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.

Abstract

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 ${\mathrm{O}}_{2}$. The reaction changes inertially the concentration of ${\mathrm{O}}_{2}$ 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 ${\mathrm{C}}_{60}$ molecules. The probability of charge-carrier generation in the reaction with ${\mathrm{O}}_{2}$ molecules is found to be higher than with ${\mathrm{C}}_{60}$ 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. \textcopyright{} 1996 The American Physical Society.