Light Trapping in Silicon Arrays of Deep Subwavelength Features for Absorption of the Solar Radiation

Absorption of the sun spectrum in silicon thin films is imperative to a range of technologies concerned with harvesting of the solar energy. Silicon surfaces decorated with subwavelength structures offer an attractive paradigm for the generation of broadband absorption enhancement. The integration of additional deep subwavelength features can provide further enhancement in broadband absorption. It was recently shown that the incorporation of nanopillar arrays with deep subwavelength sidewall structures (DSSS) can increase the omnidirectional broadband absorption performance. These arrays are referred to as DSSS arrays. In the current work, a numerical examination of the underlying light trapping mechanisms of such systems is presented. It is shown that the incorporation of DSSS concludes increase in the absorption cross-section of the individual structures composing the arrays. Particularly, enhanced absorption cross-section is obtained for sparse arrays which is indicative that the DSSS induces additional scattering inside the individual structures. In dense nanopillar arrays, the absorption cross-section decreases but the overall broadband absorption of the arrays increases due to the increase in material density. In dense DSSS arrays, further enhancement in broadband absorption, compared with the NP arrays, is calculated due to optical interactions between adjacent structures which yield an elevated absorption cross-section. Overall, the increase in broadband absorption in compact DSSS arrays is on account of the elevated light trapping induced by the DSSS; the presence of DSSS increases both the scattering inside the individual structures as well as the scattering between adjacent structures.