Controlling the Morphology and Performance of Bulk Heterojunctions in Solar Cells. Lessons Learned from the Benchmark Poly(3-hexylthiophene):[6,6]-Phenyl-C61-butyric Acid Methyl Ester System

Photovoltaic cells are an established and increasingly important technology for the production of renewable energy. Whereas organic semiconductors of low molecular weight are generally deposited by evaporation under vacuum, films of conjugated polymers are conveniently created from solution using wet processing techniques. For the production of prototypic devices, the commonly used technique of spin coating is effective on the laboratory scale, but it is not suited for large scale manufacturing because of the attendant waste of material. Among semiconducting polymers commonly explored for use in molecular solar cells, P3HT and its close analogues remain among the most interesting candidates. An important issue is the regioregularity of these polymers, which measures the extent to which the sample can be considered to be a single molecular species with a perfect alternation of alkylated and nonalkylated sites on the thiophene rings.