence (PL) of Alq3 thin film originates from its two geometrical isomers, namely, facial and meridional, which result from PL decay analysis (biexponential fit). It is also confirmed that the PL from both the isomers decreases for increasing light exposure time leading to the creation of luminescent quencher in Alq3 thin film.

Synthesis and Luminescent Properties of Bis(fluoren-9-ylidenemethyl)aromatics: Green and Red Materials in Organic Electroluminescent Devices

Derivatives of 4,4'-bis(2,2-diphenylvinyl)biphenyl (DPVBi) were prepared and investigated for application in green and red fluorescent emitters in organic electroluminescent (EL) devices. Although the green emitter showed yellowish green electroluminescence, a series of π-conjugated compounds revealed the potential for a red-green-blue (RGB) fluorescent system.

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light‐emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.

Cross‐Coupling Reactions in the Chemistry of Silole‐Containing π‐Conjugated Oligomers and Polymers.

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Preparation, Thermotropic Liquid-Crystalline and Fluorescent Properties of Semi-Rigid Homo- and Copoly(ester-imide)s Composed of 3,3”,4,4”-p-Terphenyltetracarboxdiimide and 3,3’,4,4’-Biphenyltetracarboxdiimide

Semi-rigid homo- and copoly(ester-imide)s were prepared from bismethyl ester of 3,3”,4,4”-p-terphenyltetracarboxdiimide and bisalcohols of 3,3’,4,4’-biphenyltetracarboxdiimide by melt polycondensation and their thermotropic liquid-crystalline, photo- (PL) and electroluminescent (EL) properties were investigated. Differential scanning calorimetry (DSC) measurements, polarizing microscope observations of textures and powder X-Ray analyses suggested that homopolymer having hexamethylene chain form monotropic smectic C or A phase and most of copolymers enantiotropic nematic phase. PL spectra showed that the polymers emitt blue light in the chloroform solutions and in the films. EL spectra of polymers in double-layer devices (ITO/poly(3,4-ethylene dioxythiophene) (PE DOT)/polymer/LiF or Ca/Al), with blue emission, were almost identical to the PL spectra, although luminances were very low. The poly(ester-imide)s can be used as blue light-emitting and/or electron-transporting materials for organic EL devices.