Active Matrix Electronic Paper with
Silver Electrodeposition

Noriyuki KOKEGUCHI＊, Takeshi HAKII＊, Kiyofumi HASHIMOTO＊

Abstract
In recent years, many technologies relating to electronic

１

Introduction

paper and related products have been developed or pro-

An electronic paper is expected as a medium ofposed. But the gulf between the quality of electronic paper

the next generation which has strong points of bothso far achieved and the merits of standard paper still exists.

paper and a display. That is, visibility of paper, andMany technological problems are yet to be solved, and we

re-writability of display. In order to realize the elec-

chose to focus on improving the display quality of electronictronic paper, various technologies have been known,

paper documents, which is very important to document

such as, Ch-LCD (Cholesteric-LCD), PN-LCD (Poly-readability. We developed a 3.5 inch sheet of electronic pa-

mer Network LCD), a reflective LCD, and an electro-per, driven by an active-matrix using silver electrodeposi-

phoretic display. But quality levels of these technolo-tion, and, for an electronic paper, exhibiting the highest lev-gies have not met the customer satisfaction (Fig. 1).

el of whiteness in the world. The electronic paper has a25per, and a contrast ratio of at least 20:1, which together real-20

Contrast䚭ratio

white reflectivity of over 60%, approaching that of copy paize visibility equivalent to copy paper.

for document reading
Reflective
LCD

15

PN-LCDElectrodeposition

Electrophoretic

105

Copied paper

Ch-LCD

Newspaper

00

20

4060

80

100

Reflectivity(%)
Fig.1 Reflectivity vs. contrast ratio of various media

As a display type electronic paper, technical development of the electrophoretic method and theCh-LCD method has preceded, but the reflectivity ofwhite state has been at most 40% and less than thewhiteness of paper (60% or more). On the other
hand, electrochemical methods could theoreticallyrealize a high reflectivity and a high contrast.
Among these methods,

an electrodeposition meth-od exhibited an excellent display potential １〜３）, butthey were using passive-matrix drive, and the method had various problems to be solved for use as adevice for document readings, for instance, driving＊Device Technology R&D Laboratories
KONICA MINOLTA TECHNOLOGY CENTER, INC

64stability, a gray color tone and bi-stability. The active-matrix device using electrochromic dyes has

KONICA MINOLTA TECHNOLOGY REPORT VOL.6（2009）

been known ４）, but this device has showed blue im-so that silver salt electrolyte can permeate into it.

ages against white background, therefore, a black

Polyvinyl alcoholand white display which is important for a documentof TiO2, and does not dissolve in the silver salts

reading was not realized. We will report that theplays a role of fixing the particles

electrolyte.

above-described performance of the display has
been improved by development of a new halogenfree silver salt electrolyte composition and a porouswhite scattering layer, and a black and white electronic paper driven by 3.5 inch active-matrix using
the silver electrodeposition has been developed.

2 Novel Display Structure and Mechanism
Fig. 2 shows a display structure of our silver electrodeposition display. A porous-white scattering layer
is sandwiched between a pair of electrodes, and thislayer incorporates silver salt electrolyte solution. We
used silver in this electorodepsion display, because
deposited silver exhibits pitch black, and further,because a silver ion is monovalent. On the otherFig.3 Sectional view of a porous-white scattering layer

hand, a bismuth ion is trivalent. So the electric power consumption of silver is one-third of that of bismuth. The coloring mechanism is as follows. A silver
atom and a silver ion are reversibly changed by applying voltage of within one point five volt. By an
oxidation-reduction reaction at an electrode interface, appearances of black and white states are reversibly changed. The black state appears when thesilver is deposited, and the white state appears when
the silver is dissolved in electrolyte.
Fig.4 Reflectivity vs. amount of TiO2

