Quantum dot-enhanced display having dichroic filter

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.

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.

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. 1Aillustrates a conventional liquid crystal display (LCD) with an edge lit backlight configuration. The LCD ofFIG. 1Aincludes a primary light source or backlight source102, a light guide panel (LGP)106, and a LCD panel120. The conventional LCD ofFIG. 1Auses the LCD panel120with control electronics and the backlight source102to produce color images. The backlight source102provides white light.

The liquid crystal display panel120includes color filters122arranged in subpixels, such as a red color filter, a green color filter, and a blue color filter. The red, green, and blue filters122transmit a light having a specific wavelength of white light incident from the backlight source102. The filters122transmit 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 source102is typically used. However, this generally causes an increase in power consumption by the liquid crystal display.

The LCD panel120also includes a front polarizer118, a rear polarizer114, a thin film transistor (TFT)126, and liquid crystal116as well as electrodes (not shown). The color filters122are positioned between the liquid crystal116and the front polarizer118. The TFT126is positioned between the liquid crystal116and the rear polarizer114. Each pixel has a corresponding transistor or switch for controlling voltage applied to the liquid crystal116. The liquid crystal116may 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 polarizers118and114may be set at right angles. Normally, the LCD panel120may be opaque. When a voltage is applied across the liquid crystal116, the rod-shaped polymers align with the electric field and untwist such that the voltage controls the light output from the front polarizer118. For example, when a voltage is applied to the liquid crystal116, the liquid crystal116rotates so that there is a light output from the front polarizer118.

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 source102includes 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 source102travels toward the light guide panel (LGP)106, through diffuser film110and prism108as well as double brightness enhanced film (DBEF)124, which provides a uniform light backlight for the liquid crystal display panel120. The phosphors may include transition metal compounds or rare earth compounds. Alternatively, the backlight source102may include a white LED that provides white light to the light guide panel106. The white LED may use a blue LED with broad spectrum yellow phosphor, or a blue LED with red and green phosphors.

FIG. 1Billustrates a direct lit backlight configuration for the conventional LCD. As shown, the main differences from the edge lit configuration100B include different arrangement of a number of LEDs and absence of the LGP106. More specifically, the LEDs102are arranged to directly provide light to a diffuser plate125, which is normally thicker than the diffuser film110and thus supports the diffuser film110.

FIG. 2illustrates a quantum dot-enhanced liquid crystal display with a dichroic filter (DCF) incorporated in a sample embodiment. Quantum dot-enhanced liquid crystal display200includes a light source202, a light guide panel (LGP)204, a quantum dot-enhanced film (QDEF)206, a DCF210, a LCD panel216. The QD enhanced display200may also optionally include a prism212and a double brightness enhanced film (DBEF)214. The light source202may be a blue light-emitting diode (LED) or a blue Gallium Nitride (GaN) LED.

To produce even lighting, a blue light from the light source202first passes through the LGP204that may include a series of unevenly-spaced bumps or light extraction features224and a reflector218behind the light extraction features224. The LGP204diffuses the blue light through the series of unevenly-spaced bumps or light extraction features224, as shown by blue light220. The density of the bumps or light extraction features increases further away from the light source202. The front face of the LGP204faces the LCD panel216and the back of the LGP204has the reflector218, which guides otherwise wasted light back toward the LCD panel216. In one example, the reflector218may 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 panel216also includes color filters arranged in subpixels, a front polarizer, a rear polarizer, and liquid crystal as well as electrodes, similar to the LCD panel120for the conventional LCD. Generally, there is an air space between the LCD panel216and the DBEF214.

Unlike the conventional LCD, instead of using the red phosphor110A and green phosphor110B, the QDEF206including red QDs208A and green QDs208B produces red color and green color, which are excited by the blue light from the light source202. The QDEF206converts the color while diffusing the blue light220from the light source202.

Generally, the QDEF206is configured to transmit a portion of the blue light220from the light source202such that white light222comes out of the QDEF206. The QDEF206includes a group of red quantum dots (QDs)208A and green QDs208B, which actively convert the blue light220from the LED into red light and green light through the quantum dots. When the QDs208A and208B are irradiated by the blue light from the light source202, the blue light causes the QDs208A and208B 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 QDs208A and208B.

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 inFIG. 1B. The prism212may also be removed or substituted by other brightness enhancement component in an alternative embodiment. The DBEF214may be removed in another embodiment.

