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Solar Energy Materials & Solar Cells 90 (2006) 434–443

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Effect of the organic–inorganic templateICS-PPG on sol–gel deposited V2O5

electrochromic thin film

M. Liberatorea,�, F. Deckera, A. Surca Vukb,B. Orelb, G. Drazicc

aDipartimento di Chimica Universita ‘‘La Sapienza’’and Istituto Nazionale di Fisica della Materia,

Piazzale A. Moro 5, 00185 Roma, ItalybNational Institute of Chemistry, SI-1000 Ljubliana, Slovenia

cJoseph Stefan Institute, SI-1000 Ljubliana, Slovenia

Received 30 November 2004; accepted 13 April 2005

Available online 13 June 2005

Abstract

We produced V2O5 thin films with sol–gel technique, using in the synthesis as a template

agent, the so-called ICS-PPG. We studied the electrochromic behaviour of thin films with

electrochemical techniques as slow scan cyclic voltammetry and cyclic voltammetry (SSCV,

CV), impedance spectroscopy (EIS) and potentiostatic intermittent titration (PITT). More-

over, we measured transmittance spectra, optical bandgap of the material and, with the help of

transmission electron microscopy (TEM), we found out informations about films structure.

The addition of ICS-PPG produces more transparent samples in comparison with samples

prepared without template agent. The ion storage capacity is only slightly increased and, with

ageing, some deposits appear on the surface limiting the electrochemical performances.

r 2005 Elsevier B.V. All rights reserved.

Keywords: Vanadium pentoxide; ICS-PPG; SSCV; EIS; PITT

see front matter r 2005 Elsevier B.V. All rights reserved.

.solmat.2005.04.035

nding author. Tel.: +496151 166853; fax: +49 6151 166308.

dress: massimiliano.liberatore@surface.tu-darmstadt.de (M. Liberatore).

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M. Liberatore et al. / Solar Energy Materials & Solar Cells 90 (2006) 434–443 435

1. Introduction

Thin films of V2O5 are important for development of iono-optic deviceapplications like electrochromic displays, because their layered structure allows aneasy intercalation/de-intercalation of different ions between its slabs. With variuosdeposition methods it is possible to prepare thin films with different orientation ofthe grains with respect to the substrate, different structure and nanocrystallinity.

Sol–gel prepared V2O5 thin films made by Livage synthesis [1] were extensivelystudied [2–4]. We modified this synthesis by addition of an organic–inorganic hybrid,the so-called ICS-PPG, prepared from 3-isocyanatopropyltriethoxysilane (ICS) andpoly(propylene glycol) bis-2 (ammino-propyl-ether) (2-APPG). This template wasadded into V2O5 sols in different molar percentages (1%, 2% and 5%), taking intoaccount that addition of ICS-PPG in the synthesis of WO3 gasochromic films gavecompact, nanosized grain samples, without optical defect [5].

We have investigated the behaviour of V2O5/ICS-PPG samples with electro-chemical and optical techniques to check their performances in terms of ion storagecapacity, reaction kinetics, cycling stability and transparency and to compare theobtained results with the properties of a thin V2O5 film prepared without ICS-PPG.

2. Experimental

The V2O5 films were prepared using a sol–gel alkoxide route. The precursor V-oxoisopropoxide was diluited in 2-propanol. Acetic acid in molar ratio 1:2 with V-oxoisopropoxide was added to initiate the hydrolysis and condensation reaction, andthen, ICS-PPG was added in percentages of 1%, 2% and 5%mol, respectively. Thefilms were deposited on FTO glass by ‘‘dip coating’’ method using a pulling velocityof 10 cmmin�1 and then they were heated for 1 h at 400 1C.

Electrochemical measurements were carried out in three-electrode cells hermeti-cally sealed under Ar atmosphere. Lithium foils were used as counter and referenceelectrodes and 1.0M LiClO4/PC solution as electrolyte. A Potentiostat/GalvanostatECOCHEMIE AUTOLAB 12, coupled with a frequency response analyser modulewas used to measure SSCV, CV, PITT, EIS and the software ‘‘NLSS’’ developed byB. Boukamp, was used to fit the impedance data. The transmittance spectra wererecorded on HP 8452 diode array spectrophotometer and TEM images were takenusing Jeol 2000 FX microscope operating at 200 kV.

3. Results

Slow scan cyclic voltammetry (SSCV) [6] was used to check the behaviour of allelectrodes after preparation. Here, we report the first CV at scan rate of 500 mV s�1

for the sample 1% ICS-PPG (sample A), 2% ICS-PPG and 5% ICS-PPG (sampleB), respectively. The open-circuit potential (OCP) was around 3.45V for the threedifferent compositions, in agreement with the value of 3.40V reported in literature

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for V2O5 thin film prepared with same sol–gel synthesis, but without ICS-PPG [7].The scans were performed between 2.9 and 3.7V vs. Li reference electrode, startingfrom OCP in cathodic direction (Fig. 1). All the samples presented two cathodicpeaks characteristic of crystalline V2O5. The first one around 3.4V corresponds tothe insertion of half mole of Li+ the second to insertion of another half mole inaccordance with following reactions [6]: 1

2Li++1

2e�+V2O5 ¼ Li 0.5V2O5,

12Li++1

2

e�+Li 0.5V2O5 ¼ Li V2O5.When the scan direction was inverted the anodic peaks of de-insertion reaction

appeared. The hysteresis between cathodic and anodic peaks was not pronounced atthis slow scan rate because the reaction went in quasi-reversible manner. The chargereversibly inserted during the first cycle was 30mC cm�2 for samples A and B,20mC cm�2 for 2% ICS-PPG.

After SSCV the samples A and B were aged, at scan rate of 10mV s�1, cyclingbetween 2.9 and 3.7 V (Fig. 2). The cathodic and correspondent anodic peaks of thisregion appeared much broader in comparison to those in SSCV and showed anevident potential separation. In particular, the difference of anodic peak potentialand cathodic peak potential increased with ageing, for sample B more than forsample A. After 300 cycles the sample A presented a charge fading of 24% and

-0.0001

0

5 10-5

-5 10-5

0.0001

3.73.63.53.43.33.23.12.9 3

SAMPLE A

SAMPLE B

SAMPLE 2% ICS-PPG

Cu

rren

t (

A c

m-2

)

Voltage ( V vs Li )

Fig. 1. Samples A, B and 2% ICS-PPG: first CV at 500mVs�1, Li reference and counter electrode,

electrolyte 1M LiClO4/PC, potential interval 2.9–3.7 V vs. Li.

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0

0.0008

0.0005

-0.0005

-0.001

0

0.0006

0.0004

0.0002

-0.0002

-0.0004

-0.0006

Cu

rren

t (A

cm

-2)

Cu

rren

t (A

cm

-2)

2.9 3.2 3.4 3.63.13 3.3 3.5 3.7

100th cycle

200th cycle

300th cycle

100th cycle

200th cycle

300th cycle

Voltage ( V vs Li)

(A)

(B)

Fig. 2. (A) CV of sample A at 10mVs�1 after 100, 200, 300 cycles. (B) CV of sample B at 10mVs�1 after

100, 200, 300 cycles.

M. Liberatore et al. / Solar Energy Materials & Solar Cells 90 (2006) 434–443 437

sample B of 50% in comparison with the charge inserted during the first cycles,respectively. The reversibility of the insertion reaction was less for sample B than forsample A; in order to study the limiting kinetic step of the reaction we performedelectrochemical impedance spectroscopy (EIS) and potentiostatic intermittenttitration technique (PITT) experiments. We chose PITT rather than the morecommonly used galvanostatic intermittent titration technique, because the PITTpermits to avoid parasitic reactions by strict control of working electrode potential.

We performed EIS experiments on the samples polarized at E ¼ 3:3V (Li0.5V2O5

stoichiometry), at various stages of ageing. Sample B gave us the most impressiveresults: during the first 100 cycles the impedance spectra reported in the Nyquistplots (Fig. 3A) showed a high-frequency semicircle starting from 130X, a straightline at 451 in the medium frequency range, and an almost vertical line extending to

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low frequencies. Using non-linear least-squares fitting program by Boukamp and theRandles equivalent circuit to interpret our data, we can ascribe the semicircle todouble layer capacitance (Cdl) and charge transfer resistance (Rct) of the insertionreaction, the 451 line to diffusion of Li into the thin film (ZW) and the vertical line tolimiting capacitance (CL) due to space limiting diffusion in the thin film. After 100cycles a second semi circle appeared in the higher frequency domain. Both the firstsemi circle and the second one showed increasing size with ageing, but the secondloop disappeared when the sample was washed in CH3CN (Fig. 3B). This feature istypical only for sample B. In this case, Randles equivalent circuit was modified byaddition of a resistance and a capacitance in parallel. Fit analysis suggests us thepresence of an adsorbed electrolyte thin layer on the film surface and, the increase offirst semicircle with ageing, could indicate that the charge transfer becomes theslowest step of the reaction in agreement with impedance data on V2O5 powderstaken by others [8].

Moreover, we have calculated from the low-frequency part of the spectrum the Lichemical diffusion coefficient using the formula DLiþ ¼ Z002pnl2=3RL; where Z00 isthe imaginary part of impedance at frequency n, RL limiting resistance and l filmthickness [9]. The value of DLiþ is of the order of 10�11 cm2 s�1 and does not changesignificantly with ageing.