㻷㼕㼄㼑㼖㼓㼄㼕㼈㼑㼗㻃㼖㼘㼅㼖㼗㼕㼄㼗㼈㻷㼕㼄㼑㼖㼓㼄㼕㼈㼑㼗㻃㼈㼏㼈㼆㼗㼕㼒㼇㼈㻶㼌㼏㼙㼈㼕㻃㼖㼄㼏㼗㻃㼈㼏㼈㼆㼗㼕㼒㼏㼜㼗㼈

Fig. 4 shows a relationship between the amount oftitanium dioxide and reflectivity at five hundred and㻳㼒㼕㼒㼘㼖㻐㼚㼋㼌㼗㼈㻃㼖㼆㼄㼗㼗㼈㼕㼌㼑㼊㻃㼏㼄㼜㼈㼕fifty nanometers. Reflectivity increases sharply up to
the amount of thirty grams per square meter, and ismostly saturated in the region over the amount. The㻦㼒㼘㼑㼗㼈㼕㻃㼈㼏㼈㼆㼗㼕㼒㼇㼈㻦㼒㼘㼑㼗㼈㼕㻃㼖㼘㼅㼖㼗㼕㼄㼗㼈

porous-white particles are made using the amount oftitanium dioxide of this flat region. So, even if theFig.2 Schematic cross-sectional view of silver electrodeposition display

amount of titanium dioxide varies a little in a manufacturing process, the change of reflectivity is slight.

3Porous-White Scattering Layer
Fig. 3 shows a sectional view of porous-white scat-

The porous-white scattering layer plays an importantrole. When a white pigment is incorporated in theelectrolyte liquid, the dispersion of the white pig-tering layer we developed. We have achieved thement is not fully stable, and then the reflectivity var-highest and constant whiteness of a display with

ies with time. In this case, however, such a reflectiv-

minimum amounts of titanium dioxide. The layer

ity change does not occur, because the titanium

consists of particles of titanium dioxides (TiO2) cov-dioxide is fixed. And because minute filling of the ti-

ered with polyvinyl alcohol, and has a porous form

tanium dioxide is carried out, this system gives theKONICA MINOLTA TECHNOLOGY REPORT VOL.6（2009）65

highest whiteness per unit cross-section area compared to other types of electronic paper.

4 New Halogen-Free Silver Salt Electrolyte
Formula 1 shows a general formula of the newly
developed silver salt solvent. In Formula 1, M represents a hydrogen atom, a metal atom or a quarternary ammonium; Z represents a group of atoms necessary to form a nitrogen containing heterocycle
combined with a carbon atom and a nitrogen atom;
n represents an integer from 0 to 5; and R representsa hydrogen atom or a substituent. Table 1 shows acomparison of the electrolyte compositions. In orderFig.5 Reflection spectrum dependence on applied time (electrolyte 2)

to dissolve silver, a silver halide or an alkali halide
had been used. By having developed these compounds, it became possible to remove a halogenatedcompound from silver salt electrolyte to result in enhanced performances of gray color tone, driving stability and bi-stability as a display.

Formula 1: Newly developed silver salts solvents
Table 1 Comparison of electrolyte composition
Fig.6 Reflection spectrum dependence on applied time (electrolyte 1)

When a silver salt electrolyte composition ischanged, the color tone of deposit silver will change.
It is presumed that the difference in a color tone relates to the size of the particles of deposit silver. In
the case of the electrolyte 2, the mercapto group effectively controls the size. For a black and white

5Reflectivity and Gray Color Tone
Fig. 5 shows reflection spectra of electrolyte 2

when 1.5 V was applied between a pair of electrodes
with changes of the applied time. It has reflectivity

electronic paper, electrolyte 2 is superior to electrolyte 16Bi-stabilityof at least 60% , and contrast ratio of at least 20 orFig. 7 shows a cyclic voltammogram for the silver

more. Moreover, the forms of the visible portion ofredox reaction. A peak accompanying silver reduc-the reflection spectra are mostly flat, resulting in antion (black state) is observed at the lower left. An-excellent gray color tone.
Fig. 6 shows reflection spectra of electrolyte 1 atother peak accompanying silver oxidization (whitethe same condition as the electrolyte 2. The forms

ibility of a driving repetition was excellent. As theof the visible portion of the reflection spectra arepeak shape shows the hysteresis, this electrodeposi-

not flat, leading to a dark reddish-brown color tone.

