PATENT DOCUMENT

Publication Number: US-10591774-B2
Application Number: US-201715693305-A
Country: US
Kind Code: B2

Title: Displays with collimated light sources and quantum dots

Abstract:
A display may have display layers that form an array of pixels. The display layers may include a first layer that includes a light-blocking matrix and a second layer that overlaps the first layer. The first layer may include quantum dot elements formed in openings in the light-blocking matrix. The light-blocking matrix may be formed from a reflective material such as metal. The second layer may include color filter elements that overlap corresponding quantum dot elements in the first layer. Substrate layers may be used to support the first and second layers and to support thin-film transistor circuitry that is used in controlling light transmission through the array of pixels. The display layers may include a liquid crystal layer, polarizer layers, filter layers for reflecting red and green light and/or other light to enhance light recycling, and layers with angularly dependent transmission characteristics.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 a light source configured to produce pump light, wherein the light source comprises an array of cells each of which includes a light-emitting device; 
 display layers that form an array of pixels configured to display images in response to the pump light, wherein the display layers include a first layer having at least first quantum dot elements of a first color and second quantum dot elements of a second color, a second layer having at least first color filter elements of the first color that overlap the first quantum dot elements and second color filter elements of the second color that overlap the second quantum dot elements, a first substrate layer, and a second substrate layer; and 
 a filter layer interposed between the first layer and the light source, wherein the filter layer is configured to reflect light of the first and second colors and pass the pump light and wherein each cell in the array of cells includes a transparent layer with gratings that diffract light from the light-emitting device of that cell through the first substrate layer. 
 
     
     
       2. The display defined in  claim 1  wherein the display layers comprise:
 a liquid crystal layer interposed between the first and second substrate layers, wherein the second layer is interposed between the liquid crystal layer and the second substrate layer. 
 
     
     
       3. The display defined in  claim 2  wherein the first substrate layer is interposed between the first layer and the light source. 
     
     
       4. The display defined in  claim 1  wherein the light-emitting device of each cell is a light-emitting diode configured to exhibit destructive interference in a direction perpendicular to the transparent layer and configured to exhibit constructive interference in a direction that is not perpendicular to the transparent layer. 
     
     
       5. The display defined in  claim 1  wherein the gratings include at least first and second gratings with different respective rotational orientations. 
     
     
       6. The display defined in  claim 1  wherein the light-emitting device of each cell includes a light-emitting diode with a first index of refraction and wherein the transparent layer has a second index of refraction that is within 10% of the first index of refraction. 
     
     
       7. The display defined in  claim 1  wherein the transparent layer includes openings and reflective material in the openings. 
     
     
       8. The display defined in  claim 2  wherein the filter layer is interposed between the first substrate layer and the first layer and is formed from at least one material with a refractive index of at least 1.4 and wherein the display further comprises a layer of material between the filter layer and the first layer having an index of refraction of less than 1.3. 
     
     
       9. The display defined in  claim 2  wherein the first layer has a first refractive index and wherein the display layers comprise a layer interposed between the first and second layers with a second refractive index that is lower than the first refractive index. 
     
     
       10. The display defined in  claim 2  wherein the first layer comprises a matrix of light-blocking material with openings that receive the first and second quantum dot elements. 
     
     
       11. The display defined in  claim 10  wherein the light-blocking material comprises metal. 
     
     
       12. The display defined in  claim 1  wherein the light source includes blue light-emitting diodes that are configured to produce blue pump light and wherein the first layer includes diffuser elements associated with blue pixels. 
     
     
       13. The display defined in  claim 1  wherein the light source includes ultraviolet light-emitting diodes that are configured to produce ultraviolet pump light and wherein the first layer includes blue quantum dot elements that produce blue light in response to receiving the ultraviolet pump light. 
     
     
       14. The display defined in  claim 1  wherein the display layers comprise:
 a liquid crystal layer interposed between the first and second substrate layers. 
 
     
     
       15. A display, comprising:
 a light source configured to produce pump light; 
 display layers that form an array of pixels configured to display images in response to the pump light, wherein the display layers include:
 a first layer having at least first quantum dot elements of a first color and second quantum dot elements of a second color; 
 a second layer having at least first color filter elements of the first color that overlap the first quantum dot elements and second color filter elements of the second color that overlap the second quantum dot elements; 
 a first substrate layer; and 
 a second substrate layer, wherein the second layer is interposed between the first substrate layer and the second substrate layer; and 
 
 a filter layer interposed between the first layer and the light source, wherein the filter layer is configured to reflect light of the first and second colors and pass the pump light and wherein the second substrate layer has a portion with recesses. 
 
     
     
       16. The display defined in  claim 15  wherein the second substrate comprises a transparent material with a refractive index and wherein the recesses are patterned to reduce the refractive index of the second substrate in the portion of the second substrate facing the second layer. 
     
     
       17. The display defined in  claim 16  wherein the display layers further comprise a glass layer adjacent to the recesses, wherein the glass layer is interposed between the second substrate layer and the second layer. 
     
     
       18. The display defined in  claim 15 , wherein the display layers include a liquid crystal layer interposed between the first and second substrate layers and wherein the second layer is interposed between the liquid crystal layer and the second substrate layer. 
     
     
       19. A display, comprising:
 a light source that produces pump light; 
 display layers that form an array of pixels configured to display images in response to the pump light, wherein the display layers include a first layer having at least red and green quantum dot elements and a second layer having at least red color filter elements that overlap the red quantum dot elements in red pixels of the array of pixels and green color filter elements that overlap the green quantum dot elements in green pixels of the array of pixels, wherein the first layer includes a metal matrix having openings that receive at least the red and green quantum dot elements, and wherein the red and green quantum dot elements of the first layer have a first index of refraction; 
 a filter layer interposed between the first layer and the light source that reflects at least red and green light and that passes the pump light; and 
 a layer of material between the first layer and the filter layer that has a second index of refraction that is less than the first index of refraction. 
 
     
     
       20. The display defined in  claim 19 , further comprising:
 an additional layer of material between the first layer and the second layer, wherein the additional layer of material has a third index of refraction that is less than the first index of refraction.

