| 1. |
- Arvizu, Miguel A, et al.
(författare)
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Electrochromic WO3 thin films attain unprecedented durability by potentiostatic pretreatment
- 2019
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Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 7:6, s. 2908-2918
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Tidskriftsartikel (refereegranskat)abstract
- Electrochromic windows and glass facades are able to impart energy efficiency jointly with indoor comfort and convenience. Long-term durability is essential for practical implementation of this technology and has recently attracted broad interest. Here we show that a simple potentiostatic pretreatment of sputterdeposited thin films of amorphous WO3-the most widely studied electrochromic material-can yield unprecedented durability for charge exchange and optical modulation under harsh electrochemical cycling in a Li-ion-conducting electrolyte and effectively evades harmful trapping of Li. The pretreatment consisted of applying a voltage of 6.0 V vs. Li/Li+ for several hours to a film backed by a transparent conducting In2O3: Sn layer. Associated compositional and structural modifications were probed by several techniques, and improved durability was associated with elemental intermixing at the WO3/ITO and ITO/glass boundaries as well as with carbonaceous solid-electrolyte interfacial layers on the WO3 films. Our work provides important new insights into long-term durability of ion-exchange-based devices.
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| 2. |
- Niklasson, Gunnar, 1953-, et al.
(författare)
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Electrochemical rejuvenation of Tungsten oxide electrochromic thin films : Evidence from impedance spectroscopy
- 2018
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Ingår i: 13th International Meeting on Electrochromism, IME-13. ; , s. 11-
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Konferensbidrag (refereegranskat)abstract
- A major challenge for energy-efficient smart window technology is to ensure the durability of electrochromic (EC) devices capable of render a service life significantly higher than 20 years. The development of more durable EC materials would also make it possible to increase the transmittance contrast between bleached and colored states without the risk of limiting service life. Recently, it has been shown that degraded EC coatings can be restored to their initial state by electrochemical rejuvenation treatments.1,2 In addition, it was found that tungsten oxide EC films could gain vastly improved durability after extended electrochemical treatments at high applied potentials.3 In this paper we present an attempt to unravel the complex mechanisms behind high potential rejuvenation and durability-enhancing treatments. We study EC amorphous tungsten oxide, which is the most commonly used EC oxide. It is used in most commercial device designs, often in combination with a nickel oxide-based complementary EC layer.Amorphous tungsten oxide thin films were deposited by sputtering onto conducting indium-tin oxide (ITO) coated glass substrates. Ion intercalation and diffusion in the films were studied by electrochemical impedance spectroscopy measurements in the frequency range 10 mHz-10 kHz and for potentials between 2.0 and 3.3 V vs. Li/Li+, using the film as working electrode in a Li+ containing electrolyte. Measurements were carried out for as-deposited EC tungsten oxide films, degraded and rejuvenated films as well as durability-enhanced WOx films. The impedance data were in good agreement with a Randles-type equivalent circuit containing an anomalous diffusion element.4 In this study we focus on changes at the electrolyte/EC film and EC film/ITO interfaces during degradation and after different electrochemical treatments.The most notable changes were associated with the high frequency and charge transfer resistances. The high frequency resistance increased significantly during degradation as well as extended rejuvenation treatments; a similar effect was observed in durability-enhanced WOx films. This might indicate compositional or chemical changes in the ITO backing or at the film/ITO interface. The charge transfer resistance associated with the electrolyte/film interface also increased after treatments, but in addition exhibited a strong potential dependence. The appearance of a second high-frequency process after rejuvenation is considered to be more interesting. Possible explanations include an additional adsorption step preceding ion intercalation into the EC film, or alternatively the appearance of a solid-electrolyte interphase layer of the type commonly observed in Li-ion batteries.Ion diffusion coefficients were not significantly different for rejuvenated EC films as compared to the as-deposited ones. On the other hand degraded films exhibited a completely different impedance response, which could be interpreted as being due to parasitic chemical reactions in the system.An increased understanding of ageing and rejuvenation processes will facilitate the search for more durable EC materials and preliminary results suggest that interfacial characteristics may influence durability. Eventually, improved EC coatings will be important for large-scale practical application of electrochromic materials, for example in smart windows. References[1] R.-T. Wen, C.G. Granqvist, G.A. Niklasson, Nature Mater., 14, 996 (2015).[2] H.-Y. Qu, D. Primetzhofer, M.A. Arvizu, Z. Qiu, U. Cindemir, C.G. Granqvist, G.A. Niklasson, ACS Appl. Mater. Interf., 9, 42420 (2017).[3] M.A. Arvizu, H.-Y. Qu, G.A. Niklasson, C.G. Granqvist, Thin Solid Films, 653, 1 (2018).[4] S. Malmgren, S.V. Green, G.A. Niklasson, Electrochim. Acta, 247, 252 (2017).
