Patent Publication Number: US-2021167123-A1

Title: Micro led display device

Description:
RELATED APPLICATIONS 
     This application is a continuation of Ser. No. 16/136,233, entitled “MICRO LED DISPLAY DEVICE” (now Pub. No. US 2019/0245006 A1) filed Sep. 19, 2018, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a display device. More particularly, the present disclosure relates to a micro LED display device. 
     Description of Related Art 
     Recently, a display device has been rapidly developed as an important human-machine interface. A portable electronic device, a computer or a television can represent complicated messages through the display device. 
     Owing to the demands on the large visible area, compact volume and low energy consumption, a liquid crystal display (LCD) device is getting more popular and has become a mainstream. A conventional LCD device  100  is shown in  FIG. 1 . The LCD device  100  includes, from bottom to top, a backlight module  111 , a first polarizer  112 , a first substrate  113 , a transistor layer  114 , a first electrode  115 , a liquid crystal layer  116 , a second electrode  117 , a color filter  118 , a second substrate  119  and a second polarizer  120 . The operation mechanism of the LCD device  100  is then briefly described. The liquid crystal molecules in the liquid crystal layer  116  are twisted when a voltage is applied. One or more transistors in the transistor layer  114  is/are used to control the twisted direction of the liquid crystal molecules and are functioned as a light switch. Furthermore, lights emitted from the backlight module  111  are passed through the first polarizer  112  and the second polarizer  120  for generating different polarized direction lights to incorporate with the twisted directions of the liquid crystal molecules to control the brightness variation to form a gray scale. An image with a full color can be formed by combining a plurality of pixels. Each of the pixels consists of a plurality of sub-pixel units  118   a  in the color filter  118 . However, the formation of the sub-pixel units  118   a  is determined by each transistor in the transistor layer  114 , which control the luminance of the backlight module  111 . Furthermore, an alignment film can be disposed on the first substrate  113  and the second substrate  119  for aligning the liquid crystal molecules. A voltage can be applied to the transistor layer  114  through the first electrode layer  115  and the second electrode  117 . 
     However, the power efficiency and the brightness (contrast) of such kind of LCD device  100  is low because only few lights emitted from the backlight module  111  can pass through the liquid crystal layer  116 . Furthermore, the manufacturing processes of the transistor layer  114  are complicated thereby increasing the manufacturing cost. OLED device has been reached to the market as an alternative of the LCD device  100 . The OLED device has larger viewing angle than the conventional LCD device  100 , however, issues such as light color flashing and light color decay still exist due to the material properties of the organic component, thus the lifetime of the OLED device is dramatically reduced. 
     Therefore, there is a need to develop a display device having high power efficiency, large viewing angle and long lifetime. 
     SUMMARY 
     According to one aspect of the present disclosure, a micro LED display device is provided. The micro LED display device includes a micro LED array, a light enhancing layer, a color filter and a polarizer. The micro LED array includes a plurality of micro LEDs, wherein each of the micro LEDs is independently controlled to emit a light. The light enhancing layer is located above the micro LED array, wherein the light enhancing layer includes a plurality of quantum dots. The color filter is located above the light enhancing layer, wherein properties of the light of each of the micro LEDs is converted by each of the quantum dots thereby projecting a plurality of sub-pixel units in the color filter. The polarizer is located above the color filter. 
     In one example, the quantum dots convert a color of the light of each of the micro LEDs of the micro LED array. 
     In one example, the quantum dots reduce a full width at half maximum of the light of each of the micro LEDs of the micro LED array. 
     In one example, the quantum dots increase conversion efficiency of the light of each of the micro LEDs passed through the light enhancing layer. 
     In one example, the micro LED display device includes an electrode layer, wherein the electrode layer drives the micro LED array to emit lights. 
     In one example, the micro LED display device further includes a first substrate and a second substrate, wherein the first substrate is located between the light enhancing layer and the color filter, and the second substrate is located between the color filter and the polarizer. 
     In one example, the color filter includes a light-absorptive material. 
     In one example, each of the sub-pixel units is corresponded to a red color, a green color or a blue color. 
     In one example, the light of each of the micro LEDs includes a red color, a green color or a blue color. 
     In one example, a color of the light of each of the micro LEDs is different or identical. 
     In one example, a color of each of the sub-pixel units is corresponded to a color of the light of each of the micro LEDs. 
     In one example, each of the sub-pixel units in the color filter is departed by a mask. 
     In one example, each of the micro LEDs is aligned correspondingly to each of the sub-pixel units. 
     In one example, the sub-pixel units in the color filter are aligned in a linear shape, a square shape, a triangle shape or a mosaic shape. 
