Photovoltaic Effect in Self-Assembled Molecular Monolayers on Gold: Influence of Orbital Energy Level Alignment on Short-Circuit Current Generation

Abstract

Photoinduced current generation at metal–organic monolayer interfaces is observed upon photoexcitation of a monolayer of hemicyanine molecules chemically adsorbed onto a gold electrode. A series of hemicyanines is investigated that bind to the gold via a thiol moiety, in an orientation such that the acceptor moiety of the hemicyanine is closer to the metal than its donor part. The quantum yield of short-circuit photocurrent generation in a diode using a liquid electrolyte as second contact, correlates with the strength of the donor moiety of the dyes. Modeling of the photocurrent generation using Marcus theory indicates that the net photocurrent results from asymmetry in the electron transfer rates of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with the electrodes. The quantum efficiency of short-circuit photocurrent generation decreases for the HOMO levels of the hemicyanine going deeper below the Fermi-level of the metal. The deeper HOMO level provides a ...