Patent Application
20200089035
The present invention relates to backplanes for electro-optic displays. More specifically, it is related to non-traditional pixel shapes, such as hexagonal or triangular. Many displays (LCD and electrophoretic) use arrays of electrodes to present pixels of various colorations that are perceived by a viewer as an image. Such pixel arrays traditionally use rectangular or square electrodes, sometimes with edge structures. In this case, each electrode has four nearest neighbor electrodes joined by an edge. Because of the array structure, the pixels can be quickly and individually addressed using scan and gate lines, along with coordinated controllers. Similar structures can be used for non-display purposes, such as particle sensing and electrowetting on dielectric (EWoD).
The term “electro-optic” as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range. While this application refers to devices as electro-optic devices, generally, it is to be understood that the same structure may be used for a non-optical, i.e., non-display application, such as particle sensing or electrowetting on dielectric (EWoD).
The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the E Ink patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. Indeed, as already mentioned, the change in optical state may not be a color change at all. The terms “black” and “white” may be used hereinafter to refer to the two extreme optical states of a display, and should be understood as normally including extreme optical states which are not strictly black and white, for example the aforementioned white and dark blue states. The term “monochrome” may be used hereinafter to denote a drive scheme that only drives pixels to their two extreme optical states with no intervening gray states.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published US Patent Application No. 2002/0180687 (see also the corresponding International Application Publication No. WO 02/079869) that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
The term “impulse” is used herein in its conventional meaning of the integral of voltage with respect to time. However, some bistable electro-optic media act as charge transducers and with such media an alternative definition of impulse, namely the integral of current over time (which is equal to the total charge applied) may be used. The appropriate definition of impulse should be used, depending on whether the medium acts as a voltage-time impulse transducer or a charge impulse transducer.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in these patents and applications include:
The present invention provides efficient designs for pixel arrays with non-traditional shapes that can employ traditional scan and gate lines, controllers, shift registers, etc.
The invention provides backplanes having an array of hexagonal electrodes or an array of triangular electrodes. Because the backplane designs route the gate lines along the periphery of the electrodes there is less cross talk with the surface of the electrode. These designs simplify construction and control of the electrodes and improve the regularity of the electric field above the electrode. Such electrode designs may be particularly useful in particle sensing and EWoD applications, however, there is no reason that the designs cannot be used in a more traditional display, such as an LCD display or an electrophoretic display (EPD).
A benefit of the backplane electrode structures disclosed is that they are easily coupled to standard controllers and can result in simple pinouts that are plug-and-play with existing ecosystems. This reduces the complexity of the substrates as well as interfaces with printed circuit boards, thereby reducing costs.
In one aspect, the invention provides a pixel electrode backplane including a plurality of scan lines, a plurality of gate lines, a plurality of storage capacitors, a plurality of thin film transistors, and a plurality of hexagonal electrodes. Typically, the storage capacitors are larger than 0.5 pF. In the invention, the hexagonal electrodes are arranged in a honeycomb structure, and the voltage potential of each hexagonal electrode is controllable with only one scan line and only one gate line. In some embodiments, the plurality of scan lines is coupled to a scan controller and the plurality of gate lines is coupled to a gate controller. In some embodiments, the gate lines are routed parallel to the edges of the hexagonal electrodes.
In another aspect, the invention provides a pixel electrode backplane including a plurality of scan lines, a plurality of gate lines, a plurality of storage capacitors, a plurality of thin film transistors, and a plurality of triangular electrodes. Typically, the storage capacitors are larger than 0.5 pF. In the invention, two triangular electrodes or four triangular electrodes are arranged as a square and the voltage potential of each triangular electrode is controllable with only one scan line and only one gate line. In some embodiments, the plurality of scan lines is coupled to a scan controller and the plurality of gate lines is coupled to a gate controller. In some embodiments, the gate lines are routed parallel to the edges of the triangular electrodes. In some embodiments, the scan lines are routed parallel to the edges of the triangular electrodes.
The functionality of the backplane electrodes can be expanded by disposing a dielectric layer over the pixel electrodes, as well as a hydrophobic layer over the dielectric layer. Such coated pixel electrode backplanes can be incorporated into microfluidic devices by adding a light-transmissive electrode and a spacer disposed between the pixel electrode backplane and the light-transmissive electrode.
In other embodiments, the backplane electrodes can be the basis for controlling an electrophoretic display (EPD) by adding a light-transmissive electrode and disposing an electrophoretic medium (typically charged particles in a non-polar solvent) between the backplane electrodes and the light-transmissive electrode.