Description:
This application claims the benefit of provisional patent application No. 62/483,606, filed Apr. 10, 2017, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices, and, more particularly, to electronic devices with displays. 
     Electronic devices often include displays. Backlit displays such as backlit liquid crystal displays include backlight units. A backlight unit produces light that travels outwardly through an array of pixels in a display. The pixels modulate the intensity of the light from the backlight unit to create images on the display. 
     Backlight units help ensure that displays can display images in a wide variety of ambient lighting conditions. If care is not taken, however, backlight units may produce light that does not efficiently illuminate display pixels or that does not allow the display pixels to exhibit desired levels of color performance. 
     SUMMARY 
     A display may have display layers that form an array of pixels. The array of pixels may include quantum dot elements of different colors. A light source may produce pump light for the quantum dot elements. When exposed to pump light, the quantum dot elements may emit light for forming images on the display. A liquid crystal layer, thin-film transistor circuitry, and one or more polarizer layers may be used to form pixel structures in the display layer that modulate the light intensities of individual pixels. 
     The light source may include an edge-lit light guide or an array of cells containing individually adjustable light-emitting devices. Light-emitting devices may emit unpolarized light or light that is substantially polarized. In some configurations, light-emitting devices such as light-emitting diodes formed from semiconductor dies may be covered with a high-index-of-refraction material that forms a light-distributing waveguide structure. The high-index-of-refraction material may have a refractive index that is within 10% or other suitable amount of the refractive index of the light-emitting diodes (e.g., the semiconductor material forming the semiconductor dies). The waveguide structures may have light-redirecting structures such as gratings for redistributing light from the light-emitting diodes outwards through the pixel array. The waveguide structures may also have openings in which reflective material is formed to help distribute the light within each cell. 
     The display layers may include a first layer that includes a light-blocking matrix and a second layer that overlaps the first layer. The first layer may include the quantum dot elements formed in openings in the light-blocking matrix. The light-blocking matrix may be formed from a reflective material such as metal. The second layer may include color filter elements that overlap corresponding quantum dot elements in the first layer. 
     Substrate layers may be used to support the first and second layers and to support the thin-film transistor circuitry that is used in controlling light transmission through the array of pixels. A low-index portion of the second substrate layer may be formed from recesses in a surface of the second substrate layer that faces the first and second layers. 
     The display layers may include a liquid crystal layer, polarizer layers, filter layers for reflecting red and green light and/or other light while passing pump light wavelengths to enhance light recycling, and light-collimating filter layers with an angularly dependent transmission characteristic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG. 2  is a schematic diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG. 3  is a cross-sectional side view of an illustrative display in accordance with an embodiment. 
         FIG. 4  is a cross-sectional side view of a light source having a filter layer with an angularly dependent transmission characteristic in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of light source structures including an edge-lit light guide in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of light source structures in a direct-lit lighting configuration in accordance with an embodiment. 
         FIG. 7  is a side view of an illustrative direct-lit light source cell having a light collimation structure such as a holographic element in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of a direct-lit light source cell having a light source with nanorods in accordance with an embodiment. 
         FIG. 9  is a cross-sectional side view of a direct-lit light source cell having an index-matched layer with light-redirecting features such as grating features in accordance with an embodiment. 
         FIG. 10  is a top view of an illustrative direct-lit light source cell of the type shown in  FIG. 9  in accordance with an embodiment. 
         FIG. 11  is a cross-sectional side view of an illustrative light source having a stack of multiple light-emitting devices such as light-emitting diodes or lasers of different colors in accordance with an embodiment. 
         FIG. 12  a cross-sectional side view of a portion of an illustrative display with quantum nanoparticles such as quantum dots in accordance with an embodiment. 
         FIG. 13  is a cross-sectional side view of a portion of an illustrative display with low refractive index layer to help recycle off-axis light emitted from quantum dot elements in accordance with an embodiment. 
         FIG. 14  is a cross-sectional side view of a portion of an illustrative display with a low-index-of-refraction layer to help recycle off-axis light in accordance with an embodiment. 
         FIG. 15  is a cross-sectional side view of a portion of an illustrative display with a holographic layer with an angularly dependent light transmittance in accordance with an embodiment. 
         FIG. 16  is a cross-sectional side view of an illustrative display having pixels with color filter elements in accordance with an embodiment. 
         FIG. 17  is a cross-sectional side view of an illustrative display with color filter elements and quantum dot elements of different corresponding colors for forming pixels of different respective colors in a display with a non-inverted thin-film transistor layer display configuration in accordance with an embodiment. 
         FIG. 18  is a cross-sectional side view of an illustrative display having color filter elements and quantum dot elements of different corresponding colors in an inverted thin-film transistor layer display configuration in which a layer of thin-film transistor circuitry is interposed between a display cover layer and a liquid crystal layer in accordance with an embodiment. 
         FIGS. 19, 20, and 21  show illustrative color filter and quantum dot configurations for displays in accordance with embodiments. 
         FIGS. 22, 23, and 24  show illustrative locations for low-index-of-refraction layers in a display to help enhance light recycling in accordance with embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with a display is shown in  FIG. 1 . Electronic device  10  may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, a device embedded in eyeglasses or other equipment worn on a user&#39;s head, or other wearable or miniature device, a computer display that does not contain an embedded computer, a computer display that includes an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     In the example of  FIG. 1 , device  10  includes a display such as display  14  mounted in housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. A touch sensor may be formed using electrodes or other structures on a display layer that contains a pixel array or on a separate touch panel layer that is attached to the pixel array (e.g., using adhesive). 
     Display  14  may include an array of pixels  22 . The array of pixels  22  in display  14  may form a rectangular area or an area of other suitable shapes for displaying images for a user. Pixels  22  may be formed from liquid crystal display (LCD) components, an array of electrophoretic pixels, an array of electrowetting pixels, or pixels based on other display technologies. Configurations in which display  14  is a liquid crystal display that is illuminated by a light source are sometimes described herein as an example. Liquid crystal display pixels for display  14  may have any suitable switching configuration (e.g., fringe-field switching, vertical alignment, twisted nematic, in-plane switching, etc.). Fringe-field switching displays may exhibit reduced sensitivity to touch. Twisted neumatic designs may be helpful in arrangements in which blue light is being modulated, because the retardation of liquid crystal material tends to be greater at short wavelengths. The use of liquid crystal display technology for forming display  14  is merely illustrative. Display  14  may, in general, be formed using any suitable type of pixels. 
     Display  14  may be protected using a display cover layer such as a layer of transparent glass or clear plastic. Openings may be formed in the display cover layer. For example, an opening may be formed in the display cover layer to accommodate a button, a speaker port, or other component. Openings may be formed in housing  12  to form communications ports (e.g., an audio jack port, a digital data port, etc.), to form openings for buttons, etc. In some arrangements, the display cover layer for display  14  is free of openings and/or housing  12  is free of openings for buttons, etc. 
