Patent Application
20150331294
The present invention relates to a voltage-switchable nanoparticle-dye complex, to a process for its preparation and to the use of it in electrochromic applications. More in particular, the present invention relates to a voltage-switchable nanoparticle-dye complex, to be used for electrochromic technology in electronic paper technology. The invention also relates to a display panel comprising such nanoparticle-dye complex.
The present invention relates to electronic paper technology, or “E-paper” technology, for which the website www.epapercentral.com/epaper-technologies-guide provides an overview of the various “front plane” and “back plane” technologies that are under development to address this emerging display market.
The prior art electrochromic display (ECD) technology offers the benefits of operation at low voltage (<5 volts), high reflective brightness (uses TiO2 white paint pigment for white state), and bright color. The bright color of ECD technology is due to the intrinsic color associated with the direct absorption of (a limited band of) incident light in the specific electrochromic molecular dye used. Typically, this color is a deep purple or violet for the viologen dyes used. To achieve other colors in an ECD display, overlay color filters are used which combine with the intrinsic color of the dye to cause a combination color to be viewed on the display. Black-white contrast is a desirable, even required, feature of e-book displays such as exists in the popular Kindle eBook. The Kindle, and all versions of it, uses the dominant, competing “electrophoretic display (EPD)” technology which combines electric-charged black and oppositely charged white particles to generate black-white contrast. In EPD technology, when the back plane electrode applies say a positive voltage to the EPD black plane, the positively-charged black particles physically move to the front plane and cause a black pixel (single picture element) to be viewed at the front of the EPD display. Similarly, a negative voltage applied by the backplane electrode moves the white particles to the front plane electrode and the black particles move back to the back plane electrode). One problem with this EPD technology is that the particles tend to “stick” to either the front electrode or back electrode surface thus causing the black-white contrast to appear “grey” over time. Grey pixels offer limited contrast, poor combination colors (unsaturated or “muddy” in appearance), and undesirable “readability” in general. What's needed is a black-white contrast technology for good readability and to provide a basis for good color in e-book displays. A second limitation of current EPD and ECD technology is each technology's relatively slow response speed which limits page refresh rate in e-book readers based on either EPD or ECD technology.
Black-White contrast is a highly desired feature in e-book readers which have as their main purpose to display reading materials which are commonly originated with black-white characters, drawings, and images. Black-white contrast also is necessary to depict colors in high saturation. The present display technology of choice for e-book readers is an electrophoretic technology which “moves” relatively large black or white pigmented particles up or down between two conducting substrates by the action of a medium voltage of 15 volts. EPD technology also suffers from the disadvantage of slow speed since the colored particles must move (drift) through the dispersing medium (e.g. liquid) under the action of an applied voltage between the two conducting substrates. A further limitation is the fact that these colored particles can “stick” to one of the substrate surfaces thus degrading contrast over time. One technology that overcomes the voltage limitation and allowing contrast switching at less than 5 volts is electrochromic display (ECD) technology which applies a low voltage, e.g. 2 volts, to an electrochromic dye, such as viologen, in a redox reaction where the viologen changes charge state by one charge unit which then causes a violet-white contrast to be displayed. A disadvantage of current electrochromic technology is that some of the viologen in an ECD display will undergo a two-charge change in charge state which is irreversible and leaves a ‘brownish” residue in the display, thus destroying both color and contrast over time. Another disadvantage of current electrochromic technology is that a significant charge has to be transported in order to effect color change. As a consequence, high current and therefore high energy is needed to change the image on a current electrochromic display. Furthermore, black-white contrast has also been difficult to achieve.
It will be appreciated that there exists a strong need for further improved display technology, wherein the before mentioned drawbacks have been diminished or eliminated.
An object of the present invention is therefore to provide display systems, which enable low voltage operation at 5V or less, low current change, a real true black-white contrast and a high speed switching. The invention is defined by the independent claims. The dependent claims define advantageous embodiments.
As result of extensive research and experimentation such a display system has now surprisingly been found.
