Positive Charge Carriers Research Articles

Comparison of the Mobilities of Negative and Positive Ions in Nonpolar Solutions

The mobilities of organic radical ions of different molecular volumes have been determined in squalane and hexane solutions to study the influence of the ion charge sign on the ionic mobility in a weakly polar liquid. The relative mobility of geminate radical ions was measured using the method of time-resolved electric field effect in the recombination fluorescence. To determine the mobility of cations and anions separately, a trend in the value of the relative mobility was analyzed by varying the mobility of one of the geminate partners. The ratios between the mobilities of the anion and the cation of the same molecules were found to be about 1.1. It was shown that in liquid alkanes, the solvent electrostriction was the main factor determining a decrease in the mobility of an ion as compared to the parent neutral molecule. The strong dependence of the electrostrictive effect on the radius of the ionic solvation shell allows the observed difference between negative and positive charge carriers by a small but systematic difference in the effective radii of the ions to be explained.

Solution deposited NiO thin-films as hole transport layers in organic photovoltaics

Organic solar cells require suitable anode surface modifiers in order to selectively collect positive charge carriers and improve device performance. We employ a nickel metal organic ink precursor to fabricate NiO hole transport layers on indium tin oxide anodes. This solution deposited NiO annealed at 250 °C and plasma treated, achieves similar OPV device results reported with NiO films from PLD as well as PEDOT:PSS. We demonstrate a tunable work function by post-processing the NiO with an O 2-plasma surface treatment of varied power and time. We find that plasma treatment is necessary for optimal device performance. Optimal devices utilizing a solution deposited NiO hole transport layer show lower series resistance and increased fill factor when compared to solar cells with PEDOT:PSS.

Spectroscopic diagnostics of a pulsed discharge in high-pressure argon

Results of investigation of high-pressure argon plasma excited by a high-current pulsed volume discharge are presented. The plasma diagnostics employs spatiotemporal dependences of the emission intensity in the VUV — visible range. A homogenous discharge is observed at pressures up to 10 atm. It is found that the spectrum of the UV — visible photorecombination continuum is sensitive to the discharge constriction. Change in the shape of the spectrum is caused bythe change of the type of positive charge carriers upon passing of the discharge from the uniform phase (molecular Ar2+ ions) to the arc phase (atomic Ar+ ions). Experimental data and model calculations show that the electron heating after the main excitation pulse is a highly undesirable process. It slows down the recombination flow in the plasma, which results in stretching of all the kinetic processes for all excited components in time, and hence in a decrease in the peak values of their concentrations. Electron collision-induced mixing effi-ciently converts the reservoir of long-lived Ar2* molecules in the triplet state into rapidly emitting singlet excimers. It is this mechanism that dominates the production of singlet Ar2* excimer molecules. The threshold concentration needed to obtain lasing at a wavelength of 127 nm on Ar2* excimers (1Σ+u(v=0)) was, according to calculations, about 5×1015 cm-3 for the gain 0.05 cm-1. This concentration can be achieved in the case of homogeneous pulsed discharge pumping with the peak electron concentration 2.×1016 cm-3 at the argon pressure 10 atm.

Positive electrification on the floor of an underground mine gallery at the arrival of seismic waves and similar electrification on the surface of partially stressed rocks in laboratory

Terra Nova, 22, 203–207, 2010 Abstract A large plate electrode in a mine gallery detected electric signals when seismic waves arrived. This signal is equivalent to the generation of an electric field with the upward negative and the downward positive. One possible reason for detection of the electric field is a positive electrification of the floor. Here, we studied such electrification in the laboratory using an andesite block quarried from the same mine. When partially loaded at one end, the block exhibits positive electrification on the surface of the other unstressed end. The activation of positive hole charge carriers reasonably explain the laboratory results. The positive holes are activated by breaking peroxy bonds, a common type of lattice defects in igneous rock-forming minerals. The positive holes spread to the surface, thereby causing the electric field. The same charge carriers may be activated upon arrival of seismic waves.

