This projects aims to contribute to a better understanding of the relationship between processing conditions, active layer morphology and device performance in polymer solar cells (PSCs). To this end a series of Neutron Reflectivity experiments are envisaged, to provide the most detailed understanding possible of the processing-morphology-performance relationships in polymer:fullerene solar cells based on a polymer with the best power conversion efficiencies (PCEs), namely PffBT4T-2OD (also known as PCE11). Optimized organic photovoltaic (OPV) processing, with its associated low production costs, would radically change the renewable energy paradigm that is key for opening solar energy to new markets such as those in developing countries where people would not normally think about investing in and generating their own electricity.