Optical spectroscopy of field-induced charge in self-organized high mobility poly(3-hexylthiophene)

Charge modulation spectroscopy (CMS) is an electro-optical spectroscopic technique that allows the charge carriers present in the conducting channels of field-effect transistors (FET’s) to be studied in situ. We use this technique to study the charge carriers present in regio-regular poly(3-hexylthiophene) P3HT that has been shown to exhibit high field-effect mobilities of up to 0.1 ${\mathrm{cm}}^2$/Vs, similar to that observed for amorphous silicon. We demonstrate that the CMS spectra of charge carriers in high-mobility regio-regular P3HT FET’s are independent of charge density, modulation frequency, and temperature. This is evidence for the presence of a single, intrinsic charge carrier that we identify as a singly charged polaronic species. The spectral features attributed to the charged species show a lack of vibronic structure that is in contrast to the vibronic structure present in the bleaching of the main $\pi -{\pi }^{*}$ absorption of the neutral chains. The transition energies observed in regio-regular P3HT cannot be understood as an extrapolation of charge-induced transitions in isolated short-chain oligomers to long conjugation lengths. Our results give evidence that interchain coupling in highly ordered P3HT is sufficiently strong so that the charge carriers cannot be considered to be confined to a single chain, rather, they now exhibit quasi-two-dimensional characteristics.