Solar Energy Materials & Solar Cells 84 (2004) 329–336

Electrical conductivity as a function oftemperature in amorphous lithium tungsten oxide

Lars Berggren*, Jesper Ederth, Gunnar A. Niklasson

Department of Materials Science, The (Angstr .om Laboratory, Uppsala University, P.O. Box 534, SE-751 21,

Uppsala, Sweden

Received 31 October 2003; accepted 5 February 2004

Available online 4 June 2004

Abstract

Tungsten oxide is a widely used electrochromic material for smart windows. In order to

study the charge carriers involved in the electrochromic process, it is important to characterize

the electrical transport in tungsten oxide. Substoichiometric amorphous tungsten oxide films

were prepared by DC-magnetron sputtering. The films were electrochemically intercalated

with lithium. The Li/W intercalation ratios for the tungsten oxide films were in the range 0.15–

0.53. Temperature dependent resistivity measurements were performed in the temperature

range 77–300K for samples at different lithium intercalation levels. It was found that the data

are consistent with the variable range hopping model.

r 2004 Elsevier B.V. All rights reserved.

Keywords: Tungsten oxide; DC conductivity; Temperature dependency; Polarons; Density of states

1. Introduction

Disordered tungsten trioxide is known to have good electrochromic properties andit has for a long time been considered as the cathodic base material in electrochromicdevices [1]. The oxide is suitable for optical applications and it is often deposited as athin film on a transparent substrate, although it is possible in other applications tohave for instance a mirror as the substrate. The tungsten oxide is cathodicallycolored by insertion of small ions like H+, Li+ and Na+. By extraction of the same

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*Corresponding author. Tel.: +46-18-471-6215; fax: +46-18-50-01-31.

E-mail address: lars.berggren@angstrom.uu.se (L. Berggren).

0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.solmat.2004.02.049

ions the reverse (bleaching) process can be achieved. Optical properties of LixWOy

have earlier been investigated [2] and it has been shown that the best colorationefficiency is found in substoichiometric tungsten oxide (i.e. with an oxygen/tungstenratio below 3) [3]. The underlying detailed mechanism for the coloration is still anopen question, but it has been suggested that electrochromic coloration is due tocolor centers [4] or polarons [2,5] intimately associated with the intervalence chargetransfer mechanism (IVCT) [6].Crystalline LixWOy exhibits a metal-insulator transition at a value of xE0.2 [1,7]

However in amorphous coatings, the electrical conductivity exhibits an insulator-likebehavior at much larger values of x. It has been found in amorphous evaporatedWO3 [8] and hydrogen doped WO3 [9] as well as pulsed laser deposited (PLD)sodium doped WO3 [10] that the conductivity follows the variable range hopping(VRH) equation in three dimensions (n=3) given by Mott [11]:

sðTÞ ¼ s0 expðT0=TÞ1=ðnþ1Þ; ð1Þ

where T is the temperature and n the dimension of the conduction path. Theprefactor s0 p [N(EF)]

1/2 and T0=Ca3/(kN(EF)), where C is a numerical factor,N(EF) is the density of states (DOS) at the Fermi energy, a is the coefficient of theexponential decay of the wave function and k is Boltzmann’s constant. The variablerange hopping model of Mott [11] was derived for the case of phonon-assistedtunneling (hopping) of electrons between localized electronic states. Similar modelsof the temperature dependent DC conductivity have been obtained for transport ofsmall polarons [12]. These theories predict slight numerical differences for T0 inEq. (1). The factor C=6.49 when N(EF) is constant for uncorrelated polaronhopping in the low-temperature regime, while the factor C=17.8 in the correlatedhopping case. It is difficult to distinguish electrons from polarons by means oftemperature dependent conductivity measurements.The aim of this work is to investigate whether the VRH behavior is observed also

for amorphous substoichiometric tungsten oxide films deposited by sputtering.

2. Film deposition and characterization

Our substrates were glass plates that were covered by a conductive layer ofindium–tin oxide. The conductive layer covered the whole surface except for a smallstrip, around half the substrate length and 1mm wide. The prepared substrates werecovered with a thin film of tungsten oxide by reactive DC magnetron sputtering in aBalzer UTT deposition system. The deposition chamber was baked more than 8 h atabout 106 Torr and 120C before sputtering took place. The target material was acircular plate of pure tungsten (99.99%) with a diameter of 5 cm. All the films weresputtered at a power of 200W and a pressure of 20mTorr but with two differentargon/oxygen gas flows, 50/8 and 50/22, where the flows were measured in ml/min.The composition and the density of the sputtered films have been obtained from

the composition profile performed by elastic recoil detection analysis (ERDA). Thefilms considered in this paper were found to have a composition of about WO2.89,

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with a density of 5.7 cm3, and WO2.93 with a density of 5.0 cm3, respectively [13].

