Electrochromic oxides: Electrochemical density of states, degradation kinetics and rejuvenation of degraded films
2015 (English)Conference paper, Abstract (Refereed)
Electrochromic (EC) materials change their optical properties upon the application of a voltage signal. EC devices are of interest for energy efficient smart windows, as well as for a number of niche applications. An EC device can be viewed as a thin-film electrical battery whose charging state is manifested by optical absorption. The central part of the device comprises EC thin films based on amorphous tungsten oxide (WO3) and nano-crystalline nickel oxide (NiOx) joined by a layer of polymer electrolyte. This three-layer arrangement is positioned between transparent and electrically conducting thin films of indium tin oxide (In2O3:Sn; ITO) backed by polyester foils. Applying a voltage between the ITO films induces transport of ions between the WO3 and NiO films, together with charge compensating electron transport through the outer circuit. Insertion of ions and electrons in WO3 and extraction of them from NiO lead to enhanced optical absorption. In this paper we report novel developments regarding the electronic density of states (DOS), the ageing kinetics and the rejuvenation of aged EC materials.
The optical absorption in EC oxides is due to electronic transitions between localized states close to the band edges. In order to model the optical absorption it is necessary to have a good understanding of the electronic DOS. We have found that it is possible to derive an effective DOS from electrochemical measurements, in particular from measurements of quasi-steady-state potential curves, during the ion/electron insertion process. The obtained “electrochemical DOS” exhibits good agreement with state-of-the-art density functional calculations of the DOS for a number of amorphous and nano-crystalline oxides.
EC devices are prone to slowly degrade under repeated electrochemical cycling and a thorough understanding of the kinetics of this degradation is necessary in order to estimate the lifetime of smart window products. The ageing is often ascribed to irreversible incorporation of ions at the film surfaces or inside the film. We present results which show that the degradation displays dispersive kinetics and the chemical reaction behind the process can be modelled by using a rate constant with power-law time dependence. Results obtained for WO3 and NiO thin films show both similarities and differences.
We have very recently found that rejuvenation of previously degraded WO3 films is possible. Films with heavily degraded charge capacity and optical switching were subjected to a galvanostatic treatment for a certain time. After subjecting the film to a small current for a number of hours, it regained its initial charge capacity and optical properties. The rejuvenation is ascribed to de-trapping of ions from deep traps, induced by a high electrochemical potential over the film.
The optical absorption and switching as well as the durability of EC material are key issues for the successful commercial development of smart windows. The results presented here pave the way for an increased understanding of the optical switching as well as of the degradation kinetics. The possibility of rejuvenating degraded films is highly exciting, and further studies are necessary in order to develop this idea into a practical process.
Place, publisher, year, edition, pages
Condensed Matter Physics Engineering and Technology
Research subject Engineering Science with specialization in Solid State Physics
IdentifiersURN: urn:nbn:se:uu:diva-266865OAI: oai:DiVA.org:uu-266865DiVA: diva2:868988
SCiMAN 10. 10th Symposium on Advanced Materials science and Nanotechnology, 7-9 December 2015, Universidad de Costa Rica