In this work, a teepee-like photonic crystal (PC) structure on crystalline silicon (c-Si) is experimentally demonstrated, which fulfills two critical criteria in solar energy harvesting by (i) its Gaussian-type gradient-index profile for excellent antireflection and (ii) near-orthogonal energy flow and vortex-like field concentration via the parallel-to-interface refraction effect inside the structure for enhanced light trapping. For the PC structure on 500-mu m-thick c-Si, the average reflection is only similar to 0.7% for lambda = 400-1000 nm. For the same structure on a much thinner c-Si ( t = 10 mu m), the absorption is near unity (A similar to 99%) for visible wavelengths, while the absorption in the weakly absorbing range (lambda similar to 1000 nm) is significantly increased to 79%, comparing to only 6% absorption for a 10-mu m-thick planar c-Si. In addition, the average absorption (similar to 94.7%) of the PC structure on 10 mu m c-Si for lambda = 400-1000 nm is only similar to 3.8% less than the average absorption (similar to 98.5%) of the PC structure on 500 mu m c-Si, while the equivalent silicon solid content is reduced by 50 times. Furthermore, the angular dependence measurements show that the high absorption is sustained over a wide angle range (theta(inc) = 0-60 degrees) for teepee-like PC structure on both 500 and 10-mu m-thick c-Si.
- photonic crystal; antireflection; light trapping ultrathin silicon; thin-film photovoltaics