Photovoltaic processes of singlet and triplet excited states in organic solar cells

This article reports the respective photovoltaic processes of singlet and triplet photoexcited states in dissociation and charge reactions based on the studies of magnetic-field effects of photocurrents. The magnetic-field effects of photocurrents reveal that weak donor-acceptor interactions lead to a two-step photovoltaic process: dissociation in polaron-pair states evolved from singlet excitonic states and exciton-charge reactions occurred in triplet excitonic states in the generation of the photocurrent. However, strong donor-acceptor interactions yield a one-step photovoltaic process: direct dissociation of both singlet and triplet excitons in bulk-heterojunction organic solar cells. In addition, the magnetic-field effects of photocurrents indicate that the dissociated electrons and holes form charge-transfer complexes with singlet and triplet spin configurations at donor-acceptor intermolecular interfaces. As a result, the magnetic-field effects of photocurrents can deliver a critical understanding of singlet and triplet photovoltaic processes to design advanced solar-energy materials and devices.