Double-layer Electroluminescent Devices Research Articles

Highly efficient blue phosphorescent and electroluminescent Ir(iii) compounds

Three new Ir(III) compounds with deep-blue phosphorescence have been synthesized. These molecules have the general formula of Ir(C∧N)2(L∧X), where C∧N = 2′,6′-difluoro-2,3′-bipyridine (dfpypy) and L∧X = ancillary ligand such as 2-picolinate, pic (1), acetylacetonate, acac (2), or dipivaloylmethanoate, dpm (3). The ancillary ligands have been found to significantly destabilize both HOMO and LUMO levels of the Ir(III) complexes, compared to Ir(dfpypy)3, without significantly changing the phosphorescence energy. Compounds 1–3 emit bright blue phosphorescence with λmax = 440–460 nm and quantum efficiencies of 0.60–0.95 in solution and the solid state. Double-layer electroluminescent devices using compounds 1–3 as the dopant, CDBP (4,4′-bis(9-carbazolyl)-2,2′-dimethylbiphenyl) as the host/hole transporting layer, and TPBi (1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene) as the electron transport layer have been fabricated. These EL devices show pure blue colour with high efficiency. The EL device of compound 3 at the doping level of 20 wt% shows the best performance with EQE of 10–15% at the brightness of 10–1000 cd m−2 and the maximum current efficiency of 22 cd A−1.

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

Multifunctional Hyperbranched Oligo(fluorene vinylene) Containing Pendant Crown Ether: Synthesis, Chemosensory, and Electroluminescent Properties

Multifunctional hyperbranched oligo(fluorene vinylene) (HOFC) containing pendant crown ether was synthesized from 2,4,7-trivinyl-9,9-dihexylfluorene and 2,7-dibromo-9-(15-crown-4)-9H-fluorene via Heck coupling reaction. Their chemosensory, photophysical, and electrochemical properties were investigated and compared with those of linear oligo(fluorene vinylene) (LOFC) to elucidate the effect of hyperbranched structure. Both HOFC and LOFC exhibit selective fluorescence quenching toward Fe3+ and Ru3+, with the Stern−Volmer coefficients (Ksv) to Ru3+ being 2 × 104 and 1.9 × 104 M−1, respectively. The stability constant (Ks) of forming complex with Ru3+ are 3.1 × 103 and 5 × 103 M−1 for HOFC and LOFC. Moreover, hyperbranched HOFC reveals homogeneous film morphology due to its hyperbranched structure. After thermal treatment, the cured polymer (HFC) shows better thermal stability because of higher cross-linked density. Double-layer electroluminescent devices (ITO/PEDOT:PSS/HFC or LFC/Ca/Al), using thermally cross-linked HFC or LFC as emitting layer, were fabricated to investigate their optoelectronic properties. The turn-on voltage, maximum luminance and maximum luminance efficiency of HFC device (4.1 V, 7132 cd/m2 and 1.3 cd/A) are superior to those of LFC device (6.2 V, 331 cd/m2, 0.22 cd/A), which have been attributed to its homogeneous film morphology. Current results indicate that the hyperbranched oligo(fluorene vinylene) is a promising material for chemosensors and electroluminescent devices.

Synthesis, photophysical and electroluminescent properties of donor–acceptor–donor molecules based on α-cinnamoyl cyclic ketene dithioacetals

New symmetrical π-conjugated donor–acceptor–donor functionalized molecules consisting of different electron-donating moieties but same α-cinnamoyl cyclic ketene dithioacetals (CCKDA) acceptor have been synthesized. The photophysical properties of these compounds indicate that the absorption and emission result from the intramolecular charge-transfer between the donor and the acceptor. Cyclic voltammetry data show that the band gaps of the compounds vary by introducing different donor moieties. Density functional theory calculations provide an insight into the electronic structure of the molecules. The double-layer electroluminescent devices are fabricated by using these molecular materials as light emitting layer. The electroluminescent performance of the device suggests that these donor–acceptor–donor molecules can be good candidates for the application in organic light-emitting diodes.

