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Conjugated polymers are considered to be one-dimensional semiconductors. In conjugated polymers single and double bonds alternatively bond the carbon atoms along the polymer chain. The loosely bound electrons determine the electronic properties of conjugated polymers. In order to utilise the properties of conjugated polymers in terms of a photovoltaic (PV) device application an acceptor material must be added. The acceptor material used in this study is used in buckminsterfullerence (C60). C60 was selected for this purpose due to its size and the fact that it can accept up to six additional electrons. Ultrafast charge transfers from a conducting polymer onto C60 were first reported in 1992 by Sariciftci et al. in a blend of MEH-PPV (poly (2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene) and since then it’s properties as an electron acceptor have been widely investigated. The debate of the exact mechanism of the charge transfer in polymer/fullerence blends is still on going today. The investigation of these systems is predominantly done using time resolved spectroscopy (photoinducted absorption, PIA), which is still considered to be the most effective way of investigation charge transfer between polymer/fullerence composites. However time resolved spectroscopy is expensive and is not readily available for use. The aim of this study is to explore charge transfer signatures in polymer/fullerence composites without using PIA spectroscopy. This study proposes to use steady state spectroscopic techniques coupled with electrochemical and conductivity measurements to characterise charge transfer signatures in polymer fullerence composites. The much studied MEH-PPV/C60 model system was initially employed in a systematic approach to try and elucidate charge transfer indicators between the polymers and C60 Fluorescence spectroscopy, cyclic volammetry, spectroelectrochemical and conductivity measurements provided evidence of charge transfer signatures in the model composite and were seen to be potentially viable techniques for assessing novel systems. The study was then extended to a homologous series of polymers which have continuously varying electronic and optical properties. The polymer series was synthesised in house and characterised for the first time in the solid state as part of this work. Electronic spectroscopy of the polymer series revealed that aggregation was recurring in the solid state form of the polymers. The electronic properties of the molecular and solid-state forms are different and the structure property relationships previously determined for the molecular forms could not be applied to the solid-state. It was observed that emission spectra of the polymers were all seen to be red shifted compared to those of their isolated molecular form. The fluorescence yield was also low, which also confirmed that there are aggregates within the polymers in their solid-state form. Cyclic volammetry measurements allowed calculation of the exact positioning of the HOMO-LUMO levels of each polymer. The precise levels of the HOMO-LUMO levels are important when matching the energy levels of the polymer to the energy levels of C60. Electrochemical bandgaps were in close accordance to the optical bandgaps. In-situ spectroelectrochemical measurements allowed observations of the bipolaron energy states of each polymer. Electroabsorption studies showed that each polymer was dominated by the quadratic Stark effect. The electroabsorption spectrum of each polymer closely resembled a first derivative lineshape of it’s absorption spectrum indicating a dominance of intramolecular transitions within each of the polymers. Using two of the new polymers a synthetic probe of the charge transfer mechanism was obtained. Charge transfer markers were evident in the new composites using fluorescence spectroscopy, cyclic volammetry, spectroelectrochemical measurements and conductivity measurements. For the results it was seen that the interaction of the two polymers with the C60 varied. From this systematic approach it was possible to observe which of the polymers showed the most potential for a device application by matching of the appropriate energy levels to achieve a more efficient charge transfer. In general it can be said that using a approach it was possible to match up the energy levels of a polymer/fullerence composite in order to achieve a more efficient interaction, which can be measured without the need for time resolved spectroscopy.
Moghal, J. (2008) Structure property relationships in electron donating systems for potential photovoltaic applications. Doctoral Thesis, Dublin Institute of Technology. doi:10.21427/D7P025