Monte Carlo simulation of charge transport in amorphous selenium photoconductors

The electronic properties of amorphous materials are greatly affected by the density of localized states in the mobility gap of these materials. The exact shape of the density of states (DOS) distribution in amorphous selenium (a-Se) is still unresolved despite decades of research. One of the most commonly employed methods to investigate charge transport properties in high resistivity materials is time-of-flight (TOF) transient photoconductivity experiment. The TOF transient photoconductivity technique is used to measure the induced photocurrent in the external circuit when the sample is photoexcited. Information pertaining to carrier mobility and other carrier parameters are deduced from the shape of the photocurrent. The investigation of the charge transport phenomenon is well known to be a complicated task. Monte Carlo (MC) simulation method has become a standard method for carrier transport studies in amorphous materials. The purpose of this research work is to develop a Monte Carlo simulation model for charge transport in typical TOF transient photoconductivity experiment to investigate the DOS distribution in a-Se. The MC simulations were first performed for relatively simpler models for which theoretical and analytical solutions were available. The MC model developed here is based on simulating the drift of carriers resulting from photogeneration, subject to the influence of an applied electric field and multiple trapping events. The free drift time of photocarriers and their dwell time in the traps are stochastic in nature, in accordance with the probabilities of these events. Electron time-of-flight transient photocurrents were calculated in amorphous selenium as a function of the electric field. The distribution of localized states (DOS) in a-Se has been investigated by comparing the experimentally measured and calculated transient photocurrents. The analysis of multiple-trapping transport has been done by the discretization of a continuous DOS. The DOS distribution has been optimized to produce the best agreement between the calculated and measured transient photocurrents. The resulting DOS has distinct features: A first peak at ~0.30 eV below Ec with an amplitude ~1017 eV–1 cm–3, a second small peak (or shoulder) at 0.45–0.50 eV below Ec with an amplitude 1014–1015 eV–1 cm–3, and deep states with an integral concentration 1011–1014 cm–3 lying below 0.65 eV, whose exact distribution could not be resolved because of the limitations of the available experimental data. The density of states (DOS) distribution in the vicinity of the valence band mobility edge in vacuum coated a-Se films has been investigated by calculating the MC hole transient photocurrents at different temperatures, and also the dependence of the drift mobility on the temperature and field. The calculated TOF transient photocurrents were compared with experimental data published elsewhere. It is shown that, analogous to electron transport in a-Si:H, the DOS near Ev is a featureless, monotonically decreasing distribution in energy up to Ev + 0.4 eV, without the 0.28 eV peak near the valence band which was thought to control the hole drift mobility. Such a DOS was able to account for hole TOF data reported previously by several authors to date

Novel Soft Chemistry Synthesis of TiO2 for Applications in Dye–Sensitized Solar Cells and Photocatalysis

