THE CHARGED EXCITATIONS IN THIN-FILMS OF ALPHA-SEXITHIOPHENE WITHIN SEMITRANSPARENT FIELD-EFFECT DEVICES - INVESTIGATION BY OPTICAL SPECTROSCOPY OF FIELD-INDUCED CHARGE AND BY PHOTOIMPEDANCE SPECTROSCOPY

We report measurements of optical absorption, photoimpedance and photovoltaic excitation spectra associated with field-induced (positive) charges present in semi-transparent metal-insulator-semiconductor (MIS) diodes, in which sexithiophene forms the semiconductor layer. We detect the presence of at least three types of charged excitations present under different conditions of bias voltage and device preparation. We assign them to radical cations, intramolecular dications and intermolecular pi-dimers. We discuss implications for the use of sexithiophene in thin-film transistors.