Voltage polarity manipulation of the magnetoresistance sign in organic spin valve devices

The spin transport in organic spin valve (OSV) devices has been systematically investigated by inserting a low work function material Al between ferromagnetic electrode and organic layer. The resistance and current-voltage curve symmetry are dramatically altered as increasing Al thickness, indicating that an electron-unipolar OSV is obtained. Moreover, the magnetoresistance sign depends on the voltage polarity for certain Al thickness. We attribute this phenomenon to the Fermi and the lowest unoccupied molecular orbits energies of the two electrodes responding to the spin injection and detection, respectively. These findings provide a simple approach to control both the carrier type and the spin direction simultaneously.