  Photovoltaic energy conversion in photovoltaic cells has been analyzed by the
detailed balance approach or by thermodynamic arguments. Here we introduce a
network representation to analyze the performance of such systems once a
suitable kinetic model (represented by a master equation in the space of the
different system states) has been constructed. Such network representation
allows one to decompose the steady state dynamics into cycles, characterized by
their cycle affinities. The maximum achievable efficiency of the device is
obtained in the zero affinity limit. This method is applied to analyze a
microscopic model for a bulk heterojunction organic solar cell that includes
the essential optical and interfacial electronic processes that characterize
this system, leading to an explicit expression for the theoretical efficiency
limit in such system. In particular, the deviation from Carnot's efficiency
associated with the exciton binding energy is quantified.
