WO2016150393A1

WO2016150393A1 - Optically addressed multi-stable full color liquid crystal display

Info

Publication number: WO2016150393A1
Application number: PCT/CN2016/077187
Authority: WO
Inventors: Abhishek Kumar Srivastava; Wanlong ZHANG; Vladimir Grigorievich Chigrinov; Hoi-Sing Kwok
Original assignee: The Hong Kong University Of Science And Technology
Priority date: 2015-03-24
Filing date: 2016-03-24
Publication date: 2016-09-29
Also published as: CN107430304A

Abstract

An optically addressed, multi-stable liquid crystal display (LCD) panel includes an LCD cell, an LCD printer panel, and a backlight (111). The LCD cell comprises an optically active alignment layer (107) and a liquid crystal layer (106). The LCD printer panel comprises a second liquid crystal layer. The LCD printer panel is configured to modulate light passing through the LCD printer panel with image information for addressing the LCD cell. The backlight (111) is configured to provide the light passing through the LCD printer panel for addressing the LCD cell. The modulated light provided by the LCD printer is configured to expose the optically active alignment layer (107) of the LCD cell to generate grayscale levels.

Description

OPTICALLY ADDRESSED MULTI-STABLE FULL COLOR LIQUID CRYSTAL DISPLAY

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional Patent Application No. 62/177,759, filed on March 24, 2015, which is incorporated by reference herein in its entirety.

FIELD

The described technology relates generally to an optically addressed, multi-stable liquid crystal display (LCD) panel and a method for producing an optically addressed, multi-stable liquid crystal display (LCD) panel.

BACKGROUND

Displays that are able to maintain an image without power consumption to maintain the image are known as “bi-stable” displays (generally corresponding to a binary image) or “multi-stable” displays (generally corresponding to a range of grayscales) . Most conventional devices are bi-stable, and only few are capable of achieving a multi-stable or bi-stable color image. An example of a bi-stable display device is an E-ink reader, which is based on an electrophoretic display. Recently, reflective filters have been used with respect to such E-ink readers to add color, but are limited with respect to reflectance and the quality of the reflective filters. Thus, the color triangle for such displays is limited to 7-9％of the National Television System Committee (NTSC) standard.

Some multi-stable displays are based on cholesteric liquid crystals. Cholesteric liquid crystals possess a helical structure and exhibit two stable states at zero field: the planar texture and the focal conic texture. In the planar texture, they reflect circularly polarized light, whereas in the focal conic texture, they scatter light in forward directions. They can be switched from the planar texture to the focal conic texture by a low-voltage pulse, and they can be switched from the focal conic texture to the planar texture by a high-voltage pulse. They can be used to make reflective displays that do not need a backlight and have good readability under normal ambient light. However, multi-stable displays based on cholesteric liquid crystals present difficulties with respect to material processing and alignment.

Another type of multi-stable display is a Bi-stable Nematic ( “BiNem” ) display, which has only two states and hence no grayscale. However, grayscale can be realized by driving signal modulation, which controls the amount of black and white area (which are separated by a borderline) in one pixel. Different driving signal modulations provide for the borderline to be differently located, changing the respective sizes of the black and white regions within the pixel to achieve different gray scales. Since the borderline behaves like a curtain moving down on a window, this is also referred to as a "curtain effect" . However this type of display is not optimal for high-resolution displays because the gray scale in BiNem depends on the ratio between the black and white area, creating a tradeoff between the grayscale and pixel size.

SUMMARY OF THE INVENTION

In an exemplary embodiment, an optically addressed, multi-stable liquid crystal display (LCD) panel includes: an LCD cell comprising an optically active alignment layer and a liquid crystal layer； an LCD printer panel, comprising a second liquid crystal layer, wherein the LCD printer panel is configured to modulate light passing through the LCD printer panel with image information for addressing the LCD cell； and a backlight, configured to provide the light passing through the LCD printer panel for addressing the LCD cell. The modulated light provided by the LCD printer is configured to expose the optically active alignment layer of the LCD cell to generate grayscale levels.

