Microstructure control of dual-phase inkjet-printed a-WO3/TiO2/WOX films for high-performance...

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Microstructure control of dual-phase inkjet-printed a-WO3/TiO2/WOX

films for high-performance electrochromic applications

Pawel Jerzy Wojcik,* Ana Sofia Cruz, L�ıdia Santos, Lu�ıs Pereira, Rodrigo Martins and Elvira Fortunato*

Received 27th February 2012, Accepted 23rd April 2012

DOI: 10.1039/c2jm31217d

The microstructural aspects related to crystalline or amorphous structure of as-deposited and annealed

films of sol–gel-derived WO3 are shown in the literature to be critical for electrochromic (EC)

performance. In consideration of ion insertion materials, there is a need for developing light and at the

same time nanocrystalline structures to improve both coloration efficiency and switching kinetics. By

controlling microstructure and morphology, one could design a material with optimal EC performance.

This report compares the microstructural and morphological characteristics of standard WO3 wet

deposition techniques versus inkjet printing technology (IPT), correlating these features with their

optical and electrochemical performances, emphasizing the importance of the dual-phase a-WO3/TiO2/

WOX film composition proposed in this work for high-performance EC applications. The effect of the

type and content of metal oxide nanoparticles in the precursor sols formulated in various

peroxopolytungstic acid (PTA) and oxalic acid (OAD) proportions on film properties is

comprehensively studied using multi-factorial design of experiment (DOE). To the authors’ knowledge,

no other report on sol–gel deposition of inorganic EC materials via the inkjet printing technique exists,

in which furthermore the film crystallinity can be controlled under low-temperature process conditions.

The proposed method enables development of EC films which irrespective of their composition (a-

WO3, a-WO3/TiO2 or a-WO3/TiO2/WOX) outperform their amorphous or nanocrystalline analogues

presented as the state-of-the-art due to their superior chemical and physical properties.

Introduction

Considerable effort has been devoted to the study of WO3 films

prepared from wet chemistry routes such as sol–gel processing1,2

and electrodeposition3,4 which offer several advantages over

conventional vacuum process techniques such as electron beam

evaporation5 or radio frequency magnetron sputtering.6 Besides

higher film uniformity, wet deposition allows also for micro-

structure control by modifying precursor composition and post-

treatment conditions. Although the application of IPT for WO3

nanoparticle deposition from aqueous dispersions has been

recently reported,7 no attempt has been made to print WO3

coatings from sol–gel precursors. The transition from well-

known wet coating techniques to IPT is dictated by the need to

deposit patterns on large surfaces without the need for lithog-

raphy, as those technological aspects are critical in mass

production. Moreover, IPT allows precise patterning with

reduced raw material waste, therefore significantly lowering the

costs of production. Fig. 1 attempts to show the evolution of

microstructure in WO3 films deposited via the peroxo route

Departamento de Ciencia dos Materiais, CENIMAT/I3N, Faculdade deCiencias e Tecnologia – Universidade Nova de Lisboa andCEMOP/Uninova, 2829-516 Caparica, Portugal. E-mail: p.wojcik@campus.fct.unl.pt; emf@fct.unl.pt; Fax: +351 21 294 8558; Tel: +351 21294 8562

13268 | J. Mater. Chem., 2012, 22, 13268–13278

(wherein tungsten metal is dissolved in hydrogen peroxide) by

various wet deposition techniques as a function of temperature.

The aim is to compare changes in thickness (d), crystallinity and

estimated grain size of films subjected to different post-deposi-

tion annealing temperatures. By examining those structures, we

have an idea of how this processing–property relationship works.

This brief overview will also serve as background for the evalu-

ation of the results obtained via IPT.

Studies on morphological and microstructural evolution of

spin- and dip-coated films as a function of temperature have been

rarely reported in the electrochromic literature.1,2 It has been

observed (XRD, SEM) that annealing temperature induces an

amorphous to hexagonal to triclinic structure transition of those

films.3 The beginning of crystallization was observed at 250 �Cwhen a small volume of grains (1.2–25 nm for spin- and

55–80 nm for dip-coated films) appeared (see Fig. 1a and b).

Further increase in annealing temperature up to 500 �C led to

a considerable improvement in the crystallinity of both spin- and

dip-coated films (65–100 nm and 75–100 nm, respectively).

Moreover, data show that thickness values of as-deposited and

annealed spin- and dip-coated films reduce upon raising the

annealing temperature. The thickness (da) of spin-coated films

annealed at 250 �C for 1 hour decreases by 94%, while further

annealing for an additional hour at 500 �C decreases this value

by an additional 1% compared to the initial thickness of

This journal is ª The Royal Society of Chemistry 2012

Fig. 1 Graphical review presenting the evolution of microstructure and surface morphology in WO3 films deposited via the peroxo route by (a) spin-

coating, (b) dip-coating and (c) potentiostatic electrodeposition as a function of annealing temperature. Symbols d stand for film thickness.

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as-deposited films. For dip-coated films, the thickness (db) during

the same treatment drops by 92% and an additional 2%,

respectively. The analysis of nanostructured thin films deposited

by those two techniques revealed a superior EC performance of

dip-coated films in terms of optical and electrochemical activity.1

At the same time, dip-coated films could sustain a higher number

of cycles, which is a direct consequence of grain size. It has been

ascertained that among sol–gel-derived WO3 spin- and dip-

coated films, the EC performance of films annealed at 250 �C is

far superior in comparison with their amorphous or

polycrystalline counterparts obtained upon annealing at

different temperatures.

