Achieving an Accurate Surface Profile of a Photonic Crystal for Near-Unity Solar Absorption in a Super Thin-Film Architecture Ping Kuang, † Sergey Eyderman, ‡ Mei-Li Hsieh, § Anthony Post, † Sajeev John, ‡ and Shawn-Yu Lin* ,† † The Future Chips Constellation and the Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States ‡ Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario M5S 1A7, Canada § Department of Photonics, National Chiao-Tung University, Hsinchu, Taiwan 300 * S Supporting Information ABSTRACT: 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-μm- thick c-Si, the average reflection is only ∼0.7% for λ = 400- 1000 nm. For the same structure on a much thinner c-Si ( t = 10 μm), the absorption is near unity (A ∼ 99%) for visible wavelengths, while the absorption in the weakly absorbing range (λ ∼ 1000 nm) is significantly increased to 79%, comparing to only 6% absorption for a 10-μm-thick planar c-Si. In addition, the average absorption (∼94.7%) of the PC structure on 10 μm c-Si for λ = 400-1000 nm is only ∼3.8% less than the average absorption (∼98.5%) of the PC structure on 500 μm 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 (θ inc =0-60°) for teepee-like PC structure on both 500 and 10-μm-thick c-Si. KEYWORDS: photonic crystal, antireflection, light trapping, ultrathin silicon, thin-film photovoltaics T he development and utilization of solar cells and solar power have steadily increased in recent years in the effort to create a sustainable, renewable, clean energy resource. At present, crystalline and multicrystalline silicon (c- Si and m-Si)-based photovoltaics (PVs) are still the largest constituents of worldwide solar cell and module productions. However, the utilization of c-Si solar cells is being hindered by their high dollar-per-Watt cost. In order to retain its competitiveness, the best approach is to develop different silicon PV cell designs which are capable of achieving high energy conversion efficiency while using much less high-quality c-Si material, and the thickness of silicon has to be reduced from hundreds of micrometers to a cost-effective level of tens of micrometers, or even less. Some reports have already experimentally shown that moderate conversion efficiencies have been achieved with ultrathin c-Si solar cells. 1-5 Never- theless, thinner silicon still has a major disadvantage of insufficient light absorption in the longer near-infrared (IR) range of the solar spectrum, which in turn reduces the efficiency of the solar cell device. In order to overcome low IR absorption in thin-film solar cells, various schemes of light trapping mechanisms were proposed to (i) reduce the reflection and (ii) enhance light absorption. Both approaches are critical and desirable for performance improvement and cost reduction in solar cells. For reflection reduction, antireflective coating (ARC) designs of nanowires and nanorods, 6-8 moth-eyes, 9 and graded-index multilayer films 10-12 have been extensive studied. Furthermore, textured or patterned nanostructures such as nanowires, 13,14 nanocones, 2,15,16 nanopyramids, 1,17,18 plasmonics, 19 and pho- tonic crystals (PCs) 20-23 all have been extensively investigated for enhanced light trapping and improved absorption in thin- film solar cells. One intriguing optical phenomenon is the parallel-to-interface refraction (PIR) in PCs. 21,24-28 PIR effect is a negative refraction of light inside a PC, producing nearly perpendicular light-bending phenomenon. This acute light- bending phenomenon directly results in increased optical path Received: March 17, 2016 Accepted: June 3, 2016 Published: June 3, 2016 Article www.acsnano.org © 2016 American Chemical Society 6116 DOI: 10.1021/acsnano.6b01875 ACS Nano 2016, 10, 6116-6124