In another exemplary embodiment, a method for producing an optically addressed, multi-stable liquid crystal display (LCD) panel includes: providing an LCD cell comprising an optically active alignment layer and a liquid crystal layer； providing an LCD printer panel, comprising a second liquid crystal layer, wherein the LCD printer panel is configured to modulate light passing through the LCD printer panel with image information for addressing the LCD cell； and providing a backlight, configured to provide the light passing through the LCD printer panel for addressing the LCD cell. The modulated light provided by the LCD printer is configured to expose the optically active alignment layer of the LCD cell to generate grayscale levels.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following.

Figure 1 is a schematic diagram of an exemplary optically addressed multi-stable full color liquid crystal display (LCD) panel.

Figure 2 is a schematic diagram of an exemplary optically addressed multi-stable full color liquid crystal display cell with an optical printer.

Figure 3 depicts an exemplary image displayed by an exemplary optically addressed multi-stable full color liquid crystal display panel.

Figure 4 is a schematic diagram illustrating an exemplary backlight unit used in an exemplary optically addressed multi-stable full color liquid crystal display panel.

Figure 5 is a schematic diagram illustrating an exemplary liquid crystal display (LCD) printer used in an exemplary optically addressed multi-stable full color liquid crystal display panel.

Figure 6 depicts an exemplary 4-bit gray scale image displayed on an exemplary optically addressed multi-stable full color liquid crystal display panel.

DETAILED DESCRIPTION

Exemplary embodiments of the invention provide an optically addressed multi-stable full color liquid crystal display panel that is capable of displaying a color image without continuous power consumption for the optically-addressed display (although it will be appreciated that a backlight of the display may still draw power) .

In an exemplary embodiment of a multi-stable optically addressed liquid crystal display (LCD) panel, a display cell has one optically passive alignment layer and one optically active alignment layer for which the alignment of the easy axis can be changed by a flash of light. The different images are refreshed on the LCD panel by a flash of a light encoded with a mono color image generated by a light printer, which is also a LCD panel. A light-emitting diode (LED) backlight of the LCD panel includes at least two LEDs: one LED for addressing the LCD with a desired wavelength and another LED with a different color, wherein the LEDs are switched sequentially in time for addressing and displaying the color image. This allows different gray scales to be realized for different pixels, and, with a color filter array over the optically addressed LCD panel, provides a full color image. Thus, the multi-stable optically addressed LCD panel achieves a full color LCD multi-stable image that does not require continuous power for maintaining the image.

Figure 1 is a schematic diagram of an exemplary optically addressed multi-stable full color liquid crystal display (LCD) panel. The optically addressed multi-stable full color liquid crystal display includes an ultraviolet (UV) protective layer 101, a color filter array 102, polarizers 103,

substrates

104, 108, an optically passive alignment layer 105, an optically active alignment layer 107, a liquid crystal layer 106, and an optical printer having an LCD printer panel with half wave plate 109, a quarter wave plate (not depicted) , a polarizer 110, and a backlight unit 111. As depicted in Figure 1, the liquid crystal layer 106 may include some liquid crystals in a planar alignment mode while others are in a twisted nematic alignment mode, which provides for optical contrast.

In an exemplary embodiment, the optically active alignment layer 107 is composed of azo dye, and it changes the alignment of the “easy” axis based on being irradiated by polarized light of a specific wavelength. The optically passive alignment layer 105 is fixed and is not affected by the irradiating light. The optically passive alignment layer 105 may be a photo-alignment layer, a rubbed polyimide layer, or an aligning layer formed by oblique evaporation or ion beam deposition. The liquid crystals are disposed in the liquid crystal layer 106, between the two

substrates

104, 108 coated with the

alignments layers

105, 107. The

substrates

104, 108 may be made of any commercially available material, such as transparent glass or plastic.

