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

We report on the influence of charge injection on the optical absorption and fluorescence of sexithiophene (α-6T) in a semi-transparent metal-insulator-semiconductor (MIS) device. Under negative gate voltage, positive charges are injected in α-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 α-6T+ and those at higher energies to substrate modes. Field-induced charges also modulate the α-6T fluorescence intensity which is either quenched or enhanced depending on the sign and amplitude of the applied voltage. At Vg = −15 V, we estimate that the injection of one positive charge quenches the fluorescence of approximately 200 molecules of α-6T.