Relative importance of polaron activation and disorder on charge transport in high-mobility conjugated polymer field-effect transistors

Abstract

The charge transport properties of conjugated polymer semiconductors are governed by strong electron phonon coupling, leading to polaron formation as well as the presence of structural and electronic disorder. However, the relative contribution which polaronic relaxation and disorder broadening make to the temperature activation of the mobility of these materials is not well understood. Here we present a combined study of the temperature and concentration dependences of the field-effect mobility and the optically induced electron-transfer transitions of a series of poly(3-hexylthiohene) field-effect transistors of different molecular weight. We apply a vibronic coupling model to extract the reorganization energy and the strength of electronic coupling from the optical spectra. We observe a transition from a localized to a delocalized transport regime as a function of molecular weight and crystalline quality. Polaron activation is comparable to disorder-induced activation in the low-mobility regime $[\sim {10}^{-3}{\mathrm{cm}}^2∕\left(\mathrm{V}\mathrm{s}\right)]$ and needs to be taken into account when interpreting the field-effect mobility, while disorder becomes the dominant mechanism to limit charge transport in the high-mobility regime with mobilities $>{10}^{-2}–{10}^{-1}{\mathrm{cm}}^2∕\left(\mathrm{V}\mathrm{s}\right)$.