Cholesteric display and manufacturing method and driving method therefor

Abstract

A reflective liquid crystal display, a cholesteric display, and a manufacturing method and driving method for a cholesteric display. The cholesteric display constructs a cholesteric microdroplet layer from broadband red, green and blue microdroplets, and is addressed by a TFT active matrix by means of vertical and in-plane switching. Cholesteric full-spectrum Bragg reflection ensures bright reflection and color reproduction. The cholesteric display not only provides video-speed dynamic pictures having infinite colors, but also provides excellent static images.

Description

Cholesteric display and method of manufacturing and driving the same TECHNICAL FIELD

[0001] The present invention relates to a reflective liquid crystal display, more specifically, to a cholesteric display, which is constructed from broadband red, green and blue microdroplets and addressed by vertical switching and in-plane switching TFT active matrix, the bright reflection and color reproduction of the display is ensured by the cholesteric phase full-spectrum Bragg reflection of the cholesteric display liquid crystal. Thus, the display not only provides video speed dynamic pictures with infinite colors, but also provides excellent static images. BACKGROUND

[0002] The cholesteric liquid crystal display is characterized in that the picture remains on the display even if the driving voltage is switched off. The bistability and multistability also ensure a completely flicker-free static display and the possibility of infinite multiplexing to produce a giant display and / or an ultra-high resolution display. In the cholesteric phase liquid crystal, the molecules are oriented in a helix with the periodicity characteristic of the material. In the planar state, the axis of the helix is perpendicular to the display plane. Light with a wavelength matching the pitch of the helix is reflected, the display appears bright. If an alternating current (AC) voltage is applied, the structure of the liquid crystal changes from planar texture to focal conic texture. The main feature of the focal conic state is the property of high diffuse light scattering due to the distribution of small birefringent domains, where the refractive index changes abruptly at the boundaries between these domains. This focal conic texture has no single optical axis. The focal conic texture is usually milky white (i.e., white light scattering). The planar texture and the focal conic texture can coexist in the same panel or entity, which is a very important feature for display applications, whereby gray levels can be achieved.

[0003] Current cholesteric displays exploit the "Bragg reflection", one of the inherent properties of the cholesteric phase. In Bragg reflection, only part of the incident light with the same handedness of circular polarization and within a specific wavelength band can be reflected back, thus producing a monochromatic display. However, the rest of the incident light spectrum (including 50% with opposite handedness of circular polarization and the outward Bragg reflection wavelength band) will pass through the display and be absorbed by the black coating material on the back substrate of the display to ensure contrast. The overall light utilization efficiency is quite low. The impression of Bragg reflection is that monochromatic display is one of the unique properties of cholesteric liquid crystal displays (ChLCD).

[0004] U.S. Patent No. 5,796,454 describes a black and white backlit cholesteric liquid crystal (ChLC) display, which includes a controllable ChLC structure, a first circular polarizer laminated to a first substrate having the same circular polarization as the liquid crystal, a second circular polarizer laminated to a second substrate having the opposite circular polarization as the liquid crystal, and a light source. The black and white backlit display is preferably illuminated by a light source that produces natural "white" light. Thus, when the display is illuminated by incident light, the circular polarizer transmits 50% of the components of the right circularly polarized incident light. When the ChLC is in the ON state, the light reflected by the ChLC is the portion of the incident light having a wavelength within the intrinsic spectral bandwidth and the same handedness, and the portion of the light transmitted through the ChLC is the complement of the intrinsic color of the ChLC. The transmitted light has right circular polarization, and thus it is blocked by the left circular polarizer. Therefore, the observer will perceive that the area of the display is substantially black. When the display is in the OFF state, the light transmitted through the polarizer is scattered by the ChLC, the forward scattered portion of the incident light is emitted from the controllable ChLC structure as depolarized light, and the left circularly polarized portion of the forward scattered light is transmitted through the left circular polarizer, and thus is perceived by the viewer. In U.S. Patent No. 5,796,454, the black and white display is produced by the backlight assembly, and the ambient light is only noise.

[0005] U.S. Patent No. 6,344,887 describes a method of making a full-spectrum reflective cholesteric display, which is incorporated herein by reference. This patent discloses a cholesteric display that employs a polarizer having the same polarity as the liquid crystal. The display utilizes two types of reflection: Bragg reflection (first reflection) and metallic reflection (second reflection). The display utilizes a circular polarizer and a metallic reflecting film located at the back of the display to direct the second component of the incident light back to the viewer.

[0006] U.S. Patent No. 6,873,393 describes a method of making a black and white or color cholesteric display without using Bragg reflection, which is incorporated herein by reference. This patent discloses a cholesteric display that employs a front polarizer having the opposite polarity as the liquid crystal. The function of the display cell structure is only as a light shutter to turn ON and OFF the incident light. In the black and white display mode, the white state is achieved by the metallic reflection from the cholesteric planar texture region, and the black state is obtained by the depolarization effect of the cholesteric focal conic texture region and the filtering effect of the polarizer. In the full color mode, the full color state is produced by the metallic reflector of the cholesteric planar texture region and the micro color filter, and the black state is achieved by the cholesteric focal conic texture region.

[0007] US Patent No. 7564518 describes a reflective cholesteric display employing two circular polarizers. The front circular polarizer has a predetermined polarity opposite to both the Bragg reflection of the display and the back reflective circular polarizer. A high-transmission absorbing weak polarizer is employed in the display system. In black-and-white display mode, the white state is achieved by the cholesteric focal conic texture region, and the black state is obtained by the cholesteric planar texture region. In full-color mode, the full-color state is produced by the micro color filters in the cholesteric focal conic texture region, and the black state is achieved by the cholesteric planar texture region.

[0008] Patent US20200233254A1 describes a cholesteric display employing a substrate with a mirror, which is incorporated herein by reference. Therein, the monostable liquid crystal structure comprises a field-induced nematic homeotropic texture and a cholesteric focal conic texture. SUMMARY

[0009] The main object of the present invention is to create a vertical planar switching TFT display.

[0010] Another object of the present invention is to create a wideband full-color cholesteric droplet display.

