A promising class of materials for solar energy is conjugated polymer (CP)-fullerene mixtures. They offer the future potential for replacing current inorganic systems with cheaper alternatives which can be created in flexible, wide area devices. These CP-fullerene systems are bulk-heterojunctions systems due the phase separation of the two moieties. Such heterojunctions can potentially have high device efficiency if the interfacial interactions between CP and fullerenes are controlled to form interconnected phases and optimal interface structures. Despite significant device charaterization and optimization there is a lack of fundamental information about the pahse behaviour of CP-fullerenes. We wish to address this area by studying the phase behaviour of a series of model CP-fullerene systems.