Rational design of inverted nanopencil arrays for cost-effective, broadband and omnidirectional light harvesting

Due to the unique optical properties, three dimensional arrays of silicon nanostructures have attracted increasing attention as the efficient photon harvesters for various technological applications. In this work, instead of dry etching, we have utilized our newly developed wet anisotropic etching to fabricate silicon nanostructured arrays with different well-controlled geometrical morphologies, ranging from nanopillars, nanorods, inverted nanopencils to nanocones, followed by the systematic investigations of their photon capturing properties combining experiments and simulations. It is revealed that optical properties of these nano-arrays are predominantly dictated by their geometrical factors including the structural pitch, material filling ratio and aspect ratio. Surprisingly, along with the proper geometrical design, the inverted nanopencil arrays can couple incident photons into optical modes in the pencil base efficiently in order to achieve excellent broadband and omnidirectional light harvesting performances even with the substrate thickness down to 10 μm, which are comparable to the costly and technically difficult achievable nanocone counterparts. Notably, the fabricated nanopencils with both 800 and 380 nm base diameters can suppress the optical reflection well below 5 % over a broad wavelength of 400 to 1000 nm and a wide angle of incidence between 0° and 60°. All these findings not only offer additional insight into the light trapping mechanism in these complex 3D nanophotonic structures, but also provide efficient broadband and omnidirectional photon harvesters for the next-generation cost-effective ultra-thin nanostructured photovoltaics.