Bimolecular Crystals of Fullerenes in Conjugated Polymers and the Implications of Molecular Mixing for Solar Cells

Organic photovoltaics have recently attracted tremendous attention in industry and academia since they offer the potential to significantly change energy production by drastically reducing the manufacturing cost for solar cells. Organic materials are flexible, lightweight and can be fabricated over large areas via low cost solution-based processing technologies. The most common solar cell implementation has a bulk-heterojunction (BHJ) blend as the active layer, where electron donor (typically, a light absorbing polymer) and acceptor (a fullerene) materials form a bicontinuous network as a result of self-assembly on the nanometer length-scale. Such cells have a reported power-conversion efficiency above 6%. A rapid increase in efficiency over the last decade was accomplished through materials design and morphology control. It has remained a puzzle, however, why some polymer:fullerene systems reach optimal morphology and efficiency at equal parts polymer and fullerene and other systems optimize at around 80% fullerene content. Little is known about what material properties are responsible for determining the optimal blend ratio.