     According to another aspect of the present disclosure, a micro LED display device is provided. The micro LED display device includes a micro LED array and a color filter. The micro LED array includes a plurality of micro LEDs, wherein each of the micro LEDs is independently controlled to emit a light. The color filter includes a plurality of quantum dots and is located above the micro LED array, wherein properties of the light of each of the micro LEDs is converted by each of the quantum dots thereby projecting a plurality of sub-pixel units in the color filter. 
     In one example, the micro LED display device further includes a polarizer located above the color filter. 
     In one example, the quantum dots convert a color of the light of each of the micro LEDs of the micro LED array. 
     In one example, the quantum dots reduce a full width at half maximum of the light of each of the micro LEDs of the micro LED array. 
     In one example, the quantum dots increase conversion efficiency of the light of each of the micro LEDs passed through the color filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows: 
         FIG. 1  is a schematic view showing a conventional LCD device; 
         FIG. 2  is a schematic view showing a micro LED display device according to one embodiment of the present disclosure; 
         FIG. 3  is a schematic view showing a micro LED display device according to one embodiment of the present disclosure; and 
         FIG. 4  is a schematic view showing a micro LED display device according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Micro LEDs are regarded as the next-generation display technology due to its superior properties such as high contrast, high efficiency, high resolution, high response time, etc. Furthermore, a micro LED display device has high color saturation and rapid frame rate and is particularly suitable for next-generation advanced applications (e.g. ultrahigh-definition TVs, micro projectors). An elementary type of micro LED display device only utilizes RGB LEDs to generate a full color image. However, this kind of micro LED display device has disadvantages due to natural material properties of RGB LEDs, such as low efficiency of green LEDs and seriously reduced EQE (External Quantum Efficiency) of red LEDs when LED chip size shrinks down to below 20 μm. Furthermore, owing to the different materials properties of different color LEDs, mismatch in electrical properties occurred thereby resulting in difficulties on integrating these different LEDs. Accordingly, the present disclosure provides improvements of such kind of micro LED display device. 
       FIG. 2  is a schematic view showing a micro LED display device  200  according to one embodiment of the present disclosure. The micro LED display device  200  includes a micro LED array  212 , a light enhancing layer  213 , a color filter  215  and a polarizer  217 . The light enhancing layer  213  is located above the micro LED array  212 ; the color filter  215  is located above the light enhancing layer  213 ; and the polarizer  217  is located above the color filter  215 . The micro LED display device  200  also includes a first substrate  214  and a second substrate  216 . The first substrate  214  is located between the light enhancing layer  213  and the color filter  215 . The second substrate  216  is located between the color filter  215  and the polarizer  217 . The micro LED display device  200  can further include an electrode layer  211 . The electrode layer  211  can be located under the micro LED array  212  for electrically driving the micro LED array  212  to emit lights. The light enhancing layer  213  includes a plurality of quantum dots  500 . Properties of the light emitted from each of the micro LEDs  212   a  is converted by each of the quantum dots  500  thereby projecting a plurality of sub-pixel units  215   a  in the color filter  215 . The mechanism of forming the sub-pixel units  215   a  of the micro LED display device  200  of the present disclosure is distinctly different from that of the conventional LCD device. Conventional LCD device is recognized as non-emissive display device owing to LC (Liquid Crystal) panel cannot emit light, the required light source is provided by using an extra backlight module. The micro LED display device  200  of the present disclosure is recognized as emissive display device owing to micro LEDs  212   a  can emit light itself, without requiring any extra backlight source. Accordingly, in a non-emissive LCD device  100  as in  FIG. 1 , the LC panel includes a plurality of pixels where each of the pixels consists of RGB sub-pixel units, and each RGB sub-pixel unit is addressed independently by a thin-film transistor (TFT), which regulates the luminance transmittance from the backlight module  111 . In contrast, in the emissive micro LED display device  200  as in the present disclosure, micro LEDs  212   a  both directly serves as sub-pixel units  215   a  and light source. Therefore, backlight module  111  in LCD device  100  as in  FIG. 1  doesn&#39;t participate in the formation of the sub-pixel units  118   a,  each sub-pixel unit  113   a  is formed by an unit TFT transistor in the transistor layer  114 . 