       FIG. 2  is a schematic diagram of device  10 . As shown in  FIG. 2 , electronic device  10  may have control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  18  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  18  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors (e.g., ambient light sensors, proximity sensors, orientation sensors, magnetic sensors, force sensors, touch sensors, pressure sensors, fingerprint sensors, etc.), light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input-output devices  18  and may receive status information and other output from device  10  using the output resources of input-output devices  18 . Input-output devices  18  may include one or more displays such as display  14 . 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may display images on display  14  using an array of pixels in display  14 . While displaying images, control circuitry  16  may control the transmission of each of the pixels in the array and can make adjustments to the amount of illumination for the pixel array that is being produced by light source structures in display  14 . 
     Display  14  may have a rectangular shape (i.e., display  14  may have a rectangular footprint and a rectangular peripheral edge that runs around the rectangular footprint) or may have other suitable shapes. Display  14  may be planar or may have a curved profile. 
     A cross-sectional side view of display  14  is shown in  FIG. 3 . As shown in  FIG. 3 , display  14  may include a light source such as light source  42 . Light source  42 , which may sometimes be referred to as a pump light source or backlight, may be configured to supply illumination to display layers  46  such as light  44 . In some arrangements, pixels  22  include quantum dots, quantum rods, or other quantum nanoparticles that emit light at a particular color (e.g., red, green, etc.) in response to receiving pump light at a shorter wavelength (e.g., blue pump light or ultraviolet pump light). In these configurations, light  44  serves as pump light for the quantum dots (or quantum rods or other quantum nanoparticles) and structures  42  may be referred to as a source of pump light or pump light source. In other configurations, pixels  22  do not contain quantum nanoparticles. In these configurations, light  44  serves as backlight illumination and light source  42  may be referred to as a backlight unit. 
     As shown in  FIG. 3 , light  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 3 ) away from light source  42  and is received by pixels  22  in display layers  46 . Light  44  passes through transparent structures in pixels  22  and/or appropriately colored filter elements and/or is absorbed by quantum dots in pixels  22  and re-emitted from the quantum dots at wavelengths associated with the quantum dots (e.g., red light is emitted from red quantum dots, etc.). In this way, light  44  may help illuminate images on the pixel array formed from pixels  22  in display layers  46  so that these images may be viewed by viewer  48  in direction  50 . 
     Display layers  46  may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in housing  12  or display layers  46  may be mounted directly in housing  12  (e.g., by stacking display layers  46  into a recessed portion in housing  12 ). Display layers  46  may form a liquid crystal display or may be used in forming displays of other types. 
     In a liquid crystal display, display layers  46  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Display  14  may also include polarizers. The polarizers may be formed from external polarizer layers (e.g., polarizer layers on the surfaces of layers  56  and  58  that face away from liquid crystal layer  52 ) and/or from in-cell polarizers (polarizers facing liquid crystal layer  52 ). 
     Layers  58  and  56  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  58  and  56  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, lower layer  58  may be a thin-film transistor layer that includes an array of pixel circuits based on thin-film transistors and associated electrodes (pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Upper layer  56  may be a layer that includes an array of colored pixel elements (e.g., color filter elements and/or colored quantum dot elements) for providing display  14  with the ability to display color images. If desired, the lower layer of display  14  may be a layer that includes an array of colored elements (e.g., color filter elements and/or colored quantum dot elements) and the upper layer of display  14  may be a thin-film transistor layer. Configurations in which an array of colored elements (e.g., color filter elements and/or colored quantum dot elements) are combined with thin-film transistor structures on a common substrate layer in the upper or lower portion of display  14  may also be used. 
     During operation of display  14  in device  10 , control circuitry (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed to pixels  22  using display driver circuitry (e.g., one or more display driver integrated circuits and/or thin-film transistor circuitry) while light source  42  is providing light  44  to pixels  22 . 
     It may be desirable to limit the angular spread of the light from pixels  22  to enhance display efficiency. Configurations in which light source  42  produces polarized light may also be helpful in enhancing display efficiency (e.g., polarizer losses can be reduced). 
     To help collimate light  44 , light source  42  may be provided with a filter layer having an angularly dependent light transmission characteristic. As shown in  FIG. 4 , for example, light source  42  may have a light-emitting structure such as light-emitting structure  42 L. Structure  42 L may include an edge-lit light guide, may include a direct-lit backlight structure having an array of light-emitting cells (tiles) and/or may have other suitable structures for emitting light. The light that is emitted upwards by structure  42 L may include light  44 - 1  that is aligned closely with the Z-axis of  FIG. 4  (e.g., light that is aligned with surface normal n of structure  42 L and the other layers of display  14 ). The light emitted from structure  42  may also include off-axis light  44 - 2  (e.g. light at a non-zero angle with respect to surface normal n). Light source  42  may include a light-collimating filter layer with an angularly dependent transmission characteristic such as layer  62 . Layer  62  may be a hologram, a thin-film interference filter, or other layer of one or more materials that reflects off-axis light (light not aligned with axis Z that is therefore oriented at a non-zero angle with respect to the surface normal of display layers  46  such as a non-zero angle of at least 10° or at least 20°, as examples) more than on-axis light (light aligned with axis Z). As a result, collimated light  44 - 1  will pass through layer  62  and will serve as collimated illumination for display structures  46  ( FIG. 3 ), whereas off-axis light  44 - 2  will be recycled back into light-emitting structures  42 L. If desired, a polarizer such as polarizer  60  may be used to help polarize collimated light  44 - 1 . Polarizer  60  of  FIG. 4  may be a reflective polarizer that passes light that is oriented along a desired axis (e.g., the Y axis) and that reflects orthogonally polarized light (e.g., light aligned with the X-axis). In general, display  14  may include one or more non-reflective polarizers (e.g., external polarizers and/or in-cell polarizers) may include one or more reflective polarizers, or other suitable light polarizing structures. 
     As shown in  FIG. 5 , light-emitting structures  42 L may be based on a light guide such as light guide  102 . Light guide  102  may be provided with illumination from light source  100 . Light guide  102  may be, for example, a thin transparent sheet of polymer or other transparent material having light scattering features. Light source  100  may include one or more light-emitting diodes or other light emitting components and may emit light into one or more edges of light guide  102  (light guide  102  may be an edge-lit light guide). Light that has been emitted into light guide  102  may be distributed in the X-Y plane of  FIG. 6  in accordance with the principal of total internal reflection. The light scattering features (pits, grooves, bumps, ridges, light-scattering particles, or holes in light guide  102 , printed ink patterns on light guide  102 , etc.) may scatter laterally propagating light out of light guide  102  to serve as light  44 . Reflector  104  may be used to redirect any downwardly scattered light back in the upwards (+Z) direction. 
