All solid state electrochromic devices based on WO3-NiO

An all-solid-state electrochromic device based on NiO/WO 3complementary structure and solid hybrid polyelectrolyte

J.Zhang a ,J.P.Tu a,?,X.H.Xia a ,Y.Qiao b ,Y.Lu b

a Department of Materials Science and Engineering,State Key Laboratory of Silicon Materials,Zhejiang University,Hangzhou 310027,China b

Key Laboratory of Infrared and Low Temperature Plasma of Anhui Province,Hefei Electronic Engineering Institute,Hefei 230037,China

a r t i c l e i n f o

Article history:

Received 1June 2009Accepted 29June 2009

Available online 16July 2009Keywords:

Electrochromic device Tungsten oxide Nickel oxide

Hybrid polyelectrolyte

a b s t r a c t

An all-solid-state electrochromic (EC)device based on NiO/WO 3complementary structure and solid polyelectrolyte was manufactured for modulating the optical transmittance.The device consists of WO 3?lm as the main electrochromic layer,single-phase hybrid polyelectrolyte as the Li +ion conductor layer,and NiO ?lm as the counter electrochromic layer.Indium tin oxide-(ITO)coated glass was used as substrate and ITO ?lms act as the transparent conductive electrodes.The effective area of the device is 5?5cm 2.The device showed an optical modulation of 55%at 550nm and achieved a coloration ef?ciency of 87cm 2C à1.The response time of the device is found to be about 10s for coloring step and 20s for bleaching step.The electrochromic mechanism in the NiO/WO 3complementary structure with Li +ion insertion and extraction was investigated by means of cyclic voltammograms (CV)and X-ray photoelectron spectroscopy (XPS).

1.Introduction

Electrochromism refers to the persistent and reversible change of optical properties by an applied voltage pulse.There are many transition metal oxides exhibiting electrochromism,e.g.oxides of W,Ni,Ir,V,Ti,Co and Mo [1–3].Organic materials especially some conducting polymers such as poly(aniline),poly(3,4-propylene-dioxythiophene)also received much attention for electrochromic (EC)applications [4–7].Electrochromic materials attract consider-able interest because of their potential applications,such as information displays,smart windows,variable-re?ectance mir-rors,and variable-emittance thermal radiators [8–13].For prac-tical applications,the electrochromic materials should be incorporated into an EC device,which is a multilayer structure with electronic conductor,ion conductor and ion storage.A high-performance EC device should present high optical contrast,good optical memory and chemical stability to electrochromic cycles.

Tungsten oxide is known as the typical cathodic coloration material and nickel oxide as the typical anodic coloration material.They have drawn much interest in the last decade [14–18]and a summary of earlier works was provided by Niklasson and Granqvist [19].Notably,tungsten oxide and nickel oxide can be fully transparent to visible light,and they are quite complementary because of their coloration types.In the fabricating process of EC device,however,they are chemically incompatible when tungsten oxide and nickel oxide have to be

immersed in acidic and alkaline liquid electrolytes.Moreover,liquid electrolyte is dif?cult to seal and not safe for practical application.The liquid electrolyte can be replaced by solid electrolyte such as tantalum oxide and polyelectrolyte.In the present work,an EC device with nickel oxide as ion storage and solid hybrid polyelectrolyte as ion conductor was assembled.The EC device exhibited high optical contrast and coloration ef?ciency.The electrochromic properties and mechanism were investigated in detail.

2.Experimental 2.1.Electrochromic ?lm

The WO 3?lms were deposited on indium tin oxide-(ITO)coated glass substrates (sheet resistance 25O /U ,5?6cm 2in sizes)by reactive DC magnetron sputtering of a metallic tungsten target.The sputter chamber was initially evacuated to a base pressure of 6.7?10à3Pa and then pure oxygen and argon gases were introduced into the chamber with the mass ?ow ratio of 1:4.The substrates were heated by radiant exposure and the temperature was 1501C.2.2.Counter electrode

The NiO ?lm was prepared by a chemical bath deposition (CBD)method [14].0.16mol of nickel sulfate,0.03mol of potassium persulfate and 400ml H 2O were added to a 500ml

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?Corresponding author.Tel.:+8657187952573;fax:+8657187952856.

E-mail address:tujp@https://www.360docs.net/doc/bd1804520.html, (J.P.Tu).