     The operation mechanism of the micro LED display device  200  is then described. The micro LED array  212  includes a plurality of micro LEDs  212   a  which are aligned in order. Each of the micro LEDs  212   a  is electrically driven by the electrode layer  211 , and can be independently controlled to emit a light. The electrode layer  211  can be made from conductive materials (metal or other materials), and can provide the required electric power. The lights emitted from the micro LED array  212  pass through the light enhancing layer  213  located above. The micro LED  212   a  is an inorganic LED, material such as InGaN is commonly used for blue and green LEDs, and material such as AlGaInP is commonly used for red LEDs. In the present disclosure, the light enhancing layer  213  may has many functionalities due to a plurality of quantum dots  500  are spread in the light enhancing layer  213 . 
     Quantum dots  500  are small semiconductor crystals with sizes down to nanoscale. Their properties are vastly different from bulk semiconductors. The most striking characteristics of the quantum dots  500  are the tuneability of the semiconductor bandgap by varying their size and discrete energy levels (so-called quantum confinement effect). Furthermore, when applying quantum dots  500  in micro LED displays  200 , narrow spectra can be obtained owing to their narrow emission line width (e.g. full width at half maximum (FWHM) is about 20 to 30 nm for Cdse and InP based quantum dots). High color saturation is obtained at the extrema by narrow spectra, which cover greater than 90% of the strictest Rec. 2020 color gamut standard. The narrow linewidths of the quantum dots  500  also enable them to form viable active elements of micro LEDs in high-definition displays. 
     In addition to reduce a full width at half maximum (FWHM) of the light of each of the micro LEDs  212   a  of the micro LED array  212 , the quantum dots  500  in the present disclosure can also convert a color of the light of each of the micro LEDs  212   a  of the micro LED array  212 . The quantum dots  500  also increase conversion efficiency of the light of each of the micro LEDs  212   a  passed through the light enhancing layer  213 . The emergence of quantum dot  500  has benefits of high photoluminescence quantum yield, high photostability, solution processability, low fabrication cost and color tuneability, thereby providing a powerful full-color solution for high-definition micro LED display devices. 
     The color filter  215  disposed above the light enhancing layer  213  is used for enhancing color saturation. It has been known that a full color image is generated by combining a plurality of smallest addressable elements (so called pixels), and each of the pixels includes a plurality of sub-pixel units  215   a.  In  FIG. 2 , each of the micro LEDs  212   a  generates a sub-pixel units  215   a,  and projects the sub-pixel unit  215   a  in the color filter  215 . A pixel can be formed by combining the sub-pixels  215   a.  For generating a full color image, the color of each of the micro LEDs  212   a  can be different or identical. For example, in a three primary color system, the light of each of the micro LEDs  212   a  is a red color, a green color and a blue color separately, therefore each of the sub-pixel units  215   a  is corresponded to a red color, a green color and a blue color separately and are combined to form a pixel. In a four primary color system, the light of each of the micro LEDs  212   a  is a red color, a green color, a blue color and a yellow color separately, therefore each of the sub-pixel units  215   a  is corresponded to a red color, a green color, a blue color and a yellow color separately and are combined to form a pixel. In a six primary color system, the light of each of the micro LEDs  212   a  is a red color, a green color, a blue color, a cyan color, a purple color and a yellow color separately, therefore each of the sub-pixel units  215   a  is corresponded to a red color, a green color, a blue color a cyan color, a purple color and a yellow color separately and are combined to form a pixel. In the aforementioned embodiments, each of the micro LEDs  212   a  emits different color, and the color of each of the sub-pixel units  215   a  is corresponded to the color of the light of each of the micro LEDs  212   a.  A better color saturation and color reproduction can be achieved while using more sub-pixel units  215   a  with different colors. Furthermore, an alignment form of the sub-pixel units  215   a  also has influence on the color saturation. In other word, the sub-pixel units  215   a  can be aligned in a linear shape, a square shape, a triangle shape or a mosaic shape fir obtaining different color saturation. In the situation that each micro LED  212   a  has the same color, the quantum dots  500  of the present disclosure can be used for convert the color of the light of each of the micro LEDs  212   a  for generating a full color image. For example, when each of the micro LEDs  212   a  emit the same blue color, for generating three primary color (RGB), three micro LEDs  212   a  are used in corporation with three types of colored quantum dots  500 . A first type of quantum dot  500  is used to convert blue color into red color. A second type of quantum dot  500  is used to convert blue color into green color. A third type of quantum dot  500  is used to generate the same blue color, but enhances the properties of the light (e.g. reduces emission line width, increases quantum efficiency, etc.). Furthermore, the color filter  215  also has light-transmission regions, and each of the light-transmission regions is aligned correspondingly to each of the micro LEDs  212   a.  Each of the light-transmission regions can allow the light emitted from each of the micro LEDs  212   a  to pass through. Since the light with various colors of each of the micro LEDs  212   a  passes through the light enhancing layer  213  and then passes through each of the light-transmission regions of the color filter  215 , the light passes through the color filter  215  can still keep its narrow line width (FWHM) resulted from the quantum dots  500  in the light enhancing layer  213 , therefore color saturation can be enhanced. 
