Maximizing the ultimate absorption efficiency of vertically-aligned semiconductor nanowire arrays with wires of a low absorption cross-section

Abstract

Single semiconducting nanowires with sub-wavelength diameters exhibit superior light absorption, and hence triggered a vivid discussion regarding the application of these nanostructures into future generations of high efficiency solar cells. We examine the transition from a single highly absorbing silicon wire into an array composed of such individuals in order to validate the application of these into solar harvesting devices. We use finite-difference time-domain simulations to show that the coupling of the Fabry–Perot oscillations with the waveguide resonances inside the wires has a significant effect on the array absorption. For example, the ultimate absorption efficiency of a square-tiled wire array under normal incidence (array period of 0.5µm, wire diameter of 0.4µm and wire height of 2) is 81% higher than a 2µm thin-film when the Fabry–Perot oscillations are considered and 37% higher when these oscillations are not considered. This coupling screens out the contribution of the waveguide modes to the array absorption and therefore, unlike previously published work, we eliminate the contribution of the Fabry–Perot oscillations. In this manner we demonstrate the absorption enhancement due to waveguide modes, and general correlations between the nanowire geometry and the overall array absorption are presented. First, we show that once an isolated wire with high absorption cross-section is nested inside an array its absorption decreases due to wire proximity effects. Secondly, the array absorption is maximized with relatively wide wires of low absorption cross-sections. We show that a 75nm wire inside an square-tiled array with 2µm period has an average absorption efficiency factor of 6.5 and the average relative absorption of the array is 0.5%, while the same wire nested inside an array of a 0.25µm period exhibits 2.3 average absorption efficiency factor and the array exhibits average relative absorption of 9.85%. Finally, there is an optimized wire diameter that once exceeded the array absorption converges to that of a continuous film. For example, the maximum absorption of 0.5µm array is obtained with wire diameter of 0.4µm where a decrease in relative absorption is recorded for arrays with wires exceeding 0.4µm.