Optimizing a concentrator system which uses multijunction solar cells is challenging because: (a) the conditions are variable, so the solar cells rarely operate under optimal conditions and (b) the conditions are not controlled, so any design problems are difficult to characterize. Any change in the spectral content of direct-beam sunlight as it passes through the concentrator optics is of particular interest, as it can reduce the performance of multijunction cells and is difficult to characterize. Here we show how the fill factor can be used to detect and diagnose this sort of a "spectral skewing" by the concentrator optics during outdoor operation. The work presented here is for GaInP 2 /GaAs tandem cells, but the conclusions are equally valid for GaInP 2 /GaAs/Ge triple-junction cells.
