PATENT DOCUMENT

Publication Number: US-9810942-B2
Application Number: US-201514944016-A
Country: US
Kind Code: B2

Title: Quantum dot-enhanced display having dichroic filter

Abstract:
A display device is provided. The display device includes a light source emitting a blue light and a light emitting layer including a first group of red quantum dots and a second group of green quantum dots. The light emitting layer is configured to absorb a first portion of the blue light from the light source to emit red light and green light and to transmit a second portion of the blue light. The display device also includes dichroic filter layers to improve light recycling and backlight efficiency.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 a display module; and 
 backlight structures that provide light to the display module, wherein the backlight structures include:
 blue light-emitting diodes that provide blue light; 
 a light-conversion film that converts the blue light into light of at least one different color; 
 a first dichroic filter; 
 a second dichroic filter, wherein the light-conversion film is interposed between the first and second dichroic filters; and 
 a prism layer interposed between the second dichroic filter and the display module, wherein the prism layer reflects on-axis light and collimates off-axis light. 
 
 
     
     
       2. The display defined in  claim 1  wherein the light-conversion film comprises a phosphor film. 
     
     
       3. The display defined in  claim 1  wherein the light-conversion film comprises a quantum dot film. 
     
     
       4. The display defined in  claim 1  wherein the first dichroic filter and the second dichroic filter have different transmission spectrums. 
     
     
       5. The display defined in  claim 1  further comprising a reflective polarizer laminated to the second dichroic filter. 
     
     
       6. The display defined in  claim 1  wherein the light of the different color includes red light and green light and wherein the light of the different color is reflected by the first dichroic filter. 
     
     
       7. The display defined in  claim 1  wherein the light of the different color includes yellow light that is reflected by the first dichroic filter. 
     
     
       8. The display defined in  claim 1  wherein the first dichroic filter is interposed between the light-conversion film and the light-emitting diodes. 
     
     
       9. The display defined in  claim 1  wherein the second dichroic filter is interposed between the display module and the light-conversion film. 
     
     
       10. The display defined in  claim 1  wherein the display module comprises a liquid crystal display module. 
     
     
       11. The display defined in  claim 1  wherein the blue light-emitting diodes are arranged in an array and directly light the light-conversion film. 
     
     
       12. The display defined in  claim 1  further comprising a reflective polarizer, wherein the prism layer is interposed between the second dichroic filter and the reflective polarizer. 
     
     
       13. The display defined in  claim 1  wherein the first dichroic filter passes blue light and reflects the light of the different color. 
     
     
       14. The display defined in  claim 13  wherein the second dichroic filter reflects more blue light than passes blue light and wherein the second dichroic filter passes the light of the different color. 
     
     
       15. The display defined in  claim 1  wherein the light-conversion film comprises a quantum dot film with red and green quantum dots and wherein the light of the different color comprises red and green light from the red and green quantum dots. 
     
     
       16. The display defined in  claim 15  wherein the first dichroic filter reflects at least 95% of the red and green light and passes at least 90% of the blue light. 
     
     
       17. The display defined in  claim 16  wherein the second dichroic filter reflects at least 55% of blue light exiting the quantum dot film and passes at least 95% of the red and green light from the quantum dot film. 
     