     In the illustrative configuration of  FIG. 6 , light-emitting structure  42 L has been formed using a direct-lit light source arrangement. With this arrangement, structure  42 L includes a two-dimensional array of individually adjustable cells (tiles)  110 . Each cell  110  may contain a corresponding light source  106 . Each light source  106  may include one or more light-emitting diodes. The light-emitting diodes may emit pump light (e.g., blue light, ultraviolet light, or other shorter wavelength light for pumping quantum dots, quantum rods, or other quantum nanoparticles) or, in other configurations for display  14 , may serve as backlight for pixels  22  with color filter elements (e.g., white backlight). Reflectors  108  may be used to reflect light that is emitted from each light sources  106  along the Z axis as light  44 . Reflectors  108  may have curved shapes that help collimate light  44 . 
     If desired, light  44  can be further collimated by covering each cell  110  with a filter layer such as layer  62  of  FIG. 7  that exhibits an angularly dependent transmission characteristic. With this type of arrangement, off-axis light rays that strike the lower surface of layer  62  will be recycled back towards reflector  108 , whereas collimated on-axis light rays (light rays parallel to the Z-axis) may pass outwardly through layer  62 . Layer  62  may be a holographic element (holographic filter), may be a thin-film filter, or may be other suitable filter that selectively passes on-axis light. 
     In the illustrative example of  FIG. 8 , light source  106  has been formed from nanorods  112 . Nanorods  112  may be oriented horizontally, may be oriented vertically along the Z-axis as shown in  FIG. 8 , or may have other suitable orientations. Electrodes  114  and  116  (e.g., a metal electrode from which nanorods  112  are grown and an electrode formed from a consolidated layer of nanorods and/or other conductive electrode structures) may be used in applying current through nanorods  112  so that nanorods  112  emit light  44 . Light  44  may tend to be polarized (e.g., with an electric field in the X-Y plane of  FIG. 8 ). The polarization of light  44  of cell  110  of  FIG. 8  may, if desired, be at least partially maintained by forming reflectors  108  from structures that tend to reflect the polarized upwards as collimated light  44  without altering the polarization state of the light. 
     If desired, light source  106  may include other types of light-emitting device configured to emit polarized light. Light source  106  may, for example, be a laser such as a vertical-cavity surface-emitting laser. The vertical-cavity surface-emitting laser may have a strained semiconductor die that causes the laser to emit light that is at least 70% or at least 80% linearly polarized. Polarized light may also be produced from other lasers or light-emitting diodes (e.g., strained or unstrained surface-emitting and/or edge-emitting devices with Bragg gratings and/or angled output facets). With one illustrative configuration, light source  106  may be a resonant cavity light-emitting diode (e.g., a resonant cavity light-emitting diode with strained semiconductor layers and/or other structures that enhance the polarization of output light). If desired, a polarizer structure may be formed at the output of a light source that is supplying light to a cell or edge-lit light guide (e.g., to help polarize the light before the light is distributed within the cell or light guide). 
       FIG. 9  is a cross-sectional side view of an illustrative light source cell in which a light-emitting device such as a laser or light-emitting diode is covered with an index-matching coating with light redirecting structures. As shown in  FIG. 9 , light source  106  may be formed from one or more light-emitting devices mounted on a substrate such as substrate  114  (e.g., a printed circuit, a printed circuit covered with a polymer or inorganic buffer layer such as a buffer layer with a relatively low index of refraction, etc.). 
     Light source  106  may include one or more light-emitting diodes. For example, light source  106  may include one or more blue light-emitting diodes such as group III-nitride-based light-emitting diodes. Light source  106  may be covered with a layer such as layer  118 . Layer  118  may be formed from a transparent material (e.g., a clear polymer or an inorganic layer) and may have light-redirecting structures  120  (sometimes referred to as out-coupling structures) such as gratings, thin-film filter structures, protrusions, grooves, or other light scattering structures, and/or other structures for coupling light out of the waveguide formed by layer  118 . Light-redirecting structures  120  may be formed using photolithography, molding, laser processing, machining, and/or other fabrication techniques. 
     Light that is emitted by light source  106  may be distributed laterally within layer  118  (in the X-Y plane of  FIG. 9 ) in accordance with the principal of total internal reflection. When this light reaches a light-redirecting structure (e.g., a grating or other light-redirecting structure  120 ), it is redirected (e.g., diffracted) upwardly along the Z-axis as light  44 . Light-redirecting structures  120  may each include a respective grating or other structure that is associated with a respective pixel  22  and/or may be formed from uniform and/or pseudorandomly oriented structures. The pattern of light-redirection structures  120  in layer  118  may be configured to create uniform patterns of light  44  across all of the pixels  22  of display  14 . 
     The index of refraction of semiconductor structures such as group III-nitride-based light-emitting diodes is about 2.45-2.4 at wavelengths of 400-500 nm. To reduce reflections at the output of light source  106 , layer  118  may be formed from a material with an index-matched refractive index value (e.g., a refractive index of 2.4 or other value that is within 25%, within 20%, within 15%, within 10%, within 5%, within 1%, or within other suitable amounts of the index of refraction of light source  106 ). As an example, layer  118  may be formed from titanium oxide (e.g., titania with a refractive index of 2.2-2.4), silicon nitride, silicon oxynitride, aluminum oxide, silicon oxide, other oxides, other nitrides, other inorganic materials, mixtures of these inorganic materials, and/or organic materials with appropriate refractive index values. 
     With one illustrative configuration, light source  106  may include a group III-nitride-based light-emitting diode (e.g., a micro-light-emitting diode with lateral dimensions of 15 microns or less or other suitable size or other light-emitting diode) of 1-2 microns in thickness and layer  118  may have a thickness of 2-4 microns. A thin-film reflector (e.g., a dielectric stack forming a thin-film interference filter structure, a metal layer, or other reflective thin-film structure) such as thin-film reflector  116  may be formed on layer  118  with a shape that overlaps light source  106 . The p contact of the group III-nitride-based light-emitting diode(s) of source  106  may be located adjacent to substrate  114  and may be formed from a reflective material such as silver or other suitable metal. The active layer of the group III-nitride-based light-emitting diode(s) may be positioned relative to the p contact so that vertically emitted light  44 A from the group III-nitride-based light-emitting diode(s) experiences destructive interference when reflecting off of the lower side of layer  116  and so that diagonally emitted light  44 B experiences constructive interference when reflecting off of layer  116 . Due to the destructive interference for on-axis light (parallel to the Z axis and the surface normal of cell  110  and layer  118 ) and the constructive interference for off-axis light (light propagating at non-zero angles with respect to the Z axis), light from light source  106  may be emitted into layer  118  with a desired angular spread (over a range of angles tilted with respect to the Z axis). A thin p-type layer (e.g., 60 nm of a material such as GaN or other suitable material) may be used in forming the group III-nitride-based light-emitting diodes and the silver layer forming the p contact may be formed directly on the surface of the group III-nitride-based light-emitting diode(s) to help maximize the destructive interference of light such as light  44 A and maximize constructive interference of tilted light such as light  44 B (e.g., light with an angle that is suitable for waveguide propagation within layer  118 ). Source  60  has quantum wells and the distance between the last quantum well (i.e., the quantum well that is closes to the p-GaN) and the p-contact metallization is generally less than 70 nm, and preferably less than 60 nm. Due to index matching between light-source  106  and layer  118 , the angular distribution of light emissions from light source  106  may be preserved in a desired pattern. 
     To help ensure that light  44  is distributed uniformly over each cell  110 , cells  110  may include reflective structures. Consider, as an example, the arrangement of  FIG. 10 . As shown in the top view of illustrative cell  110  in  FIG. 10 , cell  110  (e.g., layer  118 ) may have openings such as holes  118 H. Holes  118 H may have vertical sidewalls or sidewalls of other shapes and may be formed by etching (as an example). Reflective coatings (a layer of metal, etc.) such as coating  122  may be formed in cell  110  (e.g., in holes  118 H, along the edges of cell  110 , etc.) to help redistribute light  44 B emitted from light source  106 . 
