Effect of dye-doped concentration on the charge carrier recombination in molecularly doped organic light-emitting devices

The effect of the concentration of 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl- 9-enyl)-4H-pyran(DCJTB) as dopant in tris(8-hydroxyquinoline) aluminium (Alq(3)) on the charge carrier recombination was studied by transient electroluminescence (EL). The electron-hole recombination coefficient (gamma) was determined from the long-time component of the temporal decay of the EL intensity after a rectangular voltage pulse was turned off. It was found that the coefficient monotonically decreased with an increase in the DCJTB-doping concentration. The monotonic decrease is attributed to concentration quenching on the excitons and coincided well with the reduction of the EL efficiency.

Investigation of charge-carrier injection characteristics in NPB/Alq3 heterojunction devices

The effect of copper phthalocyanine (CuPc) and LiF interfacial layers on the charge-carrier injection in NN'-di(naphthalene-l-yl)N,N'-diphenyl-benzidine (NPB)/tris(8-hydroxyquinoline) aluminium (Alq(3)) organic heterojunction devices have been studied through the analysis of current-voltage characteristics. The investigation clearly demonstrated that the hole injection into NPB from anode is Fowler-Nordheim (FN) tunneling and the electron injection into Alq3 from cathode is Richardson-Schottky (RS) thermionic emission. The barrier heights obtained from the FN and RS models proved that the band alignments for charge-carrier injection are greatly improved by the CuPc and LiF interfacial layers, which should fully clarify the role of the interfacial layer on the improvement of device performance.

Studies of electron-transfer and charge-transfer coupling processes at a liquid/liquid interface by double-barrel micropipet technique

Investigation of a heterogeneous electron-transfer (ET) reaction at the water/1,2-dichloroethane interface employing a double-barrel micropipet technique is reported. The chosen system was the reaction between Fe(CN)(6)(3-) in the aqueous phase (W) and ferrocene in 1,2-dichloroethane (DCE). According to the generation and the collection currents as well as collection efficiency, the ET-ion-transfer (IT) coupling process at such an interface and competing reactions with the organic supporting electrolyte in the organic phase can be studied. In addition, this technique has been found to be an efficient method to distinguish and measure the charge-transfer coupling reaction between two ions (IT-IT) processes occurring simultaneously at a liquid/liquid interface. On this basis, the formal Gibbs energies of transfer of some ions across the W/DCE interface, such as NO3-, NO2-, Cl-, COO-, TBA(+), IPAs+, Cs+, Rb+, K+, Na+, and Li+, for which their direct transfers are usually difficult to obtain because of the IT-IT coupling processes, were quantitatively evaluated.

Studies on charge transfer across the micro-liquid/liquid interface supported at a double-barrel micropipette

In this paper, the charge transfer across the micro-liquid/liquid interface supported at the orifice of a double-barrel micropipette, namely, a theta-pipette, is reported. Simple ion transfer(TMA(+)), facilitated ion transfer (potassium ion transfer facilitated by DB18C6), and electron transfer (ferrocene and ferri/ferrocyanide system) have been investigated by cyclic voltammetry. The experimental results show that a very thin aqueous film, linking both barrels filled with the aqueous solution and the organic solution respectively, can spontaneously be formed on the outer glass surface of such a double-barrel micropipette to construct a micro-liquid/liquid interface, which provides the asymmetry of diffusion field. Such device is demonstrated experimentally which can be employed as one of the simplest electrochemical cells to investigate the charge transfer across the liquid/liquid interface.

Charge carrier injection and transport in PVK: Alq(3) blend films

We have investigated the current-voltage and electroluminescent (EL) characteristics of single-layer organic devices based on poly(9-vinylcarbazole) (PVK) and tris(8-hydroxyquinoline)aluminium (Alq(3)) blend with different PVK : Alq(3) concentrations. The experimental results from the observed thickness and temperature dependence clearly demonstrate that the current at low voltage is due to the holes injected at the anode and is space-charge limited, whereas the current at the high voltage that steeply increases is explained as the electron tunnelling injection at the cathode. The hole mobility is directly determined by space-charge-limited current at the low voltage region and decreases with increasing Alq(3) content in the blend. The EL efficiency shows concentration dependence, which is attributed to the change of the transport of electrons and holes in the blend film.

