Rational geometrical design of three-dimensional nanostructures for efficient light harvesting

Three-dimensional (3-D) array of nanostructures have drawn much attention for efficient light harvesting property with a small amount of material compared to their thin film counterparts. In our work the effect of shape, pitch and geometric parameters on absorption of 3-D nanostructure arrays are systematically studied. Particularly, it is found that light absorption of nanopillar and nanocavities can be determined by the diameter because of the existence of transverse resonance modes. Optical properties of dual-diameter nanopillar structure are also investigated, and it is found that this structure can solve the paradox of large material filling ratio and small reflectance to some extent so as to enhance the optical absorption. If the number of nanopillar segments is continually increased to form a multi-diameter nanopillar or nanotower structure, the optical absorption can compete with the optical absorption of nanocone structure with the same length. On the other hand, it is also discovered that the integrated nanopillar and nanocavity structures combining both "positive" and "negative" nanostructures show more efficient light absorption than the "positive" or "negative" nanostructures alone over a broad range of wavelengths and incident angles.