Magnetic Studies of Photovoltaic Processes in Organic Solar Cells

In this paper, we use magnetic field effects of photocurrent (MFE_PC) to study the photovoltaic processes in pristine conjugated polymer, bulk heterojunction, and double-layer solar cells, respectively, based on poly(3-alkylthiophene) (P3HT). The MFE_PC reveals that the photocurrent generation undergoes the dissociation in polaron pair states and the charge reaction in excitonic states in pristine conjugated polymers. As for the bulk-heterojunction solar cells consisting of electron donor P3HT and electron acceptor [6,6]-phenyl C61-butyric acid methyl ester (PCBM), the MFE_PC indicates that the dissociated electrons and holes inevitably form the intermolecular charge-transfer (CT) complexes at donor and acceptor interfaces. Essentially, the photocurrent generation relies on the further dissociation of intermolecular CT complexes. Moreover, we use double-layer solar cell to further study the intermolecular CT complexes with well-controlled donor-acceptor interfaces based on double-layer P3HT/TiO_x design. We find that the increase in free energies can significantly reduce the density of CT complexes upon thermal annealing.