Ion Traps and Optical Modelling: A Potential Relationship in Electrochromics

Literature seminar abstract

Dynamic, or “smart” windows are an emerging energy-saving technology that have the potential to reduce heating, cooling, and lighting energy needs for residential and commercial buildings by about ten percent.1, However, the active materials in electrochromic devices tend to degrade over time. That is, they lose the ability to reversibly change from the “colored” to “bleached” states after many cycles. As a result, these window coating materials are unable to modulate between optical transmission states that prevent infrared radiation from entering and heating buildings. For model WO3 electrochromic smart windows, the colored and bleached states correspond to the reduced W5+ and oxidized W6+ states, respectively. Here I will discuss recent literature that focuses on identifying the degradation mechanism in WO3 smart windows. Wen et al.  have recently reported that ion trap states in the material account for the performance degradation under device cycling.3  Interestingly, there is evidence to support multiple types of trap states that have specific effects on both the coloring and bleaching conditions.3,4  In this talk, I will incorporate the insight from Wen et al. and unpublished work from Evans et al. into a discussion of how the optical performance of electrochromics can be modeled to gain physical insight into kinetics of both coloring and bleaching processes. Specifically, I will discuss how one- versus two-site intercalation mechanisms can impact the coloration of an electrochromic device. Finally, I will show that the original model by Vuillemin and Bohnke5 is incapable of predicting the optical density changes in electrochromic materials that follow a multi-site model.

 

(1)           Llordes, A.; Garcia, G.; Gazquez, J.; Milliron, D. J. Tunable near-infrared and visible-light transmittance in nanocrystal-in-glass composites. Nature 2013, 500, 323−326.

(2)           DeForest, N., Shehabi, A., O’Donnell, J., Garcia, G.,Geenblatt, J., Lee, E. S.,  Milliron, D. J. United States energy and CO 2 savings potential from deployment of near-infrared electrochromic window glazings. Building and Environment 2015, 89, 107-117.

(3)           Wen, R., M., Granqvist, C. G., & Niklasson, G. A. Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films. Nature Materials, 2015, 14 (10), 996-1001.

(4)           Wen, R., Arvizu, M. A., Morales-Luna, M., Granqvist, C. G., & Niklasson, G. A. Ion Trapping and Detrapping in Amorphous Tungsten Oxide Thin Films Observed by Real-Time Electro-Optical Monitoring. Chemistry of Materials, 2016, 28 (13), 4670-4676.

(5)          Vuillemin, B., & Bohnke, O. Kinetics study and modelling of the electrochromic phenomenon in amorphous tungsten trioxide thin films in acid and lithium electrolytes. Solid State Ionics, 1994 68(3-4), 257-267.

 

Division(s): Materials

Speaker: R. Colby Evans

Speaker Institution: Colorado State University

Event Date: 09-22-2017

Event Time: 4:00 PM

Event Location: Chemistry A101

Mixer Time: 3:45 PM

Mixer Location: Chemistry B101E

Host: J. Sambur