  We developed a novel method for obtaining the distribution of trapped
carriers over their degree of localization in organic transistors, based on the
fine analysis of electron spin resonance spectra at low enough temperatures
where all carriers are localized. To apply the method to pentacene thin-film
transistors, we proved through continuous wave saturation experiments that all
carriers are localized at below 50 K. We analyzed the spectra at 20 K and found
that the major groups of traps comprise localized states having wave functions
spanning around 1.5 and 5 molecules and a continuous distribution of states
with spatial extent in the range between 6 and 20 molecules.
