Effects of concentrator chromatic aberration on multi-junction cells

Most practical photovoltaic concentrator optics have considerable chromatic aberration. Thus in a two-junction monolithic cell, the pattern of current generation in the high-bandgap junction is different from that in the low-bandgap junction, resulting in an attempted current flow parallel to the junction in the tunnel junction and adjacent layers. Sheet resistivity of these layers is too high to support this lateral current flow, so at each point on the cell, the current is limited to the current generated in the lowest-current junction. This results in a substantial loss in cell output unless the chromatic aberration of the optics is somehow compensated for. Using a software model, the authors have examined the losses caused by chromatic aberration, and have designed two types of concentrator optics which meet the spectral uniformity needs of multijunction solar cells. One is for space applications where weight is an overriding concern. The other is for terrestrial use where cost and a low sensitivity to construction and operating tolerances are paramount.