Trap-free,space-charge-limited currents in a polyfluorene copolymer using pretreated indium tin oxide as a hole injecting contact

We report time-of-flight, transient dark injection (DI) and current density versus voltage measurements on polyfluorene copolymer diode structures using pretreated indium tin oxide (ITO) as a hole injecting contact. For ITO exposed to an oxygen plasma, coated in poly(ethylenedioxythiophene)/polystyrenesulphonic acid, or with both treatments, all measurements were entirely consistent with positive carrier, trap-free, space-charge-limited current theory. For untreated ITO, the behaviour is instead injection limited.     

Recombination studies in a polyfluorene copolymer for photovoltaic applications

We present detailed continuous wave (cw) and transient photoinduced absorption (PA) measurements in thin films of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzo-thiadiazole)] (DiO-PFDTBT), and its blends with the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in weight ratios of 1:0, 4:1 and 1:4. We measure the frequency, intensity and temperature dependence of the PA signal in the frequency domain, and compare with the results obtained from the transient PA decay measurements in the time domain. In all blends, the PA spectrum shows a broad high energy PA band ranging from ∼1 eV to 2 eV as well as a low energy band peaking at ∼0.35 eV. We attribute the low energy band to the P₁ transition of polarons and part of the high energy band to the correlated P₂ transition of polarons. Both frequency and time domain measurements show that the high energy band has two decay components, a faster component in the microsecond time regime and a slower component in the millisecond time regime. The slow component is strongly dispersive, whereas the fast component is practically non-dispersive.     

Space-charge limited conduction with a field and temperature dependent mobility in Alq light-emitting devices

Electrical transport in organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinolato)aluminium (Alq) is investigated as a function of temperature and organic layer thickness. It is shown that the thickness dependence of the current provides a unique criterion to discriminate between (1) injection limited behavior, (2) trap-charge limited conduction with an exponential trap distribution and a field independent mobility, and (3) trap-free space charge limited conduction with a field and temperature dependent mobility. The observed thickness and temperature dependent current–voltage characteristics are found to be in excellent agreement with trap-free SCLC with a hopping type charge carrier mobility.     

On the field and temperature dependence of hole mobility in molecularly doped polymer

The charge transport properties of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped polycarbonate (PC) have been measured as a function of field and temperature. Hole mobility is observed to follow log μ ∝ E^1/2 (i.e. Poole–Frenkel behavior) at relatively high field strengths while at low field strengths mobility initially decreased with increase in field strength. Mobility value undergoes minima at a particular field value for a given temperature. Minima in mobility are observed to occur at higher field strength as the temperature is lowered. Mobility data and transport parameters are analyzed using the formalism of Gaussian disorder model (GDM). The mobility behavior at low field and shift of mobility minima with temperature suggests large disorder in the film. Investigation of film morphology using photoluminescence, X-ray diffraction and scanning electron microscopy suggests that large positional disorder in the film is probably due to film morphology and TPD–PC interaction.     

A study of hole mobility in diaryldiacetylenes

Thin homogeneous films exhibiting effective hole transport with drift mobility values up to 10⁻⁴ cm² V⁻¹ s⁻¹ are formed in the process of vacuum thermal deposition of monomeric diaryldiacetylenes. Basic transport characteristics and their dependence on the electric field, temperature and chemical composition of the monomers are determined.     

Confinement-induced enhancement of hole mobility in MEH-PPV

Template wetting nanofabrication was used to create high-aspect-ratio, nanotubular structures from the semiconducting polymer poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene). Carrier transport in these nanostructures was determined to be space-charge-limited and thermally activated, with low-field mobilities shown to up to three be orders of magnitude higher than those typically reported for thin-films of the same material. Ultraviolet–visible spectroscopy showed a 0.3–0.6 eV reduction in the leading-edge absorption energy of polymer nanotubular structures compared to thin-films or solutions. Photovoltaic devices constructed from the nanotubes demonstrated photovoltaic fill factors superior to those measured in similarly constructed thin-film devices. These results are indicative of a confinement-induced ordering present in the nanotubular material.     

