Materials For Electroluminescent Devices Research Articles

Bright Single-Layer Perovskite Host–Ionic Guest Light-Emitting Electrochemical Cells

Perovskite light-emitting devices have drawn considerable attention for their favorable optoelectronic properties. High carrier mobilities make perovskites excellent candidates as host materials in electroluminescent devices, but perovskites have yet to be brought to their full potential in this role. To achieve high performance in a simple single-layer device, we employed a CsPbBr3 perovskite host and a novel ionic iridium complex guest along with a polyelectrolyte to demonstrate efficient light-emitting electrochemical cells (PeLECs). For an optimal guest/host blend, 10600 cd/m2 luminance at 11.6 cd/A and 9.04 Lm/W is achieved at 4.1 V, demonstrating greater than a 2-fold overall improvement of previous efforts and over a 2-fold efficiency enhancement over host-only devices. These devices showed a range of electroluminescence color proceeding from orange-red to green, facilitated by the reconfigurable ionic materials blend. Optimized devices exhibited stable operation under constant current driving, maintaining >630 cd/m2 emission for 40 h. Our rationally designed ionic guest at an optimal concentration of the host produced efficient (>90%) Förster energy transfer and improved thin film morphologies for high-performance PeLEC operation enabled by ionic migration to interfaces.

A photophysical and spectroelectrochemical study on N-phenyl-carbazoles and their oxidized species

A series of N-phenyl-carbazole derivatives namely TCz (N-tolyl-carbazol), TCz-TCz (a 3,3´-linked dimer), and CzPh-CH2-PhCz (a methylene-bridged N-phenyl-carbazole), have been synthesized and studied by means of absorption and fluorescence spectra, DFT calculations, cyclic voltammetry and spectroelectrochemistry. It is concluded that the methylene bridge in CzPh-CH2-PhCz isolates the two TCz units, whereas TCz-TCz allows extending the electronic coupling along both carbazoles. Moreover, the phosphorescence spectrum of TCz-TCz showed a high energy triplet state (2.76 eV). It was observed that oxidation of CzPh-CH2-PhCz generates an electrochromic polymer containing TCz-TCz dimer units separated by methylene bridges. Oxidized TCz-TCz subunit was analyzed by EPR spectroscopy, and found to be a Class III mixed valence ion in Robin and Day classification. All results suggest that these molecules might be suitable as selective probes in fluorescence sensing or as host materials in electroluminescent devices.

Hole-Buffer Material Derived from Pyrene, Schiff Base and Tris to Enhance Emission Efficiency of Polymer Light-Emitting Diodes

Inserting a hole-buffer layer is an effective way to enhance emission efficiency of electroluminescence devices. We have successfully synthesized a new hole-buffer material PSB composed of pyrene, Schiff base and trihydroxy tert-butyl groups by the Suzuki-coupling reaction. The HOMO and LUMO lev-els were -6.33 eV and -2.55 eV, respectively, as estimated from cyclic volt-ammograms. In addition, homogeneous films (rms roughness ~2 nm) were readily obtained by spin-coating process. Multilayer polymer light-emitting diodes, ITO/PEDOT:PSS/PSB/SY/LiF/Al, have been fabricated using PSB as hole-buffer layer (HBL). Inserting PSB as HBL significantly enhances the per-formance (maximum luminance: 26,439 cd/m2, maximum current efficiency: 7.03 cd/A), compared with the one without PSB (9802 cd/m2, 2.43 cd/A). It is also superior to the device with conventional BCP as hole-blocking layer (ITO/PEDOT:PSS/SY/BCP/LiF/Al: 15,496 cd/m2, 5.56 cd/A). Current results strongly indicate that the PSB is a potential hole-buffer material for electrolu-minescent devices.

Efficient non-doped bluish-green organic light emitting devices based on N1 functionalized star-shaped phenanthroimidazole fluorophores

In this paper, two star-shaped fluorescent phenanthroimidazole fluorophores, tris(4-(1-(3-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIMCFTPA) and tris(4-(1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIPCFTPA) with a D–π–A structure were designed and synthesized by attaching a hole-transporting triphenylamine and an electron transporting phenanthroimidazole moiety. A detailed photophysical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical investigations (DFT) were performed to get a better understanding of the electronic structures. In particular, PIMCFTPA shows blue shifted emission due to the most twisted conformation and reduced intermolecular interaction as compared with PIPCFTPA. Both the fluorophores exhibit high glass transition temperatures and high thermal stabilities with decomposition temperatures up to 377°C. The excellent stability renders them promising materials in electroluminescent devices. Non-doped organic light-emitting devices (OLEDs) using these fluorophores as emissive materials display bluish green emissions with high efficiency (6.58cdA−1, 5.91lmW−1, 3.62% at 100cdm−2), low turn-on voltages (2.83V) (PIMCFTPA) and excellent spectral stability. Our results suggest that the molecular design strategy of integrating TPA with phenthroimdazole play an important role in the device performance.