[0011] Yet another object of the present invention is to utilize a multi-stable display during power-free static display.

[0012] Yet another object of the present invention is to achieve a high-frame-rate TFT reflective cholesteric liquid crystal display.

[0013] Yet another object of the present invention is to create an optically off state by a vertical switching voltage.

[0014] Yet another object of the present invention is to create an optically on state by a planar switching voltage.

[0015] Yet another object of the present invention is to achieve a low-voltage driving scheme.

[0016] Yet another object of the present invention is to obtain an electrically driven high-brightness full-color display.

[0017] The last object of the present invention is to convert a video dynamic picture into a power-free static image.

[0018] Embodiments of the first aspect of the present application provide a cholesteric display comprising a transparent front substrate, a front conductive layer, a cholesteric liquid crystal layer, a cholesteric droplet layer, a back substrate, and a light absorbing layer. The cholesteric liquid crystal layer and the cholesteric droplet layer are laminated in sequence to form a liquid crystal layer. The transparent front substrate, the front conductive layer, the cholesteric liquid crystal layer, the cholesteric droplet layer, the back substrate, and the light absorbing layer are juxtaposed in sequence to form a cholesteric display structure. A first voltage pulse is applied to the liquid crystal layer to perform a vertical switching of the liquid crystal layer, so that the liquid crystal layer is driven to a vertical switching region. Incident light through the vertical switching region of the liquid crystal layer is absorbed by the light absorbing layer to form an optical off state. A second voltage pulse is applied to the liquid crystal layer to perform a planar switching of the liquid crystal layer, so that the liquid crystal layer is driven to a planar switching region. Incident light through the planar switching region of the liquid crystal layer is Bragg reflected by the liquid crystal in the liquid crystal layer to form an optical on state with at least one gray level. The vertical switching and the planar switching are interchanged at a video rate, so that a viewer observes a reflective color dynamic picture.

[0019] In some embodiments, the liquid crystal in the vertical switching region is a cholesteric focal conic texture.

[0020] In some embodiments, the liquid crystal in the planar switching region is a cholesteric planar texture.

[0021] In some embodiments, the liquid crystal in the planar switching region is a cholesteric planar and focal conic coexisting texture.

[0022] In some embodiments, the first voltage pulse is a vertical switching voltage pulse, and a voltage of the vertical switching voltage pulse is higher than a saturation voltage V2 of a cholesteric planar texture.

[0023] In some embodiments, the second voltage pulse is a planar switching voltage pulse, and a voltage of the planar switching voltage pulse varies between a threshold voltage V1 of a cholesteric focal conic texture to a cholesteric planar texture and a saturation voltage V2 of the cholesteric planar texture.

[0024] In some embodiments, the cholesteric droplet layer comprises cholesteric droplets of a first color, cholesteric droplets of a second color, and cholesteric droplets of a third color.

[0025] In some embodiments, the cholesteric liquid crystal droplets of the cholesteric droplet layer comprise right-handed cholesteric liquid crystal droplets and left-handed cholesteric liquid crystal droplets.

[0026] In some embodiments, the cholesteric liquid crystal layer is invisible in a normal direction and visible in an oblique direction.

[0027] In some embodiments, the cholesteric droplet layer and the cholesteric liquid crystal layer have opposite handedness.

[0028] Embodiments of the second aspect of the application provide a method of manufacturing a cholesteric display, comprising: a. applying a first color cholesteric mixture on a TFT substrate to form a first coating; b. performing a first exposure on the first coating to generate cholesteric droplets of the first color; c. applying a first chiral doping layer on the TFT substrate to form a second coating; d. performing a second exposure on the second coating to generate cholesteric droplets of a second color; e. applying a second chiral doping layer on the TFT substrate to form a third coating; f. performing a third exposure on the third coating to generate cholesteric droplets of a third color; and g. laminating and curing the TFT substrate, the cholesteric droplet layer formed by the cholesteric droplets of the first color, the cholesteric droplet layer formed by the cholesteric droplets of the second color, the cholesteric droplet layer formed by the cholesteric droplets of the third color, and a transparent front substrate to form a cholesteric display.

[0029] In some embodiments, the applying a first color cholesteric mixture on a TFT substrate to form a first coating comprises: mixing a cholesteric liquid crystal, a polymer mixture, and a photoinitiator to form the first color cholesteric mixture; and applying the first color cholesteric mixture on the TFT substrate to form the first coating.

[0030] In some embodiments, the performing a first exposure on the first coating to generate cholesteric droplets of the first color comprises: performing a polymerization reaction on the first coating under a UV light station; disposing a first photomask on the first coating such that a light-transmitting region of the first photomask covers a first pixel region of the TFT substrate, and a light-blocking region of the first photomask covers a second pixel region and a third pixel region of the TFT substrate; and performing a first exposure on the first coating to form cholesteric droplets of the first color on the first coating corresponding to the first pixel region.

[0031] In some embodiments, the applying a first chiral doping layer on the TFT substrate to form a second coating comprises: spin-coating a first chiral doping material on the TFT substrate to form a first thin layer of a first predetermined thickness; and in-situ mixing a liquid crystal polymer mixture of the second pixel region and the third pixel region of the TFT substrate with the first chiral doping material within the first predetermined thickness of the first thin layer to form a second color cholesteric mixture, such that the first thin layer containing the second color cholesteric mixture forms the second coating.

[0032] In some embodiments, the exposing the second coating a second time to generate the second color of cholesteric droplets comprises: performing a second polymerization reaction on the second coating under the UV light station; disposing a second photomask on the second coating such that a light-transmissive region of the second photomask covers the second pixel region of the TFT substrate and a light-blocking region of the second photomask covers the third pixel region of the TFT substrate; and exposing the second coating a second time to form the second color of cholesteric droplets on the second coating corresponding to the second pixel region.

[0033] In some embodiments, the coating a second chiral dopant layer on the TFT substrate to form a third coating comprises: spin-coating a second chiral dopant material on the TFT substrate to form a second thin layer of a second predetermined thickness; and a liquid crystal polymer mixture of the third pixel region of the TFT substrate is mixed in-situ with the first chiral dopant material and the second chiral dopant material within the second predetermined thickness of the second thin layer to form a third color of cholesteric mixture, such that the second thin layer comprising the third color of cholesteric mixture forms the third coating.

