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 electrontransfer 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 [10−3 cm2 /V s] 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 cm2 /V s.