Peak efficiency of multijunction photovoltaic systems

The maximum efficiency of multijunction photovoltaic systems is determined for independently connected solar cells that are optically in series. The maximum efficiency is computed as a function of solar concentration using both the Shockley-Queisser detailed balance radiative limit for the reverse saturation current density and a simple empirical expression for the reverse saturation current density obtained from published “state-of-the-art” solar cells performance characteristics. It is shown that there are an optimal number of junctions for peak efficiency after which the system efficiency will decrease as the number of junctions increases.