Opportunities for enhanced omnidirectional broadband absorption of the solar radiation using deep subwavelength structures

Abstract

Omnidirectional absorption is important to non-tracking systems designed for the harvesting of solar energy. Presently, we examine both experimentally and numerically the broadband absorption under oblique illumination driven by deep sidewall subwavelength structures (DSSS) in silicon nanopillar arrays (DSSS arrays). Specifically, we target DSSS geometries that are a built-in side effect of the top-down cyclic Bosch dry etch process employed to realize high aspect ratio silicon nanopillar (NP) arrays. We numerically compare the DSSS array with an optically-optimized straight-sidewall nanopillar array (SSNP array) under oblique illumination. We show how the presence of DSSS generates a higher absorptivity particularly for the spectral range >600 nm, induces the formation of absorptivity peaks at the near-infrared spectral range, and overall provides an enhanced omnidirectional broadband absorption. We experimentally show that the specular reflectivity and the total reflectivity of silicon DSSS arrays that were fabricated using a top-down Bosch dry etch process is significantly lower compared with that of the corresponding SSNP arrays. Specifically, we show a decrement in the broadband specular reflection of up to 30% for certain angles of illumination, and about 13% decrement in the total reflectivity.