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| 3. |
- Qu, Hui-Ying, et al.
(författare)
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Potentiostatically pretreated electrochromic tungsten oxide films with enhanced durability : Electrochemical processes at interfaces of indium–tin oxide
- 2019
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 682, s. 163-168
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Tidskriftsartikel (refereegranskat)abstract
- Recent work has shown that electrochromic WO3 films, backed by In2O3:Sn (ITO) and immersed in a lithium-ion-conducting electrolyte, can attain unprecedented electrochemical cycling durability after potentiostatic pretreatment at high voltage. Here we demonstrate that this intriguing feature is associated with changes in the properties of the ITO film. Specifically, we studied thin films of ITO and WO3/ITO immersed in an electrolyte of LiClO4 in propylene carbonate at potentials up to 6.0 V vs. Li/Li+ by cyclic voltammetry and impedance spectroscopy and present evidence that electrochemical reactions occur under these conditions. X-ray photoemission spectroscopy indicated that the ITO film was partly dissolved at high voltages and that the dissolution reaction promoted diffusion of In and Sn into the WO3 film.
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| 4. |
- Rojas González, Edgar Alonso, et al.
(författare)
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Charge coloration dynamics of electrochromic amorphous tungsten oxide studied by simultaneous electrochemical and color impedance measurements
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The coloration mechanisms in electrochromic (EC) systems can be probed by comparing the dynamics of the electrical and optical responses. In this paper, the linear frequency-dependent electrical and optical responses of an amorphous tungsten oxide thin film were measured simultaneously by a combination of two techniques—that is, electrochemical impedance spectroscopy (EIS) and the so-called color impedance spectroscopy (CIS). This was done at different bias potentials, which can be associated with different intercalation ratios. Equivalent circuit fitting to the EIS spectra was used to extract the Faradaic components participating in the total impedance response. The latter were assigned to an intermediate adsorption step before the intercalation and to the diffusion of the electron-ion couple in the film. A quantity denoted complex optical capacitance is compared to the complex electrical capacitance—in particular, their expressions are related to the Faradaic processes. The coloration at low intercalation levels followed both the adsorption and diffusion phenomena. Conversely, the diffusion contribution was dominant at high intercalation levels and the adsorption one seemed to be negligible in this case. For perfectly synchronized electrical and optical responses, their complex spectra are expected to differ only by a multiplying factor. This was the case at low intercalation levels, apart from small deviations at high frequencies. A clear departure from this behavior was observed as the intercalation level increased. A combination of frequency-dependent techniques, as presented in this work, can help in the understanding of the dynamics of the coloration mechanisms in EC materials at various conditions—for example, at different intercalation levels and optical wavelengths.
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| 5. |
- Rojas González, Edgar Alonso, et al.
