We study solar-cell designs using nano-grating on both top (transmission) and bottom (reflection) of the
solar cell. First, we perform simulations based on rigorous coupled wave analysis (RCWA) to evaluate
the diffraction top gratins. In RCWA method, we calculate up to 20 harmonics, and sweep the launch
angle of incident light from 0 to 90 degree. The incident light varies from100nm to 1200nm wavelength.
Triangular grating can achieve higher light absorption compared to the rectangular grating. The best top
grating is around 200nm grating period, 100nm grating height, and 50% filling factor, which responses to
37% improvement for triangular grating and 23% for rectangular grating compared to non-grating case.
Then, we use Finite-Difference Time-Domain (FDTD) to simulate transmission/reflection double grating
cases. We simulated triangular-triangular (top-bottom) grating cases and triangular-rectangular (top-bottom)
grating case. We realize solar cell efficiency improvement about 42.4%. For the triangular-triangular
(top-bottom) grating case, the 20% efficiency improvement is achieved. Finally, we present
weighted-light simulation for the double grating for the first time and show the best grating can achieve
104% light improvement, which is quite different from traditional non-weighted calculation.