Photogeneration of charge in solid films of α-sexithiophene

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 alpha-sexithiophene (alpha-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/(alpha-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 alpha-6T film allows to characterize dynamic parameters of triplet excitons. (C) 1998 Elsevier Science B.V. All rights reserved.