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Titolo: Low dimension polymer-based nanostructures for photovoltaics
Data di pubblicazione: 5-giu-2013
Abstract: In this thesis it is discussed the effect of the low dimensionality on several physical properties of hybrid nanostructures for new generation photovoltaics. We investigated several hybrid and organic model systems, consisting of an organic polymer playing the role of electron donor and a low-dimension nanostructure (1D or 0D) which acts as electron acceptor and transporter. Model potential molecular dynamics has been used to characterize the polymer morphology onto the nanostructured substrates. In particular, the wrapping phenomena on one-dimensional structures (ZnO nanoneedles and carbon nanotubes) have been analyzed as a function of several physical variables such as temperature, substrate crystallography, polymer chain length and density. It has been thus possible to observe that wrapped configurations are only metastable on carbon nanotubes at room temperature and in absence of solvents. Nevertheless, wrapped configurations induced by the solvent can be frozen due to the interactions among neighboring polymer chains. According to this study, it is possible to enhance the polymer-nanotube alignment (and thus improving the polymer transport properties) through a suitable tuning of the synthesis parameters. Conversely, wrapped geometries are stable on ZnO nanoneedles, due to the small polymer mobility on the ZnO surface. The results obtained on the morphology of polymer-ZnO hybrids have then been used as a starting point to evaluate the electronic structure and the optical absorption properties. Hybrid models consisting in a 120-atoms ZnO nanoparticle and a set of oligothiophenes have been studied through the density functional theory, and the energy-level alignment has been obtained by using the Δ-self-consistent-field method. 120-atoms ZnO nanoparticles have been synthesized and found to be particularly stable. They therefore not only represent a useful model for computational studies, but are also of potential technological interest. An important result thus obtained is to demonstrate that the interaction between the two organic/inorganic moieties shifts the energy levels, giving rise to a nonstaggered junction. This phenomenon is not present in planar ZnO substrates, but it is rather induced by the nanostructuration of the hybrid polymer/metal oxide system. From the methodological standpoint, a simplified model to predict the energy-level alignment at the interface has been developed, allowing to spare computational resources. Finally, since the atomic configuration of the 120-atoms ZnO nanoparticles is unknown, we calculated the optical absorption spectra in the near ultra-violet of a set of (ZnO)60 isomers: this information can be compared to experimental spectroscopic data and can thus be used to elucidate the most abundant structure of this cluster. The results of the present work suggest that the use of nanostructures, although opening interesting technological possibilities such as increasing the donor/acceptor interface, also requires a critical readdressing of our understanding of morphologies and electronic level alignment in low-dimension systems.
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