Mie resonances of dielectric nanostructures for light trapping and spectral conversion applications in photovoltaics
Final Report Abstract
Strong light absorption is indispensable for high-efficiency crystalline silicon solar cells. This becomes even more important as solar cells become thinner and thinner with the aim of decreased costs and increased material efficiency. Conventional light management means are not applicable anymore to very thin solar cells. Most new approaches on light trapping rely on nanostructuring the absorber interface. Unfortunately, the gain in light absorption achieved by these structures perishes as the nanostructured interface strongly degrades the electronic performance of the absorber. Therefore, the basic idea of this project was "planar black silicon". The goal was to find nanostructures that increase light absorption in the absorber, while leaving the absorber electronically intact, i.e. planar. Mie resonators have attracted increasing interest in the photonics community due to properties such as strong and spectrally broadband light scattering that are also appealing for solar cells. In this project, I investigate high-refractive index nanostructures that support Mie resonances applied to the backside of crystalline silicon planar solar cells for light trapping. It could be shown that for the considered solar cell design, i.e. amorphous silicon scatterers embedded into a transparent spacer layer at the backside of the silicon absorber terminated by an aluminum mirror, light scattering of Mie resonators is strongly enhanced. This enhancement is due to an efficient coupling of the modes of the Mie resonator with Fabri-Perot modes of the spacer layer. The enhanced light scattering of the individual scatterer translates into strong and spectrally broadband light diffraction when the scatterers are arranged into a dense array. The resulting absorption gain in thick silicon absorbers reveals a performance close to that of a (lossy) lambertian scattering backside, which is remarkable considering that the absorber layer is planar. Therefore, the results of this project indicate that amorphous silicon Mie resonators at the backside of solar cells for light trapping are attractive candidates to achieve strong light absorption in future ultrathin planar silicon solar cells. This work consitutes an important step towards "planar black silicon".
