We investigated the effect of spin-orbit coupling on magnetoresistance in nonmagnetic organic semiconductors. A Lorentz-type magnetoresistance is obtained from spin-orbit coupling-dependent spin precession under the condition of a space-charge-limited current. The magnetoresistance depends on the initial spin orientation of the electron with respect to the hole in electron-hole pairs, and the increasing spin-orbit coupling slows down the change in magnetoresistance with magnetic field. The field dependence, the sign and the saturation value of the magnetoresistance are composite effects of recombination and dissociation rate constants of singlet and triplet electron-hole pairs. The simulated magnetoresistance shows good consistency with the experimental results.

Charge carriers in organic semiconductor are different from that of traditional inorganic semiconductor. Based on three-current model, considering electrical field effect, we present a theoretical model to discuss spin-polarized injection from ferromagnetic electrode into organic semiconductor by analyzing electrochemical potential both in ferromagnetic electrode and organic semiconductors. The calculated result of this model shows effects of electrode's spin polarization, equilibrium value of polarons ratio, interfacial conductance, bulk conductivity of materials and electrical field. It is found that we could get decent spin polarization with common ferromagnetic electrode by increasing equilibrium value of polarons ratio. We also find that large and matched bulk conductivity of organic semiconductor and electrode, small spin-dependent interfacial conductance, and enough large electrical field are critical factors for increasing spin polarization.