To further investigate the diffusion of lithium ions we have used the PITTtechnique [10]. Analysing the chronoamperometric curve in terms of It1/2 vs. log t, wecan find out from the plateau of the curve the DLiþ, the chemical diffusioncoefficient of Li+ ions. In the case of thin films it is reasonable to assume thediffusion path length is equal to the film thickness.

The results show for the sample A slightly lower diffusion coefficient with respectto sample 2% ICS-PPG and sample B. Generally, the values of DLiþ are in

0

50

300

250

200

150

100

0

50

300

250

200

150

100

100 150 200 250 300 350

10 cycles 100 cycles

200 cycles

300 cycles 400 cycles

-Z''

/ O

hm

-Z''

/ O

hm

Z' / Ohm Z' / Ohm150 200 250 300 350 400

400 cycles before washing

400 cycles after CH3CN washing

(A) (B)

Fig. 3. (A) Sample B Nyquist plots recoded at E ¼ 3:3V vs. Li after 10, 100, 200, 300, 400 cycles. (B)

Sample B Nyquist plots recorded at E ¼ 3:3V vs. Li after 400th cycle, before and after CH3CN washing.

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agreement with those calculated for thin films of V2O5 without ICS-PPG. The curveof DLiþ vs. the cell potential does not present trend with two minima coincident withcathodic peaks. Aurbach studied V2O5 thin films prepared by evaporative vacuumdeposition [11,12] and interpreted the behaviour of DLiþ considering the Frumkinintercalation isothermal model [13]. Our data about V2O5 thin films prepared bysol–gel method without ICS-PPG, shown recently, [14] were in agreement with thisresults. Therefore, it is possible to hypothesize that the presence of ICS-PPG in thelattice reduces the high attractive electron–ion interaction of Aurbach’s model. In(Fig. 4) part B, we can see the behaviour of DLiþ with ageing for sample B. All thecurves present a quite similar trend: an oscillation around the value of 10�11 cm2 s�1.The ageing does not seem to considerably influence the Li+ ions diffusion into thefilm.

We have recorded UV–visible spectra in air, before the electrochemicalexperiments, for film without ICS-PPG (sample 0) samples A and B respectively(Fig. 5). A common feature of the spectra is the very low value of transmittancebetween 300 and 400 nm, while around 500 nm, fundamental absorption edgeappears and shifts to higher wavelengths with increasing percent of ICS-PPG.Beyond 600 nm the transmittance values increased with the percent of addedtemplate. This could also be due to different samples thickness: 100 nm for sample 0,150 nm for sample A and 200 nm for sample B. From transmittance spectra we

10-15

10-14

10-13

10-12

10-11

10-10

Sample ASample BSample 2% ICS-PPG

10-13

10-12

10-11

10-10

10-9

10-8

3.1 3.2 3.3 3.4 3.5

10 cycles100 cycles200 cycles300 cycles

DL

i+ (

cm2

s-1)

DL

i+ (

cm2

s-1)

Voltage ( V vs Li )

(A)

(B)

Fig. 4. (A) Diffusion coefficients of samples A, B and 2% ICS-PPG as a function of cell potential,

calculated from PITT after first CV cycle. (B) Diffusion coefficient of sample B as a function of cell

potential, calculated from PITT after 10, 100, 200, 300 cycles.

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0

10

20

400 500 600 700 800

30

40

50

60

70

80

Sample 0% ics-ppg

Sample A

Sample B

T%

/nm

Fig. 5. Samples A, B and 0% ICS-PPG: Transmittance spectra (350–820nm) recorded in air on as

deposited samples.

M. Liberatore et al. / Solar Energy Materials & Solar Cells 90 (2006) 434–443440

calculated the optical bandgap Eg using the following equation: ðahnÞn ¼ Bðhn� EgÞ

[15,16] where a is the optical absorption coefficient hn the energy of incident photon,B the absorption edge width parameter, Eg the optical bandgap, n a factor dependingfrom the nature of transition (Fig. 6). We have obtained a good fit using the modelwith n ¼ 2=3 that suggest direct forbidden transition across the energy bandgap ofthe material.

The value of Eg increases from 2.44 eV for 0% film to 2.48 eV for sample A andfinally to 2.52 eV for sample B. This shift could indicate a decrease of a grain size ofthe films.

To investigate the morphology of our samples we used the transmission electronmicroscope (TEM). We have taken images of cross section of the films on silicon/silicon oxide/platinum substrate prepared using a Gatan cross sectional TEMspecimen preparation kit. The images (Fig. 7) show that the V2O5 and V2O5 (2%)films consist of randomly oriented grains of V2O5 with dimensions in order up to100 nm. The diffraction patterns, here not reported, show a less crystalline phase for2% ICS-PPG sample.

