Light Trapping Characteristics of Photonic Crystal Constructs and Randomly Textured Thin Silicon

We present an experimental study investigating the light trapping properties of ultra-thin silicon with inverted pyramidic photonic crystals of nano-scale mesas, as well as random pyramidal textured silicon. Using conventional industry compatible technologies - photolithography and wet alkaline etching - photonic crystals are fabricated with uniformity, homogeneity, high reproducibility wherein the mesas are of nanoscale widths (i.e., below 40 nm). For thin-silicon foils of 20 and 40 µm thicknesses, both structures demonstrate significant enhancement in absorption compared to flat thin silicon, especially after the addition of double-layer anti-reflection coating and a metallic back reflector. For photonic crystals, the enhancement is due to the significant increase in optical path length especially at long wavelengths where effects such as parallel to interface refraction are observed. For random pyramids, the wide range of pyramid sizes obtained here (from 500 nm to 5 µm) show a broadband absorption enhancement where small pyramids suppress the reflection at short wavelengths and large pyramids contribute to trapping the long-wavelength photons. The maximal photocurrent densities for these structures are comparable to that for the Lambertian surface and exceed 40 mA/cm2 for silicon foil thickness down to 20 µm,