Synthesis, Structure, Photoluminescence, and Electroluminescence of Four Europium Complexes: Fabrication of Pure Red Organic Light‐Emitting Diodes from Europium Complexes

Four new europium complexes were prepared by treating europium(III) trifluoro(thenoyl)acetonate trihydrate with new tridentate ligands, based on dipyrazolyltriazine and utilized as emitting materials in electroluminescent devices. The complexes were characterized by elemental analysis, FTIR spectroscopy, UV/Vis spectrophotometry, and 1H NMR spectroscopy. The coordinated ligands serve as light‐harvesting chromophores in the complexes, with absorption maxima in the range 334–402 nm [ε = (8.9–81.4) × 103 m–1 cm–1]. The ligands efficiently sensitize europium luminescence, with maximum quantum‐yield (QLEu) and observed lifetime (τobs) values of 34 % and 600 µs in the solid state and 35 % and 528 µs in toluene, respectively. The radiative lifetimes of Eu (5D0) are in the range 1170–1281 µs and the ligand‐to‐metal energy‐transfer efficiency (ηsens) is in the range 71–92 % for those complexes in the solid state and in the range 68–97 % for those in solution. Additionally, organic light‐emitting diodes (OLEDs), which exhibited pure red emission, were fabricated with europium(III) complexes. It is shown that by modifying the [Eu(tta)3·3H2O] molecule with ancillary ligands, one can tune and control its electroluminescence spectra, along with its electrical properties, such as the current/voltage characteristics of OLED devices based on [Eu(tta)3(L)]. The best OLED presented a maximum luminance of 3156 cd/m2 and a maximum efficiency of 0.7 cd/A at an applied voltage of 8 V.

Effect of fluorocarbon (trifluoromethyl groups) substitution on blue electroluminescent properties of 9,9′-bianthracene derivatives with twisted intramolecular charge-transfer excited states

By introducing a highly fluorescent 9,9′-bianthracene, fluorocarbon (trifluoromethyl groups) with the different linkage mode on the periphery of the 9,9′-bianthracene core, two novel blue-emissive materials (BAn-(3)-CF3 and BAn-(4)-CF3) were synthesized and characterized. The high luminescence efficiency and excellent stability rendered both promising materials in electroluminescent devices. The twisted intramolecular charge transfer state was demonstrated to enhance the occurrence of singlet excitons in the fluorescent emitter, which was based on the 9,9′-bianthracene moiety. Specifically, the meta-linkage BAn-(3)-CF3 device doped with sky blue fluorescent material 4,4-bis[4-(di-p-tolylamino)styryl]biphenyl showed that the singlet generation fraction was more than 25%, and presented an impressive luminous current efficiency of 5.88 cd A−1 and an external quantum efficiency of 3.15% with a high luminance of over 12,000 cd m−2. Noticeably, the para-linkage BAn-(4)-CF3 device exhibited an excellent efficiency stability with almost unchanged roll-off on the current and power efficiencies, even when the brightness approached 12,000 cd m−2.

Synthesis and Characterization of Carbazole Dendrimers as Solution‐Processed High Tg Amorphous Hole‐Transporting Materials for Electroluminescent Devices

AbstractWe report on an efficient and facile synthesis of up to 4th generation carbazole dendrons by using a mild Ullmann coupling reaction. The dendrimers, namely GnC (n = 1–4), are thermally stable amorphous materials with high glass transition temperatures (Tg) up to 347 °C and exhibit chemically stable redox properties. The compounds show great potential to work as solution‐processed hole‐transporting materials for electroluminescent devices. Double‐layer Alq3‐based organic light‐emitting diodes employing these dendrimers as the hole‐transporting layers with the device configuration of ITO/PEDOT: PSS/GnC(spin‐coating)(40 nm)/Alq3(50 nm)/LiF(0.5 nm):Al(150 nm) emit a bright green color (λem = 514 nm) from the Alq3 layer, with maximum luminance, maximum current efficiency, and turn‐on voltage of 29,262 cd m–2, 5.11 cd A–1, and 3.0 V, respectively. The abilities of GnC to act as hole‐transporting materials in terms of thermal property and device performance are superior to common hole‐transporters, N,N′‐diphenyl‐N,N′‐bis(1‐naphthyl)‐(1,1′‐biphenyl)‐4,4′‐diamine (NPB) and N,N′‐bis(3‐methylphenyl)‐N,N′‐bis(phenyl)benzidine (TPD).

