ELECTRO-OPTIC DEVICE HAVING ELECTROPHORETIC MEDIUM COMPRISING AN ORGANIC ELECTROACTIVE COMPOUND

Abstract

An electro-optic device is disclosed that comprises an electrophoretic medium including electrically charged pigment particles, a charge control agent, an organic electroactive compound, and a non-polar liquid. The electrophoretic medium is included in a plurality of microcells. The composition of the electrophoretic medium enables the reduction of remnant voltage of the electro-optic device and the degradation of the electrodes and other components of the device even after operation using DC-imbalance waveform.

Claims

1. An electro-optic device comprising: a first electrode layer comprising a light-transmissive electrode; a second electrode layer comprising a plurality of pixel electrodes; a microcell layer disposed between the first electrode layer and the second electrode layer, the microcell layer comprising a plurality of microcells, each microcell of the plurality of microcells comprising an electrophoretic medium, the electrophoretic medium being in contact with the light-transmissive electrode of the first electrode layer and at least one of the plurality of pixel electrodes of the second electrode layer, the electrophoretic medium including: (a) electrically charged pigment particles, (b) a non-polar liquid; (c) a charge control agent; and (d) an organic electroactive compound, wherein the organic electroactive compound is present in the electrophoretic medium in an oxidized form and a reduced form, the oxidized form of the organic electroactive compound is electrochemically reducible at the surface of one of the first and second electrode layers, the reduced form of the organic electroactive compound is electrochemically oxidizable at the surface of one of the first and second electrode layers, and the oxidized form and the reduced form of the organic electroactive compound (i) are soluble in the non-polar liquid of the electrophoretic medium, or (ii) are part of reverse micelle structures that are present in the non-polar liquid of the electrophoretic medium.

2. The electro-optic device of claim 1, wherein the molecular structure of the reduced form of the organic electroactive compound is represented by Formulas I to IX, ##STR00008## ##STR00009## wherein R.sub.1 to R.sub.39 may be hydrogen, substituted or unsubstituted alkyl, alkenyl, or aryl groups, and substituents R.sub.1 and R.sub.2 (taken together), and/or R.sub.3 and R.sub.4, and/or R.sub.5 and R.sub.6, and/or R.sub.6 and R.sub.7, and/or R.sub.7 and R.sub.8, R.sub.9 and R.sub.10, and/or R.sub.11 and R.sub.12, and/or R.sub.14 and R.sub.15, and/or R.sub.15 and R.sub.16, and/or R.sub.16 and R.sub.17, and/or R.sub.19 and R.sub.20, and/or R.sub.21 and R.sub.22, and/or R.sub.25 and R.sub.26, and/or R.sub.26 and R.sub.27, and/or R.sub.27 and R.sub.28, may form a ring; and at least one of R.sub.31 and R.sub.32 is an aryl group, at least one of R.sub.33 and R.sub.34 is an aryl group, and at least one of R.sub.36 and R.sub.37 is an aryl group.

3. The electro-optic device of claim 2, wherein at least one of R.sub.1 to R.sub.4 of Formula I, at least one of R.sub.5 to R.sub.8 of Formula II, at least one of R.sub.9 to R.sub.13 of Formula III, at least one of R.sub.14 to R.sub.18 of formula IV, at least one of R.sub.19 to R.sub.24 of Formula V, at least one of R.sub.25 to R.sub.30 of Formula VI, at least one of R.sub.31 and R.sub.32 of Formula VII, at least one of R.sub.33 to R.sub.35 of Formula VIII, and at least one of R.sub.36 to R.sub.39 of Formula IX is an alkyl or alkenyl group having at least 10 carbon atoms.

4. The electro-optic device of claim 2, wherein at least one of R.sub.9 to R.sub.12 of Formula III, at least one of R.sub.14 to R.sub.17 of formula IV, at least one of R.sub.19 to R.sub.22 of Formula V, at least one of R.sub.25 to R.sub.28 of Formula VI, at least one of R.sub.33 and R.sub.34 of Formula VIII, and at least one of R.sub.36 and R.sub.37 of Formula IX is an alkyl or alkenyl group having at least 10 carbon atoms.

5. The electro-optic device of claim 3, wherein the alkyl or alkenyl group has from 10 to carbon atoms to 100 carbon atoms.

6. The electro-optic device of claim 5, wherein the alkyl or alkenyl group comprises an isoprene dimer or oligomer comprises from 1 to 20 isoprenyl units (CH.sub.2CHC(CH.sub.3)CH.sub.2).

7. The electro-optic device of claim 5, wherein the alkyl or alkenyl group comprises an isoprene dimer or oligomer comprising from 1 to 13 isoprenyl units (CH.sub.2CHC(CH.sub.3)CH.sub.2).

8. The electro-optic device of claim 6, wherein the molecular structure of the reduced form of the organic electroactive compound is represented by Formula X, ##STR00010## wherein R.sub.1, R.sub.2, and R.sub.3 may be hydrogen, alkyl, alkenyl, or alkoxy groups, and n is an integer from 2 to 20.

9. The electro-optic device of claim 8, wherein R.sub.1 and R.sub.2 are methoxy groups, and R.sub.3 is methyl group.

10. The electro-optic device of claim 9, wherein the reduced form of the organic electroactive compound is ubiquinol-10.

11. The electro-optic device of claim 8, wherein R.sub.1 and R.sub.2 are methyl groups, and R.sub.3 is hydrogen.

12. The electro-optic device of claim 11, wherein the reduced form of the organic electroactive compound is represented by Formula XI. ##STR00011##

13. The electro-optic device of claim 1, wherein the molecular structure of the oxidized form of the organic electroactive compound is represented by Formula XII, ##STR00012## wherein R.sub.40, R.sub.41, R.sub.42, R.sub.43, and R.sub.44 may be hydrogen, alkyl, alkenyl, or alkoxy groups, and m is an integer from 2 to 20.

14. The electro-optic device of claim 1, wherein the molecular structure of the oxidized form of the organic electroactive compound is represented by Formula XIII or Formula XIV, ##STR00013## wherein R.sub.45, R.sub.46, R.sub.47, R.sub.48, R.sub.49, R.sub.50, R.sub.51, and R.sub.52 may be hydrogen, alkyl, alkenyl, or alkoxy groups, p is an integer from 2 to 20, and q is an integer from 2 to 20.

15. The electro-optic device of claim 14, wherein the molecular structure of the oxidized form of the organic electroactive compound is represented by Formula XIV, R.sub.48, R.sub.49, R.sub.50, R.sub.51 are hydrogens, and R.sub.52 is methyl.

16. The electro-optic device of claim 15, wherein q is selected from the group consisting of 3, 4, 7, and 9.

17. The electro-optic device of claim 1, wherein the oxidized form of the organic electroactive compound has a reduction potential that is not greater than 1.0 V relative to a standard hydrogen electrode.

18. A method of operating an electro-optic device, the method of operating comprising: providing the electro-optic device according to claim 1; driving the electro-optic device using a DC-imbalanced waveform.

19. The method of operating the electro-optic device of claim 18, wherein the remnant voltage of the electro-optic device is lower than the remnant voltage of a control electro-optic device, the control electro-optic device including a control electrophoretic medium comprising no organic electroactive compound.