Charge transfer across the water/nitrobenzene interface by three-electrode system

A droplet of aqueous solution containing a certain molar ratio of redox couple is first attached onto a platinum electrode surface, then the resulting drop electrode is immersed into the organic solution containing very hydrophobic electrolyte. Combined with reference and counter electrodes, a classical three-electrode system has been constructed, Ion transfer (IT) and electron transfer (ET) are investigated systematically using three-electrode voltammetry. Potassium ion transfer and electron transfer between potassium ferricyanide in the aqueous phase and ferrocene in nitrobenzene are observed with potassium ferricyanide/potassium ferrocyanide as the redox couple. Meanwhile, the transfer reactions of lithium, sodium, potassium, proton and ammonium ions are obtained with ferric sulfate/ferrous sulfate as the redox couple. The formal transfer potentials and the standard Gibbs transfer energy of these ions are evaluated and consistent with the results obtained by a four-electrode system and other methods.

Synthesis, properties and X-ray crystal structure of a new heteropolyacid supermolecular charge-transfer complex [(VB1)(2)DMFHPMo12O40 center dot 5DMF]

The title complex [(VB1)(2)DMFHPMo12O40.5DMF, VB1 = vitamin B-1 (thiamine chloride), DMF = N,N-dimethylformamide] has been synthesized and characterized by elemental analysis, IR, UV-Vis, electron spin resonance, X-ray photoelectron spectroscopy and cyclic voltammetry methods. The X-ray crystal structure revealed that there is one independent molecule in the unit cell of the title complex that contains one mixed-valence heteropolyanion, two VB1+ cations and six DMF molecules. The title complex possesses a centrosymmetrical arrangement in the unit cell, with the P atom at the symmetry center of the heteropolyanion and with eight O atoms surrounding the central P atom, such that two sets of PO4 tetrahedra are formed. The PO4 tetrahedra and MoO66-(7-) octahedra are disordered in the heteropolyanion. The bond distances of P-O-a and Mo=O-d are in the ranges 1.57 (4)-1.70 (4) Angstrom and 1.61 (2)-1.67 (2) Angstrom, respectively.

Synthesis, characterization and crystal structure of a charge transfer salt based on 1.3-cyclopentadiene tetrabutylammoniumdodecamolybdosilicoate

A charge transfer salt, (Bu4N)(4) (C5H6)[(HSiMo11MoO40)-Mo-VI-O-V] has been photochemically synthesized from (Bu4N)(4)SiMo12O40 and 1.3-cyclopentadiene and Characterized, by elemental analysis, IR spectra, solid diffusion reflectance electronic spectra, CV and ESR. The X-ray crystal structure revealed that the title complex crystal data are as follows: triclinic, space group P (1) over bar, a = 14.347(3), b = 14.423(3), c = 27.158(5) Angstrom, alpha = 96.90(3), beta = 104.18(3), gamma = 98.20(3)degrees, V = 5322(2) Angstrom (3), Z = 2, M-r = 2855. 30, D-c = 1.782g.cm(-3), F(000) = 2860, R = 0.0719, wR = 0.198. The title compound is composed of 1.3-cyclopentadiene, four tetrabutylammonium and [(SiMo11MoO40)-Mo-VI-O-V](4-) anion.

Charge carrier mobility and electroluminescence in a green-emitting alternating block copolymer with a methoxy Bi-substituted chromophore

We determine the mobility of positive and negative charge carriers in a soluble green-emitting alternating block copolymer with, a methoxy bi-subsbituted conjugated segment. The negative charge carrier mobility of 6 x 10(-11) cm(2)/V.s is directly determined using space-charge-limited current analytical expressions. Positive charge carrier transport is also space-charge-limited, with a mobility of I x 10(-8) cm(2)/V.s. The electron trap distribution is exponential, with a characteristic energy of similar to 0.12 eV. A hole trap with energy similar to 0.4 eV was observed. This copolymer is used as emissive material in organic light-emitting diodes that present brightness of similar to 900 cd/m(2) at 12.5 V.

