Light trapping in ultrathin silicon photonic crystal superlattices with randomly-textured dielectric incouplers.

We report here several different superlattice photonic crystal based designs for 200nm thick c-Si solar cells, demonstrating that these structures have the ability to increase broadband absorption from λ = 300nm to 1100nm by more than 100% compared to a planar cell with an optimized anti-reflection coating. We show that adding superlattices into photonic crystals introduces new optical modes that contribute to enhanced absorption. The greatest improvements are obtained when combining a superlattice photonic crystal with a randomly textured dielectric coating that improves incoupling into the modes of the absorbing region. Finally, we show that our design methodology is also applicable to layers 1 to 4 microns in thickness, where absorbed currents competitive with conventional thick Si solar cells may be achieved.