We have performed magnetoresistance measurements on polyfluorene sandwich devices in weak magnetic fields as a function of applied voltage, device temperature (10K to 300K), film thickness and electrode materials. We observed either negative or positive magnetoresistance, dependent mostly on the applied voltage, with a typical magnitude of several percent. The shape of the magnetoresistance curve is characteristic of weak localization and antilocalization. Using weak localization theory, we find that the phase-breaking length is relatively large even at room temperature, and spin-orbit interaction is a function of the applied electric field.
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