Materials Science and Engineering A286 (2000) 144–148

Nanostructured metal oxides for printed electrochromic displays

Jingyue Liu * , James P. ColemanMonsanto Corporate Research, Monsanto Company, 800 N. Lindbergh Bl6d., U1E, St Louis, MO 63167, USA

Abstract

Electrochromic devices are able to change their optical properties reversibly under the action of applied voltages. Theconventional method of fabricating electrochromic devices utilizes a ‘sandwich’ configuration of electrodes. We developed a‘side-by-side’ design for fabricating electrochromic display devices without the use of conductive, transparent electrodes. A simpleprinting technology can be used to produce commercial scale, flexible electrochromic displays. We have also discovered that tinoxide nanocrystallites heavily doped with antimony exhibit a high level of electrochromism. The high contrast ratio ofnanostructured antimony–tin oxide (ATO) electrochromic displays is attributed to an accessible antimony energy state in theband gap of the mixed oxide. The fast switching rate can be attributed to the high surface area of, and high number density ofgrain boundaries in, the nanophase ATO materials. The interfacial regions between ATO nanocrystallites facilitate the transportof ions in and out of the electrochromic layer. The dynamics of the electrochromic displays is critically dependent on thenanostructure of the electrochromic layer. The design strategy for commercial production of printed, flexible electrochromicdisplays will be discussed. © 2000 Elsevier Science S.A. All rights reserved.

Keywords: Nanostructure; Electrochromism; Display device; Metal oxide; Electron microscopy

www.elsevier.com/locate/msea

1. Introduction

Electrochromic devices, such as displays or smartwindows, are able to change their color reversibly un-der the action of an applied voltage pulse. An elec-trochromic device is essentially a rechargeable batterywith the electrochromic electrode separated by a solid,gel, or liquid electrolyte from a charge-balancing coun-ter-electrode. When an external potential is appliedbetween the working and counter electrodes the chargeor discharge of this electrochemical cell induces colorchanges of the electrochromic device. The modulationof the optical properties of electrochromic devices islargely determined by the capacity of the elec-trochromic materials for charge injection and expul-sion. The contrast ratio, which is an importantparameter for evaluating the performance of displaydevices, is defined as the ratio of the intensity of lightdiffusely reflected through the bleached state of thedisplay to the intensity of light diffusely reflected fromthe colored state. Commercial devices require elec-trochromic materials with a high contrast ratio, high

coloration efficiency, long cycle life, high write–eraseefficiency, and fast response time.

The conventional method of fabricating elec-trochromic devices utilizes a ‘sandwich’ configurationof electrodes [1]. The electrochromic electrode of thesedevices generally consists of a thin film of elec-trochromic materials deposited onto a conductive,transparent glass such as indium–tin oxide (ITO)coated glass substrate. Disadvantages of this type ofdevice design include high cost, low conductivity,difficulty in fabricating sophisticated display designs,and electrochemical instabilities.

To overcome these difficulties, we developed an inter-digitated electrode approach. The construction of dis-play devices utilizes a low cost printing or coatingprocess. The production of raw materials involves onlysimple solution and dispersion processes. With this newdesign approach electrochromic display devices can becommercially produced without the use of transparentelectrodes. A wide range of device types has beendemonstrated with an interesting range of colors andprototype electronic drivers [2].

A large number of electrochromic materials includingtransition metal oxides, conducting polymers, metal-lopolymers, Prussian Blue systems, etc. are now avail-

* Corresponding author. Fax: +1-314-6942408.E-mail address: jingyue.liu@monsanto.com (J. Liu)

0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.PII: S0921 -5093 (00 )00719 -X

J. Liu, J.P. Coleman / Materials Science and Engineering A286 (2000) 144–148 145

able [1,3]. Transition metal oxides such as WO3, MoO3,TiO2, and V2O3 show considerable variations in stoi-chiometry and display inherent electrochromic proper-ties [1]. Thin films of antimony–tin oxide (ATO) andindium–tin oxide have also been studied in the past toexplore their electrochromic properties [4,5]. Both ofthese thin films exhibited very weak or no elec-trochromism. Antimony-doped tin oxide powders havebeen used as oxidation catalysts [6] or as electricallyconductive pigments [7]. These powders, however, havenot been investigated for their electrochromicproperties.

