Nanoimprint lithography of light trapping patterns in sol-gel coatings for thin film silicon solar cells

Abstract

For thin-film silicon solar cells, light trapping schemes are of uppermost importance to harvest all available sunlight. Typically, randomly textured TCO front layers are used to scatter the light diffusively in <i>p-i-n</i> cells on glass. Here, we investigate methods to texture the back contact with both random and periodic textures, for use in <i>n-i-p</i> cells on opaque foil. We applied an electrically insulating SiOx-polymer coating on a stainless steel substrate, and textured this barrier layer by nanoimprint. On this barrier layer the back contact is deposited for further use in the solar cell stack. Replication of masters with various random and periodic patterns was tested, and, using scanning electron microscopy, replicas were found to compare well with the originals. Masters with U-grooves of various sub micrometer widths have been used to investigate the optimal dimensions of regular patterns for light trapping in the silicon layers. Angular reflection distributions were measured to evaluate the light scattering properties of both periodic and random patterns. Diffraction gratings show promising results in scattering the light to specific angles, enhancing the total internal reflection in the solar cell.