Correlation between interdomain carrier hopping and apparent mobility in polycrystalline organic transistors as investigated by electron spin resonance

Microscopic charge dynamics inside microcrystal domains and across domain boundaries in polycrystalline organic thin-film transistors (OTFTs) were resolved by angle- and temperature-dependent field-induced electron spin resonance (FI-ESR) spectroscopy. We utilized small-molecule and polymer-based polycrystalline OTFTs, both of which are composed of planar microdomains oriented uniaxially on the substrates. Two types of motional narrowing effect are observed at magnetic fields perpendicular and parallel to the substrate: In the former, we observed the motional narrowing effect due to intradomain transport, while in the latter, we observed another motional narrowing effect due to the carrier hopping across the domain boundaries, which averaged out the variation in g factors between respective microdomains. The analyses revealed that the activation energies for the intradomain transport are between 5 and 21 meV, while those for the interdomain carrier hopping are between 45 and 90 meV. The latter ones coincide with those for the apparent mobility for the respective OTFTs. These results demonstrate that the charge transport is limited by the domain boundaries, which form rate-determining barrier potential both in small-molecule and polymer-based polycrystalline OTFTs.