     
       18. A display, comprising:
 a liquid crystal display module; 
 backlight structures that provide backlight for the liquid crystal display module, wherein the backlight structures comprise:
 a blue light-emitting diode that produces blue light; 
 a quantum dot film that receives the blue light and that produces light of a different color; 
 a first dichroic filter; 
 a second dichroic filter, wherein the quantum dot film is interposed between the first and second dichroic filters; and 
 a prism layer interposed between the second dichroic filter and the liquid crystal display module, wherein the prism layer reflects on-axis light and collimates off-axis light.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of patent application Ser. No. 13/630,785, filed Sep. 28, 2012, which claims the benefit of provisional patent application No. 61/660,501, filed Jun. 15, 2012, which are hereby incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to display devices. More specifically, the invention relates to a display having a dichroic filter. 
     BACKGROUND 
     A quantum dot-enhanced liquid crystal display uses quantum dots to facilitate display of electronic information. As one example, Quantum dots (QDs) are semiconductors in the form of nanocrystals that provide an alternative display. The electronic characteristics of the QDs are generally governed by the size and shape of the nanocrystals. Quantum dots of the same material, but with different sizes, can emit light of different colors when excited. More specifically, the emission wavelength of the QDs varies with a size and shape of the quantum dots. As one example, larger dots may emit longer wavelength light, such as red light while smaller QDs may emit shorter wavelength light, such as blue light or violet light. For example, quantum dots formed from cadmium selenide (CdSe) may be gradually tuned to emit light from the red region of the visible spectrum for a 5 nm diameter quantum dot, to the violet region for a 1.5 nm quantum dot. By varying dot size, the entire visible wavelength, ranging from about 460 nm (blue) to about 650 nm (red), may be reproduced. 
     One of the common issues with quantum dots is that they are potentially toxic. Cadmium-free quantum dots or heavy metal-free quantum dots may be desirable for consumer goods applications. In other words, it may be useful to reduce the cadmium (Cd) content in a product below a threshold such that the cadmium is present only in trace or minimal amounts. Quantum dots with a stable polymer coating may be nontoxic. Another issue is the high production cost for the quantum dots in the display. 
     There remains a need for designing the quantum dot-enhanced liquid crystal display to achieve reduced toxicity, improved performance, and lower cost in fabrication. 
     SUMMARY 
     Embodiments described herein may provide a dichroic filter (DCF) on quantum dot-enhanced film (QDEF) in a liquid crystal display for transmitting a red light and a green light and a small portion of a blue light but reflecting most of the blue light, such that a white light is produced. The DCF helps reduce the density of quantum dots and thus may reduce toxic content, such as Cd content. The DCF also improves color and luminance uniformity. The DCF may also reduce quenching and thus manufacturing cost. 
     In one embodiment, a display device is provided. The display device includes a light source emitting a blue light, and a light emitting layer including a first group of red quantum dots and a second group of green quantum dots. The light emitting layer is configured to absorb a first portion of the blue light from the light source to emit red light and green light and to transmit a second portion of the blue light. The display device also includes a dichroic filter layer configured to reflect a portion of the transmitted second portion of the blue light such that the reflected portion of the blue light is recycled in the light emitting layer and to transmit a remaining portion of the transmitted second portion of the blue light to output a white light. 
     In another embodiment, a display device includes a light source emitting a blue light and a light emitting layer comprising a first group of red quantum dots and a second group of green quantum dots. The light emitting layer configured to absorb a first portion of the blue light from the light source to emit red light and green light and to transmit a second portion of the blue light. The display device also includes a dichroic filter layer configured to reflect a portion of the transmitted second portion of the blue light such that the reflected portion of the blue light is recycled in the light emitting layer and to transmit a remaining portion of the transmitted second portion of the blue light to output a white light. The display device further includes a liquid crystal display including a front polarizer, a rear polarizer, and a liquid crystal layer between the first polarizer and the second polarizer. The liquid crystal display also includes a plurality of color filters between the front polarizer and the liquid crystal layer. The liquid crystal display is configured to control passage of the white light from the dichroic filter through the color filters arranged in subpixels. 
     Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a conventional liquid crystal display (LCD) with an edge lit configuration (Prior art). 
         FIG. 1B  illustrates a conventional liquid crystal display (LCD) with a direct lit configuration (Prior art). 
         FIG. 2  illustrates a quantum dot-enhanced display with a dichroic filter in an embodiment. 
         FIG. 3  illustrates a detailed structure of the dichroic filter (DCF) of  FIG. 2  in an embodiment. 
         FIG. 4  illustrates transmittance versus wavelength for the dichroic filter of  FIG. 3  and an emission curve of QDs in an embodiment. 
         FIG. 5  illustrates the recycling of blue light in the QDEF by the dichroic filter in an embodiment. 
         FIG. 6  illustrates color gamuts for the quantum dot-enhanced display of  FIG. 2  and the LCD of  FIG. 1  in an embodiment. 
         FIG. 7  is a graph of illustrative an illustrative backlight emission spectrum and transmission curves for illustrative dichroic filters in accordance with an embodiment. 
         FIGS. 8, 9, and 10  are cross-sectional side views of illustrative displays with multiple dichroic filters in accordance with embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure may be understood by reference to the following detailed description, taken in conjunction with the drawings as briefly described below. It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale. 
       FIG. 1A  illustrates a conventional liquid crystal display (LCD) with an edge lit backlight configuration. The LCD of  FIG. 1A  includes a primary light source or backlight source  102 , a light guide panel (LGP)  106 , and a LCD panel  120 . The conventional LCD of  FIG. 1A  uses the LCD panel  120  with control electronics and the backlight source  102  to produce color images. The backlight source  102  provides white light. 
     The liquid crystal display panel  120  includes color filters  122  arranged in subpixels, such as a red color filter, a green color filter, and a blue color filter. The red, green, and blue filters  122  transmit a light having a specific wavelength of white light incident from the backlight source  102 . The filters  122  transmit wavelengths of light corresponding to the color of each filter, and absorb other wavelengths. Accordingly, a light loss is generated in the liquid crystal display by the color filters. In order to display images having a sufficient brightness, the backlight source  102  is typically used. However, this generally causes an increase in power consumption by the liquid crystal display. 
     The LCD panel  120  also includes a front polarizer  118 , a rear polarizer  114 , a thin film transistor (TFT)  126 , and liquid crystal  116  as well as electrodes (not shown). The color filters  122  are positioned between the liquid crystal  116  and the front polarizer  118 . The TFT  126  is positioned between the liquid crystal  116  and the rear polarizer  114 . Each pixel has a corresponding transistor or switch for controlling voltage applied to the liquid crystal  116 . The liquid crystal  116  may include rod-shaped polymers that naturally form into thin layers with a natural alignment. The electrodes may be made of a transparent conductor, such as an indium-tin-oxide material (commonly referred to as “ITO”). The front and rear polarizers  118  and  114  may be set at right angles. Normally, the LCD panel  120  may be opaque. When a voltage is applied across the liquid crystal  116 , the rod-shaped polymers align with the electric field and untwist such that the voltage controls the light output from the front polarizer  118 . For example, when a voltage is applied to the liquid crystal  116 , the liquid crystal  116  rotates so that there is a light output from the front polarizer  118 . 
     For a conventional LCD, a white LED, a cold-cathode fluorescent lamps (CCFL) or an incandescent backlighting may be used. Generally, a brighter light source may have a shorter life time and generate more heat. 
     As an example, the backlight source  102  includes one or more blue LEDs and yellow phosphor pumped by the blue LEDs to emit white light for the LCD. The white light from the backlight source  102  travels toward the light guide panel (LGP)  106 , through diffuser film  110  and prism  108  as well as double brightness enhanced film (DBEF)  124 , which provides a uniform light backlight for the liquid crystal display panel  120 . The phosphors may include transition metal compounds or rare earth compounds. Alternatively, the backlight source  102  may include a white LED that provides white light to the light guide panel  106 . The white LED may use a blue LED with broad spectrum yellow phosphor, or a blue LED with red and green phosphors. 
       FIG. 1B  illustrates a direct lit backlight configuration for the conventional LCD. As shown, the main differences from the edge lit configuration  100 B include different arrangement of a number of LEDs and absence of the LGP  106 . More specifically, the LEDs  102  are arranged to directly provide light to a diffuser plate  125 , which is normally thicker than the diffuser film  110  and thus supports the diffuser film  110 . 
       FIG. 2  illustrates a quantum dot-enhanced liquid crystal display with a dichroic filter (DCF) incorporated in a sample embodiment. Quantum dot-enhanced liquid crystal display  200  includes a light source  202 , a light guide panel (LGP)  204 , a quantum dot-enhanced film (QDEF)  206 , a DCF  210 , a LCD panel  216 . The QD enhanced display  200  may also optionally include a prism  212  and a double brightness enhanced film (DBEF)  214 . The light source  202  may be a blue light-emitting diode (LED) or a blue Gallium Nitride (GaN) LED. 
