Absorption in thin Si films with randomly formed subwavelength structures

Thin-film (<25 μm) solar cells are desirable due to their low cost and reduced bulk recombination losses. However, due to weak near infrared optical absorption of c-Si, cell efficiency is poor. Due to their large feature dimensions, geometrical texturing techniques are not suitable for thin film solar cells. We have investigated subwavelength diffractive optical approach to reduce surface reflection as well as enhance near IR optical absorption. For experimental evaluation, thinned Si films were bonded to pyrex and randomly textured to form subwavelength surfaces. A metal film was deposited on the back surface to achieve optical confinement. Near-incidence spectral optical measurements showed ∼100% optical absorption over 400-1200-nm spectral range. Rigorous coupled wave analysis was used to demonstrate diffractive scattering into diffraction orders was responsible for optical absorption.