To avoid undesirable effects of ambient light, the UV protective layer 101 is provided to prevent ambient light from affecting the optically active alignment layer 107. The optically addressed multi-stable full color liquid crystal display cell is thus addressed only by the addressing beam from the backlight 111.

An exemplary optically addressed multi-stable full color LCD panel (e.g., as depicted in Figure 1) includes three main stages: an optically-addressable multi-stable liquid crystal display cell (discussed in further detail below in connection with Figures 2-3) ； a backlight unit (discussed in further detail below in connection with Figure 4) ； and an LCD printer panel of a light printer (discussed in further detail below in connection with Figures 2 and 5) .

Figures 2 and 3 illustrate working principles of an optically addressed multi-stable full color liquid crystal display cell.

Figure 2 is a schematic diagram of an exemplary optically addressed multi-stable full color liquid crystal display cell. The display cell includes an optically active layer 202 and an optically passive layer 203, along with substrates 204 and a liquid crystal medium 205. Polarized light passes through an LCD optical printer 201, wherein the opaque region 206 of the LCD optical printer 201 does not change the polarization azimuth of the system while the the transparent region 207 of the LCD optical printer 201 changes the polarization azimuth of the impinging light and thus addresses the multi-stable display cell (i.e., causes the liquid crystals of the multi-stable display cell to assume a desired configuration for producing desired grayscales) . The relative sizes/shapes of the opaque region 206 and transparent region 207 for the LCD optical printer 201 correspond to a mono-color image uploaded to the LCD optical printer 201, and thus the relative sizes/shapes of the opaque region 206 and transparent region 207 will vary based on the mono-color image that is uploaded.

Controlling the irradiating light thus allows for precise control of the easy axis of the optically active alignment layer 203, which allows the liquid crystal orientation in the display cell to be changed. Thus, optical contrast among neighboring pixels of the LCD panel, and consequently an image corresponding thereto, are generated by the LCD panel.

Figure 3 depicts an exemplary image displayed by an exemplary optically addressed multi-stable full color liquid crystal display panel. The exemplary image includes twisted nematic region 301 (corresponding to lighter areas) and planar alignment region 302 (corresponding to darker regions) . Spatial generation of grayscale is also illustrated in Figure 3, which is achieved for each pixel by controlling the ratio of the two regions (i.e., the opaque region and the transparent region) within a pixel (similar to a dithering process) .

Figure 4 is a schematic diagram illustrating an exemplary backlight unit used in an exemplary optically addressed multi-stable full color liquid crystal display panel. The backlight unit is used to provide the color image and to address each pixel. The backlight unit includes a light guide 401, LEDs 402 of different colors (e.g., red and green) , and an LED 403 for addressing a pixel of the display panel.

The wavelength for the LED 403 depends on the optically active alignment layer. In one exemplary implementation, for an SD1 alignment layer having two absorption bands around 365nm and 450nm, the wavelength is 450nm (or any wavelength in the range of 300 to 450 nm) . The two additional LEDs 402 (e.g. of red and green colors) are combined with blue light (from the addressing beam from LED 403) to provide white balance with respect to the backlight.

The backlight unit further includes at least one polarizer which is configured to maintain the degree of the polarization of the light and maintain the electro-optics of the LCD panel. Because the opaque and the transparent regions of the LCD optical printer is controlled based on the polarization azimuth of light passing through the LCD optical printer, it is important to control the degree of the polarization using the polarizer.

The three LEDs work sequentially in time. First, the addressing LED 403 is ON while the color LEDs 402 are OFF. The addressing LED 403 is ON with high intensity for addressing pixels of the optically addressed multi-stable full color liquid crystal display panel (i.e., setting the pixel to a desired stable configuration) . After the pixels are addressed, the intensity of the addressing LED 403 is decreased, and the color LEDs 402 are switched ON to maintain the white balance of the backlight and to provide a color image (i.e., via a color filter layer while the backlight is kept on) .

