  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.