     To produce even lighting, a blue light from the light source  202  first passes through the LGP  204  that may include a series of unevenly-spaced bumps or light extraction features  224  and a reflector  218  behind the light extraction features  224 . The LGP  204  diffuses the blue light through the series of unevenly-spaced bumps or light extraction features  224 , as shown by blue light  220 . The density of the bumps or light extraction features increases further away from the light source  202 . The front face of the LGP  204  faces the LCD panel  216  and the back of the LGP  204  has the reflector  218 , which guides otherwise wasted light back toward the LCD panel  216 . In one example, the reflector  218  may be made of highly reflective material, such as white polyethylene terephthalate (PET) and in one embodiment reflects about 97% of all light impacting its surface. 
     The LCD panel  216  also includes color filters arranged in subpixels, a front polarizer, a rear polarizer, and liquid crystal as well as electrodes, similar to the LCD panel  120  for the conventional LCD. Generally, there is an air space between the LCD panel  216  and the DBEF  214 . 
     Unlike the conventional LCD, instead of using the red phosphor  110 A and green phosphor  110 B, the QDEF  206  including red QDs  208 A and green QDs  208 B produces red color and green color, which are excited by the blue light from the light source  202 . The QDEF  206  converts the color while diffusing the blue light  220  from the light source  202 . 
     Generally, the QDEF  206  is configured to transmit a portion of the blue light  220  from the light source  202  such that white light  222  comes out of the QDEF  206 . The QDEF  206  includes a group of red quantum dots (QDs)  208 A and green QDs  208 B, which actively convert the blue light  220  from the LED into red light and green light through the quantum dots. When the QDs  208 A and  208 B are irradiated by the blue light from the light source  202 , the blue light causes the QDs  208 A and  208 B to photoluminescence and thereby produce secondary light. The color of the secondary light is generally a function of the size, size distribution and composition of the QDs  208 A and  208 B. 
     It will be appreciated by those skilled in the art that the QD enhanced display may vary in configuration. For example, other lit configurations may be used, including a direct lit configuration in some embodiments, similar to the direct lit configuration shown in  FIG. 1B . The prism  212  may also be removed or substituted by other brightness enhancement component in an alternative embodiment. The DBEF  214  may be removed in another embodiment. 
     The QDEF  206  may include a host matrix. The QDEF  206  may also include red QDs and green QDs  208 A and  208 B disposed in the host matrix. The host matrix allows light from the light source  202  to pass through. The host matrix may be any polymer, such as polyacrylate, polystyrene, polyimide, polyacrylamide, polyethylene, polyvinyl, poly-diacetylene, polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone, polypyrrole, polyimidazole, polythiophene, polyether, epoxies, silica glass, silica gel, siloxane, polyphosphate, hydrogel, agarose, cellulose, and the like. 
     Widely used methods of forming the QD include a chemical wet method or a chemical vapor. The chemical wet method mixes precursors with an organic solvent and grows particles to form the QD through a chemical reaction. 
     Enhancement or quenching of the radiation of the QDs may be achieved by adjusting the size of the QD, changing structure or adding other materials. Quenching may help increase light efficiency. Higher efficiency means that more red light or green light will be produced from red QDs and green QDs by using the same light source  202 . When QDs are stuck to each other, for example, a red QD is stuck to a green QD, the red QD may be re-excited by the green QD, which may increase the light efficiency of the red light, but may reduce the light efficiency of the green light. Thus, it is desirable to have the QDs separated from each other in the host matrix. When the QD density is reduced because of the recycling of the blue light through use of the DCF, there will be less likelihood for the QDs to stick to each other and thus to improve light efficiency. Therefore, quenching may be minimized to reduce manufacturing cost. 
     As an example of the QD, a group II-VI compound, such as CdSe, CdTe, CdS, ZnSe, ZnTe, ZnS, HgTe, or HgS, may be used. Also, the QD may also have a core-shell structure. The core comprises at least one selected from the groups consisting of CdSe, CdTe, CdS, ZnSe, ZnTe, ZnS, HgTe, and HgS, and the shell comprises at least one selected from the groups consisting of CdSe, CdTe, CdS, ZnSe, ZnTe, ZnS, HgTe, and HgS. Further, a group III-V compound such as GaN, InAs, GaAs, GaInP may be applied to the core or shell. 
     As shown in  FIG. 2  the DCF  210  is used in front of the QDEF  206  to recycle the blue light  220 . The DCF  210  recycles the blue light  220  from the light source  202  within the QDEF  206  by reflecting a portion of the blue light  220 , although the DCF  210  still transmits a small portion of the blue light  220  to provide the white light  222  to the LCD panel  216 . This recycling of the blue light  220  helps reduce the CdSe content, and also improves color and luminance uniformity. Quenching will also be reduced as a result of reduced QD density. 
