Poly(3-hexylthiophene) nanofibers for organic photovoltaic applications

Abstract

Dwindling non-renewable energy resources, as well as global warming and air pollution, have forced human beings to seek for alternative resources. Solar energy is one of the most attractive candidates to replace fossil fuels in years to come. Therefore, the research into improving the efficiency, the lifetime and the cost of solar cells has gained momentum worldwide in recent years. Organic solar cells are highly viable as cheaper options for energy harvesting. At present, the best power conversion efficiency (PCE) of close to 10% has been demonstrated although further development is still required to push the efficiency beyond the limit required for commercialization. The application of one-dimensional organic nano-structures based on conjugated polymers for organic solar cells has gained some interest recently. These nanostructures enable highly efficient solar cell devices, through improved photon harvesting and enhanced charge transport. In particular, solar cell devices based on the nanostructures of poly(3-alkylthiophene) derivatives have exhibited PCEs in the range of 3 – 4%. Nevertheless, the performance of organic nanostructure devices generally falls below those that undergo the conventional thermal annealing. The work presented in this thesis focuses on poly(3-hexylthiophene) (P3HT) nanofibers and their applications for organic solar cells, and is expected to drive the research of other organic nanostructures with high aspect ratios. The optical, electrical and morphological characterizations of the P3HT nanofiber blends reveal the potential as well as the limitations of the organic nanostructure system. It was found that the morphology of the blends of P3HT nanofibers and fullerene molecules should be optimized to reduce holes recombination prior to extraction to external electrodes. The poorer device performance could be attributed to the poor interconnectivity between the nanofibers. Subsequently, with the application of solvent additives with high boiling points and with the suitable interaction energies with the active materials, improvement in the morphology results in a better-connected nanofiber blend. The benefit of organic nanostructures is more pronounced in an all-polymer blend system, which may suffer either a more severe phase separation or intermixing. It is shown that the pre-formed P3HT nanofibers could maintain their crystallinity in the all-polymer blend, thus enabling improved blend absorption, exciton dissociation and charge transport. Besides all-polymer blends, in terms of morphological stability, organic nanostructures also have an edge for ternary blend systems. The negative impact of the dopant molecules on the original blends is moderated when the active materials are transformed into the more morphologically stable organic nanostructures.