Magneto-conductance of π-conjugated polymer based unipolar and bipolar diodes

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 = Bm; 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, ΔB. 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 Bm and ΔB 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.