  Conduction in pristine conjugated polymers (other than polyacetylene) is by
polaron hopping between sites corresponding to conjugation lengths. The strong
increase of current $I$ with voltage $V$ observed for both emission-limited and
ohmic contacts is due in large part to mobility increase as increasing field
makes it more possible to overcome internal barriers, such as energy
differences between sites. For emission-limited contacts an additional source
of nonlinear increase of $I$ with increasing $V$ is greater ability to escpe
return to the injecting electrode due to the image force. For ohmic contacts
additional nonlinearity comes from space charge effects. We are able to fit $I$
vs. $V$ for electron or hole conduction in some poly($p$-phenylene vinylene),
PPV, derivatives with ohmic contacts for reasonable values of the parameters
involved.