Oxygen diffusion in relation to p-type doping in uranium dioxide

In this paper we show how electrical conductivity and intrinsic oxygen diffusion coefficient measurements can be used in conjunction to further our understanding of oxygen related point defects in UO2. Electrical conductivity measurements have enabled an estimate to be made of the positive charge carrier concentration in two sets of samples containing different levels of doping agents. The gas–solid isotopic exchange method was then used to load the samples with 18O tracer atoms the concentration profile of which were subsequently characterised using SIMS and chromatic confocal microscopy. At the oxygen potential and temperature studied (750 °C), application of point-defect theory to our experimental results points to oxygen migration proceeding via an interstitial mechanism and to the fact that impurities control the concentration of negatively charged point defects responsible for atomic migration.

Electrical and optical properties of poly(2-dodecanoylsulfanyl-p-phenylenevnylene) and its application in electroluminescent devices

We describe the optical and electrical characterization of a poly(p-phenylenevinylene) derivative: poly(2-dodecanoylsulfanyl-p-phenylenevinylene) (12COS-PPV). The electrical characterization was carried out on devices with the FTO\PEDOT:PSS\12COS-PPV/Al structure. Positive charge carrier mobility μ h of ~1.0 × 10−6 cm2 V−1 s−1 and barrier height φ of ~0.1 eV for positive charge carrier injection at the PEDOT:PSS/12COS-PPV interface were obtained using a thermionic injection model. FTO\PEDOT:PSS\12COS-PPV/Ca devices exhibited green-yellow electroluminescence with maximum emission at λ = 540 nm.

VUV–VIS spectroscopic diagnostics of a pulsed high-pressure discharge in argon

High-pressure argon plasma, excited by a high-current pulsed volume discharge, has been investigated. Spatial-time VUV–VIS emission kinetics were used for the plasma diagnostics. A homogeneous discharge was obtained at a pressure of up to 10 bar. It was revealed that the spectral shape of the UV–VIS photorecombination continuum is a sensitive diagnostic tool for the constriction of the discharge. This shape changes because of the difference of the positive charge carriers in the arc (atomic Ar+ ions) and homogeneous (molecular ions) phases of the discharge. The intensity of this continuum is proportional to the square of the electron density. The experimental data and modelling show that the heating of electrons after the main excitation pulse is a very undesirable process. It suppresses the recombination flow in plasma, thus the kinetics of all excited species are spread in time with a decrease in the excimers densities. The electron collision-induced mixing effectively converts the reservoir of long-lived triplet molecules to fast-emitted singlet excimers. This mechanism is dominant in the production of singlet excimers.A realistic threshold density for the lasing of excimers of about 5 × 1015 cm−3 was estimated (the gain coefficient is 0.05 cm−1). This criterion could be realized in 10 bar of Ar by a homogeneous single pulse discharge pumping with a peak electron density of 2.4 × 1016 cm−3.

Crystal structure, morphology and physical properties of LaCo 1−xTi xO 3±δ perovskites prepared by a citric acid assisted soft chemistry synthesis

The influence of the partial substitution of Co by Ti in the LaCoO 3 perovskite system is studied by evaluating the electrical conductivity, the Seebeck coefficient and the thermal conductivity of the compounds up to T = 1273 K. The X-ray diffraction patterns of the LaCo 1− x Ti x O 3± δ (0.01 ⩽ x ⩽ 0.5) phases show two structural modifications depending on the Ti content. Compounds with x < 0.3 crystallize in the rhombohedrally distorted perovskite structure while samples with x ⩾ 0.3 possess an orthorhombic unit cell. The oxidation state of the Co ions is studied by X-ray absorption near edge structure (XANES) spectroscopy. A negative thermoelectric power is found in the LaCoO 3 system for low level Ti substitution ( x = 0.01). In contrast, samples with higher Ti content show a large positive Seebeck coefficient, indicating positive majority charge carriers in the system. The electrical resistivity of the studied materials reveals a semiconducting-like behaviour. The lattice thermal conductivity was found to be low and nearly temperature-independent. The samples exhibit very small crystallite sizes in the range of few nanometres. Therefore, the low thermal conductivity can be assigned to an enhanced phonon scattering on grain boundaries.