[0034] In some embodiments, the exposing the third coating a third time to generate the third color of cholesteric droplets comprises: performing a third polymerization reaction on the third coating under the UV light station; and exposing the third coating a third time to form the third color of cholesteric droplets on the third coating corresponding to the third pixel region.

[0035] In some embodiments, the coating thickness of the first color of cholesteric mixture is in a range of 3 pm to 10 pm.

[0036] In some embodiments, the average size of the first color of cholesteric droplets, the second color of cholesteric droplets, and the third color of cholesteric droplets are each in a range of 0.4 pm to 3.0 pm.

[0037] In some embodiments, the first chiral dopant material and the second chiral dopant material are each cholesteric liquid crystal CB15.

[0038] In some embodiments, the coating thickness of the first chiral dopant material and the second chiral dopant material are each in a range of 0.5 pm to 1.0 pm.

[0039] In some embodiments, the first color of cholesteric droplets, the second color of cholesteric droplets, and the third color of cholesteric droplets are red cholesteric droplets, green cholesteric droplets, and blue cholesteric droplets, respectively.

[0040] Embodiments of the third aspect of the present application provide a driving method for a cholesteric display, the cholesteric display comprising a liquid crystal layer formed by laminating a cholesteric liquid crystal layer and a cholesteric droplet layer in sequence, wherein the driving method comprises: a. vertical switching; b. planar switching; and c. memory freeze; wherein the vertical switching comprises applying a vertical electric field signal to all sub-pixels of the liquid crystal layer to drive the display into a cholesteric focal conic texture to form an optically off state; wherein the planar switching comprises applying a planar electric field signal to all sub-pixels of the liquid crystal layer to drive the display into a planar texture or a focal conic and planar coexisting texture to form an optically on state; wherein the vertical switching and the planar switching are interchanged at a video rate; wherein the memory freeze comprises fixing all focal conic textures, planar textures and focal conic and planar coexisting textures by switching all the sub-pixels to zero electric field at the same time; whereby a viewer observes a video display and a bistable display.

[0041] In some embodiments, the video rate is 60 frames / second.

[0042] The display of the present application not only provides a video speed dynamic picture with infinite colors, but also provides an excellent static image. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1A shows a schematic structure of a vertical field orientation of a cholesteric display of the present application.

[0044] Figure 1B shows a schematic structure of a planar field orientation of a cholesteric display of the present application.

[0045] Figure 2 shows a schematic structure of a cholesteric display of the present application.

[0046] Figure 3 shows a flow chart of a manufacturing method of a cholesteric display of the present application, wherein reference number 3A is a schematic coating process of a cholesteric mixture of a first color, reference number 3B is a generation process of cholesteric droplets of the first color, reference number 3C is a schematic coating process of a first chiral doping layer, reference number 3D is a generation process of cholesteric droplets of a second color, reference number 3E is a schematic coating process of a second chiral doping layer, and reference number 3F is a generation process of cholesteric droplets of a third color. DETAILED DESCRIPTION

[0047] The cholesteric display of the present application comprises a transparent front substrate, a front conductive layer, a cholesteric liquid crystal layer, a cholesteric droplet layer, a back substrate, and a light absorbing layer; wherein the cholesteric liquid crystal layer and the cholesteric droplet layer are laminated in sequence to form a liquid crystal layer; the transparent front substrate, the front conductive layer, the cholesteric liquid crystal layer, the cholesteric droplet layer, the back substrate, and the light absorbing layer are juxtaposed in sequence to form a cholesteric display structure;

[0048] wherein applying a first voltage pulse to the liquid crystal layer to perform a vertical switching of the liquid crystal layer causes the liquid crystal layer to be driven into a vertically switched region, and wherein incident light through the vertically switched region of the liquid crystal layer is absorbed by the light absorbing layer to form an optically off state;

[0049] wherein applying a second voltage pulse to the liquid crystal layer to perform a planar switching of the liquid crystal layer causes the liquid crystal layer to be driven into a planar switched region, and wherein incident light through the planar switched region of the liquid crystal layer is Bragg reflected by the liquid crystals within the liquid crystal layer to form an optically on state having at least one gray scale level;

[0050] wherein the vertical switching and the planar switching are interchanged at a video rate, whereby a viewer observes a reflective color dynamic picture.

[0051] Referring first to FIG. 1A, a cross-sectional structure of a vertical field oriented cholesteric display of the present application is shown. The liquid crystal layer 110 is driven into a vertically switched region, at which time the liquid crystal layer 110 includes at least one focal conic texture region 111. The liquid crystal layer 110 is positioned between a transparent front substrate 101 having a front conductive layer, i.e., a transparent rectangular electrode 103, and a back substrate 102 having a set of parallel strip pixel electrodes 104 and a common electrode 105 with a thin film transistor (TFT) array to form a cell structure having a thickness in the range of 2 μm to 10 μm, and more preferably, in the thickness range of 2.5 μm to 3.5 μm. The liquid crystal layer 110 includes a cholesteric phase liquid crystal layer and a cholesteric phase droplet layer, which are sequentially laminated to form the liquid crystal layer 110. The pixel electrodes 104 are connected to the source of the TFT, and the common electrode 105 is connected to the ground of the display. The front substrate 101 can be made of glass or plastic having a thickness in the range of 0.1 mm to 1.1 mm, and the back substrate 102 can be made of glass or plastic having the same thickness as the front substrate 101. A light absorbing layer, i.e., a black absorbing layer 130, is positioned outside the back substrate 102.

[0052] The cholesteric phase liquid crystal is made of nematic molecules having positive dielectric anisotropy, and chiral molecules. When a first driving voltage pulse 121 is applied between the transparent rectangular electrode 103 and the common electrode 105, the liquid crystal layer 110 is addressed at the focal conic texture region 111, the first driving voltage pulse being a signal for vertical field driving of the liquid crystal layer. Incident light 140 through the transparent front substrate 101 is scattered into diffuse light, wherein about 5% of the incident light is backscattered to the viewer, and thus 95% of the incident light will become forward scattered light components, which are substantially eliminated when impinging on the black absorbing layer 130. Therefore, the display assumes an optically off state.

