Ageing of electrochromic tungsten oxide thin films studied by impedance spectroscopy.
2016 (English)In: / [ed] M. Labardi and E. Tombari, Pisa, 2016, O-89- p.Conference paper, Abstract (Refereed)
Tungsten oxide is the most widely used cathodic electrochromic material for smart window applications and a major challenge is to ensure the durability over a service life of more than 20 years. Hence, the degradation under operating conditions must be better understood and preferably prevented. Recent progress in this field includes studies of the power-law kinetics of the degradation process  as well as the possibility to rejuvenate degraded films, thereby restoring them to their initial state . In this paper we use impedance spectroscopy to investigate electrochemical ageing of sputter deposited tungsten oxide thin films. Impedance spectroscopy measurements were carried out in a Li+-containing electrolyte, from the open circuit potential at about 3.2V vs. Li down to (1) 2.2 V, which is in the reversible intercalation range; (2) 1.5 V and (3) 1.0 V; at these latter voltages Li intercalation has been found to be irreversible. Subsequently, measurements were carried out for increasing voltages up to the starting point at 3.2 V vs. Li. The intercalated charge capacity was unaffected when potentials below 2.0 V were avoided, while it decreased drastically on the return path from 1.5 V and 1.0 V vs. Li. We interpret this ageing behaviour as an effect of trapping of Li ions irreversibly in the film. Secondly, after the films had been subjected to the low potentials known to induce degradation, the charge transfer resistance in the usual operating range above 2.0 V vs. Li showed a marked increase. This is interpreted as a signature of the development of a solid-electrolyte interface. In conclusion, our measurements suggest that the development of the solid-electrolyte interface may be responsible for the slow degradation observed under normal operating conditions, while Li ion trapping is mainly responsible for severe degradation at potentials significantly below 2.0 V vs. Li.
 R.-T. Wen, C. G. Granqvist, and G. A. Niklasson: Cyclic voltammetry on sputter-deposited films of electrochromic Ni oxide: Power-law decay of the charge density exchange. Appl. Phys. Lett. 105 163502 (2014)
 R.-T. Wen, G.A. Niklasson and C.G. Granqvist: Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films. Nature Materials 14 996-1002 (2015)
Place, publisher, year, edition, pages
Pisa, 2016. O-89- p.
Research subject Engineering Science with specialization in Solid State Physics
IdentifiersURN: urn:nbn:se:uu:diva-305385OAI: oai:DiVA.org:uu-305385DiVA: diva2:1037598
9th International Conference on Broadband Dielectric Spectroscopy and its Applications, BDS 2016, Pisa, Italy, September 11-16, 2016
FunderSwedish Research Council