Optical characterization of the coloration process in electrochromic amorphous and crystalline WO3 films by spectroscopic ellipsometry

Accepted Manuscript Title: Optical characterization of the coloration process in electrochromic amorphous and crystalline WO3 films by spectroscopic ellipsometry Author: Guangzhong Yuan Chenzheng Hua Li Huang Christophe Defranoux Peter Basa Yong Liu Chenlu Song Gaorong Han PII: DOI: Reference:

S0169-4332(16)32314-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.10.176 APSUSC 34270

To appear in:

APSUSC

Received date: Revised date: Accepted date:

21-8-2016 9-10-2016 26-10-2016

Please cite this article as: Guangzhong Yuan, Chenzheng Hua, Li Huang, Christophe Defranoux, Peter Basa, Yong Liu, Chenlu Song, Gaorong Han, Optical characterization of the coloration process in electrochromic amorphous and crystalline WO3 films by spectroscopic ellipsometry, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.10.176 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Optical characterization of the coloration process in electrochromic amorphous and crystalline WO3 films by spectroscopic ellipsometry Guangzhong Yuan1, Chenzheng Hua1, Li Huang2, Christophe Defranoux 3, Peter Basa 3, Yong Liu1*, Chenlu Song1, Gaorong Han1* 1.

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027,

China

2.

Semilab China Co., Ltd., A: 3rd Floor, B2 Building, BETWIN, No.889 Shangcheng Rd. Pudong, Shanghai 200120, China

Semilab Semiconductor Physics Laboratory Co., Ltd., Prielle Kornelia ut 2, H-1117 Budapest, Hungary

*Corresponding author: Yong Liu *Co-corresponding author: Gaorong Han State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China Tel: +86 (0) 571 87951842, Fax: +86 (0) 571 87951842 Email address: [email protected] Email address: [email protected]

Graphical abstract

1

Amorphous WO3 2.2 0V

2.0 1.9

-0.2 V -0.4 V -0.6 V

1.8

-0.8 V

1.7 400

Voltage

500

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-1.0 V 700

2.4

p

WO3 film

800

s

s

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0V -0.2 V

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Refractive index

Refractive index

2.3

2.1

Crystalline WO3

Electrochemical Workstation

2.0 1.8

-0.4 V

1.6 1.4

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Wavelength (nm)

Wavelength (nm)

H2SO4

800

Highlights 1. Coloration processes of amorphous and crystalline WO3 films are studied by SE. 2. Amorphous WO3 can store more H+ ions compared to crystalline one at same voltage. 3. The optical parameters of crystalline WO3 are much more sensitive to the H+ ions. 4. Difference of electrochromic mechanisms between amorphous and crystalline WO3 is proved by SE.

Abstract Amorphous and crystalline electrochromic WO3 films exhibit quite different optical properties during coloration process. In the present work, amorphous and crystalline electrochromic WO3 films prepared by a solution method were characterized using X-ray diffraction, scanning electron microscope, and transmission electron microscope techniques. A double-layer model with sharp interfaces was established for the fitting of the ellipsometry parameters. The results show that the proton favors amorphous films more than crystalline WO3 films. The refractive indices of both amorphous and polycrystalline WO3 films decrease while extinction coefficients increase with the inserting of H+ during the coloration process. But the optical parameters of the latter are much more sensitive to the H+ ions injected compared to the amorphous WO3 during the coloration process. That is the refractive index modulation of the crystalline WO3 films is about 53% at 633 nm while that of the amorphous films about 15% at the same wavelength. The Drude-like free electron model for crystalline WO3 and hopping mechanism of small polaron for amorphous WO3 are used to explain the difference in detail. These results are very helpful for the better understanding of the coloration process and for the design of electrochromic devices. Key words: spectroscopic ellipsometry; WO3; electrochromic film; coloration process