Figure 5 is a schematic diagram illustrating an exemplary liquid crystal display (LCD) printer used in an exemplary optically addressed multi-stable full color liquid crystal display panel. The LCD printer (or “light printer” or “optical printer” ) provides a polarization plane rotation stage. The LCD printer includes a backlight unit 501 (e.g., such as the backlight unit depicted in Figure 4) and a polarizer 502, along with an LCD printer panel with half wave plate 503 for the addressing beam and a quarter wave plate (QWP) 504 for the same wavelength.

In an exemplary embodiment, the LCD printer panel is a planar aligned LCD printer panel with its optical axis making an angle with the polarization azimuth of the impinging light from the backlight 501. After light from the backlight passes through the LCD printer panel, it passes through the QWP 504 and thus the polarization azimuth of the emerging light can be controlled by controlling the retardation of the planar aligned liquid crystal display cell by an applied voltage. The angle between the easy axis of the planar aligned LCD printer panel and QWP 504 is 45° in an exemplary implementation. This provides continuous control on the polarization azimuth of the emerging light beam and thus the gray scale of the optically addressed multi-stable full color liquid crystal display cell.

In another embodiment, the LCD printer is based on a half wave planar aligned LCD printer panel that works in binary mode. For an ON pixel, the polarization azimuth of the impinging light is rotated by a certain angle and in an OFF state the LCD does not affect the polarization azimuth of the light. In this mode the gray scale of the display can be controlled either spatially or by the time of the irradiation.

The optically addressed multi-stable full color liquid crystal display cell can be optically addressed as follows. A mono-color image is first uploaded to the light printer. Then, the addressing LED of the backlight unit is switched ON. The beam of the addressing LED passes through the light printer, addresses the optically addressed multi-stable full color liquid crystal display cell (corresponding to a group of pixels of the LCD panel or all pixels of the LC panel) , and generates different gray levels. Next, the addressing LED and the light printer are switched OFF and the color LEDs are switched ON to generate white light. The light passes through the optically addressed multi-stable full color liquid crystal display cell and color filter array on the top and displays different gray scale and color for different pixels, and thus a full color image is achieved. The resolution and the color provided by the optically addressed multi-stable full color liquid crystal display panel depends on the LCD printer and color filter arrays.

Figure 6 depicts an exemplary 4-bit gray scale image displayed on an exemplary optically addressed multi-stable full color liquid crystal display panel. The gray scale of the pixels seen in Figure 6 is generated by controlling the twist angle between the easy axis of the optically active and pasive alignment layers (e.g., via modulated light provided by the LCD printer which exposes the optically active alignment layer, causing twist angles which translate to gray scales being generated) . It will be appreciated that, in an alternative implementation, another way to control the gray scale provided by pixels is to change the twist angle of the impinging light.

In an embodiment, the invention provides an optically addressed multi-stable full color liquid crystal display panel. The panel includes: a backlight unit to illuminate and address the panel, polarizers to provide the proper optical modulation, an optically addressed liquid crystal display cell that has at least one optically active alignment layer, a liquid crystal display printer panel to address the optically addressed liquid crystals display cell, a color filter array to show pixel with distinct color, and a UV filter sheet (or “protective layer” ) to protect the optically addressed liquid crystal display panel from ambient UV light. The display panel provides a full color multi-stable image.

In an embodiment, the backlight unit comprises light emitting diodes for at least two colors, wherein at least one LED has a desired wavelength to address the pixels on optically addressed bi-stable full color liquid crystal display cell and additional LEDs are provided for white balancing. In an exemplary implementation, blue light is used for the addressing of different pixel on the optically addressed bi-stable full color liquid crystal display cell, and the additional LEDs (e.g., two white LEDs； or one red LED and one green LED) are of other color (s) to provide white balancing.

