Three-dimensional nanotub submicrometer diffraction gratings for solar cells.

Diffraction gratings are a promising approach for reducing reflection and achieving light-trapping in solar cells. Using square lattices as a base structure, we investigate a novel bi-periodic nanotub three-dimensional grating structure and compare it with established textured structures for thin-film and wafer applications. For wafer application, simulations show that optimal AR coated nanotubs demonstrated solar weighted reflectance (SWR) of 2% compared to AR coated square pyramids with values 1.9%. Nanotubs also show SWR below 8% for polar angles to 60°. Simulated short-circuit current thin-film cells with nanotubs using smaller dimensions show higher yields (3-6  mA/cm2 average) compared to square pyramids. For periods greater than 700 nm at aspect ratios of 0.7 and greater, nanotubs have reduced current attributed to the increased planar surface area of the nanotub base, and evident in increased SWR. A simple nanoimprint lithography process was employed in experiments to define a square array of circular holes, utilizing a polydimethylsiloxane (PDMS) stamp applied onto a sol-gel imprint resist. The underlying silicon was then wet etched to produce the nanotub textures of 200 nm height and 513 nm period. AR coated nanotub wafers were produced via plasma enhanced chemical vapor deposition (PECVD), with an experimental and theoretical SWR of 6.4% and 5.4%, respectively.