2,5-Functionalized Spiro-Bisiloles as Highly Efficient Yellow-Light Emitters in Electroluminescent Devices

New spiro-bisilole molecules functionalized with nitrogen-containing heterocyclic groups including 7-azaindolyl, indolyl, and 2,2′-dipyridylamino have been synthesized. These molecules are found to display good chemical and thermal stability. They are luminescent in solution and in the solid state with an emission color ranging from blue–green to yellow, depending on the functional group. In the solid state, they display high photoluminescence quantum efficiency (32–40 %). The electroluminescence properties for one of the new molecules, 2,3,3′,4,4′,5-hexaphenyl-2′,5′-bis(p-2,2′-dipyridylaminophenyl)spiro-bisilole, have been investigated by fabricating single-layer and double-layer electroluminescent devices. The double-layer device, in which N,N′-bis(1-naphthyl)-N,N′-diphenylbenzidine acts as the hole-transport layer and the functionalized spiro-bisilole functions as the emitter (emission wavelength = 566 nm) and the electron-transport layer, displays a brightness of 8440 cd m–2 at 9 V with a current efficiency of 1.71 cd A–1. No evidence of exiplex emission is observed.

Novel Poly(2,3-diphenyl-1,4-phenylenevinylene) Derivatives Containing Long Branched Alkoxy and Fluorenyl Substituents: Synthesis, Characterization, and Their Applications for Polymer Light-Emitting Diodes

Two series of poly(2,3-diphenyl-1,4-phenylenevinylene) (DP-PPV) derivatives containing long branched alkoxy chains or fluorenyl substituents were synthesized. The branched alkoxy groups were introduced to enhance the solubility for spin-coating processes. Bulky fluorenyl groups with two hexyl chains on the 9-position were introduced to increase steric hindrance and prevent close packing of the main chain. By controlling the feed ratio of different monomers during polymerization, DP-PPV derivatives with high molecular weights were obtained. All synthesized polymers possess high glass transitions temperatures and thermal stabilities. The maximum photoluminescent emissions of the thin films are located between 498 and 564 nm. Cyclic voltammetric analysis reveals that the band gaps of these light-emitting materials are in the range from 2.4 to 2.5 eV. A double-layer electroluminescent device with the configuration of ITO/PEDOT/polymer 2/Ca/Al exhibited a low turn-on voltage (4.0 V), a very high external quantum efficiency (3.39 cd/A), and a high brightness (16 910 cd/m2).

Organoboron Compounds with an 8-Hydroxyquinolato Chelate and Its Derivatives: Substituent Effects on Structures and Luminescence

Four new luminescent organoboron complexes have been synthesized and fully characterized. These compounds are four-coordinate boron chelated by either 8-hydroxyquinolato (q) or functionalized 8-hydroxylquinolato ligands, including BPh2(5-(1-naphthyl)-q) (1), BPh2(5-(2-benzothienyl)-q) (2), B(2-benzothienyl)2q (3), and B(2-benzothienyl)2(2-Me-q) (4). All four compounds have a tetrahedral geometry as established by X-ray diffraction analyses. In solution, compounds 1-4 have an emission maximum at 534, 565, 501, and 496 nm, respectively, at room temperature. They emit similar colors in the solid states without red shifts of the emission band due to the lack of significant intermolecular interactions in the crystal lattices. The substituent group at C5 or C2 position of the 8-hydroxyquinolato ligand has been observed to have a significant impact on the emission energy and the emission quantum efficiency of the boron complexes. Molecular orbital calculations (Gaussian 98) showed that the electronic transition of 1 and 2 is a pi-pi* transition centered on the functionalized 8-hydroxyquinolato group and the electronic transition of 3 and 4 is an interligand charge transfer from the 2-benzothienyl ligand to the hydroxyquinolato ring. A double-layer electroluminescent device using 3 as the emitter has been fabricated, which produced a broad emission band with a significant contribution of exciplex emission.

Preparation of Poly(silylene-p-phenylene)s Containing a Pendant Fluorophor and Their Applications to PL Imaging

Poly(silylene-p-phenylene)s bearing an anthrylethynyl, pyrenylethynyl, or terthienyl group as the pendant were obtained in moderate yield, by substitution reactions of poly[ethoxy(methyl)silylene-p-phenylene] with the corresponding organolithium reagents in THF. The polymers exhibited photoluminescent (PL) properties in solutions as well as in the films. They were photoactive and irradiation of the films in air with a low-pressure mercury lamp led to a decrease of PL efficiencies, being applicable to PL imaging of the films. The present polymers can be used also as hole-transporting materials in double-layer electroluminescent (EL) devices and the devices with the structure of ITO/polymer film/Alq3/Mg−Ag emitted a green EL arising from Alq3 emission.

