We propose to study thin-film blends of the polymer:fullerene system PTB7:PC70BM using neutron reflectivity (NR). This system currently exhibits the highest power conversion efficiency of all polymer:fullerene blends when fabricated into a solar cell device. We propose to use NR to measure the compositional depth profile of PTB7:PC70BM films. These films will be deposited by spray-casting onto substrates with a range of interfacial layers in order to mimic the structures present in a solar cell device. Our NR measurements will be complemented by a novel microscopy technique developed in Sheffield that can probe photo-physical properties of samples with a depth resolution of ~ 20 nm. Combining depth-dependent structural and photophysical measurements of our samples will enable us to develop a deeper understanding of device performance using these state-of-the-art materials.