Advantages of using organic materials as the active layer in photovoltaics include their suitability for low cost and large area processes and their mechanical flexibility. In particular, an accurate control of the nanoscale morphology of the active layer is paramount for optimizing the performance of the devices. Bulk-heterojunction devices are the best perfoming at present but are multicomponent systems, with plenty of unknowns for optimising performance. We propose to use Neutron Reflectometry, to probe the nanoscale morphology and interdiffusion of the donor-acceptor pair on a simple bilayer device with similar performance. We will use a polymer donor with high hole mobility and interesting solubility behaviour, and monitor the structure as a function of annealing temperature and device preparation method.