Optical Absorption and Photocurrent Spectroscopy for Investigating Important Length Scales in Molecular Electronics

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.