Potentiostatic electrodeposition is unique among wet pro-

cessing methods in that it allows the low-temperature deposition

of nanocrystalline WO3 films, which additionally are chemically

more stable than their amorphous counterparts while having

a similar performance.3 Detailed studies of annealing tempera-

ture effects on the structure and EC performance of electro-

deposited WO3 films have been reported by M. Deepa et al.3,4

They observed (XRD, TEM) that an as-deposited WO3 film is

J. Mater. Chem., 2012, 22, 13268–13278 | 13269

composed of nanosized grains with an estimated size of 10–20 nm

embedded in a nanoporous amorphous matrix. Increasing the

temperature (>100 �C) causes the crystal size to grow systemat-

ically. A significant improvement in chemical stability due to

a uniform distribution of grains (12–32 nm) was observed for

films annealed at 250 �C (Fig. 1c). Successive annealing up to

500 �C led to a further particle size increase (63–85 nm). Simul-

taneously, the thermal treatment resulted in a slight decrease in

the film volume. The film thickness (dc) after heat treatment at

250 �C for 1 hour was about 90% of the thickness value of the as-

deposited layer, while further annealing for an additional hour at

500 �C decreased this value down to 85%. The fastest switching

kinetics of the as-deposited film the authors attributed to the

presence of nanocrystallites, which increase the active surface

area, while the highest optical modulation was observed in

amorphous films dried at 60 �C.With crystallization at high temperature the structure not only

becomes ordered, but increases in density, which is known to

inhibit electrochromism. Sensitive to the microstructure, lithium

ion diffusion is significantly limited in heat-treated dense or

crystalline films which contain a limited number of insertion sites

to accommodate the Li+ ions.3 This situation is presented in

corresponding micrograph models of films annealed at 500 �Cwhich reveal a dense, compactly packed microstructure of

polycrystalline metal oxide irrespective of the deposition tech-

nique. When small nanocrystals are dispersed in an amorphous

matrix, EC properties depend on degree of crystallinity and

density of both phases. Therefore, optimal EC properties of the

films are a balance among network formation, organic burnout,

porosity, and crystallization. This dependence has been observed

in all the above examples.1–4 Taylor et al.1 reported that films

annealed at temperatures which promote nanocrystal creation

exhibit lower optical modulation due to the fact that there is less

amorphous material with fewer sites available for coloration.

They concluded that crystallinity does not prohibit electro-

chromism and that the lower EC performance of annealed films

is attributed to the microstructure.

Optimal EC performance of wet processed WO3 films is

a balance among chemical stability, response time and trans-

mission modulation, ion-storage capacity and coloration effi-

ciency. If fast coloring/bleaching operation is required for an

application, then a nanocrystalline film is most suited but at the

penalty of lower optical modulation. On the other hand, an

amorphous film is the most sensible choice when high coloration

efficiency is desired and kinetics is of secondary importance.

Concluding, spin- and dip-coating techniques do not offer much

room for EC performance optimization, due to the film densifi-

cation caused by the high temperature involved in nanocrystal

creation. Electrodeposition partially overcomes the limitations

of those techniques by reducing process temperature for dual-

phase film creation. However, the deposition process is restricted

by the substrate shapes, the expensive equipment and poor

crystallinity of final products. The disadvantages of all described

deposition methods are even more apparent when the large-area

assembly of EC films is considered.

In this work the sol–gel films are processed by IPT which offers

flexibility of deposited film composition, high throughput and

simplicity in material selection, when compared to the conven-

tional techniques. However, fully optimizing the printing process

13270 | J. Mater. Chem., 2012, 22, 13268–13278

by evaluating all possible combinations of factors using single-

factor experiments is prohibitively time-consuming. As there are

several factors to evaluate and potential interactions exist

between the factors, multi-factorial design of experiment (DOE)

should be explored as an alternative to traditional single-variable

experiments. This technique is a time- and cost-effective

approach for testing the effects of many variables

simultaneously.

Materials and methods

Materials

Peroxopolytungstic acid (PTA) was synthesized based on the

procedure reported by T. Kudo et al.8 Tungsten metal mono-

crystalline powder (Aldrich, 0.6–1 mm, 99.9%) was carefully

added to a 50 ml mixture (50 : 50) of distilled water (Millipore)

and hydrogen peroxide (Sigma-Aldrich, 30% by weight). Cooling

was employed and the solution was kept slowly stirring for 24 h

in a refrigerator to prevent thermal changes due to the strong

exothermic nature of dissolution. The excess tungsten powder

was then removed by filtration (Roth, 0.45 mm syringe filter)

leading to a transparent solution. In order to remove the excess

hydrogen peroxide the solution was dried at 65 �C and washed

several times with distilled water. After drying, a water-soluble

WO3$xH2O2$yH2O orange crystal powder (PTA)9 was obtained

as the final result. Two types of commercially available WOX

nanoparticles were used as a source of crystalline phase: WO3,

referred to as yellow (Aldrich, <100 nm); and WO2.9, referred to

as blue (Super Conductor Materials, 99.99%, ceramic oxide

target). Titania paste (Solaronix, Ti-Nanoxide T/SC, 15–20 nm,

3% by weight) was used as a source of nanocrystalline TiO2.

These nanoparticles were used in the form of aqueous (Millipore)

alcohol (Merck, 2-propanol, 99.8%) dispersions in a fixed

proportion of 30 : 70 and solid content of 0.01% by weight.

Oxalic acid dihydrate (Merck) and Triton X-100 (JT Baker

Chemical) were used as received.

