Organic electroluminescent device fabricated with chemical vapor deposited bis(8-hydroxy-5-quinolyl)-methane aluminum chelate polymer film

In this paper chemical vapor deposition (CVD) method is presented for the preparation of chelate polymer film as constituent for organic electroluminescent (EL) devices. The chelate polymer film, poly(bis(8-hydroxy-5-quinolyl)methane aluminum), was thermally converted into by simultaneously co-evaporated aluminum acetylacetonate and bis(8-hydroxy-5-quinolyl)-methane in the gas phase in the CVD process. Two kinds of EL devices using the chelate polymer as the emitting material were fabricated, and their EL properties were characterized and discussed.     

Efficient flexible devices using a statistical copolymer of oxadiazole containing PPV

Flexible light-emitting diodes, with simple device architectures, fabricated using a random copolymer of hole transporting dialkoxy-substituted phenylenevinylene (PV) with an electron transporting oxadiazole containing PV derivative as the emissive layer and higher work function aluminum cathodes have been examined and compared with control devices on glass substrates. In all devices poly(3,4-ethylenedioxythiophene) with poly(styrenesulfonate) (PEDOT:PSS) was used as the hole injection layer and a thin layer of cesium fluoride or lithium fluoride has been used at the polymer/cathode interface to aid electron injection. Devices on plastic substrates with a lithium fluoride interlayer performed the best, exhibiting an average external quantum efficiency (EQE) of 0.8% and luminance of 1600 cd/m² at 40 mA/cm² (7.8 V). Stability of this device and morphology of the emissive film have also been investigated.     

OLEDs based on new oxadiazole derivatives

The absorption, photoluminescence and electroluminescence properties of systematically modified poly(1,3,4-oxadiazole)s are reported and compared with low molar mass compounds. Poly(1,3,4-oxadiazole)s are functionalised by introducing pendent alkyl side chains and tetraphenylsilane or hexafluoroisopropylidene group (6F) into the main chain, respectively. The photo-and electroluminescence of single layer devices was found to be in the blue and green spectral range. A further strategy was to start with the optimisation of the substituents of low molar mass compounds followed by bonding the optimised structure to a flexible PMMA main chain. It was demonstrated that it is possible to preserve the optical properties of the oxadiazole unit and at the same time to improve film forming properties of the final polymer.     

Improved efficiencies of light-emitting diodes through incorporation of charge transporting components in tri-block polymers

9,9-Di-n-hexylfluorene-co-anthracene (DHF-ANT) random copolymer and triphenylamine (TPA)- and oxadiazole (OXA)-containing tri-block copolymers along with a crosslinked TPA polymer are used to fabricate single- and double-layer light emitting diodes (LEDs). In both single-layer and double-layer devices, TPA hole transporting components improve hole injection and transport, leading to improved device performance when low work function calcium (Ca) is used as the cathode, whereas OXA electron transporting components improve electron injection when high work function aluminum (Al) is used. This suggests that DHF-ANT copolymer and DHF-ANT blocks in the two tri-block polymers are hole-limited in devices with Ca cathodes, and electron-limited with Al cathodes. Furthermore, double-layer devices including a separate hole transporting crosslinked TPA layer increase device efficiencies by at least one order in magnitude over the corresponding single-layer devices, due to the improved charge injection, charge confinement and charge recombination.     

Synthesis and characterization of new electroluminescent molecules containing carbazole and oxadiazole units

The synthesis of new molecules containing both electron and hole transporter units is reported. This class of compounds, named OC, may be used for assembling electroluminescent devices made by a single organic layer. The active moieties are the carbazole, as the hole transporter unit, and the oxadiazole, as the electron transporter unit. The chemical formulation and the complex geometry of the molecular frame allow good solubility in chlorinate solvents and the preparation of homogeneous films by spinning technique.Photoluminescence of molecules, both in solution and in film, occurs in the blue region of visible spectra, the exact peak position of emission depending on the pendants attached to the oxadiazole unit. The electroluminescence occurs in a higher wavelength region, with a blue-green emission. The electroluminescent devices consist in the simple sequence ITO–OC–Al and ITO–PEDOT–OC–Al.     

