Re-defining Non-tracking Solar Cell Efficiency Limits with Directional Spectral Filters.

Optical filters that respond to the wavelength and direction of incident light can be used to increase the efficiency of tracking solar cells. However, as tracking solar cells are more expensive to install and maintain, it is likely that nontracking solar cells will remain the main product of the (terrestrial) solar cell industry. Here we demonstrate that directional spectral filters can also be used to increase the efficiency limit of nontracking solar cells at the equator beyond what is currently understood by up to ∼0.5% (relative ∼1.8%). We also reveal that such filters can be used to regulate the energy output of solar cells throughout a day or year, and can reduce the thickness of the absorber layer by up to 40%. We anticipate that similar gains would be seen at other latitudes. As this filter has complex wavelength-direction functionality, we present a proof-of-concept design based on Luneburg lenses, demonstrating these filters can be realized. Our results will enable solar cells with higher efficiency and more stable output while using less material.