Metal-oxide films for electrochromic applications: present technology and future directions

ABSTRACT:The quest for superior and low-cost electrochromic (EC) thin films, for applications in smart windows, remains strong owing to their large importance for energyefficient buildings. While the development of new EC materials for improved devices is important, diminishing or reversing degradation is another key issue, and electrical rejuvenation of degraded EC materials can offer new opportunities. Here we demonstrate that cathodically coloring EC thin films of TiO 2 , which normally suffer from ion-trapping-induced degradation of charge capacity and optical modulation upon extensive electrochemical cycling, can recover their initial EC performance by a rejuvenation procedure involving Li + ion de-trapping. Thus the initial performance can be regained, and refreshed TiO 2 films exhibit the same degradation features as as-deposited films upon prolonged electrochemical cycling. The rejuvenation was carried out by using either galvanostatic or potentiostatic treatments. Our study may open avenues to explore the recovery not only of EC materials and devices based on those but also for other ion-exchange-based devices.

mentioning

confidence: 99%

Eliminating Electrochromic Degradation in Amorphous TiO₂ through Li-Ion Detrapping

Wen

Niklasson

Granqvist

2016

“…1,2 The largest application, at least with regard to surface area, is for EC architectural "smart widows" capable of providing energy efficiency in actively cooled buildings and simultaneously yielding comfortable indoor conditions. [3][4][5] EC devices for these applications typically use a WO 3 thin film separated from a Ni-oxide-based thin film by an ion-conductor, usually for Li + , in thin-film form or being a layer of a polymer electrolyte, and electrical powering is made possible by embedding this three-layer configuration between thin films of transparent electrical conductors. [5][6][7] Optical modulation ensues when ions are shuttled between the WO 3 film, which colors cathodically by ion insertion, and the Ni-oxide-based film with complementary anodic coloration under ion extraction.…”

mentioning

confidence: 99%

Galvanostatic Ion Detrapping Rejuvenates Oxide Thin Films

Arvizu¹,

Wen²,

Primetzhofer³

et al. 2015

Ion trapping under charge insertion−extraction is wellknown to degrade the electrochemical performance of oxides. Galvanostatic treatment was recently shown capable to rejuvenate the oxide, but the detailed mechanism remained uncertain. Here we report on amorphous electrochromic (EC) WO 3 thin films prepared by sputtering and electrochemically cycled in a lithium-containing electrolyte under conditions leading to severe loss of charge exchange capacity and optical modulation span. Time-of-flight elastic recoil detection analysis (ToF-ERDA) documented pronounced Li + trapping associated with the degradation of the EC properties and, importantly, that Li + detrapping, caused by a weak constant current drawn through the film for some time, could recover the original EC performance. Thus, ToF-ERDA provided direct and unambiguous evidence for Li + detrapping.

“…[1][2][3] These devices normally utilize joint transport of small ions and electrons between a thin film of tungsten oxide and a counter electrode based on nickel oxide, basically in the same way as in an electrical battery, and the optical transmittance is low when the charge resides in W oxide while the transmittance is high when the charge is in Ni oxide. Ni oxide films suffer from optical degradation and limitation in modulation span under extended electrochemical cycling, [4][5][6] and several recent studies aimed at alleviating these deficiencies have been reported.…”

mentioning

confidence: 99%

Strongly Improved Electrochemical Cycling Durability by Adding Iridium to Electrochromic Nickel Oxide Films

Wen

Niklasson

Granqvist

2015

Anodically coloring Ni oxide thin films are of much interest as counter electrodes in Woxide-based electrochromic devices such as "smart windows" for energy efficient buildings.However, Ni oxide films are prone to suffer severe charge density degradation upon prolonged electrochemical cycling, which can lead to insufficient device lifetime. Therefore means to improve the durability of Ni-oxide-based films is an important challenge at present.Here we report that the incorporation of a modest amount of Ir into Ni oxide films [Ir/(Ir + Ni) = 7.6 at.%] leads to remarkable durability, exceeding 10,000 cycles in a Li-conducting 2 electrolyte, along with significantly improved optical modulation during extended cycling.Structure characterization showed that the fcc-type NiO structure remained after Ir addition.Moreover, the crystallinity of these films was enhanced upon electrochemical cycling. 3Electrochromic thin-film devices are of much current interest for "smart windows" capable of varying their throughput of visible light and solar energy so that buildings can be rendered both energy efficient and comfortable. 1-3 These devices normally utilize joint transport of small ions and electrons between a thin film of tungsten oxide and a counter electrode based on nickel oxide, basically in the same way as in an electrical battery, and the optical transmittance is low when the charge resides in W oxide while the transmittance is high when the charge is in Ni oxide. Ni oxide films suffer from optical degradation and limitation in modulation span under extended electrochemical cycling, 4-6 and several recent studies aimed at alleviating these deficiencies have been reported. Beneficial effects of additives to the Ni oxide have been studied, specifically for Li, 7,8 C, 9,10 N, 11 W, 12 (Li,W), 13 (Li,Al), 14 (Li,Zr), 15 and several more. Nevertheless, the most crucial problem for Ni-oxide-based films, viz., their cycling durability, is still far from solved. In a previous paper, 4 we showed that the degradation of charge density in Ni-oxide follows a power-law decay model upon cycling in 1 M LiClO 4 dissolved in propylene carbonate (Li-PC), and long-term degradation hence does not only curtail cycle-life but also gradually erodes the optical modulation span of the device.In this Letter, we report that the charge density exchange in Ni-oxide-based thin films can be significantly increased during extended electrochemical cycling if the films contain a modest amount of iridium, and optical modulation is concomitantly enhanced.We deposited Ir-Ni oxide films by co-sputtering from metallic iridium and nickel targets in argon-oxygen atmosphere onto unheated glass coated with In 2 O 3 :Sn (i.e., ITO, 60 Ω/square).Films were also prepared on carbon substrates for composition determination. Film thickness t was determined by surface profilometry across a step edge. The iridium content in the Ni oxide was characterized by Rutherford backscattering spectroscopy (RBS). Ni oxide before and after 10,000 and 2,600 CV cycles, respe...