The optical physics of dielectric nanostructures: enabling improved photovoltaic designs

Abstract

Sunlight is, by far, the world’s most abundant source of energy, and may be converted directly into electricity without the emission of any greenhouse gasses using photovoltaic solar cells (SCs). While the cost of SCs has decreased tremendously in the last decade with the creation of superb economies of scale, there remains a wide scope for technological advancements to improve the cost competitiveness of SCs by reducing costs and increasing efficiencies. We investigate how nanophotonic structures may enable these developments. A common thread of our studies is to begin by examining the basic interaction of light with a nanostructure, as embodied by the structure’s modes, before designing optimised structures for a given application. The simulation tool that we have developed for our investigations is well aligned with this philosophy, and has been made freely available. We begin by studying thin films, which are inexpensive but whose efficiency is limited by low absorption. To address this deficiency we pattern the absorbers at the nanometer and micrometer scale so that the sunlight excites resonant nanophotonic modes in which it is trapped and strongly absorbed. In particular we study nanowire and nanohole arrays with bi-periodic inclusions of high and low refractive index respectively. By understanding the optical physics of these structures we arrive at a semi-analytic routine for optimising their absorption, and we also study the effects of disorder, showing how including NWs with varying diameter into the array increases the absorption. An alternative approach is to combine many physically separated SCs into a multi-junction SC to reach efficiencies in excess of 50%. A similar approach may also be used to achieve efficiencies of over 30% using affordable subcells, for instance made from perovskite and silicon. A crucial element in these devices is a wavelength selective filter, which may be made of dielectric gratings with regions of near 100% reflectance. We show these reflections are fundamentally due to the symmetries of these structures’ Fano resonances. Finally, we demonstrate that gratings of deeply subwavelength thickness, composed of metals or relatively weakly-absorbing semiconductors, can absorb nearly 100% of light at a target wavelength. These findings open up many practical applications because, unlike previous demonstrations that used complicated metallic metamaterials and plasmonics, weakly-absorbing semiconductors are abundantly found in nature, are compatible with optoelectronic applications, and can be patterned using standard techniques. Our theoretical findings are experimentally validated at visible wavelengths in a grating made of antimony sulphide.