Photoexcited carrier relaxation dynamics in pentacene probed by ultrafast optical spectroscopy: Influence of morphology on relaxation processes

We present a comparative study of ultrafast photoexcited state relaxation in pentacene single crystals and in pure and C60C60-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 C60C60-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.