20. An electro-optic device comprising: a first electrode layer comprising a light-transmissive electrode; a second electrode layer comprising a plurality of pixel electrodes; a microcell layer disposed between the first electrode layer and the second electrode layer, the microcell layer comprising a plurality of microcells, each microcell of the plurality of microcells comprising an electrophoretic medium, the electrophoretic medium being in contact with the light-transmissive electrode of the first electrode layer and at least one of the plurality of pixel electrodes of the second electrode layer, the electrophoretic medium including: (a) electrically charged pigment particles, (b) a non-polar liquid; (c) a charge control agent; and (d) an organic electroactive compound, wherein the organic electroactive compound is present in the electrophoretic medium at least in one of an oxidized form or a reduced form, the oxidized form of the organic electroactive compound is electrochemically reducible at the surface of one of the first and second electrode layers, the reduced form of the organic electroactive compound is electrochemically oxidizable at the surface of one of the first and second electrode layers, and the oxidized form and the reduced form of the organic electroactive compound (i) are soluble in the non-polar liquid of the electrophoretic medium, or (ii) are part of reverse micelle structures that are present in the non-polar liquid of the electrophoretic medium.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0055] FIG. 1 is a side view of an electro-optic display comprising electrophoretic medium encapsulated in microcapsules from prior art.

[0056] FIG. 2 is a side view of an electro-optic display comprising electrophoretic medium encapsulated in microcells from prior art.

[0057] FIG. 3A is a simplified schematic of the movements of charged species that are present in the electrophoretic medium and the adjacent layers of an electro-optic display in response to an electric field that may result in charge accumulation at the electrode interfaces and in the reduction of the image quality of the display. The figure is disclosed in the prior art.

[0058] FIG. 3B is a simplified schematic of the movements and transformations of important species that are present in the electrophoretic medium and the adjacent layers of an electro-optic display in response to an electric field. The figure is disclosed in the prior art, which is relevant to the mitigation of charge accumulation and remnant voltage at electrode interfaces.

[0059] FIG. 4 is a side view of an example of an electro-optic device according to the present invention.

[0060] FIG. 5 is a scheme of the redox reactions in which an organic electroactive compound is involved.

[0061] FIG. 6 shows the oxidized and reduced forms of ubiquinol-10 and the redox reactions that are involved in the transformation between the two forms.

[0062] FIG. 7 is a scheme of the redox reactions in the electrophoretic medium at an initial period upon the introduction of the reduced form of an organic electroactive compound.

[0063] FIG. 8 is a graph of the electric current versus time flowing through an inventive electrophoretic medium (but without including electrically charged pigment particles) over 48 hours under continuous application of a DC voltage across the liquid.

[0064] FIG. 9 is a graph of remnant voltage versus time that was measured across an inventive electrophoretic medium, but without including electrically charged pigment particles, after application of a DC voltage across the liquid over 48 hours and removal of the applied voltage.

[0065] FIG. 10 is a graph of the measured electric current flowing through a control and an inventive electrophoretic medium versus applied voltage after driving the medium using various DC voltages and removal of the voltage.

[0066] FIG. 11 is a graph of the measured remnant voltage through a control and an inventive electrophoretic medium versus time after driving the medium using various DC voltages and removal of the voltage.

[0067] FIG. 12 is a waveform that was applied to electro-optic device media for measuring the color gamut of the corresponding electrophoretic medium.

[0068] FIG. 13 is the measured color gamut of a control electro-optic device.

[0069] FIG. 14 is the measured color gamut of an inventive electro-optic device.

DETAILED DESCRIPTION OF THE INVENTION

[0070] The term oxidized form referring to an organic electroactive compound is used herein to include the fully oxidized form of the organic electroactive compound and other molecules that are more oxidized than the fully reduced form of the organic electroactive compound. The term reduced form referring to an organic electroactive compound is used herein to include the fully reduced form of the organic electroactive compound and other compounds that are more reduced than the fully oxidized form of the organic electroactive compound.

[0071] The term unsubstituted alkyl group refers to branched and unbranched group containing one or more single CC bonds, and not a double CC bond or a triple CC bond. The term unsubstituted alkyl group includes cyclic groups containing single CC bonds and not a double CC bond or a triple CC bond or an aromatic group. The term unsubstituted alkenyl group refers to branched and unbranched groups containing one or more double CC bonds. The term unsubstituted aryl group refers to a group containing an aromatic ring. The term substituted alkyl group refers to an alkyl group, in which one or more hydrogen is replaced by a substituent, the substituent not being an alkyl group. The substituent may be an alkenyl group, an aryl group, or a group that includes a heteroatom, the heteroatom being an atom of groups V-VII of the periodic table. The term substituted alkenyl refers to an alkenyl group, in which one or more hydrogen is replaced by a substituent, the substituent not being an alkenyl group or an alkyl group. The substituent may be an aryl group or a group that includes a heteroatom, the heteroatom being an atom of groups V-VII of the periodic table. The term substituted aryl refers to an aryl group, in which one or more hydrogen is replaced by a substituent. The substituent may be an alkyl group, an alkenyl group, an aryl group or a group that comprises a heteroatom, the heteroatom being an atom of groups V-VII of the periodic table. Non-limiting examples of heteroatom are nitrogen, phosphorus, oxygen, sulfur, selenium, fluorine, chlorine, bromine, and iodine. Non-limiting examples of groups that comprises a heteroatom are hydroxyl, amino, alkoxy, carboxy, thio, cyano, nitro, and others.

[0072] A typical electro-optic display may have an electro-optic material layer that comprise electrophoretic medium, which is encapsulated in microcapsules or microcells, as described in U.S. Pat. No. 6,982,178. FIG. 1 shows a side view of an example of the basic structure of a portion of an electro-optic display having microcapsules. Electro-optic display 100 comprises a front plane 116, a backplane 118, and an adhesive layer 110 that connects front plane 116 and backplane 118. Front plane 116 comprises a front substrate 102, a light transmissive electrically conductive layer 104 (also called in the literature as first electrode or top electrode or common electrode), optionally a polymeric layer 106, and an electro-optic material layer 108. Backplane 118 comprises a plurality of pixel electrodes 112 and a rear substrate 114. An adhesive layer 110 connects the electro-optic material layer 108 of front plane 116 with the backplane 118. Light transmissive electrically conductive layer 104 acts as the front electrode of the display. The electro-optic material layer 108 comprises a plurality of microcapsules. Each microcapsule has a microcapsule wall and includes electrophoretic medium having charge pigment particles in a non-polar liquid. Typically, the plurality of microcapsules are retained within a polymeric binder. A viewer can view the image of electrophoretic medium of electro-optic display 100 from the side of the front substrate of the display.

[0073] FIG. 2 is a side view of an example of the basic structure of a portion of an electro-optic display having microcells. Electro-optic display 200 has a viewing side 230 and comprises a front substrate 202, a light transmissive electrically conductive layer 204, an electro-optic material layer 208, a sealing layer 209, and a backplane 218, the backplane comprising a plurality of pixel electrodes. The electro-optic material layer 208 comprises a plurality of microcells 230A-D. Each microcell 230A-D of the electro-optic material layer 208 comprises electrophoretic medium 220. Electrophoretic medium 220 comprises electrically charged pigment particles 225 in a non-polar liquid. Each microcell 230A-D has a bottom layer 231, microcell walls 232, and an opening. Microcell walls 232 separate adjacent microcells from each other. Sealing layer 209 spans each opening of the microcells and seals electrophoretic medium 220 inside the microcells.

[0074] In the electro-optic display of FIG. 1, which comprises microcapsules, front substrate 102 is light transmissive. Front substrate 102 may be a plastic film, such as a sheet of poly(ethylene terephthalate) (PET) having thickness of from 25 to 200 m. Although not shown in FIG. 1, front substrate 102 may further comprise one or more additional layers, for example, a protective layer to absorb ultraviolet radiation, barrier layers to prevent ingress of oxygen or moisture into the display, and anti-reflection coatings to improve the optical properties of the display. The same is true for the electro-optic display of FIG. 2, which comprises microcells. That is, in the electro-optic display of FIG. 2, which comprises microcells, front substrate 202 is light transmissive. Front substrate 202 may be a plastic film, such as a sheet of poly(ethylene terephthalate) (PET) having thickness of from 25 to 200 m. Front substrate 202 may further comprise one or more additional layers, for example, a protective layer to absorb ultraviolet radiation, barrier layers to prevent ingress of oxygen or moisture into the display, and anti-reflection coatings to improve the optical properties of the display.

