Correlation between physical properties and electrochromic performances of DC magnetron sputtered a-WOx thin films

Tungsten oxide thin films were grown on different substrates by the conventional process of direct current magnetron sputtering. A series of amorphous WOx (a-WOx) films, with variable stoichiometry and porosity, were achieved with different total pressure (Pt) induced by oxygen flow rate (Q(O2)) variation. The objective of this study is to determine the couple (Q(O2); Pt) necessary to achieve optimal electrochromic performances of WOx and also study the correlation between physical properties and electrochromic performances. The WOx films optical transmittance evolved from opaque to transparent with the increase of Q(O2). A degradation of films refractive index and the increase of their porosity, both mainly due to the increase of Pt, were observed. The Raman spectroscopy indicated the presence of different W valence states in the as deposited a-WOx films. They all contain W4+, but W6+ were found only into transparent or semi-transparent films. A weak peak of W5+ was observed into the semi-transparent film. Electrical properties studied by Metal-Oxide-Semiconductor structures showed a decrease of the oxide capacitance when Q(O2) and Pt increase. The WOx transparent and optically reversible films, deposited with lower values of (Q(O2); Pt), i.e., Q(O2) = 4.5 sccm and 6 sccm, and Pt = 1.13 Pa and 1.37 Pa, respectively, exhibited the highest values of WOx/Si interface traps average density and positive ions density (Nss). The positive Nss reflects the oxygen deficiency amount in the films. These films were resistive to the discoloration process but showed the highest coloration efficiency CE632nm = 44.4 cm2/C and 42.2, respectively. Their coloration times were tc = 19 s and 22 s, and their discoloration times were td = 180 s and 75 s, respectively. They exhibited the highest optical modulation close to Delta T632nm = 76 %.