  We derive kinetic equations describing injection and transport of spin
polarized carriers in organic semiconductors with hopping conductivity via an
impurity level. The model predicts a strongly voltage dependent
magnetoresistance, defined as resistance variation between devices with
parallel and antiparallel electrode magnetizations (spin valve effect). The
voltage dependence of the magnetoresistance splits into three distinct regimes.
The first regime matches well known inorganic spintronic regimes, corresponding
to barrier controlled spin injection or the well known conductivity mismatch
case. The second regime at intermediate voltages corresponds to strongly
suppressed magnetoresistance. The third regime develops at higher voltages and
accounts for a novel paradigm. It is promoted by the strong non-linearity in
the charge transport which strength is characterized by the dimensionless
parameter $eU/k_BT$. This nonlinearity, depending on device conditions, can
lead to both significant enhancement or to exponential suppression of the spin
valve effect in organic devices. We believe that these predictions are valid
beyond the case of organic semiconductors and should be considered for any
material characterized by strongly non-linear charge transport.
