Experimental characterisation of irradiance and spectral non-uniformity and its impact on multi-junction solar cells: Refractive vs. reflective optics

The impact of non-uniform illumination on triple junction solar cells equipped with either refractive or reflective optics without secondary optical elements is characterised by varying the concentrator-to-receiver distance. Indoor experiments covering both electrical measurements and photographs with a charge-coupled device camera are performed. The non-uniformities are examined by differentiating two spectral bands, those affecting the top and middle subcells. This allows the irradiance and spectral distributions to be analysed. The analysis is complemented with ray tracing simulations. Results show a different behaviour for the refractive and reflective systems. Irradiance non-uniformities in the case of the reflective system around the on-focus position are higher than those measured in the refractive system. However, the spectral non-uniformities that characterise the refractive system constitute only a residual effect in the reflective system. The drop in the solar cell fill factor caused by non-uniformities is mainly driven by the degradation of the slope of the I?V curve near the shortcircuit point in the case of the refractive system, while it is mainly driven by the degradation of the slope of the I?V curve near the open-circuit point in the case of the reflective system. Both systems are very sensitive to the concentrator-to-receiver distance, a critical parameter that must be accurately designed in any concentrator system. The findings will allow the trade-off between increasing performance and increasing cost strategies to mitigate the non-uniformities in concentrator photovoltaic systems to be better evaluated.