The invention provides for arrays of hexagonal and triangular electrodes that individually addressable with conventional scan/gate driving. Such arrays may be useful in creating displays, such as liquid crystal displays (LCD) or electrophoretic displays (EPD). Such arrays may also be useful in non-display applications, such as particle (e.g., photon) sensors or electrowetting on dielectric (EWoD), which may be used for microfluidic applications such as lab-on-a chip assays.
Creating digital pictures requires the ability to address individual spatial elements (pixels) quickly. In some instances, each pixel is its own color source (e.g., full color electrophoretic displays described in U.S. Pat. No. 9,921,451), in other instances, several sub-pixels act in concert to give the illusion of a range of colors for the pixel. While it is possible to wire each pixel to control the voltage state individually, it is more common to provide an array of non-linear elements, such as transistors or diodes, with at least one non-linear element associated with each pixel (or sub-pixel), to produce an “active matrix” display. An addressing or pixel electrode, which addresses one pixel, is connected to an appropriate voltage source through the associated non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode could be connected to the source of the transistor. Conventionally, in high-resolution arrays, the pixels are arranged in a two-dimensional array of rows and columns, such that any specific pixel is uniquely defined by the intersection of one specified row and one specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; again, the assignment of sources to rows and gates to columns is conventional but essentially arbitrary, and could be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that at any given moment only one row is selected, i.e., that there is applied to the selected row electrode a voltage such as to ensure that all the transistors in the selected row are conductive, while there is applied to all other rows a voltage such as to ensure that all the transistors in these non-selected rows remain non-conductive. The column electrodes are connected to column drivers, which place upon the various column electrodes voltages selected to drive the pixels in the selected row to their desired optical states. (The aforementioned voltages are relative to a common front electrode that is conventionally provided on the opposed side of the electro-optic medium from the non-linear array and extends across the whole display.) After a pre-selected interval known as the “line address time” the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed to that the next line of the display is written. This process is repeated so that the entire display is written in a row-by-row manner. (Throughout this document, the gate lines are horizontal and the scan lines are vertical.) Because driving electronics are typically arranged as a matrix, the pixel electrodes coupled to the driving electronics are commonly square or rectangular and arranged in a rectangular array in order to maximize the area covered by the electrodes.
Processes for manufacturing active matrix displays are well established. Thin-film transistors, for example, can be fabricated using various deposition and photolithography techniques. A transistor includes a gate electrode, an insulating dielectric layer, a semiconductor layer and source and drain electrodes. Application of a voltage to the gate electrode provides an electric field across the dielectric layer, which dramatically increases the source-to-drain conductivity of the semiconductor layer. This change permits electrical conduction between the source and the drain electrodes. Typically, the gate electrode, the source electrode, and the drain electrode are patterned. In general, the semiconductor layer is also patterned in order to minimize stray conduction (i.e., cross-talk) between neighboring circuit elements.
Liquid crystal displays commonly employ amorphous silicon (“a-Si”), thin-film transistors (“TFTs”) as switching devices for display pixels. Such TFTs typically have a bottom-gate configuration. Within one pixel, a thin film capacitor typically holds a charge transferred by the switching TFT. Electrophoretic displays can use similar TFTs with capacitors, although the function of the capacitors differs somewhat from those in liquid crystal displays; see the aforementioned copending application Ser. No. 09/565,413, and Publications 2002/0106847 and 2002/0060321. Thin film transistors can be fabricated to provide high performance. Fabrication processes, however, can result in significant cost.
In TFT addressing arrays, pixel electrodes are charged via the TFTs during a line address time. During the line address time, a TFT is switched to a conducting state by changing an applied gate voltage. For example, for an n-type TFT, a gate voltage is switched to a “high” state to switch the TFT into a conducting state.
It should be appreciated that the backplanes described herein may be extended to an electro-optic display comprising a layer of electro-optic medium disposed on the backplane and covering the pixel electrode. Such an electro-optic display may use any of the types of electro-optic medium previously discussed; for example, the electro-optic medium may be a liquid crystal, a rotating bichromal member or electrochromic medium, or an electrophoretic medium, preferably an encapsulated electrophoretic medium. In some embodiments, when an electrophoretic medium is utilized, a plurality of charged particles can move through a suspending fluid under the influence of an electric field. Such electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays.
