  We report the optical absorption characteristics and the photocurrent spectra
for large-area molecular junctions containing different molecular structures
and thicknesses. Through experimental assessment of the optical absorption of
molecules in solution and chemisorbed onto transparent, conductive carbon
films, the energy ranges and intensity of absorption for a variety of molecular
layers as a function of thickness were determined. Our findings show that
optical absorption is red-shifted for a chemisorbed molecular layer relative to
the molecule in solution, indicating electronic coupling between the molecule
and the surface. However, we also find that from 2-22 nm in thickness, the
absorption characteristics follow Beers law, with no further shifts in the
wavelength of maximum absorption or low-energy absorption onset. For energies
where no absorption occurs, an internal photoemission process takes place
resulting in photocurrent that can be sustained over a limited range. For
energies at which the molecular component absorbs light, photocurrent can
proceed over longer distances, and the trends in photocurrent yield versus
thickness shows interesting trends that may be related to the distance scale
over which charge carriers can communicate with the conductive contacts. This
method is particularly relevant for strongly coupled systems that rely on
molecular properties to mediate charge transport, since excitation takes place
directly within the molecular layer, and provides a direct probe of energetics
in systems that show bulk-limited transport.
