Abstract
The magnetoconductance (MC) of unipolar and bipolar organic diodes based on active layers made of the π-conjugated polymer DOO-PPV is presented in the range of magnetic field strength, B below about 100 mT, and for various polymer exchange isotopes. The MC( B) response is composed of two main regions: (i) a “sign-reversal” region for | B| < 1–2 mT, where MC( B) reverses its sign reaching a maximum absolute value |MC| m at B = B m; and (ii) a monotonic region for | B| > 2 mT, where MC( B) monotonically increases (decreases) for bipolar (unipolar) devices with an approximate Lorentzian line shape of width, Δ B. Similar behavior has been observed for the magneto-electroluminescence in bipolar devices. We found that MC( B) in both field regions is strongly isotope dependent; in particular both B m and Δ B are larger for nuclei with larger hyperfine interaction (HFI) constant. The complete MC( B) response, including the novel sign-reversal component is explained by a model of spin-coupled pairs of either same or opposite charge polarons, in which the HFI is explicitly included in the electronic spin Hamiltonian.
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