A study of the charged excitations in thin films of a-sexithiophene by voltage-modulation spectroscopy and photoimpedance measurements

We report on the spectra of charged excitations present in solid-state thin films of α-sexithiophene. We use Voltage - Modulation Spectroscopy to charge the oligomer chains electrically by the field effect in MetalInsulator - Semiconductor (MIS) devices in which a-sexithiophene is the active semiconductor layer. This technique allows us to probe the charged excitations in a solid state thin film device environment, whilst avoiding the steric and coulombic perturbations due to counter-ions present in chemically doped solutions. We observe the existence of two distinct charged excitations in addition to the radical cation. Photoimpedance spectra and some preliminary low temperature spectra provide further insight into these charged excitations.