Accordingly, the present invention relates to a voltage-switchable, nanoparticle-dye complex, obtainable by reacting at least ZnO nanoparticles, TiO2 nanoparticles and a bipyridine compound wherein each pyridine ring bears at least one carboxyl or carboxaldehyde group, further on to be indicated by “dye” throughout the specification. Said herein before described voltage-switchable nanoparticle-dye complex enables low voltage operation at 5V or less and more in particular in the range from 2 to 5V, a true black-white contrast and high speed switching. In this connection it is assumed that a single charge transfer causing the contrast change is between two semiconducting nanoparticles and an attached dye between two different semiconducting particles and provides a field-induced color change.
It will be appreciated that said system does not show the contrast degrading double redox reaction which is common in ECD's.
Surprisingly such nanoparticles-dye complex switches reversibly under low voltage drive waveform from a pure white state to a pure black state. A black-white state in a pyridine-based dye is totally unexpected.
Moreover, another important advantage of the present invention is formed by a faster refresh enabling video performance, being a desirable factor in e-book displays. Preferred nanoparticle-dye complexes according to the present invention are those derived from a bipyridine dicarboxylic acid or a bipyridine dicarboxaldehyde as dye. More preferred nanoparticle-dye complexes are derived from symmetrical bipyridine compounds.
It will be appreciated that in the bipyridine dicarboxylic acid or bipyridine dicarboxaldehyde, the respective carboxylic acid groups or carboxaldehyde groups may be present in the ortho, meta or para position, relative to the bonding of the pyridine rings, and preferably are present in the para position.
Specific examples of the nanoparticle-dye complexes according to the present invention are obtained from 2,2′-bipyridine-5,5′-dicarboxylic acid, 2,2′-bipyridine-3,3′-dicarboxylic acid or 2,2′-bipyridine-4,4′-dicarboxaldehyde. More preferred are complexes obtained from 2,2′-bipyridine-5,5′-dicarboxylic acid and 2,2′-bipyridine-3,3′-dicarboxylic acid. Moreover, the size of the starting ZnO and TiO2 particles has been found to be critical.
A preferred range of the average size of starting ZnO particles is from 1 nm to 100 nm. A more preferred range runs from 5 nm to 100 nm and still more preferably the average size is from 10 nm to 100 nm.
A preferred range of the average size of starting TiO2 particles is from 1 nm to 100 nm. A more preferred range runs from 5 nm to 100 nm and still more preferably the average size is from 10 nm to 100 nm.
Said preferred complex of ZnO, TiO2 and 2,2′-bipyridine-5,5′-dicarboxylic acid (bipy-dca) shows a characteristic peak in its Raman spectrum near 1500 cm−1 and in particular between 1500 cm−1 and 1750 cm−1.
Another aspect of the present invention is formed by a composition for use in electrochromic applications in electronic paper technology or “E-paper” technology, which comprises at least one voltage-switchable nanoparticle-dye complex according to the present invention.
More in particular, said composition can be used in the fields of flat panel displays, optical switches and sensors (optical, chemical, thermal). The largest industrial application is in the field of flat panel displays, in particular e-book readers and tablet PCs.
It will be appreciated that the initial small scale testing experiments were performed by a combination of nanoparticle-dye complex and a dispersing medium such as mineral oil, water and an electrolyte.
Said electrolyte may be selected from a variety of chemical compounds such as inorganic alkali metal or alkaline earth metal salts like sodium chloride, potassium chloride; buffer systems derived from fatty acids and their salts such as oleic acid/sodium oleate/sodium chloride; ionic liquids comprising organic salts which are liquid at ambient temperature and are containing an ammonium ion, imidazolium ion, pyridinium ion, piperidinium ion, pyrrolidinium ion, phosphonium ion or sulphonium ion. Examples thereof are 1-alkyl-2-methylpyridinium, 1-alkyl-3-methylpyridinium, 1-alkyl-4-methylpyrridinium, wherein the alkyl group can be selected from ethyl, n-butyl, isobutyl or 1-propylpyridinium, 1-butylpyridinium, 1-ethylpyridinium, 1-hexylpyridinium. Preferred electrolytes are sodium chloride or potassium chloride.