In order to obtain 3 g of gel electrolyte, 0.36 g of PMMA

(Fluka), 0.21 g of PEO–PPO (Zeon Chemicals), 0.33 g of LiClO4

(Fluka) and 1.75 g of propylene carbonate (Fluka, 99%) were

mixed with 4 ml of tetrahydrofuran (Aldrich, 99.9%) followed by

stirring for about 4 h until a transparent uniform lightly viscous

gel was obtained.10

Inkjet printing

An ink was formulated according to a DOE recipe for each lab-

testing device with 3 wt% addition of Triton X-100. Particular

layers (1 cm2) were printed 5 times using a conventional desktop

printer (Canon PIXMA IP4850) in regular intervals of around

1 min while being exposed to a relative humidity of 50%, at 28 �C,on ITO PET substrates (Sigma-Aldrich, 1000 �A of ITO, 60 U

sq�1, T > 75% at 550 nm). All films were dried at room temper-

ature for 24 h and annealed in air at 120 �C (EHRET, TK4067,

Germany) for 1 h.

Encapsulation

The devices were assembled as a sandwich structure which

included an inkjet-printed EC film (working electrode), a gel

Table 1 The list of experimental trials according to mixture design withone non-mixture component methodology

Run wsolvent wPTA wOAD wTiO2wWOX

1 0.539 0.010 0 0 0.451 Yellow2 0 0.200 0 0 0.800 Blue3 0 0.113 0.050 0.837 0 Blue4 0 0.200 0 0.800 0 Yellow5 0.800 0.200 0 0 0 Yellow6 0.575 0.010 0 0.415 0 Yellow7 0 0.010 0.050 0 0.940 Blue8 0 0.010 0.050 0.940 0 Yellow9 0 0.010 0 0.498 0.492 Blue10 0.940 0.010 0.050 0 0 Yellow11 0 0.010 0 0 0.990 Yellow12 0 0.200 0.050 0 0.750 Blue13 0 0.010 0 0 0.990 Blue14 0 0.010 0 0.990 0 Yellow15 0 0.200 0 0 0.800 Yellow16 0.465 0.010 0.026 0 0.499 Blue17a 0.940 0.010 0.050 0 0 Blue18a 0.800 0.200 0 0 0 Blue19 0.866 0.109 0.025 0 0 Yellow20 0.441 0.010 0.026 0.523 0 Blue21 0 0.081 0.024 0.445 0.451 Yellow22a 0 0.010 0 0.990 0 Blue23 0 0.200 0.023 0.777 0 Blue24 0 0.200 0.050 0.750 0 Yellow25 0.990 0.010 0 0 0 Blue26a 0 0.010 0.050 0.940 0 Blue27 0.351 0.200 0.050 0 0.399 Yellow28 0 0.010 0.050 0 0.940 Yellow29 0.401 0.108 0 0.491 0 Blue30 0.750 0.200 0.050 0 0 Blue

a Duplicates.

electrolyte for ion storage, and two transparent conductors (ITO

PET foil) separated by double-sided tape spacer (1 mm thick)

which were utilized to establish electrical contacts. The device

was sealed with thermoplastic glue.

Characterization

The structural analysis of EC printed films was supported by

SEM measurements (Auriga SEM-FIB, Zeiss). Optical

measurements were performed using a spectrometer set-up con-

sisting of an HR4000 high-resolution spectrometer (Ocean

Optics), halogen light source HL-2000-FHSA (Mikropack) and

high current source measure unit (Keithley 238). The electro-

chemical measurements (Gamry Reference 600 Potentiostat,

Gamry Instruments) were performed on assembled devices in

two-electrode configuration in which working and working sense

were connected to a working electrode and reference and counter

were connected to a second (ITO) electrode. Such a simplified EC

system in which ITO coating serves as both transparent

conductor and counter electrode is capable of giving a fairly high

optical modulation in the visible and solar spectral regions,

although it has low charge capacity. An ITO counter electrode,

at which an electrochemical process similar to the one on the

working electrode occurs, causes no significant colour change, as

was demonstrated in the state of the art.11–13

Software

The Design of experiment and statistical data analysis were

performed using JMP 8.0 (SAS Institute Inc., Cary, NC, USA).

Design of experiments

A screening design was performed according to the Design of

experiment methodology using SAS JMP 8.0 statistical software.

The flexible JMP Custom Designer was applied to determine the

main factors and interaction effects and to investigate the

changes of the responses by varying each factor in order to

predict device performance for all possible ink recipe combina-

tions. A screening design method was selected to characterize the

inkjet printing process of dual-phase a-WO3/TiO2/WOX films

because the studied problem involves a large number of input

factors. Designed mixtures can be described as quinary systems

of solvent–PTA–OAD–TiO2–WOX with one additional non-

mixture factor X which stands for experimentally determined

WOX particle oxygen content (dual-level categorical factor with

values of 2.9 for blue and 3 for yellow nanopowder). Mixtures

(inks) must meet strict physicochemical property requirements

(viscosity, surface tension and maximum particle size) which

impose several constraints. An ink composition based on solvent

with isopropyl alcohol to water ratio of 0.3 : 0.7 by weight con-

taining less than 20 wt% of PTA and less than 5 wt% of OAD

results in viscosity and surface tension values of 1.5–2 cP and

30–40 dyne cm�1, respectively, acceptable values for a conven-

tional office printer. The maximum nanoparticle size should be at

least one hundred times smaller than the nozzle diameter (9 mm)

which gives 90 nm (according to Canon FINE technology

specification). Additionally, all samples must contain PTA

component ($1 wt%) in order to ensure the existence of amor-

phous phase which guarantees an EC effect by allowing

electronic conduction to take place in a low-temperature process.

Based on imposed restrictions, the values of individual weight

fraction ranges and categorical levels of factors were defined as

follow: 0# wsolvent # 0.99; 0.01# wPTA # 0.2; 0# wOAD # 0.05;

0 # wTiO2# 0.99; 0 # wWO3

# 0.99; X ¼ {blue, yellow}. Due to

the complexity of such a constricted experimental area, the

application of D-optimal algorithm for experimental design

seems to be essential. The computer-aided D-optimal design for

six factors studied, shown in Table 1, was composed of 30

experiments and the order of runs was fully randomized to

minimize the effect of uncontrolled factors that could affect the

final results. Since wWOXand X factors are dependent, each pair

of runs (5, 18), (8, 26), (10, 17) and (14, 22) describe the same

experiments. Those experiments cannot be merged, because

deleting the duplicates would cause the need for a new D-optimal

model generation.