Coherent Holes in a Semiconductor Quantum Dot

Quantum states of positive charge carriers may be more stable to information loss than those of electron-based systems.

Photoprogrammable Organic Light‐Emitting Diodes

Enlightening the memory: The integration of a crosslinkable photochromic dithienylperfluorocyclopentene (DTE) into organic light-emitting diodes (OLED) allows for the individualization of the emissive area of the OLED device, for example, for signage applications. The operation principle is based on switching the injection barrier for holes (positive charge carriers). Very large ON/OFF ratios of up to 3000 for current as well as electroluminescence have been achieved.

Molecular Dynamics Simulations of Electrosprayed Water Nanodroplets: Internal Potential Gradients, Location of Excess Charge Centers, and “Hopping” Protons

Water nanodroplets charged with excess protons play a central role during electrospray ionization (ESI). In the current study molecular dynamics (MD) simulations were used for gaining insights into the nanodroplet behavior based on classical mechanics. The SPC/E water model was modified to permit the inclusion of protons as highly mobile point charges at minimum computational cost. A spherical trapping potential was assigned to every SPC/E oxygen, thereby allowing the formation of protonated water molecules. Within a tightly packed nanodroplet the individual potential wells merge to form a three-dimensional energy landscape that facilitates rapid proton hopping between water molecules. This approach requires short-range modifications to the standard Coulomb potential for modeling electrostatic proton-water interactions. Simulations on nanodroplets consisting of 1248 water molecules and 10 protons (radius, ca. 21 A) result in a proton diffusion coefficient that is in agreement with the value measured in bulk solution. Radial proton distributions extracted from 1 ns MD runs exhibit a large peak around 14 A, in addition to substantial population density closer to the droplet center. Similar radial distributions were found for nanodroplets charged with Na+ ions. This behavior is dramatically different from that expected on the basis of continuum electrostatic theory, which predicts that excess charge should be confined to a thin layer on the droplet surface. One important contributor to this effect seems to be the ordering of water molecules at the liquid/vacuum interface. This ordering results in an electrical double layer, generating a potential gradient that tends to pull positive charge carriers (such as protons, but also others such as Na+ ions) toward the droplet interior. This deviation from the widely assumed surface charge paradigm could have implications for the mechanism by which protonated analyte ions are formed during ESI.

Water-induced polaron formation at the pentacene surface: Quantum mechanical molecular mechanics simulations

Water is an omnipresent polar impurity that is expected to be the origin of many electric degradation phenomena observed in organic semiconductors. Here, we describe a microscopic model for polaron formation in the outermost layer of a pentacene crystal due to the polarization of a nearby water layer. The efficient coupling of a classical force field that describes the liquid with a tight-binding model that represents the $\ensuremath{\pi}$ system of the organic layer permits the calculation of nanosecond length trajectories. The model predicts that the reorientation of water dipoles stabilizes positive charge carriers on average by 0.6 eV and thus leads to a polaron trap state at the liquid interface. Thermal fluctuations of the water molecules provoke two-dimensional diffusive hopping of the charge carrier parallel to the interface with mobilities of up to $0.6\text{ }{\text{cm}}^{2}\text{ }{\text{s}}^{\ensuremath{-}1}\text{ }{\text{V}}^{\ensuremath{-}1}$ and lead to an amorphous broadening of the valence-band tail. As a consequence, water-filled nanocavities act as trapping sites in pentacene transistors. Instead, a complete wetting of the organic film is expected to result in fast thermally activated hopping transport. Polaron trapping is thus not expected to be a limiting factor for transistor-based sensors that operate under water.

Intra-molecular mobility of holes along rod-like helical Si backbones in optically active polysilanes

Intra-molecular mobility of positive charge carriers in poly( n-hexyl-[ S]-2-methylbutylsilane) (PHMBS) films doped with N, N′-bis(2,5-di- tert-buthylphenyl)-3,4,9,10-perylenedicarboximide (PDI) is studied by time-resolved microwave conductivity (TRMC) measurements. PHMBS with optically active side chains has a rod-like tightly locked Si catenation with unlikely long persistence length compared with conventional dialkyl-substituted polysilanes. It was found that PDI is a suitable electron acceptor for PHMBS as it provides the high product of photo carrier generation yield ϕ as ∼0.08% under an excitation at 355 nm. The efficient electron transfer reaction from PHMBS to PDI gives clear conductivity transients ascribed to the positive charges on the Si catenation of PHMBS by TRMC measurements. The estimated value of isotropic mobility along the tightly locked Si catenation is 0.36 cm 2 V −1 s −1, suggesting the presence of mobile holes on the backbones of PHMBS.