[0053] Referring first to Figure IB, a planar field oriented cross-sectional structure of the cholesteric display of the present application is shown. The liquid crystal layer 110 is driven into a planar switching regime, in which the liquid crystal layer 110 comprises at least one planar texture region 112. The liquid crystal layer 110 is located between a transparent front substrate 101 having a front conductive layer, i.e., a transparent rectangular electrode 103, and a back substrate 102 having a set of parallel strip pixel electrodes 104 and a common electrode 105 with a thin film transistor (TFT) array to form a cell structure with a thickness in the range of 2 μm to 10 μm, more preferably, in the thickness range of 2.5 μm to 5.0 μm. The pixel electrodes 104 are connected to the source of the TFT, and the common electrode 105 is connected to the ground of the display. The front substrate 101 can be made of glass or plastic with a thickness in the range of 0.1 mm to 1.1 mm, while the back substrate 102 can be made of glass or plastic with the same thickness as the front substrate 101. A black absorbing layer 130 is located outside the back substrate 102.

[0054] The cholesteric phase liquid crystal is made of nematic molecules with positive dielectric anisotropy and chiral molecules with a helical pitch tuned within the visible wavelength, e.g., green light with a center wavelength λ = 550 nm. Then, when a second drive voltage pulse 122 is applied between the pixel electrode 104 and the common electrode 105, the liquid crystal layer 110 is addressed in the planar texture region 112, the second drive voltage pulse being a signal for horizontal field driving of the liquid crystal layer. As the incident light 140 through the transparent front substrate 101, the components with the same polarity as the liquid crystal and within the Bragg reflection bandwidth will be reflected by the cholesteric planar texture as circularly polarized light 141, while the rest will be absorbed by the black absorbing layer 130. Thus, the display presents bright green light, i.e., an optically on state.

[0055] The above described optical on and off states switching between the vertical and planar fields provide bright color contrast for the reflective display. The texture transition between the cholesteric focal conic texture and the cholesteric planar texture only reflects the rotation process of the optical screw axis, not the unwinding process in the prior art. There is no cholesteric-nematic phase transition and associated relaxation in the present application. The switching response time mainly depends on the applied voltage fast enough to obtain a video rate display.

[0056] Turning now to Figure 2, it shows a schematic cross-sectional structure of a full-color cholesteric display, in which a cholesteric microdroplet layer 206 is located between a front substrate 201 with a transparent conductive electrode 203, i.e. a front conductive layer, and a TFT back substrate 202. The cholesteric microdroplet layer has an array of colored microdroplets with a red, green and blue pattern. The wavelengths of R red, G green and B blue light are 650 nm, 550 nm and 450 nm, respectively, and the bandwidth of the color is up to 100 nm (70 nm < Δλ < 100 nm). Each display pixel is composed of three sub-pixels of R, G and B, and each sub-pixel is composed of a plurality of microdroplets. The average size, i.e. the microdroplet diameter, of the microdroplets is in the range of 0.4 μm - 3.0 μm, more preferably 0.8 μm - 2.0 μm. As shown in Figure 2, the cholesteric microdroplet layer 206 comprises cholesteric microdroplets of a first color, cholesteric microdroplets of a second color and cholesteric microdroplets of a third color, which are red, green and blue, respectively.

[0057] A cholesteric liquid crystal layer 207, i.e. a light-enhancing liquid crystal thin layer, is located between the cholesteric microdroplet layer 206 and the front substrate 201. The cholesteric liquid crystal layer 207 and the cholesteric microdroplet layer 206 are laminated in sequence to form a liquid crystal layer. The cholesteric liquid crystal layer 207 is composed of cholesteric liquid crystals with a spin direction opposite to that of the cholesteric microdroplets of the cholesteric microdroplet layer 206, and the center wavelength of the cholesteric liquid crystal layer 207 is adjusted to 750 nm. In this case, the cholesteric microdroplets respectively comprise right-handed liquid crystals and an enhancement layer left-handed liquid crystals. The thickness of the cholesteric liquid crystal layer 207 is in the range of 1 μm - 2 μm. A TFT active matrix comprising a source electrode 204 and a common electrode 205 is made on the inner side of the back substrate 202. There is a TFT circuit unit within each sub-pixel, in which a comb-shaped drain electrode and a comb-shaped common electrode are alternated throughout the display area to generate a planar electric field. Finally, a black absorbing layer 210 is attached to the outer side of the back substrate 202.

[0058] As shown in FIG. 2, there are two groups of electrical signals leading to the display: the first driving signal 230 is directly connected to all pixels of the display panel, while the second driving signal 220 is connected to the TFT source of each sub-pixel. In the TFT structure, on the back substrate 202, such as a transparent insulating substrate, there are formed the gate lines transmitting the scanning signals from outside, the gates being branches of the gate lines, and the storage capacitor electrodes parallel to the gate lines, thus forming the gate insulating layer on the transparent insulating substrate. The data lines perpendicular to the gate lines and transmitting the display signals from outside are formed on part of the gate insulating layer. The semiconductor and N+ layers are formed on the gate insulating layer and the gates. The source and the drain are formed on the layer with ohmic contact, and the source is connected with the data line. In this context, the gates, the source, the drain, the gate insulating layer, and the semiconductor and N+ layers form the TFT active matrix, and the channel of the TFT is generated in the amorphous silicon (a-Si) layer part between the source and the drain. When the scanning signal is applied to the gate through the gate line, the TFT is turned on; and the display signal reaches the source through the data line, and then flows into the drain through the channel in the amorphous silicon layer.