Solar Energy Materials &Solar Cells 93(2009)1840–1845

pyrex beaker with magnetic stirring to form a dark green solution. ITO-coated glass substrates masked with polyimide tape to prevent deposition on the nonconductive sides,placed vertically in the solution.Then40ml of aqueous ammonia(25–28%)was added to the solution under constant vigorous stirring.The ITO glass substrates deposited with precursor?lms were washed with deionized water.After removing the tape masks,the coated samples were dried at751C and then annealed at3001C in air for1.5h.

2.3.Solid hybrid polyelectrolyte

The solid hybrid polyelectrolyte was prepared based on a sol–gel procedure following Refs.[20,21].0.2mol of citric acid (CA)monohydrate was dissolved in100ml anhydrous ethyl alcohol under stirring at room temperature.0.1mol of tetra-ethyl-orthosilicate(TEOS)and5g lithium carbonate were added to the solution.Then20g ethylene glycol(EG)was added to citrate solution,to promote polymerization reaction.The mixture was heated up to601C for1h to form a gel,which would be introduced into the device and solidi?ed.

2.4.Electrochromic devices

The EC device in the form of glass/ITO/WO3/polyelectrolyte/ NiO/ITO/glass was fabricated by the following process:two coated substrates assembled together with epoxy glue and they were separated by a frame of plexiglass.The distance between the electrodes was 2.0mm and then the hybrid electrolyte with viscosity of about30cps was?lled into the cell through a small hole.The device was heated at601C for24h to solidify the electrolyte.The effective area of the device is5?5cm2.The schematic diagram of the device is shown in Fig.1.

2.5.Characterization

The optical transmission spectrum was measured over the range from200to900nm with a SHIMADZU UV-240spectro-photometer.The electrochemical measurements were carried out with a CHI660c electrochemical workshop.X-ray photoelectron spectroscopy(XPS)data were obtained with an ESCALab220i-XL electron spectrometer from VG Scienti?c using300W Al-K a radiation.The base pressure was about3?10à9mbar.The binding energies were referenced to the C1s line at284.6eV from adventitious carbon.

3.Results and discussion

Fig.2a–c shows the bleached and colored states(applied voltages2.5,à1.5andà2.5V,respectively)of the EC device.The color of the device changes from transparent(bleached state)to deep blue(colored state).The views through the device applied at different voltages are shown in Fig.2d–f.The colors are quite gentle,and can meet application of architecture windows.Fig.3 shows the spectral transmittance of the device applied at2.5and à2.5V for20s in the10th and1000th cycle,respectively.The transmittance variation of the device is55%at550nm,which is much higher than the one with WO3/CeO2–TiO2structure[22].In bleached state,both the devices exhibit similar transmittance (about75%at550nm).But in colored state,the transmittance of EC device base on WO3/NiO complementary structure in this work is much lower than that of EC device based on WO3/CeO2–TiO2 structure(20%vs.45%at550nm).The extra optical absorption in EC device based on WO3/NiO complementary structure is attributing to the NiO layer.

In order to evaluate the performance of the device,the color ef?ciency is calculated.Coloration ef?ciency(CE)is de?ned as the change of optical density(D OD)per unit of inserted(or

extracted) Fig.1.Schematic diagram of the electrochromic

device.

Fig.2.The photographs of device with bleached and colored states,applied at(a)2.5V,(b)à1.5V and(c)à2.5V;and the corresponding views through the device.

J.Zhang et al./Solar Energy Materials&Solar Cells93(2009)1840–18451841

charge (D Q ),i.e.

CE ?D OD =D Q e1T

D OD ?log eT b =T c T

e2T

where T b and T c refer to transmittance of EC device in bleached and colored states,respectively.A high value of CE indicates that the EC device exhibits large optical modulation with small charge inserted (or extracted).After 10cycles,the performance of the device tends to be stable,and the CE value is 87cm 2C à1at 550nm.This value is comparable to the result of Anh et al.’s work (84cm 2C à1,with proton-conducting solid polymer electrolyte)[23]and also Zhang et al.’s work (84cm 2C à1,with polyamine as the counter electrode)[4],much higher than the EC device with Ce–Ti oxides as counter electrode (35cm 2C à1)[22].In this work,the NiO ?lm not only performs as ion storage layer,but also as complementary electrochromic layer.We already know that the NiO ?lms prepared by CBD method have a porous morphology and exhibit excellent electrochromic properties (CE ?42cm 2C à1)[14].Sum up of the electrochromic effects of WO 3and NiO layers,the device achieved much higher coloration ef?ciency.

