Organic photovoltaic (OPV) cells have the potential to be the basis of a renewable energy source that is inexpensive and portable. Current state-of-the-art solution-processed OPV cells are based on the so-called bulk heterojunction (BHJ) architecture, consisting of an active layer that is a ¿blend¿ between a conjugated polymer (electron donor D) and a fullerene (electron acceptor A) forming a nanoscale interpenetrating network structure due to self-organized phase separation. This organization is critical to device performance and in this experiment we propose to use SANS to study the phase separation and aggregation in cells comprising different donor and acceptor materials fabricated using two methods - blended materials and sequential bilayer formation. Annealing conditions will also be investigated in order to understand the factors that lead to optimized device performance.