Fast transient ''photoconductivity'' in semiconducting polymers: Free carrier photocurrent or displacement current generated by electric-field-induced polarization of bound excitons?

We have re-examined the displacement current interpretation of the fast transient photocurrent in semiconducting polymers. We find that our earlier calculation was in error by a factor of 4 pi (arising from incorrectly converting from MKS units). We re-evaluate the displacement current contribution using the earlier data obtained from spin-cast PPV and data from tensile drawn samples of PPV. The drawn and oriented PPV yields a fast transient photocurrent that is three orders of magnitude larger than that obtained from spin-cast films (when normalized to the same incident photon flux and to the same applied electric field). For the best samples, analysis of the data in terms of a displacement current from bound excitons would require an exciton polarizability, alpha(exc) > 10(7) Angstrom(3), far too large to arise from bound excitons. For comparison, we summarize the data available from other conjugated polymers as well.