Etude ab initio des proprietes electroniques et optiques d'un systeme donneur-accepteur organique utilise dans les cellules photovoltaiques

The search for new sources of clean and renewable energy has recently been encouraged by the growing energy demand caused by the industrialization of developing countries and by population growth. In this context, the generation of electricity through the exploitation of solar energy with photovoltaic cells is particularly interesting, since this energy source is largely unused compared to its full potential. Nevertheless, large scale electricity generation with the current design of photovoltaic cells based on silicon is hindered by the large manufacturing cost of these devices. A new generation of photovoltaic cells, which includes organic photovoltaic cells that use semiconducting polymers, is under intense development in order to significantly reduce the manufacturing costs. The replacement of conventional materials with conjugated polymers in photovoltaic cells opens the possibility of using large scale manufacturing processes to produce large-area devices at low cost. However, the power conversion efficiency and the lifetime of organic photovoltaic cells are currently too low for these devices to be cost effective. A better understanding of the organic photovoltaic process is therefore necessary to improve the power conversion efficiency of these devices.;The operating principle of photovoltaic cells requires the charge transfer between a polymer acting as an electron donor and a molecule acting as an electron acceptor to enable the dissociation of photogenerated excitons into free charge carriers. Furthermore, to ensure that the majority of the photogenerated excitons dissociates, the active region of an organic photovoltaic cell is typically formed by a bulk heterojunction between the donor and the acceptor. Many experimental studies have shown that the power conversion efficiency of these devices, which is proportional to the product of their short-circuit current Isc with their open circuit potential Voc, is strongly governed by the microstructure of the bulk heterojunction defined as the local order of the two phases and the organization of the donor-acceptor interfaces. Even though these studies have helped to increase the efficiency of organic photovoltaic cells, the relations linking the microstructure of the bulk heterojunction to their electronic and optical properties are still to be established.;The objective of the research project is to computationally study the electronic and optical properties of organic bulk heterojunctions composed of regioregular poly(3-hexylthiophene) (rrP3HT) and C60, two materials typically used in organic photovoltaic cells. In this study, the microstructure of the donor-acceptor systems can be directly controlled, which facilitates the systematic study of the influence of this parameter on the electronic and optical properties of the organic bulk heterojunctions. The density functional theory (DFT) is used to study the ground state geometric and electronic properties of multiple bulk heterojunction systems, while the time dependent density functional theory (TDDFT) is used to study the optical properties of these systems. The SIESTA software package is used to study periodic systems representing perfectly crystalline materials.;The results obtained in this research project show that the power conversion efficiency of organic photovoltaic cells is strongly modulated by the microstructure of the bulk heterojunctions. Indeed, the size of the rrP3HT crystalline domains must be optimized to maximize the efficiency of the photovoltaic devices, since Voc and Isc have opposite behaviors with respect to π-stacking of the rrP3HT chains. In addition, the efficiency of organic photovoltaic cells could be improved by imposing geometrical constraints in the bulk heterojunctions through manufacturing methods in order to increase the value of Voc without altering the value of Isc. (Abstract shortened by UMI.).