The backplanes described herein may also be used for electrowetting on dielectric (EWoD). An EWoD device typically includes a cell filled with an oil and at least one aqueous droplet. The cell gap is typically in the range 50 to 200 μm, but the gap can be larger. In a basic configuration, a plurality of propulsion electrodes (pixels) are disposed on one substrate and a singular top electrode is disposed on the opposing surface. The cell additionally includes hydrophobic coatings on the surfaces contacting the oil layer, as well as a dielectric layer between the propulsion electrodes and the hydrophobic coating. (The upper substrate may also include a dielectric layer). The hydrophobic layer prevents the droplet from wetting the surface. When no voltage differential is applied between adjacent electrodes, the droplet will maintain a spheroidal shape to minimize contact with the hydrophobic surfaces (oil and hydrophobic layer). Because the droplets do not wet the surface, they are less likely to contaminate the surface or interact with other droplets except when that behavior is desired. By individually addressing the electrodes in the active matrix, it is possible to move droplets of water, split the droplets, and merge droplets. Because the water droplets are compatible with biological molecules, it is possible to perform bioassays on minute quantities of samples. Conventional rectangular arrays can limit functionality in EWoD devices, because the only options are up/down/left/right whereas a hexagonal electrode of the invention has six nearest neighbor electrodes for more directions of movement and forces can be applied at 60°, 120° and linearly.
The layout of the controller for the hexagonal array is shown in
The layout of the controller for the triangular array is also shown in
Of course the arrangements of the electrodes in the figures is exemplary, and the geometrical dimensions of the pixel electrode and/or the gate line and/or scan lines can be modified for special applications or as the number of pixels is limited by available scan controllers or backplane size. For example, the size of the pixel electrode may be reduced to enlarge the gap space between the electrode and the data line. In some other embodiments, the electrical properties of the material between the pixel electrode and the data line may be altered to reduce crosstalk. For example, one may increase the thickness of the insulating thin film between the pixel electrode and its neighboring data lines to reduce capacitive coupling.
In some embodiments, backplanes of the invention can be incorporated into an electrowetting on dielectric (EWoD) device including a backplane of the invention coupled to a light-transmissive electrode and separated by a spacer. The fundamental operation of an EWoD device is illustrated in the sectional image of
The dielectric 908 must be thin enough and have a dielectric constant compatible with low voltage AC driving, such as available from conventional image controllers for LCD displays. For example, the dielectric layer may comprise a layer of approximately 20-40 nm SiO2 topped over-coated with 200-400 nm plasma-deposited silicon nitride. Alternatively, the dielectric may comprise atomic-layer-deposited Al2O3 between 2 and 100 nm thick, preferably between 20 and 60 nm thick. The TFT is constructed by creating alternating layers of differently-doped a-Si structures along with various electrode lines, with methods know to those of skill in the art. The hydrophobic layer 907 can be constructed from materials such as Teflon® AF (Sigma-Aldrich, Milwaukee, Wis.) and FlurorPel™ coatings from Cytonix (Beltsville, Md.), which can be spin coated over the dielectric layer 908.
While it is possible to have a single layer for both the dielectric and hydrophobic functions, such layers typically require thick inorganic layers (to prevent pinholes) with resulting low dielectric constants, thereby requiring more than 100V for droplet movement. To achieve low voltage actuation, it is better to have a thin inorganic layer for high capacitance and to be pinhole free, topped by a thin organic hydrophobic layer. With this combination, it is possible to have electrowetting operation with voltages in the range +/−10 to +/−50V, which is in the range that can be supplied by conventional TFT arrays.
When a voltage differential is applied between adjacent electrodes, the voltage on one electrode attracts opposite charges in the droplet at the dielectric-to-droplet interface, and the droplet moves toward this electrode, as illustrated in
As shown in
As shown in
In EWoD applications, the gate (select) lines are scanned for line-at-a time addressing, while the data lines carry the voltage to be transferred to propulsion electrodes for electrowetting operation. If no movement is needed, or if a droplet is meant to move away from a propulsion electrode, then 0V will be applied to that (non-target) propulsion electrode. If a droplet is meant to move toward a propulsion electrode, an AC voltage will be applied to that (target) propulsion electrode.
An exemplary electrophoretic display (EPD) incorporating a pixel electrode backplane of the invention is show in
From the foregoing, it will be seen that the present invention can provide a backplane with arrays of hexagonal or triangular electrodes. It will be apparent to those skilled in the art that numerous changes and modifications can be made to the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative and not in a limitative sense.
This application claims priority to U.S. Provisional Patent Application No. 62/732,421, filed Sep. 17, 2018. All references, patents, and patent applications disclosed herein are incorporated by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| 62732421 | Sep 2018 | US |
