Photonic light trapping in silicon nanowire arrays: deriving and overcoming the physical limitations

TL;DR

This paper investigates how silicon nanowire arrays enhance light absorption and concentration for solar applications, identifying physical limitations and proposing inverted silicon nanocones as a superior design to optimize performance.

Contribution

It introduces a new understanding of the physical limitations in SiNW arrays and demonstrates that SiNC arrays can overcome these, enabling better independent optimization of absorption and concentration.

Findings

SiNW arrays have a maximum absorption at an ideal density.

Mode suppression limits light concentration in SiNW arrays.

SiNC arrays allow higher degrees of freedom for optimization.

Abstract

Hexagonally aligned, free-standing silicon nanowire (SiNW) arrays serve as photonic resonators which, as compared to a silicon (Si) thin film, do not only absorb more visible (VIS) and near-infrared (NIR) light, but also show an inherent photonic light concentration that enhances their performance as solar absorbers. Using numerical simulations we show, how light concentration is induced by high optical cross sections of the individual SiNWs but cannot be optimized independently of the SiNW array absorption. While an ideal spatial density exists, for which the SiNW array absorption for VIS and NIR wavelengths reaches a maximum, the spatial correlation of SiNWs in an array suppresses the formation of optical Mie modes responsible for light concentration. We show that different from SiNWs with straight sidewalls, arrays of inverted silicon nanocones (SiNCs) permit to avoid the mode suppression. In fact they give rise to an altered set of photonic modes which is induced by the spatial correlation of SiNCs in the array, and therefore show a higher degree of freedom to independently optimize light absorption and light concentration. Apart from explaining the good light absorbing and concentrating properties of SiNC arrays, the work justifies a revaluation of SiNW arrays as optical absorbers.