  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.