(författare)
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Charge coloration dynamics of electrochromic amorphous tungsten oxide studied by simultaneous electrochemical and color impedance measurements
- 2021
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 129:5
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Tidskriftsartikel (refereegranskat)abstract
- The coloration mechanisms in electrochromic systems can be probed by comparing the dynamics of the electrical and optical responses. In this paper, the linear frequency-dependent electrical and optical responses of an amorphous tungsten oxide thin film were measured simultaneously by a combination of two techniques-that is, electrochemical impedance spectroscopy (EIS) and the so-called color impedance spectroscopy. This was done at different bias potentials and their associated intercalation levels. Equivalent circuit fitting to the EIS spectra was used to extract the Faradaic components from the total impedance response. The latter were assigned to an intermediate adsorption step before the intercalation and to the diffusion of the electron-ion couple in the film. A quantity denoted complex optical capacitance was compared to the complex electrical capacitance-particularly, their expressions are related to the Faradaic processes. The coloration at low intercalation levels followed both the adsorption and diffusion phenomena. Conversely, the diffusion contribution was dominant at high intercalation levels and the adsorption one seemed to be negligible in this case. The complex spectra of perfectly synchronized electrical and optical responses are expected to differ only by a multiplying factor. This was the case at low intercalation levels, apart from small deviations at high frequencies. A clear departure from this behavior was observed as the intercalation level increased. A combination of frequency-dependent techniques, as presented here, can help to elucidate the dynamics of the coloration mechanisms in electrochromic materials at various conditions-for example, at different intercalation levels and optical wavelengths.
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| 6. |
- Rojas González, Edgar Alonso, et al.
(författare)
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Coloration of tungsten oxide electrochromic thin films at high bias potentials and low intercalation levels
- 2020
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Ingår i: Materials Letters: X. - : Elsevier BV. - 2590-1508. ; 7:100048
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Tidskriftsartikel (refereegranskat)abstract
- The optical absorption of amorphous tungsten oxide thin films was studied at low intercalation levels. The electronic density of states was obtained by an electrochemical method and bias potential regions were assigned to the conduction band and gap states according to the experimentally estimated conduction band edge. Differences between the coloration in the conduction band and gap states were observed when comparing the experimental results to a theoretical site-saturation model that considers electronic transitions between localized tungsten sites. The model could reproduce the optical response due to conduction band states. However, it underestimated the rate of change of the optical absorption coefficient with intercalation level for gap states. This discrepancy is discussed in the context of small-polaron optical absorption.
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| 7. |
- Rojas González, Edgar Alonso, et al.
(författare)
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Continuous-Variable Triple-Photon States Quantum Entanglement
- 2018
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 120:4
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the quantum entanglement of the three modes associated with the three-photon states obtained by triple-photon generation in a phase-matched third-order nonlinear optical interaction. Although the second-order processes have been extensively dealt with, there is no direct analogy between the second and third-order mechanisms. We show, for example, the absence of quantum entanglement between the quadratures of the three modes in the case of spontaneous parametric triple-photon generation. However, we show robust, seeding-dependent, genuine triple-photon entanglement in the fully seeded case.
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| 8. |
- Rojas González, Edgar Alonso, et al.
(författare)
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Differential coloration efficiency of electrochromic amorphous tungsten oxide as a function of intercalation level : Comparison between theory and experiment
- 2020
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 127:20
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Tidskriftsartikel (refereegranskat)abstract
- Optical absorption in amorphous tungsten oxide (aWO3), for photon energies below that of the bandgap, can be rationalized in terms of electronic transitions between localized states. For the study of this phenomenon, we employed the differential coloration efficiency concept, defined as the derivative of the optical density with respect to the inserted charge. We also made use of its extension to a complex quantity in the context of frequency-resolved studies. Combined in situ electrochemical and optical experiments were performed on electrochromic aWO3 thin films for a wide lithium intercalation range using an optical wavelength of 810 nm (1.53 eV). Quasi-equilibrium measurements were made by chronopotentiometry (CP). Dynamic frequency-dependent measurements were carried out by simultaneous electrochemical and color impedance spectroscopy (SECIS). The differential coloration efficiency obtained from CP changes sign at a critical intercalation level. Its response exhibits an excellent agreement with a theoretical model that considers electronic transitions between W4+, W5+, and W6+ sites. For the SECIS experiment, the low-frequency limit of the differential coloration efficiency shows a general trend similar to that from CP. However, it does not change sign at a critical ion insertion level. This discrepancy could be due to degradation effects occurring in the films at high Li+ insertion levels. The methodology and results presented in this work can be of great interest both for the study of optical absorption in disordered materials and for applications in electrochromism.