[0075] In the electro-optic displays of FIG. 1, which comprises microcapsules, the light transmissive electrically conductive layer 104 may be a thin continuous coating of electrically conductive material with minimal intrinsic absorption of electromagnetic radiation in the visible spectral range such as indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), graphene or the like. The same is true for the electro-optic display of FIG. 2, which comprises microcells. That is, the light transmissive electrically conductive layer 204 may be a thin continuous coating of electrically conductive material with minimal intrinsic absorption of electromagnetic radiation in the visible spectral range such as indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), graphene or the like.

[0076] The electrophoretic medium of electro-optic display 100 of FIG. 1 and the electrophoretic medium of electro-optic display 200 of FIG. 2 may comprise two types of electrically charged pigment particles. The electrophoretic medium of the displays may comprise three types, four types, five types, or more than five types of electrically charged pigment particles. Each type of electrically charged pigment particles may have a different color. Each type of electrically charged pigment particles may have a positive or negative charge. If the electrophoretic medium comprises two types of electrically charged pigment particles, i.e. a first and second type of electrically charged pigment particles, the first type of the electrically charged pigment particles may be white and negatively charged (or positively charged) and the second type of the electrically charged pigment particles may be black and positively charged (or negatively charged). Upon application of an electric field across the electro-optic material layer via the light transmissive electrically conductive layer and a pixel electrode of the backplane, the white particles of the electrophoretic medium move towards the positive electrode and the black particles move towards the negative electrode, so that an observer viewing the display from the viewing side (the side of front substrate), sees the portion of the display that corresponds to the pixel electrode either white or black, depending upon whether the light transmissive electrically conductive layer is positive or negative relative to the pixel electrode.

[0077] As described in U.S. Pat. No. 6,982,178, adhesive layer 110 of the electrophoretic medium of electro-optic display 100 is disposed between electro-optic material layer 108, which comprises encapsulated electrophoretic medium, and the plurality of pixel electrodes 112 of backplane 118 (see FIG. 1). The adhesive layer 110 enables the construction of an electro-optic display by combining two subassemblies, such as front plane 116 and backplane 118 of FIG. 1. Adhesive layer 110 may be formed by coating an adhesive composition on the surface of electro-optic material layer 108, connecting front plane 116 and backplane 118, and then curing the adhesive composition thermally or via UV curing. In the case of the microcell electro-optic display of FIG. 2, sealing layer 209 is disposed between the plurality of microcells, which include electrophoretic medium 220, and the pixel electrodes 212 of backplane 218.

[0078] Adhesive layer 110 of the microcapsule electro-optic display 100 of FIG. 1 and sealing layer 209 of the microcell electro-optic display 200 of FIG. 2 are in the electrical path separating the pixel electrodes of backplane from the light transmissive electrically conductive layer. Thus, the electrical properties of the adhesive layer of the microcapsule electro-optic display and the electrical properties of the sealing layer of the microcell electro-optic display are important for the operation of the display and they must be carefully tailored. For example, the sealing layer may comprise, in addition to a polymeric material, conductive filler particles to adjust its volume resistivity.

[0079] Polymeric layer 106 of microcapsule electro-optic display 100 of FIG. 1 may be a lamination adhesive layer with similar properties to those described above with reference to adhesive layer 110. However, because polymeric layer 106 is adjacent to the non-pixelated, light transmissive common electrode 104, its electrical conductivity may be higher than that of lamination adhesive layer 110, which is adjacent to the pixelated backplane electrodes 112 and cannot be so conductive as to lead to significant currents flowing from one backplane electrode to its neighbors when they are held at different potentials during switching of the display. When polymeric layer 106 is an adhesive layer, it may be used to affix electro-optic material layer 108 to electrode layer 104 during manufacture of the front plane as described in detail in the aforementioned U.S. Pat. No. 6,982,178.

[0080] FIG. 3A is reproduced from the prior art. Specifically, this figure was published as FIG. 2 of U.S. Pat. No. 9,726,957. It is a simplified illustration of the movements of charged species that are present in the electrophoretic medium of the electro-optic material layer and its adjacent layers of electro-optic display 100 of FIG. 1 in response to an electric field applied by means of electrodes 104 and 112. For convenience, only polymeric layer 106, electro-optic material layer 108 and adhesive layer 110 are shown, although, as will be clear to one of ordinary skill in the art, other layers are present in the display. Mobile charged species in each layer are shown generically as positively charged species A, C and E, and negatively charged species B, D and F. Species A and B in polymeric layer 106 could arise, for example, from an ionic dopant added to enhance the conductivity of polymeric layer 106 or could be present adventitiously in the materials used to form polymeric layer 106. For example, if water is present in polymeric layer 106, species A could correspond to a proton arising from the ionization of the water. Species A may comprise more than one mobile, cationic species. In this discussion, species

[0081] Species A refers to any mobile, positively charged species in polymeric layer 106. Likewise, species E and F refer to positively charged and negatively charged mobile species in adhesive layer 110. Species C and D refer to mobile, charged entities in the electrophoretic medium of electro-optic material layer 108; such entities include charged pigment particles, whose motion changes the optical state of the display, and charged species, whose motion has no direct optical effect, such as micellar charges that are well known in the art.

[0082] Charged species may cross the boundaries between the various layers of the display. This is shown schematically in FIG. 3A, wherein positively charged species E is shown as crossing the boundary between adhesive layer 110 and electro-optic material layer 108. Furthermore, negatively charged species B is shown as crossing the boundary between polymeric layers 106 and electro-optic material layer 108. If charged entities are displaced within their respective layers and cannot cross the boundaries between layers, charge will accumulate at the impermeable boundary, being stored as if in an electrolytic capacitor. After the applied electric field is removed and electrodes 104 and 112 are grounded, discharge of the stored charge at boundaries within the display will occur, changing the electric field experienced by the electrophoretic medium of electro-optic material layer 108 and potentially changing the optical state of the display.

[0083] Even if mobile ionic charges can flow freely across the boundaries between the layers within the display (without accumulating at the internal boundaries), there is still the difficulty that ionic species cannot cross the boundaries between the interior layers of the display and the electrodes 104 and 112. The only likely mechanism for charge transfer across these boundaries is electron transfer, i.e., reduction/oxidation chemical processes. If electron transfer between electrodes 104 and 112 and the interior layers is blocked, ionic charges will inevitably build up at the boundaries between electrode 104 and layer 106 and between layer 110 and electrode 112. If the display is driven with a DC imbalanced drive scheme a substantial charge build-up at these locations may occur. Relaxation of the built-up charge when the electrodes are brought to a common potential may lead to a flow of charge carriers through the electro-optic material layer 108. This flow of charge carriers may lead to a change in the optical state of the display, which may be undesirable.