Synthesis of organosilanylene–pentathienylene alternating polymers and their application to the hole-transporting materials in double-layer electroluminescent devices

Polymers composed of an alternating arrangement of organosilanylene and pentathienylene units, [SiBu 2-X-SiBu 2-(2,5-C 4H 2S) 5] n ( 1: X=none, 2: X=O) were prepared and their application to the hole-transport in double-layer electroluminescent (EL) devices was studied. The EL devices with the structure of ITO/ 1 or 2/Alq/Mg:Ag emitted a green light resulting from Alq emission by applying the bias voltage. The device with 1 showed a lower turn-on voltage than that with 2, while almost the same maximum luminance of 4000 cd m −2 was obtained from both of the devices.

Syntheses, structures, and electroluminescence of Ln(2)(acac-azain)(4)(mu-acac-azain)(2) [acac-azain = 1-(N-7-azaindolyl)-1,3-butanedionato, Ln = Tb(III) and Y(III)

Two new luminescent lanthanide complexes Ln(2)(acac-azain)(4)(mu-acac-azain)(2) [acac-azain = 1-(N-7-azaindolyl)-1,3-butanedionato, Ln = Tb(III), 1, Y(III), 2] have been synthesized and structurally characterized. These two dinuclear complexes are isostructural with the two lanthanide ions being bridged by two acac-azain ligands. Each of the two metal ions is further chelated by four oxygen atoms from two acac-azain ligands, resulting in a coordination number eight for each metal ion. 1 displays characteristic Tb(III) emission bands while 2 displays weak blue luminescence attributable to the ligand. Single-layer and double-layer electroluminescent devices for compound 1 were fabricated, where compound 1 doped PVK layer functions as both the emitting layer and the hole transport layer and PBD functions as an electron transport layer (in the double-layer device), demonstrating that compound 1 is a promising green emitter in electroluminescent devices.

Blue emission from light-emitting diodes based on lithium complex

We report the optical and electroluminescent properties of the lithium complex of (2,3-dimethyl-8-hydroxyquinoline) lithium (LiMMq), exhibiting intense photoluminescence (PL), peaked at about 458 nm. As the result of electron-donating methyl groups in 8-hydroxyquinoline, the PL spectrum is blue-shifted relative to (8-hydroxyquinoline) lithium (Liq). The double-layer electroluminescent devices with a novel polymer poly(N,N'-diphenyl-N,N'-bis(4-methylphenyl)-1,4-phenylenediamine-1,3-diisopropenylbenzene) as hole transporting layer and the LiMMq as emitting layer, sandwiched between cathode of Mg : Ag alloy and anode of indium-tin oxide, were fabricated, and the bright blue electroluminescence with luminance of more than 8000 cd m-2 was obtained. The properties indicate that the LiMMq is a potential blue emitting material for the application in light-emitting diodes.

NARROW BANDWIDTH BRIGHT SCARLET ORGANIC ELECTROLUMINESCENT DEVICE BASED ON N,N'-BIS[4-(N,N-DIMETHYLAMONO)-BENZYLIDENE]-DIAMINOMALEONITRILE DYE

A novel red dye, N, N'-bis[4-(N,N-dimethylamino)-benzylidene]diaminomaleonitrile (BAM), was prepared by reacting diaminomaleonitrile with 4-(N,N-dimethylamino)-benzaldehyde and were characterized by 1H NMR, UV absorption and photoluminescence. The BAM dye showed an absorption peak wavelength of 530 nm and bright photoluminescence with a peak wavelength at 675 nm. It was used as the doped emitter for fabricating a bright scarlet organic electroluminescent (EL) device. The structure of the double-layer EL device consisted of a hole-transport layer and a luminescent layer between ITO glass and magnesium electrodes. The hole-transport layer was a poly(N-vinylcarbazole) (PVK) film. The luminescent layer consisted of a host material, 8-hydroxyquinoline aluminum (Alq3), and BAM dye as the dopant. A bright light with the peak of 620 nm and narrow bandwidth of 50 nm was obtained in the device with a maximum luminance of 6230 cd/m2. The emission spectra almost unchanged as the luminance increased with increasing injection current and the bias voltage. A tentative explanation from both the electronic distribution viewpoint and the molecular geometric analysis for the narrow bandwidth of this red dye was offered.