Although the high cost of solar cells prevents them being a primary candidate for energy production, great attention has been paid towards them because of the depletion of the conventional energy sources–fossil fuels–and the global warming effect, and the need to provide power to remote communities disconnected from the power grid. To reduce the cost, thin film technologies for silicon solar cells have also been investigated and commercialized, but dye sensitized solar cells (DSSC) have been considered as a promising alternative even for the silicon thin films with efficiency exceeding 10%. Compared with silicon-based photovoltaic devices, DSSCs are quite complex systems that require an intimate interaction among components. Within the last few years, conclusive smart solutions have been provided to improve the efficiency of these cells, with solar efficiency that makes them potential competitors against silicon devices. The most successful systems use titanium oxide as a core material tuned to collect and transmit the electrons generated by the photo-excitation of dye molecules. However, most of the solutions demonstrated so far require a thermal treatment of the TiO2 photoelectrodes at temperatures that preclude using any flexible organic substrate. This treatment prevents development of any roll-to-roll manufacturing process, which would be the only way to achieve cost effective large scale production. In order to overcome this major drawback, a novel synthesis of TiO2 at room temperature is described in the present document. This synthesis leads to 4-6 nm nanocrystalline anatase, the desired phase of titanium oxide for photoactive applications. An intensive study was carried out to explore the properties of these nanoparticles, via a mixture design study designed to analyze the influence of the starting composition on the final TiO2 structure. The influence of a post-synthesis thermal treatment was also explored. This 4 nm nanocrystalline TiO2 exhibits a high specific surface area and a good porosity that fulfills the requirements for an efficient photoanode; a high surface area allows high dye loading, and, hence, increases photocurrent and photo-conversion efficiency. Another important result of this study is the band gap, as it confirmed that nanocrystalline anatase has an indirect band gap and a quantum confinement for a crystal size of less than 10 nm. This result, well-known for bulk materials, had been discussed in some previous publications that claimed the effectiveness of a direct band gap. Following this synthesis and the structural and spectroscopic analyzes carried out in parallel, photocatalytic study was an important tool to further explore the semiconducting properties of this material. Additionally, our material gave very promising results in photocatalytic dye degradation, compared to the commercial products, even if it was not initially synthesized for this application. We assign these performances to the improved crystallinity resulting from thermal activation, without changing the crystal size, and to the ability to optimize the surface. This photocatalytic study gave us insights into the methods that optimize the electronic structure of the titanium oxide. Hence, we decided to thermally activate the nanoparticles before the preparation of films to be inserted into DSSCs. At this stage, as the thermal activation applies to the powder, the resulting material can still be used with flexible substrates. We have successfully integrated these nanoparticles in dye sensitized solar cells. Various organic additives were added to the TiO2 paste used to prepare photoelectrode films, to increase the porosity of the film and have a crack–free film with good attachment to the substrate. We demonstrated that the dye was chemically attached to the TiO2 surface, which led to better electron transport. Different treatment methods (UV and thermal) were applied to the film to cure it from organic additives and improve the electronic connectivity between the particles. When the UV treatment was applied as a single method, i.e. without thermal treatment, the cell performance was lower, but a combination of thermal treatment and UV enhanced this performance. We compared our nanoparticles to the reference material used in most of the studies on DSSC, that is, TiO2 Degussa, with cells prepared the same way. Our nanoparticles revealed higher overall conversion efficiency. As the dye attachment to the TiO2 surface is an important parameter that enhances the cell efficiency, so we checked via ATR-FTIR how the dye attached to the TiO2 surface. In addition, FTIR, UV-Vis, and IV measurements revealed that the amount of dye adsorbed was increased through HCl treatment of the photoelectrode. We also checked the internal resistance of the cell using impedance spectroscopy, and the analysis proved a successful integration of the nanoparticles in dye–sensitized solar cells as there was an increase in both the electron life time and the recombination resistance, and a decrease in the charge transfer resistance compared to the commercial powder.1 yea

MS FT-2-2 7 Orthogonal polynomials and quadrature: Theory, computation, and applications

Quadrature rules find many applications in science and engineering. Their analysis is a classical area of applied mathematics and continues to attract considerable attention. This seminar brings together speakers with expertise in a large variety of quadrature rules. It is the aim of the seminar to provide an overview of recent developments in the analysis of quadrature rules. The computation of error estimates and novel applications also are described

Нелінійна динаміка — 2013

The book of Proceedings includes extended abstracts of presentations on the Fourth International conference on nonlinear dynamics

Theory of fluctuations in disordered systems

The thesis is devoted to the study of various aspects of disordered and glassy systems. In the first part of the thesis, we have studied the problem of charachterizing the critical dynamical fluctuations of structural glasses at the dynamical transition point. A field theory approach has been developed combined with the replica method and it has been introduced an effective theory that is capable to describe the dynamical heterogeneities at the dynamical transition point. A Ginzburg criterion has been derived to understand the region of validity of the mean field approach. These results are valid for the critical behavior of the dynamics in the beta regime. To understand what happens in the alpha regime we have developed a Boltzmann Pseudodynamics approach to structural glasses that is able to cupture the quasi-equilibrium nature of the glassy dynamics in the long time regime. The third part of the thesis is devoted to the study of the glass and jamming physics of hard spheres in the infinite dimension limit. In this context we show that this model displays a Gardner transition that affects deeply the jamming part of the phase diagram. This means that to describe correctly the jamming properties of hard spheres we need to take into account the full replica symmetry breaking effects. The full replica symmetry breaking formalism for hard sphere systems is completely developed. The last part of the thesis is devoted to study mode coupling dynamics around a quasi-continuous transition