Improved expression of charge-carrier mobility in disordered semiconducting polymers considering dependence on temperature,electric field and charge-carrier density

The unified expression of mobility μ for semiconducting polymers based on the Gaussian disorder model is improved. The original expression was proposed by Pasveer et al. [W.F. Pasveer, J. Cottaar, C. Tanase, R. Coehoorn, P.A. Bobbert, P.W.M. Blom, D.M. de Leeuw, M.A.J. Michels, Phys. Rev. Lett. 94 (2005) 206601] and has non-Arrhenius temperature dependence ln(μ) ∝ 1/T² at low carrier density. The improved one has Arrhenius temperature dependence ln(μ) ∝ 1/T at low carrier density, and can describe the dependence of mobility on temperature, carrier density, and electric field solved from the master equation model fairly well. Especially the precision at high carrier density and high electric field is prominently improved. The formalism is applied to solve diffusion current equation and Poisson equation for the NRS-PPV and OC₁C₁₀-PPV polymers. By adjusting three parameters, the calculated current–voltage curves are in good agreement with the experimental data. The results confirm recent claims of Preezant and Tessler [Y. Preezant, N. Tessler, Phys. Rev. B 74 (2006) 235202] that the dependence of mobility on electric field and charge density is important to correctly describe the electric properties of ordered polymers.     

三元合金中界面迁移率的相场法计算(英文)

基于定量相场模型,提出一种新方法来计算界面迁移率,并将该方法应用于 Ti-6Al-4V 合金的α/β界面。相场模拟表明,更高的界面迁移率将导致更快的相转变速率,但在扩散控制的条件下,只有唯一的界面迁移率能匹配扩散方程。通过比较采用相场模拟和经典扩散方程所得到的相变动力学,可以得到不同温度下的界面迁移率。结果表明,计算所得的界面迁移率随着温度的升高而增加,且与 Arrhenius 方程吻合得很好。     

On theory of electric conductivity of low-conducting liquid in the field of injecting electrodes (mobility dependence)

Calculation of the mobility of free electrons at the injection of electrons from high voltage needle electrodes on a liquid surface is conducted. Applicability of the methods of solid state physics is validated. The dependence of the mobility on the electric field intensity ～E ^?1/3 is achieved on account of the dependence of the free electron wave function on the electric field intensity. It is substantiated that the distribution function with respect to the energies of electrons does not depend on the electric field intensity.     

Grain size dependent mobility in polycrystalline organic field-effect transistors

Thin film transistors were made with vacuum evaporated sexithiophene and octithiophene. Scanning electron microscopy shows that the films are polycrystalline, with a grain size that increases when the temperature of the substrate is increased. The carrier mobility of the film increases with grain size. It is thermally activated for small grains, but becomes temperature independent for larger grains. These data are interpreted in terms of a model where charge transport is limited by trapping and thermal release at localized states located at grain boundaries.     

Condensation of a gas-vapor mixture in an electric field

Condensation of a gas-vapor mixture in a direct electric field is discussed in this paper. At a noncondensable gas concentration of 5%, an approximately two-fold increase in the condensed liquid amount is obtained. The condensation process enhancement is explained by the cooperative effect of the condensate film turbulization and the decrease in the diffusion resistance of the noncondensable gas layer in an electric field. The obtained results can be used for the design of electrohydrodynamic generators and compact vapor condensers.     

Magnetic field in electric arc welding

It is shown that the longitudinal magnetic field of welding current in welding of two sheets transforms to the field of a single sheet and this confirms the electromagnetic nature of the interatomic binding forces. At a gap of 0.002 m and as a result of the separation of the lines of force, the induction of the longitudinal magnetic field of welding current rapidly increases and the arc starts to rotate. The electromagnetic force and magnetic pressure directed forward and to the side edges of the weld pool then appear. This results in the movement of liquid metal to the front part of the crater and to the side edges, formation of welded joints without undercutting, and confirms the electromagnetic nature of the formation of undercutting in one-sided high-speed welding. A method of one-sided high-speed welding with a compound electrode on a flux cushion with a gap of 0.002 m has been developed. The method improves the stability of the process and impact toughness of welded joints.     

Laser trepanning of a small diameter hole in titanium alloy: Temperature and stress fields

High stresses formed around the laser cut edges limit the practical applications of the parts produced. In the present study, laser cutting of small diameter hole into Ti-6Al-4V alloy is carried out. Temperature and stress fields developed in the cutting region are simulated, in line with the experimental parameters, via incorporating the finite element code. The temperature predictions are validated with the thermocouple data. The residual stress developed at the vicinity of the cut surface is obtained using the XRD technique. The features of the hole section are examined using the optical and scanning electron microscopes. It is found that temperature decay is gradual, due to the annealing effect, in the region where the cutting ends; in which case, the peak value of von Mises stress reduces in this region. The residual stress predicted agrees well with the data obtained from the XRD technique.     