In an embodiment, the thickness of the liquid crystal layer is configured to provide a wave-guide regime for the propagation of the impinging light.

In an embodiment, the optically active alignment layer is made of azo dye. The optically passive alignment layer is one of the group comprising: a photo-alignment layer, a rubbed polyimide layer, and an aligning layer formed by oblique evaporation or ion beam deposition.

In an embodiment, the liquid crystal display printer panel is operated in at least two states, which are designed to provide a half-wave plate or zero retardation for the addressing beam.

In an embodiment, the optically addressed liquid crystal display cell comprises a liquid crystal layer sandwiched in between a pair of substrates, with one substrate covered with optically active alignment layer and another substrate is covered with optically passive alignment layer. The substrates may be made of a transparent glass/plastic covered with or without conducting layer or conducting polymer.

In an embodiment, the LCD printer panel includes at least one liquid crystal layer sandwiched between a pair of conducting transparent substrates and a source of electrical driving voltage pulses, which is applied to the electrodes of the LCD printer panel.

In an embodiment, the conducting transparent substrates of the LCD printer panel are made of transparent glass or plastic covered with conducting layers or conducting polymer.

In an embodiment, the LCD printer panel is operated in at-least two states with distinct polarization azimuths, or many states with different polarization azimuths (i.e., the LCD printer panel with half wave plate in combination with the quarter wave plate are configured to provide a plurality of different polarization azimuths based on applications of different electric fields) .

In an embodiment, a first operation state of the LCD printer panel rotates the polarization azimuth of the impinging beam by a certain angle, and the second state of the LCD printer panel does not affect the polarization azimuth of the impinging light.

In an embodiment, the optically addressed multi-stable full color liquid crystal display panel includes a color filter array that has been placed on the top of the optically addressed liquid crystal display cell to show distinct color pixels and achieve a full color image.

In an embodiment, the optically addressed liquid crystal display cell and the LCD printer panel are sandwiched in between the two polarizers (e.g.,

polarizers

103 and 110 in Figure 1) . In an exemplary implementation, the two polarizers have crossed or parallel polarization axes.

In an embodiment, the optically addressed multi-stable full color liquid crystal display panel includes a UV filter sheet on the top of the optically addressed multi-stable full color liquid crystal display cell to protect the panel from the ambient UV light.

In an embodiment, a mono-color image is uploaded to the LCD printer panel using blue light generated by the backlight unit. The blue light is uniform prior to passing through the LCD printer panel and becomes modulated after passing through the LCD printer. This allows for grayscales to be achieved when the modulated blue light exposes the optically active layer (which generates different twist angles for the liquid crystals of the liquid crystal display cell, translating into different grayscales) .

In an embodiment, LEDs of the backlight unit operate sequentially in time for addressing and display.

It will be appreciated that exemplary embodiments of the invention provide an advantageous and cost-effective structure for a multi-stable display utilizing, for example, optical photoalignment layers capable of being optically altered by modulated light provided from an optical printer, which allows a displayed image with different grayscale levels to be maintained on the display without an electronic driver for the optically-addressed panel. Further, because the resolution of the display corresponds to the resolution of the optical printer, a high resolution level can be provided by using an optical printer with high resolution.

All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing embodiments of the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B” ) is to be construed to mean one item selected from the listed items (Aor B) or any combination of two or more of the listed items (Aand B) , unless otherwise indicated herein or clearly contradicted by context. The terms “comprising, ” “having, ” “including, ” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to, ” ) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as” ) provided herein, is intended merely to better illuminate embodiments of the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