[0059] As shown in FIG. 2, the display works in the optically on state. Therein, when the second voltage pulse, i.e., the planar switching voltage pulse 220, is applied between the source electrode 204 and the common electrode 205, the planar switching of the liquid crystal layer formed by the cholesteric liquid crystal layer 207 and the cholesteric droplet layer 206 laminated in sequence is such that the liquid crystal layer is driven to be a planar switching region, at this time, the liquid crystal layer is addressed in planar texture. When the incident light 240 passes through the transparent front substrate 201 and interferes with the R red droplet region, the component of the incident light 240 having the same polarity as the liquid crystal of the liquid crystal layer and within the red Bragg reflection bandwidth will be reflected by the cholesteric planar texture as the first circularly polarized light 241, while the remaining component of the incident light 240 will be absorbed by the black absorbing layer 210. Therefore, the first display region of the liquid crystal layer presents bright red light (λ0= 650 nm, Δλ = 100 nm), and in the case that the left-handed droplets and the right-handed droplets are made in the red droplet region of the cholesteric droplet layer, the full red bandwidth of the incident light 240 will be reflected.

[0060] Similarly, when the incident light 240 passes through the transparent front substrate 201 and interferes with the G green droplet region, the component of the incident light 240 having the same polarity as the liquid crystal of the liquid crystal layer and within the green Bragg reflection bandwidth will be reflected by the cholesteric planar texture as the second circularly polarized light 242, while the remaining component of the incident light 240 will be absorbed by the light absorbing layer, i.e., the black absorbing layer 210. Therefore, the second display region of the liquid crystal layer presents bright green light (λ = 550 nm, Δλ = 80 nm), and in the case that the left-handed droplets and the right-handed droplets are made in the green droplet region of the cholesteric droplet layer, the full green bandwidth of the incident light 240 will be reflected.

[0061] Similarly, when incident light 240 passes through the transparent front substrate 201 and interferes with the B blue droplet region, the components of the incident light 240 that are of the same polarity as the liquid crystal of the liquid crystal layer and within the blue Bragg reflection bandwidth will be reflected by the cholesteric planar texture as third circularly polarized light 243, while the remaining components of the incident light 240 will be absorbed by the black absorbing layer 210. Thus, the third display region of the liquid crystal layer presents bright blue light (λ = 450 nm, Δλ = 70 nm), which will reflect the full blue bandwidth of the incident light 240 in the case of the manufacture of left-handed and right-handed droplets in the blue droplet region of the cholesteric droplet layer.

[0062] Most importantly, the three colors of the first, second and third circularly polarized light 241, 242 and 243 within the pixel will mix and reappear as a full spectrum of Bragg reflected light (410 nm < λ < 700 nm). Thus, the observer 250 will see a bright white color on the display.

[0063] On the other hand, when the liquid crystal layer is addressed by the vertical electric signal 230, the display operates in an optically off state. In this case, all the pixels of the display panel will switch to the cholesteric focal conic texture as shown in Fig. 1A. The incident light passing through the transparent front substrate 201 is scattered into diffuse light, wherein about 5% of the incident light will be backscattered to the observer, and 95% of the incident light will become forward scattered light components that are substantially eliminated when impinging on the black absorbing layer 210. Thus, the display presents an optically off state.

[0064] Between the optically on and optically off extreme states as described above, when the voltage level of the electric signal 220 varies between VI and V2 (VI < V < V2), the display operates in a gray scale, wherein VI represents the threshold voltage from the cholesteric focal conic texture to the cholesteric planar texture, and V2 represents the saturation voltage of the cholesteric planar texture. This cholesteric phase transition from the focal conic texture to the planar texture allows the display to arrange an infinite gray scale image. During the transition, the helical pitch of the cholesteric structure remains unchanged, but its helical axis becomes more ordered with respect to the normal direction of the cholesteric droplet layer as the voltage increases. The gray scale optical state is a coexistence state of the planar texture and the focal conic texture.

[0065] The cholesteric liquid crystal layer is invisible in the normal direction and visible in the oblique direction. The "normal direction" refers to the direction perpendicular to the liquid crystal. "Invisible in the normal direction" means that the normal direction is in the infrared region, so there is no visible light reflection. The "oblique direction" refers to the direction at an angle to the normal. "Visible in the oblique direction" means that the cholesteric liquid crystal layer reflects red light in the oblique direction of the liquid crystal, thus acting as a light enhancer.

[0066] The present application also provides a driving method for a vertical switching and plane switching cholesteric display, the cholesteric display comprising a liquid crystal layer formed by laminating a cholesteric phase liquid crystal layer and a cholesteric phase droplet layer in sequence. The driving method comprises:

[0067] 1. Vertical switching

[0068] The display is vertically switched, i.e. a vertical electric field signal is applied to all sub-pixels of the liquid crystal layer to drive the display into a cholesteric focal conic texture, thereby forming an optically off state. Either a brand new display from the manufacturer or a display in a power-off idle state, a first voltage pulse with a sufficient pulse width is applied to all pixels of the display panel to drive the liquid crystal layer of the display into a cholesteric focal conic texture, i.e. the display is in an optically black state. The first voltage pulse is a vertical switching voltage pulse, and the voltage of the vertical switching voltage pulse is higher than the saturation voltage V2 of the cholesteric phase plane texture.

[0069] 2. Plane switching

[0070] Immediately after the vertical pulse, the display is horizontally switched, i.e. a plane electric field signal is applied to all sub-pixels of the liquid crystal layer to drive the display into a cholesteric phase plane texture or a coexistence of focal conic and plane texture, thereby forming an optically on state. Specifically, a second voltage pulse, i.e. a plane switching signal, with a voltage level V (V1 < V < V2) is latched to each individual TFT source of the sub-pixel in a line-to-line scanning sequence controlled by the TFT gate signal to generate a horizontal electric field between the in-plane drain of the liquid crystal layer and the common electrode. The liquid crystal molecules in the TFT array will be addressed to a predetermined optical ON and gray level instantaneously. The gray level or total color is determined by an external hardware trapezoidal circuit and an internal pulse width modulation (PWM). For example, if the trapezoidal circuit comprises a series of resistors and an operational amplifier IC generating 8 voltages (v0, v1, v2, v i ,…v7), and the PWM provides 8 levels of Vrm, the total number of colors will be 4th power of 8 or 4096. The bias voltage v i (i = 0 ~ 7) represents the gray level voltage. The values of the resistors are determined by gamma correction to achieve the linear gray level of the human eye. The second voltage pulse is a plane switching voltage pulse, and the voltage V of the plane switching voltage pulse varies between the threshold voltage V1 of the cholesteric phase focal conic texture to the cholesteric phase plane texture and the saturation voltage V2 of the cholesteric phase plane texture.