Nonlinear resonance interaction of waves on a jet in a radial electric field

Nonlinear analytic asymptotic second-order calculations show that the motion of a charged jet relative to a material medium results in the appearance of rise-time periodic wave motions of the interface (Kelvin-Helmholtz instabilities), as well as an internal nonlinear resonance interaction of waves, whose parameters (the intensity and the characteristic time of interaction) depend on the physical parameters of the system: the electric charge density at the jet, its velocity relative the medium, the mass density, the values of the wavenumbers of the interacting waves, and the value of the surface tension coefficient of the boundary.     

Ultrasonic and electric discharge machining to deep and small hole on titanium alloy

Being a difficult-to-cut material, titanium alloy suffers poor machinability for most cutting process, let alone the drilling of small and deep holes using traditional machining methods. Although electric discharge machining (EDM) is suitable to handle titanium alloys, it is not ideal for small and deep holes due to titanium alloys’ low heating conductivity and high tenacity. This paper introduces ultrasonic vibration into micro-EDM and analyzes the effect of ultrasonic vibration on the EDM process. A four-axis EDM machine tool which combines ultrasonic and micro-EDM has been developed. A wire electric discharge grinding (WEDG) unit which can fabricate a micro-electrode on-line, as well as a measuring unit, is set up on this equipment. With a cylindrical tool electrode, made of hard carbide, which has high stiffness, a single-side notch was made along the electrode. Ultrasonic vibration is then introduced into the micro-EDM. Experiments have been carried out and results have shown that holes with a diameter of less than Ø0.2 mm and a depth/diameter ratio of more than 15 can be drilled steadily using this equipment and technology.     

钢基模具喷涂层温度场和应力场模拟分析

采用有限元分析软件计算不同时刻以及不同位置涂层内的温度场、应力场,分析电弧喷涂方法制造模具时的沉积过程.在建立传热模型过程中,采用在厚度方向以微小层逐层叠加来模拟涂层的增厚,并应用生死单元法逐层激活层单元参与计算过程,以模拟真实的喷涂沉积过程,获得了喷涂层内温度场、应力场的分布情况.并在此计算的基础上,分析了应力的分布对涂层失稳以及残余应力分布的影响,为模具尺寸设计和喷涂工艺制订提供了依据.     

Optical and morphological investigations of non-homogeneity in polyfluorene blends

We report investigations of organic light-emitting diodes with an active layer consisting of poly(9,9-dioctylfluorene), PFO, blended with a green emitting derivative (F8BT). We used indium tin oxide (ITO) anodes with and without a hole transporting layer, consisting of doped poly(3,4-ethylene dioxythiophene). We also studied the incorporation of an oxadiazole based hole-blocking/electron transporting layer, HBL, namely 2-(4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole, PBD, between the emissive layer and the Ca/Al cathodes. The presence of this layer reduces the range of radiative recombination in a region closer to the HBL interface, and therefore, allows investigation of depth-dependent structural non-uniformities of the emissive layer. We find marginal alterations of the electroluminescence spectra depending on the weight concentration of the F8BT in the range 5–25%. Efficient energy transfer from the host (PFO) to the guest (F8BT) ensures that the emission spectra are dominated by the green component. However, we find that blue emission from the PFO host is much more intense for the structures incorporating the HBL than for those without it. We propose that phase separation phenomena of the blend induced by the spin-coating of the PBD-based layer, are responsible for the higher weight of the blue component.     

Heat transfer at the condensation of a gas-vapor mixture in an electric field

The problems associated with the investigation of heat transfer processes at the condensation of vapor from a vapor-air mixture in the presence of an electric field have been considered. It is established that even very small additions of air impair vapor condensation and, hence, the heat transfer. It is shown that the application of an electric field to a vapor-air mixture can be an effective factor for eliminating the negative effects of the presence of air on the condensation process. It has been found out that a corona discharge specially created by means of notches on the surface of the inner electrode of a cylindrical system and its effects are the main reason for this. The experimental characteristics of the relative heat transfer coefficient at condensation as functions of the air concentration, the rate of the mixture’s delivery into the vapor condenser, and the specific heat flux at different electric field intensities have been obtained. The results are explained by the electric charging of the medium in the corona charge field, by the electric wind, and by the charging of the “vapor-liquid” interface. There has been derived a formula according to which the process of condensation is strongly influenced by the molecule effective diameter (ψ∼d _*⁶) increasing in the electric field due to the solvation of ions by water molecules resulting in a new mechanism for the intensification of the condensation process in an electric field.     

The electric field in the vicinity of a charged jet: Nonlinear calculations

The electric field intensity of a self-charge in the vicinity of an infinite cylindrical liquid jet on the surface of which waves with finite amplitude with arbitrary symmetry are running is calculated.