The proposal aims to determine the degree of interfacial mixing and roughness at an interface between two conjugated polymers, as part of a broder program to understand how to optimise the interfacial structure and nanoscale morphology in thin films of mixed conjugated polymers, including all-conjugated block copolymers, to achieve improved performance in photovoltaic devices. Neutron reflection will be used to determine the interfacial widths between thin films of deuterium labelled PFM and F8BT as a function of processing conditions (annealing time and temperature) and film thickness, and interpret these in terms of classical theories of polymer interfacial width, determining in this way estimates for the value of the thermodynamic interaction parameter