     Each micro LED  212   a  will emit light in both the upward and downward directions. To utilize downward light, the electrode layer  211  may include a reflective electrode deposited at the bottom of each micro LED  212   a.  However, such reflective electrode also reflects or scatters the incident ambient light, which could degrade the Ambient Contrast Ratio (ACR). For solving this issue, one strategy is to adopt tiny LED chips to reduce the aperture ratio and cover the non-emitting area with a mask (black matrix) to absorb the reflected or scattered ambient light. Another strategy to suppress the reflected or scattered ambient light is using a color filter  215  having light-absorptive material to absorb unwanted lights. 
     A polarization angle and a polarization direction of a light can be adjusted when the light passes through the polarizer  217 . In high luminance situation (i.e., the micro LED array  212  emits ultrahigh brightness), a circular polarizer  217  is commonly registered above the color filter  215  to block the reflected ambient light from the bottom electrode layer  211  in order to suppress the ambient light reflection from the bottom electrode layer  211 . The polarizer  217  may be removed when the color filter  215  includes light-absorptive material (absorptive color filter). However, in the embodiment of  FIG. 2 , while the color filter  215  is an absorptive color filter, the ambient light reflection from the bottom electrode layer  211  can be further reduced by the corporation of the polarizer  217  and the color filter  215 . 
       FIG. 3  is a schematic view showing a micro LED display device  300  according to one embodiment of the present disclosure. In  FIG. 3 , similar as the micro LED display device  200  in  FIG. 2 , the micro LED display device  300  includes an electrode layer  311 , a micro LED array  312 , a light enhancing layer  313 , a first substrate  314 , a color filter  315 , a second substrate  316  and a polarizer  317 . The light enhancing layer also includes a plurality of quantum dots  500 . The details of the functions and the alignment order of each layer are similar as that in  FIG. 2 , and are not addressed herein. The difference between the micro LED display device  200  and the micro LED display device  300  is that each of the sub-pixel units  315   a  in the micro LED display device  300  is departed by a mask  315   b.  The mask  315   b  can be used to block a scattered or reflected ambient light for preventing the interference. The mask  315   b  can be made of black materials to form a so-call black matrix. A plurality of masks  312   b  are also located between each of the micro LEDs  312   a.  Furthermore, each of the sub-pixel units  315   b  is aligned from each other, and each of the micro LEDs  312   a  is aligned correspondingly to each of the sub-pixel units  315   b.    
       FIG. 4  is a schematic view showing a micro LED display device  400  according to one embodiment of the present disclosure. The micro LED display device  400  includes an electrode layer  411 , a micro LED array  412 , a substrate  414  and a color filter  415 . The color filter includes a plurality of quantum dots  500 . Mask  312   b,    315   b  is also used for reducing the reflected or scattered ambient light. In  FIG. 4 , the light enhancing layer  313  in  FIG. 3  is removed. In  FIG. 2  and  FIG. 3 , the light enhancing layer  213 ,  313  filled with quantum dots  500  is used for increasing the light performance. However, in  FIG. 2  and  FIG. 3 , the light enhancing layer  213 ,  313  located above the light source (micro LED array  212 ,  312 ) may possibly absorbs the light emitted from the micro LEDs  212   a,    312   a,  thus the light conversion efficiency may be decreased, resulting in the decrease of the color gamut. For further improving the light performance, the color quantum dots  500  are emerged into to the color filter  415 , as shown in  FIG. 4 . Furthermore, the volume of the micro LED display device  400  can be further reduced for obtaining a more compact size. 
     In the micro LED display device  200 ,  300 ,  400  of the present disclosure, the emitted light is provided by the micro LEDs  212   a,    312   a,    412   a  of the micro LED array  212 ,  312 ,  412 , and the micro LEDs  212   a,    312   a,    412   a  are also serve as sub-pixel units  215   a,    315   a,    415   a.  The mechanism of generating full color image of such micro LED display device  200 ,  300 ,  400  is significantly different from the conventional LCD device. Therefore, in the micro LED display device  200 ,  300 ,  400  of the present disclosure, extra backlight module is not required, therefore the manufacturing cost can be reduced. Furthermore, the micro LED display device  200 ,  300 ,  400  of the present disclosure also has higher power efficiency, wider viewing angle, longer lifetime and higher color gamut than the conventional LCD device. 
     Although the present disclosure has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims.