Nanostructured cupric oxide (CuO) thin films have been deposited on copper (Cu) substrates at different substrate temperatures and oxygen to argon gas ratios through direct current (DC) reactive magnetron sputtering. The deposited CuO thin films are characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), profilometry, and spectrophotometry techniques. The crystalline phases, morphology, optical properties, and photothermal conversion efficiency of the CuO thin films are found to be significantly influenced by the change in substrate temperature and oxygen to argon gas ratio. The variations in the substrate temperature and oxygen to argon gas ratio have induced changes in Cu+ and Cu2+ concentrations of the CuO thin films that result in corresponding changes in their optical properties. The CuO thin film prepared at a substrate temperature of 30°C and O2 to Ar gas ratio of 1？:？1 has exhibited high absorptance and low emittance; thus, it could be used as a solar selective absorber in solar thermal gadgets. 1. Introduction Solar collectors have gained immense interest owing to their potential applications in the field of water and air heating systems and cooling of buildings [1]. A thin film coated on a metal substrate having selective spectral response in the solar radiation is called selective coating. An ideal selective coating should have high solar absorptance ( ) in the visible and near infrared region (0.3–2？μm) and low thermal emittance ( ) in the infrared region (2–20？μm) of the solar spectrum in order to fully utilize the high energy radiation as well as to minimize undesired thermal losses [2, 3]. Hence, the effective use of solar energy for thermal applications requires the development of optically efficient solar selective coatings. The economic viability of the conversion process of solar energy into thermal energy depends on low-cost production and high durability of the selective coating under severe operational conditions along with efficient collection of solar radiation [4]. In this connection, several solar selective coatings have been developed in order to use them as selective absorbers in flat-plate collectors. Cupric oxide (CuO) is a p-type semiconductor with bandgap energy of 1.2？eV [5]. It has been widely investigated for various applications such as solar energy conversion, optoelectronics, batteries, sensors, semiconductors, and catalysis [6, 7]. The nontoxicity of CuO and abundant availability of its constituents make it an advantageous and promising material for
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