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.