In situ charge-modulation spectroscopy of oligothiophene field-effect diodes: From sexithiophene towards polythiophene

We have studied the charged excitations in a series of oligothiophenes of increasing conjugation length, namely α-6T, α-8T and substituted α-12T, in order to gain insight into how charge is stored in polythiophenes and to investigate how the polaron and bipolaren concepts relate to molecular cations and dications. We measured optical charge-modulation spectra of thin solid films of oligomers within field-effect diodes, because of the direct relevance to organic thin-film transistors and also the ability to induce unipolar charge carriers by purely electrostatic means, without introducing chemical dopants. By modulating the gate voltage and probing the resulting modulation of optical absorption at the same frequency, we have observed optical transitions characteristic of the charged excitations of these oligomers in the solid state.