2

Introduction

Electrochromic (EC) materials reversibly change their optical properties (darken/lighten) driven by a small voltage, which have been applied widely as smart windows, antiglare automobile rear-view mirrors and displays for their low cost and low energy consumption. With the advantages of high contrast ratio [1], non-toxic nature [2], and high coloration efficiency [3-6], WO3 is recognized as one of the most promising cathode electrochromic materials for commercial applications. In the past few decades,

large amount of research has

been conducted on electrochromic WO3 films. It has been widely accepted that amorphous and crystalline WO3 films exhibit quite different electrochromic performance. For example, amorphous tungsten oxides usually have higher coloration efficiencies and faster response time than crystalline ones [5, 7] while the durability of crystalline WO3 is usually better [4, 8]. After coloration, the peak absorption of WO3 films is at 1.2 eV for amorphous but 0.7 eV for crystalline material [7]. The reflectance of near-infrared is reduced for amorphous tungsten oxides whereas it is enhanced for crystalline tungsten oxides [9-10]. Many models have been proposed to explain the mechanism of the coloration process of tungsten oxides. For example, oxygen vacancies or F-like color centers, intervalence charge transfer (IVCT), polaronic absorption and interband excitations for disordered films [11-15]. The coloration in crystalline WO3 is generally attributed to a Drude-like free electron absorption which is very similar to a heavily doped semiconductor [16]. Spectroscopic ellipsometry (SE), based on measuring the change of polarization state of incident light, is a rapid, precise, and non-destructive technique to investigate optical and structural properties of films [17]. As optical parameters reflect internal properties of 3

materials, SE can be used to study oxygen vacancies, charge carrier density, and polaron and phonon properties [17-20]. Considering previous experiments upon the electrochromic WO3 films using SE [21-23], only variation of refractive index and extinction coefficient of amorphous tungsten oxides were investigated. Crystalline tungsten oxides and the difference between amorphous and crystalline WO3 are still in lack. Moreover, the dependence of the optical parameters of electrochromic WO3 on the coloration mechanism is also not fully studied. In this work, amorphous and crystalline WO3 films were prepared by a simple solution method. By controlling voltage applied, various amounts of H+ ions were injected into the amorphous and crystalline electrochromic WO3 films and the coloration process were characterized by SE in detail. With the help of scanning electron microscopy (SEM) and some other characterization techniques, a double-layer model with a sharp interface was established for the regression of the films. The results show that the amorphous and crystalline WO3 films exhibit quite different optical evolution during the coloration process and the mechanism has been discussed in detail.

Experiments 200 mg ammonium metatungstate was dissolved in 1.5 mL N,N-dimethylformamide, then 0.5 mL ethanol was added to adjust the viscosity of the solution propitious for spin coating. ITO substrates (3 cm × 3 cm) were ultrasonically cleaned by detergent and ethanol each for 30 min in sequence and dried at 80 oC. WO3 films were deposited onto ITO substrates with a spin coating method. After annealing process at 350 oC for 1 hour (Sample1) and 400 oC for 12 hours (Sample 2), amorphous and crystalline electrochromic WO3 films 4

were obtained respectively. The as-prepared WO3 films were characterized by X-Ray diffraction (XRD, Empyrean 200895, PANalytical B.V with Cu Kα radiation) and high resolution transmission electron microscopy (HRTEM, Tecnai F20, FEI with an accelerating voltage of 200k eV). The morphology and thickness of the films were observed by scanning electron microscopy (SEM, S-4800, Hitachi with an accelerating voltage of 15 kV). The transmittance and reflectance spectrum of the films were measured by an UV-Vis-NIR spectrophotometer (Carry5000, Agilent). The SE measurements (SE, GES-5E, Semilab) were carried out to analyze the optical parameters of the WO3 films. A double-layer structure model with a sharp interface for WO3/ITO was employed for the regression after the acquisition of ellipsometric parameters. The regression work was conducted using the software package WinElli II. A three-compartment system containing 0.5M H2SO4 as electrolyte, Pt foil as the counter electrode and Ag/AgCl as a reference electrode was employed for electrochromic characterization (Fig. 1).

