Designing transparent nanophotonic gratings for ultra-thin solar cells

Integration of a rear surface nanophotonic grating can increase photocurrent in ultra-thin solar cells. Transparent gratings formed of dielectric materials and high bandgap semiconductors can offer efficient diffraction with lower parasitic absorption than more widely studied metal/dielectric equivalents. In these systems, the maximum photocurrent which can be obtained for a grating made of a given combination of materials is shown to follow a simple empirical model based on the optical constants of these materials and independent of grating dimensions. The grating dimensions still require optimization in order to maximize the photocurrent for a given active layer thickness by balancing the effects of diffraction outside the front surface escape cone and the tuning of waveguide modes in long wavelength regions which are poorly absorbed in an ultra-thin film. The optimal grating pitch is shown to be of particular relevance for both effects, changing nonmonotonically as the absorber gets thicker in order to track favourable waveguide mode resonances at wavelengths near the absorber bandgap. These trends together with the empirical model for material selection drastically reduce the design space for highly efficient light trapping with transparent gratings. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License.

Automated summary

Integration of a rear surface nanophotonic grating can enhance the photocurrent in ultra-thin solar cells. Transparent gratings made of dielectric materials and high bandgap semiconductors offer efficient diffraction and lower parasitic absorption. The maximum photocurrent in these gratings depends on the optical constants of the materials used, regardless of the grating dimensions. However, optimization of the grating dimensions is still necessary to maximize the photocurrent for a given active layer thickness.