KEYWORDS: Reactive ion etching, Absorption, Silicon, Etching, Photonic crystals, Solar energy, Solar cells, Scanning electron microscopy, Ultraviolet radiation, Reflection
We show a unique design of teepee-like photonic crystal (TP-PC) structure that possesses a true gradient, Gaussian-type surface profile for smooth and accurate index matching between air and silicon for near-perfect light trapping. Such funnel-like, inverse-conical topography is capable of achieving near-zero optical reflection and near-unity solar absorption with excellent angular response over the entire visible light wavelength range. The fabrication only requires standard microelectronics reactive-ion etching (RIE) process. We demonstrate how various process parameters, such as RIE gas mixture ratio, RIE power, thickness of silicon dioxide (SiO2) coatings, and lattice constant of the photonic crystal, can impact the details of the “Gaussian” profile and further improve the optical performance of the TP-PC structure at broad-λ, broad-θ, especially in the ultraviolet (UV) wavelength range. Our finite-difference time-domain (FDTD) simulation of the TP-PC structure reveals existence of multiple absorption resonances in the 800- to1100-nm wavelength range. Poynting vector plots show that such strong absorption enhancements at the resonant frequencies are due to long-lifetime photonic modes arising from parallel-to-interface refraction of the incoming sunlight and formation of vortex-like energy flow pattern inside the TP-PC structure. Our design will lead the way for future development of ultrathin, high-efficiency c-Si solar photovoltaics.
A novel technique based on the two polymer micro-transfer molding (2-P μTM) for fabricating one dimensional (1D)
high aspect ratio nanoscale metallic structures is presented and experimental characterization is described. Glancing
angle metal deposition and physical argon ion milling (etching) techniques were also employed in processing. The
resulting metallic structures have high transmission (~80%) in the visible spectrum and have superior electrical
conductivity (resistance from 2.4 -7.3 Ω) compared to standard indium-tin oxide (ITO) glass. Thus, the high aspect ratio
metallic structures are a promising alternative with potentially superior performances to ITO glass as transparent
electrodes for organic solar cells.
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