Results and discussion

The XRD results of the films are given in Fig. 2. The diffraction peaks of ITO substrates can be observed for both samples. There are no WO3 diffraction peaks found in the sample 1 annealed at 350 oC, indicating an amorphous WO3 film. On the other hand, For for sample 2 annealed at 400 oC, besides the ITO peaks, the diffraction peaks matching very well with triclinic WO3 (PDF#20-1323) can be found. The average grain size calculated by Scheller’s equation is about 30 nm. HRTEM image of sample 2 in Fig.3 also indicates a fine crystalline WO3 lattice fringe image with an interplanar spacing of about 0.38 nm, corresponding to (002) plane of PDF#20-1323. It is worth to note that no obvious structural transformation was found 5

in annealed ITO substrates based on XRD results (S1). The SEM cross sectional images in Fig.4 show that amorphous and crystalline films exhibit a similar double-layer structure, ITO transparent conductive layer (about 165 nm) and WO3 layer (about 42 nm). As the surface of the as-prepared films are very smooth (S2), a simple double-layer structure model with a sharp interface for WO3/ITO was applied for the regression to obtain the optical parameters. The thickness of WO3 layers was fixed at 42 nm according to the SEM results and thickness mapping analysis (S5) to improve the regression accuracy. As shown in Table 1，the combination of Cauchy term and a Lorentz oscillator, accounting for the UV terms of the dispersion law, was employed for both amorphous and crystalline WO3 layers at the bleached state, generating very good regression results (S3). For colored state, the choosing of dispersion law terms is much trickier due to the polaron formation driven by H+ injection makes a complex optical absorption process. After trying several theoretical models (S4), suitable dispersion laws of a Cauchy term and four Lorentz oscillators (totally 15 parameters were fitted) for amorphous WO3 and a Drude term with one Lorentz oscillator (6 parameters were fitted) for crystalline WO3 were established. The regression results are quite good that all the coefficients of determination are larger than 0.98, as shown in Fig 5. To further validate the optical models, the calculated transmittance spectra of the amorphous and crystalline samples at the bleached and colored states are shown in Fig.6, in agreement with the measured data. Turning the focus on the coloration/bleach process of WO3 films, which is in accordance with intercalation/deintercalation of H+ into / out of the WO3 films: 𝑥𝐻 + + 𝑊𝑂3 + 𝑥𝑒 − ↔ 𝐻𝑥 𝑊𝑂3

(1)

By applying various voltages ranging from 0 V to -1.0 V (versus Ag/AgCl reference 6

electrode), the WO3 films are colored from clear to dark blue. Based on Fig.6, the average electrochromic transmittance modulation between the clear and blue states in the visible spectrum (400 nm ~ 800 nm) are 14% and 19% for the as-prepared amorphous and crystalline WO3 samples, respectively. More details about the coloration process of WO3 films can be found from the variation of refractive index (n) and extinction coefficient (k) with the applied voltages, as shown in Fig 7. The n of the uncolored crystalline WO3 film is higher than that of the amorphous WO3, being consist with the fact that crystalline films are usually denser than that of the amorphous ones, while the extinction coefficients of both the uncolored films are close to 0. After applying voltage for both crystalline and amorphous WO3 films, the n decreases while the k increases at the same time owing to the Kramers–Kronig consistency [23]. But the modulations of n and k of crystalline films are greater than that of amorphous ones. Taken the refractive index and extinction coefficient at 633 nm for example, the refractive index of amorphous WO3 changes from 2.06 to 1.75 while that of crystalline WO3 from 2.20 to 1.04, corresponding to 17% and 53% modulation rates respectively. By integrating the current for time during the whole coloration process at the specific applied voltage, the amount of the injected H+ ions can be calculated. Considering the packing density of films yielded by [23]  n f 2 -1   nb 2  2  P 2  n  2   n 2  1    f  b

(2)

where nf and nb are the refractive indices of the film and bulk WO3 (n550nm=2.5). The calculated packing densities of amorphous and crystalline WO3 are 82.6% and 89.5%, respectively. Taking the density of bulk WO3 as 4.9g/cm2 [21], the molar ratio of the injected H+ ion to W were calculated as shown in Fig.8, accompanied with the n at 633nm for both amorphous and crystalline WO3. The results show that amorphous WO3 can store much more 7

H+ ions compared to crystalline one at the same applied voltage, but the refractive index of the latter is more sensitive to the injected H+ ions, suggesting different EC mechanisms in amorphous and crystalline WO3. The absorption coefficient can be calculated from refractive index and extinction coefficient with the following equation [15]: a(λ) =