The QDEF206may include a host matrix. The QDEF206may also include red QDs and green QDs208A and208B disposed in the host matrix. The host matrix allows light from the light source202to 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 source202. 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 inFIG. 2the DCF210is used in front of the QDEF206to recycle the blue light220. The DCF210recycles the blue light220from the light source202within the QDEF206by reflecting a portion of the blue light220, although the DCF210still transmits a small portion of the blue light220to provide the white light222to the LCD panel216. This recycling of the blue light220helps 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 display200may optionally include brightness enhancement components, such as a prism212and a double brightness enhanced film (DBEF)214. The brightness enhancement components are optically transparent. The prism212helps reduce beam angle and thus increases light intensity through the DBEF214. The DBEF may be provided by manufacturers, such as 3M® among others.

The DBEF214is a reflective polarizer film which increases efficiency by repeatedly reflecting any unpolarized light back, which would otherwise be absorbed by the LCD's rear polarizer. The DBEF214is placed behind the liquid crystal display panel216without any other film in-between. The DBEF214may be mounted with its transmission axis substantially parallel to the transmission axis of the rear polarizer. The DBEF214helps recycle the white light222that would normally be absorbed by the rear polarizer (not shown) of the liquid crystal panel216, and thus increases the brightness of the liquid crystal display panel216.

FIG. 3illustrates a detailed structure of the DCF210in an embodiment. The DCF210is optically transparent. The DCF210may include alternating layers302and304of optical coatings with different refractive indexes and a glass substrate306. For example, layer302has a first refractive index and layer304has a second refractive index different from the first refractive index. The interfaces between the alternating layers302and304of different refractive indexes produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. In this disclosure, each of layers302and304has a thickness of about ¼ of the wavelength of the blue light.

The DCF210may be fabricated by vacuum deposition. The DCF210may be coated on the QDEF206. The DCF210may also be a separate sheet which is placed over the QDEF206. The transmission and reflection band of the DCF210may need to be properly aligned with the spectra of red, green and blue light.

FIG. 4illustrates an emission curve422of QDs and a transmittance curve420for the DCF210. As shown in the emission curve422, each of the red, green, and blue color bands412A-C emitted from the QDs is separated from each other. In other words, an emission color bandwidth410of the QDs is relatively narrow. Each of the emission color bandwidth410A-C for respective quantum dots412A,412B,412C varies with color and material. For example, CdSe quantum dots may have a blue emission bandwidth410of about 35 nm. InP quantum dot may have a blue emission bandwidth410of about 45 nm, which is wider than that of the CdSe quantum dot.

As shown inFIG. 4, the DCF210also has a low transmittance over a blue region402(see transmittance curve420). Typically, the transmittance of the DCF210is not zero. A small portion of the blue light220as shown inFIG. 2is transmitted through the DCF210, such that the white light222also shown inFIG. 2will be produced by combining the blue light from the light source202with the red and green lights from the QDs208A and208B. The transmittance in the blue region402may be at certain percentage, for example 25%, but may not be more than 50%. The transmittance of the blue region402may vary with the number of the layers302and304in the DCF210(seeFIG. 3). As shown inFIG. 4, the transmittance over a region404including the green and red regions as well as beyond the red region404is nearly 100%. The transmission curve420also includes a transition region or slope406A or406B may vary with a light incident angle. For example, slope406A at 0 degree angle of incidence (AOI) is steeper than slope406B at 45 degree AOI.

The transmittance blue region402has a reflection band bandwidth408, which may vary with coating materials for layers302of a first refractive index and layers304of a second refractive index in the DCF210. The reflection band width408often increases with the refractive index difference between the two coating materials having the first refractive index and the second refractive index.

The DCF210has a different function from the color filters122for the conventional LCD, because the conventional color filters122absorb a light in a color band while the DCF210reflects a light in the reflection band408. Thus, the DCF210generates less heat than the color filters122. The DCF210also has a longer life than the color filters122.

FIG. 5illustrates recycling blue light in the QDEF in an embodiment. A larger portion502B of the incoming blue light502A is reflected by the DCF210. A portion of this reflected blue light will excite the red QDs208A and green QDs208B in the QDEF206to increase the output of the red light and green light. This is a first order recycling. The red and green QDs208A and208B also diffuse the incoming blue light502A more to help increase uniformity of a white light222through the DCF210, which is one of the benefits of including the DCF in the QD enhanced display200. A remaining portion502C of the reflected blue light502B will enter the light guide panel112(not shown) and reflected from the reflector218at the bottom of the LGP204and will re-enter the QDEF206again and excite more red and green QDs208A and208B. Again, a portion of the remaining portion502C will be reflected again by the DCF210, which is the second order recycling. This recycling will continue until no blue light will be reflected by the dichroic filter210.

