  Nonlinear photocurrent carriers in conjugated polymers, such as polarons,
bipolarons and solitons, are considered at low photon energies where a
tunnelling process is necessary. We show that polarons usually dominate the
photocurrent I due to a novel electric field assisted tunnelling for which
ln(I) ~ -E^{-2/3}. For near degenerate polymers an electric field E which
exceeds the confinement potential and frequencies above twice the soliton
energy, soliton tunnelling is favored. Photocurrent data can then be used to
identify the remarkable phenomenon of soliton conduction.
