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We report the optical absorption characteristics and the photocurrent spectrafor large-area molecular junctions containing different molecular structuresand thicknesses. Through experimental assessment of the optical absorption ofmolecules in solution and chemisorbed onto transparent, conductive carbonfilms, the energy ranges and intensity of absorption for a variety of molecularlayers as a function of thickness were determined. Our findings show thatoptical absorption is red-shifted for a chemisorbed molecular layer relative tothe molecule in solution, indicating electronic coupling between the moleculeand the surface. However, we also find that from 2-22 nm in thickness, theabsorption characteristics follow Beers law, with no further shifts in thewavelength of maximum absorption or low-energy absorption onset. For energieswhere no absorption occurs, an internal photoemission process takes placeresulting in photocurrent that can be sustained over a limited range. Forenergies at which the molecular component absorbs light, photocurrent canproceed over longer distances, and the trends in photocurrent yield versusthickness shows interesting trends that may be related to the distance scaleover which charge carriers can communicate with the conductive contacts. Thismethod is particularly relevant for strongly coupled systems that rely onmolecular properties to mediate charge transport, since excitation takes placedirectly within the molecular layer, and provides a direct probe of energeticsin systems that show bulk-limited transport.

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