     Structures  120  may include gratings. With one illustrative configuration, a first portion (e.g., a first half) of structures  120  may have gratings that run parallel to a first dimension (e.g., axis X) and a second portion (e.g., a second half) of structures  120  may have gratings that run parallel to a second dimension (e.g., axis Y). Diversifying the rotational orientation of the gratings of structures  120  may help reduce the mean path length before light  44 B is extracted as light  44 , thereby minimizing waveguide losses in layer  118 . The shape (e.g., the outline when viewed from above) of light source  106  may be configured so that light source  106  fits between structures  120  (e.g., structures  120  associated with respective overlapping pixels  22 ). For example, light source  106  may have a cross-shaped footprint as shown in  FIG. 10  with arms that run between respective pairs of structures  120  (and therefore between respective pairs of overlapping pixels  22 ). 
     Light  44 B that is emitted by source  106  may have polarization in which the electric field of light  44 B lies predominantly in the X-Y plane of  FIG. 10  (e.g., in the plane of cell  110 ) or other suitable polarization. Light with certain ranges of angular orientations with respect to the Z axis (e.g., off-axis light) may exhibit constructive interference and certain range of angular orientations with respect to the Z axis (e.g., on-axis light) may exhibit destructive interference. Constructive interference may, for example, create lobes of higher intensity light  44 B along predefined orientations with respect to axis Z. Gratings in structures  120  can be configured to redirect light  44 B having certain predefined k-vectors vertically along axis Z. Because constructive interference dominates in certain angular ranges and because the gratings of structures  120  are appropriately configured to handle this light, out-coupled light  44  will exhibit a well-defined beam divergence (e.g., 20-30° or narrower) as established by the properties of the gratings forming structures  120  and the spectral bandwidth of light source  106 . 
     As shown in  FIG. 11 , multiple waveguide layers  118  may be stacked (e.g., high-index materials such as titania layers may be formed with intervening lower-index buffer layers such as silicon oxide layers or polymer layers and/or other stacked waveguides may be formed) and each of these multiple waveguide layers may receive light from a light source of a respective color (e.g., a first of layers  118  may receive red light R from a light source  106  that produces light at red wavelengths, a second of layers  118  may receive blue light B from a light source  106  that produces light at blue wavelengths, and a third of layers  118  may receive green light G from a light source  106  that produces light at green wavelengths). Each layer  118  may guide light of a different wavelength to a set of tuned light redirecting structures (gratings)  120  in that layer, thereby allowing light of multiple colors (R, G, B) to be redirected outwardly as light  44  to illuminate corresponding pixels  22 . Light sources  106  may be centrally located within each cell  110  and/or may emit light into the edge of waveguide layers such as layers  118 . If desired, a single waveguide layer  118  may carry multiple wavelengths of light (e.g., red, green, and/or blue) supplied by respective light sources  106 . 
     A cross-sectional side view of a portion of an illustrative display for device  10  is shown in  FIG. 12 . In the example of  FIG. 12 , display  14  includes a layer with an array of colored pixel elements such as layer  56 . Layer  56  may include layers such as transparent substrate layer  130  (e.g., a glass layer or plastic layer with a thickness of 0.3-0.7 mm, a thickness of at least 0.2 mm, a thickness of less than 1 mm, etc.). Layer  130  may be the outermost substrate layer of display  14  and/or may be an internal substrate layer. Layer  132  may be formed on layer  56  (e.g., on an inner surface of layer  130 ) and may include an array of colored elements (colored pixel elements for corresponding pixels  22 ). The colored elements of layer  132  may, in general, include colored filter elements and/or quantum nanoparticle elements (e.g., elements formed from quantum dots or quantum rods). 
     With the illustrative arrangement of  FIG. 12 , layer  132  includes a blue (B) color filter element  134  and includes quantum dot elements such as red (R) and green (G) quantum dot elements  136 . If desired, other photoluminescent materials and color filtering arrangements may be used can be used (e.g., phosphorous and color filters can be used in forming elements  136 , quantum rods may be used, etc.). Layer  132  allows display  14  to display color images. Light  44  from light source  42  may be blue light. The intensity of light  44  in each of pixels  22  may be modulated using liquid crystal pixel structures formed from thin-film transistors, thin-film capacitors, thin-film electrodes, and/or other thin-film circuitry (e.g., thin film circuitry in layer  58 ). In an arrangement in which layer  130  is the outermost layer of display  14 , the intensity of blue light  44  is modulated before reaching layer  132 . In flipped panel arrangements, light can be modulated after passing through colored elements in layer  132 . 
     In the blue pixels of display  14 , blue light  44  passes through blue color filter element  134 . Color filter element  134  may contain blue polymer (e.g., polymer containing blue dye or pigment) and may allow blue light  44  from light source  42  to pass. If desired, diffuser layers and/or clear polymer layers may be incorporated into element such as element  134  instead of using blue polymer or in addition to using blue polymer to form element  134 . The blue light  44  from light source  42  serves as pump light for quantum dot elements  136 . Each quantum dot element may have cadmium selenide particles or other quantum nanoparticles (e.g., quantum dots or quantum nanorods) that are configured to emit light of a desired wavelength in response to absorbing pump light  44 . For example, red elements  136  may include red quantum dots in a polymer binder that emit red light when pumped with blue light  44  and green elements  136  may include green quantum dots in a polymer binder that emit green light when pumped with blue light  44 . Layer  132  may have a thickness of 10 microns, 5-15 microns, at least 2 microns, at least 7 microns, less than 20 microns, or other suitable thickness. 
     To help enhance display performance and efficiency, display  14  may include one or more filter layers such as filter layer  140 . Filter layers in display  14  such as filter layer  140  may be formed from materials with desired spectral transmission characteristics (e.g., a transmission characteristic that absorbs one or more wavelengths of light and that transmits one or more other wavelengths of light based on the bulk optical properties of the materials and/or may be formed from stacks of dielectrics, semiconductors, and/or metals to form a thin-film interference filter that passes and blocks desired wavelengths of light. Cholesteric liquid crystal filters may also be formed with desired spectral transmission characteristics for use as filter layer  140 . 
     Filter (filter layer)  140  of  FIG. 12  may, as an example, be configured to pass blue light while reflecting red and green light. With this type of arrangement, blue pump light  44  may pass through filter  140  to pump quantum dots  136  in red and green pixels and may pass through filter  140  and blue color filter elements  134  in blue pixels. Red and green light that is generated in the red and green quantum dots of layer  134  and that is propagating in the −Z direction may be recycled back in the +Z direction by reflecting off of filter  140 . 
     Light  44  may be polarized when exiting light source  42  and/or upon passing through a lower polarizer layer in display  14  (e.g., a polarizer interposed between liquid crystal layer  52  and light source  42 ). An upper polarizer for display  14  may be formed using a layer of polarizer material located above layer  56  and/or at other suitable location above layer  52  or be formed using an in-cell polarizer such as in-cell polarizer  140  of  FIG. 12 . 
     If desired, one or more low refractive index layers may be incorporated into display  14  to help recycle off-axis light. This may enhance the collimation of emitted light and improve display efficiency. As an example, consider the illustrative configuration of  FIG. 13 . In the example of  FIG. 13 , low refractive index layer  142  has been interposed between layer  130  (e.g., a display cover layer or other substrate) and layer  132 . Layer  130  may be formed from a material such as glass and may have a refractive index of 1.5 (as an example). Layer  142  may have a refractive index that is lower than the refractive index of layer  130 . For example, the refractive index of layer  142  may be 1.1-1.4, at least 1.2, at least 1.3, less than 1.4, less than 1.35, less than 1.3, etc. The refractive index of layer  132  may be about 1.5-1.7, at least 1.4, less than 1.7, etc. Air in region  154  may have an index of refraction of 1, which gives rise to an index-of-refraction difference with layer  130  at glass-air interface  130 U (the upper surface of layer  130 ). 