[0084] The above-mentioned U.S. Pat. No. 9,726,957 disclosed a technology that mitigates the problem of charge accumulation by adding an electrochemically active compound into a layer of the electro-optic display that is adjacent to the electro-optic material layer. The electrochemically active compound is oxidizable or reducicble. The invention of U.S. Pat. No. 9,726,957 is schematically illustrated in FIG. 3B, which was published as FIG. 3 of the patent. As shown in FIG. 3B, charge accumulation at electrode interfaces is mitigated by electrochemical oxidation/reduction reactions involving electron transfer from the electrodes to materials that are included in polymeric layer 106 and adhesive layer 110 of the electro-optic display of FIG. 1. At the cathode, an electron is transferred to reduce a component in the adjacent layer (reduction of neutral species G to provide anion G is illustrated), while at the anode an electron is transferred to the electrode resulting in oxidation of a component in the adjacent layer (electron transfer from neutral species J to produce cation J+ is shown). The injected charges are of the opposite sign to the charges displaced during initial polarization of the display, and thus the charge buildup at the electrode interfaces is mitigated. As indicated in FIG. 3B, when a potential is applied to electro-optic display 100 of FIG. 1, migration of ions towards the electrodes leads to accumulation of charge in thin diffuse layers near the electrodes, the thickness of these layers being of the order of the Debye screening length, as is well known in the electrochemical art. Within these diffuse layers, the gradients in electrical potential are very steep. After a certain time, the potential gradient becomes sufficient to cause electron transfer reactions. The ease of electron transfer is determined by, among other things, the redox potentials of the materials present in the vicinity of the electrodes and their concentrations.

[0085] As mentioned above, in the above-mentioned U.S. Pat. No. 9,726,957, the electrochemically active compound that is used to control charge injection at the electrode interface is incorporated into an external phase layer of the display, that is, in a layer different from the electro-optic material layer, which includes the electrophoretic medium. However, it was found that, in practice, only those components that are within a sufficiently short distance from effective electron transfer to or from the electrodes would be effectively oxidized or reduced. Thus, the components undergoing electron transfer at an electrode might be depleted faster than the time required to be replenished by diffusion onto the electrode surface, given that diffusion of typical electroactive compounds through polymers are too slow to sustain a practical electric current in the display. For this reason, the electro-optic device structure disclosed in U.S. Pat. No. 9,726,957 is not optimal to mitigate the problem of charge accumulation at the electrodes and the resulting remnant voltage.

[0086] The inventors of the present invention surprisingly found that a microcell elector-optic device having an electrophoretic medium in contact with the first electrode layer and at least one of a plurality of pixel electrodes, the electrophoretic material comprising an organic electroactive compound, provides an effective mitigation of remnant voltage following extended DC-imbalanced driving. In another example, an electrophoretic medium in contact with the first electrode layer and all of the pixel electrodes of the plurality of pixel electrodes. The electro-optic device of the present invention comprises (1) a first electrode layer comprising a light transmissive electrode, (2) a second electrode layer comprising a plurality of pixel electrodes, and (3) a microcell layer disposed between the first electrode layer and the second electrode layer. The microcell layer comprises a plurality of microcells, each microcell of the plurality of microcells comprising an electrophoretic medium, the electrophoretic medium being in contact with the light transmissive electrode of the first electrode layer and at least one of the plurality of pixel electrodes of the second electrode layer (or more than one, or all, the pixel electrodes of the second electrode layer). The electrophoretic medium includes (a) electrically charged pigment particles, (b) a non-polar liquid, (c) a charge control agent, and (d) an organic electroactive compound. The organic electroactive compound is present in the electrophoretic medium in an oxidized form and a reduced form. The oxidized form of the organic electroactive compound is electrochemically reducible at the surface of one of the light transmissive electrodes of the first electrode layer and a pixel electrode of second electrode layers. The reduced form of the organic electroactive compound is electrochemically oxidizable at the surface of one of the light transmissive electrodes of the first electrode layer and a pixel electrode of the second electrode layer. The oxidized form and the reduced form of the organic electroactive compound are soluble in the non-polar liquid of the electrophoretic medium. Alternatively, the oxidized form and the reduced form of the organic electroactive compound are included in reverse micelle structures that are present in the electrophoretic medium.

[0087] The content of the organic electroactive compound in the electrophoretic medium may be from 0.1 to 10 weight percent by weight of the electrophoretic medium. The content of the organic electroactive compound in the electrophoretic medium may be from 0.2 to 8 weight percent, 0.3 to 6 weight percent, from 0.4 to 5 weight percent, from 0.5 to 4 weight percent, from 0.6 to 3 weight percent, or from 0.7 to 2 weight percent by weight of the electrophoretic medium.

[0088] Reverse micelle structures in a non-polar liquid (first liquid) are droplets of a second liquid or droplets of a dispersion (particles in a third liquid) that are surrounded by amphiphilic molecules. The droplets form the core of the reverse micelle structures. Amphiphilic molecules are molecules having both a hydrophobic group (head) and a hydrophobic group (tail) in their molecular structure. Typically, the beads of the amphiphilic molecules are oriented towards the core of each reverse micelle structure and the tails are oriented towards the outside surface of the reverse micelle structure, as the tails are hydrophobic and more compatible with the non-polar liquid. The second liquid or the third liquid are less hydrophobic than the tail of the amphiphilic molecule and the non-polar fluid and more compatible with the head of the amphiphilic molecule. The non-polar liquid is called continuous phase of the reverse micelle structures/non-polar liquid mixture.

[0089] FIG. 4 is a side view of an example of an electro-optic device according to the present invention. The electro-optic device 300 comprises a front substrate 202, a first electrode layer 204 having a light transmissive electrode, an electro-optic material layer 208, and a second electrode layer 318. Electro-optic device 300 has a viewing side 230. The second electrode layer 318 comprises a plurality of pixel electrodes 319. The electro-optic material layer 208 comprises a plurality of microcells 230A-D. Each microcell 230A-D of the electro-optic material layer 208 comprises electrophoretic medium 320. Electrophoretic medium 320 comprises electrically charged pigment particles 225 in a non-polar liquid. Microcell walls 232 separate adjacent microcells from each other. In electro-optic device 300, the light transmissive electrode of the first electrode layer 204 and at least one of pixel electrodes 319 of the plurality of pixel electrodes of second electrode layer 318 are in contact with electrophoretic medium 320. More than one pixel electrode 319 of the plurality of pixel electrodes of the second electrode layer 318 may be in contact with the electrophoretic medium. All the pixel electrodes 319 of the plurality of pixel electrodes of the second electrode layer 318 may be in contact with the electrophoretic medium 320. The second electrode layer 318 is typically part of a backplane, the backplane comprising circuitry and a substrate. The fact that the electrophoretic medium is in contact with light transmissive electrode of the first electrode layer and with the pixel electrodes of the second electrode layer means that electro-optic device 300 may not comprise a sealing layer, which is present in commercially available microcell electro-optic displays that comprise electrophoretic media.

[0090] Front substrate 202 of electro-optic device 300 of FIG. 4 may be a plastic film, such as a sheet of poly(ethylene terephthalate) (PET) having thickness of from 25 to 200 m. Front substrate 202 may further comprise one or more additional layers, for example, a protective layer to absorb ultraviolet radiation, barrier layers to prevent ingress of oxygen or moisture into the display, and anti-reflection coatings to improve the optical properties of the display.

[0091] The light transmissive electrode of first electrode layer 204 of electro-optic device 300 of FIG. 4 may be a thin continuous coating of electrically conductive material with minimal intrinsic absorption of electromagnetic radiation in the visible spectral range such as indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), graphene or the like.

[0092] The electrophoretic medium of electro-optic display 300 of FIG. 4 may comprise two types of electrically charged pigment particles. The electrophoretic medium of the display may comprise three types, four types, five types, or more than five types of electrically charged pigment particles. Each type of electrically charged pigment particles may have a different color. Each type of electrically charged pigment particles may have a positive or negative charge. If the electrophoretic medium comprises two types of electrically charged particles, i.e. a first and second type of electrically charged pigment particles, the first type of pigment particles may be white and negatively charged (or positively charged) and the second type of electrically charged pigment particles may be black and positively charged (or negatively charged). Upon application of an electric field across the electro-optic material layer via the light transmissive electrode of the first electrode layer and a pixel electrode of the second electrode layer, the white particles of the electrophoretic medium move towards the positive electrode and the black particles move towards the negative electrode, so that an observer, who views the display from viewing side (the side of front substrate), sees the portion of the display that corresponds to the pixel electrode either white or black, depending upon whether the light transmissive electrode is positive or negative relative to the pixel electrode.