Double-layer electroluminescent devices with Langmuir–Blodgett films of amphiphilic 8-hydroxyquinoline lanthanum as emitter

A double-layer organic thin film electroluminescent (EL) device has been prepared by using the LB film technique. Its basic structure consists of a hole-transport layer of material, poly ( N-vinylcarbazole) (PVK) and an electron-transport layer of material, the Langmuir–Blodgett films of bis [ N-hexadecyl-8-hydroxy-2-quinolinecarboxamide] lanthanum (LaL 2Cl). Comparing with the luminance of the single-layer EL device (ITO/LB film/Al), the double-layer EL device (ITO/PVK/LB film/Al) has a higher luminance (873 cd m −2) and operates at a lower driving voltage (8 V). The deposited surface pressure of the LB film had strong influence on the EL intensity of the devices. The luminancent mechanism of the EL devices was discussed.

A Novel Bipolar Electroluminescent Poly(arylene ethynylene) Consisting of Carbazole and Diethynylthiophene Units

A novel bipolar conjugated copolymer poly[(2, 5-diethynylthienylene)-alt-co-(9-hexyl-3,6-carbazole)] (PCT) bearing electron-affinitive moiety of 2,5-diethynylthiophene and hole-transport moiety of carbazole was synthesized via a Pd-catalyzed coupling reaction. The polymer possesses a weight average molecular weight of 23 000 with a polydispersity index of 2.5. PCT exhibits good thermal stability with a glass-transition temperature of 103°C and an onset decomposition temperature of 338°C. The HOMO and LUMO energy levels of PCT were determined to be –5.36 and –2.46 eV, respectively, using cyclic voltammetry. Single-layer and multilayer light-emitting diodes emit yellow light (peak at ≈ 580 nm). Double-layer electroluminescent device with the structure of ITO/copper phthalocyanine/PCT/Ca/Ag has an external quantum efficiency of 0.01%, higher than that of three-layer device with the structure of ITO/copper phthalocyanine/PCT/Alq3/Al, indicating that the electron injection is the limiting factor for the performance of PCT in polymer light-emitting diodes.

Organic electroluminescence of silole-incorporated polysilane

We have examined the electroluminescent (EL) device utilizing a new type of polysilane that incorporates siloles with a high electron affinity, poly(2′,3′,4′,5′-tetraphenyl-1′-silacyclopenta-2′,4′-diene-1′,1′-ylidene-1,1,2,2,3,3,4,4-octamethyltetrasilanylene) (PDMS-S). PDMS-S based single-layer EL device exhibited blue emission with a peak at 488 nm and better electrical properties compared to the device using poly(methylphenylsilane) (PMPS) without silole rings. The maximum external quantum efficiency of the device with PDMS-S was 0.001% and the maximum luminance reached was 5 cd/m 2 at 22 V. The hole-transporting properties of PDMS-S were almost equal to that of PMPS from the hole-mobility with the standard time-of-flight technique and the current–voltage characteristics of double-layer EL device which comprised of PDMS-S as a hole-transporting layer and tris (8-quinolinolato) aluminum (III) complex as an electron-transporting emitting layer. It is clear from these results of the improvement in the single-layer device with PDMS-S and the hole-transporting properties that the electron-transporting property of PDMS-S with silole rings is higher than that of PMPS.