     During operation, on-axis light that is emitted light from quantum dot elements  136  such as light ray  144 , may pass outwardly parallel to the Z-axis (and parallel to display surface normal n). Because light  144  is parallel to the Z-axis, light  144  will pass through interface  130 U without being reflected inwardly due to total internal reflection. If off-axis light were present at interface  130 U, this off-axis light might reflect at interface  130 U due to total internal reflection and might then travel significant lateral distances (distances in the X-Y plane) before striking a colored pixel element in layer  132  due to the relatively large thickness of layer  130 . This could cause a green ray of light to enter a red quantum dot (as an example), leading to crosstalk between pixels. 
     With the configuration of  FIG. 13 , off-axis rays such as ray  146  that are emitted from a quantum dot element, are reflected inwardly (at least somewhat towards the −Z direction) as shown by reflected ray  148  due to total internal reflection at the interface between layer  132  and layer  142  (e.g., when the refractive index of layer  142  is less than the refractive index of layer  132 ) and/or due to reflection at the interface between layers  142  and  130  due to refractive-index mismatch between layers  142  and  130 . Layer  142  is relatively thin. For example, the thickness of layer  142  may be 2-5 microns, at least 1 micron, at least 2 microns, at least 5 microns, fewer than 100 microns, fewer than 20 microns, fewer than 10 microns, or other suitable thickness. As a result, the interface between layer  142  and layer  130  is relatively close to the upper surface of layer  132 , so even if light  146  reflects off of the interface between layer  142  and  130  rather than the interface between layer  132  and layer  142 , reflected light rays such as ray  148  will not tend to travel excessively in lateral directions X and Y before being returned to layer  132 . This may help prevent recycled light that is associated with a pixel of one color from spreading into adjacent pixels of different colors. As shown in  FIG. 13 , for example, inwardly directed rays such as ray  148  may be scattered and/or absorbed and reemitted by quantum dots such as quantum dot  150  in the same quantum dot element from which ray  146  was emitted. For example, green light  146  may be reabsorbed and/or scattered by green quantum dot  150  rather than impinging on a red quantum dot in an adjacent pixel. After scattering from dot  150  and/or being reemitted from dot  150 , this light has a renewed opportunity to form collimated light that will pass through interface  130 U (see, e.g., collimated light  152 ). 
     As shown in the illustrative configuration of  FIG. 14 , low-index layer  142  may be formed by patterning the inner surface of layer  130 . This creates areas of low refractive index (e.g., areas from which portions of the glass or other material forming layer  130 , which may have index values of about 1.5, have been selectively removed and replaced with vacuum, air, or other gases, which may have index values of about 1.0). The selectively removed portions are small and therefore do not create optically noticeable individual features but rather cause the overall effective index for the patterned region to be reduced (e.g., the index in region  142  is an average of the index of the 1.5 index regions with the index of the 1.0 index regions and may be characterized by a average (effective) value of 1.25 (as an example). 
     Etching techniques and/or other fabrication techniques may be used in forming recesses such as recesses  142 H (e.g., holes, grooves, etc.) among unrecessed portions of layer  130  such as regions  142 C. Recesses  142 H may have lateral dimensions of 0.5-2 microns, at least 0.1 microns, at least 0.5 microns, at least 1 micron, fewer than 10 microns, fewer than 4 microns, or fewer than 2 microns. Recesses  142 H may have depths of 2-5 microns, at least 1 micron, at least 2 microns, at least 5 microns, fewer than 100 microns, fewer than 20 microns, fewer than 10 microns, or other suitable depths. Optional glass plate  142 ′ may be incorporated into layer  142  to help prevent portions of layer  132  from intruding into recesses  142 H. Layer  142 ′ may be at least 1 micron thick, at least 2 microns thick, fewer than 25 microns thick, etc. 
     In some configurations for display  14 , a holographic element or other optical element that serves as a filter with an angularly dependent transmission may be used to help collimate light exiting layers  46 .  FIG. 15  shows how a holographic layer such as layer  142 E may be interposed between layer  130  and layer  132 . Layer  142 E may have a higher transmission for on-axis light (light in a range of angles parallel to axis Z and surface normal n) and lower transmission for off-axis light (light at wider angles). On-axis light rays exiting layer  132  such as light ray  144  will pass through layer  142 E and upper surface (interface)  130 U (where layer  130  meets surrounding air) and, due to the on-axis nature of these light rays, will pass through layer  130  without being reflected due to total internal reflection. Off-axis light rays exiting layer  132  such as light ray  144  will be reflected by layer  142 E close to layer  132 . As a result, off-axis light can be recycled by the quantum dots of layer  132  to form on-axis light  152  that passes through interface  130 U. If desired, prism films and other films with optical features in the films may be used in collimating light  44 . 
     If desired, layer  142  may be formed from low-index-of-refraction structures other than recesses  142 H. For example, layer  142  may be formed from a polymer layer (e.g., a polymer layer with an index of refraction of 1.25-1.35, at least 1.25, at least 1.3, at least 1.35, less than 1.4, less than 1.35, etc. such as a fluorinated polymer or siloxane polymer). As another example, layer  142  may be formed from air-infused silica (e.g., silica with bubbles, silica into which hollow microspheres or other low-index particles have been incorporated, etc.). Air-infused silica layers may have index-of-refraction values of 1.2, at least 1.25, at least 1.3, less than 1.4, less than 1.35, less than 1.3, less than 1.25, etc. 
     If desired, light  44  may be ultraviolet light and each blue color filter element  134  may be a blue quantum dot element formed from blue quantum dots in a layer of polymer binder. Color filter elements may, if desired, be included in pixels  22  that also include quantum dot elements. In the arrangements of  FIGS. 12-14 , elements  136  are formed from quantum dots and element  134  is formed from blue color filter material. This is, however, merely illustrative. Each colored pixel element in layer  132  may include color filter elements, quantum nanoparticle elements, diffuser, clear material, and/or other suitable structures. 
     Layer  142 E may have high transmission for light that is oriented parallel to axis Z (and surface normal n) such as light  144  and may reflect (and therefore recycle) off-axis light such as light  146 . In the example of  FIG. 15 , a layer such as holographic layer  142 E (e.g., a hologram) with a transmission characteristic that favors on-axis light over off-axis light has been incorporated into layers  46  instead of incorporating low-index layers such as layer  142 . If desired, a layer such as layer  142 E may be incorporated into display  14  in addition to incorporating one or low-index layers such as layer  142  into layers  46 . In the example of  FIG. 15 , holographic layer  142 E has been interposed between layer  132  and layer  130 . Layers such as layer  142 E may be incorporated elsewhere in layers  46 , if desired. 
     A cross-sectional side view of an illustrative display for device  10  is shown in  FIG. 16 . In the configuration of  FIG. 16 , display  14  has a light source such as light source  42 L that emits light  44  such as white light. If desired, light source  42 L may include a filter layer that exhibits an angularly dependent transmission characteristic to help recycle off-axis light as described in connection with layer  62  of  FIG. 4  (for an edge lit light guide layer configuration) and layer  62  of  FIG. 6  (for a direct-lit configuration having an array of individually adjustable light-source cells  110 ) or may use other suitable light source structures. As shown in  FIG. 16 , display  14  may include upper polarizer  154  and lower polarizer  60 . In-cell polarizers may also be used in display  14 , if desired. In configurations in which light source  42 L emits polarized light, lower polarizer structures such as illustrative external lower polarizer  60  of  FIG. 16  may be omitted or the strength of the lower polarizer structures may be reduced. 
