Lingyi Meng

Giant magnetic-field-induced strains in Heusler alloy NiMnGa with modified composition

A giant magnetic-field-induced strain (MFIS) of −3100 ppm has been obtained in Heusler alloy Ni52Mn22.2Ga25.8 single crystal in the [001] direction at a temperature from 23 to 31 °C. This MFIS reaches saturation in an applied field about 6 kOe, and exhibits the same amplitude with an opposite sign while the field is perpendicular to the samples. According to a previous model, this MFIS associates with the twin boundary motion. The martensitic self-strain has been found to be 2%, implying a preferential orientation of martensite variants. Results related to the magnetic properties are discussed.

Dynamic Monte Carlo Simulation for Highly Efficient Polymer Blend Photovoltaics

We developed a model system for blend polymers with electron-donating and -accepting compounds. It is found that the optimal energy conversion efficiency can be achieved when the feature size is around 10 nm. The first reaction method is used to describe the key processes (e.g., the generation, the diffusion, the dissociation at the interface for the excitons, the drift, the injection from the electrodes, and the collection by the electrodes for the charge carries) in the organic solar cell by the dynamic Monte Carlo simulation. Our simulations indicate that a 5% power conversion efficiency (PCE) is reachable with an optimum combination of charge mobility and morphology. The parameters used in this model study correspond to a blend of novel polymers (bis(thienylenevinylene)-substituted polythiophene and poly(perylene diimide-alt-dithienothiophene)), which features a broad absorption and a high mobility. The I-V curves are well-reproduced by our simulations, and the PCE for the polymer blend can reach up to 2.2%, which is higher than the experimental value (>1%), one of the best available experimental results up to now for the all-polymer solar cells. In addition, the dependency of PCE on the charge mobility and the material structure are also investigated.

An improved dynamic Monte Carlo model coupled with Poisson equation to simulate the performance of organic photovoltaic devices

We describe a new dynamic Monte Carlo model to simulate the operation of a polymer-blend solar cell; this model provides major improvements with respect to the one we developed earlier [J. Phys. Chem. B 114, 36 (2010)] by incorporating the Poisson equation and a charge thermoactivation mechanism. The advantage of the present approach is its capacity to deal with a nonuniform electrostatic potential that dynamically depends on the charge distribution. In this way, the unbalance in electron and hole mobilities and the space-charge induced potential distribution can be treated explicitly. Simulations reproduce well the experimental I-V curve in the dark and the open-circuit voltage under illumination of a polymer-blend solar cell. The dependence of the photovoltaic performance on the difference in electron and hole mobilities is discussed.

Water Transport and Purification in Nanochannels Controlled by Asymmetric Wettability

Biomimetic asymmetric nanochannels have recently attracted increasing attention from researchers, especially in the aspect of the asymmetric wettability (a hydrophilic-hydrophobic system), which can be utilized to control the wetting behavior of aqueous media and to offer a means for guiding water motion. By using molecular dynamics simulations, a design for a potentially efficient water filter is presented based on (n, n) single-walled carbon nanotubes, where n = 6, 8, 10 and 12, asymmetrically modified with hydrophilic groups (carboxyl, -COOH) at one tip and hydrophobic groups (trifluoromethyl, -CF(3) ) at the other. The reduced water density on the hydrophobic sides of the functionalized nanotubes are observed in both pure water and aqueous electrolyte solution, except for the functionalized (6, 6) tube, due to the change of dipole orientation of the single-file water wire within it. The functionalized (8, 8) tube can significantly maintain the low water density on the hydrophobic side. Both (6, 6) and (8, 8) tubes have relatively high energy barriers at their tips for ion permeation, which can be obtained by calculating the potential of mean force. Such tip functionalization of a nanotube therefore suggests the great possibilities of water transport and filtration, dominated by asymmetric wettability. The functionalized (8, 8) tube could act as a nanofluidic channel for water purification, not only for ion exclusion but also as a stable water column structure.

Magnetomechanical properties of single crystal Terfenol-D

Quasi-static and dynamic magnetomechanical measurements have been made on a twin-free oriented single crystal of Tb/sub 0.27/Dy/sub 0.73/Fe/sub 2/ (Terfenol-D). Unusually large values of dynamic strain coefficient, d/sub 33/, and magnetomechanical coupling coefficient, k/sub 33/, of 19.1 nm A/sup -1/ and 0.83 respectively were recorded with an oscillating field of amplitude 500 A m/sup -1/ rms at room temperature. The static magnetostriction saturated at 1688/spl times/10/sup -6/, however 90% of this strain was achieved with a field of 45 kA m/sup -1/. The strain versus magnetization curves exhibit unusually linear regions, allowing confirmation that 71/spl deg/ domain wall motion is primarily responsible for the high coupling coefficient of this material.

Device simulation of low-band gap polymer solar cells: Influence of electron-hole pair dissociation and decay rates on open-circuit voltage

We simulated the performance of recently developed highly efficient bulk heterojunction photovoltaic cells with poly [N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] as the donor and [6,6]-phenyl C70-butyric acid methyl ester as the acceptor, using a device model. The simulated current-voltage curve is in excellent agreement with the experiment. This enables us to analyze how microscopic processes of excitons and charges govern the device performance. The influence of dissociation rate and decay rate of photoinduced electron-hole pairs on the open-circuit voltage VOC is investigated. It is shown that a high dissociation rate relative to decay rate will lead to enhanced VOC.

Effects of size constraint on water filling process in nanotube

Molecular dynamics (MD) simulation and the potential of mean force (PMF) analysis are used to investigate the structural properties of water molecules near the end of nanotube for the whole process from the initial water filling up to the configuration stabilization inside the carbon nanotubes (CNTs). Numerical simulations showed that when a small-sized nanotube is immersed into the water bath, the size constraint will induce a prevailing orientation for the water molecule to diffuse into the tube and this effect can persist approximately 3.3 angstroms from the end of CNT. As the structure within the CNTs stabilizes, the ambient structural properties can indirectly reflect their corresponding properties inside the nanotube. Our results also showed that there exists a close correlation between the PMF analysis and the results of MD simulations, and the properties at nanometer scale are closely related to the size-constraint effect.

Theoretical Modeling of the Optical and Electrical Processes in Polymeric Solar Cells

The elementary processes occurred in organic solar cell include optical absorption, excitation energy transfer, photoinduced charge transfer, charge transport, and charge collection at the electrodes. Even though modern quantum chemistry has achieved great success in electronic structure calculations, it is still not enough to describe these elementary processes at first-principles. We describe in this chapter our recent progresses toward quantitative theoretical understanding of the optical and electronic processes in organic photovoltaic materials, including optical absorption and emission spectra for conjugated oligomers, energy transfer in polymers, charge transport in organic semiconductors, and device modeling of heterojunction solar cells based on dynamic Monte Carlo simulation and the continuum model.