[0093] The electrophoretic medium of electro-optic display 300 of FIG. 4 may further comprise one or more charge control agents (CCA). The CCA may be a low molecular weight surfactant, polymeric agent, or blends of one or more components. The CCA is used to stabilize or otherwise modify the sign and/or magnitude of the charge on the electrically charged pigment particles. The CCA is typically a molecule comprising ionic or other polar functional groups (head groups). At least one of the positive or negative ionic head groups is preferably attached to a non-polar chain (tail group), which is typically a hydrocarbon chain. It is thought that the CCA forms reverse micelle structures in the electrophoretic medium.

[0094] During the operation of the electro-optic device, the organic electroactive compound may be present in the electrophoretic medium in an oxidized form and in a reduced form. The oxidized form of the organic electroactive compound is electrochemically reducible at the surface, or near the surface, of the light transmissive electrode of the first electrode layer or a pixel electrode of the second electrode layer. The reduced form of the organic electroactive compound is electrochemically oxidizable at the surface, or near the surface, of the light transmissive electrode of the first electrode layer or a pixel electrode of the second electrode layer. That is, the oxidized form is electrochemically reversibly reduced to the reduced form and the reduced form is electrochemically reversibly oxidized to the oxidized form. The conversion of the oxidized form to the reduced form and from the reduced form to the oxidized form via redox reactions. Thus, organic electrochemically active compounds may also be called redox materials.

[0095] Although organic electroactive compounds that are capable of at least partial reversible redox reactions are preferred to those undergoing irreversible reactions, the ability to undergo fully reversible redox reactions is not an absolute requirement of the invention. The presence of the reduced form or the oxidized form of the organic electroactive compound at sufficient concentration mitigates premature degradation of the device and reduces the remnant voltage for a period of time.

[0096] A scheme that shows some important elements that are related to the operation of the electro-optic device of the invention, including the redox reactions at the electrodes is presented in FIG. 5. FIG. 5 shows a cross section of a microcell having two electrodes and electrophoretic medium between the electrodes, the electrophoretic medium comprising electrically charged pigment particles and the organic electroactive compound. Typically, the distance between the electrodes are in the range of 5-25 m. The electrophoretic fluid comprises a non-polar liquid, typically having a low dielectric constant, electrically charged pigment particles (P+ and P). The symbols P+ and P indicate that the electrophoretic medium contains positively charged pigment particles. In practice, the electrophoretic medium may contain more than two types of electrically charged pigment particles having charges or various magnitudes. FIG. 5 also shows that the electrophoretic medium contains charged reverse micelle structures M+ and M. These reverse micelle structures may be formed from surfactant-like molecules, such as CCA, which are added into the electrophoretic medium in order to facilitate and maintain pigment charging. When a voltage is applied across the microcells via the electrodes, the positively charged pigment particles and the positively charged reverse micelle structures migrate to the more negatively charged electrode (hereinafter referred to as the cathode), while the negatively charged pigment particles and the negatively charged reverse micelle structures migrate towards the more positively charged electrode (hereinafter referred to as the anode). This migration of charges can screen the electric field that is experienced by any of the electrically charged pigment particles or the charged reverse micelle structures, which are present in parts of the electrophoretic medium that are not near the electrodes. Thus, a potential gradient may be formed in the electrophoretic medium. That is, the potential may be higher near the surface of the electrodes than in spaces way form the surface of the electrodes, as the electrically charged pigment particles and the charged reverse micelle structures migrate, which may enable electron transfer reactions involving components of the electrophoretic medium.

[0097] The organic electroactive compounds of the electrophoretic medium of the present invention are represented in FIG. 5 as Q (oxidized form) and QH2 (reduced form). The oxidized form Q and the reduced form QH2 of the organic electroactive compound form the redox couple of the electrophoretic medium. As shown in FIG. 5, QH2 loses two electrons to form Q and two protons at the anode. At the cathode, Q gains two electrons and with the addition of two protons, the reduced form QH2 is formed. An electric current can be sustained, when the reduced form QH2 that is formed at the cathode diffuses to the anode, and the oxidized form Q that is formed at the anode diffuses to the cathode. These redox reactions at the electrodes prevent the depletion of the redox couple Q/QH2.

[0098] An electro-optic device must be able to be driven in both directions. That is, application of voltage of one polarity should cause migration of positively charged pigment particles to the viewing side, and application of voltage of the opposite polarity should cause migration of negatively charged pigment particles to the viewing side. In the present invention, a current can be sustained whichever polarity is used to address the device. That is, the protons formed at the anode will equilibrate with the negatively charged pigment particles and negatively charged reverse micelle structures that are driven to the anode, while protons are consumed at the cathode (or, equivalently, anionic materials that consume protons are formed at the cathode). Thus, the electrochemical reactions at the electrodes may at least partially neutralize the electrically charged pigment particles and the charged reverse micelle structures that have been migrated to the electrodes as the microcell is initially polarized. This charge neutralization may lead to a reduction in the remnant voltage that would otherwise cause electrical kick-back and image drift when the display is grounded after having been driven. Because electrical current may be sustained indefinitely in the electro-optic device of the present invention, there is no reason to expect that remnant voltage will increase substantially after an initial polarization and an establishment of a steady state. Indeed, the present inventors have surprisingly found that, in a model system, a current can pass through a microcell that contains organic electroactive compounds of the invention for 48 hours without a substantial increase in remnant voltage and without degradation of the electrodes.

[0099] Any compound that (a) has oxidized and reduced forms that may be at least partly reversibly formed and (b) is soluble, or can be part of reverse micelle structures, in the non-polar liquid of the electrophoretic medium may be used in the present invention. Preferred compounds include substituted quinone/hydroquinone pairs (or the corresponding catechols) in which side reactions are minimized by suitable substitution on the benzene ring or rings. One particularly preferred pair of compounds are ubiquinol/ubiquinone, having the structure shown in FIG. 6. Ubiquinol, the structure at the top of FIG. 6, is the reduced form and ubiquinone, the structure at the bottom of FIG. 6, is the oxidized form.

[0100] It is possible that an electrophoretic medium of a device is initially charged with only one of the two forms (oxidized form or reduced form of a redox pair), because the corresponding partner compound will be formed as the device is driven. A reaction scheme of such formation is shown in FIG. 7. At first, as the device is driven, the reduced form of the organic electroactive compound QH2, which was initially included in the electrophoretic medium, is converted to the oxidation for Q at the anode. Thus, even if, initially, there was no Q present at the cathode to sustain a current, another reduction may occur instead, for example, the reduction of a proton from water to form hydrogen ions, as shown in FIG. 7. This reaction may not be reversible. However, as this reaction proceeds in the cathode, more of the oxidized form Q of the organic electroactive compound will be produced and be build up in the microcell until there is sufficient concentration in the electrophoretic medium to sustain a continuous electric current. Thus, the organic electroactive compound may be added into the electrophoretic medium of an electro-optic device in either their reduced or their oxidized form, or a mixture of the two forms. Furthermore, it is possible to add a combination of two (or more) organic electroactive compounds into the electrophoretic medium. For example, the oxidized form of one organic electroactive compound A and the reduced form of another organic electroactive compound B may be used. In this case, it may be desirable that the standard reduction potential of the oxidized form of organic electroactive compound Abe less positive than that of the oxidized form of organic electroactive compound B. This may prevent the reaction between the two organic electroactive compounds.