     In the illustrative configuration of  FIG. 16 , layers  56  and  58  are sandwiched between polarizers  154  and  60 . Liquid crystal layer  52  is interposed between layers  56  and  58 . As shown in  FIG. 16 , layer  58  may include substrate  160  (e.g., a layer of clear glass or polymer), a layer of thin-film transistor circuitry such as thin-film transistor circuitry  158  on substrate  160 , and liquid crystal alignment layer  156  (e.g., a polyimide layer). Layer  56  may include substrate  162  (e.g., a clear glass or polymer layer that is used in forming display cover layer  130  and/or an internal layer in display layers  46 ). Colored pixel element layer  132  may be formed on substrate  162  and may include color filter elements  150  such red color filter elements for red pixels  22 R that pass red light, green color filter elements  150  for green pixels  22 G that pass green light, and blue color filter elements  150  for blue pixels  22 B that pass blue light. 
     Color filter elements  150  may be formed within respective openings in light-blocking matrix  152 . Matrix  152  may have the shape of a grid with openings for pixels (e.g., rectangular pixel openings, chevron-shaped pixel openings, etc.). The material for forming matrix  152  may be opaque, so that matrix  152  blocks stray light from adjacent pixels and prevents color crosstalk. Matrix  152  may be formed from black ink (e.g., a polymer binder containing black particles such as particles of carbon black or other dark pigments or a polymer containing dark dyes), may be formed from metal, or may be formed from other opaque masking materials. Liquid crystal alignment layer  157  (e.g., a polyimide layer) may be formed on the inner surface of layer  56  facing liquid crystal layer  52 . 
     In the illustrative configuration of  FIG. 17 , display  14  has an array of quantum nanoparticle elements such as quantum dot elements. To pump the quantum dot elements, light source  42 L may be configured to emit pump light light  44 . Pump light  44  may be emitted using a direct-lit configuration with multiple individually adjustable cells in light source  42 L or in an arrangement for light source  42 L that includes an edge-lit light guide layer. Pump light  44  may have a wavelength that is sufficiently short to excite the quantum dots in layer  132 . For example, if layer  132  includes red, green, and blue quantum dots, pump light  44  may be ultraviolet light. If layer  132  includes red and green quantum dots and no blue quantum dots, pump light  44  may be blue light. 
     Colored pixel element layer  132  may include an array of colored pixel elements each of which includes two or more sublayers of materials. For example, each pixel  22  in display  14  may have a first layer with quantum dots and a second layer with color filter material that helps filter out stray light (e.g., unconverted pump light). As shown in  FIG. 17 , for example, layer  56  may include a colored pixel element layer such as layer  132  that includes a first layer such as layer  132 A with quantum dot structures and a second layer such as layer  132 B with color filter elements. Liquid crystal layer  52  may be interposed between layer  56  and layer  58 . Lower polarizer  60  may be formed on the lower surface of substrate  160  or may be omitted in configurations in which light source  42 L produces polarized light. The upper polarizer in display  14  may, if desired, be formed using an in-cell polarizer configuration, as shown by in-cell polarizer  154 C. An in-cell polarizer may also be used in forming lower polarizers for displays  14 , if desired. 
     Filter layer  164  of  FIG. 17  may be configured to pass pump light (e.g., blue or ultraviolet light) while reflecting and thereby recycling light at other wavelengths (e.g., red and green light produced by red and green quantum dots). In some arrangements, filter layer  164  may be formed from a stack of dielectric layers forming a thin-film interference filter and may have materials with an index of refraction of 1.4-1.6, at least 1.4, at least 1.45, less than 2, less than 1.7, etc. (as examples). Filter layer  164  may also be formed using other filter configurations (e.g., a cholesteric liquid crystal filter arrangement). 
     Red pixels  22 R may each include a red quantum dot element  136 R (e.g., an element with red quantum dots in a polymer binder) in layer  132 A and a red color filter element  150 R in layer  132 B. Green pixels  22 G may each include a green quantum dot element  136 G (e.g., an element with green quantum dots in a polymer binder) in layer  132 A and a red color filter element  150 G in layer  132 B. Blue pixels  22 B may be configured appropriately for the type of pump light being produced by light source  42 L. 
     With a first illustrative configuration for display  14  of  FIG. 17 , light source  42 L is configured to produce ultraviolet pump light  44 . In this configuration, elements  134 B in layer  132 A of blue pixels  22 B may be blue quantum dot elements that produce blue light when pumped with ultraviolet light and elements  150 B in layer  132 B may be blue color filter elements. Filter layer  164  in this configuration may be an ultraviolet pass filter that passes ultraviolet pump light  44  while reflecting red, green and blue light to enhance output efficiency. For example, filter layer  164  may be a thin-film interference filter or may be a cholesteric liquid crystal filter that is configured to pass ultraviolet light while reflecting red, green, and blue light. To help reduce the output of unconverted ultraviolet light from the front of display  14 , an ultraviolet-light cut filter material may be incorporated into the elements of layer  132 B and/or may be interposed at other locations between layer  132 A and the front of display  14  (e.g., an ultraviolet cut filter may be interposed between layers  132 A and  132 B, may be interposed between layer  132 B and substrate  162 , etc.). 
     With a second illustrative configuration for display  14  of  FIG. 17 , light source  42 L is configured to produce blue pump light  44 . In this configuration, blue pixels  22 B may contain light-diffusing material in elements  134 B (e.g., elements  134 B may be diffuser elements) to diffuse blue pump light so that the angular distribution of output light in the blue pixels matches that of the red and green light being emitted from the quantum dots in the red and green pixels. Blue pixels  22 B may also contain, if desired, clear polymer in elements  150 B or blue color filter material in elements  150 B. Filter layer  164  in this configuration may be a blue light pass filter (e.g., a thin-film filter or cholesteric liquid crystal filter) that reflects red and green light to enhance the output efficiency of the red and green pixels. If desired, elements  150 R and  150 G may be formed from yellow color filter material instead of red and green color filter material. The yellow color filter material may be used to absorb residual (unconverted) blue pump light and thereby ensure that the red and green pixels do not appear too bluish in color. 
     Light-blocking matrix  152  may be interposed laterally (in dimensions X and Y) between the colored pixel elements in layer  132 . To help improve lateral light recycling, matrix  152  may be formed from a reflective material such as reflective metal (e.g., matrix  152  may be a light-reflecting matrix formed from aluminum or other metal). As quantum dots in the elements of layer  132 A emit light laterally, this emitted light will be reflected back towards the quantum dots by the reflective material of matrix  152  and scattered and/or absorbed and reemitted, thereby enhancing efficiency. 
       FIG. 18  is a cross-sectional side view of display  14  in an illustrative inverted display panel configuration (sometimes referred to as a flipped thin-film transistor panel or FTP configuration). In this type of arrangement, light  44  from light source  42  passes through layer  132  of layer  56  before passing through liquid crystal layer  52  and thin-film transistor layer  58 . Light source  42 L may produce quantum dot pump light such as blue or ultraviolet light. 
     Layer  56  may include substrate  162 , filter layer  164 , colored pixel element layer  132 , in-cell polarizer  154 C, and liquid crystal alignment layer  157 . Liquid crystal alignment layer  157  may be interposed between liquid crystal layer  52  and polarizer  154 C. Layer  132  may include layer  132 A (e.g., a layer in which some or all of the pixels contain quantum dots) and layer  132 B (e.g., a layer with color filter material). Light-blocking matrix  152  may be a reflective matrix formed from metal or may be formed from an opaque material such as black polymer. 
     Layer  58  may include a clear substrate such as substrate  160 . Upper polarizer  60  may be formed on the outer surface of substrate  160 . Thin-film transistor circuitry  158  (e.g., electrodes and transistors for pixel circuits in the array of pixels in display  14 ) may be formed on the inner surface of substrate  160 . A light-blocking matrix such as matrix  152 T may be formed from an opaque material in layer  158  to help block stray light from adjacent pixels. Matrix  152 T may be aligned with matrix  152  in layer  56 . Liquid crystal alignment layer  156  may be interposed between thin-film transistor circuitry layer  158  and liquid crystal layer  52 . 