Copolymers of carbazole- and indolocarbazole-containing phenylquinolines as new materials for electroluminescent devices

The photophysical and electrical properties of copolymers based on 2,6-polyphenylquinolines containing a bridging oxygen or phenylamine group between quinoline rings and octyl- or dodecylcarbazole or dioctylindolo[3,2-b]carbazole fragments in the aromatic radical are studied. It is shown that the introduction of carbazole and indolo[3,2-b]carbazole fragments emitting in the visible spectral range into the aromatic radical of a copolymer makes it possible to obtain white luminescence. The intensity of this luminescence exceeds, for copolymers in a photoinert matrix, that of the initial homopolymers. This circumstance, combined with rather high carrier mobilities on the order of 5 × 10−6 cm2 V−1 s−1, makes the copolymers promising for the development of electroluminescent devices on their basis.

Syntheses and characterization of 6,6′-diaryl-9,9′-dialkyl[3,3′]bicarbazoles as materials for electroluminescent devices

Electro-active materials containing 6,6′-diaryl-substituted [3,3′]bicarbazole core were synthesized by multistep synthetic rote and characterized. The derivatives were examined by differential scanning calorimetry and demonstrated formation of amorphous materials with rather high glass transition temperatures. The synthesized compounds have been tested as hole transporting layers in simple OLED devices with Alq3 as the emitter/electron transporting layer. The green devices containing hole transporting films of diphenyl-9,9′-diethyl-[3,3′]bicarbazole exhibited the best overall performance (turn-on voltage: 3.5V, maximum photometric efficiency: 4.2cd/A, maximum brightness: ∼12,200cd/m2).

Green, near-infrared electroluminescence of novel yttrium tetrazole complexes

Four novel yttrium complexes with the molecular formula Y(Tz)n(x)2(SCN)3−n(H2O), n = 2, 3 and x = phen, bpy, were synthesized by the reactions of yttrium salt with 5-phenyl tetrazole (Tz), 1,10-phenanthroline (phen), 2,2-bipyridine (bpy) and thiocyanate ions, and were used as the emitting material in electroluminescent devices. The structures of these complexes were determined by UV-visible and FT-IR spectroscopy, 1H NMR, CHN analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The monodentate mode of the Tz complexes was investigated by FT-IR spectroscopy. The different structures of the compounds suggest the importance of the tetrazole and auxiliary ligands in the emission properties of the yttrium complexes. It was found that the Y–polypyridine complexes dressed by tetrazole antennas have highly efficient intra-energy transfer. The Y complexes exhibited broad ligand-centered electroluminescence with maximum near 550 nm, and near-IR electroluminescence at 800, 900 and 950 nm. The electroluminescence spectra of the yttrium complexes indicated a long red shift in the visible region compared to the PVK:PBD blend. The reaction details and features were described and discussed.

Synthesis and characterization of 9,10-substituted anthracene derivatives as blue light-emitting and hole-transporting materials for electroluminescent devices

New 9,10-substituted anthracene derivatives were designed and synthesized for application as blue-emitting and hole-transporting materials in electroluminescent devices. They were characterized by H NMR, C NMR, FTIR, UV–vis, PL spectroscopy, and mass spectrometry. The theoretical calculation of three-dimensional structure and the energy densities of HOMO and LUMO states, as well as optical properties of these new obtained materials, supported the claim that they had non-coplanar structures. Their optical, thermal, and electrochemical properties could be tuned by varying the peripheral substituents. All of them were electrochemically and thermally stable molecules. Materials having electron donating triphenylamine as peripheral substituents showed promising potential as both blue light-emitting materials and hole-transporting materials for electroluminescent devices. Efficient blue and Alq3-based green OLEDs with maximum luminance efficiencies and CIE coordinates of 1.65cd/A and (0.15, 0.16) and 6.25cd/A and (0.26, 0.49) were achieved, respectively.

