  Non-equilibrium transport properties and energy conversion performance of a
molecular photo-voltaic cell are analyzed using the Lindblad master equation
within the open quantum systems approach. The method allows us to calculate the
dynamics of a system driven by several non-equilibrium sources (a situation we
call "strong non-equilibrium"), which is the natural operating condition of
photovoltaic cells. We include both coherent and incoherent processes and treat
electrons, photon, and phonons on an equal footing. We find that decoherence
plays a crucial role in determining both the overall efficiency of the
photovoltaic conversion and the optimal energy configuration of the system.
Specifically, decoherence leads to better performance, due to a faster
relaxation of the excited electrons to the electrodes. We also examine the
effect of coherent interference on the efficiency. The approach we propose in
this letter is suitable for studying transport and energy conversion in other
nanoscale systems at non-equilibrium, where both coherent and incoherent
processes take place.
