We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C-60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.