Ultimately, there were 26 independent runs and for each of

them printable solution and targets were freshly prepared just

before use. The studied responses, obtained from measurements

of the developed devices, were mechanical, optical, electrical,

fluid (ink) and overall performance parameters, 16 in total. Due

to the complexity of the analysis and the number of studied

responses, the following considerations are limited to the main

parameters describing switching kinetics and overall perfor-

mance of EC films.

Mathematical fitting based on multiple regression was per-

formed for each categorical factor level using a first-order

J. Mater. Chem., 2012, 22, 13268–13278 | 13271

polynomial function expressed by eqn (1), which helps to trans-

form the complexity of the studied problem into an easy-to-

handle mathematical form:

Y1#k16 ¼X5

biXi þX5

j¼1þ1

bijXiXj þ 3 (1)

Surface responses Yk (1 # k # 16) were determined by five

controlled mixture components Xi (1 # i # 5), coefficients of

the main factors and the coefficients of the first-order interac-

tions bi, bij, respectively, and 3which represents the random error

of the method. This model can be used to predict the response for

any condition within the experimental space.

Results and discussion

Data fitting and factorial analysis

Measured values of selected parameters, namely coloration time

(scol), bleaching time (sbl), optical density (DOD) and coloration

efficiency (CE), corresponding to each experimental run are

presented in Table 2.

The model optimality was evaluated based on a summary of fit

(least squares regression) and analysis of variance (ANOVA).

The linear responses fitting offered by the screening design

appeared to be sufficient for optical density (DOD) and colora-

tion efficiency (CE) because of its ability to supply high

Table 2 Measured values of selected parameters corresponding to eachexperimental run

Run scol [s] sbl [s] DOD [�10�2] CE [cm2 C�1]

1 14 9 36.18 188.422a 5 3 2.69 9.33a 3 4 97.71 548.954 2 3 4.25 29.135 3 4 11.5 83.956 4 5 46.05 225.727 9 4 8.25 70.518 4 4 3.84 34.879 10 7 37.73 474.0110 4 3 10.23 73.611 4 2 23.31 187.9912 11 9 38.72 175.213 10 7 26.85 80.8814 3 3 50.98 383.3115a 2 2 4.24 22.2216 8 6 4.5 15.2917a 30 30 3.88 20.7418 6 2 1.36 2.8419 4 8 75.3 404.8220 2 4 6.95 43.7221 14 12 82.64 454.0922 4 5 46.44 305.5623 4 2 20.72 199.2224 5 5 18.21 130.0525 3 2 17.56 91.9626 8 5 9.39 46.4827a 30 30 130.93 534.2728 9 2 13.12 48.0429a 7 7 5.72 21.1230 5 6 55.93 321.43

a Experimental runs excluded from subsequent statistical analysis due tothe device malfunction caused by uncontrollable factors (incorrectencapsulation, film failure, etc.).

13272 | J. Mater. Chem., 2012, 22, 13268–13278

coefficient of determination (R2-adj) values of 0.87 and 0.98,

respectively, indicating a good correspondence between the

model prediction (fitting) and the experiments, while simulta-

neously minimizing the overall number of coefficients. The

ANOVA significance probabilities of 0.02 and 0.0006, respec-

tively, support the accuracy of the model. In the case of colora-

tion time (scol) and bleaching time (sbl), regression coefficients

were 0.51 and 0.72, while significance probabilities were 0.22 and

0.09, respectively, which means that the models for those

responses are just slightly better than the overall response mean

and indicates a small number of significant regression factors. As

no nonlinear response was detected for scol and sbl, a more

complex design-type is not required, and the lack of fit comes

from difficulties associated with accurate measurements of those

responses. All response models have been graphically analysed

using leverage plots in order to test the significance of all effects

(factors and first-order interactions).14 The leverage plots for

each factor illustrate the residuals as they are and as they would

be if that component was removed from the model. Analysis

results of all effects based on the leverage plots method and

Student’s t-test statistics for scol, sbl, DOD and CE responses lead

to the conclusions shown in Table 3. Elements in column (a)

marked by (+) indicate coefficients that were determined to be

significant, and thus which factor (or factor interactions) had

a major effect on a particular response value quantifying their

importance. As the emphasis of DOE was screening, the impor-

tance column (b) indicates the most significant effects (starting

from value 1) sorted by the absolute value of the t-ratio, which

lists the test statistics for the hypothesis that each parameter is

zero. Each t-ratio value expresses the ratio of a particular

parameter estimate to its standard error, and has a Student’s

t-distribution, if the hypothesis is true.

