Liquid crystal display device and backlight

A liquid crystal display device includes a liquid crystal display panel and a backlight. The backlight includes a light guide plate, an LED, and a wavelength converter between the light guide plate and the LED. The wavelength converter includes semiconductor quantum dots in a supporter, to which a reflection film is applied except surfaces that face the LED and the light guide plate.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent Application JP 2015-226324 filed on Nov. 19, 2015, the content of which is hereby incorporated by reference into this application.

BACKGROUND

The present invention relates to a display device, and more particularly, to a liquid crystal display device configured to use a backlight equipped with a wavelength converter having semiconductor quantum dots for improving light utilization efficiency.

The liquid crystal display device includes a TFT substrate having pixel electrodes and thin film transistors (TFT) formed in a matrix, a counter substrate which faces the TFT substrate, and a liquid crystal interposed between the TFT substrate and the counter substrate. An image is formed by controlling the luminous transmittance of liquid crystal molecules for each pixel.

The backlight is disposed on the back surface of the liquid crystal display panel since the liquid crystal by itself is unable to emit light rays. The LED (Light Emitting Diode) is employed as the light source for the backlight of the liquid crystal display device for the mobile phone. The LED is disposed on the side surface of the light guide plate, and various types of optical sheets are applied to the light guide plate. The backlight is constituted by storing the above-described optical components in the mold.

Generally, the LED emits the monochromatic light contrary to the requirement of the backlight for the liquid crystal display device to emit white light. Consequently, the LED for emitting UV light or blue light, and the optical converter for generating the light ray with longer wavelength have been employed to constitute the white light emitting structure.

Japanese Unexamined Patent Application Publication No. 2015-106487 discloses the wavelength converter disposed at the light emission side of the LED, which includes the dichroic mirror at the side that faces the LED so as to prevent the light with converted wavelength from returning to the LED for the purpose of preventing reflection of the light with the converted wavelength toward the LED.

Japanese Unexamined Patent Application Publications Nos. 2014-137961, 2002-116440, and 2006-84757 disclose the structure having the reflection sheet disposed behind the LED for the purpose of preventing the light emitted from the LED from leaking out of the back side of the LED serving as the light source. Japanese Unexamined Patent Application Publication No. 2006-184347 discloses the structure having the reflection sheet disposed behind the cold-cathode tube for the purpose of preventing the light from leaking out of the back side of the cold-cathode tube serving as the light source.

SUMMARY

As the method for generating white light by using the wavelength converter for the backlight having the LED as the light source, the semiconductor quantum dot has been used as the wavelength converter with excellent color expression property. The aforementioned method allows the converted wavelength of the light to be changed in accordance with the diameter of the semiconductor quantum dot (hereinafter referred to as quantum dot).

In this case, the wavelength converter is configured by sealing the quantum dot which has been dispersed in the resin within a supporter such as the glass. It is important to efficiently make incidence of the converted light on the light guide plate side from the wavelength converter. It is an object of the present invention to improve the light utilization efficiency of the backlight by using the wavelength converter having the quantum dot.

The present invention is intended to overcome the aforementioned problem by specific structures as described below.

(1) The present invention provides a liquid crystal display device which includes a liquid crystal display panel and a backlight. The backlight includes a light guide plate, an LED, and a wavelength converter between the light guide plate and the LED. The wavelength converter is configured to have a semiconductor quantum dot in a supporter. A reflection film is applied to the wavelength converter except surfaces that face the LED and the light guide plate.
(2) The present invention provides a liquid crystal display device which includes a liquid crystal display panel and a backlight. The backlight includes a light guide plate, an LED, and a wavelength converter between the light guide plate and the LED, and a reflection sheet is applied to lower sides of the light guide plate and the wavelength converter. The wavelength converter includes a semiconductor quantum dot in a supporter. The reflection sheet has an erected part which covers a surface of the wavelength converter, which does not face the LED at a side of the surface opposite to the one that faces the light guide plate.

DETAILED DESCRIPTION

The present invention will be described in detail in reference to the embodiments.

First Embodiment

FIG. 1is an exploded perspective view of a liquid crystal display device which includes a liquid crystal display panel300and a backlight400. The liquid crystal display panel300is configured to interpose the liquid crystal between a TFT substrate100having the TFTs and pixels with pixel electrodes formed in a matrix, and a counter substrate200. A lower polarization plate101is attached to the lower part of the TFT substrate100, and an upper polarization plate201is attached to the upper part of the counter substrate200. The backlight400is disposed on the back surface of the liquid crystal display panel300.