[0101] As mentioned above, the reduced form of the organic electroactive compound, which is used in the present invention, is oxidizable at the anode. However, it is preferable that this organic electroactive compound does not readily react with oxygen.

[0102] The molecular structure of the reduced form of the organic electroactive compound may be represented by Formulas I to IX. In these formulas, R.sub.1 to R.sub.39 may be hydrogen, substituted or unsubstituted alkyl, alkenyl, or aryl groups, and substituents R.sub.1 and R.sub.2, taken together, and/or R.sub.3 and R.sub.4, and/or R.sub.5 and R.sub.6, and/or R.sub.6 and R.sub.7, and/or R.sub.7 and R.sub.8, R.sub.9 and R.sub.10, and/or R.sub.11 and R.sub.12, and/or R.sub.14 and R.sub.15, and/or R.sub.15 and R.sub.16, and/or R.sub.16 and R.sub.17, and/or R.sub.19 and R.sub.20, and/or R.sub.21 and R.sub.22, and/or R.sub.25 and R.sub.26, and/or R.sub.26 and R.sub.27, and/or R.sub.27 and R.sub.28, may form a ring; and at least one of R.sub.31 and R.sub.32 is an aryl group, at least one of R.sub.33 and R.sub.34 is an aryl group, and at least one of R.sub.36 and R.sub.37 is an aryl group.

[0103] At least one of R.sub.1 to R.sub.4 of Formula I, at least one of R.sub.5 to R.sub.8 of Formula II, at least one of R.sub.9 to R.sub.13 of Formula III, at least one of R.sub.14 to R.sub.18 of formula IV, at least one of R.sub.19 to R.sub.24 of Formula V, at least one of R.sub.25 to R.sub.30 of Formula VI, at least one of R.sub.31 and R.sub.32 of Formula VII, at least one of R.sub.33 to R.sub.35 of Formula VIII, and at least one of R.sub.36 to R.sub.39 of Formula IX may be an alkyl or alkenyl group having at least 10 carbon atoms.

[0104] At least one of R.sub.9 to R.sub.12 of Formula III, at least one of R.sub.14 to R.sub.17 of formula IV, at least one of R.sub.19 to R.sub.22 of Formula V, at least one of R.sub.25 to R.sub.28 of Formula VI, at least one of R.sub.33 and R.sub.34 of Formula VIII, and at least one of R.sub.36 and R.sub.37 of Formula IX may be an alkyl or alkenyl group having at least 10 carbon atoms. The alkyl or alkenyl group may have from 10 to carbon atoms to 100 carbon atoms. The alkyl or alkenyl group comprises an isoprene dimer or oligomer comprises from 1 to 20 isoprenyl units (CH2CHC(CH.sub.3)CH.sub.2). The alkyl or alkenyl group may comprise an isoprene dimer or oligomer comprising from 1 to 13 isoprenyl units (CH2CHC(CH.sub.3)CH.sub.2).

[0105] The molecular structure of the reduced form of the organic electroactive compound may be represented by Formula X. Groups R.sub.1, R.sub.2, and R.sub.3 may be hydrogen, alkyl, alkenyl, or alkoxy groups, and n may be an integer from 2 to 20. Groups R.sub.1 and R.sub.2 may be both methoxy groups, and R.sub.3 may be methyl group. Groups R.sub.1 and R.sub.2 may be both methyl groups, and R.sub.3 may be hydrogen.

[0106] The reduced form of the organic electroactive compound may be ubiquinol-10. The reduced form of the organic electroactive compound may be represented by Formula XI.

[0107] The molecular structure of the oxidized form of the organic electroactive compound may be represented by Formula XII. Groups R.sub.40, R.sub.41, R.sub.42, R.sub.43, and R.sub.44 may be hydrogen, alkyl, alkenyl, or alkoxy groups, and m may be an integer from 2 to 20.

[0108] The molecular structure of the oxidized form of the organic electroactive compound may be represented by Formula XIII or Formula XIV Groups R.sub.40, R.sub.41, R.sub.42, R.sub.43, and R.sub.44 may be hydrogen, alkyl, alkenyl, or alkoxy groups, p may be an integer from 2 to 20, and q may be an integer from 2 to 20. Integer q may be selected from the group consisting of 3, 4, 7, and 9. If the organic electroactive compound is represented by Formula XIV, groups R.sub.48, R.sub.49, R.sub.53, R.sub.50, and R.sub.51 may be hydrogens, and R.sub.52 may be methyl.

[0109] The oxidized form of the organic electroactive compound may have a reduction potential that is not greater than 1.0 V relative to a standard hydrogen electrode. Preferably, the oxidized form of the organic electroactive compound has a reduction potential that is not greater than 0.2 V relative to a standard hydrogen electrode. There are organic electroactive compounds meeting this reduction potential criterion, the organic electroactive compounds being soluble in the non-polar liquid of the electrophoretic medium, so that they can rapidly diffuse through the electrophoretic medium to participate in electrochemical reactions at the electrodes to reduce remnant voltages observed following extended DC-imbalanced driving.

[0110] It is believed, though the invention is in no way limited by this belief that, in the absence of an organic electroactive compound in accordance with the present invention, oxidation of water to form oxygen (a half-cell reaction with a standard reduction potential of more than 1 V) may occur. Because the organic electroactive compound used in the present invention is more easily reduced than water, when such organic electroactive compound is present, less ionic polarization is required to produce a sufficiently steep potential gradient in the electrode double layer for electron transfer to take place, and the remnant voltage experienced by the electro-optic material is consequently lower.

[0111] As mentioned above, the use of an organic electroactive compound in accordance with the present invention is intended to control the build-up of charge near the electrodes. Such a buildup of charge is typically a reversible process. The present invention also seeks to control the nature of the Faradaic reactions that occur at the electrode interfaces, so as to enable DC imbalanced driving of a display without incurring irreversible electrode damage. The organic electroactive compound of the present invention introduces a competitive redox pathway to allow Faradaic reactions to occur at the electrode interfaces without degradation of the electrodes. Without the use of the organic electroactive compound in accordance with the invention, unwanted Faradaic reactions may occur, such as electrolysis of water, leading to the formation of byproducts such as hydrogen and oxygen gas. Even worse, the electrode materials themselves may participate in redox reactions. Some materials that are used in transparent electrodes are prone to oxidation. For example, silver metal nanowires or grids may be readily oxidized to silver cations. Other materials that are widely used in transparent electrodes are prone to reduction. For example, conductive polymers, such as PEDOT:PSS, lose their conductivity when reduced. Indium tin oxide (ITO), another common material used in transparent electrodes, may be irreversibly reduced to metallic tin or metallic indium, leading to discoloration (yellowing) of the transparent electrodes and, eventually, to complete failure. The present invention seeks to introduce competitive redox chemistry to allow Faradaic reactions to occur at the electrode interfaces without degradation of the electrodes. In the case of ITO electrodes, it is desirable that the oxidized form of the organic electroactive compound used in the present invention is more easily reduced than ITO. That is, preferential reduction of the organic electroactive compound preserves the ITO electrode from irreversible electrochemical degradation. On the other hand, it is desirable that the oxidized form of the organic electroactive compound is not very easily reduced that it thermally oxidizes a reduced form, unless such an oxidation produces a new reduced form (i.e., the reduced form corresponding to the oxidized form).

[0112] The electro-optic device of the present invention may be operated by a method of operating that comprises the steps: (a) providing the electro-optic device; and (b) driving the electro-optic device using a DC-imbalanced waveform. The remnant voltage of the electro-optic device is reduced in comparison to the remnant voltage of a control electro-optic device.