The model predicts in general that EC performance of printed

films is enhanced by dual-phase composition, which means that

Table 3 Factorial analysis results obtained via leverage plots methodand Student’s t-test statisticsa

Effect

scol sbl DOD CE

a b a b a b a b

wsolvent � � + 6 + 3wPTA �* +* 6 +* 2 +* 7wOAD �* �* �* �*wTiO2

� + 2 + 1 + 1wWOX

+ 1 + 1 + 5 + 4wsolvent$wPTA � + 5 + 3 + 10wsolvent$wOAD � � � �wsolvent$wTiO2

� � � + 6wsolvent$wWOX

+ 2 + 3 � + 16wsolvent$X � � � + 5wPTA$wOAD � � � + 14wPTA$wTiO2

� + 7 + 5 + 12wPTA$wWOX

� + 4 + 4 + 13wPTA$X � � � + 9wOAD$wTiO2

� � � �wOAD$wWOX

� � � �wOAD$X � � � + 8wTiO2

$wWOX� � � + 2

wTiO2$X � � � + 15

wWOX$X � � � + 11

a a – result:� not significant, + significant, * collinearity; b – importance.

metal oxide nanocrystal concentration is crucial. In detail, the

significance analysis leads to the conclusion that the scol dependsmainly on the quantity ofWOX nanocrystals in the EC layer. The

sbl is influenced by the quantity of WOX and TiO2 nanocrystals

and also depends on WO3 amorphous content formed from PTA

precursor. DOD, which describes the optical contrast between

colored and bleached states, mainly depends on the quantity of

TiO2 nanocrystals and WO3 amorphous content. The significant

factors, with regards to their impact on CE which describes

overall device performance, depend to a greater or lesser extent

on almost all mixture components and their interactions, but the

most significant are TiO2 and WOX crystalline nanoparticles.

Collinearity indicated by (*) obtained for the main effects derived

from PTA and OAD components indicates that those two

coefficients have no independent variation to support fitting of

any investigated response variation, because they are linear

functions of other factors. While the role of PTA is evident from

coefficients of the first-order interactions, the impact of OAD on

EC performance is negligible. The OAD component may there-

fore be eliminated from consideration without detriment to the

EC effect, causing simplification of the method by reducing the

number of ingredients.

Application of a computer-aided D-optimal design success-

fully reduced the number of overall experimental trials from 162

defined for three-level full factorial design or 72 defined for

second-degree extreme vertices design down to 30 runs in the

present studies. The proposed experimental design methodology

appeared to offer a sufficiently accurate mathematical modelling

with a complete coverage of experimental trials while the total

number of designed experiments has been significantly reduced.

Fig. 2 SEM images of inkjet-printed (a) amorphous (a-WO3) and (b)

dual-phase (a-WO3/TiO2/WOX) deposited on ITO PET substrates. The

pattern boundaries are indicated by dashed lines. (Insets: ULC-magnified

cross-sectional images; BRC-comparison of printed dual-phase films to

chocolate with nuts).

Microstructure and morphology

The micrograph of the inkjet-printed amorphous a-WO3 film

shown in Fig. 2a represents a continuous film. Sporadically, the

cracks caused by stress are revealed, which are shown here in

order to expose the film cross-section. The difference obvious at

a glance between the morphological characteristics of WO3 films

deposited via standard wet techniques and IPT at low magnifi-

cations is that while continuous layers are observed in the first

case for annealed nanocrystalline films,15 their inkjet-printed

dual-phase a-WO3/TiO2/WOX counterparts are characterized by

a discontinuous pattern as may be seen in Fig. 2b. Film discon-

tinuity is caused by the existence of solid nanoparticles in the

liquid sol–gel precursor, which significantly changes its rheo-

logical behaviour. A clear distinction between the nanocrystal-

line grains distributed randomly throughout an amorphous

phase can be observed in the resultant ‘chocolate with nuts’-like

structure. At high magnifications, however, the surfaces of

printed films exhibit a rather homogeneous topography. The

stress caused by the decomposition of the precursor and the

solvent removal at elevated temperature leads to the creation of

cracks. According to studies conducted by Yamanaka et al.,8

those cracks have no significant effect on electrochemical prop-

erties of sol–gel-processed films.

According to the electrochromism theory, the optical modu-

lation in crystalline WO3 phase arises due to the increasing

Drude-type (metallic) reflection, observed especially in the IR

region, with increasing free electron/lithium injection.16 In the

amorphous phase, the most widely accepted model assumes that

the optical modulation upon double injection occurs through

increasing absorption arising from the transfer of localized

electrons between W5+ and W6+ sites, so-called small polaron

absorption.17,18 However, the improvement in EC performance

of printed films shown in the next paragraph is related to the film

microstructure, which is not subjected to densification during

deposition and post-treatment, rather than to local electronic

states.

Because of the low temperatures involved, no shrinkage

occurs, which differentiates inkjet-printed amorphous (Fig. 3a)

J. Mater. Chem., 2012, 22, 13268–13278 | 13273

Fig. 3 Microstructure and morphology of inkjet-printed (a) amorphous (a-WO3) and (b) dual-phase (a-WO3/TiO2/WOX) films after drying at room

temperature and annealing for 1 h at 120 �C.

Fig. 4 Lab-testing EC device in (a) bleached and (b) colored state.

and dual-phase (Fig. 3b) films from those developed in other wet

deposition techniques. Regardless of the composition of printed

layers, annealing at 120 �C does not significantly affect the

thickness of the layer. Systematic solvent evaporation between

passes and the low temperature applied cause low shrinkage.

Such a light structure with nanocrystalline phase dispersed in

amorphous matrix tends to reduce the diffusion path length in

the film and improve kinetics, as switching speed is predomi-

nantly governed by the solid-state diffusion of lithium ions in

WO3. Crystallinity both induced by high temperature from sol–

gel-derived tungsten oxide1 and incorporated into the film via

IPT exists as small crystallites dispersed in an amorphous matrix

of tungsten oxide and not as continuous crystalline phase. The

grain size, crystallinity and stoichiometry of nanoparticles

dispersed in inkjet-printed films can be modified by merely

changing the source of particles loaded into the ink. Aside from

the electrolyte-accessible surface area and crystallinity, the

oxygen deficiency in the crystalline phase also affects the EC

properties of printed dual-phase films. It has been observed that

the stoichiometry of WOX nanoparticles (X¼ 3 or X¼ 2.9) leads

to slightly different electrochemical and EC behaviour, indi-

cating that the choice of WO3 nanoparticles results in higher

overall performance of printed EC devices. However, several

important aspects concerning the WOX particle stoichiometry

remain unclear and further studies are going to be performed as

regards this matter.

