Electrochromic display

Details Electrochromic display Electrochromic display

2000-09-01

2004-08-31

Spector, David N. (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S268000, C359S269000, C359S270000, C359S274000

Reexamination Certificate

active

06785036

ABSTRACT:

The present invention relates to electrooptical displays, particularly those having large switchable areas in the form of electrically switchable mirrors, windows or display elements.
In principle, electrooptical displays are constructed of two parallel capacitor plates which together with a sealing frame located between them define a flat chamber in which there is a medium which changes its optical anistropy, its absorption behaviour and/or its colour under the action of a voltage applied to the capacitor plates. At least one of the capacitor plates comprises a light-transparent material, for example glass or plastic (e.g. polycarbonate) which is provided on the side which is in contact with the electrochromic medium with an electrically conductive, light-transparent layer, in particular indium-tin oxide (ITO).
Such displays are known in principle from U.S. Pat. No. 3,451,741, EP-A 240 226 and WO 94/23 333.
In the case of large electrooptical switching areas in which the electrooptical medium additionally has a certain conductivity, particularly in the case of electrochromically switchable areas, irregularities in the coloration of the area occur. On the one hand, it is then often not possible to produce large areas having a uniform intensity of colour. On the other hand, on switching, the coloration spreads like a wave across the area. Such wave-like colour intensity fluctuations sometimes display the character of vibrations which occur even independently of the switching process if the power, as is customary, is supplied via the edge of the switching area.
These irregularities in the coloration have been able to be attributed to the comparatively high specific resistance of the transparent, conductive layers which is typically from 1 to 200 ohm per square.
It is known from FR 2 689 655, EP 618 477 and EP 294 756 that metal strips or metal grids can be applied to the counterelectrode of electrochromic cells based on electrochromic films. In the case of electrochromic cells based on liquid electrochromic media and having two equivalent electrodes, such a structuring of one or both electrodes has not previously been proposed since it was not clear a priori whether preferred current paths through the liquid medium would be formed so that, on application of a voltage, a more distinct coloration would occur in the region of the grid than between the grid lines.
The display cell of the invention contains an electrochromic medium which can be a liquid, a gel or a solid. Preference is given to liquids and gels. They contain one or more electrochromic substances, a solvent, one or more UV absorbers, if desired one or more conductance salts and, if desired, one or more thickeners.
Electrochromic substances which are suitable for the purposes of the invention are pairs of redox substances of which one is reducible and the other is oxidizable. Both are colourless or only slightly coloured. On application of a voltage to the display, one substance is reduced and the other is oxidized, with at least one becoming coloured. After switching off the voltage, the two original redox substances are formed again and the display decolorizes.
It is known from U.S. Pat. No. 4,902,108 that suitable pairs of redox substances are those in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least two chemically reversible oxidation waves. Such substances are suitable for the purposes of the invention.
WO 97/30134 discloses redox systems in which RED
1
and OX
2
or OX
1
and RED
2
are covalently bound via a bridge B. Such substances are likewise suitable for the purposes of the invention.
Likewise suitable for the purposes of the invention are redox systems in which the reversible transition between RED and OX or vice versa is associated with the breaking or the formation of a &sgr; bond. Such substances are known, for example, from WO 97/30135.
Also suitable for the purposes of the invention are metal salts or metal complexes of those metals which exist in at least two oxidation states. The two oxidation states advantageously differ by 1.
Likewise suitable for the purposes of the invention are oligomers and polymers which contain at least one of the redox systems mentioned or else pairs of such redox systems as are defined above.
Likewise suitable for the purposes of the invention are mixtures of the above-described substances, provided that these mixtures contain at least one reducible and at least one oxidizable redox system.
Suitable solvents, conductance salts, UV absorbers and thickeners are likewise known from the above-cited patent applications and can be utilized for the purposes of the invention.
According to the invention, it is now proposed that a pattern of strips or a grid made of a material having metallic conductivity be located above or below the transparent, electrically conductive layer of the display cell of the invention.
The pattern of strips or grid serves to even out local resistance fluctuations in the transparent, electrically conductive layer and to avoid voltage decreases in the layer. Surprisingly, this does not result in the formation of preferred current paths, but instead a uniform and rapid, uniform coloration of the switching area is produced.
The present invention accordingly provides an electrochromic display element which contains an electrochromic liquid between two electrode sheets, where at least one of the electrode sheets is transparent and has a transparent, electrically conductive layer, characterized in that the transparent electrode sheet or sheets has/have a pattern of strips or grid made of a material having metallic conductivity. Preferably less than 10%, particularly preferably less than 3%, of the display switching area is covered by the pattern of strips or grid having metallic conductivity.
The width of the strips and their spacing is advantageously determined as a function of the distance from which the display is viewed, as derived from the intended purpose of the display. The width of the metallic strips is advantageously selected such that the strips are not perceived by the viewer. If the display is configured as an electrically dimmable rear view mirror for motor vehicles, the strips preferably have a width of from 10 to 50 &mgr;m. Wider strips can be accepted if the display is configured as, for example, a display board in airports or the like where the viewing distance is large.
In the case of such large-area displays, it is also possible to employ regular grids in which the line spacing is greater than 3 mm.
Typically, the ratio of the width of the strips to the distance between the strips can be from 1/10 to 1/10,000, preferably from 1/10 to 1/1000, particularly preferably from 1/10 to 1/100.
The metal grid can be applied in various ways. It can be applied to the substrate by sputtering, vapour deposition including chemical vapour deposition (CVD) or by adhesive bonding of a finished grid. The metal itself should have a sufficient conductivity which should be greater than 10
4
/ohm.
This grid is coated with a layer of a metal oxide, with preference being given to transparent materials having good conductivity, e.g. In
2
O
3
or SnO
2
or ITO or ZnO. These metal oxides can also be doped with traces of fluorides, antimony or aluminium to improve the conductivity.
The substrate has the function of supporting the conductive layer. In addition, it has to be transparent and serve as a barrier against chemical reactants of the electrochromic medium, in particular oxygen and water. Suitable substrates are glass or plastics, including in flexible form. The plastics in particular can be coated with layers which greatly reduce the permeation of the materials in question. This coating can be ITO of sufficient thickness or else comprise layers of non-conducting dielectrics such as TiO
2
.
As an inexpensive alternative to coating the grid with a metallic oxide, it is also possible to use the reverse procedure, namely applying the metallic gri

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