Fill Factor as a Probe of Current-Matching for GaInP2/GaAs Tandem Cells in a Concentrator System During Outdoor Operation

Designing a tandem solar cell for use in a concentrator system is challenging because: (a) the conditions are variable, so solar cells rarely operate under optimal conditions, and (b) the conditions are not controlled, so any design problems are difficult to characterize. Here, we show how the fill factor can be used as a diagnostic tool to either verify correct system design and operation or to help identify a problem. We give particular attention to the detection of spectral skewing by the concentrator optics, as this can reduce the performance of GaInP₂/GaAs tandem cells and is difficult to characterize. The conclusions are equally valid for GaInP₂/GaAs/Ge triple-junction cells.