Abstract
Near perfect absorbers find application in many areas including solar cells, energy harvesting and antireflection coatings for space applications. Here we report the use of optical gradation concept to fabricate a near perfect absorber on etched Si wafer. As a proof of concept, 99.4% absorption is achieved in the broad range of 300 nm to 2000 nm. Moreover, absorption capacity of optically graded surface remains higher than 99% up to beam incident angle of 50°. While carbon nanotubes (index ~1.1) are used as top layer, subsequent layers with increasing optical index across the thickness are chosen so as to satisfy zero reflection condition on multilayered assembly. Inward bending of incident beam and total internal reflection of reflected beam caused due to optical index gradient contributes to absorb the incident beam more efficiently. In addition, multiple scattering of incident beam due to the presence of multiscale feature size in graded assembly helps to absorb shorter as well as longer wavelengths of incident light. The graded assembly shows contact angle of 160° with roll-off angle equal to 5° implying that the graded absorber is not only super black but also superhydrophobic and self-cleaning in nature. The combination of properties shown by the super absorber makes it very attractive, especially for next generation solar cells to harness energy in the wavelength range of 1000 nm to 2000 nm.
Highlights
Super black absorbers are attractive in many technologically important areas such as photovoltaics, stealth technology and antireflection coatings for space and defence applications[1,2,3,4]
The reverse optical gradient will partially block the reflected beam to escape from the substrate through total internal reflection (TIR)
The choice of materials at a particular location in the multilayer assembly shown in Fig. 1a is motivated by the condition n1n3 = reflection surface as well as to ensure impedance matching at the top layer. n2
Summary
Near perfect absorbers find application in many areas including solar cells, energy harvesting and antireflection coatings for space applications. A super absorber should ideally have extremely low reflectance (R < 0.2%) and very high absorption (A > 99%) over broadband frequency range and angle of incident beam[5].Various processes as well as materials have been developed to bring down the refractive index of objects to unity and increase their absorption capacity[6,7,8,9]. We report the design and fabrication of an optically graded superhydrophobic and self-cleaning surface with near perfect absorption (99.4%) in the broad wavelength of 300 nm to 2000 nm with incident beam angle in the range of 0–60°. Fabrication of such an absorber has not been so far realized to the best of our knowledge.
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