Optical spectroscopy of field-induced charge in self-organized high mobility poly(3-hexylthiophene) - art. no. 125204

Charge modulation spectroscopy (CMS) is an electro-optical spectroscopic te chnique that allows the charge carriers present in the conducting channels of field-effect transistors (FET's) to be studied in situ. We use this tech nique to study the charge carriers present in regio-regular poly(3-hexylthi ophene) P3HT that has been shown to exhibit high held-effect mobilities of up to 0.1 cm(2)/Vs, similar to that observed for amorphous silicon. We demo nstrate that the CMS spectra of charge carriers in high-mobility regio-regu lar P3HT FET's are independent of charge density, modulation frequency, and temperature. This is evidence for the presence of a single. intrinsic char ge carrier that we identify as a singly charged polaronic species. The spec tral features attributed to the charged species show a lack of vibronic str ucture that is in contrast to the vibronic structure present in the bleachi ng of the main pi-pi* absorption of the neutral chains. The transition ener gies observed in regio-regular P3HT cannot be understood as an extrapolatio n of charge-induced transitions in isolated short-chain oligomers to long c onjugation lengths. Our results give evidence that interchain coupling in h ighly ordered P3HT is sufficiently strong so that the charge carriers canno t be considered to be confined to a single chain, rather, they now exhibit quasi-two-dimensional characteristics.