Electrochromics for smart windows: Oxide-based thin films and devices

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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.

mentioning

confidence: 99%

“…5 The substrate was a glass plate precoated with a transparent and electrically conducting layer of In 2 O 3 :Sn (known as ITO). The same technique was used in earlier work of ours 9,16 and is described, along with other technical details, in Supplementary Information.…”

mentioning

confidence: 99%

Galvanostatic Ion Detrapping Rejuvenates Oxide Thin Films

Arvizu¹,

Wen²,

Primetzhofer³

et al. 2015

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“…5-7 These windows are able to vary their throughput of visible light and solar radiation by the application of a low electrical voltage and can provide energy efficiency along with indoor comfort in buildings. [8][9][10] Amorphous WO 3 is the most widely studied EC material, and its optical absorption can be varied through Li + ion intercalation 11 and accompanying insertion of charge balancing electrons. This mechanism can be expressed, schematically, as 11…”

mentioning

confidence: 99%

Sustainable Rejuvenation of Electrochromic WO₃ Films

Wen

Niklasson

Granqvist

2015

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Devices relying on ion transport normally suffer from a decline of their long-term performance due to irreversible ion accumulation in the host material, and this effect may severely curtail the operational lifetime of the device. In this work, we demonstrate that degraded electrochromic WO 3 films can sustainably regain their initial performance through galvanostatic de-trapping of Li + ions. The rejuvenated films displayed degradation features similar to those of the as-prepared films, thus indicating that the de-trapping process is effectively reversible so that long-term performance degradation can be successfully avoided.De-trapping did not occur in the absence of an electric current. 2Energy conservation is widely recognized as an essential part of a sustainable global energy system. 1 This realization brings attention to the buildings sector, which is responsible for a large and growing part of the global use of energy. 2,3 Energy-efficient fenestration is of considerable interest in this context, 4 and electrochromic (EC) "smart windows" are of particular importance. 5-7 These windows are able to vary their throughput of visible light and solar radiation by the application of a low electrical voltage and can provide energy efficiency along with indoor comfort in buildings. [8][9][10] Amorphous WO 3 is the most widely studied EC material, and its optical absorption can be varied through Li + ion intercalation 11 and accompanying insertion of charge balancing electrons. This mechanism can be expressed, schematically, as 11where e -denotes electrons. Thin films of amorphous WO 3 change from optically transparent to dark blue when Li + ions are inserted, and the films return to their transparent state if these ions are extracted.High optical modulation and long-term durability are needed for EC-based fenestration.These requirements are not easily compatible, and repeated insertion and extraction of large amounts of Li + ions lead to a gradual accumulation of Li + -ions in the host material 12-15 (often referred to as "ion-trapping") with ensuing loss of EC performance. Removal of the trapped ions to re-gain the initial EC properties is essential for maintaining good device performance.It has been proposed that the host structure contains different types of intercalation sites: 13, 14 a network of connected sites with low inter-site barriers, which allows fast diffusion of the intercalated ions throughout the film, and other sites with high energy barriers which are able to trap the diffusing ions. It has been suggested by Bisquert 14, 15 that the high-energy barrier trapping sites can be filled by Li + ions having sufficient energy or by waiting for a long enough time.

“…[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

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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...