We discovered that when dispersed onto inert inor-ganic supports such as titania, silica, or aluminananocrystallites of tin oxide heavily doped with anti-mony exhibited surprisingly high levels of elec-trochromism [8]. The contrast ratio of printed reflectivedisplays depends on the antimony doping level, the typeof support, and the synthesis processes and the anneal-ing temperature of the ATO precursor materials. Thesynthesis processes and the structural evolution of ATOnanocrystallites have been reported elsewhere [9]. Inthis paper, we will discuss the design strategy, thedevice performance, and the nanostructure of printed,flexible electrochromic displays using interdigitatedelectrodes.

2. Design strategy of printed display devices

The design objective is to print the display on flexiblepolymer films utilizing commercially viable conductive

inks in an interdigitated electrode structure. Fig. 1ashows a schematic diagram illustrating the cross-sec-tional view of a display consisting of several layers ofprinted materials. An appropriate circuit can be de-signed and screen-printed on a thin polymer film withsilver–carbon ink as working and counter electrodes.Then, a layer of carbon ink is printed, completelycovering the silver–carbon layer to provide corrosionprotection. Without this conforming carbon layer pro-tection, the display can not maintain a long cycle life.Electromigration of silver microcrystals may occur un-der prolonged use of the display device. When silvermicrocrystals interact with other components the dis-play device is degraded.

After printing the conforming carbon layer, the con-necting circuits are then covered with an insulatingmaterial. The actual electrode surface is then printedwith light-colored conductive metal oxide powders dis-persed in a polymer binder. The electrochromic mate-rial can be mixed into, or printed on top of, theconductive metal oxide dispersion. A layer of anaqueous gel electrolyte is then put down before thewhole system is sealed with a transparent polymer film.Fig. 1b shows a backscattered electron image of thecross-sectional view of a display device consisting of aMylar support, Ag/C electrodes, and an ATO–TiO2

electrochromic layer. The total thickness of the elec-trochromic layer and the conductive metal oxide disper-sion layer is about 50 mm.

When an external voltage is applied between theworking and the counter electrodes, electric currentmoves vertically through the conductive metal oxidedispersion to the interface between the electrochromicmaterial and the gel electrolyte. Electrochromic colorchange takes place in this interfacial region. Ionic cur-rent flow through the electrolyte completes the electricalcircuit. If the inter-electrode spacing is kept significantlylarger than the thickness of the metal oxide coatinglayer, then the leakage current within the metal oxidedispersion layer can be minimized. This leakage currentcan be further reduced by printing a thin layer ofinsulator between the working and counter electrodesas illustrated in Fig. 1a.

For this ‘side-by-side’ display design to work, highconductivity of the electrolyte is required to maximizecurrent flow through the desired path. Detailed discus-sions on circuit designs and the selection of materialsfor various printed layers have been previously reported[2]. Sophisticated patterns can be designed using com-mercially available graphic design programs. Desiredpatterns can be transformed into display devices byutilizing specialized screen printing technology with ahigh production rate and low cost. Fig. 2a shows amicrograph of a printed numerical display, revealingthe uncovered Ag/C electrodes, covered connectingleads, and the insulator layer. Fig. 2b shows the same

Fig. 1. (a) Schematic diagram illustrates the ‘side-by-side’ design ofprinted electrochromic display devices. (b) Backscattered electronimage shows the cross-sectional view of a printed display device.

