Optical Properties of Nanostructures

Written by : Martin Foldyna

Vertical nanowire arrays exhibit excellent light absorption due to their unique structure combining individual optical properties with the collective behavior of the arrays.

At LPICM, we are working on the optimization of the light absorption inside nanowire arrays. By combining individual nanowire properties with the closely packed periodic arrangement, we have designed double-diameter nanowire arrays enabling us to efficiently trap the light from the broader part of the solar spectrum and provide performance beyond the optimized single diameter periodic arrays [1].

Variation of theoretically achievable Jsc for dual diameter vertical nanowire arrays in a hexagonal arrangement as a function of two diameters. Diagonal correspond to the equal (single) nanowire diameter arrangement in periodic array.

We have developed advanced models to describe accurately the optical response of vertical nanowire arrays with imperfections (diameter variation, surface roughness, etc.) prepared by metal assisted etching (MACE) method.

Scanning electron microscopy image of silicon nanowire arrays prepared using MACE taken from the top.

The optical model is based on a rigorous coupled-wave analysis approach which enables to obtain the rigorous optical response of the perfectly periodic nanostructures. Under the assumption of the measured response being mutually incoherent when coming from different parts of the illuminated spot, the total Mueller matrix can be acquired as a linear combination of Mueller matrices from small regions assumed to be perfectly periodic. Therefore, we model this imperfect structure as a statistical combination of the responses from locally periodic areas. We have demonstrated that this extension to the standard model for perfectly periodic nanostructures enables us to describe the measured spectrally resolved optical data and gives a good estimate of the average nanowire diameter as well as its statistical dispersion [2].

Best fits of the optical model of measured data comprising of spectrally resolved total reflectance and Mueller matrix data.

Contact: martin.foldyna@polytechnique.edu

[1] M. Foldyna, L. Yu, and P. Roca i Cabarrocas, “Theoretical short-circuit current density for diﬀerent geometries and organizations of silicon nanowires in solar cells,” Solar Energy Materials & Solar Cells 117, 645 (2013).

[2] M. Foldyna, A. S. Togonal, Rusli, Pere Roca i Cabarrocas, “Optimization and optical characterization of vertical nanowire arrays for core-shell structure solar cells,” Solar Energy Materials and Solar Cells 159, 640 (2017).