Performance of luminescence down shifting for CdTe solar cells as a function of the incident solar spectrum

In this work we address the performance of luminescence down shifting (LDS) layers in combination with cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells as a function of the solar spectrum irradiance and power distribution, as would be the case in a real outdoor situation. To this purpose we have simulated the module efficiency when a CdS/CdTe mini-module is illuminated with a solar spectrum characteristic of different hours of the day and for five distinct days. Our results indicate that the LDS layer improves the conversion efficiency of the module in all scenarios (between 6 and 10%), where the improvement is most prominent at dawn and dusk (more than 20% on cloudy/summer days). The reason for this variation lies in the power distribution of the spectra, having a greater contribution of short-wavelength light in the morning or late in the afternoon. Under such blue-rich spectra the LDS layers operate very efficiently. Furthermore, we find that the relative efficiency improvement induced by an LDS layer has a roughly linear dependence with the average photon energy of the solar spectrum.