2, University of York, York, , United Kingdom
3, Max-Planck-Institut f. Kohlenforschung, Muehlheim an der Ruhr, , Germany
Raman spectroscopy is a powerful tool to characterize electron-phonon coupling in various semiconductors. Here, we investigate the polaronic nature of two well-known high-mobility, thiophene-based polymers (PBTTT and DPPT-TT) by Raman spectroscopy combined with Density Functional Theory (DFT) calculations. Chemical and electrochemical hole doping of these polymers leads to characteristics changes in the intensity ratios of prominent Raman-active modes but no significant frequency shifts. The data suggest a localization of positive polarons on the electron-rich thienothiophene cores that are present in both polymers. DFT calculations also show that the mode intensity ratio variations are most likely caused by the local electric field that originates from negatively charged dopant molecules or electrolyte anions and the positive polaron on the polymer chain. The characteristic changes of the Raman mode intensities with the degree of doping also enable in-situ mapping of charge carrier concentration in the channel of electrolyte-gated polymer transistors with high spatial resolution.