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 di fferent system states) has been constructed. Such network representation allows one to decompose the steady state dynamics into cycles, characterized by their cycle affinities. Both the maximum achievable efficiency and the open-circuit voltage of the device are 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 systems. In particular, the deviation from Carnot's efficiency associated with the exciton binding energy is quantified. (Figure Presented).