The backlight400includes a light source having an LED60and a wavelength converter10, a light guide plate30, a reflection sheet20attached to the lower surface of the light guide plate30, and an optical sheet group40disposed on the light guide plate30. The light ray emitted from the LED60is made incidence on the wavelength converter10which uses the quantum dot. The light having the wavelength converted by the wavelength converter10is made incidence on the light guide plate30.

The light incident on the light guide plate30is emitted from its major surface to the liquid crystal display panel300. The light from the light guide plate30, directed opposite to the liquid crystal display panel300is reflected by the reflection sheet20so as to be directed to the liquid crystal display panel300. The reflection surface of the reflection sheet20is constituted by the dielectric multi-layer film, for example. The reflection sheet20has its thickness ranging from 40 μm to 100 μm. The optical sheet group40is disposed on the light guide plate30. The optical sheet group40is constituted by a lower diffusion sheet41, a lower prism sheet42, an upper prism sheet43, and an upper diffusion sheet44, for example, from the light guide plate30.

The light emitted from the light guide plate30mostly has luminance unevenness. The lower diffusion sheet41serves to make the light emitted from the light guide plate30uniform. The prism sheet has a microprism with a prism-like cross section extending in the predetermined direction. The pitch of the microprism may be set to 50 μm, for example. The prism sheet serves to direct the light which is about to proceed diagonally with respect to the major surface of the liquid crystal display panel toward the major surface thereof, thus improving the light utilization efficiency. The lower prism sheet42has the microprism extending along the x-direction as shown inFIG. 1, and the upper prism sheet43has the microprism extending along the y-direction as shown inFIG. 1.

The light emitted from the prism sheet has a repetitive pattern of the bright and dark parts in the microscopic view under the influence of the prism. Interference of such light with the scanning line or the video signal line on the liquid crystal display panel may generate the moire. The upper diffusion sheet44serves to prevent generation of the moire by diffusing the light emitted from the prism sheet.

FIG. 1shows the optical sheet group40just for exemplifying purpose. The component to be used will be different depending on usage of the liquid crystal display device. There may be the case where the single prism sheet is only used, or the single diffusion sheet is only used. Alternatively, only the diffusion sheet is used by omitting the use of the prism sheet. Each of the respective optical sheets has the thickness of approximately 60 μm, for example.

FIG. 2is a perspective view showing a part around the light source of the backlight according to the present invention. The backlight is disposed in a resin mold50. Referring toFIG. 2, the LED60is disposed in the concave part of the mold50. The wavelength converter10is arranged to face the LED60. The wavelength converter10is configured by sealing the quantum dots12distributed in the resin and the like inside the supporter11constituted by a thin glass. The light guide plate30is disposed to face the wavelength converter10.

The LED60emits blue light, and the backlight for the liquid crystal display panel requires white light. The blue light incident on the wavelength converter10is partially converted into the red light and the green light by the quantum dot12.FIG. 3is a schematic view of the quantum dot12used in the present invention. The quantum dot12is the semiconductor microparticle, making the wavelength of the converted and emitted light variable depending on the particle size. For example, if the particle size d is 2 nm, the green light will be emitted. If the particle size d is 8 nm, the red light will be emitted. Generally, the quantum dot size d is equal to or smaller than 20 nm.

Codes P1and P2shown inFIG. 3denote semiconductors. For example, the P1may be a spherical CdSe, circumference of which is covered with the P2as ZnS. The quantum dot12seals the incident light so as to emit the light with longer wavelength than that of the incident light. The incident light emitted from the LED60is either the blue light or the UV light. The white light may be derived by changing the ratio of the quantum dots with different sizes depending on the case whether the incident light is the blue light or the UV light. The code L shown inFIG. 3represents so-called Ligand which serves to facilitate dispersion of the quantum dots12in the resin.

FIG. 4is a perspective view of the wavelength converter10configured by sealing the quantum dots12dispersed in the resin or the like within the supporter11as the glass. For example, Si resin, epoxy resin and the like may be employed as the resin. Referring toFIG. 4, the wavelength converter10has a square cross section. The cross section of the wavelength converter10has to be changed so as to be adapted to the shape of the liquid crystal display device or the backlight. The distance g across which the light passes through the wavelength converter10will be changed correspondingly in the range from 0.01 mm to 10 mm, for example. The quantity of light required to be converted may vary depending on the wavelength of the incident light. In other words, wavelength of the incident light and the distance g across which the light passes through the wavelength converter vary the ratio of the quantum dots with different diameters in the wavelength converter10, or the quantum dot density.