To evaluate the response time of the EC device,a double potential step chronoamperometric (CA)experiment was per-formed (E 1?2.0V,E 2?à2.0V).The resultant current–time curve is shown in Fig.4a.The response time for coloring step it is found to be about 10s and for bleaching step is about 20s,which is a little longer than the results of Ref.[23].The difference may attribute to different electrolyte used in the present work.Li +ions are more dif?cult to move than H +in the solid hybrid electrolyte.The cyclic durability of EC device was also studied by a CA measurement taken by applying à2.0V/30s to +2.0V/30s.As shown in Fig.4b,the device has a stable electrochromic performance from the ?rst cycle up to 1600cycles.The transmittance of the device after 1000CA cycles changes little (see Fig.3).

Fig.5shows the cyclic voltammograms (CV)of EC device at a scan rate of 20mV s à1between à2.5and 2.5V.The dark blue colored state is obtained by polarizing at à2.5V,and the transparent state is obtained by polarizing at 2.5V.In order to investigate the electrochemical process of the device during cyclic voltammograms,the device was disassembled and studied by X-ray photoelectron spectroscopy.Fig.6shows the wide scanning XPS spectra of samples:(a)WO 3?lms in bleached and colored

states and (b)NiO ?lms in bleached and colored states.The binding energies of the samples were corrected using a value of 284.6eV for the C 1s peak of carbon.There is no other contaminated element except C in the WO 3?lms.The NiO ?lms are contaminated by Fe,Co and Mn,which is due to the impurity of the precursors.

The W 4f core-level spectra comprising the well-resolved spin obit split doublet peaks pertaining to W 4f 5/2and W 4f 7/2states are shown in Fig.7.The results of ?tting analysis are shown in this ?gure,and the ?tting parameters are listed in Table 1.The higher binding energy peaks at 35.6eV (in bleached state)and 35.4eV (in colored state)can be ascribed to (W 4f 7/2)6+in accordance with the XPS data by Refs.[24–26].The peaks at 35.1eV (in bleached state)and 34.9eV (in colored state)can be ascribed to (W 4f 7/2)5+[25,26].The binding energy at 33.5eV indicates that a third component exists in the colored ?lm.The component would correspond to (W 4f 7/2)4+[25,26].Typically,the small polaron transitions between W 5+and W 6+states can be used to explain the mechanism of electrochromism in WO 3?lms [27].The XPS spectrum shows that W 5+and W 6+states exist in the WO 3?lm in bleached state,which,however,is transparent.From Table 1,it can be found that the areas under (W 4f)6+and (W 4f)5+decrease from 72.7%and 27.3%in bleached state to 69.8%and 18.9%in colored

300

0102030405060

708090T r a n s m i t t a n c e /%)

Wavelength /nm

400

500

600700800

900

Fig.3.Optical transmittance spectra of EC device of 10th and 1000th CA cycles.

-25

-20-15-10-505

10C u r r e n t /m A

Time /S

120160200240280320360400

-20

-15-10-505

10P e a k c u r r e n t /m A

Number of cycles

200

400

60080010001200

1400

1600

Fig.4.Step chronoamperometric curves of the device:(a)current–time response curve of the device and (b)peak current during the step chronoamperometric cycles.

J.Zhang et al./Solar Energy Materials &Solar Cells 93(2009)1840–1845

1842

-0.3

-0.2

-0.1

0.0

0.1

C u r r e n t (m A /c m 2)

Potential (V)

Fig.5.Cyclic voltammograms of EC device at a scan rate of 20mV s à1.

900

Binding Energy (eV)

I n t e n s i t y (a .u .)

O1s

W4p

C1s

W4d

W4f

Bleached

Colored

I n t e n s i t y (a .u .)

Binding Energy (eV)

Bleached Colored

O1s

Ni3p

Ni2p

2/3C1s

Ni3s

Mn2p

Fe2p

2/3Co2p

800

700

600500400300200

100

900800700

6005004003002001000

Fig.6.Wide scanning XPS spectra of samples:(a)WO 3?lm in bleached state and colored state;(b)NiO ?lm in bleached state and colored state.

I n t e n s i t y (a .u .)

Binding Energy (eV)

I n t e n s i t y (a .u .)

Binding Energy (eV)

414039

383736353433323130

42414039

383736353433323130

Fig.7.Tungsten 4f region XPS spectra of the ?lms in bleached (a)and colored (b)states:The results of ?tting analysis are shown.The ?tting parameters are reported in Table 1.The data have been best ?tted by means of convoluted Lorentzian–Gaussian doublets.The total interpolating curve (solid line)is superimposed to the (scattered)experimental points.The lowermost curve (dot line)shows the residual difference between the total interpolating curve and the experimental data.

