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

We report on the influence of field-effect charge injection on optical absorption and fluorescence properties of alpha-sexithiophene (alpha-6T) thin films. We have performed electromodulated absorption (EMA) and electromodulated fluorescence (EMF) measurements on metal-semiconductor (MS) devices based on the molecular semiconductor alpha-6T. Analyzing the influence of the incidence angle on EMA spectra enables a discrimination of the contribution of different layers of the metal-insurator-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 alpha-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 alpha-6T from the aluminum electrode, which is unnecessary when gold is used. A large relative change of the fluorescence efficiency of alpha-6T shows that each held-effect positive charge quenches the fluorescence of approximately 200 alpha-6T molecules.