Browsing by Subject "Electron"
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Item Charge and energy interactions between nanoparticles and low pressure plasmas.(2010-05) Galli, FedericoIn this work, the interactions between low-pressure plasmas and nanoparticles are studied with numerical models aimed at understanding the phenomena that affect the nanoparticles charge, charge distribution, heating, and crystallization dinamycs. At the same time other phenomena that affect the plasma properties resulting from the presence of nanoparticles are also studied: they include the power-coupling to the plasmas, the ion energy distribution and the electron energy distribution. An analytical model predicting the nano-particle charge and temperature distributions in a low pressure plasma is developed. The model includes the effect of collisions between ions and neutrals in proximity of the particles. In agreement with experimental evidence for pressures of a few Torr a charge distribution that is less negative than the prediction from the collisionless orbital motion limited theory is obtained. Under similar plasma conditions an enhanced ion current to the particle is found. Ion-electron recombination at the particle surface, together with other particle heating and cooling mechanisms typical of silane-argon plasmas, is included in a particle heating model which predicts the nano-particle temperature. The effect of plasma parameters on the nano-particle temperature distribution is discussed and the predictive power of the model is demonstrated against experimental evidence of temperature induced crystallization of silicon nano-particles. The power coupled to the plasma is measured together with the impedance nature of the plasma, in the case of a pristine and dusty plasma. Nanoparticles are shown to strongly affect the electrical properties of the plasma, resulting in a much more resistive discharge. A study of the ion energy distribution of ions impinging the sruface of nanoparticles is carried out and shows that ion-neutral collisions in proximity of the surface of the nanoparticle not only affects the particle charge but also the average energy of ions bombarding the particle surface. Finally the presence of nanaparticle in the plasma and their ability to selectively interact with electrons in a specific energy range is studied to the extent of investigating the effects of the presence of particles on the electron energy distribution of electrons.Item Electron transport and recombination in nanowire dye-sensitized solar cells.(2010-02) Enache-Pommer, EmilThe dye-sensitized solar cell (DSSC) is a promising low cost photovoltaic device. A typical DSSC consists of a porous film made out of TiO2 nanoparticles, a monolayer of dye adsorbed on the TiO2 surface and a liquid electrolyte. The electrolyte fills the pores of the nanoparticle film forming a semiconductor-dye-electrolyte interface with large surface area. During illumination of the cell, the dye molecules inject electrons into the TiO2 nanoparticles. The injected electrons diffuse through the nanoparticle network by hopping from particle to particle until they are collected at a transparent conductive oxide (TCO) anode. Meanwhile, the charged dye molecules are reduced through an electrochemical reaction with a reductant in the electrolyte. The oxidized ionic species diffuse to the counter electrode and are reduced by electrons that have been collected at the anode and have traveled through the load to complete the circuit. Currently, dye-sensitized solar cells have reached efficiencies above 11 %, but further improvement is limited by electrons recombining with the electrolyte during their transport through the semiconductor nanoparticle network. Nanowire DSSCs have been recently introduced and have the potential to overcome the limitations of nanoparticle DSSCs, since the electron percolation through the nanoparticle network is replaced by a direct electron pathway from the point of injection to the TCO. Understanding the electron transport and recombination mechanisms in nanowire DSSCs is one of the key steps to improving DSSC efficiency. Towards this end polycrystalline TiO2, single-crystalline TiO2 and single crystalline ZnO nanowire DSSCs were fabricated and analyzed using current-voltage characteristics, optical measurements, and transient perturbation techniques such as intensity modulated photocurrent spectroscopy, photocurrent decay and open-circuit photovoltage decay. For single-crystal ZnO nanowire DSSCs, the measured electron transport time constants are independent of light intensity but change with nanowire length, seeding method and annealing time. Even if the measured transients are limited by the RC time constant of the solar cell, using the measured time constants as an upper limit for the actual electron transport time leads to the conclusion that the electron transport rate in ZnO nanowires is at least two orders of magnitude faster than the recombination rate. This indicates that the charge collection efficiency in ZnO nanowire DSSCs is nearly 100 %. These results show that films can be made out of 100 μm long ZnO nanowires while maintaining efficient charge collection. For DSSCs based on polycrystalline anatase TiO2 nanowires, the electron transport times show a power-law dependence on illumination intensity similar to that reported for TiO2 nanoparticle DSSCs. The magnitude of the electron transport times is also comparable to that of nanoparticle DSSCs, indicating that electron trapping and detrapping determine transport times for polycrystalline TiO2 nanowire DSSCs. Surprisingly, even for single-crystal rutile TiO2 nanowire DSSCs, the electron transport rate is on the order of the electron transport rate in nanoparticle-based DSSCs and not as fast as would be expected. Electron transport is slow and light intensity dependent indicating that trapping and detrapping, most likely in surface traps, still play an important role in electron transport even in single-crystal rutile TiO2 nanowires.