Anisotropic optical properties of silicon nanowire arrays based on the effective medium approximation

Abstract

In search of next-generation solar cells, silicon nanowire arrays have attracted great attention since they are cost-effective and may absorb more light compared to current thin-film silicon solar cells. Theoretical studies using finite-difference time-domain and transfer matrix methods have been performed to investigate the optical properties of silicon nanowire (SiNW) arrays. However, these methods are computationally intensive and require periodic conditions, which may not be satisfied with most fabricated samples. In the present study, an effective medium analysis considering the anisotropic nature of vertically aligned SiNWs is performed to study their optical properties in the wavelength range from 310 nm to 1100 nm, which is of the most importance for solar photovoltaic cells. The effective dielectric functions of the SiNW layer for both ordinary and extraordinary waves are obtained from the Bruggeman approximation. Thin-film optics formulae incorporating the anisotropic wave propagation in uniaxial media are employed to calculate the reflectance and absorptance of the SiNWs on silicon substrates for different polarizations. The effect of geometric parameters such as filling ratio and wire length is investigated. In addition to modeling the directional radiative properties at various angles of incidence, the hemispherical properties are also calculated to understand the light absorption and to facilitate the optimal design of high-performance SiNW solar cells.