[0071] While the current image is displayed, the next frame data is recovered in the frame buffer transferred from the shift register and the DA (Digital to Analog) converter. Once the plane switching of the previous frame is completed, the vertical switching of the next frame is started. The video rate can be 60 frames / sec. As a result, the viewer will recognize the video display. To cancel the DC (Direct Current) component, frame-to-frame or line-to-line inversion can be used in the driving scheme. It is noted that the present driving method works only in the cholesteric texture rotation without the phase change and relaxation as occurred in the cholesteric display of the prior art.

[0072] 3. Memory freeze

[0073] When the video display is switched to the bistable display, the control signal will be sent to the frame buffer to lock the predetermined image and display the image for at least one frame period. Once the data addressing of the final line is completed, the image is fixed by switching all the sub-pixels to zero voltage suddenly and simultaneously. As a result, the viewer will recognize the power-free bistable image.

[0074] The present invention also provides a method of manufacturing a cholesteric display, the manufacturing method of the cholesteric phase droplet layer of the cholesteric display is shown by reference numerals 3A-3F in FIG. 3. The manufacturing method comprises:

[0075] a. applying a cholesteric phase mixture of a first color on a TFT substrate to form a first coating layer;

[0076] b. performing a first exposure on the first coating layer to generate cholesteric phase droplets of the first color;

[0077] c. applying a first chiral doping layer on the TFT substrate to form a second coating layer;

[0078] d. performing a second exposure on the second coating layer to generate cholesteric phase droplets of a second color;

[0079] e. applying a second chiral doping layer on the TFT substrate to form a third coating layer;

[0080] f. performing a third exposure on the third coating layer to generate cholesteric phase droplets of a third color; and

[0081] g. laminating and curing the TFT substrate, the cholesteric phase droplet layer formed by the cholesteric phase droplets of the first color, the cholesteric phase droplets of the second color, the cholesteric phase droplets of the third color, and a transparent front substrate layer to form a cholesteric display.

[0082] Now turning to FIG. 3, reference numeral 3A illustrates a schematic coating process of the first color cholesteric mixture. In a class 100 clean room, a cholesteric liquid crystal having positive dielectric anisotropy and a right-handed chiral agent with a concentration conforming to the wavelength of visible light λ = 650 nm are weighed in a beaker and placed in a heating station at 50 °C while stirring to complete dissolution, and then moved to a clean oven at 50 °C overnight. A prepolymer mixture is prepared in a predetermined ratio, stirred uniformly in a beaker at room temperature, and then pressure-filtered through a 0.2 μ stainless steel filter. During the film forming operation, all raw materials including the cholesteric liquid crystal, the polymer mixture, and the photoinitiator are weighed and mixed uniformly without generating bubbles. The uniform mixture is then coated onto a TFT glass substrate 301 to form a first coating layer 302. A slit die coater is used during the coating process, and the resulting first coating layer 302 has a thickness in the range of 3 μm to 10 μm.

[0083] The mixing ratio of the cholesteric liquid crystal, the polymer mixture, and the photoinitiator is shown in Table 1.

[0084] Table 1

[0085] FIG. 3, reference numeral 3B illustrates a generation process of the first color cholesteric droplets.

[0086] The first coating layer 302 is subjected to a polymerization reaction under a UV light station 320. Exemplarily, the polymerization reaction is continuously performed for 150 seconds under the UV light station 320 having a main wavelength λ = 365 nm and an intensity of 2 mW / cm 2

[0087] The first photomask 310 is disposed on the first coating layer 302, i.e., the first photomask 310 is positioned on top of the coated TFT substrate 301 such that the light-transmitting region of the first photomask 310 covers the first pixel region of the TFT substrate 301, and the light-blocking region of the first photomask 310 covers the second and third pixel regions of the TFT substrate 301.

[0088] The first coating layer 302 is subjected to a first exposure to form the first color cholesteric droplets on the first coating layer 302 corresponding to the first pixel region. Exemplarily, the first coating layer 302 is exposed for 150 seconds under a nitrogen purge to form a red droplet region 303. The first color is red.

[0089] Optionally, the average size of the first color cholesteric droplets is in the range of 0.4 μm to 3.0 μm.

[0090] FIG. 3, reference numeral 3C illustrates a schematic coating process of the first chiral doping layer.

[0091] ​The first chiral dopant material is spin-coated on the TFT substrate 301 to form a first thin layer 304 of a first predetermined thickness; the LC-polymer mixture in the second pixel region and the third pixel region of the TFT substrate 301 is mixed in situ with the first chiral dopant material within the first predetermined thickness of the first thin layer 304 to form a cholesteric mixture of a second color, such that the first thin layer 304 containing the cholesteric mixture of the second color forms a second coating layer.

[0092] Exemplarily, the cholesteric liquid crystal CB15, i.e. the first chiral dopant material, is spin-coated on top of the TFT substrate 301 which has been exposed to the first light to form a first thin layer 304 of a predetermined thickness, which is just enough to mix the LC-polymer mixture in the unexposed regions, i.e. the second pixel region and the third pixel region, in situ with the newly coated cholesteric liquid crystal CB15 to form a green cholesteric mixture.

[0093] Optionally, the coating thickness of the first chiral dopant material is between 0.5 μm and 1.0 μm.

[0094] Reference 3D of FIG. 3 shows the process of generating the cholesteric droplets of the second color.

[0095] The second coating layer is subjected to a second polymerization reaction under the UV light station 320; the second photomask 311 is arranged on the second coating layer such that the light-transmitting region of the second photomask 311 covers the second pixel region of the TFT substrate 301 and the light-blocking region of the second photomask 311 covers the third pixel region of the TFT substrate 301; the second coating layer is subjected to a second exposure to form the cholesteric droplets of the second color on the second coating layer corresponding to the second pixel region.

