Photoconductivity of poly(2,5-diheptyloxy-p-phenylene vinylene) in the air atmosphere: Magnetic-field effect and mechanism of generation and recombination of charge carriers

The photoconductivity of poly(2,5-diheptyloxy-p-phenylene vinylene) (HO-PPV) increases upon application of an external magnetic field. This magnetic-field effect (MFE) is significantly enhanced in the presence of the oxygen of air. Two types of MFE are revealed in samples of HO-PPV. The first one is a prompt MFE connected with the existence of coupled pairs of charge carriers, as precursors for photoconductivity: these pairs dissociate thermally and/or in an external electric field, producing mobile carriers. The second MFE is delayed, and is shown to be connected with the reaction of triplet and singlet excitons with O-2. The reaction changes inertially the concentration of O-2 molecules dissolved in the bulk of the polymer, its rate being dependent on magnetic field. MFE allowed us to clarify the mechanism of photogeneration of HO-PPV: Positive mobile charge carriers responsible for the photocurrent are produced in the dissociation of primary singlet excitons by oxygen, photo-oxidation products, and other weak dopants like C-60 molecules. The probability of charge-carrier generation in the reaction with O-2 molecules is found to be higher than with C-60 clusters. An enhancement of the photoconductivity of samples in air is connected with that factor and also with a significant increase of the lifetime of charge carriers before recombination. Recombination of positive charge carriers with oxygen negative ions is very slow, giving quasipersistent photoconductivity of the polymer in air.