4𝜋𝑛kλ

(3)

𝑐

As shown in Fig.9, the absorption coefficients increase with the injection of H+ ions. But for amorphous WO3, the absorption coefficient increases almost linearly, while for crystalline WO3 a non-linearly increasing behavior in the short wavelength range can be found. Such tendencies can be explained by the different EC mechanisms of amorphous and crystalline WO3. The coloration process of amorphous WO3 is attributed to a hopping mechanism of small polaron. According to the small polaron theory, there are three absorption peaks due to electrons hopping from W6+ to W5+, from W6+ to W4+, and from W5+ to W4+ states. [23-25] The total absorption band is the superposition of three transitions. According to the statistical model developed by Berggren et al [25], three peaks are at about 1.4 eV, between 2.45 and 2.70 eV, and 3.37 eV, also, respectively, also explaining that extra three Lorentz oscillators are required to model the dispersion law of colored amorphous WO3 film. For crystalline WO3, it is usually regarded as a heavily doped semiconductor with ionized impurities [7, 26-28]. Therefore, the basic coloration mechanism in crystalline WO3 films is attributed to a Drude-like free electron absorption [28]. Only one absorption peak corresponding to surface plasmon resonance is identified, and usually located at near-infrared wavelength which results in absorption in the whole visible spectrum, leading to that the absorption of crystalline WO3 is relatively weak at short wavelength range [7, 29].

8

Conclusion The coloration process of amorphous and crystalline electrochromic WO3 films were characterized by SE in detail. A double layers model was established suitable for the fitting of the ellipsometry parameters. The refractive indices of both amorphous and crystalline WO3 films decrease with the injecting of H+ ions decrease while extinction coefficients increase during the coloration process. The optical parameters of crystalline WO3 films vary much more significantly during the coloration process compared to that of the amorphous WO3 films, while the amorphous WO3 can store much more H+ ions than the crystalline one at the same applied voltage. Such tendency is explained by the coloration of crystalline WO3 relating to a scattering mechanism of Drude-like free electron while coloration of amorphous WO3 is attributed to a hopping mechanism of small polaron. The absorption coefficient further demonstrates that different electrochromic mechanisms in amorphous and crystalline WO3. These results are very helpful for the better understanding of the coloration mechanism of WO3 and the design of electrochromic devices.

Acknowledgements The work was financially supported by National Natural Science Foundation of China (No. 51572236), National Plan for Science & Technology Support of China (2013BAE03B02) and Fundamental Research Funds for the Central Universities of China.

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Electrochemical Workstation Pt foil

Ag/AgCl

p

p

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Voltage 0V -0.2 V -0.4 V -0.6 V -0.8 V -1 V

WO3 film

0.5M H2SO4

Fig.1 The sketch of ellipsometry spectroscopic measurement of electrochromic WO3 film. 

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Fig.3 Transmission electron microscope image of the sample annealed at 400 oC. Crystalline

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Fig.4 Cross sectional scanning electron microscopy images of the amorphous and crystalline films.

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Fig.5 The measured and fitted ellipsometric parameters of amorphous and crystalline films at full colored state.

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80 70 60 Amorphous-bleach- measured Amorphous-color- measured Crystalline-bleach- measured Crystalline-color- measured Amorphous-bleach- simulated Amorphous-color- simulated crystalline-bleach- simulated Crystalline-color- simulated

50 40 30 20 400

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Fig.6 Transmittance of the films at bleached and colored state. The dots represent for the measured data while the lines for the simulated data.

A

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Fig.7 The evolution of refractive index and extinction coefficient of WO3 film with various applied voltages. Figure A and B are for amorphous WO3 while C and D for crystalline WO3. 1.0

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Fig.8 The applied voltage dependence of the H/W molar ratio and the refractive index at 633nm of amorphous and crystalline WO3.

16

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Fig.9 The evolution of absorption coefficients of WO3 films with various applied voltages.

Table.1 Desperation laws employed for WO3 films. Amorphous film Crystalline film Bleach State Cauchy+Lorentz Cauchy+Lorentz Colored State Cauchy+4Lorentz Drude+Lorentz

17