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Polymer based photovoltaic devices promise solar technologies that are inexpensive enough to be widely exploited and therefore provide a significant fraction of the future energy needs. There are many promising polymer-fullerene mixtures that are promising materials candidates for achieving high performance devices, but their exploitation requires and improved understanding of their structure-property relationships. Of particular relevance is the phase behavior of the mixtures. The phase behavior of donor-acceptor materials for photovoltaic applications is of key importance [1,2]: i) to gain a fundamental understanding and control of morphology development in the donor-acceptor blends; ii) to appropriately choose the operating window for thermal annealing; iii) to understand the long-term stability of the blended film morphology and consequently of the photovoltaic performance of the corresponding solar cells. In this work the phase behavior of polymer-fullerene mixtures is being studied using Differential Scanning Calorimetry (DSC), Wide-Angle X-Ray Scattering (WAXS), Small-Angle Neutron Scattering (SANS) and theoretical ab initio Density Functional Theory (DFT) calculations.

Organic or polymer based photovoltaic devices promise solar technologies that are inexpensive enough to be widely exploited and therefore provide a significant fraction of the future energy needs. There are many promising polymer-fullerene mixtures that are promising materials candidates for achieving high performance device, but their exploitation requires an improved understanding of their structure-property relationships. Of particular relevance is the molecular structural as well as mesoscale phase behaviour. In order to guide the use of electron acceptor fullerenes in these systems we are using multi-scale molecular modelling coupled with neutron and X-ray scattering to determine the structure behavior of model polymer-fullerene mixtures. Neutron scattering is particularly useful for these types of studies since the fullerene generally have a high scattering contrast with respect to most polymers. This natural contrast, enhanced by careful selective deuteration allows us to carefully probe the atomic and molecular interactions in these complex systems. We are studying model polymer systems to establish fullerene-polymer phase behaviour. To establish how fullerene addition changes polymer phase behaviour, using advanced scattering techniques we are studying phase morphology and molecular structure. Control of morphology development is being examined with particular relevance to solid state processing.

If the future global future energy needs are to be met, renewable energies must be exploited. The possibility of cheap, but efficient organic or polymer based photovoltaic (OPV) devices are among the competing technologies, which are actively being studied. To date the most efficient OPV devices are based on conjugated polymer-fullerene bulk heterojunctions (BHJ). The choice of polymer and fullerene is a balance of electronic (donor-acceptor) properties, but due to the nature of the solubility of the species, also includes solubilising ligands. Whilst these ligands certainly induce improved solution processability, they can also affect the electronic characteristics and also morphology of the polymers and fullerenes. However, potentially interesting polymer-fullerene combinations are not being explored due to difficulties in solution processing of the mixtures. We are exploring the use of pressure on the phase behaviour of polymer-fullerene blends, following on from previous workers who showed that melt processing of polymers can be greatly influenced by application of pressure. Under moderate pressure, we have demonstrated that equivalent phase behavior in polymer-fullerene pairs observed by conventional solution processing can be induced in solid-state processing. Using a combination of experiments and molecular modeling we are exploring the effect that pressure plays in inducing phase miscibility in otherwise intractable polymer-fullerene mixtures. To date we have been exploring a number of model-fullerene mixtures, but have demonstrated that the effects of this solid-state processing method are ubiquitous. The results of this fundamental study are expected to lead to alternative methods of processing of conjugated polymers without the need for solvents.