Incorporation of nano-features into surface photoactive arrays for broadband absorption of the solar radiation

Surface arrays of photoactive materials with subwavelength dimensions are important for energy harvesting applications due to their capacity for broadband and omnidirectional absorption of the solar radiation. It was recently proposed and demonstrated that the absorption performance of such arrays can be further amplified with the introduction of additional nano-features. In the current work we consider the incorporation of nano-features into nanopillar (NP) arrays. The considered nano-features are: quasi-nanolenses, sidewall nano-decorations, and decreasing the NP bottom diameter to transform it into a light funnel (LF). The underlying various light trapping mechanisms are examined using near-field microscopy. Near-field microscopy is used to obtained both high-resolution far-field imaging, as well as mapping of the near-field photon distributions of the arrays. Firstly, it is shown that the main contribution to far-field reflected photons is from the ambient surrounding the silicon structures. Secondly, the incorporation of the various nano-features induces a decrease in the reflected far-field photons which originate from this ambient region. Also, it is shown how the incorporation of quasi-nanolenses concludes a decrease in far-field photons reflected in the oblique directions with high polar angles, whereas the incorporation of sidewall nano-decorations leads to a decrease in far-field photons reflected in directions of small polar angles. Also, the incorporation of both qNL and sidewall nano-decorations is examined, and light trapping with the LF geometry is discussed and demonstrated. Finally, understanding the underlying light trapping mechanisms will support the deterministic design of subwavelength arrays incorporated with nano-features for efficient harvesting of the solar radiation.