Maximum efficiency of state-space models of molecular scale engines
The performance of nano-scale energy conversion devices is studied in the framework of state-space models where a device is described by a graph comprising states and transitions between them represented by nodes and links, respectively. Particular segments of this network represent input (driving) and output processes whose properly chosen flux ratio provides the energy conversion efficiency. Simple cyclical graphs yield Carnot efficiency for the maximum conversion yield. We give general proof that opening a link that separate between the two driving segments always leads to reduced efficiency. We illustrate this general result with a simple model of an organic photovoltaic cell, where such an intersecting link corresponds to non-radiative carriers recombination and where the reduced maximum efficiency is manifested as a smaller open-circuit voltage.