Both films are considered as porous compared to monoclinic WO3 that has a bulkdensity of 7.16 g/cm3. The former composition is known to have good electro-chromic coloration efficiency. The thickness of all the tungsten oxide films wasestimated by a Tencor Alpha step 200 profilometer to be around 320+/30 nm.The tungsten oxide films were electrochemically intercalated with lithium in a

three electrode setup, with an electrolyte containing 1M lithium perchlorate(LiClO4) that was dissolved in propylene carbonate (PC). Pure lithium foils wereused for the counter and reference electrodes while the working electrode was thesample. The lithium insertion was performed in an argon filled glove box thatcontained a humidity less than 5 ppm. A constant current of 0.1mA was used for allthe intercalations and it was applied for 200 s, in the case of the lowest intercalatedcharge. During intercalation, lithium ions can readily diffuse from the electrolyteinto the film above the conductive backing. Diffusion of lithium ions from theelectrolyte can also occur into the film above the gap in the conducting substrate,provide that there are electrons already present there. The diffusion of electrons fromthe gap edges to the center takes less than a few seconds, as inferred from theestimated electron mobility in the film (Section 4). The gradual coloration of the gapcould be observed visually. In all cases the color of the films was visually uniformwhen the intercalation procedure was completed. The samples were cut, outside theglove box, in such a way that there was no conductive connection between the twosides of the strip/gap.

3. Electrical measurements

We used a four point measuring method. The set up consisted of a measuringprobe with four wires inside a small metal box. The wires were connected to thetungsten oxide film by silver glue in such a way that the wires were applied in arectangular pattern, two on each side, at the edge of the non-conducting gap. Aconstant current was applied between two of the wires and the voltage was measuredbetween the other two wires [14]. The voltage did not exceed 3V in any of themeasurements. A container of liquid nitrogen was used for cooling when the voltagewas measured during the cooling. We observed some instabilities in the signal.Resistance measurements were also performed when the sample was heated to roomtemperature by simply removing the liquid nitrogen filled container. This signal wasmore stable as seen in Fig. 1, which indicates that equilibrium was attained duringthe measurements.

4. Results and discussion

Fig. 1 shows the resistance divided by the resistance at 285K as a function oftemperature for two lithium doped films, WO2.89 and WO2.93. The temperature wasincreased from nitrogen boiling temperature. We saw, during the initial cooling, that

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the resistance was generally lower than during heating. The resistance of the filmsincreased between 9 and 20% at room temperature after a full cycle of cooling andheating. We interpret the increase in resistance as due to a small loss of active chargein the films.

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0

5

10

15

20

25

30

35

50 100 150 200 250 300

Li0.27

W O2.93

Li0.39

W O2.93

Li0.52

W O2.93

Res

ista

nce

no

rmal

ized

to

R a

t 28

5 K

0

5

10

15

20

25

30

35

Li0.15

W O2.89

Li0.19

W O2.89

Li0.28

W O2.89

Li0.37

W O2.89

Li0.41

W O2.89

Li0.53

W O2.89

Res

ista

nce

no

rmal

ized

to

R a

t 28

5 K

Temperature [K]

50 100 150 200 250 300

Temperature [K]

(a)

(b)

Fig. 1. Resistance normalized with the resistance at 285K as a function of temperature during heating

from 77K toBroom temperature for LixWOy. The curves are obtained with different lithium doping and

for two samples of different oxygen content, WO2.93 (a) and WO2.89 (b).

L. Berggren et al. / Solar Energy Materials & Solar Cells 84 (2004) 329–336332

There is a possibility that the lithium could migrate inside the film during themeasurements under the influence of the applied filed. An estimation of themigration drift speed can be made from the diffusion coefficient, which has beenestimated, by several authors, to be around 1 1011 cm2/s [1]. Since the appliedvoltage is below 3V, the electric field should be below 0.3V/cm in the gap. Thelithium ion migration drift speed, much less than 1 nm/s, should not influencethe measurements since one cycle (cooling and heating process) takes little less thanan hour.The films with a doping level less than 0.3 Li/W and the Li0.39WO2.93 sample show

a large resistivity below 170K compared to the samples with higher doping levels.This transition to a more conducting state could be visualized, as the films turnedmore bluish, the more lithium that was inserted in the films. However, even the filmswith xD0.52 showed an insulating behavior with the resistance decreasing withtemperature. We observe no evidence of a metal-insulator transition in the case ofamorphous tungsten oxide films. The resistivity is in general higher for films with lesslithium doping. This is not necessarily so when comparing films with differentoxygen content, as seen in Fig. 2. An attempt to determine the electron mobility, inour lithium doped amorphous tungsten oxide films, from the resistance data at roomtemperature (see Fig. 2) have shown values of around 1.3 105 cm2V1 s1 for thehigh resistance (106O) films and 1.3 103 cm2V1 s1 for the low resistance (104O)films. These values are below the electron mobility value, 3.7 102 cm2V1 s1,found in as-deposited evaporated amorphous tungsten oxide [15] and3 102 cm2V1 s1 found in the metallic region of evaporated amorphous WO3intercalated with protons [9].Our measured resistance is proportional to the resistivity, although the

geometrical proportionality factor is rather approximate. With this statement in

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103

104

105

106

107

0.1 0.2 0.3 0.4 0.5 0.6

WO2.89

WO2.93

Res

ista

nce

at

285

K [

Ω Ω]

x [Li/W]

