Photogeneration of charge in solid films of α-sexithiophene Original Research Article

The modulating electric field effect on the luminescence (EML) and photoconduction (PC) have been studied in order to reveal the charge photogeneration mechanisms in solid films of α-sexithiophene (α-6T). It is found that the EML is due to energy-dependent electric field-mediated branching ratio between the luminescence and dissociation of the emitting excited states. Thus produced free charge carriers are detected as the bulk-generated photocurrent. The results interpreted in terms of the Onsager formalism show the charge separation process to proceed through a very efficient molecular interlayer charge transfer; the charge carrier motion forming the measured current is underlaid by hopping of positive charge carriers (holes) among a hopping site manifold with the energy width determined by torsional disorder. In addition, a dominating hole injection component of PC can be detected under positive bias of the illuminated sandwich system →Al/(α-6T)/Al/glass. It is attributed to annihilation of triplet excitons at the anode. A detailed analysis of this current as a function of excitation light intensity and its penetration in α-6T film allows to characterize dynamic parameters of triplet excitons.