Influence of field-effect charges on the optical properties of α-sexithiophene thin films

We report on the influence of field-effect charge injection on optical absorption and fluorescence properties of α-sexithiophene (α-6T) thin films. We have performed electromodulated absorption (EMA) and electromodulated fluorescence (EMF) measurements on metal—semiconductor (MS) devices based on the molecular semiconductor α-6T. Analyzing the influence of the incidence angle on EMA spectra enables a discrimination of the contribution of different layers of the metal—insulator—semiconductor (MIS) device. An EMA peak at 1.5 eV can thus be attributed to an optical absorption modulation located in the channel, induced by charges stored as radical cations α-6T·+. We have also compared the EMA properties with either gold or aluminum as contact electrodes. An excitation light must assist the injection of charges into α-6T from the aluminum electrode, which is unnecessary when gold is used. A large relative change of the fluorescence efficiency of α-6T shows that each field-effect positive charge quenches the fluorescence of approximately 200 α-6T molecules.