Interfacial exciplex electroluminescence between diamine derivatives with starburst molecular structure and tris(acetylacetonato)-(mono-phenothroline) thulium

The authors demonstrate the interfacial exciplex electroluminescence (EL) between tris(acetylacetonato)-(mono-phenothroline) thulium [Tm(AcA)₃phen] and two diamine derivatives with starburst molecular structure- 4,4′,4″-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA) and 4,4′,4″-tris[3-methyl-pheny(phenyl)-amino]triphenyl-amine (m-MTDATA), both of which have the same ionization potential (IP) (approximately 5.1 eV). When the Tm-complex and the two diamine derivatives are respectively used as the electron accepter and donors, the two EL devices exhibit different exciplex emissions, which verifies our previously reported opinion regarding the effect of the different substitutes on exciplex emission [W.M. Su, W.L. Li, Q. Xin, Z.S. Su, B. Chu, D.F. Bi, H. He, J.H. Niu, Appl. Phys. Lett. 91 (2007) 043508]. When the mixture of the two diamine derivatives is used as a donor, a white EL device with the Commission International de l’Eclairage (CIE) coordinates of (0.277, 0.323) is achieved. The exciplex formation mechanisms of the devices with the two different donors are discussed.     

透明导电薄膜ZnO:Al(ZAO)的性能及其在有机发光二极管中的应用

研究了ZAO透明导电薄膜的微观组织结构、化学成分及其电学光学特性.结果表明,多晶ZAO薄膜呈柱状晶并具有(002)面择优取向, c轴晶格常数大于0.52000 nm;ZAO薄膜不同微区之问的成分不均匀导致了电学性能的不均匀; ZAO薄膜的电阻率和在可见光区的平均透射率分别为1.39×10-4 Ω·cm和80.8％.其透射率和吸收率曲线均具有明显的波动性,该波动性影响以ZAO为阳极的有机发光二极管的发射光谱,在5.38 A/m2电流密度下二极管的发光效率大于2 cd/A.     

Fluorene-based oxadiazoles: thermally stable electron-transporting materials for light-emitting devices

We have synthesized and characterized a series of fluorene-based oxadiazole derivatives, in each of which two identical oxadiazole moieties are linked to a fluorene unit via an sp³-hybridized carbon atom (C-9) to form a rigid, nonplanar structure. This structural feature leads to a reduction in the tendency to crystallize and an increase in the glass transition temperature. Electrochemical studies revealed that these materials undergo reversible reductions and have high electron affinities. Each fluorene-based oxadiazole exhibited reasonably good performance when it was used as the electron-transporting layer in a multi-layer organic EL device.     

Intermolecular energy transfer in photo- and electroluminescence properties of a europium(III) complex dispersed in poly(vinylcarbazole)

The photoluminescence (PL) and electroluminescence (EL) properties of Eu(TTA)₃Phen (TTA: thenoyltrifluoroacetonate; Phen: 1,10-phenanthroline) dispersed in poly(N-vinylcarbazole) (PVK) in multilayer organic light emitting diodes (OLEDs) have been investigated. Both Gd(TTA)₃Phen and tris(8-hydroxyquinoline) aluminum (Alq₃) were used as an electron transporting layer in order to compare their electron energy transfer properties in OLEDs.     

Ab initio determination of electron-molecular vibration constants (em-v) in bis(ethylenedithio) tetrathiafulvalene (BEDT-TTF) and tetrathiafulvalene (TTF)

We present results from an ab initio density functional calculation of the electron-vibronic coupling strengths in neutral and charged bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecules. Results for tetrathiafulvalene (TTF) are also presented and compared with known experimental values as a measure of the accuracy of our calculation. We find that the highest occupied molecular orbital (HOMO) in ET couples strongly to modes involving the double-bonded carbon atoms as well as the sulfur atoms and that the contribution of the vibration modes involving the hydrogen atoms to the vibronic coupling is negligible. The implication of this result for the mechanism of superconductivity is discussed in the light of recent isotope effect experimental results.     

