Field-effect modulated Seebeck coefficient in pentacene and rubrene

We report on the first study of the charge carrier concentration and the temperature dependence of the Seebeck coefficient $S$ for two prototypical organic semiconductors measured in a field-effect transistor (FET) structure. As a basic transport property of solids, the Seebeck coefficient provides deep insights into the nature and dynamics of charge carriers. Using a FET structure enables the variation of the Fermi-level position in the active semiconductor region while measuring $S$, which is essential for determining individual contributions to the thermopower. The sign of the measured Seebeck coefficient is consistent with hole transport, and $S$ is in the range of 0.3-1 mV/K, it is independent of temperature between 295 K and 200 K, and interestingly it decreases for both semiconductors as $S \propto $\textit{$\vert $V}$_{g}$\textit{$\vert $}. The measured $S$ is quantitatively described by $S=(k/e)(E(V_g )/kT+A)$. The Fermi-level position $E(V_{g})$ is obtained from analyzing the transistor's characteristic which then allows to calculate the parameter $A$. For both semiconductors we find that $A$ is in the range of 1.7-3.6, just as in conventional semiconductors. The results are well described by solely considering a realistic density of in-gap trap states and band-like transport of quasiparticles that are subjected to scattering. There is no need to invoke self-trapping of massive charge carriers.