Light-assisted deep-trapping of holes in conjugated polymers.

The injection of positive charge carriers (holes) into a single conjugated polymer chain was observed to be light-assisted. This effect may underlie critical, poorly understood organic electronic device phenomena such as the build-up of functional deeply trapped charge layers in polymer light emitting diodes. The charging/discharging dynamics were investigated indirectly by a variety of single molecule electro-optical spectroscopic techniques, including an "image-capture" approach.

Space charge behavior at physical interfaces in cross-linked polyethylene under DC field

Space charge behavior at physical interfaces in cross-linked polyethylene (XLPE) films subjected to a dc field has been investigated as a function of the nature of the interfaces using the pulsed electro-acoustic technique. Aluminum, gold and carbon black-loaded polyethylene were used as electrodes to investigate space charge build-up in a single dielectric layer. A sandwich structure constituted by two dielectric films of the same nature was used to study the charging propensity of the dielectric/dielectric interface. Samples with uncharged and charged interfaces were investigated. The time dependence of the space charge distribution was subsequently recorded at different temperatures under field (polarization) and short circuit conditions (depolarization). Charge injection rather than ionic species migration is seen to control space charge distribution in single and double- layer samples in agreement with the storage conditions of the XLPE films. It is shown that the sign and distribution of the space charge depend drastically on the nature and the polarity of the electrodes and that dielectric/dielectric interfaces behave differently depending on the sign of the charge, providing deeper traps for positive charge carriers.

Photoconductive novel mesomorphic oxotitanium phthalocyanines

The synthesis and photoconductivity properties of the alkylthia and triethyleneoxysulfonyl substituted oxotitanium(IV) phthalocyanines ( 1a and 2a) are reported for the first time. The new compounds have been characterized by elemental analysis, IR, 1H and 13C NMR spectroscopy, electronic spectroscopy and mass spectroscopy. The mesogenic properties of these new materials were studied by differential scanning calorimetry and optical microscopy. The electrical dark conductivities ( σ d) and photoconductivities ( σ ph) of deposited films of 1a and 2a were investigated in various media with different concentrations of oxygen. Molecular oxygen increases photoconductivities ( σ ph) significantly. The photoconductivity mechanism and formation of Pc + which is positive charge carrier under the light irradiation of the phthalocyanine molecules are demonstrated using the theoretical calculations. The geometries of the oxotitanium(IV) phthalocyanines (TiOPc) are optimized with PM3 semi-empirical method, and their visible absorption maxima are calculated with ZINDO/S method. The results agree well with the observed values. It was found that for the calculation of visible absorption of neutral and positively charged substituted TiOPc molecules using ZINDO/S method could rapidly yield excellent results. Dipole moment, HOMO, LUMO energies and atomic charges are also calculated for clarification of the oxygen effect on the photoconductivity using PM3 and ZINDO/S methods.

Influence of intensive light exposure on the complex impedance of polymer light-emitting diodes

In this work we investigated the effect of visible radiation on the electrical properties of poly[(2-methoxy-5-hexyloxy)-p-phenylenevinylene]- MH-PPV films and light emitting diodes. Complex impedance measurements of (Au or ITO)/MH-PPV/(Au or Al) samples were carried out at room temperature and exposed to white light. Over the frequency range from 100 mHz to 2 MHz, the electrical results of Au/MH-PPV/Au was dominated by the Cole-Cole approach, where the electrode influence is negligible. However, some additional influence of the interface was observed to occur when Al was used as electrode. These effects were observed under both dark and visible-light illumination conditions. A simple model based on resistor-capacitor parallel circuits was developed to represent the complex impedance of the samples, thereby separating bulk and interface contributions. We observed that the polymer electrical resistivity decreased while the dielectric constant of the polymer and the thickness of the Al/MH-PPV layer were almost constant with increasing light intensity. The decrease of the polymer layer resistance comes from a better charge injection due to a light induced dissociation of positive charge carriers at the electrode.

