Optimization of Photolithographic Fabrication of Photonic Crystals and their Use in High Efficiency Solar Cells

In this work we present an optimized process for the photolithographic fabrication of inverted pyramid photonic crystals (PC) with 3.1 µm periodicity on Si(001)-substrates to improve the light trapping in single junction solar cells. Anisotropic alkaline etch was used to form the pyramids with (111)-sidewalls using partial surface masking with lithographically structured SiO₂. Ridge widths between the pyramids down to (150 ± 50) nm were achieved, while ensuring a yield of multiple (2 × 2) cm² areas per wafer sample. After deposition of an antireflection stack consisting of AlO_x, SiN_y, and SiO_z with different thickness optimizations a weighted reflection approaching that of a random pyramid reference sample could be shown. We demonstrate a path length enhancement of 25 at a wavelength of 1200 nm for our cell with PCs. This is en par with but not superior to the respective value for the reference sample with random pyramids, and still below the Lambertian limit. Furthermore, we present the first POLO²-IBC (interdigitated back contact) solar cells with such photonic crystals on the front sides. These solar cells feature a power conversion efficiency of 22.9%.