Synthesis of copolyfluorenes with high fluorenone contents and its application in electroluminescent device by simple blending

Poly(fluorene-co-fluorenone)s containing high contents of fluorenone chromophore were synthesized by the Suzuki coupling reaction to study their electroluminescent (EL) properties. The copolymers are thermally stable below 430°C (Td) in nitrogen atmosphere. In film state, their absorption and photoluminescence spectra (peaked at 373–382 nm and 562–564 nm, respectively) are mainly originated from fluorenone units, because of its efficient energy transfer. Both LUMO and HOMO energy levels, estimated from their cyclic voltammograms, are lowered slightly (−3.17−3.23 eV, −5.84−5.89 eV) with increasing contents of electron-withdrawing fluorenone units. Double-layer EL devices, using the copolyfluorenes or their blends with poly(9,9-dihexylfluorene) (PF) as emitting layer, show exclusive emission originated from fluorenone chromophore (565 nm) when its content is high. Blending 0.02–5 wt % of PF-33 (fluorenone fraction: 0.37) with PF greatly enhances device performance (610 cd m−2 4400 cd m−2 and 0.45 cd A−1 1.52 cd A−1). Our results demonstrate that the copolyfluorenes are promising emitting materials for EL devices by simple blending. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Single crystal X-ray structure and optical properties of anthraquinone-based dyes

AbstractThis study focuses on the preparation, single crystal X-ray diffraction, characterization, and optical properties of some anthraquinone-based dyes. The anthraquinone-based antimicrobial dye N-{2-[(9,10-dioxo-9,10-dihydroanthracen-1-yl)amino]-2-oxoethyl}-N,N-dimethylbutan-1-aminium chloride monohydrate (III) was obtained from 1-aminoanthraquinone (I) via 2-chloro-N-(9,10-dioxo-9,10-dihydroanthracen-1-yl)acetamide (II) using known preparation and characterization methods. Single crystal X-ray diffraction analysis of III revealed a monoclinic system, space group P21/n, Z = 4. Photoluminescence properties of anthraquinone dyes I–III were also investigated. These dyes gave an intense emission (λmax = 341 nm) upon the irradiation by UV light and showed photoluminescence quantum yields of 73 %, 66 %, and 61 % with long excited-state lifetimes of 6.87 ns, 6.14 ns, and 5.69 ns, respectively. These anthraquinone dyes are of interest as an organic light emitting material for electroluminescent devices.

Synthesis, thermal, electrochemical, and photophysical characterization of 1,5-bis(diarylamino)naphthalene derivatives as potential hole transport OLED materials

Novel 1,5-bis(diarylamino)naphthalene derivatives (1–9), which have potential as hole-transporting materials for electroluminescent devices, were obtained through palladium-catalyzed coupling of diarylamines and 1,5-dibromonaphthalene. The thermal, electrochemical, and photophysical properties of these compounds were examined and the effects of the N-aryl substituents on these properties were investigated. These materials possess glass transition temperatures (Tg) that range from 70–131 °C and these values are related to the identity of the aryl substituents. Cyclic voltammetric measurements demonstrated that these compounds possess two reversible oxidation processes and were further used to estimate the HOMO energy levels of these materials by comparison with the ferrocene/ferrocenium couple. The intramolecular charge mobility, as gauged by the difference between the two oxidation potentials, indicates that compounds 1–8 have a similar degree of delocalization as meta-diaminobenzene derivatives while that of 9 is similar to TPD. Compounds 1–9 emit in the blue-green region and optical absorption and emission data for these materials can be rationalized in terms of the electronic donating properties of the aryl substituents. This data when combined with the electrochemically determined HOMO energies allowed estimation of the LUMO energy levels.Key words: hole transport materials, diaminonaphthalene, thermal analysis, palladium-catalyzed arylation.

New Organic Electroluminescent Materials

ABSTRACT Spectral and electric properties of five new materials for electroluminescent devices are described including materials for emitting layers and a high-Tg hole-transporting material.

