Stencil-nanopatterned back reflectors for thin-film amorphous silicon n-i-p solar cells

With the aim of testing different periodic back reflectors for thin-film amorphous silicon (a-Si:H) n-i-p solar cells, we use stencil-lithography technique for patterning two different back reflector configurations. One is a plasmonic grating, composed of an array of isolated silver nanodots embedded in a zinc oxide layer. The other is a metallic grating with nanodots on top of a silver layer. The stencil lithography technique allows us to perform a fair comparison as both configurations lead to the same geometry of the silicon films. The stencils themselves were fabricated by wafer-scale nanosphere lithography. We found that both back reflectors have a photocurrent enhancement compared to their planar morphology references. The metallic grating shows a better performance than the plasmonic grating, with 19% short circuit current density enhancement compared to a flat reflector. We conclude that pure grating coupling is more efficient than coupling through localized surface plasmons for light trapping. Our best results in n-i-p solar cells are still obtained on random textured back reflector, which demonstrate higher short circuit current density. We present a single a-Si:H cell with 10.4% initial efficiency on textured silver substrate and a tandem cell a-Si:H/a-Si:H with 11.1% initial efficiency on a UV-embossed plastic substrate.