J. Liu, J.P. Coleman / Materials Science and Engineering A286 (2000) 144–148146

Fig. 2. Components of a word display illustrating the printing processof different layers.

area after printing a layer of ATO–TiO2 electrochromicmaterials, covering the whole display device except theconducting leads for external connections.

3. Electrochromic ATO nanocrystallites

The electrochromic layer is the key element in deter-mining the quality of electrochromic devices. Althoughvarious electrochromic materials can be formulated intoprintable ink, we discovered that tin oxide nanocrystal-lites heavily doped with antimony gave surprisinglygood electrochromic properties with high contrast ratioand fast switching rate [8]. The detailed synthesis pro-cesses of ATO nanocrystallites and the structural evolu-tion of these nanophase materials with annealingtemperature and doping level have been reported else-where [8,9].

The electrochromic layer is composed of ATOnanocrystallites coated onto light-color supportingpowders such as titania, silica, alumina, etc. Fig. 3ashows a schematic diagram illustrating ATO nanocrys-tallites highly dispersed on particulate titania. Thisdesign more effectively utilizes the ATO nanocrystal-lites and the ATO-support powders can be easily for-mulated into printable ink with appropriate polymerbinders. Fig. 3b shows a backscattered electron imageof a cross-sectional view of ATO–TiO2 powders, re-vealing the coating of ATO nanocrystallites onto theTiO2 particulate support.

Fig. 3c shows an atomic resolution transmission elec-tron micrograph of an ATO–silica powder. Small ATOcrystallites with an average size of about 4 nm wereclosely packed onto the silica support, forming a con-tinuous coating layer with a thickness ranging from 10to 100 nm. The ATO nanocrystallites encapsulated theparticulate silica support. Because of the smaller size ofthe ATO crystallites, the total surface area of theATO-support powders is very high (:40 m2 g−1).Twins, dislocations, and other types of defects arepresent in individual ATO nanocrystallites. There is nopreferential crystallographic orientation among theATO nanocrystallites. A significant amount of disorderis present at the grain boundaries formed by thesenanocrystallites. The average size of the ATO nanocrys-tallites changes with antimony doping levels and an-nealing temperatures of ATO precursor materials [9].

4. Device performance and discussion

The quality of an electrochromic device depends onmany parameters including the contrast ratio, switchingrate, and cycle life. High contrast ratio is preferred forall commercial display devices. Fig. 4 shows a worddisplay printed with nanostructured ATO–silica elec-trochromic materials: in (a) and (b) the lettering is

Fig. 3. (a) Schematic diagram illustrates dispersion of ATO nanocrys-tallites onto inert inorganic particulate supports such as titania. (b)Backscattered electron image of a cross-sectional view of ATO–tita-nia particulate powders revealing the coating of ATO nanocrystallitesonto titania powders. (c) High-resolution transmission electron mi-crograph of a cross-sectional view of ATO–silica powders revealingthe sizes of, defects within, and grain boundaries between, ATOnanocrystallites.

J. Liu, J.P. Coleman / Materials Science and Engineering A286 (2000) 144–148 147

Fig. 4. Word display using ATO–silica nanostructured elec-trochromic materials (91.5 V).

polarized cathodically and anodically, respectively, at1.5 V. A high contrast ratio is clearly obtained.

The contrast ratio of nanostructured ATO displaysdepends on the ATO loading, the type of support, thedoping level of antimony, the average size of the ATOnanocrystallites, the oxidation state of the antimonydopant, and the spatial distribution of antimony con-taining species [8,9].

Fig. 5b shows variations of the contrast ratio withthe change of antimony doping level for a displaydevice prepared with materials consisting of ATOnanocrystallites supported on TiO2 powders. At lowdoping levels the contrast ratio rapidly increases withthe antimony content. At higher doping levels the con-trast ratio slowly increases to a maximum at about 43mol.% antimony, and then slowly decreases with fur-ther increase of the antimony concentration.