Since different WOX nanomorphologies can be obtained in

a range of crystal phases via various material synthesis tech-

niques,19 almost unlimited selection of nanocrystalline content

for ink formulation is possible. Furthermore, doping of WOX

nanoparticles during synthesis is another important area worth

further exploration, because it opens new opportunities for ion

intercalation with significantly enhanced chromic response. The

combination of dopedWOX nanocrystal synthesis with the newly

presented method would allow for dual-phase film development

with different nanoscale composition of each phase.

13274 | J. Mater. Chem., 2012, 22, 13268–13278

Electrochromic performance

The lab-testing reversible device shown in Fig. 4 has been

developed as a four-layer stack between two flexible PET

substrates. The optically active film is printed onto a transparent

conducting ITO electrode and coupled to a gel electrolyte (ionic

conductor with dispersed lithium ions) and second electrode. All

optical and electrochemical measurements presented below were

performed in situ with such encapsulated devices. Application of

a voltage drives the ions into the a-WO3/TiO2/WOX film, where

they are intercalated causing the chromatic effect. Reversing the

voltage returns them to the electrolyte, which also serves as ion

storage. Cathodic polarization irreversibly reduces the peroxo

complexes to a-WO3 which is typical for PTA precursor

regardless of the deposition method. This process was manifested

by poor initial responses which improved and stabilized after

a few cycles.

Contour plots on a trilinear coordinate scale were used to

present predictions of experimental results obtained via multiple

regression fitting. This graphical methodology provides an easy

way to demonstrate the efficacy of the enhanced performance of

printed EC films. Ternary plots were generated as a function of

TiO2/WOX nanocrystalline content, while the concentrations of

other components (PTA, OAD) were kept constant (Fig. 5–7).

The third axis stood for pure alcoholic aqueous solvent in order

to quantify the amount of nanocrystals dispersed in the ink and

afterwards transferred to the printed films. Comparison between

corresponding surface responses plotted for different WOX

stoichiometry (WO2.9 and WO3) provides information regarding

the role of oxygen content in the nanocrystalline phase. In the

whole range of studied concentration, films containing WO3

nanoparticles outperform their WO2.9 analogues, while both

show similar relationships between factors and responses. Such

a dual-phase a-WO3/TiO2/WOX microstructure aided ion

movement manifested in coloration/bleach kinetics, optical

density and coloration efficiency faster and to a greater extent

than previously reported sol–gel-derived WO3 films. The appli-

cation of a negative potential causes the active film to turn deep

blue over scol (Fig. 5a) and the initial bleached state can be

restored by applying a reverse potential within sbl (Fig. 5b).

Those parameters were defined as the requisite time for the

transmittance change by 80% of the total difference between final

states. The highest measured colouring and bleaching time values

do not exceed 14 and 12 seconds, respectively. A substantial

reduction in coloration and bleaching time is observed when

increasing the nanocrystalline TiO2 content, reaching values

more than three times lower (scol < 3 and sbl < 2.5). Coloration

kinetics is observed to be slower than bleaching kinetics for all

Fig. 5 Response contour plots presenting variation in (a) coloration and (b)

varied (wPTA ¼ 0.01, wOAD ¼ 0, X ¼ 3{yellow}). The faded regions indicate

the films under investigation, which is in agreement with the well-

defined, but different mechanism governing the two processes.

While the exchange of current density at the EC film–electrolyte

interface controls coloration kinetics, the space charge-limited

Li+ ion diffusion current governs the bleaching time.18

As a consequence of dual-phase microstructure, there is a large

interface between the oxide layer and gel electrolyte, which

promotes ion diffusion through the porous film. In order to

evaluate the difference in electrochemical properties, one needs

to refer to the values of the total charge inserted during the

coloration cycle (QINS). Our findings allowed us to estimateQINS

values and demonstrate them in a ternary plot as a function of

TiO2/WOX nanocrystalline content (Fig. 6a). The contradiction

between results and the postulated microstructure-dependent

mechanism for films with high nanoparticle loading is due to film

discontinuity which causes overestimation of surface area. In

order to mark these concentrations for further considerations,

ternary plots which present parameters by definition calculated

per unit area and current flowing through the EC device contain

faded rounded regions. QINS higher than 1.4 mC cm�2 was

measured for films containing a high amount of WOX nano-

particles implying a higher anodic current flow though the

crystalline fraction and higher Li+ ion mobility of the film. Very

well-defined anodic (bleaching) peak (ibleaching) for those films

(Fig. 6b) is evidence of improved kinetic performance caused by

the highly porous nanocrystalline structure that greatly reduces

the diffusion length of lithium ions. On the other hand, these

films manifest high values of reduction (coloring) peak maxima

(icoloring), which may indicate that Li+ ion insertion/extraction is

not completely reversible (Fig. 6c). The value of icoloring decreases

significantly with an increase of TiO2 particle loading. By

enhancing the operating voltage, the change in transmittance

between colored and bleached states becomes increasingly

evident. When the voltage exceeds a certain value, the trans-

mittance of the bleached films is markedly lower than that before

coloration which means that the device does not bleach

bleaching time as the weight fractions of TiO2 and WOX dispersions are

unreliable measurements due to the low optical modulation.