     The QD enhanced display  200  may optionally include brightness enhancement components, such as a prism  212  and a double brightness enhanced film (DBEF)  214 . The brightness enhancement components are optically transparent. The prism  212  helps reduce beam angle and thus increases light intensity through the DBEF  214 . The DBEF may be provided by manufacturers, such as 3M® among others. 
     The DBEF  214  is a reflective polarizer film which increases efficiency by repeatedly reflecting any unpolarized light back, which would otherwise be absorbed by the LCD&#39;s rear polarizer. The DBEF  214  is placed behind the liquid crystal display panel  216  without any other film in-between. The DBEF  214  may be mounted with its transmission axis substantially parallel to the transmission axis of the rear polarizer. The DBEF  214  helps recycle the white light  222  that would normally be absorbed by the rear polarizer (not shown) of the liquid crystal panel  216 , and thus increases the brightness of the liquid crystal display panel  216 . 
       FIG. 3  illustrates a detailed structure of the DCF  210  in an embodiment. The DCF  210  is optically transparent. The DCF  210  may include alternating layers  302  and  304  of optical coatings with different refractive indexes and a glass substrate  306 . For example, layer  302  has a first refractive index and layer  304  has a second refractive index different from the first refractive index. The interfaces between the alternating layers  302  and  304  of different refractive indexes produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. In this disclosure, each of layers  302  and  304  has a thickness of about ¼ of the wavelength of the blue light. 
     The DCF  210  may be fabricated by vacuum deposition. The DCF  210  may be coated on the QDEF  206 . The DCF  210  may also be a separate sheet which is placed over the QDEF  206 . The transmission and reflection band of the DCF  210  may need to be properly aligned with the spectra of red, green and blue light. 
       FIG. 4  illustrates an emission curve  422  of QDs and a transmittance curve  420  for the DCF  210 . As shown in the emission curve  422 , each of the red, green, and blue color bands  412 A-C emitted from the QDs is separated from each other. In other words, an emission color bandwidth  410  of the QDs is relatively narrow. Each of the emission color bandwidth  410 A-C for respective quantum dots  412 A,  412 B,  412 C varies with color and material. For example, CdSe quantum dots may have a blue emission bandwidth  410  of about 35 nm. InP quantum dot may have a blue emission bandwidth  410  of about 45 nm, which is wider than that of the CdSe quantum dot. 
     As shown in  FIG. 4 , the DCF  210  also has a low transmittance over a blue region  402  (see transmittance curve  420 ). Typically, the transmittance of the DCF  210  is not zero. A small portion of the blue light  220  as shown in  FIG. 2  is transmitted through the DCF  210 , such that the white light  222  also shown in  FIG. 2  will be produced by combining the blue light from the light source  202  with the red and green lights from the QDs  208 A and  208 B. The transmittance in the blue region  402  may be at certain percentage, for example 25%, but may not be more than 50%. The transmittance of the blue region  402  may vary with the number of the layers  302  and  304  in the DCF  210  (see  FIG. 3 ). As shown in  FIG. 4 , the transmittance over a region  404  including the green and red regions as well as beyond the red region  404  is nearly 100%. The transmission curve  420  also includes a transition region or slope  406 A or  406 B may vary with a light incident angle. For example, slope  406 A at 0 degree angle of incidence (AOI) is steeper than slope  406 B at 45 degree AOI. 
     The transmittance blue region  402  has a reflection band bandwidth  408 , which may vary with coating materials for layers  302  of a first refractive index and layers  304  of a second refractive index in the DCF  210 . The reflection band width  408  often increases with the refractive index difference between the two coating materials having the first refractive index and the second refractive index. 
     The DCF  210  has a different function from the color filters  122  for the conventional LCD, because the conventional color filters  122  absorb a light in a color band while the DCF  210  reflects a light in the reflection band  408 . Thus, the DCF  210  generates less heat than the color filters  122 . The DCF  210  also has a longer life than the color filters  122 . 
       FIG. 5  illustrates recycling blue light in the QDEF in an embodiment. A larger portion  502 B of the incoming blue light  502 A is reflected by the DCF  210 . A portion of this reflected blue light will excite the red QDs  208 A and green QDs  208 B in the QDEF  206  to increase the output of the red light and green light. This is a first order recycling. The red and green QDs  208 A and  208 B also diffuse the incoming blue light  502 A more to help increase uniformity of a white light  222  through the DCF  210 , which is one of the benefits of including the DCF in the QD enhanced display  200 . A remaining portion  502 C of the reflected blue light  502 B will enter the light guide panel  112  (not shown) and reflected from the reflector  218  at the bottom of the LGP  204  and will re-enter the QDEF  206  again and excite more red and green QDs  208 A and  208 B. Again, a portion of the remaining portion  502 C will be reflected again by the DCF  210 , which is the second order recycling. This recycling will continue until no blue light will be reflected by the dichroic filter  210 . 