tion display has bi-stability66state) is observed at the upper right. The reproduc-

KONICA MINOLTA TECHNOLOGY REPORT VOL.6（2009）

paper is under the prototypes. The reflectivity is
63% and the contrast ratio is 20 or more, and thenvisibility equivalent to copied paper is realized. Thefeatures of the prototypes are summarized in Table 2.
Table 2 Display characteristics

Fig.7 Cyclic voltammogram for the silver redox reaction

Fig. 8 shows an evaluation result of bi-stability. The
horizontal axis shows an amount of change from
10% of reflectivity per minute measured with a circuit between opposing electrodes open. The bi-stability of an electrolyte 2 was also improved by about
10 times compared to the electrolyte 1. Therefore,
the removal of halogen from the electrolyte sharply
improved the bi-stability. The improvement in bistability is a very important element for the stable
drive of this electro-deposition display.
Fig.9 I mages of electronic paper (3.5 inch & 113 ppi) with silver
electrodeposition driven by LTPS-TFTs

Fig.8 Bi-stability increase by removing halogen from electrolyte
Fig.10 Image of the prototype when viewed nearly end-on

7Prototype Active Matrix Driving
Fig. 9 shows prototypes of our electronic paper (3.5inch & 113 ppi) with an active-matrix drive using
the newly developed electrolyte. The prototypes arecomposed of a back plane (LTPS-TFT), a porouswhite scattering layer, silver salt electrolyte, a transparent electrode, and a transparent substrate. Thesilver salt electrolyte can penetrate into the porouswhite scattering layer. Between the pair of opposed
electrodes, voltage of -1.5 (black state) or +1.5
(white state) is applied. In figure 9, a sheet of copied

Fig.11 Image of the prototype compared with a newspaper

KONICA MINOLTA TECHNOLOGY REPORT VOL.6（2009）67

Fig. 10 shows an image of the prototype whenviewed nearly end-on. Visibility is good even if
viewed from a different angle from at right angle.
Fig. 11 shows an image of the prototype comparedwith a newspaper. The reflectivity of the white stateis at most 20% brighter than a newspaper.

8ConclusionAn electronic paper of 3.5 inch with an active-ma-trix drive using the silver electrodeposition has beendeveloped. By having developed a new halogen-freesilver salt electrolyte composition, driving stability, agray color tone, and bi-stability have been considerably improved, and having developed porous-white
scattering layer structure, excellent properties (reflectivity of at least 60%, and contrast ratio of atleast 20) realized visibility equivalent to paper. So
this device can display images with the highest
whiteness and contrast as an electronic paper. We
are going to decrease the response time, develop adevice having a large area, and increase the durability.
References

１）H. J. Stocher, et.al., “An SEM Study of the Nature of theElectrochemical Deposit in Reversible Electro- Deposition
Displays”, J. Electrochem. Soc., pp. 746-748, 1981.
２）Micyiya Nakajima, et.al., "Naturally White Electrochromic
Display Using Organic/Inorganic Nanocomposite Pulp", SID
05 DIGEST, pp. 1678-1681, 2005.
３）K . Shinozaki, "Electrodeposition Device for Paper-like
Displays", SID02 DIGEST, pp. 39-41, 2002.
４）Simon W-B Tam, et.al., “The Design and Driving of ActiveMatrix Electrochromic Displays Driven by LTPS TFTs”, SID
06 DIGEST, pp. 33-36, 2006

(An additional note: The contents described in this paper waspresented at Society for Information Display 2008 International
Symposium (Digest of technical papers Vol.39,p.1022))

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KONICA MINOLTA TECHNOLOGY REPORT VOL.6（2009）