Table 1

XPS binding energies,full-width at half-maximum (FWHM)and composition (area %)of components obtained by deconvolution of the W 4f region spectra.Bleached state Colored state Position/eV

Area (%)FWHM/eV Position/eV Area (%)FWHM/eV 37.772.7 1.237.569.8 1.535.6

35.437.327.3 1.037.118.9 1.435.134.9–

–

35.811.3

1.4

33.5

J.Zhang et al./Solar Energy Materials &Solar Cells 93(2009)1840–1845

1843

state,respectively.The amount of W 5+and W 6+states decreased while the optical absorption increased.So it is not reasonable to explain by the mechanism of small polaron transitions between W 5+and W 6+states,at least,it is not the main mechanism.A certain amount of W 6+and W 5+is reduced to a third component W 4+(the area under (W 4f)4+is 11.3%,see Table 1)after coloration.This phenomenon indicates that the optical absorption is due to the small polaron transition between W 5+and W 4+states instead of W 5+and W 6+states,which is in agreement with the chromic mechanism proposed by Zhang et al.[28].h n +W 5+(A)+W 4+(B)-W 5+(B)+W 4+(A)

Fig.8shows the XPS spectra of NiO ?lms in bleached and colored states.As shown in Fig.8a,the binding energy of Ni 2p 3/2for bleached NiO ?lm is 855.2eV,and two peaks are observed at 855.5and 854.2eV for colored NiO ?lm.The O 1s region spectra and the results of ?tting analysis are shown in Fig.8b and c,and the ?tting parameters are listed in Table 2.The binding energies of 529.2eV in O 1s region and 854.2eV in Ni 2p 3/2region are consistent with the peak of NiO.When the ?lm was colored,the intensity of binding energy at 529.2eV increased (area from 63.1%to 66.3%,see Table 2),which was related to the increase in NiO amount.When the ?lm was bleached,the intensity of binding energy at 531.0–531.1eV increased (area from 23.9%to 27.8%,see Table 2),which might attribute to the intercalation of Li +ion into NiO ?lm [29,30].The binding energies of 532.4–532.8eV may correspond to the contribution of the oxygen in water molecule (H–O–H)[31],and the amount (area from 9.98%to 9.08%,see Table 2)changed slightly during the EC cycles.

4.Conclusions

An all-solid-state electrochromic device based on NiO/WO 3complementary structure and solid polyelectrolyte was manufac-tured for modulating the optical transmittance.The device showed an optical modulation of 55%at 550nm and achieved a coloration ef?ciency of 87cm 2C à1.The response time of the device is about 10s for coloring step and 20s for bleaching step.The optical absorption in colored WO 3?lms is due to the polaron transition between W 5+and W 4+,while the coloration in NiO ?lms is due to the Li +insertion.The views through the device applied at different voltages are quite gentle.The EC device based on NiO/WO 3complementary structure and solid polyelectrolyte has potential applications in smart windows.

Acknowledgement

This work was supported by Zhejiang University K.P.Chao’s High Technology Development Foundation (Grant no.2008ZD001).

Table 2

XPS binding energies,full-width at half-maximum (FWHM)and composition (area %)of components obtained by deconvolution of the O 1s region spectra.Bleached state Colored state Position/eV Area (%)FWHM/eV Position/eV Area (%)FWHM/eV 532.49.98 1.9532.89.08 1.7531.066.3 2.1531.163.1 2.1529.2

23.9

1.3

529.2

27.8

1.2

890I n t e n s i t y (a .u .)

Binding Energy (eV)

536I n t e n s i t y (a .u .)

Binding Energy (eV)

I n t e n s i t y (a .u .)

Binding Energy (eV)

880

870860850

534

532530528526

536534532530528526

Fig.8.XPS spectra of the NiO ?lm:(a)Ni 2p 3/2region spectra for the ?lm in bleached state and colored state;O 1s region spectra for the ?lm in (b)bleached state and (c)colored state.The results of ?tting analysis of O 1s region are shown.The ?tting parameters are reported in Table 2.The data have been best ?tted by means of convoluted Lorentzian–Gaussian doublets.The total interpolating curve (solid line)is superimposed to the (scattered)experimental points.The lowermost curve (dot line)shows the residual difference between the total interpolating curve and the experimental data.

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