Anisotropic magnetoconductance in polymer thin films

The dependence of the magnetic field sensitive current on the orientation of the magnetic field has been investigated in organic semiconductor devices where the active layer consists of the poly(p-phenylene vinylene) derivative "Super Yellow." Previous work on Alq(3) suggested that the anisotropy was caused either by anisotropic spin-spin interactions or by anisotropic hyperfine fields, but no discrimination could be made. In the present work, the anisotropy at the hyperfine field scale is best explained by dipolar coupling between the spin of polarons. In addition, a high field anisotropy is found with an opposite sign, different angle, and voltage dependence. Spin density matrix calculations were performed of polaron pair interactions for the low field effect, and a Delta g-mechanism, triplet-polaron, or triplet-triplet interaction for the high field effect. The simulations confirm that the low field anisotropy can indeed be explained by dipolar coupling. However, the proposed models can not entirely account for the high field anisotropy. These results show that, although contemporary models can account for (anisotropic) magnetic field effects in organic semiconductors at low field scales, more experimental and theoretical research of high field effects is highly desirable.