Polymers for electro-optic applications
Polymer based photovoltaic cells are being intensively investigated. In such cells three key processes need to occur; namely light absorption, charge separation of the exciton, and transport of the separated charges to the electrodes. Light absorption is reliant on the optical density of the polymer. In general charge separation is achieved by blending an electron acceptor with the polymer film. However, blending materials gives rise to potentially unreliable manufacturing and lifetime issues. This thesis describes the preparation of poly(l,4-phenylenevinylene) derivatives containing dipoles in which the process of charge separation can be achieved intramolecularly. The dipole was created with the use of electron donating alkoxy groups attached to the polymer backbone, and electron withdrawing nitro group attached to the fluorenyl side chains. These groups are believed to facilitate the dissociation of the photogenerated exciton, and potentially stabilise the holes and electrons that are formed when the exciton is separated. The fluorenyl side chains were attached to the polymer backbone via biphenyl or vinyl linkages. The polymers were primarily formed using the Gilch method and the conjugated polymers were obtained either via a soluble precursor route or directly from the monomer. The photophysical properties were studied for polymers with the fluorenyl side-chains as they were found to be more easily formed and stable. For poly[2-(7-nitro-9,9-dipropylfluorenyl)-5-(2'- ethylhexyloxy)-l,4-phenylenevinylene] it was found that the photoluminescence quantum yield dropped by a factor of eight relative to the polymer without the nitro group. It was further elucidated that this was due to the exciton being separated. Solar cells containing the polymers from this study showed modest performance.