Revealing Single-Molecule\nPhotocurrent Generation\nMechanisms under On- and Off-Resonance Excitation

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