Engineering inverse woodpile and woodpile photonic crystal solar cells for light trapping.

We demonstrate that inverse woodpile and woodpile photonic crystal nanocrystalline silicon structures may be engineered for light trapping in solar cells. We use finite-difference tim-domain simulations to show that the geometry of these photonic crystals may be varied such that absorption in the infrared, visible, and ultraviolet parts of the spectrum may all be improved. The short-circuit current density and ultimate efficiency are also improved. We found a 77.1% and 106% absorption enhancement in the optimized inverse woodpile and woodpile structures respectively, compared to a nanocrystalline silicon thin film of the equivalent thickness. The inverse woodpile structures may be approximated as a thin film with effective index of refraction, whereas the woodpile structures exhibit resonances from the coupling of TE and TM leaky modes in the stacked cylinders. Woodpile photonic crystal structures exhibit improved performance compared to inverse woodpile structures over a range of equivalent thicknesses and incidence angles. The performance of woodpile structures is also generally insensitive to the diameter, pitch and number of layers, whereas inverse woodpile structures are much more sensitive to morphology.