The supporter11employed for the wavelength converter10is not limited to the glass, but may be the transparent plastics such as acrylic and polycarbonate. In the case where the present invention is not applied, the light incident on the wavelength converter10is directed toward the light guide plate30, but partially makes boundary reflectance on the supporter11, and is radiated outside without being incident on the light guide plate30as shown inFIG. 5. Referring toFIG. 5, the light incident on the wavelength converter10from the LED60reflects on the boundary at the side of the light guide plate30, which is then radiated outside owing to difference in the refractive index between the supporter11and air. Meanwhile, the light directed to the side surface of the wavelength converter10refracts, and is radiated outside. The resultant light leakage may deteriorate the light utilization efficiency of the backlight.

In the present invention, a reflection film13is formed on the surface of the wavelength converter10except the parts that face the LED60and the light guide plate30so as to guide the light incident on the wavelength converter10and the light having the wavelength converted by the quantum dots entirely into the light guide plate30. In this way, the light utilization efficiency of the backlight may be improved.

FIG. 6is a perspective view of the wavelength converter10according to the present invention when seen from the LED60.FIG. 7is a perspective view of the wavelength converter10when seen from the light guide plate30. Referring toFIG. 6, the reflection film is applied to the supporter of the wavelength converter10except the surfaces that directly face the LEDs60.FIG. 6only shows the upper side of the supporter11. However, the reflection film is also applied to the lower side of the supporter11. The reflection film13is applied to the side surface of the wavelength converter10.

FIG. 7is a perspective view of the wavelength converter10when seen from the light guide plate30. The reflection film13is applied to the surface of the wavelength converter except the part that directly faces the light guide plate30. Although not shown inFIG. 7, the reflection film is applied to the lower side of the wavelength converter10. AsFIGS. 6 and 7show, the present invention is configured to allow all the incident light on the wavelength converter10to be made incidence on the light guide plate30. This makes it possible to improve the light utilization efficiency of the backlight.

FIG. 8is a plan view representing the state where the LEDs60, the wavelength converter10, and the light guide plate30are stored in the mold50. Referring toFIG. 8, the wavelength converter10is entirely covered with the reflection film13except the parts that directly face the LEDs60, and the light guide plate30. The hatching part and the side surface indicated by the bold line represent the part of the wavelength converter10, to which the reflection film13is applied. Although not shown inFIG. 8, the reflection film is applied to the lower surface of the wavelength converter10.

FIG. 9is a schematic view representing the optical path of the light emitted from the LED60. The light which has been emitted from the LED60, and partially reflecting on the part at which the wavelength converter10faces the light guide plate30will make reflection on the reflection film13applied to the wavelength converter10so as to be incident on the light guide plate30. The light directed to the side surface of the wavelength converter10is reflected by the reflection film13so as to be incident on the light guide plate30. Compared with the case where the reflection film is not applied to the wavelength converter10as shown inFIG. 5, the present invention allows significant improvement in the utilization efficiency of light emitted from the LED.

The reflection film13may be formed by sputtering or vapor depositing the metal film with high reflectance such as Al. Alternatively, the reflection film may be formed by applying the white resin with high reflectance, or the resin with metallic luster. It may also be formed by using the multi-layer film of the dielectric with different refractive indexes. The multi-layer film is formed by vapor deposition or sputtering. It is preferable to form the reflection surface into the mirror surface. In view of formation of the mirror surface, it is advantageous to use the glass for forming the supporter.

The method of manufacturing the wavelength converter10includes the step of forming the thin supporter. The quantum dot12dispersed in the liquid resin is injected into the supporter with vacuum injection method, which will be cured by heat or UV light. The thus formed quantum dot12having thin bar-like shape may be called “quantum dot bar”.

FIG. 4shows the wavelength converter10with square cross section as an example. However, the cross section may be arbitrarily shaped so as to be adapted to the shape of the liquid crystal display device.FIG. 10shows the wavelength converter10with rectangular cross section, to which the reflection film13is applied except the side that faces the light guide plate. The cross section as shown inFIG. 10has a longer side in the y-direction. It is also possible to allow the cross section to have the longer side in the x-direction as required by the liquid crystal display device. Likewise the case as described above, the reflection film is not applied to the surface that faces the light guide plate.