An optically addressed, multi-stable liquid crystal display (LCD) panel, comprising:

an LCD cell comprising an optically active alignment layer and a liquid crystal layer；

an LCD printer panel, comprising a second liquid crystal layer, wherein the LCD printer panel is configured to modulate light passing through the LCD printer panel with image information for addressing the LCD cell； and

a backlight, configured to provide the light passing through the LCD printer panel for addressing the LCD cell；

wherein the modulated light provided by the LCD printer is configured to expose the optically active alignment layer of the LCD cell to generate grayscale levels.
The optically addressed, multi-stable LCD panel according to claim 1, further comprising:

two polarizers, wherein the LCD cell and the LCD printer panel are disposed between the two polarizers.
The optically addressed, multi-stable LCD panel according to claim 2, wherein the two polarizers have crossed or parallel polarization axes.
The optically addressed, multi-stable LCD panel according to claim 1, further comprising:

an ultraviolet filter sheet, configured to protect the optically addressed, multi-stable LCD panel from ambient ultraviolet light.
The optically addressed, multi-stable LCD panel according to claim 1, further comprising:

a color filter array, configured to provide colors for an image displayed by the optically addressed, multi-stable LCD panel.
The optically addressed, multi-stable LCD panel according to claim 1, wherein an image displayed by the optically addressed, multi-stable LCD panel is multi-stable, such that the displayed image is maintained without continuous power consumption for maintaining the image.
The optically addressed, multi-stable LCD panel according to claim 1, wherein the backlight comprises a first light source configured to provide the light passing for addressing the LCD cell.
The optically addressed, multi-stable LCD panel according to claim 7, wherein the backlight comprises one or more additional light sources configured to provide white balancing.
The optically addressed, multi-stable LCD panel according to claim 8, wherein the first light source and the one or more additional light sources are configured to be operated sequentially in time for addressing and for display.
The optically addressed, multi-stable LCD panel according to claim 8, wherein the first light source and the one or more additional light sources are light-emitting diodes (LEDs) of different colors.
The optically addressed, multi-stable LCD panel according to claim 1, wherein the LCD cell further comprises an optically passive alignment layer on a side of the liquid crystal layer opposite of the optically active alignment layer.
The optically addressed, multi-stable LCD panel according to claim 11, wherein the optically passive alignment layer is selected from the group consisting of: a photoalignment layer, a rubbed polyimide layer, an alignment layer formed by oblique evaporation, and an alignment layer formed by ion beam deposition.
The optically addressed, multi-stable LCD panel according to claim 1, wherein the optically active alignment layer comprises azo-dye.
The optically addressed, multi-stable LCD panel according to claim 1, wherein the LCD printer panel further comprises electrodes and a voltage source configured to provide electrical driving voltage pulses to the electrodes.
The optically addressed, multi-stable LCD panel according to claim 1, wherein the LCD printer panel is configured to be operated in at least two states with different polarization azimuths.
The optically addressed, multi-stable LCD panel according to claim 15, wherein the at least two states include a first state corresponding to rotating a polarization azimuth of an impinging beam by an angle and a second state corresponding to a polarization azimuth of an impinging beam being unaffected.
The optically addressed, multi-stable LCD panel according to claim 15, wherein the at least two states include a first state corresponding to a half wave plate and a second state corresponding to zero retardation for the light for addressing the LCD cell.
A method for producing an optically addressed, multi-stable liquid crystal display (LCD) panel, comprising:

providing an LCD cell comprising an optically active alignment layer and a liquid crystal layer；

providing an LCD printer panel, comprising a second liquid crystal layer, wherein the LCD printer panel is configured to modulate light passing through the LCD printer panel with image information for addressing the LCD cell； and

providing a backlight, configured to provide the light passing through the LCD printer panel for addressing the LCD cell；

wherein the modulated light provided by the LCD printer is configured to expose the optically active alignment layer of the LCD cell to generate grayscale levels.
The method according to claim 18, further comprising:

providing an ultraviolet filter sheet, configured to protect the optically addressed, multi-stable LCD panel from ambient ultraviolet light.
The method according to claim 18, further comprising:

providing a color filter array, configured to provide colors for an image displayed by the optically addressed, multi-stable LCD panel.