Organic electroluminescent devices using organosilicon polymers containing phenylene or diethynylanthracene units

Double-layer electroluminescent (EL) devices composed of an alternating polymer with mono-, di-, or tri-silanylene and phenylene units, [(Si R) m (C 6 H 4 )] n (R = alkyl, m = 1-3) as a hole-transporting layer, and tris(8-quinolinolato) aluminium(III) complex (Alq) as an electron-transporting-emitting layer were fabricated. The longer silanylene chain lengths in the polymer, on going from m = 1 to m = 2 and 3, result in better electrical properties for the EL devices, implying that the σ-π conjugation in the polymers plays an important role in the hole-transporting properties, including the hole-injection efficiency from an anode. This is in marked contrast to the improved hole-transporting properties that occur in response to reducing the silanylene chain length of silanylene-diethynylanthracene polymers previously reported. The UV absorption maxima of silanylene-phenylene polymers shift to longer wavelengths with increasing m, and their oxidation peak potentials in cyclic voltammograms shift to lower potential with increasing m, in accordance with the improved electrical properties of the device that are observed with the polymers containing the longer silanylene chain. A triple-layer EL device with a hole-transporting layer of monosilanylene-diethynylanthracene polymer, an electron-transporting-emitting layer of Alq, and an electron-blocking layer of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) exhibited a maximum efficiency of 1.0 lm W -1 and a maximum luminance of 14750 cd m -2 , both of which are much higher than the values obtained from a conventional EL device with TPD/Alq.

Organic single- and double-layer electroluminescent devices based on substituted phthalocyanines

Organic electroluminescent diodes containing tetra- tert-butylphthalocyanine, tetrakis(3-phenylneopentyloxy)-phthalocyanine and tetrakis(3-phenylneopentyloxy)-phthalocyaninatocopper(II) were prepared by the vacuum deposition method. The current–voltage ( I– U) characteristics and the electroluminescence (EL) spectra were measured.

Tri ( p-terphenyl-4-yl) amine as a novel blue-emitting material for organic electroluminescent devices

A novel amorphous molecular material, tri( p-terphenyl-4-yl)amine ( p-TTA), was found to function as a morphologically and thermally stable blue-emitting material for organic electroluminescent (EL) devices. A double-layer EL device consisting of an emitting layer of p-TTA and a hole-transport layer of 1,3,5-tris[ N-(4-diphenylaminophenyl)phenylamino] benzene sandwiched between an alloy of magnesium and silver (about 10:1) and indium-tin-oxide (ITO) electrodes emitted bright blue light resulting from p-TTA. The EL device exhibited a maximum luminance of about 350 cd m −2 at a driving voltage of 13 V. The external quantum efficiency was estimated to be 0.4%.

A novel yellow-emitting material, 5,5′′-bis{4-[bis(4-methylphenyl)amino] phenyl}-2,2′:5′,2′′-terthiophene, for organic electroluminescent devices

A novel amorphous molecular material, 5,5′′-bis{4-[bis(4-methylphenyl)amino]phenyl}2,2′:5′,2′′-terthiophene (BMA-3T), has been found to function as a yellow-emitting material in organic electroluminescent (EL) devices. Both the single layer EL device using BMA-3T alone and the double layer EL device consisting of an emitting layer of BMA-3T and an electron-transport layer of tris(8-quinolinolato)aluminum sandwiched between indium-tin-oxide (ITO) and an alloy of magnesium and silver (∼10:1) electrodes emitted a bright yellow light resulting from BMA-3T. The double layer EL device showed much better performances than the single layer EL device, exhibiting a maximum luminance of ∼13 000 cd m−2 at a driving voltage of 18 V, and a luminous efficiency of 1.1 lm W−1 at a luminance of 300 cd m−2.

Fabrication and performances of a double-layer organic electroluminescent device using a novel starburst molecule, 1,3,5-tris[N-(4-diphenylaminophenyl)phenylamino]benzene, as a hole-transport material and tris(8-quinolinolato)aluminum as an emitting material

A double-layer organic electroluminescent (EL) device was fabricated using a novel starburst molecule, 1,3,5-tris[N-(4-diphenylaminophenyl)phenylamino]benzene (p-DPA-TDAB), as a hole transport material and tris(8-quinolinolato) aluminum (Alq/sub 3/) as an emitting material, and its performance characteristics were investigated. It was found that p-DPA-TDAB, which forms a stable amorphous glass with a glass-transition temperature of 108/spl deg/C, functions as a good hole-transport material and that the EL device is thermally stable, operating at a temperature of 120/spl deg/C.