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Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films

Abstract

There is keen interest in the use of amorphous WO3 thin films as cathodic electrodes in transmittance-modulating electrochromic devices1,2,3,4. However, these films suffer from ion-trapping-induced degradation of optical modulation and reversibility on extended Li+-ion exchange. Here, we demonstrate that ion-trapping-induced degradation, which is commonly believed to be irreversible, can be successfully eliminated by constant-current-driven de-trapping; that is, WO3 films can be rejuvenated and regain their initial highly reversible electrochromic performance. Pronounced ion trapping occurs when x exceeds 0.65 in LixWO3 during ion insertion. We find two main kinds of Li+-ion-trapping site (intermediate and deep) in WO3, where the intermediate ones are most prevalent. Li+ ions can be completely removed from intermediate traps but are irreversibly bound in deep traps. Our results provide a general framework for developing and designing superior electrochromic materials and devices.

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Figure 1: Rejuvenation assessment for WO3 films.
Figure 2: Electrochromic performance of WO3 films under varied operations.
Figure 3: Dynamics of ion trapping and de-trapping in WO3 thin films.
Figure 4: Ion-trapping-induced optical performance of WO3 thin films.

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Acknowledgements

We acknowledge support with RBS measurements from D. Primetzhofer and the staff of the Tandem accelerator laboratory at Uppsala University. Assistance was received from M. Arvizu and C. Triana for sample preparation. Financial support was received from the European Research Council under the European Community’s Seventh Framework Program (FP7/2007–2013)/ERC Grant Agreement No. 267234 (‘GRINDOOR’).

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R.-T.W conceived the idea, designed and performed the experiments. R.-T.W. carried out the data analysis with help from G.A.N. and C.G.G. R.-T.W., G.A.N. and C.G.G. co-wrote the manuscript. G.A.N. and C.G.G. supervised and supported the work.

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Correspondence to Rui-Tao Wen.

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Wen, RT., Granqvist, C. & Niklasson, G. Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films. Nature Mater 14, 996–1001 (2015). https://doi.org/10.1038/nmat4368

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