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) (H O-PPV) increases upon application of an external magnetic held. This m agnetic-field effect (MFE) is significantly enhanced in the presence o f the oxygen of air. Two types of MFE are revealed in samples of HO-PP V. The first one is a prompt MFE connected with the existence of coupl ed pairs of charge carriers, as precursors for photoconductivity: thes e pairs dissociate thermally and/or in an external electric field, pro ducing 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 dis solved in the bulk of the polymer, its rate being dependent on magneti c field. MFE allowed us to clarify the mechanism of photogeneration of HO-PPV: Positive mobile charge carriers responsible for the photocurr ent are produced in the dissociation of primary singlet excitons by ox ygen, photo-oxidation products, and other weak dopants like C-60 molec ules. The probability of charge-carrier generation in the reaction wit h O-2 molecules is found to be higher than with C-60 clusters. An enha ncement of the photoconductivity of samples in air is connected with t hat factor and also with a significant increase of the lifetime of cha rge carriers before recombination. Recombination of positive charge ca rriers with oxygen negative ions is very slow, giving quasipersistent photoconductivity of the polymer in air.