  Understanding the movement of charge within organic semiconducting films is
crucial for applications in photo-voltaics and flexible electronics. We study
the sensitivity of the electrical conductance of a silicon nanowire to changes
of charge states within an organic semiconductor physisorbed on the surface of
the nanowire. Elastic scattering caused by motion of charge carriers near the
nanowire modifies the mean-free path for backscattering of electrons
propagating within it, which we have mathematically expressed in terms of the
causal Green's functions. The scattering potential has been computed using a
combination of the polarizable continuum model and density functional theory
with the range-separated exchange-correlation functional for organic molecules
and the semi-empirical tight-binding model for silicon. As an example, the
sensitivity to charge state changes in tetracene is computed as a function of
operating temperature and geometrical parameters of a nanowire. For a single
molecule, ultra-thin silicon nanowires with characteristic sizes of the
cross-section below 2 nm produce a detectable conductance change at room
temperature. For larger nanowires the sensitivity is reduced, however the
conductance change grows with the number of charged molecules: with sub-4 nm
nanowires being sensitive enough to detect several tens of charge carriers. We
propose using noise spectroscopy to access the temporal evolution of the charge
states. Information regarding the spatial distribution of charge carries in
organic thin films can be obtained using a grid of nanowire resistors and
electric impedance tomography.