White organic light-emitting diodes based on blue fluorescent bis(2-(2-hydroxyphenyl)benzoxazolate)zinc [Zn(hpb)2] doped with DCM dye

Bright white organic light-emitting diodes (WOLEDs) with single active layer has been demonstrated from blue emitting zinc complex bis(2-(2-hydroxyphenyl)benzoxazolate)zinc [Zn(hpb)₂] doped with orange luminescent 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM) dye. White light electroluminescence (EL) spectrum from Zn(hpb)₂ has been achieved by adjusting the concentration of DCM dye. WOLED with a structure of ITO/α-NPD/Zn(hpb)₂:DCM (x%)/BCP/Alq₃/LiF/Al has been fabricated. The EL spectra covering the whole visible spectra range of 400–700 nm, with two peaks at 446 and 555 nm has been measured. The device emits white light at 10 V with Commission Internationale de I’ Eclairage (CIE) coordinates (0.27, 0.31) and brightness 1083 Cd/m². The maximum current efficiency of the device was 1.23 Cd/A at 9.5 V and maximum luminance reaches 2210 Cd/m² at 12 V.     

Synthesis and electrochemical characterization of a new polymer constituted of alternating carbazole and oxadiazole moieties

We report herein the synthesis and electrochemical characterization of a novel copolymer containing an electron rich carbazole moiety and an electron deficient oxadiazole unit—poly[N-(2′-ethylhexyl)-carbazole-3,6-diyl-1″,3″,4″-oxadiazole-2″,5″-diyl] (PCO). PCO can be dissolved in THF, CHCl₃, xylene, and DMSO. The structure of the polymer is confirmed by FT-IR, NMR and elemental analysis. The optical and electronic properties of the polymer are investigated by UV–Vis absorption spectroscopy and photoluminescence spectroscopy as well as cyclic voltammetry. The results show that the polymer films emit greenish-blue light (λ_max 485 nm) upon UV excitation. Both p-doping and n-doping processes are observed in cyclic voltammetric investigation. A comparison between the properties of polycarbazole and polycarbazole–oxadiazole has been presented.     

Enhanced photovoltaic cells efficiency via incorporation of high electron-deficient oxadiazole moieties on side chains of poly(phenylene vinylene)s and poly(fluorene)s

High charge mobility and fast exciton dissociation rate are essential for photovoltaic cells. In this study, we demonstrate effects of the electron-deficient oxadiazole (OXD) moiety attached on the side chain on the efficiency of photovoltaic cells for poly(phenylene vinylene)s (PPVs) and poly(fluorene)s (PFs) as the active layer by luminescence decay and time of flight measurement (TOF). We find that OXD moiety can increase exciton dissociation rate and promote electron transport. Therefore, the efficiencies of incident photon converting to electron of the OXD-attached polymers, POPD-MEH-PPV(37/63) and OXD50PFO, are higher than those of MEH-PPV and PFO by factors of 2.1 and 29, respectively.     

Photoluminescent and electroluminescent properties of DCM-1 dispersed poly(p-phenylene vinylene) derivatives

The single layer devices utilizing poly[2-(carbazol-9-yl)-5-(2-ethylhcxyloxy)-1,4-phenylene vinylene] (CzEh-PPV) and poly[2-{4-[5-(4-tert-butylphenyl)-l,3,4-oxadiazolyl]-phenyl}-5-(2-ethythexyloxy)-l,4-phenylene vinylene] (OxdEh-PPV) doped with varying weight percent of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM-1) were fabricated and their photo luminescence (PL) and electroluminescence (EL) properties were discussed in this investigation. The PL spectra of DCM-1 doped polymers show that the emission is mostly from DCM-1 and Förster energy transfer may occur between DCM-1 and the two polymers. On the other hand, field-dependence of the emission spectra of EL devices was observed in detail. For the CzEh-PPV/DCM-1, the emission at the wavelength of 534 nm remains unchanged as the level of DCM-1 increases, whereas the peak at 572 nm is intensified with increasing both the additive level and applied electric field. For the OxdEh-PPV/DCM-1, the main peak is red-shift as the level of DCM-1 increases and blue-shift as the applied electric field does.     

High photoluminescence and photoswitch of bis(8-hydroxyquinoline) zinc nanoribbons

Highly luminescent bis(8-hydroxyquinoline) zinc nanoribbons with average width of 150 nm and thickness of 50 nm and length up to several hundreds of micrometers have been synthesized via a facile solvothermal method. The room-temperature photoluminescence spectrum of the products showed a strong and stable emission centered at 515 nm under excitation at 386 nm. The nanoribbons were found to be sensitive to light. The photocurrent enhanced by ca. 24 times under irradiation of an incandescence lamp with power density of 2.5 mW/cm². This conclusion was interesting and might found potential application in light-controlled devices in the future.     

