Embedded biomimetic nanostructures for enhanced optical absorption in thin-film solar cells.

Light-management is critical to thin film solar cells due to their usually limited optical absorption in the active layer. Conventional approaches involve employing separate techniques for anti-reflection and light trapping. Here, we demonstrate an embedded biomimetic nanostructure (EBN) that achieves both effects for hydrogenated amorphous silicon (a-Si:H) solar cells. The fabrication of EBNs is accomplished by patterning an index-matching silicon-nitride layer deposited on a glass substrate using polystyrene nanospheres lithography, followed by reactive ion etching. The profile of EBN is then reproduced layer by layer during the deposition of a-Si:H cells. We show that a solar cell with an optimized EBN exhibits a broadband enhanced external quantum efficiency due to both anti-reflection and light-trapping, with respect to an industrial standard cell using an Asahi U glass substrate which is mostly optimized for light trapping. Overall, the cell with an optimized EBN achieves a large short-circuit current density of 17.74 mA/cm(2), corresponding to a 37.63% enhancement over a flat control cell. The power conversion efficiency is also increased from 5.36% to 8.32%. Moreover, the light management enabled by the EBN remains efficient over a wide range of incident angles up to 60°, which is particularly desirable for real environments with diffused sun light. The novel patterning method is not restricted to a-Si:H solar cells, but is also widely applicable to other thin film materials.