Selective solar absorption is critical for efficient solar-thermal energy conversion.1 By absorbing solar energy while suppressing thermal emission in the IR, efficient solar to thermal energy conversion can be achieved. Plasmonic and metamaterial absorbers have shown broadband absorption but are difficult to fabricate on large scales and have yet to survive high temperatures.2,3Previously presented metallic photonic crystal structures have shown absorption in the near infrared region and high temperature durability (>1000°C), but are limited in the visible domain due to diffraction losses.4 Here, we present a 6” wafer-scale fabricated 2D metallic dielectric photonic crystal (MDPhC) with an average measured absorption of 85% for 250 nm < λ < 1770 nm and <10% for λ > 3.1 μm. The MDPhC has a period of 780 nm with a minimum structure width of 40 nm fabricated via atomic layer deposited (ALD) sidewall lithography across the 6" wafer. Measurements, theory, and simulations are presented to explain the broadband absorption profile.

The MDPhC uses dielectric filled cavity with an anti-reflection coating (ARC) to extend the absorption bandwidth of the MDPhC from the near-UV to the near-IR domain. Finite-difference time-domain (FDTD) simulations show that the dielectric filled cavities red shift the higher order cavity modes into the visible spectrum to create a high density of modes. This overlap of the multiple low-Q cavity modes creates the broadband absorption profile. The cut-off mode of the optical cavities in the near-IR suppresses absorption/emission at longer wavelengths for efficient solar to thermal energy conversion.

The high density of optical cavity modes allows for improved absorption in the visible spectrum, but can be further enhanced via a thin ARC coating. By using the experimentally fitted complex permittivity of ruthenium, an ARC layer of HfO2designed for the visible spectrum is calculated via the Fresnel reflection equation to range from 22 nm to 72 nm. With the addition of such a layer, both experiments and simulations confirm the improved absorption in the visible domain with a peak measured absorption of 96% at 688 nm. Thus together with the ARC coating and the high density of optical states, a highly absorbing profile across the solar spectrum is accomplished.

The experimentally demonstrated broadband absorption in metal can lead to major improvements in thermal emitters and photoelectrolysis for advanced solar energy conversion applications.

Resources:

(1) Lenert, A.; Bierman, D. M.; Nam, Y.; Chan, W. R.; Celanovi?, I.; Solja?i?, M.; Wang, E. N. Nat. Nanotechnol. 2014, 9, 126-130.

(2) Aydin, K.; Ferry, V. E.; Briggs, R. M.; Atwater, H. A. Nat. Commun. 2011, 2, 517.

(3) Cui, Y.; Fung, K. H.; Xu, J.; Ma, H.; Jin, Y.; He, S.; Fang, N. X. Nano Lett. 2012,12, 1443-1447.

(4) Yeng, Y. X.; Ghebrebrhan, M.; Bermel, P.; Chan, W. R.; Joannopoulos, J. D.; Solja?i?, M.; Celanovic, I. Proc. Natl. Acad. Sci. 2012, 109, 2280-2285.

Jeffrey Chou

Massachusetts Institute of Technology

Appearances