Role of surface modification of CdS nanoparticles on the performance of hybrid photovoltaic devices based on p-phenylenevinylene derivate

The role of organic capping ligand of semiconductor nanoparticles in dictating the interfacial charge transfer processes in hybrid semiconductor nanoparticles/polymer-based photovoltaic devices is investigated. Morphology, optical and structural study of the CdS nanoparticles and the hybrid material were accomplished using X-ray diffraction (XRD), absorption (UV–vis), atomic force microscopy (AFM), transmission electron microscopy (TEM), photoluminescence (PL) and time resolved photoluminescence spectroscopy (PLRT). A broad band absorption in UV–visible region and considerable fluorescence quenching of MEH-PPV in the composites are noted indicating a photo-induced charge transfer and dissociation of excitons. Time-resolved photoluminescence measurements indicating decreased lifetime further confirm this process. The solar cells open-circuit voltage and short-circuit current were improved using thiophenol modified CdS nanoparticles as electron acceptor in comparison to MEH-PPV only device demonstrating a promising approach to enhance charge transport in the hybrid nanoparticles–polymer composite photovoltaic cells (PV).     

Microscopic view of charge injection in an organic semiconductor

We have measured the chemical potential and capacitance in a disordered organic semiconductor by electric force microscopy, following the electric field and interfacial charge density microscopically as the semiconductor undergoes a transition from Ohmic to space-charge limited conduction. Electric field and charge density at the metal-organic interface are inferred from the chemical potential and current. The charge density at this interface increases with electric field much faster than is predicted by the standard diffusion-limited thermionic emission theories.     

Effect of meson exchange current and three-body force on the charge form factor of3He

We report a calculation of meson exchange current contribution to the charge form factor of³He by the hyper spherical harmonic expansion method with the inclusion of two-pion exchange three nucleon force. Results indicate that the charge form factor is changed appreciably in the right direction at high momentum transfer, due to the inclusion of the meson exchange current.     

Dependence of charge injection on temperature, electric field, and energetic disorder in an organic semiconductor

In order to determine energetic disorder's role in facilitating charge injection from gold into a molecularly doped polymer, we have examined the dependence of current on the local electric field, measured using electric force microscopy, at temperatures ranging from 250 to 330 K. From these data we infer, in a single experiment, the temperature dependence of the main factors governing the injection current: the electric-field induced lowering of the image-potential barrier, the interfacial charge density, and the mobility. In this system, the Schottky effect is anomolously large, and the interfacial charge density is larger than expected and strikingly non-Arhennius. Our analysis indicates that these effects are all a consequence of the Gaussian density of states in the organic.     

Analysis of the photovoltaic efficiency of a molecular solar cell based on a two-level system

A molecular solar cell is modelled as a two-level system connected to electrodes by chains of electron-transporting and hole-transporting orbitals. Light absorption and emission are simulated using the generalised Planck equation and intermolecular charge transfer using non-adiabatic Marcus theory. Quantum efficiency–voltage characteristics, open-circuit voltage and monochromatic power-conversion efficiency are calculated as a function of the following parameters: charge-separation rate, interfacial recombination rate, charge mobility, light intensity and built-in bias. We find that slow charge separation, fast recombination and low mobility all contribute to a decrease in efficiency compared to the ideal (detailed balance) limit. When charge-separation and interfacial recombination rates are related through the intermolecular coupling, maximum efficiency is achieved at some optimum, but not the maximum, charge-separation rate. Two regimes are distinguished for the open-circuit voltage: when interfacial recombination is important, V_oc varies approximately linearly with the donor–acceptor energy gap; but when recombination is insignificant, V_oc is determined by the optical gap. Including exciton binding energy in the driving force for charge separation reduces V_oc. In systems with significant recombination, V_oc first increases and then saturates with increasing light intensity. Low mobility and interfacial recombination are the main avoidable sources of loss when realistic parameters are used, but the effects of low mobility can be partly compensated by applying a built-in bias between the electrodes. PACS 72.40.+w; 73.40.Lq; 72.80.Le     

Extended phase diagram of La1-xCaxMnO3 by interfacial engineering

The interfacial enhanced ferromagnetism in maganite/ruthenate system is regarded as a promising path to broaden the potential of oxide-based electronic device applications. Here, we systematically studied the physical properties of LaLa_1-xCa_xMnO₃/SrRuO₃ superlattices and compared them with the LaLa_1-xCa_xMnO₃ thin films and bulk compounds. The LaLa_1-xCa_xMnO₃/SrRuO₃ superlattices exhibit significant enhancement of Curie temperature (T_C) beyond the corresponding thin films and bulks. Based on these results, we constructed an extended phase diagram of LaLa_1-xCa_xMnO₃ under interfacial engineering. We considered the interfacial charge transfer and structural proximity effects as the origin of the interface-induced high T_C. The structural characterizations revealed a pronounced increase of B-O-B bond angle, which could be the main driving force for the high T_C in the superlattices. Our work inspires a deeper understanding of the collective effects of interfacial charge transfer and structural proximity on the physical properties of oxide heterostructures.     