(1-Naphthyl)phenylamino functionalized three-coordinate organoboron compounds: syntheses, structures, and applications in OLEDs

New three-coordinate organoboron compounds functionalized by a (1-naphthyl)phenylamino group, B(mes)2(dbp-NPB) (1), B(db-NPB)3 (2), and B(dbp-NPB)3 (3), have been synthesized. A variable temperature 1H NMR study showed that the aryl groups around the boron center in these compounds have a rotation barrier ∼70 kJ mol−1. The new boron compounds are amorphous solids with Tg being 110 °C, 171 °C and 173 °C, respectively. The electronic properties of the new boron compounds were investigated by cyclic voltammetry and UV–visible spectroscopy. All three boron compounds are blue emitters in the solid state. In solution the emission spectra of the boron compounds shift toward a longer wavelength with increasing solvent polarity. In CH2Cl2, the emission quantum efficiency of the three compounds was determined to be 0.22, 0.27 and 0.23, respectively. Several series of electroluminescent (EL) devices where compounds 1–3 are used as either an emitter/electron transport material, a hole transport material, or a hole injection material have been fabricated and their performance has been compared to the corresponding devices of BNPB, a previously investigated molecule, NPB, a commonly used hole transport material, and CuPc, a commonly used hole injection material. The EL results indicate that the new boron compounds are not suitable as emitters/electron transport materials, but they are promising as hole transport and hole injection materials in EL devices.

Dendrimer complexes: Fine control of metal assembly in macromolecules

New phenylazomethine dendrimers have been developed, with each dendrimer having a unique single structure. From ultraviolet-visible absorption spectra, we determined that stannous chloride added to dendritic polyphenylazomethine (DPA) was assembled in a stepwise manner from the core shell to the periphery. The selective binding was achieved by the electron density gradient formed in the dendrimer, and it was confirmed by shell-selective imine reduction, transmission electron microscopy, and NMR measurements. The complex formation constants of the respective shells were estimated to be considerably different by a simulation analysis of ultraviolet-visible absorption spectra. The reversible assembly and release of iron into and out of the dendrimer were possible by electrochemical oxidation and reduction, respectively. DPA with a porphyrin at the core was found to function as an effective reduction catalyst of carbon dioxide. DPA with aryl amine at the core was demonstrated to function as a hole-transporting material in electroluminescent devices.

From Blue to Red: Syntheses, Structures, Electronic and Electroluminescent Properties of Tunable Luminescent N,N Chelate Boron Complexes

AbstractA comprehensive study of a series four‐coordinate boron compounds with the general formula of BPh2(N,N), where N,N are bidentate chelate ligands containing both neutral and negatively charged nitrogen donor atoms has been conducted. The structures of the boron complexes were examined via single‐crystal X‐ray diffraction. The series of molecules display bright luminescence with emission maxima λmax ranging from blue to red, depending on the nature of the N,N chelate ligand. The electronic effects and their consequences on the luminescent properties of the complexes due to the CH replacement of the chelate ligand by a nitrogen atom, the increase of conjugation, or the change of substituents on the chelate ligand have been examined using electrochemical analysis, UV‐visible, and fluorescence spectroscopic methods, and by molecular orbital calculations (Gaussian 98). Experimental data and MO calculation results established that the emission of this class of compounds is caused by π–π* transitions centered on the chelate ligand. Furthermore, the experimental and theoretical results consistently and conclusively established that electron withdrawing groups on the negatively charged N‐donor portion of the chelate ligand causes a decrease in the highest occupied molecular orbital (HOMO) energy level, thus increasing the energy gap. The CH replacement by a nitrogen atom on the negatively charged portion of the chelate ligand causes a dramatic decrease of the HOMO energy level, and the increase of conjugation in the chelate ligand significantly decreases the energy gap. Blue and red electroluminescent (EL) devices were fabricated successfully using two representative boron compounds from the series. The new boron compounds have been found to be able to function as both emitters and electron transport materials in EL devices.

Synthesis and properties of PPP-based copolymers containing phenothiazine moiety

Novel soluble alternating conjugated copolymers (PFSP and PFSR) comprised of phenothiazine unit are synthesized by palladium-catalyzed Suzuki coupling reactions. Their thermal stability, photoluminescence, electroluminescence, hole injection and transport properties are investigated. The resulting copolymers exhibit good thermal stability and excellent hole-injection ability (about -5.2eV), which are closely matched to the work function of ITO. Double-layer devices demonstrate that PFSP is a promising hole-transporting material for electroluminescent devices.

Non‐amine‐Based Furan‐Containing Oligoaryls as Efficient Hole Transporting Materials.

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