4. Applications of Ordered Si Nanowire Array to Solar Energy Harvesting and NEMS

Nanostructured silicon thin-film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top–down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance of \({\sim }{99}\,\%\) (upper bound) over wavelengths 400–1100 nm by a thickness of only \(5\,\upmu \mathrm{{m}}\), is realized by our recently reported technique self-powered parallel electron lithography that has high throughput and high resolution. High-efficiency (up to 10.8 %) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays. Moreover, these ordered nano-structures have been successfully integrated into nano-electro-mechanical system (NEMS), enabling high-efficiency and broad-band optical actuation for NEMS devices. The first-ever nanopillar membrane acoustic speaker, using nano-scale photonic crystal optical absorbers for thermo-mechanical excitation of speaker membrane, is demonstrated.