Reduced charge transfer exciton recombination in organic semiconductor heterojunctions by molecular doping

We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer-fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by ~70%. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states.     

Effect of non-uniform reaction rates at solid–oxide interfaces on the electrochemical impedance

An impedance model is developed for a porous oxygen electrode on top of a solid oxide conductor, taking into account adsorption and surface transport along the pore walls, interfacial diffusion and reaction along the interface, as well as current distribution (2D) in the electrolyte. All parameters are in principal measurable. Simulated impedance spectra typically exhibit two semicircles, one related to the charge transfer reaction at the interface (high frequencies), and one related to mass transfer limitations (low frequencies). The resolution of these two semicircles, however, depend on the relative contributions of these two processes to the overall potential losses, and the magnitude of the interfacial capacitance relative to the other kinetic and transport parameters, as well as geometrical parameters.     

Probing reaction mechanisms in mixed phase TiO2 by EPR

Charge separation processes in mixed phase TiO₂ photocatalysts are investigated by electron paramagnetic resonance (EPR) spectroscopy. The mechanisms of interfacial electron transfer, subsequent charge migration and recombination at surface sites, and other interfacial effects on chlorophenol/TiO₂ chemistry have been probed. Distorted interfacial sites have been observed and are proposed as catalytically reactive hot spots. This detailed knowledge of charge transfer processes is critical to the nanoscale design of catalysts and subsequent improvement of catalytic efficiency.     

界面耦合对DNA分子电荷输运性质的影响

在三维紧束缚模型下,采用传递矩阵的方法研究了不同界面耦合对DNA分子电荷输运性质的影响.结果表明：界面耦合理想时,透射率和电子定域长度都很大,分子呈现出良好的导电性;反之,透射率和电子定域长度减小,分子呈现出较差的导电性.当分子与金属电极之间理想耦合时,从伏安特性上可以看出,poly(dG)-poly(dC) DNA分子的开启电压小于poly(dA)-poly(dT) DNA分子的开启电压,并且在相同的偏压下前者的电流值要大于后者.因此,poly(dG)-poly(dC) DNA分子的导电性优于poly( 关键词： 界面耦合 透射率 定域长度 导电性     

The Electric Field Outside a Stationary Resistive Wire Carrying a Constant Current

We present the opinion of some authors who believe there is no force between a stationary charge and a stationary resistive wire carrying a constant current. We show that this force is different from zero and present its main components: the force due to the charges induced in the wire by the test charge and a force proportional to the current in the resistive wire. We also discuss briefly a component of the force proportional to the square of the current which should exist according to some models and another component due to the acceleration of the conduction electrons in a curved wire carrying a dc current (centripetal acceleration). Finally, we analyze experiments showing the existence of the electric field proportional to the current in resistive wires.     

Screen charge transfer by grounded tip on ferroelectric surfaces

We have investigated polarization reversal and charge transfer effects by a grounded tip on 50 nm thick ferroelectric thin films using piezoelectric force microscopy and Kelvin force microscopy. We observed the polarization reversal in the center of written domains, and also identified another mechanism, which is the transfer of screen charges toward the grounded tip. In order to overcome these phenomena, we successfully applied a modified read/write scheme featuring a bias voltage. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron injection dynamics in dye-sensitized semiconductor nanocrystalline films

We have summarized recent ultrafast spectroscopic studies on phenomena associated with dye-sensitization of semiconductor metal oxide nanoparticles, especially TiO₂ nanocrystalline film from a surface science perspective with a strong relation to mechanism of electron injection in dye-sensitized solar cells, which are attracting much interest from both viewpoints of pure science and applied science.A lot of chemical and physical processes are involved in this solar cell, such as light harvesting by molecules and nanostructures, interfacial electron transfer, charge migration in solid and electrolyte, degradation of the materials, and so on. Among them, the very primary process initiated by photoabsorption by sensitizing dye molecules; that is, electron injection from excited adsorbates into the conduction band of semiconductor metal oxides is significantly important, because this process must be 100% efficient with a minimum driving force for high current and voltage generation.We have first focused on details of experimental methods used in this research area, and then in the following Sections, have organized this review by concentrating on each parameter that influences dynamics of electron injection in dye-sensitized semiconductors. Finally we have emphasized it is important to measure actual DSSCs for the precise comparison between electron injection dynamics and device performance.     

