The photoconductivity of blends of the polymer poly(3-hexylthiophene) (P3HT) and the fullerene-based acceptor [6,6]-phenyl C61-butyric acid methyl ester (PCBM), the prototypical active layer in organic photovoltaic devices, has been investigated using the contactless Flash-Photolysis Time-Resolved Microwave Conductivity (FP-TRMC) technique. Upon this firm foundation, bulk heterojunctions in which the electron acceptor has been replaced by carbon nanotubes have also been prepared. Critical to the efficacy of these structures is the generation and quenching of excitons, and transport, trapping and recombination of carriers. We develop a kinetic scheme to describe photoinduced exciton and free carrier generation and decay, which permits comparison with existing models for pristine polymer films and bulk heterojunctions. We show that exciton relaxation involves a first order process, resulting in free carrier generation, and that quenching of excitons by holes limits free carrier generation at high light intensities, and discuss the ramifications on the operation of bulk-heterojunction photovoltaic devices.