Optical spectroscopy properties and charge compensation of BaLiF3 doped with Ce3+

The excitation and emission spectra of the BaLiF3:Ce3+ phosphors synthesized through solid state reaction have been measured. By investigating the properties of the excitation spectra we point out that the variation in the excitation spectra with the amount of CeF3 dopant results from the different patterns of charge compensation in the matrices. The vacancies of Li+ ions are the favorable charge compensation pattern at low concentration of CeF3 doped, but interstitial F- ions are the major charge compensation pattern when the concentration of CeF3 doped goes beyond a certain value. (C) 2000 Academic Press

Charge transport in a blue-emitting alternating block copolymer with a small spacer to conjugated segment length ratio

We analyze current versus voltage data obtained using single carrier injection in several metal/polymer/metal sandwich structures. The polymer used in each case is a soluble blue-emitting alternating block copolymer. Our experimental results demonstrate that the electron transport is space-charge limited by the high density of traps having an exponential energy distribution (temperature dependent characteristic energy) in the copolymer. The electron mobility of 8x10(-10) cm(2)/V s is directly determined using space-charge-limited current analytical expressions. Hole transport is also space-charge limited, with a mobility of 2x10(-6) cm(2)/V s. A hole trap with energy 0.17 eV is observed. We compare these results with those obtained for related block copolymers with different spacer and conjugated segment lengths and discuss the influence of spacer length and conjugated segment length on the charge transport properties. (C) 2000 American Institute of Physics. [S0021-8979(00)04501-1].

Photochemical synthesis and crystal structure of a charge transfer salt [HMDH2][(H2PMoMo11O40)-Mo-v]center dot 2AA center dot 3H(2)O center dot DMF

The title supramolecular compound, [HMDH2][(H2PMoMo11O40)-Mo-V] . 2AA . 3H(2)O . DMF (HMD = hexamethylene diamine; AA=acetaldehyde; DMF=N,N-dimethyl formamide), has been photochemically synthesized by using elemental analysis, IR, solid diffusion reflectance, electronic spectra, ESR spectra and X-ray single-crystal analysis. The crystallographic data: triclinic, P (1) over bar, a=14.092(2), b=14.347(3), c=14.358(3)Angstrom, alpha = 75.10(3), beta = 80.70(3), gamma = 80.73(3)degrees, V = 2746.6(10)Angstrom (3), Z = 2, M-r = 2081.68, D-c=2.517g/cm(3), F(000) =1970, mu (MoK alpha) =2.766mm(-1). The structure has been refined to R =0.0832 and wR=0.2638, by full-matrix least-squares method. The title compound is composed of hexamethylene diamine, two acetaldehyde molecules, three water molecules, one N,N-dimethylformamide and [(H2PMoMo11O40)-Mo-V](2-) heteropoly anion.

Characteristics in frequency domain by inductively coupled plasma atomic emission spectrometry with charge injection detection

The charactesistics of two-dimension spectra obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) with charge injection detection (CID) in frequency domain were studied in the present paper. The measurement spectra were Fourier transformed and the frequency distribution of the spectra was obtained. Results showed that the spectra in frequency domain could he divided into two parts:high frequency and low frequency signals. The later stood for measurement spectra and the former for background and noises. However, the high frequecny signals could not be smoothed simply to reduce noises because the background was deteriorated even though the spectral signal did not change significantly.

Synthesis, properties and crystal structure of charge-transfer salt (DBTTF)(6)HSiMo12O40H2O

Charge-transfer salt (DBTTF)(6)HSiMo(12)O(40)4H(2)O was synthesized by electrocrystallization and characterized by IR spectrum and electronic spectrum. Its magnetic property, conductivity and crystal structure were determined. The title compound consists of heteropoly anions, water molecules and DBTTF columns which are formed by repeated arrangement of tetramer (DBTTF), in the direction of 15 degrees to the a axis in the tunnel constituted by the anions and other type of DBTTF. The title compound is paramagnetic and semiconducting. (C) 1998 Elsevier Science Ltd. All rights reserved.

Preliminary study on the charge transfer and bond ionicity of binary crystals

Charge transfer and bond ionicity of some monovalent, divalent, and trivalent binary crystals of A(N)B(8-N) type have been investigated using the self-consistent method. The method divides the binary crystal systems into two subsystems which contain only one kind of element each in physical space. The charge transfer values are obtained by adjusting the charge in a self-consistent way. Based on the obtained charge transfer values, an empirical formula for bond ionicity has been proposed. It has been shown that the present results for bond ionicity are in good agreement with the previous theoretical study delivered by Levine and Pauling. The results also indicate that a large magnitude of charge transfer (or less excess charge in the bonding region) gives rise to high bond ionicity (or low bond covalency); this agrees well with the viewpoint that the excess charge in the bonding region is the origin of the formation of bond covalency. (C) 1998 American Institute of Physics. [S0021-9606(98)00837-X].