Environment-Assisted and Environment-Hampered Efficiency at Maximum Power in a Molecular Photocell

The molecular photocell, i.e., a single-molecule donor-acceptor complex placed between two external leads, besides being technologically important, is a paradigmatic example of a many-body system operating in strong nonequilibrium. The quantum transport properties and the photovoltaic energy conversion efficiency of the photocell are investigated within the open quantum system approach by solving the Lindblad master equation. The interplay of the localized vibrational environment corresponding to the molecule (via the electron-phonon interaction) and the soft vibrational environment implemented via local dephasing shows its signature in the efficiency at maximum power. We find vibration-assisted electron transport in the medium to a strong electron-phonon coupling regime when the system does not experience dephasing. Exposure to dephasing hampers such a vibration-assisted electron transport in a specific and broad range of dephasing rates. Our results demonstrate that while a single (either vibrational or dephasing) environment can assist quantum transport, the combined effect of more than one environment can, quite counterintuitively, hamper the efficiency.