International Journal of Modern Physics B

In comparison with traditional solid p-n junction solar cells, the process of light-to-electric transformation in dye-sensitized solar cells is complicated. In order to obtain a comprehensive understanding of the physical and chemical mechanism in the complicated process, people have proposed some models to describe electron injection, diffusion and recombination occurred in the process. In this paper, we will give a brief review on these models. The electrical characteristic of dye-sensitized solar cell can be well described by the diffusion model, which was originally proposed by Södergren and later further developed by Ferber, Anta, Bisquert et al. The electron injection, diffusion and recombination manifest themselves via three parameters: injection efficiency η_inj, diffusion coefficient D and recombination rate (time) K (τ) in the diffusion equation. Meanwhile, some microscopic models have also been developed to evaluate η_inj, D and K. The dynamical behavior of electron injection can be described by a kinetic theory, and corresponding η_inj can be understood from a conduction-band fluctuation model or a two-energy-level model. The power-law dependence of D and K on electron density can be well explained by trapping model, but the temperature behavior of D cannot be explained by this model. In the potential barrier model, a weak electron-density-dependent D is obtained, and the observed temperature dependence of D in experiment is naturally expected. Although currently the relevant experimental results cannot be consistently explained within one model, we believe that these models still are important for us to understand the physical and chemical mechanism in these microscopical processes and are helpful for us to further improve the photovoltaic performance of dye-sensitized solar cell.

Nonlinear Discrete Klein–Gordon (NDKG) model is a nonlinear lattice model used to investigate different nonlinear phenomena raised in many physical contexts. In the present study we use this model to observe linear phonon scattering by stable spatially inhomogeneous equilibrium solution in the piece-wise linear (PWL) version (asymmetric double well) of nonlinearity instead of the conventional cubic or other types of nonlinearities. Considering one-site symmetric solution in such a system we are able to construct the exact expressions for the transmittance (T) and reflectance using the transfer matrix formalism. From the condition of perfect transmission (T = 1, hence reflectance, R = 0) in the elastic scattering process, we observe in our model, such a situation happens to appear at the threshold of a localized mode at the band-edge of the extended eigen modes. The advantage of the PWL version of nonlinearity is that all the results derived are exact.

Although atomic Bose gases are experimentally investigated in the dilute regime, interparticle interactions play an important role on the transition temperatures of Bose–Einstein condensation. In this study, Bose–Einstein condensation is handled using fractional calculus for a Bose gas consisting of interacting bosons which are trapped in a three-dimensional harmonic oscillator. In this frame, in order to introduce the nonextensive effect, fractionally generalized Bose–Einstein distribution function which features Mittag–Leffler function is adopted. The dependence of the transition temperature of Bose–Einstein condensation on α (a measure of fractality of space) has been established. The transition temperatures for the dilute ⁸⁷Rb, ²³Na and ⁷Li atomic gases have been obtained in consistent with experimental data and the nature of the interactions in the Bose–Einstein condensate has been enlightened. In the course of our investigations, we have arrived to the conclusion that for α < 1 attractive interactions and for α > 1 repulsive interactions are predominant.

First principles total energy calculations have been performed using full potential linear augmented plane wave method (FP-LAPW) within density functional theory to study the structural, electronic and optical properties of MgS_xSe_1-x, MgS_xTe_1-x and MgSe_xTe_1-x alloys in the rock salt crystallographic phase. The generalized gradient approximation parameterization scheme has been used for calculating the ground state structural parameters and their deviation from the Vegard's law has been discussed. Full relativistic electronic band structures and density of states have been calculated to study the electronic properties of the end binary compounds and ternary alloys MgS_xSe_1-x, MgS_xTe_1-x and MgSe_xTe_1-x (0.25 < x < 0.75). Optical bowing for these semiconductor alloys has been discussed in term of volume deformation, electronegativity and structural relaxation. Optical properties of the binary and ternary magnesium chalcogenides have been calculated in terms of the complex dielectric function and the results are compared with available theoretical and experimental data.

A continuum field model describing the electrical characteristics of polycrystalline semiconductors ceramics is suggested. Taking into account the continuum theory, a static differential equation about electron level on the base of Poisson equation is established. The one-dimensional calculation is carried out using the Runge–Kutta method. The effect of grain size, temperature and donor concentration on the current–voltage characteristic and specific capacitance of the material is calculated quantitatively using ZnO ceramics as an example. The results pointed out that current and voltage characteristics divide into three regions: Linear region before breakdown field, nonlinear region near breakdown field and upturn region after breakdown field. As the applied voltage increases, the grain boundary barrier and the grain boundary capacitance in the nonlinear zone drop drastically. The results are compared with experimental data. An interesting phenomenon is that the Schottky barrier has a small offset along the direction of the applied electric field.

We consider a system consisting of very small colloidal particles clothed each by f end-grafted flexible polymer chains we regarded as star polymers, and hard spherical colloidal particles in a good solvent. Our main objective is to determine the expression of the interaction force between a spherical colloid and a star polymer as a function of distance between them. We limit ourselves to the case where the star polymer is smaller than the colloid.

In the first part, the system is dissolved in a melt of short linear chains of polymerization degree P<N, where N denotes the polymerization degree of grafted chains. To compute the expected force, we consider two regimes: (1) high-grafting density and (2) small-grafting density (f < f*). For (f > f*), we show that the expression of the expected force coincides exactly with that of the case of a small molecular weight solvent. For (f < f*), we show that there is a change in behavior.

In the second part, we assume that the lengths of the f grafted chains were randomly distributed and there is talk of a polydisperse star polymer. We show that the computation of the expected force depends on the relative values of the polymerization degree of longest grafted chain, N, when it is compared to the typical one N_c ~ f^1/(α-1). Here α is the polydispersity exponent. We distinguish two regimes depending on whether N < N_c or N > N_c. For the regime with N < N_c, and comparing the expression of the force obtained for the monodisperse case, we can say that the polydispersity of grafted chains induce a drastic change of the force expression. For the regime with N > N_c, we found the existence of two distance-ranges. For small distances, the effective force expression is identical to that relative to the above case (N < N_c). But for high distances, the effective force expression is similar to the monodisperse case.

In the present paper, we compute the superfluid density of two-dimensional weakly interacting boson system at zero temperature using one-loop renormalization group calculations under the scheme of newly derived formula [M. Holzmann and G. Baym Phy. Rev. B 76, 092502 (2007)]. We find that the interactions between the boson particles are marginally irrelevant in the renormalization group sense. The fluctuations of high-energy scale enhance superfluid density compared with the result of the mean-field approximation. The correction has (ln μ) form, where μ is the chemical potential of the boson system.