Revealing Single-Molecule Photocurrent Generation Mechanisms under On- and Off-Resonance Excitation.

We investigate photocurrent generation mechanisms in a pentacene single-molecule junction using subnanometer resolved photocurrent imaging under both on- and off-resonance laser excitation. By employing a wavelength-tunable laser combined with a lock-in technique, net photocurrent signals are extracted to elucidate photoinduced electron tunneling processes. Under off-resonance excitation, photocurrents are found to arise from photon-assisted tunneling, with contributions from three distinct frontier molecular orbitals at different bias voltages. In contrast, under resonance excitation, photocurrents are found to be significantly enhanced at negative bias voltages, exhibiting a spatial distribution linked to molecular electronic transitions and associated transition dipoles. This study reveals the contributions of different frontier molecular orbitals in the photon-assisted tunneling processes under off-resonance excitation, while molecular optical transitions are also found to be important at negative bias under resonance excitation. These findings could be instructive to the design and optimization of advanced optoelectronic devices.