  Charge-carrier transport at the semiconductor-gate dielectric interface in
organic field-effect transistors is critically dependent on the degree of
disorder in the typically semi-crystalline semiconductor layer. Charge trapping
can occur at the interface as well as in the current-carrying semiconductor
layer itself. A detailed and systematic understanding of the role of grain
boundaries between crystallites and how to avoid their potentially detrimental
effects is still an important focus of research in the organic electronics
community. A typical macroscopic measurement technique to extract information
about the energetics of the grain boundaries is an activation energy
measurement. Here, we compare detailed experiments on the energetic properties
of monolayer thin films implemented in organic field-effect transistors, having
controlled numbers of grain boundaries within the channel region to kinetic
Monte-Carlo simulations of charge-carrier transport to elucidate the influence
of grain boundaries on the extracted activation energies. Two important
findings are: 1) whereas the energy at the grain boundary does not change with
the number of grain boundaries in a thin film, both the measured and simulated
activation energy increases with the number of grain boundaries. 2) In
simulations where both energy barriers and valleys are present at the grain
boundaries there is no systematic relation between the number of grain
boundaries and extracted activation energies. We conclude, that a macroscopic
measurement of the activation energy can serve as general quality indicator of
the thin film, but does not allow microscopic conclusions about the energy
landscape of the thin film.