FIG. 11shows the wavelength converter with circular cross section. Referring toFIG. 11, the reflection film13is not applied to the side that faces the light guide plate.FIG. 12shows the wavelength converter with oval cross section. Referring toFIG. 11, the reflection film13is not applied to the side that faces the light guide plate.FIG. 12shows that the oval cross section has the long axis in the y-direction. However, it is also possible to form the oval cross section having the long axis in the x-direction so as to be adapted to the shape of the liquid crystal display device. In such a case, the reflection film13is not applied to the surface that faces the light guide plate as well. Referring toFIGS. 10 to 12, the reflection film is not applied to the surface of the wavelength converter, which directly faces the LED.

Second Embodiment

In the first embodiment, the reflection film is applied to the wavelength converter for improving the light utilization efficiency of the backlight. Such structure requires the process step of applying the reflection film to the wavelength converter. In this embodiment, the reflection sheet is partially used to form a reflector of the wavelength converter. The aforementioned structure may improve the light utilization efficiency of the backlight while omitting the process of applying the reflection film to the wavelength converter.

FIG. 13is a plan view of the backlight according to the embodiment. Referring toFIG. 13, the LEDs60, the wavelength converter10, and the light guide plate30are stored in the mold50. The LED60is disposed in the concave part of the mold50, and the wavelength converter10is disposed to face the LEDs60. The light guide plate30is disposed to face the wavelength converter10. AsFIG. 13shows, an erected part21of the reflection sheet20is disposed between the LEDs60while facing the wavelength converter10. The erected part21of the reflection sheet20allows reflection of the light directed opposite to the light guide plate30so that the light is directed toward the light guide plate30. This makes it possible to improve the light utilization efficiency of the backlight.

FIG. 14is a sectional view taken along line A-A ofFIG. 13. Referring toFIG. 14, the wavelength converter10is disposed to face the LEDs60, and the light guide plate30is disposed to face the wavelength converter10. The reflection sheet20is applied to the lower side of the light guide plate30. AsFIG. 14shows, the LED60is connected to a flexible wiring substrate70, and the optical sheet group40including two sheets is applied to the upper side of the light guide plate30. The use of two optical sheets for constituting the optical sheet group in this embodiment is a mere example. Four sheets may be used for constituting the optical sheet group as shown inFIG. 1. A light shielding tape80is applied to shield the LEDs60and the wavelength converter10as well as the peripheral area of the optical sheet group40.

FIG. 15is a sectional view taken along line B-B ofFIG. 13.FIG. 15is different fromFIG. 14in the erected part21of the reflection sheet20for shielding the back surface of the wavelength converter10. Referring toFIG. 15as a sectional view of the part where the LED60does not exist, the incident light from the LED60is not shielded by the erected part21of the reflection sheet20. Any other structures are similar to those described referring toFIG. 14.

AsFIG. 15shows, in this embodiment, the erected part of the reflection sheet20serves to return the light proceeding from the inside of the wavelength converter10in the direction opposite to the light guide plate30to the inside of the wavelength converter10again. This makes it possible to improve the light utilization efficiency of the backlight. The erected part21of the reflection sheet20may be formed simultaneously with shaping of the reflection sheet20. It is therefore possible to reduce the load to the process compared with the case where the reflection film13is applied to the wavelength converter10.

FIG. 16is a detailed exploded perspective view of the erected part of the reflection sheet20. Referring toFIG. 16, the backlight with high light utilization efficiency may be configured only by disposing the LEDs60and the wavelength converter10on the reflection sheet20having the erected part21.

FIG. 17is an exploded perspective view of another example of the backlight according to the embodiment. Referring toFIG. 17, the width of the erected part21of the reflection sheet20is increased so as to be partially bent to allow the reflection sheet to be disposed not only at the side that faces the wavelength converter10but also at the side surfaces of the LEDs60at least partially. The LED60may cause the light leakage from the lateral side rather than from the lower surface. In such a case, the reflection sheet is applied to the side surface of the LED to prevent the light leakage therefrom so as to ensure improved light utilization efficiency of the backlight.

FIG. 18is an exploded perspective view of another example of the backlight according to the present invention. Referring toFIG. 18, the length of the erected part21of the reflection sheet20is made longer compared with the case as shown inFIG. 16. A folded part22of the reflection sheet20covers not only a part of the side surface but also the upper surface of the wavelength converter10at least partially. The configuration as shown inFIG. 18reflects the light proceeding from the upper surface of the wavelength converter10to the outside to be returned to the inside thereof, thus further improving the light utilization efficiency of the backlight compared with the case as shown inFIG. 16.

The explanation has been made with respect to application of the present invention to the backlight of the liquid crystal display device. However, the present invention may be applied to any other display device requiring the backlight with no limitation.