Highly reflecting molybdenum thin films having significant solar absorptance

Abstract

A high solar absorptance coupled with a high IR reflectance is necessary for efficient photothermal solar conversion. The most common selective blacks operate on the principle of the absorber-reflector tandem, in which a highly reflecting film is overcoated with a layer which absorbs over the solar spectral range. Molybdenum films fabricated by chemical vapor deposition (CVD) under oxidizing conditions and subjected to post-deposition annealing and passivation provide both optical functions—high IR reflectance together with sufficient solar absorptance—in a single layer. Film deposition proceeds under atmospheric pressure and at 300°C through pyrolytic decomposition of molybdenum carbonyl (Mo(CO) 6) in the presence of an oxygen bleed. Films thus deposited contain a high concentration of oxygen and exhibit typically a solar absorptance of a = 0.77 and a thermal emittance for 500°C black-body radiation of e = 0.31. In subsequent annealing in a reducing atmosphere at 770°C this high solar absorptance is nearly retained whereas the thermal emittance is lowered to values of the order of e = 0.08. The passivation of these black molybdenum films against deterioration through operation in open air requires an overcoating. In this case Si 3N 4 deposited by CVD was used. This passivation layer can be used as an antireflection coating if it is of the proper thickness, raising the solar absorptance to values greater than a = 0.91 while only slightly increasing the thermal emittance to e = 0.11. Changes in the optical properties of black molybdenum during annealing are related to alternations in structure and composition. Since annealing proceeds at temperatures in excess of 700°C no thermal deterioration processes are expected in the absence of oxygen at the lower operating temperature of 500°C. This was demonstrated through testing at 500°C in a roughing pump vacuum; no changes in the films' optical characteristics were observed after 1000 h. All depositions and anneals proceed at atmospheric pressure, facilitating a continous flow-through procedure important for large-scale economic fabrication.