Still another aspect of the present invention is formed by a process for the manufacture of a nanoparticle-dye complex of the present invention. Such a process comprises:
It will be appreciated that the bipyridine dicarboxylic acid dye or bipyridine dicarboxaldehyde dye can be added before or after the heating or both, but according to a preferred embodiment of said process, the bipyridine dicarboxylic acid dye or bipyridine dicarboxaldehyde dye is only added before heating the composition in a coated layer.
According to a particularly preferred embodiment a display coated with said nanoparticle-dye complex is provided by a process, comprising:
It will be appreciated that the applied organic acid can be selected from a variety of acids which are liquid at ambient temperature, such as acetic acid, oleic acid, citric acid, oxalic acid, glycolic acid, lactic acid, uric acid, tosylic acid and the like.
A preferred acid is acetic acid.
An alternative process for the manufacture of a nanoparticle-dye complex of the present invention comprises:
It will be appreciated that the dye can be added before or after the heating step or both, but preferably the dye is only added before heating the composition in a coated layer. According to a particularly preferred embodiment a display coated with said nanoparticle-dye complex is provided by a process comprising:
The hereinbefore mentioned organic aprotic polar solvent may be selected from halogenated hydrocarbons, ethers, sulfoxides and nitrile compounds.
More specific examples are dichloromethane, trichloromethane (chloroform), tetrahydrofuran (THF), acetonitrile and dimethylsulfoxide (DMSO).
A more preferred solvent is DMSO.
Another aspect of the present invention is formed by a display panel comprising:
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
In the drawings:
The present invention is further illustrated by the following examples, however without restricting its scope to these specific embodiments.
A ZnO—TiO2 paste was made by ultrasonically mixing for 3 minutes, 100 nm ZnO nanoparticles and an equal quantity of 26 nm TiO2 nanoparticles blended with an amount by volume of 5% glacial acetic acid (pH˜3-4). The mixture was allowed to settle leaving a white paste covered by a thin layer of acetic acid. The excess acetic acid was siphoned off, and the remaining paste was coated onto the ITO pixel of a front plane ITO glass substrate as follows:
Thin, Scotch tape was placed on the ITO side of the front plane ITO glass substrate to define the thickness and outline of ZnO—TiO2 paste layer.
The ZnO—TiO2 paste was coated and knifed to thickness using the edge of a glass slide. The ZnO—TiO2 paste/ITO/glass substrate was then heated to 130° C. for 30 minutes on a hotplate, well below the temperature used to fire a typical ceramic coating.
After cooling, a few drops of a 1% dye suspension of bipy-dca (1% of 2,2′-bipyridine-5,5′-dicarboxylic acid, denoted as a Linker dye, in DI water) is added to the ZnO—TiO2 coated ITO front plane and allowed to soak for 30 minutes, and then dried for 30 minutes at 80° C. Alternatively, the Linker dye can be added to the ZnO—TiO2/acetic acid paste that is coated onto the ITO front plane and baked at 130° C. in the previous step.
This ZnO—TiO2-dye paste can also be applied to the backplane (ITO or metal). Before assembly and edge sealing, the oil-salt-water solution is added. Optionally, a small amount of Linker dye solution may be added when the oil-salt-water solution is added. The backplane ITO electrode was prepared as follows:
A thin coat of ZnO—TiO2 paste was knife coated onto the ITO backplane and dried on a hotplate at 130° C. for 30 minutes.
Just before laminating the backplane to the front plane, an edge seal of epoxy adhesive was applied to the front plane. A KCl electrolyte composed of mineral oil, water and 5% KCl was formulated and a few drops were placed inside the seal area of the front plane. The backplane was laminated to the front plane, weighted down on a hotplate and heat cured at 80° C. for 2 hours, and allowed to cool before testing.