Fig. 5a shows variations of the contrast ratio withthe change of annealing temperature. This displaydevice was prepared using ATO nanocrystallites (43mol.% Sb) supported on alumina powders. The con-trast ratio first increases with the increase of annealingtemperature, reaches a maximum at about 600°C, andthen decreases with further increase of annealing tem-perature. It is clear that the contrast ratio of the printeddisplay device crucially depends on the doping level ofantimony and the annealing temperature of the ATOprecursor materials.

Although transition-metal oxides, especially thinfilms of tungsten trioxide, have been extensively andintensively studied, the fundamental mechanism of theirelectrochromism is still not well understood. The mi-crostructure of these thin films may play a significantrole in determining their electrochromic performance.Different models of the nanostructure of thin films havebeen proposed [10]. For example, depending on thepreparation method, a tungsten trioxide film can beviewed as a nanocomposite rather than a truly amor-phous material. The grain sizes or domains of thenanocomposite film can be a few nanometers orsmaller. It is also suggested that columnar structures inthin films enhance the electrochromic properties ofthese films [10]. The intercolumnar regions may alloweasy transport of ions across the films. The presence ofnanoclusters or nanocrystallites in thin films may alsomodify the band structure of these materials.

The improved electrochromic properties of the ATO-support powder materials can be attributed to theformation of nanocrystallites with a large number ofantimony-rich grain boundaries. At a nanoscopic level,these nanostructured materials provide a high-surfacearea that allows easy transport of mobile ions acrossthe ATO nanocrystallites. The fast switching rate of theprinted displays is attributable to the high surface areaand large number of grain boundaries of nanostruc-tured metal oxide powders because more ATO sites can

Fig. 5. (a) Contrast ratio versus annealing temperature for ATO (43%antimony) supported on alumina. (b) Contrast ratio versus antimonydoping level for ATO supported on titania.

J. Liu, J.P. Coleman / Materials Science and Engineering A286 (2000) 144–148148

be quickly accessed during the switching (charge injec-tion and expulsion) process. Furthermore, the majorityof the antimony dopant may be preferentially located ator near the grain boundaries. An increased level ofelectronic states at these grain boundaries may be re-sponsible for the color formation and high contrastratio observed in ATO electrochromic devices.

Electron microscopy results showed that variationsof the contrast ratio with the change of antimonydoping level and annealing temperature can be corre-lated to the average size of the ATO nanocrystallitesand the intimate mixing of antimony with tin oxide [9].Antimony not only acts as a dopant to modify theelectronic structure of tin oxide but also inhibits thegrowth of tin oxide nanocrystallites at annealing tem-peratures B700°C. At low dopant level or higher an-nealing temperatures, the average size of the ATOnanocrystallites increases significantly. Consequently,the total number of grain boundaries and the amountof antimony available to provide electron charge carri-ers are reduced. Thus, the contrast ratio of the displaydevices is reduced. Furthermore, at higher annealingtemperatures or very high antimony doping levels, anti-mony can migrate out of tin oxide nanocrystallites toform separate phases. This will change the electronicstructure as well as the conductivity of the ATO materi-als. The electrochromic properties of ATO nanostruc-tured materials are optimized at an antimony-dopinglevel of about 43% and an annealing temperature ofabout 600°C.

5. Summary

We have developed a commercially viable process forfabricating electrochromic display devices without usingtransparent electrodes. The simple interdigitated print-

ing process can be easily scaled up for commercialproduction of flexible displays. The nanostructure ofATO-support powders determines the performance ofthis type of electrochromic devices. The enhanced elec-trochromic performance of ATO nanocrystallites is at-tributable to high-surface area, easily accessible sitesthrough grain boundaries, and increased level of elec-tronic states introduced by higher levels of antimonydopant.

Acknowledgements

The authors are grateful to Peter Crozier and JaneTseng for assisting with high-resolution electron mi-croscopy experiments. Part of the electron microscopywork was performed through the Industrial AssociateProgram of Arizona State University.

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