Charge transfer from ammonia physisorbed on nanotubes

We report the use of nanotube field-effect transistor devices for chemical sensing in a conducting liquid environment. Detection of ammonia occurs through the shift of the gate voltage dependence of the source-drain current. We attribute this shift to charge transfer from adsorbed ammonia molecules, with the amount of charge estimated to be as small as 40 electrons for the smallest shift detected. Using the concentration dependence of the response as an adsorption isotherm, we are able to measure the amount of charge transfer to be 0.04 electron per ammonia molecule.     

Effect of oxygen vacancy on Cu(111)/ZnO interface

ABSTRACT

Using the first-principles calculation, we explore the interfacial characteristics of ZnO monolayer on the Cu(111) surface with and without oxygen vacancy. It is found that electrons transfer from the Cu substrate towards the ZnO monolayer and the different position of O atoms relative to the Cu surface determine the interfacial interaction and then turn a flat graphitic ZnO monolayer into an asymmetrical dumpling structure. The oxygen vacancy is not only the result of substrate stabilisation effect but also strengthen the interfacial interaction to make charge transfer mechanism and dumpling effect dominant to the compression effect, thus resulting in an overall increase of Cu work function and the decreasing of potential step.     

Organic/metal interfaces in self-assembled monolayers of conjugated thiols: A first-principles benchmark study

We present an extensive theoretical study of a self-assembled monolayer of a prototype π-conjugated thiol on Au(1 1 1). A series of experimentally accessible quantities are calculated and compared to available experimental data. After assessing the methodology and re-evaluating experimental data in the light of our theoretical results, we focus on the interfacial phenomena determining the modification of the effective workfunction of the substrate and link our findings to the energetic alignment of the frontier molecular orbitals with the metal Fermi level. Since we find no evidence of charge transfer between metal and molecule, the observed interfacial phenomena are rationalized in terms of charge fluctuations and electrostatics at the atomic length scale. The microscopic picture established in this work provides a deeper understanding of the interfacial processes that govern the working principle of single-molecule electronics and organic electronic devices.     

Interfacial charge current in the ferromagnet/two-dimensional electron gas junction with Rashba spin orbital interaction

We report a theoretical study on the interfacial electron transport in the ferromagnet/two-dimensional electron gas (FM/2DEG) hybrid junction at zero bias, where the Rashba spin-orbit interaction (RSOI) is considered in 2DEG region. It is shown that a nonzero charge current can spontaneously flow in the interface of the junction due to broken time reversal symmetry and spin-dependent scattering of electron at interface. This interfacial charge current can be modulated by system parameters such as the magnetization of FM, RSOI strength, and interface barrier, moreover, it can be optimized as the magnetization of FM in 2DEG plane is perpendicular to interface whereas it can vanish as the FM magnetization is parallel to interface.     

Electrophoretic mobility without charge driven by polarisation of the nanoparticle–water interface

Polarisation of the interface, spontaneously occurring when water is in contact with hydrophobic solutes or air, couples with the uniform external field to produce a non-zero force acting on a suspended particle. This force exists even in the absence of a net particle charge, and its direction is affected by the first-order, dipolar and the second-order, qudrupolar orientational order parameters of the interfacial water. The quadrupolar polarisation gives rise to an effectively negative charge. The corresponding surface charge density is inversely proportional to the area of the shear surface. As a result, the overall contribution from the quadrupolar polarisation to the particle mobility becomes negligible compared to experimentally reported values for particles exceeding a few nanometres in size. In contrast, the contribution of the dipolar order of the interface to the effective surface charge scales inversely with the particle size and dominates the zero-charge mobility of submicron particles. The corresponding electrokinetic charge is determined by the preferential orientation of interfacial dipoles relative to the surface normal.     

激光加速质子束空间电荷力研究

激光加速器可以输出具有独特品质的质子束,例如μm尺寸、ps脉冲长度和高峰值电流。强流粒子束的空间电荷力效应较强,对面向应用的束流传输提出了挑战。通过二维PIC模拟研究了激光加速后与质子速度接近的电子的影响。采用椭球模型估算空间电荷力的影响,比较不同电荷分布的差异。结果表明每束团质子数超过1010后空间电荷力显著影响质子束传输,甚至严重破坏束流品质。空间电荷力的影响在20 ps后显著减弱,离开靶约1.2 mm。     

PULSED LASER-INDUCED TRANSIENT SUAFACE PHOTOVOLTAIC TECHNIQUE FOR STUDYING THE INTERFACIAL CHARGE TRANSFER PROCESSES

In this paper,the processes of the interfacial charge transfer between theP-Si(111) and CuTsPc supramolecular film(L-B)have been directly observed by usinga pulsed laser-induced transient surface photovoltaic technique. We also disoovered thatthe SiO_2 film on the surface of P-Si(111)influences the photovoltaie response.