Optimal wavelength scale diffraction gratings for light trapping in solar cells

Dielectric gratings are a promising method of achieving light trapping for thin crystalline silicon solar cells. In this paper, we systematically examine the potential performance of thin silicon solar cells with either silicon (Si) or titanium dioxide (TiO₂) gratings using numerical simulations. The square pyramid structure with silicon nitride coating provides the best light trapping among all the symmetric structures investigated, with 89% of the expected short circuit current density of the Lambertian case. For structures where the grating is at the rear of the cell, we show that the light trapping provided by the square pyramid and the checkerboard structure is almost identical. Introducing asymmetry into the grating structures can further improve their light trapping properties. An optimized Si skewed pyramid grating on the front surface of the solar cell results in a maximum short circuit current density, J_sc, of 33.4mAcm², which is 91% of the J _sc expected from an ideal Lambertian scatterer. An optimized Si skewed pyramid grating on the rear performs as well as a rear Lambertian scatterer and an optimized TiO₂ grating on the rear results in 84% of the J_sc expected from an optimized Si grating. The results show that submicron symmetric and skewed pyramids of Si or TiO₂ are a highly effective way of achieving light trapping in thin film solar cells. TiO ₂ structures would have the additional advantage of not increasing recombination within the cell.