     The dichroic filter  210  helps reduce the density of the red and green quantum dots  208 A and  208 B through recycling the blue light  502 , which increases the emissions from each red QD  208 A and green QD  208 B. This reduction in QD density leads to less CdSe used in the quantum dot-enhanced liquid crystal display  200 , which helps achieve a lower Cd content to meet the Cd free requirement of consumer goods. 
       FIG. 6  illustrates color gamuts for the quantum dot-enhanced display and the LCD of  FIG. 2 . A color gamut is a portion of the color space that may be reproduced or represented. For example, color gamuts  602  (CdSe QD),  604  (InP QD),  606  (Adobe RGB) and  608  (standard RGB or sRGB) are a portion of real color space  610 . Currently, conventional LCD meets the standard color gamut  608 , also labeled as “sRGB”. To provide a more full color than the conventional LCD, newer color gamut  606 , also labeled as “Adobe RGB”, is desired, because newer color gamut  606  has a larger area than the standard color gamut  608  and is closer to real color space  610 . As shown in  FIG. 6 , the QD enhanced LCD  200  is better than the conventional LCD (e.g. sRGB), because the CdSe QD color gamut  602  and InP QD color gamut  604  for the QD enhanced LCD  200  have larger triangle areas than the standard color gamut  608  of the conventional LCD. 
     Additionally, the color saturation of the CdSe QD enhanced display is more close to the desired Adobe RGB or better than the InP QD enhanced display. Although InP QD enhanced display  200  is a Cd free consumer product, its color gamut is not as good as the CdSe QD enhanced display. For the CdSe QD enhanced display with wider color gamut, the Cd content may be reduced below a threshold by including the DCF  210  to recycle blue light and thus reduce QD density, which is essentially considered Cd free, or comparable to the InP QD enhanced display. 
     The QD enhanced display  200  with the QDEF  206  and the DCF  210  is characterized by better color accuracy and narrow bandwidth as well as or wider color gamut than the conventional LCD. The conventional LCD can&#39;t produce pure red, green and blue for the display. Instead, the LCD needs to add a few other colors to the Red, Green and Blue colors. 
     The QD enhanced display  200  is generally much brighter than the conventional LCD display as a result of its wider color gamut. The quantum dot-enhanced display  200  using a blue LED as a backlight, may have a power efficiency similar to the conventional LCD using a white LED backlight. However the QD-enhanced display  200  typically has a much wider color gamut than the conventional LCD backlit with a white LED. In other words, for the conventional LCD to achieve the same color gamut as the quantum dot-enhanced display  200 , power efficiency would be much lower than the quantum dot-enhanced display. 
     Furthermore, some light loss may occur during blue light recycling by the DCF  210 . However, by using highly transmissive materials in some color regions such as red and green and highly reflected materials in another color region such as the blue region for the DCF  210 , light loss may be reduced and/or minimized. In addition, the DCF may help reduce quenching and thus may increase power efficiency. These two factors affect the power efficiency and may cancel each other when taken together in a system, such that about the same electrical power may be consumed by an LCD using the DCF  210  as without the DCF  210 . 
     The quantum dot includes a material selected from the group consisting of a group II-VI compound, a group III-V compound, a group IV-VI compound, a group IV compound, and mixtures of these groups. 
     The group II-VI compound includes a material selected from the group consisting of CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, and HgZnSTe. 
     The group III-V compound includes a material selected from the group consisting of GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, and InAlPSb. 
     The group IV-VI compound comprises a material selected from the group consisting of SnS, SnSe, SnTe, PbS, PbSe, PbTe, SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, SnPbSSe, SnPbSeTe, and SnPbSTe. 
     The group IV compound comprises a material selected from the group consisting of Si, Ge, SiC, and SiGe. 
     If desired, display  200  may have backlight structures (sometimes referred to as a backlight unit) that include two dichroic filters to enhance light recycling and backlight efficiency. For example, a first dichroic filter may be placed below film  206  and a second dichroic filter may be placed above film  206 . Film  206  may be a light-conversion film that converts light of one color (e.g., blue light) into light of at least one different color (e.g., yellow light or red and green light). Film  206  may be a quantum dot film with red and green quantum dots or may be a phosphor film with red and green phosphor particles in place of red and green quantum dots. Light-conversion film  206  may be edge lit or may be directly lit by blue light-emitting diodes (or light-emitting diodes of other colors). 
