Efficient Black Silicon Solar Cell with a Density-Graded Nanoporous Surface: Optical Properties, Performance Limitations, and Design Rules

We study optical effects and factors limiting performance of our confirmed 16.8% efficiency "black silicon" solar cells. The cells incorporate density-graded nanoporous surface layers made by a one-step nanoparticle- catalyzed etch and reflect less than 3% of the solar spectrum, with no conventional antireflection coating. The cells are limited by recombination in the nanoporous layer which decreases short-wavelength spectral response. The optimum density-graded layer depth is then a compromise between reflectance reduction and recombination loss. Finally, we propose universal design rules for high-efficiency solar cells based on density-graded surfaces.