Fig. 2. The resistance at 285K of the WO2.89 and the WO2.93 films on a logarithmic scale as a function of

lithium doping. The error bars reflect the uncertainty due to sample to sample variations in the

measurement geometry.

L. Berggren et al. / Solar Energy Materials & Solar Cells 84 (2004) 329–336 333

mind we use Eq. (1) and plot the logarithm of the normalized resistance, given byFig. 1, as a function of T1/4. The result is seen in Fig. 3. The match with the VRHtheory is good except at small discontinuities at certain temperatures.The slopes in Fig. 3 represent T0

1/4 in Eq. (1). Fig. 4 shows T01/4 as a function of the

level of inserted lithium for the films WO2.93 and WO2.89. It is seen that T0 decreaseswith increasing lithium doping level. The decrease in T0 is probably mostly due to an

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0

2

4

6

8

0.24 0.26 0.28 0.3 0.32 0.34

Li0.15

WO2.89

Li0.27

WO2.93

Li0.19

WO2.89

Li0.28

WO2.89

Li0.39

WO2.93

Li0.37

WO2.89

Li0.41

WO2.89

Li0.53

WO2.89

Li0.52

WO2.93L

n(R

/R28

5 K

)

T-1/4 [K-1/4 ]

Fig. 3. The logarithm of the normalized resistance as a function of T1/4 for the films WO2.89 and WO2.93at different lithium doping levels. The diagram shows also a linear fit to the measured data.

0

20

40

60

80

100

0.1 0.2 0.3 0.4 0.5 0.6

WO

WO

T01/

4 [K

1/4 ]

x [Li/W]

2.93

2.89

Fig. 4. T01/4 as a function of x (lithium doping level) for the films WO2.89 and WO2.93. The lines are linear

fits. They are drawn for convenience.

L. Berggren et al. / Solar Energy Materials & Solar Cells 84 (2004) 329–336334

increase of the DOS at eF. The parameter T0 is usually larger for the film with lessoxygen vacancies at the same lithium doping level. This suggests that N(eF) may belower for the WO2.93 film as compared to the WO2.89 film.We can estimate N(eF) by applying approximate values for the other parameters

involved. By giving a1 a value of 2 (A, letting T01/4 decrease from 100 to 20K1/4

during lithium insertion and putting C to 10 we get an increase of N(eF) fromapproximately 1 1020 to 1 1023 eV1 cm3 as the Li/W ratio increases from 0.15to 0.5 Li/W. Bandstructure calculations for crystalline [16,17] and amorphous [18]WO3 have been reported. A theoretical calculation of the DOS at the Fermi level forcubic crystalline Li(1)WO3 have been made [16]. It gave a value of 2 10

22 eV1 cm3

at a Fermi level of 1.3 eV above the bottom of the conduction band. The monoclinicphase may be more relevant to the amorphous DOS than the cubic are since it isstable at room temperature. The monoclinic unit cell contains eight W atoms but it isalso larger in size than the corresponding cubic cell. The DOS in the lower part of theconduction band appears to be higher than for the cubic phase [19]. The DOS alsodecreases very rapidly towards the bottom of the conduction band for themonoclinic structure [19]. To sum up, the values of N(eF) inferred from ourelectrical measurements appear to be of the correct order of magnitude, but moredetailed comparisons with computations, as well as with electron spectroscopy (see[1] and references quoted therein) are needed.

5. Conclusions

We have in this work measured the resistance as a function of temperature forsputtered thin films of amorphous LixWOy with two different oxygen contents, asubstoichiometric (y=2.89) film and a less oxygen deficient (y=2.93) film. It hasbeen found that the conductance behavior follows the variable range hopping lawfor each lithium doping level. It is not possible to determine whether the chargecarriers are electrons or polarons, by resistance measurements, alone. No evidencefor an insulator-metal transition was found for Li/W ratios as high as 0.53.

Acknowledgements

The financial support from the Swedish Science Council, the Swedish Foundationfor Strategic Environmental Research (MISTRA) and the National EnergyAdministration (STEM) are gratefully acknowledged. We would also like to thankMr. G. Gustavsson for discussions regarding the substrate preparations.

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