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| 9. |
- Rojas González, Edgar Alonso
(författare)
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Dynamic and quasi-stationary electrochromic response of amorphous tungsten oxide thin films : In situ combined electrochemical and optical measurements during lithium intercalation
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electrochromic (EC) materials can adjust their optical properties, reversibly, by means of an external electrical stimulus. They have relevant technological applications; for example, energy-efficient smart windows, which can adapt dynamically—according to the given environmental conditions—to control the heat and visible light fluxes between the interior and exterior of a building. EC applications are currently available on the market. However, there are still questions concerning the fundamental processes responsible for the EC effects.This thesis focuses on EC inorganic oxide materials in the thin film form; particularly, amorphous tungsten oxide. In this case, the electrochromism is induced by the intercalation of small ions (such as lithium ions) into the material and the insertion of electrons from an external circuit due to charge neutrality requirements within the film. These electrons are the ones causing the optical changes. This work centers its attention to the quasi-equilibrium and dynamic EC processes. They were studied by in situ simultaneous electrochemical and optical measurements at different conditions—that is, for a wide range of intercalation levels and bias potentials.The experimental results from quasi-equilibrium measurements were in accordance with a phenomenological optical absorption model for amorphous tungsten oxide that is based on electronic transitions between states localized on neighboring tungsten atoms. In this case, the consideration of W4+ sites in the model was needed to properly reproduce the experimental results.The dynamic measurements used an experimental setup which can acquire simultaneously the frequency-dependent electrical and optical responses of the EC system. In the frequency domain, different mechanisms with various characteristic times and responses can be isolated. Here, the coloration was mainly assigned to the ion and electron diffusion within the film. However, adsorption-related phenomena were also found to contribute to the coloration, especially at high bias potentials—corresponding to low intercalation levels. Interestingly, the dynamic optical response was in-phase with the electrical one at high bias potentials. Nevertheless, a delay between the former and the later was noticed as the bias potential decreased—that is, increasing intercalation level.The methods and results from this thesis provide new perspectives into the fundamental coloration mechanisms in EC systems. In addition, studies like those presented here can be readily extended to different materials at diverse conditions—for example, at various optical wavelengths, material compositions, and film thicknesses.
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| 10. |
- Rojas González, Edgar Alonso, et al.
(författare)
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Setup for simultaneous electrochemical and color impedance measurements of electrochromic films : Theory, assessment, and test measurement
- 2019
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Ingår i: Review of Scientific Instruments. - : AMER INST PHYSICS. - 0034-6748 .- 1089-7623. ; 90:8
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Tidskriftsartikel (refereegranskat)abstract
- Combined frequency-resolved techniques are suitable to study electrochromic (EC) materials. We present an experimental setup for simultaneous electrochemical and color impedance studies of EC systems in transmission mode and estimate its frequency-dependent uncertainty by measuring the background noise. We define the frequency-dependent variables that are relevant to the combined measurement scheme, and a special emphasis is given to the complex optical capacitance and the complex differential coloration efficiency, which provide the relation between the electrical and optical responses. Results of a test measurement on amorphous WO3 with LED light sources of peak wavelengths of 470, 530, and 810 nm are shown and discussed. In this case, the amplitude of the complex differential coloration efficiency presented a monotonous increase down to about 0.3 Hz and was close to a constant value for lower frequencies. We study the effect of the excitation voltage amplitude on the linearity of the electrical and optical responses for the case of amorphous WO3 at 2.6 V vs Li/Li+, where a trade-off should be made between the signal-to-noise ratio (SNR) of the optical signal and the linearity of the system. For the studied case, it was possible to increase the upper accessible frequency of the combined techniques (defined in this work as the upper threshold of the frequency region for which the SNR of the optical signal is greater than 5) from 11.2 Hz to 125.9 Hz while remaining in the linear regime with a tolerance of less than 5%. (C) 2019 Author(s).
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