Photoconductivity and current producing states in molecular semiconductors

We present a methodology for computing photocurrent production in molecular semiconducting molecules. Our model combines a single-configuration interaction picture with the nonequilibrium Green's function approach to compute the current response of a molecular semiconducting wire following excitation. We give detailed analysis of the essential excitonic, charge transfer, and dipole states for poly-(phenylenevinylene) chains of length 32 and 48 repeat units under an electric field bias and use this to develop a reduced dimensional tunneling model which accounts for chain-length and field-dependent behavior. In this paper, we consider the decay of an excited electron/hole state on a molecular wire under bias attached to semiconducting leads at either end. We find that the current produced by the decay of an excitation depends not only upon the lifetime of the state, as given by the imaginary part of its complex eigenvalue, but also upon the net charge on terminal ends of the molecule. We also find that weakly bound electron/hole charge-transfer pairs can decay into the continuum via field induced tunneling and produce a net current whereas excitonic states decay via tunneling but give no net current contribution. (C) 2005 American Institute of Physics.