     The emission spectrum of the blue light-emitting diodes and the red and green emissive elements (quantum dots or phosphors) may be characterized by an intensity versus wavelength characteristic such as curve  700  of  FIG. 7 . If desired, the red and green elements in film  206  may be replaced by yellow phosphors or yellow quantum dots that produce a yellow spectral peak such as peak  701 . Illustrative configurations for the backlight structures of display  200  that include red and green emissive structures that produce respective red and green output peaks are described herein as an example. 
     The first dichroic filter may have a high transmission in the blue (B) portion of the spectrum while reflecting red (R) and green (G) light, as shown by the illustrative transmission spectrum DCF 1  of  FIG. 7 . This allows blue light to pass through the first dichroic filter into film  206  while helping to recycle any downwardly reflected red and green light back into film  206 . As indicated by transmission characteristic DCF 2  for the second dichroic filter, the second dichroic filter may have a moderate transmission for blue (B) light (so that blue light is recycled through film  206  rather than being immediately emitted from the upper surface of film  206 ) and high transmission for red (R) and green (G) light (so as not to block red and green light being emitted by film  206 ). 
     Illustrative configurations for incorporating dichroic filters DCF 1  and DCF 2  into the backlight structures of display  200  are shown in  FIGS. 8, 9, and 10 . In these examples, display  200  is directly lit (i.e., blue light is produced by an array of light-emitting diodes  202  so that display  200  can be locally dimmed to enhance dynamic range). Light-emitting diodes  202  may emit light into backlight structure  706  (e.g., an air gap, a clear panel, a diffuser panel, or other structure). Edge-lit configurations may be used, if desired (e.g., by placing one or more blue light-emitting diodes  202  along the edge of a light-guide plate). The configurations of  FIGS. 8, 9, and 10  are merely illustrative. 
     As shown in  FIG. 8 , dichroic filter DCF 1  may be placed below film  206 . The blue light transmission properties of filter DCF 1  (e.g., blue light transmission of about 70-90%, 80% or more, 90% or more, 95% or more, etc.) allow blue light from diodes  202  to efficiently pass into film  206 . Some of the blue light is converted into red and green light by film  206 . Outwardly directed red and green light from film  206  may pass through filter DCF 2 , which preferably transmits 80% or more, 90% or more, or 95% or more of the light at red and green light wavelengths. Inwardly directed red and green light from film  206  may be reflected back outwards by filter DCF 1  (which preferably transmits less than 5-20% of red and green light and therefore reflects at least 80%, 90% or more, or 95% or more of the red and green light outward through display module  216 ). Filter DCF 2  reflects excess blue light (i.e., filter DCF 2  preferably inwardly reflects about 55% or more, 65% or more, or 75% or more of the outwardly directed blue light), so that blue light that has not been converted into red or green light is reflected back through film  206 , thereby enhancing blue light recycling. 
     Light recycling may be further enhanced using other light recycling structures. For example, prism layer  704  may recycle on-axis light downwards through film  206  (i.e., layer  704  may reflect light that is oriented directly upwards in the orientation of  FIG. 8  back in the downwards/inwards direction) while collimating off-axis light. Reflective polarizer  702  may have a pass axis that is aligned with the pass axis of the lower polarizer of liquid crystal display unit  216 . Light with different polarizations will be reflected back through film  206  and recycled. 
     If desired, the thickness of the backlight structures may be minimized by using a thin reflective polarizer film that is supported by dichroic filter DCF 2  rather than stand-alone reflective polarizer  702  of  FIG. 8 . In the example of  FIG. 9 , thin reflective polarizer layer  702 ′ has been laminated to the upper surface of dichroic filter DCF 2 , so that layer  702 ′ is interposed between prism layer  704  and filter DCF 2 . In the example of  FIG. 10 , thin reflective polarizer layer  702 ′ has been laminated to the lower surface of dichroic filter DCF 2 . 
     Having described several embodiments, it will be recognized by those skilled in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention. 
     Those skilled in the art will appreciate that the presently disclosed embodiments teach by way of example and not by limitation. Therefore, the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.

Metadata:
Filing Date: 20151117
Publication Date: 20171107
Grant Date: 20171107
Priority Date: 20120615
Inventors: YOU CHENHUA
QI JUN
HSU FAN-CHUNG
YIN VICTOR H.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02F1/133609", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F2202/108", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F2001/133614", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133615", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133615", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133614", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133609", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133615", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133609", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F2202/108", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F2202/108", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133614", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 55437387