We present a comparative study of ultrafast photoexcited state relaxation in pentacene single crystals and in pure and C-60-doped pentacene films using optical pump-probe spectroscopy. The photoinduced absorption spectra in pentacene crystals is consistent with a dominant singlet-triplet fission decay channel for above-gap excitation. This decay channel is suppressed in thin films and even further suppressed by electron trapping in C60-doped films. Thus we show that suppression of triplet state production, which is necessary for free carrier formation and thus photovoltaic and photodiode performance, is controllable via sample morphology. (C) 2009 Elsevier B.V. All rights reserved.