     As with the illustrative arrangement for display  14  of  FIG. 17 , display  14  of  FIG. 18  may use either blue or ultraviolet light  44  as pump light for quantum dots in layer  132 . Filter layer  164  may be configured to pass the pump light while reflecting light at other wavelengths (e.g., red and green light produced by red and green quantum dots when the pump light is blue or red, green, and blue light produced by red, green, and blue quantum dots when the pump light is ultraviolet). 
     Red pixels  22 R may each include a red quantum dot element  136 R in layer  132 A and a red color filter element  150 R in layer  132 B. Green pixels  22 G may each include a green quantum dot element  136 G in layer  132 A and a red color filter element  150 G in layer  132 B. Blue pixels  22 B may be configured appropriately for the type of pump light being produced by light source  42 L. 
     With a first illustrative configuration for display  14  of  FIG. 18 , light source  42 L is configured to produce ultraviolet pump light  44 . In this configuration, blue pixels  22 B may contain blue quantum dot elements (elements  136 B) in layer  132 A that produce blue light when pumped with ultraviolet light and may contain blue color filter elements (elements  150 B) in layer  132 B. Filter layer  164  in this configuration may be an ultraviolet pass filter that passes ultraviolet pump light  44  while reflecting red, green and blue light to enhance output efficiency. Filter layer  164  may be a thin-film interference filter or may be a cholesteric liquid crystal filter (as examples). To help reduce the output of unconverted ultraviolet light from the front of display  14 , an ultraviolet-light cut filter material may be incorporated into the elements of layer  132 B and/or may be interposed at other locations between layer  132 A and the front of display  14  (e.g., an ultraviolet cut filter may be interposed between layers  132 A and  132 B, may be interposed between layer  132 B and polarizer  154 C, etc.). 
     With a second illustrative configuration for display  14  of  FIG. 18 , light source  42 L is configured to produce blue pump light  44 . In this configuration, blue pixels  22 B may contain light-diffusing material in elements  134 B (e.g., elements  134 B may be diffuser elements) to diffuse blue pump light so that the angular distribution of output light in the blue pixels matches that of the red and green light being emitted from the quantum dots in the red and green pixels. Blue pixels  22 B may also contain, if desired, clear polymer in elements  150 B or blue color filter material in elements  150 B. Filter layer  164  in this configuration may be a blue light pass filter that reflects red and green light to enhance the output efficiency of the red and green pixels. If desired, elements  150 R and  150 G may be formed from yellow color filter material (e.g., instead of red and green color filter material) to absorb residual (unconverted) blue pump light and thereby ensure that the red and green pixels do not appear too bluish in color. 
     Illustrative configurations for the colored pixel elements and filtering layers of display  14  are shown in  FIGS. 19, 20, and 21 . In  FIGS. 19, 20, and 21 , layers  132  are shown as being incorporated into the lower display layers of an inverted display of the type shown in  FIG. 18 . If desired, layers  132  of  FIGS. 19, 20, and 21  may be used in non-inverted configurations for display  14  such as the configuration for display  14  of  FIG. 17 . 
     As shown in  FIG. 19 , matrix  152  may have openings that receive elements  136 R,  136 G, and  134 B. Matrix  152  of  FIG. 17  laterally separates filter elements  150 R,  150 G, and  150 B, whereas matrix  152  of  FIG. 19  laterally separates only elements  136 R,  136 G, and  134 B. 
       FIG. 20  shows how yellow color filter material such as filter element  150 Y may be used in place of individual red and green filter elements. Yellow elements  150 Y may overlap red quantum dot element  136 R and green quantum dot element  136 G and may be used as a blue cut filter to absorb unconverted blue pump light  44 . Element  150 B in the arrangement of  FIG. 20  may be formed from clear polymer (as an example). 
       FIG. 21  shows how a blanket (global) filter layer such as an ultraviolet cut filter layer that overlaps pixels of all colors ( 22 R,  22 G,  22 B) may be used in forming layer  132 B. Layer  132 B of  FIG. 21  may be formed from a thin-film interference filter or a cholesteric liquid crystal filter (as examples). 
     In general, any suitable pixel configuration may be used for display  14  (pixelated red, green, and blue color filters, a yellow color filter structure for red and green pixels and clear structure for blue pixels, a ultraviolet cut filter when light  44  is ultraviolet light, and/or other suitable pixel arrangements) and these pixel configurations may be used for inverted displays (see, e.g., display  14  of  FIG. 18 ) and for non-inverted displays (see, e.g., display  14  of  FIG. 17 ). 
     If desired quantum dots in layer  132  may be replaced by other photoluminescent structures such as quantum rods. The polarization of light emitted by the quantum rods may be oriented along the lengths of the quantum rods, so quantum rod configurations for display  14  may, if desired, use quantum rods that are aligned with the pass axis of a corresponding polarizer (e.g., an in-cell polarizer) in display  14 . Quantum dots and/or quantum rods may be formed from CdSe nanostructures and/or nanostructures of other suitable materials. 
     As described in connection with  FIG. 14 , it may be desirable to incorporate a low-index layer such as layer  142  between layer  132  and adjacent higher index layers (e.g., layer  130 ). In the example of  FIG. 22 , low-index layer  142  has been incorporated between layer  162  and layer  132  in an arrangement in which layer  132  includes layer  132 A and layer  132 B. Layers  132 A and/or  132 B and/or layer  162  may have refractive indices higher than the refractive index of low-index layer  142 . Layer  132 A may include red, green, and blue quantum dot elements or may include red and green quantum dot elements and diffuser elements for blue pixels. Layer  132 B may include red or yellow color filter elements for red pixels, green or yellow color filter elements for green pixels, and clear elements for blue pixels. A low index layer (layer  142 ) may also be incorporated between filter layer  164  and layer  132  and may have a lower refractive index than layer  164  and/or layer  132 . With configuration such as these, the presence of layer(s)  142  will help create light reflections for off-axis light leaving layer  132 . These light reflections will help recycle light in layer  132  (e.g., in quantum dots in layer  132 A) without undesired lateral light spreading and will therefore help enhance display efficiency without creating crosstalk between pixels. If desired, layer  132 B may be a diffuser layer (e.g., in an configuration with color filter elements and/or in a configuration without color filter elements). 
     The cross-sectional side view of  FIG. 23  shows another illustrative configuration for incorporating low-index layers  142  into display  14 . In the arrangement of  FIG. 23 , layer  132 A is covered with an optional diffuser layer such as layer  132 A′ (e.g., a layer with light-scattering structures to help diffuse light, etc.). Layer  132 B may include color filter elements. Filter  164  may be supported on a substrate such as substrate  162 . Polarizer  154 C may be, for example, an in-cell polarizer. Low-index layers  142  may be interposed between layer  132 B and layer  132 A′ and between layer  132 A and filter  166  (as an example) and may have a lower index of refraction than these adjacent layers to help improve quantum dot light recycling. Arrangements of the type shown in  FIG. 23  may be used in inverted displays of the type shown in  FIG. 18  (as an example). 
     In the illustrative arrangement of  FIG. 24 , substrate  162  is located above layers  132 A and  132  (e.g., as in non-inverted configurations of the type shown in  FIG. 17 ). Layer  132 B (e.g. a layer with color filter elements) has been formed on the inner surface of substrate  162 . Layer  132 A (e.g., a quantum dot layer) may be covered with optional diffuser layer  132 A′. Liquid crystal layer  52  may be interposed between layer  132 A and thin-film transistor layer  58 . To enhance light recycling for off-axis light rays emitted from quantum dot layer  132 A, low-index layers  142  may be incorporated into display  14  at locations such as an upper location between layer  132 B and layer  132 A′ and a lower location such as between layer  132 A and layer  52 . 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20170831
Publication Date: 20200317
Grant Date: 20200317
Priority Date: 20170410
Inventors: DROLET, JEAN-JACQUES P.
CHEN, YUAN
Steckel, Jonathan S.
BITA, ION
SIZOV, DMITRY S.
TAI, CHIA HSUAN
LEONARD, John T.
WANG, LAI
LYNGNES, OVE
XIN, Xiaobin
GE, ZHIBING
Assignee: APPLE INC
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Family ID: 63710914