  We explore the magnetoresistance (MC) effect in an organic semiconductor
device based on the magnetic field related bipolaron formation. By establishing
a group of dynamic equations, we present the transition among spin-parallel,
spin-antiparallel polaron pairs and bipolarons. The transition rates are
adjusted by the external magnetic field as well as the hyperfine interaction of
the hydrogen nuclei. The hyperfine interaction is addressed and treated in the
frame work of quantum mechanics. By supposing the different mobility of
polarons from that of bipolarons, we obtain the MC in an organic semiconductor
device. The theoretical calculation is well consistent to the experimental
data. It is predicated that a maximum MC appears at a suitable branching ratio
of bipolarons. Our investigation reveals the important role of hyperfine
interaction in organic magnetic effect.
