The aim of our experiment is to understand the link between interfacial width and the photophysics at the interface to the operation of a polymer solar cell. We propose to extend our previous neutron reflectivity studies of conjugated polymer interfaces to a simple bilayer device of P3HT and C60. We will use neutron reflectivity to characterise the interfacial width between the solar cell materials P3HT and C60 for various thermal processing conditions. This interface is of crucial importance since it is where excitons are split into the holes and electrons in a polymer solar cell. We will use the measurement of interfacial width and correlate this with photoluminescence measurements and ultrafast pump probe experiments to understand the nature of these short lived excited states.