[0096] Exemplarily, the second polymerization reaction is performed under the UV light station 320 with a main wavelength λ = 365 nm and an intensity of 2 mW / cm 2 for 150 seconds. The second photomask 311 is on top of the coated TFT substrate. The coating layer is exposed for 150 seconds under a nitrogen purge to form the green droplet region 305.

[0097] Optionally, the average size of the cholesteric droplets of the second color is in the range of 0.4 μm to 3.0 μm.

[0098] Reference 3E of FIG. 3 shows a schematic coating process of the second chiral dopant layer.

[0099] A second chiral dopant material is spin-coated on the TFT substrate 301 to form a second thin layer 306 of a second predetermined thickness; the LC-polymer mixture of the third pixel region of the TFT substrate 301 is in-situ mixed with the first chiral dopant material and the second chiral dopant material within the second predetermined thickness of the second thin layer 306 to form a cholesteric mixture of a third color, such that the second thin layer 306 containing the cholesteric mixture of the third color forms a third coating layer.

[0100] Exemplarily, the cholesteric liquid crystal CB15 is spin-coated on the top of the TFT substrate after two exposures to form a second thin layer 306 of a predetermined thickness, which is just enough to in-situ mix the LC-polymer mixture in the unexposed region, i.e., the third display region, with the newly coated second chiral dopant material CB15 to form a blue cholesteric mixture.

[0101] Optionally, the coating thickness of the second chiral dopant material is between 0.5 μm and 1.0 μm.

[0102] Optionally, the average size of the cholesteric droplets of the third color is in the range of 0.4 μm to 3.0 μm.

[0103] Reference 3F of FIG. 3 shows the generation process of the cholesteric droplets of the third color.

[0104] The third coating layer is subjected to a third polymerization reaction under the UV station 320. Exemplarily, the third polymerization reaction is performed under the UV station 320 with a main wavelength λ = 365 nm and an intensity of 2 mW / cm 2 .

[0105] The third coating layer is subjected to a third exposure to form cholesteric droplets of the third color on the third coating layer corresponding to the third pixel region. Exemplarily, the third coating layer is exposed for 150 seconds under the condition of nitrogen purging to form a blue droplet region 307.

[0106] Then, the coated fabric is subjected to a spin cleaning, rinsing and drying process to form a cholesteric droplet layer, and finally laminated with a cholesteric liquid crystal layer 308, a top substrate 302. The cholesteric liquid crystal layer 308 is arranged between the substrate 302 and the cholesteric droplet layer. Then, the sealing ring of the display panel is finally cured by a UV exposure machine to obtain a complete TFT unit structure. The cholesteric liquid crystal layer 308 is composed of cholesteric liquid crystals with a spin direction opposite to that of the cholesteric droplets, and the center wavelength is adjusted to 750 nm. In this case, the cholesteric droplets respectively contain right-handed liquid crystals and enhanced left-handed liquid crystals. The thickness of the cholesteric liquid crystal layer 308 is in the range of 1 μm to 2 μm.

[0107] The functional explanation of the cholesteric liquid crystal layer 308 is as follows. For normal incident light, no visible light is reflected from the cholesteric liquid crystal layer 308. In other words, the cholesteric liquid crystal layer 308 is invisible to incident light. Thus, the liquid crystal display reflects white or true color from the cholesteric droplets without color shift. But under oblique illumination conditions, the Bragg reflection tends to shift towards short wavelengths (called blue shift). As a result, the red light is attenuated with the increase of the oblique angle. On the other hand, the introduction of the cholesteric liquid crystal layer 308 will significantly compensate the blue shift and provide enough red color so that the display looks whiter and brighter in the oblique illumination environment.

[0108] While the application and its advantages have been described in detail, those skilled in the art will understand that various modifications, substitutions, and alterations can be made herein without departing from the spirit and scope of the application in its broadest form.

Claims

1. A cholesteric display, characterized in that include: Transparent front substrate; front conductive layer; a cholesteric liquid crystal layer; Cholesteric phase droplet layer; back substrate; as well as light absorbing layer; The cholesteric liquid crystal layer and the cholesteric droplet layer are sequentially laminated to form a liquid crystal layer; the transparent front substrate, the front conductive layer, the cholesteric liquid crystal layer, the cholesteric droplet layer, the back substrate, and the light absorption layer are sequentially juxtaposed to form a cholesteric display structure; wherein a first voltage pulse is applied to the liquid crystal layer to vertically switch the liquid crystal layer so that the liquid crystal layer is driven into a vertical switching region, and incident light passing through the vertical switching region of the liquid crystal layer is absorbed by the light absorbing layer to form an optically closed state; wherein a second voltage pulse is applied to the liquid crystal layer to perform in-plane switching on the liquid crystal layer so that the liquid crystal layer is driven into an in-plane switching region, and incident light passing through the in-plane switching region of the liquid crystal layer is Bragg reflected by liquid crystals in the liquid crystal layer to form an optically on state having at least one grayscale level; The vertical switching and the in-plane switching are interchanged at a video rate, so that the viewer observes a reflected color dynamic picture.

2. The cholesteric display according to claim 1, wherein The liquid crystal in the vertical switching region has a cholesteric focal conic texture.

3. The cholesteric display according to claim 1, wherein The liquid crystal in the plane switching region has a cholesteric plane texture.

4. The cholesteric display according to claim 1, wherein The liquid crystal in the plane switching region has a coexistence texture of cholesteric phase plane and focal conic.

5. The cholesteric display according to claim 1, wherein The first voltage pulse is a vertical switching voltage pulse, and the voltage of the vertical switching voltage pulse is higher than the saturation voltage V2 of the cholesteric plane texture.

6. The cholesteric display according to claim 1, wherein The second voltage pulse is a plane switching voltage pulse, and the voltage of the plane switching voltage pulse varies between a threshold voltage V1 of the cholesteric focal conic texture to the cholesteric planar texture and a saturation voltage V2 of the cholesteric planar texture.

7. The cholesteric display according to claim 1, wherein The cholesteric droplet layer includes cholesteric droplets of a first color, cholesteric droplets of a second color, and cholesteric droplets of a third color.

