Asymmetric back contact nanograting design for thin c-Si solar cells

•We perform optical modeling for thin c-Si wafers with back contact gratings.•Asymmetric backcontact gratings perform better than symmetric ones.•Optimal designs of gratings lead to over 97% of the Lambertian absorption limit.

Texturing of silicon wafers with periodic gratings in submicron scale is one of effective light trapping routes especially for thin crystalline silicon solar cells. The grating can be very effective for trapping of photons near band gap of crystalline silicon when its period is suitably chosen. The asymmetric gratings provide higher light trapping efficiency than symmetric ones because the asymmetric one suppress the escape of internally reflected light in the silicon wafers. In this study, we conceptually show that asymmetric grating can be fabricated by simple stacking of a dielectric layer onto the symmetric silicon grating structures. Optical simulations were performed to calculate optical absorptions in thin crystalline silicon of a 5 μm thickness with symmetric and asymmetric grating structures. We demonstrate that with the asymmetric grating structures combined with highly efficient antireflection coating, optical absorptions in the thin silicon wafers can reach over 97% of the Lambertian absorption limit.