Photo-induced Current through Molecular Junction in the Weak Molecule-Lead Coupling Limit

Photo-induced electron transport through a single molecule is studied in the limit of weak molecule-lead coupling using the Pauli master equation method that propagates the probabilities of the many-body states of the molecular Hamiltonian. Assuming a continuous wave (CW) light source in resonance with the singlet-singlet transition, we show that optical excitation modifies the current by inducing populations of excited electronic states. In contrast with previous studies, the triplet state and singlet-triplet transition are considered in our model, and they are found to play an important role in the photo-induced conduction. The steady current in the sequential transmission regime is calculated for the dependence on gate voltage, external field, asymmetric coupling ratio and light polarization. In different regimes that vary by the energy level of the charged state, we find that the optical excitation may switch on current by pumping up population in the triplet state, or suppress current by reducing the population of the singlet ground state. A near-zero onset voltage of photo-induced conduction is observed when the transport gap is aligned with the energy level of the triplet state.