Too Hot for Photon-Assisted Transport: Hot-Electrons Dominate Conductance Enhancement in Illuminated Single-Molecule Junctions

We investigate light-induced conductance
enhancement in single-molecule junctions via photon-assisted
transport and hot-electron transport. Using 4,4′-bipyridine
bound to Au electrodes as a prototypical single-molecule
junction, we report a 20−40% enhancement in conductance
under illumination with 980 nm wavelength radiation. We
probe the effects of subtle changes in the transmission function
on light-enhanced current and show that discrete variations in
the binding geometry result in a 10% change in enhancement.
Importantly, we prove theoretically that the steady-state
behavior of photon-assisted transport and hot-electron transport is identical but that hot-electron transport is the dominant
mechanism for optically induced conductance enhancement in single-molecule junctions when the wavelength used is absorbed
by the electrodes and the hot-electron relaxation time is long. We confirm this experimentally by performing polarizationdependent conductance measurements of illuminated 4,4′-bipyridine junctions. Finally, we perform lock-in type measurements of
optical current and conclude that currents due to laser-induced thermal expansion mask optical currents. This work provides a
robust experimental framework for studying mechanisms of light-enhanced transport in single-molecule junctions and offers tools
for tuning the performance of organic optoelectronic devices by analyzing detailed transport properties of the molecules involved.