Microstructure tuning facilitated photo-efficiency enhancement and environmental benign nature of HfO2/Mo/HfO2 multilayer films. (15th May 2018)

• Record Type: Journal Article
• Authors: Dubey, P.; Gomez, J.; Manandhar, S.; Shutthanandan, V.; Ramana, C.V.
• Is Part Of: Solar energy. Volume 166(2018)
• Page Start: 146
• Page End: 158
• Publication Date: 2018-05-15
• Subjects: Efficient window -- Hydrophobicity -- Optical properties -- Mechanical properties; Solar energy -- Periodicals; Solar engines -- Periodicals; 621.47
• DOI: 10.1016/j.solener.2017.12.021
• Languages: English
• ISSNs: 0038-092X

Abstract

Graphical abstract: Highlights: Vertically aligned columnar structure facilitates solar spectral selectivity of HfO2 /Mo/HfO2 . Aligned microstructure of the layers in HfO2 /Mo/HfO2 induces the high hydrophobicity. Mechanical properties support flexible and wear resistive nature of HfO2 /Mo/HfO2 . Understanding the structure-property relationship is important for the development of an energy-efficient product. Abstract: Nano-layered, multilayered films consisting of HfO2 (dielectric)/Mo(metal)/HfO2 (dielectric), (D/M/D architecture) with controlled layer thickness, chemistry and microstructure were demonstrated for efficient utilization of solar energy with well-designated spectral control. The effect of Mo metal interlayer on the energy performance and efficiency of HfO2 /Mo/HfO2 multilayer films made by electron-beam deposition onto silicon and glass substrates has been evaluated. The Mo interlayer thickness [Mo( t )] was varied in the range of 5–25 nm while the top/bottom HfO2 layer thickness was kept constant at ∼50 nm. Structural, morphological and optical characterization was performed to understand the effect of Mo( t ) induced microstructure on mechanical durability and optical behavior of the D/M/D multilayer films. Structural studies revealed that D/M/D multilayer films with relatively low Mo( t ) were amorphous, while higher Mo( t ) induces the amorphous-to-nanocrystalline microstructure transformation, which also induces surface roughening effects. The D/M/D multilayer film with Mo( t ) = 20 nm, where the nano-columnar morphology of the D-M-D layers are aligned perpendicular to the substrate surface and exhibits the optimum spectral selectivity. Such microstructure and morphological tailoring also facilitates the environmentally friendly (self-cleaning/stain repellent) nature of these multilayer films by the high contact angle (∼102°) achieved. Molybdenum interlayer facilitated higher contact angle leads to super-hydrophobic nature of the HfO2 /Mo/HfO2 multilayered film surfaces. The low surface roughness values (0.2–0.8 nm), which are primarily due to Mo at the interface, of the HfO2 /Mo/HfO2 multilayered films satisfy the general requirements of heat mirror applications. In addition, the microstructure of intermediate Mo layer and HfO2 -Mo interfaces significantly control the mechanical properties of D/M/D multilayer films. Optimal mechanical properties (hardness = 30 GPa, elastic modulus = 312 GPa, wear resistance = 0.094, adhesion strength = 2050 µN) have been obtained for Mo(t) = 20 nm D/M/D multilayer film. The mechanical response strongly supports flexible and wear resistive nature of HfO2 /Mo/HfO2 multilayer films.