Photoluminescence and electroluminescence in thin films of polyconjugated organic molecules

Summary form only given. We describe the formation and deactivation of excited singlet states in vapor deposited films of oligothienylenes, oligophenylenevinylenes and oligophenylenes as a function of the molecular chain-length, film thickness and molecular orientation in the film. The molecular order within the films is determined by angular resolved polarized UV/VIS absorption and fluorescence spectroscopy. The deactivation of excited states is investigated by means of photo- and electroluminescence, photoconductivity and singlet oxygen production. D.C. and A.C. impedance methods in the dark and under illumination are applied to the characterization of the interface between organic film and contacting metal. Vapor deposited oligothienylene films on polar substrates possess a high degree of molecular order which decreases with growing layer thickness and also upon introduction of bulky alkyl substituents. Concomitantly the fluorescence quantum yields increase from 10-5 to 10-3 while the photoconductivity decreases. No singlet oxygen production is observed upon irradiation, indicating that excited singlet states decay mainly by radiationless recombination via the singlet manifold or by charge carrier separation. Sexithiophene-aluminum junctions exhibit a diode-like behavior in the dark and a photovoltaic effect under illumination. Under forward bias, electroluminescence is observed with an onset voltage of 3V for unsubstituted and 10 V for alkylated sexithiophene.