4. Discussion and conclusions

Information about structure, ion storage capacity and intercalation kinetics ofICS-PPG samples were obtained firstly from SSCV and CV. ICS-PPG samples havepresented in CV, a set of reversible peaks corresponding to intercalation of 1mol of

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0

1000

2000

3000

4000

5000

6000

7000

2.4 2.5 2.6 2.7 2.8 2.9 3

Sample 0% ICS-PPGSample ASample B

Eg = 2.44 eV

Eg = 2.48 eV

Eg = 2.51 eV

h //eV

( h

)2

/ / 3

Fig. 6. Sample A, B and 0% ICS-PPG: Linear fits of plots ðahnÞ2=3 vs. photon energy (hn), derived from

transmittance spectra. The extrapolated intercept on x-axis gives the bandgap value.

Fig. 7. (A) TEM image of sample 0% ICS-PPG cross section on Si/SiO2/Pt substrate. (B) TEM image of

sample 2% ICS-PPG cross section on Si/SiO2/Pt substrate.

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lithium ions. The two cathodic peaks indicated the occupancy of differentintercalation sites in the crystalline structure by the ions. The addition of ICS-PPG has improved slightly the ion storage capacity in comparison with that ofsamples without ICS-PPG. In particular, samples A and B showed better insertioncapability during first cycles. With prolonged cycling the CV of ICS-PPG sampleshave shown two main trends: a remarkable charge fading, and an increase of thedistance between cathodic and anodic peaks, those features being more evident forsample B. We can deduce that the process related to the onset of peaks became moreirreversible and slower.

The modification of kinetic properties observed with CV was confirmed in the EISand PITT experiments. From impedance spectroscopy it was possible to betterindividuate various factors limiting the intercalation process. The main aspects ofEIS spectra were the mentioned presence of a semicircle increasing with ageing and,for the sample B, the onset of a second semicircle at high frequency. We couldexplain this experimental evidence considering the addition of ICS-PPG. Thetemplate should produce more porous and nanocrystalline thin films, in consequenceelectrodes with very reactive surface. Therefore, a multiple CVs have produced onsurface some deposits responsible of the increasing of Rct. In case of sample B, therichest in ICS-PPG, the very reactive surface is also responsible for a fast adsorptionphenomenon. At certain degree of ageing it is difficult to distinguish the relevanttime constant of charge transfer step and of solid state diffusion step and in thespectra the semicircle overlapped with Warburg straight line. To have a betterevaluation of DLiþ, PITT measurements were performed. We can notice that there isno decrease of DLiþ in correspondence to the phase transitions of the material andthat the ageing has changed only slightly the values of DLiþ. We could deduce thatthe diffusion is not responsible for the slackening of insertion reaction.

We also have performed optical measurements and TEM images to have a bettercorrelation between thin film morphology and electrochemical data. The samplesanalysed in the as-deposited state have shown a shift of optical bandgap to higher eVwith increase of ICS-PPG percentages. This confirms the structural effect of ICS-PPG that changes the crystallinity of the films. Another confirmation of thishypothesis is due to TEM images in which sample 2% ICS-PPG presents lesscrystalline phases and crystallite size smaller than in the sample without ICS-PPG.

Our study has pointed out that the addition of ICS-PPG implements only slightlyion storage capacity and accelerates the Li ions diffusion into thin film by surfacestructure modification. The reactive surface of ICS-PPG sample B becomes aged dueto deposits that increases the charge transfer resistance at electrolyte/electrodeinterphase.

References

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[3] A. Surca, B. Orel, Electrochem. Acta 44 (1999) 3051.

[4] A. Surca Vuk, S. Bencic, B. Orel, F. Decker, J. Sol–gel Sci.Technol. 23 (2002) 53.

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[5] U. Opara Krasovec, R. Jese, B. Orel, G. Grdadolnik, G. Drazic, Monatshefte Chem. 133 (8) (2002)

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[11] M.D. Levi, Z. Lu, D. Aurbach, J. Power Sources 97 (2001) 482.

[12] M.D. Levi, Z. Lu, D. Aurbach, Solid State Ionics 143 (2001) 309.

[13] M.D. Levi, D. Aurbach, Electrochim. acta 45 (1999) 167.

[14] M. Liberatore, F. Decker, A. De Bonis, R. Guascito, A. Salvi, Congresso SCI, Torino 2003, Poster

session.

[15] K.I. Arshak, G.A. Landers, R. Perrem, Int. J. Electron. 76 (6) (1994) 1011.

[16] C.V. Ramana, R.J. Smith, O.M. Hussain, Phys. stat. solid. 199 (1) (2003) R4.