We present a comparative study of ultrafast photo-conversion dynamics intetracene (Tc) and pentacene (Pc) single crystals and Pc films using opticalpump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals isactivated and temperature-independent respectively, demonstrating dominantsinglet-triplet exciton fission. In Pc films (as well as C$_{60}$-doped films)this decay channel is suppressed by electron trapping. These resultsdemonstrate the central role of crystallinity and purity in photogenerationprocesses and will constrain the design of future photovoltaic devices.
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