Roll-to-Roll sputtering of thermochromic VO2-based coatings onto ultrathin flexible glass

Thermochromic vanadium dioxide (VO2)-based materials could enable large energy savings in windows and glass facades, especially for passive cooling in warmer climates.
VO2 undergoes a metal-to-semiconductor transition (MST) at a transition temperature Ttr, resulting in a change in the transmittance of infrared light. This temperature dependent, reversible effect in VO2-based coatings can temperature dependent passively regulate the amount of the solar heat transmitted into buildings. At temperatures lower than the Ttr, transmittance of the semiconductive monoclinic VO2 in the infrared region of the solar spectrum is high. At temperatures above the Ttr, the transmittance of the metallic tetragonal VO2 in that wavelength range decreases significantly. The change of transmittance does not occur in the visible range of the solar spectrum, so the colour of the coating remains the same. For the application in different climates, one option for tailoring the MST is the use of dopants, such as tungsten (W). Although thermochromic VO2-based coatings have been widely studied at laboratory scale, fundamental challenges remain for industrial manufacturing.
The present work sets out to explore the prospects of the deposition of W doped VO2-based coating using an existing laboratory scale layer stack design of ZrO2/V1-xWxO2/ZrO2 onto ultra-thin glass in an upscaled roll-to-roll process using unipolar pulsed and high power impulse magnetron sputtering HiPIMS. In this stack, the bottom ZrO2 layer acts as a template for the thermochromic V1-xWxO2 film doped with W, while the top ZrO2 layer protects it from mechanical and chemical factors. Both ZrO2 layers influence the optical properties of the stack.
For the deposition of the thermochromic phase of V1-xWxO2 a new oxygen control system was developed. It is based on the control of the discharge current, plasma emission intensity and glass back-side temperature after the deposition. Furthermore, another aim of the study was to investigate the properties of the produced coating regarding the energy savings potential and structure with UV-Vis-NIR spectrophotometry, Rutherford backscattering spectrometry, X-ray diffraction, atomic force microscopy and field emission scanning electron microscopy. The newly designed optical modelling investigated the UV-Vis-NIR spectra and allowed to calculate the dispersion relation of the layers and their electrical properties. One of the most important findings of this thesis is that the composition of the V1-xWxO2 layers depends strongly on the conditions in the sputtering chamber, such as working point, target history and the deposition temperature. Further, it has been shown that the enhanced doping content of W in VO2 layers led to lower Ttr and higher deposition rate, lower deposition temperature increased the Ttr and the lower film thickness reduced the thermochromic properties. The V1-xWxO2 layers grew on ultra-thin glass in the island growth mode (Volmer-Weber).