Organic π -conjugated polymers constitute an important class of functional materials. [ 1 ] Their ability to act as semiconductors and to facilitate charge transport has led to their use as the active material in various optoelectronic devices, including fi eld-effect transistors, polymer light-emitting diodes, and organic solar cells (OSCs). [2][3][4] Of particular current interest is the possibility of manufacturing OSCs via solution processing methods. [ 5 , 6 ] To date, the most promising active-layer architecture is the bulk heterojunction (BHJ), which consists of an interpenetrating network of electron donor and electron acceptor domains. [ 6 , 7 ] The most effi cient BHJ solar cells have employed π -conjugated polymers as the donor and fullerene derivatives, such as [6,6]-phenyl-C 71butyric acid methyl ester (PC 71 BM), as the acceptor. [ 8 , 9 ] Power conversion effi ciencies (PCEs) approaching 5% have been realized using poly(3-hexylthiophene) (P3HT) and above 7% with narrow-bandgap donor/acceptor (D/A) copolymers comprising a backbone with alternating electron-rich and electron-defi cient structural units. [ 10 , 11 ] These latter materials exhibit absorption characteristics with good overlap with the solar terrestrial spectrum. The most common method for fabricating the BHJ layer involves mixing the polymer donor (D) with the fullerene acceptor (A) in solution and allowing the formation of the interpenetrating matrix immediately at the point of fi lm formation. If this is not effi ciently accomplished, then subsequent processing steps may be required.
As-cast solution-processed polymer/fullerene solar cells can exhibit poor performance due to the miscibility of the BHJ components, which leads to poor separation of the donor and acceptor phases. Thermal annealing, a process which can induce phase-separation of the active layer, has proven to be an excellent technique for increasing the performance of P3HT/fullerene solar cells. [ 14 , 15 ] Recently, the incorporation of small quantities of solvent additives during the fi lm-forming