J. Mater. Chem., 2012, 22, 13268–13278 | 13275

Fig. 6 Response contour plots showing variation in (a) charge densities, (b) bleaching current maximum peak and (c) coloration current maximum

peak as the weight fractions of TiO2 and WOX dispersions are varied (wPTA ¼ 0.01, wOAD ¼ 0, X ¼ 3{yellow}). The green contour shows the

concentrations of highest electrical response. The regions of rounded faded marks indicate concentrations for which discontinuous patterns were

obtained. (d) Site saturation effect as a function of operating voltage; DTSS is defined as the difference in transmittance of the bleached films to its value

before coloration for particular operating voltage (DTSS ¼ 0 for fully reversible optical effect).

completely. This specific value of coloring voltage called the

optimum voltage20 (Vopt) lies in the range of �1.5 V to �1 V for

a-WO3, �1 V to 0 V for a-WO3/WOX and 0 V to 0.5 V for

a-WO3/TiO2 films. Color-bleached characteristics recorded at

900 nm show that the EC effect was fully reversible for operating

voltages lower than Vopt, while after applying a higher voltage

the film did not bleach completely, due to the intensive charge

trapping particularly evident for films containing nanocrystalline

WOX phase. Increasing the WOX nanocrystal concentration in

the film increases the volume of the grain boundaries, which

contributes to more trapping of free charge carriers. In addition,

lattice strain and crystal distortion can also affect the Li+ ion

insertion/extraction, causing reversible deterioration. It has been

also reported before that while increasing the TiO2 doping

concentration the amount of residual charges in the WO3-based

13276 | J. Mater. Chem., 2012, 22, 13268–13278

film decreases due to the enhancement in the reversibility of the

redox process.21 The positive impact of TiO2 nanoparticles on the

reversibility of printed EC films is demonstrated in Fig. 6d, which

represents the so-called site saturation effect.22

As a merit of optical performance, values of optical density

change (DOD) and quantitative coloration efficiency (CE) are

given as surface responses shown in Fig. 7. The DOD values are

calculated as a natural logarithm of the ratio of transmittance in

the bleached state to transmittance in the colored state of the

films taken at a certain wavelength.16 In order to reflect the

potential of this method for EC window application, the optical

parameters were measured at high wavelengths, in the NIR

region (l¼ 900 nm). The DOD surface response shown in Fig. 7a

indicates that films containing a large amount of TiO2 nano-

particles with a smaller share of WOX content in amorphous

Fig. 7 Response contour plots presenting variation in (a) optical density and (b) coloration efficiency as the weight fractions of TiO2 and WOX

dispersions are varied (wPTA ¼ 0.01, wOAD ¼ 0, X ¼ 3{yellow}). The regions of rounded faded marks indicate concentrations for which discontinuous

patterns were obtained.

matrix exhibit the highest transmission modulation due to the

modification of the internal film structure. The Li+ conductive

network of interconnected TiO2 nanoparticles facilitates ionic

transport into the EC layer. At the same time, mesoscopic TiO2

structure significantly increases the surface area of a-WO3

matrix. As both TiO2 and WOX nanoparticles are hydrophilic,

films with high nanocrystalline content become more hydrated.

Such physisorbed water gives a significant enhancement in ion

dynamics through an amorphous phase,18 consequently leading

to an increase in DOD up to 0.82. The overall device performance

expressed by CE is defined as a ratio between DOD and injected/

ejected charge measured for a specified film area.16 According to

this definition, an ideal coating would exhibit a large optical

modulation with a small amount of operating charge, giving rise

to a large CE value. This is a very convenient performance

indicator; however, its measurement methods vary between

research groups, making difficulties in comparison between

various deposition systems. The application of IPT brings

additional complications in this topic due to the printed film

discontinuity. CE should be then redefined in order to describe

not only continuous films, but also inhomogeneous printed

patterns, in which the sizes of electrochemically active islands

and distances between them are comparable. Such an extended

definition would refer to the active area of the EC device

measured in the device length scale instead of microscale as is

commonly practiced.

Inkjet-printed EC devices irrespective of composition exhibit

higher CEs than sol–gel-derived WO3 films deposited via stan-

dard wet methods. CE values calculated according to the rede-

fined formula for printed amorphous films which are observed to

be continuous vary between 230 and 260 cm2 C�1 while patterns

containing a high amount of TiO2/WOX nanoparticles exhibit

high efficiencies of 440 to 470 cm2 C�1 (Fig. 7b). Although the

accuracy of CE is affected by the method, for which optical and

electrochemical measurements were not performed at the same

time, these values reflect the real EC performance of printed

devices. It should be also noted that the EC performance of the

presented devices could be increased by applying a counter

electrode with higher charge capacity than ITO.23 In such

a system, the maximal value of charge that can be reversibly

shuttled between counter electrode and active film would be

higher resulting in increased coloration.

The presented data show that dual-phase a-WO3/WOX

microstructure is most favourable for a rapid ion insertion/

extraction process as these printed films exhibit very well-defined

anodic peak and high charge densities. In addition to the

microstructure, the amount of dopant, namely TiO2, plays an

important role manifested in decreased charge trapping effect as

well as in enhanced optical modulation.

TritonX-100 as non-ionic surfactantwas used in order to adjust

the ink surface tension to a value acceptable for printing systems

(30–40 dyne cm�1). The choice of this organic compound was

dictatedby two factors.Firstly, cationic surfactants are unsuitable

for PTA precursor, because their molecules precipitate Keggin-

like PTA polyanions, which prevent the synthesis of a stable

coating solution.24 Secondly, Triton X-100 is a well-known

surfactant often used with success for inkjet-printable inorganic

dispersions.25 It is noteworthy that non-ionic surfactants also play

a key role in the formation of WO3 mesostructures in the organic

template sol–gelmethod.26–28As the coating solutions presented in

those studies are similar to our sol–gel printable precursor, we can

expect that printed hybrid films, annealed at 120 �C, may also

exhibit a mesostructure. However, this phenomenon has not been

investigated and its influence on the EC performance of printed

a-WO3/TiO2/WOX is not known yet.

