Accurate on-chip measurement of the Seebeck coefficient of high mobility small molecule organic semiconductors

Abstract

We present measurements of the Seebeck coefficient in two high mobility organic small molecules, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C 8 -BTBT) and 2,9-didecyl-dinaphtho[2,3-b:2 ′,3 ′ -f]thieno[3,2-b]thiophene (C 10 -DNTT). The measurements are performed in a field effect transistor structure with high field effect mobilities of approximately 3 cm 2 /V s. This allows us to observe both the charge concentration and temperature dependence of the Seebeck coefficient. We find a strong logarithmic dependence upon charge concentration and a temperature dependence within the measurement uncertainty. Despite performing the measurements on highly polycrystalline evaporated films, we see an agreement in the Seebeck coefficient with modelled values from Shi et al. [Chem. Mater. 26, 2669 (2014)] at high charge concentrations. We attribute deviations from the model at lower charge concentrations to charge trapping.