This thesis describes the development of donor-acceptor conjugated copolymer fullerene blends for applications in bulk heterojunction organic solar cells. The characterisation of the optoelectronic properties of the blends as well as the optimisation of such materials into organic photovoltaic (OPV) devices is described. The use of a composite cathode structure (in which a thin layer of calcium is backed by an optically thick layer of aluminium) for OPV application is presented. It is shown that this cathode structure optimise the power conversion efficiencies of PCDTBT based OPVs. The optimisation of the cathode structure was confirmed using a reflectivity model that described the electromagnetic field within the OPV devices. The solubility of a number of polymers was increased ising octyloxy side-chain substituents with device optimisation studies indicating the necessity of a thermal annealing treatment to fully optimise device performance. Selenophene based conjugated polymers were also investigated that had red-shifted absorption characteristics compared to comparable thiophene based materials. Despite a reduction in the optical energy gap, it was found that these polymers exhibited a lower molar absorption coefficient and reduced hole mobility, features that ultimately lead to poorer device performance. One selenophene polymer however was shown to have similar power conversion efficiency compared with its thiophene equivalent. Fluorene based copolymers were also investigated and were shown to result in efficient OPV devices through an increase in the device open-circuit voltage. Finally, a conjugated polymer containing a fluorene unit together with additional thiophene moieties and octyloxy substituents was characterised. OPV devices were prepared using a simple preparation method with power conversion efficiencies demonstrated exceeding 6%.