The dichroic filter210helps reduce the density of the red and green quantum dots208A and208B through recycling the blue light502, which increases the emissions from each red QD208A and green QD208B. This reduction in QD density leads to less CdSe used in the quantum dot-enhanced liquid crystal display200, which helps achieve a lower Cd content to meet the Cd free requirement of consumer goods.

FIG. 6illustrates color gamuts for the quantum dot-enhanced display and the LCD ofFIG. 2. A color gamut is a portion of the color space that may be reproduced or represented. For example, color gamuts602(CdSe QD),604(InP QD),606(Adobe RGB) and608(standard RGB or sRGB) are a portion of real color space610. Currently, conventional LCD meets the standard color gamut608, also labeled as “sRGB”. To provide a more full color than the conventional LCD, newer color gamut606, also labeled as “Adobe RGB”, is desired, because newer color gamut606has a larger area than the standard color gamut608and is closer to real color space610. As shown inFIG. 6, the QD enhanced LCD200is better than the conventional LCD (e.g. sRGB), because the CdSe QD color gamut602and InP QD color gamut604for the QD enhanced LCD200have larger triangle areas than the standard color gamut608of 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 display200is 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 DCF210to 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 display200with the QDEF206and the DCF210is characterized by better color accuracy and narrow bandwidth as well as or wider color gamut than the conventional LCD. The conventional LCD can'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 display200is generally much brighter than the conventional LCD display as a result of its wider color gamut. The quantum dot-enhanced display200using 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 display200typically 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 display200, power efficiency would be much lower than the quantum dot-enhanced display.

Furthermore, some light loss may occur during blue light recycling by the DCF210. 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 DCF210, 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 DCF210as without the DCF210.

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 IV compound comprises a material selected from the group consisting of Si, Ge, SiC, and SiGe.

If desired, display200may 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 film206and a second dichroic filter may be placed above film206. Film206may 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). Film206may 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 film206may 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 curve700ofFIG. 7. If desired, the red and green elements in film206may be replaced by yellow phosphors or yellow quantum dots that produce a yellow spectral peak such as peak701. Illustrative configurations for the backlight structures of display200that 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 DCF1ofFIG. 7. This allows blue light to pass through the first dichroic filter into film206while helping to recycle any downwardly reflected red and green light back into film206. As indicated by transmission characteristic DCF2for the second dichroic filter, the second dichroic filter may have a moderate transmission for blue (B) light (so that blue light is recycled through film206rather than being immediately emitted from the upper surface of film206) and high transmission for red (R) and green (G) light (so as not to block red and green light being emitted by film206).

Illustrative configurations for incorporating dichroic filters DCF1and DCF2into the backlight structures of display200are shown inFIGS. 8, 9, and 10. In these examples, display200is directly lit (i.e., blue light is produced by an array of light-emitting diodes202so that display200can be locally dimmed to enhance dynamic range). Light-emitting diodes202may emit light into backlight structure706(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 diodes202along the edge of a light-guide plate). The configurations ofFIGS. 8, 9, and 10are merely illustrative.

As shown inFIG. 8, dichroic filter DCF1may be placed below film206. The blue light transmission properties of filter DCF1(e.g., blue light transmission of about 70-90%, 80% or more, 90% or more, 95% or more, etc.) allow blue light from diodes202to efficiently pass into film206. Some of the blue light is converted into red and green light by film206. Outwardly directed red and green light from film206may pass through filter DCF2, 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 film206may be reflected back outwards by filter DCF1(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 module216). Filter DCF2reflects excess blue light (i.e., filter DCF2preferably 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 film206, thereby enhancing blue light recycling.

Light recycling may be further enhanced using other light recycling structures. For example, prism layer704may recycle on-axis light downwards through film206(i.e., layer704may reflect light that is oriented directly upwards in the orientation ofFIG. 8back in the downwards/inwards direction) while collimating off-axis light. Reflective polarizer702may have a pass axis that is aligned with the pass axis of the lower polarizer of liquid crystal display unit216. Light with different polarizations will be reflected back through film206and 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 DCF2rather than stand-alone reflective polarizer702ofFIG. 8. In the example ofFIG. 9, thin reflective polarizer layer702′ has been laminated to the upper surface of dichroic filter DCF2, so that layer702′ is interposed between prism layer704and filter DCF2. In the example ofFIG. 10, thin reflective polarizer layer702′ has been laminated to the lower surface of dichroic filter DCF2.

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.