Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat Hafnium carbide based solar absorber coatings with high spectral selectivity Kushagra Hans a,b , S. Latha a , P. Bera a , Harish C. Barshilia a, ⁎ a Nanomaterials Research Laboratory, Surface Engineering Division, CSIR-National Aerospace Laboratories, Bangalore 560017, India b Birla Institute of Technology and Science-Pilani, Goa Campus, NH-17B, Zuarinagar, Goa 403726, India ARTICLE INFO Keywords: Hafnium carbide Solar absorber coating High selectivity Sputtering ABSTRACT Single layer coatings of hafnium carbide were deposited on stainless steel, copper and silicon substrates by means of reactive sputtering. The deposition process was optimized for maximum solar absorptance by varying the process parameters. In the case of coated stainless steel substrates, we obtained a maximum solar absorp- tance of 0.925 along with a thermal emittance of 0.13. For coated copper substrates we obtained a maximum solar absorptance of 0.948 along with a thermal emittance of 0.03. Reflectance spectra of SS and Cu substrates coated at optimized parameters were measured in the wavelength range 250 nm to 20 μm. Detailed x-ray photoelectron spectroscopy (XPS) studies were carried out to investigate the chemical composition of the coatings. XPS analysis of coated samples showed that because of the tendency of HfC to oxidize, the coating had a surface consisting mostly of HfO 2 but the extent of oxidation reduced with depth and partially oxidized HfC was present on the subsurface layers of the coatings. Along with the absorbing nature of HfC and appropriate coating thickness, the high solar absorptance of the coating can be partially attributed to the presence of HfO 2 near the surface which acts similar to an antireflection layer. Assuming a two-layer model of the coating, an upper HfO 2 layer and a HfC layer underneath it, reflectance spectra of the coated samples were simulated using SCOUT software and compared to the experimentally obtained spectra. For all the samples, best fit values of coating thicknesses were in the range 120–130 nm. 1. Introduction Electricity production is responsible for around one-quarter of the total greenhouse gas emission globally [1]. Concentrated solar power (CSP) systems are a vital cleaner alternative to fossil fuels based systems of electricity production. These systems are predicted to provide around 11% of the global electricity by the year 2050 [2]. A key component of CSP systems is the absorber surface which intercepts solar radiation. The efficiency of these systems is enhanced by application of a solar selective coating on the absorber surface [3]. Transition metal carbides and nitrides have been proposed as pro- mising candidates for solar absorber coatings because of their inherent selectivity long ago [3]. Optical properties of these materials are dif- ferent than pure metals because of the contribution to dielectric func- tion from interband transitions of bound electrons. In metals a steep drop in reflectance is observed at the plasma frequency. Whereas, in case of transition metal carbides and nitrides this drop in reflectance is shifted towards lower frequencies because of the contribution from interband transitions [4]. This extends the high absorption frequency range in these materials which leads to higher solar absorption. Sputter deposited films of TiN, ZrN, ZrC, ZrC x N y and ZrO x N y have been reported to have solar absorption ranging from 0.80 to 0.93 [5]. Hafnium carbide (HfC) is one the most refractory binary compounds known. It has a melting point of 3928 °C, which is among the highest melting point for binary compounds [6]. Sayir [7] has reported the mechanical properties of a carbon fiber - HfC composite material, which has a potential to be used as a structural material for high-temperature aerospace applications. To make the composite, HfC was deposited on substrates made of graphite and carbon fiber cloth using CVD process. The fact that HfC can withstand high-temperature environment makes it more suitable to be used in solar absorber coatings. Structure and mechanical properties of HfC films deposited using other processes have also been reported by many researchers. Teghil et al. [8] have reported the hardness of HfC coating deposited by pulsed laser ablation. Shuo et al. [9] sputter deposited HfC films on Si substrates and observed how hardness, microstructure and tribological behavior change with a change in the carbon content of the film. Oxidation of hafnium carbide has also been studied thoroughly [10–12]. At an oxygen pressure of 1.3 kPa, oxidation of HfC begins at 430 °C. During the oxidation of HfC, a dense hafnium oxide layer with carbon dissolved in it (HfC x O y ) is formed, which separates HfO 2 from HfC [10]. This layer acts as an ef- fective diffusion barrier for oxygen. Sani et al. [13] have investigated https://doi.org/10.1016/j.solmat.2018.05.005 Received 5 March 2018; Received in revised form 1 May 2018; Accepted 3 May 2018 ⁎ Corresponding author. E-mail address: harish@nal.res.in (H.C. Barshilia). Solar Energy Materials and Solar Cells 185 (2018) 1–7 0927-0248/ © 2018 Elsevier B.V. All rights reserved. T