Localized luminescence of zinc oxide nanostructures

The photoluminescence (PL) of materials has been widely used to investigate their optical properties. Recently, cathodoluminescence (CL) spectroscopy has also attractedmuch attention for the characterization of nanostructures such as nanowires, nanorods, and nanoribbons.1–3 This technique records luminescence after creating pairs of negative and positive charge carriers (so-called electron-hole pairs) in a material by high-energy electron bombardment. CL spectra are typically captured using a scanning electron microscope. The highly focused beam of electrons impinges on the sample and induces it to emit light from a localized area, with the result that CL achieves higher spatial resolution than conventional photoluminescence (PL) techniques. Furthermore, CL can also provide additional information on local stress and impurities in nanostructures, which is usually beyond the capability of other methods. Recently, we used CL to investigate zinc oxide (ZnO) nanostructures.2 ZnO has a direct bandgap of∼3.3eV and an excitonic binding energy (i.e., of the electron-hole pair) of 60meV. It has become a focus of interest owing to its potential for a wide variety of optoelectronic applications, including nanowires and nanorods. A ZnO luminescence spectra typically consists of a sharp band at ∼380nm, due to near band edge excitonic recombinations, and a broad green emission band in the visible at ∼510nm due to deep states inside the bandgap.4 The stronger luminescence of the ∼380nm-band relative to that of the visible emission is often considered indicative of defect-free, crystalline ZnO structures. Figure 1(a) shows a CLmeasurement recorded for a single isolated ZnO nanowire with a diameter of ∼25nm. Monochromatic CL images were also simultaneously recorded at 380 and 520nm, as shown in Figure 1(b) and (c). These images clearly demonstrate that low-level luminescence signals from a nanostructure can be obtained. Figure 1(d) shows that the nanowire spectrum displays the two characteristic ZnO luminescence peaks. The Figure 1. (a) Scanning electron microscope image of a ZnO nanowire on a silicon substrate. (b) Monochromatic CL image of (a) at 380nm. (c) Monochromatic CL image of (a) at 520nm. (d) CL spectrum obtained from the same area as in (a).

Bipolar charge transport model with trapping and recombination: an analysis of the current versus applied electric field characteristic in steady state conditions

A bipolar model of transport intended to describe the behaviour of polyethylene under dc stress in steady state conditions is presented. The model is based on Poisson's equation and the conservation law of charges, with trapping, detrapping and recombination phenomena being taken into account. Electric field and carrier concentrations are formulated for double-carrier injection and boundary conditions are given by the Schottky injection law. This system of equations with partial differential functions is solved using a finite volume method. The current versus applied electric field characteristic is obtained and analysed for different parameter sets of the model in order to understand their role in the predicted macroscopic behaviour. To do so, we consider two approaches. In a first step, the simulation has been realized with symmetric parameters for positive and negative charge carriers in order to facilitate the interpretation. In a second step, asymmetric parameters have been considered in the model as it constitutes a more realistic situation. Results show that the recombination plays an essential role in order to obtain the two changes in slope, forming a sigmoid-like shape in the current–voltage characteristic as observed experimentally in this kind of material.

Ambipolar Charge Carrier Transport Properties in the Homologous Series of 2,3,6,7,10,11-hexaalkoxytriphenylene

In this study, ambipolar charge carrier transport properties were investigated for a homologous series of triphenylene by time-of-flight (TOF) technique for the homeotropic domains. Ambipolar electronic charge carrier transport was observed and the both positive and negative charge carrier mobilities decrease with the increase of the alkyl chain length. Ionic conduction was also observed for the negative charge carrier transport except for C4OTP and any remarkable dependency of the alkyl chain length was not seen. It was reasonably found that both the positive and negative charged carrier mobilities by electronic hopping process is affected by the intra columnar order.