Conclusions

Tungsten oxide is the most extensively studied inorganic EC

material due to its interesting electrochemical and physical

properties observed for films developed via vacuum as well as wet

techniques. However, for use in practical high-performance EC

J. Mater. Chem., 2012, 22, 13268–13278 | 13277

devices, WO3-based systems require further improvement in film

composition and processing. In order to augment EC properties,

dual-phase a-WO3/TiO2/WOX deposited via IPT using various

ink formulations has been investigated. It was indeed surprising

to observe that in spite of employing for IPT the same WO3

sol–gel precursor known from former wet deposition techniques

either with nanoparticles or without them, the EC performances

of resulting films exhibit dramatic differences from those

described in the state-of-the-art, which is indicative of the major

role played by the deposition technique.

As-deposited inkjet-printed amorphous a-WO3 films prepared

from PTA printable precursor and dual-phase a-WO3/TiO2/

WOX prepared from WOX and TiO2 nanoparticles dispersed in

the same precursor transform into chemically stable films upon

annealing at 120 �C. The grain size, crystallinity and stoichi-

ometry of the particles are dependent only on the origin of the

crystals and are defined at the ink formulation stage. This flexi-

bility in material selection not only endows printed devices with

a better chemical stability, but is also responsible for the superior

EC performance reflected by high optical density (DODz 0.82),

enhanced coloration efficiency (CE > 400 cm2 C�1 at l¼ 900 nm)

and fast color-bleach kinetics (scol < 3 s and sbl < 2.5).

Furthermore, an inkjet-printed EC window exhibits peak power

consumption at the level of 10 mW cm�2 in the switching process,

which is 40 times lower when compared to the commercial SAGE

Electrochromic’s window29 developed by a physical route.

Considering just the main effects of each variable in eqn (1), it

is clear that the amounts of WOX and TiO2 nanoparticles and

WO3 sol–gel precursor (PTA) have the highest impact on EC

performance expressed by scol, sbl, DOD and CE, while the

impact of OAD is negligible. Besides, first-order interactions also

occurred between WOX, TiO2 and PTA factors justifying the

validity of the applied first-order polynomial model.

Interconnected TiO2/WOX nanocrystals of printed dual-phase

films lead to higher values of transmission modulation and

coloration efficiency over the visible and solar spectral regions as

compared to the poor EC performance of a-WO3 films owing to

their amorphous structure. With an increase in TiO2 nano-

particle loading, kinetics of intercalation and deintercalation

mechanisms improves and significantly reduces the deleterious

site saturation effect. The Li+ conductive network of inter-

connected TiO2 nanoparticles facilitates ionic transport into the

EC layer and increases significantly the active surface area of the

a-WO3 component. These results demonstrate that IPT is an

excellent method for the production of inorganic chromogenic

films with controlled composition and dual-phase microstructure

for low-temperature, direct-write fabrication of high-perfor-

mance EC devices. The presented data provide strong support

for the general paradigm that to achieve the best EC performance

an amorphous phase must be combined with nanocrystalline

content in a light structure that is easily penetrable by cations.

Furthermore, this work demonstrated that combination of

IPT as a novel deposition technique and DOE methodology

provided an effective means to explore and predict the behavior

of high-performance EC devices. The use of D-optimal mixture

design with one no-mixture component could reduce the total

number of experimental trials, while still maintaining high

accuracy of analysis. The identification of the critical process

factors via a screening experiment was essential for method

13278 | J. Mater. Chem., 2012, 22, 13268–13278

optimization and resulted in a reduced number of system

components greatly simplifying the technological process.

Acknowledgements

This work was funded by the Portuguese Science Foundation

(FCT-MCTES) through project Electra, PTDC/CTM/099124/

2008 and the PhD grant SFRH/BD/45224/2008 given to P. J.

Wojcik. Moreover, this work was also supported by E. For-

tunato’s ERC 2008 Advanced Grant (INVISIBLE contract

number 228144), ‘‘APPLE’’ FP7-NMP-2010-SME/262782-2 and

‘‘SMART-EC’’ FP7-ICT-2009.3.9/258203.

Notes and references

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9 B. Pecquenard, S. Castro-Garcia, J. Livage, P. Y. Zavalij,M. S. Whittingham and R. Thouvenot, Chem. Mater., 1998, 10,1882–1888.

10 P. C. Barbosa, L. C. Rodrigues, M. M. Silva, M. J. Smith,P. B. Valente, A. Goncalves and E. Fortunato, Polym. Adv.Technol., 2011, 22, 1753–1759.

11 V.-V. T. F. Benkhelifa, P. V. Ashrit, G. Bader and F. E. Girouard,Thin Solid Films, 1993, 232, 83–86.

12 B. C. H. S. S. J. Golden, Solid State Ionics, 1988, 28–30, 1733–1737.13 Z. Wang and X. Hu, Thin Solid Films, 2001, 392, 22–28.14 J. Sall, Am. Stat., 1990, 44, 308–315.15 M. Deepa, T. Saxena, D. Singh, K. Sood and S. Agnihotry,

Electrochim. Acta, 2006, 51, 1974–1989.16 C. G. Granqvist, Sol. Energy Mater. Sol. Cells, 2000, 60, 201–262.17 Y. Krasnov, Electrochim. Acta, 2004, 49, 2425–2433.18 B. W. Faughnan, R. S. Crandall and M. A. Lampert, Appl. Phys.

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29 Lawrence Berkeley National Laboratory, PIER Final Project Report:A Design Guide for Early-Market Electrochromic Windows, CaliforniaEnergy Commission, Berkley, 2006.

Microstructure control of dual-phase inkjet-printed a-WO3/TiO2/WOX films for high-performance...

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