[0113] A control electro-optic device is a device that comprises a control electrophoretic medium. A control electrophoretic medium is similar to an inventive electrophoretic medium in composition, but it does not comprise an organic electroactive compound. The inventive electro-optic device comprises a first electrode layer comprising a light-transmissive electrode, a second electrode layer comprising a plurality of pixel electrodes, and a microcell layer, the microcell layer being disposed between the first electrode layer and the second electrode layer. The microcell layer comprises a plurality of microcells, each microcell of the plurality of microcells comprising an electrophoretic medium, the electrophoretic medium being in contact with the light-transmissive electrode of the first electrode layer and at least one of the plurality of pixel electrodes of the second electrode layer. The electrophoretic medium includes (a) electrically charged pigment particles, (b) a non-polar liquid, (c) a charge control agent, and (d) an organic electroactive compound. The organic electroactive compound is present in the electrophoretic medium in an oxidized form and a reduced form. The oxidized form of the organic electroactive compound is electrochemically reducible at the surface of one of the first and second electrode layers. The reduced form of the organic electroactive compound is electrochemically oxidizable at the surface of one of the first and second electrode layers. The oxidized form and the reduced form of the organic electroactive compound are soluble in the non-polar liquid of the electrophoretic medium or are included in reverse micelle structures that are present in the electrophoretic medium.

EXAMPLES

Example 1

[0114] A solution was prepared containing (a) 1 weight % of charge control agents by weight of the solution, (b) 1 weight % of ubiquinol by weight of the solution, and (c) Isopar E. The charge control agent was prepared as described in Example 2 of U.S. Patent Application Pub. 2020/0355978. Ubiquinol-10 is supplied by Biosynth, Staad, Switzerland. The solution was introduced into a cell comprising two opposing glass plates, each coated with a transparent conductive coating of indium tin oxide (ITO) spaced apart by a distance of 25 m, with a surface area of 6.5 cm.sup.2. The cell was sealed using an epoxy composition and then driven with 15V DC for 48 hours at 25 C. During the driving period, the electrical current that passed through the cell was continually measured. After this time, the cell was driven at 0V for 50 ms, following which it was allowed to float and the remnant voltage was measured.

[0115] The electric current flowing during the 48-hour driving period is provided in FIG. 8. The graph of FIG. 8 is the measured electric current versus time. It can be seen from this graph that only a modest decrease in current was observed during the 48-hour driving period. The cell remained transparent throughout the driving period, although ITO degrades, turning yellow, under the same conditions if a control solution is used, wherein control solution comprises only CCA in Isopar E (without the organic electroactive compound). The remnant voltage during 300 s of floating following the brief drive at 0V is shown in FIG. 9, which is a graph of remnant voltage over time. In conclusion, it was shown that continuous direct current can be passed through a model composition, which includes an organic electroactive compound of the present invention) without degrading the electrodes or the model composition itself.

Example 2

[0116] An electrophoretic medium was prepared comprising a white pigment similar to pigment W1, a magenta pigment similar to pigment M1, and a cyan pigment similar to C1 in U.S. Pat. No. 10,678,111. The yellow pigment was a surface-treated Pigment Yellow 180, prepared by dispersion polymerization as described in Example 5 of U.S. Patent Application Pub. 2023/132958. The electrophoretic medium also contained the CCA of Example 1. The total amount of this CCA in the electrophoretic compositions was 1.8 weight % by weight of the electrophoretic medium. The electrophoretic medium also contained 1 weight % of polydimethylsiloxane polymer (PDMS DMS-T72 having molecular weight of approximately 700,000, supplied by Gelest Corporation) by weight of the electrophoretic medium as an image stabilizer. The weight % content of each pigment particle type by weight of the electrophoretic medium is shown in Table 1.

TABLE-US-00001 TABLE 1 Content of Pigment Particles in the Electrophoretic Medium. Pigment Wt % White Pigment 31.5 Cyan Pigment 3.2 Magenta Pigment 2.4 Yellow Pigment 3.2

Example 3

[0117] The electrophoretic medium from Example 2 was separated into two portions 2A and 2B. Sample 2A was loaded into a cell similar to that described above in Example 1, except that the distance between the two electrodes was 10 m and the surface area was 1 cm.sup.2. Into Sample 2B ubiquinol and ubiquinone were added (1 weight % of ubiquinol by weight of the electrophoretic medium and 1 weight % of ubiquinol by weight of the electrophoretic medium) to prepare Sample 3B. Ubiquinol and Ubiquinone are available from AmBeed, IL, USA. Sample 3B was loaded into a cell (same type of cell as the one that Sample 2A was loaded). Cells containing Samples 2A and 3B were driven with DC voltages having amplitude of 1, 5, 15 and 30V. Each voltage was applied for 240 seconds, after which they were driven at 0V for 100 ms. Then, the cells were allowed to float while the remnant voltage was measured. FIG. 10 shows the currents measured for Samples 2A and 3B as a function of the applied voltage, while FIG. 11 shows the remnant voltages measured during the final float state. It can be seen that: (i) the current that passed through Sample 3B (containing the organic electroactive compounds) is higher than the current that passed thought Sample 2A (control), and (ii) the remnant voltage measured in Sample 3B (containing the organic electroactive compounds) is lower than that measured in Sample 2A (control). That is, electrophoretic media comprising an organic electroactive compound according to the invention show improved performance (lower remnant voltage).

[0118] The color gamut of the electro-optic device prepared from electrophoretic media of Sample 2A (control) and Sample 3B (inventive) were also measured. The waveform of FIG. 12, which is the voltage applied to the back plane relative to the front plane, was used to probe the total color gamut (i.e., the volume of all colors addressable by the device). The waveform of FIG. 12 was used for testing purposes, even if it is not a practical waveform for a commercial display. The waveform is built from dipoles, i.e., pairs of pulses of opposite polarity, whose duration and magnitude are systematically varied as shown by the dark envelope in FIG. 12. The voltages explored were +/3.5, 6.1, 9.4, 13.4, 18.2, 23.7, and 30V and the pulse length durations were 50, 80, 120, 190 and 300 milliseconds. For every voltage pair (+,), every pulse duration pair was visited once. This was done in such a way that the pulse duration pair in one dipole had exactly one value changed in the next dipole, and this value was adjacent in the ordered list of pulse duration values. The voltages were explored in a similar fashion. In this way, the waveform was as smooth in its variations as possible, in that successive dipoles were as similar as possible to each other. The reflection spectra were obtained throughout the waveform (not only at its conclusion) and converted to CIEL*a*b* units. The color gamut observed from the electro-optic device that comprised the control electrophoretic medium is shown in FIG. 13 and the color gamut observed from the electro-optic device that comprised the inventive electrophoretic medium is shown in FIG. 14. It can be seen from FIGS. 13 and 14 that the color gamuts of control and inventive devices are very similar. That is, the organic electroactive compounds of the invention can be added to electrophoretic media without significantly affecting their electro-optical performance, but allowing for DC driving with reduced buildup of remnant voltage.

Clauses

[0119] Clause 1: An electro-optic device comprising: a first electrode layer comprising a light-transmissive electrode; a second electrode layer comprising a plurality of pixel electrodes; a microcell layer disposed between the first electrode layer and the second electrode layer, the microcell layer comprising a plurality of microcells, each microcell of the plurality of microcells comprising an electrophoretic medium, the electrophoretic medium being in contact with the light-transmissive electrode of the first electrode layer and at least one of the plurality of pixel electrodes of the second electrode layer, the electrophoretic medium including: (a) electrically charged pigment particles, (b) a non-polar liquid; (c) a charge control agent; and (d) an organic electroactive compound, wherein [0120] the organic electroactive compound is present in the electrophoretic medium in an oxidized form and a reduced form, [0121] the oxidized form of the organic electroactive compound is electrochemically reducible at the surface of one of the first and second electrode layers, [0122] the reduced form of the organic electroactive compound is electrochemically oxidizable at the surface of one of the first and second electrode layers, and [0123] the oxidized form and the reduced form of the organic electroactive compound (i) are soluble in the non-polar liquid of the electrophoretic medium, or (ii) are part of reverse micelle structures that are present in the non-polar liquid of the electrophoretic medium.