8. The cholesteric display according to claim 1, wherein The cholesteric liquid crystal droplets of the cholesteric droplet layer include right-handed cholesteric liquid crystal droplets and left-handed cholesteric liquid crystal droplets.

9. The cholesteric display according to claim 1, wherein The cholesteric liquid crystal layer is invisible in a normal direction but visible in an oblique direction.

10. The cholesteric display according to claim 1, wherein The cholesteric droplet layer and the cholesteric liquid crystal layer have opposite handedness.

11. A method for manufacturing a cholesteric display, characterized in that: include: a. coating a first color cholesteric mixture on the TFT substrate to form a first coating; b. exposing the first coating layer for a first time to generate cholesteric droplets of a first color; c. coating a first chiral doped layer on the TFT substrate to form a second coating; d. exposing the second coating layer a second time to generate cholesteric droplets of a second color; e. coating a second chiral doped layer on the TFT substrate to form a third coating; f. exposing the third coating a third time to generate cholesteric droplets of a third color; and g. Laminating and curing the TFT substrate, the cholesteric droplets of the first color, the cholesteric droplets of the second color, the cholesteric droplet layer formed by the cholesteric droplets of the third color, and a transparent front substrate to form a cholesteric display.

12. The manufacturing method according to claim 11, characterized in that: The step of coating a first color cholesteric mixture on the TFT substrate to form a first coating layer comprises: mixing a cholesteric liquid crystal, a polymer mixture, and a photoinitiator to form a cholesteric mixture of the first color; and The cholesteric mixture of the first color is coated on the TFT substrate to form a first coating layer.

13. The manufacturing method according to claim 12, characterized in that: The step of exposing the first coating layer to a first color for a first time to generate cholesteric droplets of a first color comprises: subjecting the first coating to polymerization under ultraviolet light; placing a first photomask on the first coating layer so that a light-transmitting area of ​​the first photomask covers a first pixel area of ​​the TFT substrate, and a light-shielding area of ​​the first photomask covers a second pixel area and a third pixel area of ​​the TFT substrate; and The first coating layer is exposed for the first time to form cholesteric droplets of a first color on the first coating layer corresponding to the first pixel area.

14. The manufacturing method according to claim 13, characterized in that: The step of coating the first chiral doping layer on the TFT substrate to form a second coating layer comprises: Spin coating a first chiral dopant material on the TFT substrate to form a first thin layer with a first predetermined thickness; and The liquid crystal polymer mixture of the second pixel area and the third pixel of the TFT substrate is in-situ mixed with the first chiral doping material within the first predetermined thickness of the first thin layer to form a cholesteric mixture of a second color, so that the first thin layer containing the cholesteric mixture of the second color forms a second coating.

15. The manufacturing method according to claim 14, characterized in that: The step of exposing the second coating layer for a second time to generate cholesteric droplets of a second color comprises: performing a second polymerization reaction on the second coating layer under the ultraviolet light station; placing a second photomask on the second coating layer so that a light-transmitting area of ​​the second photomask covers the second pixel area of ​​the TFT substrate and a light-shielding area of ​​the second photomask covers the third pixel area of ​​the TFT substrate; and The second coating layer is exposed for a second time to form cholesteric droplets of a second color on the second coating layer corresponding to the second pixel area.

16. The manufacturing method according to claim 15, characterized in that: The coating of the second chiral doping layer on the TFT substrate to form a third coating layer comprises: Spin coating a second chiral dopant material on the TFT substrate to form a second thin layer with a second predetermined thickness; and The liquid crystal polymer mixture of the third pixel area of ​​the TFT substrate is in-situ mixed with the first chiral doping material and the second chiral doping material within the second predetermined thickness of the second thin layer to form a cholesteric mixture of a third color, so that the second thin layer containing the cholesteric mixture of the third color forms a third coating layer.

17. The manufacturing method according to claim 16, characterized in that: The step of exposing the third coating layer to a third exposure to generate cholesteric droplets of a third color comprises: subjecting the third coating layer to a third polymerization reaction under the UV light station; and The third coating layer is exposed for a third time to form cholesteric droplets of a third color on the third coating layer corresponding to the third pixel area.

18. The manufacturing method according to claim 11, characterized in that The coating thickness of the cholesteric mixture of the first color is in the range of 3 μm to 10 μm.

19. The manufacturing method according to claim 11, characterized in that: The average sizes of the cholesteric droplets of the first color, the cholesteric droplets of the second color, and the cholesteric droplets of the third color are all within the range of 0.4 μm to 3.0 μm.

20. The manufacturing method according to claim 16, characterized in that The first chiral doping material and the second chiral doping material are both cholesteric liquid crystal CB15.

21. The manufacturing method according to claim 16, characterized in that The coating thickness of the first chiral doping material and the second chiral doping material are both between 0.5 μm and 1.0 μm.

22. The manufacturing method according to any one of claims 11 to 21, characterized in that: The cholesteric droplets of the first color, the cholesteric droplets of the second color, and the cholesteric droplets of the third color are red cholesteric droplets, green cholesteric droplets, and blue cholesteric droplets, respectively.

23. A driving method for a cholesteric display, characterized in that: The cholesteric display includes a liquid crystal layer formed by sequentially laminating a cholesteric liquid crystal layer and a cholesteric droplet layer, wherein the driving method includes: a. Vertical switching; b. Plane switching; and c. Memory freeze; The vertical switching includes applying a vertical electric field signal to all sub-pixels of the liquid crystal layer to drive the display into a cholesteric focal conic texture, thereby forming an optically closed state; wherein the in-plane switching comprises applying an in-plane electric field signal to all sub-pixels of the liquid crystal layer to drive the display into a planar texture or a focal conic and planar coexistence texture to form an optically turned-on state; wherein the vertical switching and the in-plane switching are interchanged at a video rate; The memory freeze includes fixing all focal conic textures, planar textures, and focal conic and planar coexisting textures by switching all sub-pixels to zero electric field simultaneously; thereby the viewer observes a video display and a bi-stable display.

24. The driving method according to claim 23, wherein: The video rate is 60 frames per second.

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