Electroluminescence enhancement by blending PVK with an alternating copolymer containing triphenylamine and phenylene units

The PL and EL properties of a novel light-emitting alternating copolymer composed of triphenylamine (TPA) and phenylene vinylene derivate (MEHPPV), that is, TPA-MEHPPV and its blend with PVK were demonstrated. A considerable enhancement of the EL intensity was observed by using the polymer blends as the emission layer in the organic light-emitting devices (OLEDs). The energy transfer process in the polymer blends was also discussed.     

A novel material based on polyaniline doped with [Cs][In(dmit)2], (cesium) [bis(1,3-dithiole-2-thione-4,5-dithiolato)indium (III)]

A novel conducting hybrid material based on polyaniline doped with a monoanion derived from [cesium][bis(1,3-dithiole-2-thione-4,5-dithiolato)indium(III)], ([Cs][In(dmit)₂]), is reported. Fourier transform infrared (FTIR), UV–vis and X-ray photoelectron spectroscopies showed that the [Cs][In(dmit)₂] acts as the doping agent for the emeraldine base (insulating form of the polyaniline). This new material presents lower resistivity than the starting materials, and forms a flexible, self-supporting film with good homogeneity, which is an important feature for the development of new devices to be useful in the electroelectronic industries.     

Light-emitting properties of photo-curable polyfluorene derivatives

An alternating copolymer composed of bis-(4-octyloxyphenyl)fluorene and bis((3-hexyloxy-3′-ethyl)oxetane) fluorene [poly(PF-alt-OXTF)], and a terpolymer composed of the above two monomers with N-hexylphenotiazine [poly(PF-co-OXTF-co-PTZ)] have been synthesized by Suzuki coupling polymerization. The synthesized poly(PF-alt-OXTF) and poly(PF-co-OXTF-co-PTZ) films became insoluble after UV irradiation in the presence of a proper photo-acid generator. The UV–vis absorption and PL emission spectra of the polymers before and after photo-cross-link reaction were almost same. The EL devices were constructed with ITO/PEDOT:PSS/polymer/LiF/Al configuration. The EL devices using the photo-cross-linked polymer films also showed lower operating voltages than the devices using the corresponding polymer films without cross-link. Moreover, the EL device of the poly(PF-co-OXTF-co-PTZ) film with photo-cross-link showed better device performances than the device without cross-link, and the maximum brightness and power efficiency of the device were 4750 cd/m² and 0.68 cd/A, respectively. Furthermore, we obtained a well-patterned thin film with poly(PF-co-OXTF-co-PTZ) after a successive lithographic process.     

White polymer light-emitting diode materials with efficient electron injection backbone containing polyfluorene,oxadiazole and quinoxaline derivatives

Copolymer including dithienylquinoxaline (TQ) (<0.1 mol%) as a dopant and oxadiazole derivative as a side chain has been synthesized on a Polyfluorene (PF) backbone based on the Suzuki coupling reaction. The UV–vis spectra of polymers showed similar behaviors in the solution and on the film. However, PL spectra were similar to that of PF in solution, but the peak around 524 nm increased as the amounts of TQ were increased in the casting film. With increase in TQ, effective Forster energy transfer (from PF and PFOxd15 to TQ) was observed. According to CV measurement, HOMO and LUMO levels declined as the amount of oxadiazole derivative increased. In case of PFOxd15TQ10, the luminous efficiency and power efficiency were 1.23 cd/A and 0.64 lm/W, respectively with a maximum brightness of 6940 cd/m². CIE coordinates were close to pure white (0.33, 0.39).     

Charge transporting block copolymer for morphological control in light emitting device based on polymer blends

Bipolar charge transporting block copolymer composing of carbazole and oxadiazole monomers as hole and electron transporting units, respectively, was synthesized by nitroxide mediated radical polymerization. It is found that the current efficiency significantly increased with the addition of the block copolymer in the device based on polymer blend system. AFM measurement revealed that a phase-separated structure in the polymer blend layer changed to suitable morphology in the presence of block copolymer.