Nanoimprinted Diffraction Gratings for Light Trapping in Crystal-Silicon Film Photovoltaics

Crystal-silicon (c-Si) film photovoltaics hold the promise of combining the advantages of state-of-the-art wafer-silicon technology with the scalability and the inherently much lower cost of thin-film solar technologies. However, thin-film silicon is transparent to red and near-infrared (NIR) light which comprises >37% of solar radiation. In thin-film technologies effective light trapping is essential to absorb sufficient red and near-infrared (NIR) light and reach targeted efficiencies of 16%–18%, as defined by the U.S. National Solar Technology Roadmap.

One solution is diffractive light trapping, which, at least in certain wavelength ranges, can theoretically outperform light trapping through random scattering at a rough surface or interface. In this project, we developed a nanoimprinting process for Titanium Dioxide, a high-refractive-index dielectric material, and fabricated diffraction gratings as back-reflectors for thin-film solar cells.

Nanoimprinted Titanium Dioxide diffraction gratings on a three-inch silicon wafer.

Electron Micrographs of nanoimprinted Titanium Dioxide diffraction gratings (pillar arrays) and corresponding Silicon master molds. Left Column: Micrograph of Titanium Dioxide pillar arrays from 0 degree angle; Center: Titanium Dioxide pillar arrays from 88 degree; Right: Silicon master pattern. All images have the same respective magnification.

This work was supported by the United States Department of Energy (DOE) Photovoltaic Supply Chain and Cross-Cutting Technologies Grant, DEEE0000586, and matching funds by the Washington Technology Center (Seattle). Work at the National Renewable Energy Laboratory (NREL) was supported by the United States DOE under contract DE-AC36-99GO10337.