Effect of surface roughness on the solar photothermal conversion efficiency of spray-coated CuCo2O4 films

Abstract

Mixed transition metal oxide films are emerging as efficient and inexpensive potential alternatives to multilayer cermet spectrally selective coatings. However, to replace the current standards involving a complex metal–dielectric structure, oxides must be optimized in terms of their electronic structure and mainly their film morphology. In the present work, a simple ultrasonic nebulized spray pyrolysis technique is used to deposit CuCo2O4 films for solar absorber coatings. Their photothermal efficiencies are studied for solar thermal energy harvesting for different film thicknesses obtained by varying the deposition time. The film surface attributes are studied using atomic force microscopy and scanning electron microscopy. The films deposited for 5 and 10 min show relatively high visible absorptance (∼0.79) and relatively low thermal emittance (∼0.1) and thus are promising candidates for spectrally selective coatings. Meanwhile, increasing the deposition time (>10 min) increases the thickness, thereby increasing the solar absorptance. However, this results in an uncontrolled increase in the surface roughness, which affects the spectral selectivity adversely, leading to the films having higher thermal emittance of between 0.1 and ∼0.25. Analysis of the specular reflection contribution shows that this deterioration is governed predominantly by interference effects due to surface attributes. This study is important for the technological applications of spectrally selective coatings and makes a significant quantitative contribution to emphasize the importance of surface morphology in optics.

Realizing an Optical Micro‐Cavity in a CuCo2O4‐W‐CuCo2O4 Thin Film Stack for Spectrally Selective Solar Absorbers

“…The surface morphology, particularly the roughness, is highly significant for optical coatings. [ 17 ] Therefore, morphological analysis of the films was done using scanning electron microscopy (SEM) after the deposition of each layer. SEM images showed a uniform film for each layer.…”

“…The higher binding energy peak is present due to chemisorbed oxygen and lattice defects present. [ 17 ]…”

“…The higher binding energy peak is present due to chemisorbed oxygen and lattice defects present. [17] The presence of octahedral and tetrahedral Cu and Co suggests the incorporation of Cu ions to the mixed spinel oxide formed by Co. Co 3 O 4 has a normal spinel structure, where both octahedral and tetrahedral Co ions are present. Nonetheless, the incorporation of Cu ions forms the ternary spinel oxide structures, where anti-site defects can occur.…”

“…Anjana et al showed that controlling the roughness of CCO film deposited by spray pyrolysis can reach 80% absorption and 20% emittance without any anti-reflecting coating. [17] Amun Amuri et al reported a copper-cobalt oxide thin film by dip-coating with increased absorption of 86% by changing the Cu:Co ratio. [18] So these oxides can be incorporated as solar absorbing coatings because of their intrinsic absorption and tunability which is lacking in the case of metal-dielectric cermet and dielectric-metal-dielectric layers.…”