[0124] Clause 2: The electro-optic device of clause 1, wherein the molecular structure of the reduced form of the organic electroactive compound is represented by Formulas I to IX, wherein R.sub.1 to R.sub.39 may be hydrogen, substituted or unsubstituted alkyl, alkenyl, or aryl groups, and substituents R.sub.1 and R.sub.2, taken together, and/or R.sub.3 and R.sub.4, and/or R.sub.5 and R.sub.6, and/or R.sub.6 and R.sub.7, and/or R.sub.7 and R.sub.8, R.sub.9 and R.sub.10, and/or R.sub.11 and R.sub.12, and/or R.sub.14 and R.sub.15, and/or R.sub.15 and R.sub.16, and/or R.sub.16 and R.sub.17, and/or R.sub.19 and R.sub.20, and/or R.sub.21 and R.sub.22, and/or R.sub.25 and R.sub.26, and/or R.sub.26 and R.sub.27, and/or R.sub.27 and R.sub.28, may form a ring; and at least one of R.sub.31 and R.sub.32 is an aryl group, at least one of R.sub.33 and R.sub.34 is an aryl group, and at least one of R.sub.36 and R.sub.37 is an aryl group.

[0125] Clause 3: The electro-optic device of clause 2, wherein at least one of R.sub.1 to R.sub.4 of Formula I, at least one of R.sub.5 to R.sub.8 of Formula II, at least one of R.sub.9 to R.sub.13 of Formula III, at least one of R.sub.14 to R.sub.18 of formula IV, at least one of R.sub.19 to R.sub.24 of Formula V, at least one of R.sub.25 to R.sub.30 of Formula VI, at least one of R.sub.31 and R.sub.32 of Formula VII, at least one of R.sub.33 to R.sub.35 of Formula VIII, and at least one of R.sub.36 to R.sub.39 of Formula IX is an alkyl or alkenyl group having at least 10 carbon atoms.

[0126] Clause 4: The electro-optic device of clause 2, wherein at least one of R.sub.9 to R.sub.12 of Formula III, at least one of R.sub.14 to R.sub.17 of formula IV, at least one of R.sub.19 to R.sub.22 of Formula V, at least one of R.sub.25 to R.sub.28 of Formula VI, at least one of R.sub.33 and R.sub.34 of Formula VIII, and at least one of R.sub.36 and R.sub.37 of Formula IX is an alkyl or alkenyl group having at least 10 carbon atoms.

[0127] Clause 5: The electro-optic device according to clause 3 or clause 4, wherein the alkyl or alkenyl group has from 10 to carbon atoms to 100 carbon atoms.

[0128] Clause 6: The electro-optic device according to any one of clause 2 to clause 5, wherein the alkyl or alkenyl group comprises an isoprene dimer or oligomer comprises from 1 to 20 isoprenyl units (CH.sub.2CHC(CH.sub.3)CH.sub.2).

[0129] Clause 7: The electro-optic device according to any one of clause 2 to clause 5, wherein the alkyl or alkenyl group comprises an isoprene dimer or oligomer comprising from 1 to 13 isoprenyl units (CH.sub.2CHC(CH.sub.3)CH.sub.2).

[0130] Clause 8: The electro-optic device according to any one of clause 2 to clause 7, wherein the molecular structure of the reduced form of the organic electroactive compound is represented by Formula X, wherein R.sub.1, R.sub.2, and R.sub.3 may be hydrogen, alkyl, alkenyl, or alkoxy groups, and n is an integer from 2 to 20.

[0131] Clause 9: The electro-optic device of clause 8, wherein R.sub.1 and R.sub.2 are methoxy groups, and R.sub.3 is methyl group.

[0132] Clause 10: The electro-optic device of clause 9, wherein the reduced form of the organic electroactive compound is ubiquinol-10.

[0133] Clause 11: The electro-optic device of clause 8, wherein R.sub.1 and R.sub.2 are methyl groups, and R.sub.3 is hydrogen.

[0134] Clause 12: The electro-optic device of clause 11, wherein the reduced form of the organic electroactive compound is represented by Formula XI.

[0135] Clause 13: The electro-optic device according to clause 1, wherein the molecular structure of the oxidized form of the organic electroactive compound is represented by Formula XII, wherein R.sub.40, R.sub.41, R.sub.42, R.sub.43, and R.sub.44 may be hydrogen, alkyl, alkenyl, or alkoxy groups, and m is an integer from 2 to 20.

[0136] Clause 14: The electro-optic device of clause 1, wherein the molecular structure of the oxidized form of the organic electroactive compound is represented by Formula XIII or Formula XIV, wherein R.sub.45, R.sub.46, R.sub.47, R.sub.48, R.sub.49, R.sub.50, R.sub.51, and R.sub.52 may be hydrogen, alkyl, alkenyl, or alkoxy groups, p is an integer from 2 to 20, and q is an integer from 2 to 20.

[0137] Clause 15: The electro-optic device of clause 14, wherein the molecular structure of the oxidized form of the organic electroactive compound is represented by Formula XIV, R.sub.48, R.sub.49, R.sub.50, R.sub.51 are hydrogens, and R.sub.52 is methyl.

[0138] Clause 16: The electro-optic device of clause 15, wherein q is selected from the group consisting of 3, 4, 7, and 9.

[0139] Clause 17: The electro-optic device according to any one of clause 1 to clause 16, wherein the oxidized form of the organic electroactive compound has a reduction potential that is not greater than 1.0 V relative to a standard hydrogen electrode.

[0140] Clause 18: A method of operating an electro-optic device, the method of operating comprising: (a) providing the electro-optic device according to clause 1; and (b) driving the electro-optic device using a DC-imbalanced waveform.

[0141] Clause 19: The method of operating the electro-optic device of clause 18, wherein the remnant voltage of the electro-optic device is lower than the remnant voltage of a control electro-optic device, the control electro-optic device including a control electrophoretic medium comprising no organic electroactive compound.

[0142] Clause 20: An electro-optic device comprising: a first electrode layer comprising a light-transmissive electrode; a second electrode layer comprising a plurality of pixel electrodes; a microcell layer disposed between the first electrode layer and the second electrode layer, the microcell layer comprising a plurality of microcells, each microcell of the plurality of microcells comprising an electrophoretic medium, the electrophoretic medium being in contact with the light-transmissive electrode of the first electrode layer and at least one of the plurality of pixel electrodes of the second electrode layer, the electrophoretic medium including: (a) electrically charged pigment particles, (b) a non-polar liquid; (c) a charge control agent; and (d) an organic electroactive compound, wherein [0143] the organic electroactive compound is present in the electrophoretic medium at least in one of an oxidized form or a reduced form, [0144] the oxidized form of the organic electroactive compound is electrochemically reducible at the surface of one of the first and second electrode layers, [0145] the reduced form of the organic electroactive compound is electrochemically oxidizable at the surface of one of the first and second electrode layers, and [0146] the oxidized form and the reduced form of the organic electroactive compound (i) are soluble in the non-polar liquid of the electrophoretic medium, or (ii) are part of reverse micelle structures that are present in the non-polar liquid of the electrophoretic medium.