Absorption and fluorescence electro-modulation in alpha-sexithiophene metal-insulator-semiconductor devices

We report on the influence of charge injection on the optical absorption and fluorescence of sexithiophene (alpha-6T) in a semi-transparent metal-insulator-semiconductor (MIS) device. Under negative gate voltage, positive charges are injected in alpha-6T and three new bands peaking at 2.25, 2.00 and 1.50 eV appear in the electro-modulated absorption spectrum. Changing the incidence angle of the probe beam by rotating the MIS device allows the unambiguous assignment of the band at 1.50 eV to the radical cation alpha-6T(+.) and those at higher energies to substrate modes. Field-induced charges also modulate the alpha-6T fluorescence intensity which is either quenched or enhanced depending on the sign and amplitude of the applied voltage. At V-g = -15 V, we estimate that the injection of one positive charge quenches the fluorescence of approximately 200 molecules of alpha-6T.