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) an d electromodulated fluorescence (EMF) measurements on metal-semiconduc tor (MS) devices based on the molecular semiconductor alpha-6T. Analyz ing the influence of the incidence angle on EMA spectra enables a disc rimination of the contribution of different layers of the metal-insura tor-semiconductor (MIS) device. An EMA peak at 1.5 eV can thus be attr ibuted to an optical absorption modulation located in the channel, ind uced by charges stored as radical cations alpha-6T(.+). We have also c ompared the EMA properties with either gold or aluminum as contact ele ctrodes. 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 alp ha-6T shows that each held-effect positive charge quenches the fluores cence of approximately 200 alpha-6T molecules.