The test cell structure is depicted in
The formation of the ZnO-bipy-dca-TiO2 complex was confirmed by the Raman spectrum, near 1500 cm−1 (between 1500 cm−1 and 1750 cm−1), was found to be changed in comparison to the Raman spectrum of a ZnO—TiO2 nanoparticle complex.
Said ZnO-bipy-dca-TiO2 complex showed a true black-white contrast and high speed reversibly switching under low voltage (<5 volts) from a pure white state to a pure black state. Such a black-white state in a pyridine-based dye is totally unexpected to a person skilled in the art. Moreover, said system offered a faster refresh and improved readability.
In the same way as described in Example 1, a ZnO-Linker dye-TiO2 was prepared and tested, wherein the Linker dye was 2,2′-bipyridine-3,3′-dicarboxylic acid.
In the same way as described in Example 1, a ZnO-Linker dye-TiO2 was prepared and tested wherein the Linker dye was 2,2′-bipyridine-4,4′-dicarboxaldehyde.
A ZnO—TiO2 paste was made by ultrasonically mixing for 3 minutes 100 nm ZnO nanoparticles and an equal quantity of 26 nm TiO2 nanoparticles.
This mixture was blended with an amount by volume of 5% glacial acetic acid (pH up to 4) and a few drops of 1% by volume of a solution of 2,2′-bipyridine-5,5′-dicarboxylic acid denoted as Linker dye in DMSO were added.
The mixture was allowed to settle, leaving a paste by a thin layer of acetic acid and solvent.
The excess of solvent and acetic acid was siphoned off and the remaining paste was coated onto the ITO of a front plane ITO glass substrate as follows:
Thin Scotch tape was placed on the ITO side of the glass substrate to define the thickness and outline of the paste layer.
the ZnO-Linker dye-TiO2 paste was coated and knifed to thickness using the edge of a glass slide.
The Zn—O-Linker dye-TiO2 paste /ITO/ glass substrate was then heated to 130° C. for 30 minutes on a hotplate, well below the temperature used to fire a typical ceramic coating. Said ZnO-Linker dye-TiO2 paste was also applied to the backplane (ITO or metal)
The back plane ITO electrode as prepared as follows:
A thin coating of e ZnO-Linker dye-TiO2 paste was knife coated onto the ITO backplane and dried on a hotplate at 130° C. for 30 minutes.
Just before laminating the backplane to the front plane, an edge seal of epoxy adhesive was applied to the front plane. An electrolyte composed of mineral oil, water and 5% KCl was formulated and a few drops were placed inside the seal area of the front plane. The backplane was laminated to the front plane, weighted down on a hotplate and heated at 80° C. for two hours, and allowed to cool before testing.
The formation of the ZnO-Linker dye-TiO2 complex was confirmed by the Raman spectrum near 1500 cm−1 (between 1500 cm−1 and 1750 cm−1), was found to be changed in comparison to the Raman spectrum of a ZnO—TiO2 nanoparticle.
Said ZnO-Linker dye-TiO2 complex showed a true black-white contrast and high speed reversibly switching under low voltage (<5 volts) from a pure white state to a pure black state. Moreover, said system offered a faster refresh and improved readability.
In the same way as described in Example 4 a ZnO-Linker dye-TiO2 was prepared and tested, wherein the Linker dye was 2,2′-bipyridine-3,3′-dicarboxylic acid.
The test structure of
In display technology a rough distinction may be made between in-plane technology and out-of-plane technology. In-plane technology implies that the addressing electrodes for each pixel are located on the same substrate (either front plate or back-plate), i.e. are located in plane. Out-of-plane technology implies that the addressing electrodes for each pixel are located on opposite substrates (i.e. one on the front plate and one of the back plate). The next figures are all simplified in order to facilitate the understanding of the invention. For instance, only a single pixel in a monochromatic display is illustrated in each figure, with the exceptions of
The insulating layer 70 is separated from the switchable layer 50 by the electrolyte 60 (similar to
The embodiments shown in
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or an preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12154610.5 | Feb 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/052550 | 2/8/2013 | WO | 00 |
