We experimentally compare the light trapping efficiency of dielectric and metallic backscattering patterns in thin-film a-Si:H solar cells. We compare devices with randomly patterned Ag back contacts that are covered with either flat or patterned aluminum-doped ZnO (AZO) buffer layers and find the nanostructure at the AZO/a-Si:H interface is key to achieve efficient light trapping. Simulations show that purely dielectric scattering patterns with flat Ag and a patterned AZO/a-Si:H interface can outperform geometries in which the Ag is also patterned. The scattering from the dielectric patterns is due to geometrical Mie resonances in the AZO nanostructures. The optimized dielectric geometries avoid parasitic Ohmic losses due to plasmon resonances in the Ag, and open the way to a large number of new light trapping designs based on purely dielectric resonant light scattering.
