Oxygen-induced quenching of photoexcited states in polythiophene films

Oxygen causes reversible and irreversible detrimental effects to the performance of organic (opto-)electronic devices. In order to get some insight into the mechanisms of these effects, we investigated the kinetics of fluorescence quenching (FQ) in thin films (d approximate to 100 nm-12 mum) of regioregular polyalkylthiophenes upon exposure to oxygen. The kinetics of FQ consists of a fast component, which is fully reversible, as well as a slow component, which is partially irreversible. The fast reversible component leads to a loss of fluorescence intensity of approximately 2% at an oxygen partial pressure of 1 bar within milliseconds. It is independent of the intensity of the exciting light and is ascribed to collisional quenching of excited singlet states after oxygen diffusion into the bulk of the film. The diffusion coefficients of oxygen and singlet excitons were determined as D(O-2) = 1.5(3) x 10(-7) cm(2) s(-1) and D(S-1) = 5 x 10(-4) cm(2) s(-1), respectively. The slow reversible component, whose amplitude depends on light intensity, occurs on a time-scale of minutes. It is assigned to the formation of charge-transfer complexes between excited singlet states of polythiophene and oxygen. Femtosecond pump-probe experiments show that effective quenching of excited singlet states by oxygen takes place on a picosecond time-scale, leading to the enhanced formation of charged states. (C) 2004 Elsevier B.V. All rights reserved.