Patent Application
20080180618
This application is based on Japanese patent application No. 2002-129411, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a thin type display device to be installed in a mobile phone, mobile information terminal, laptop computer, etc., more specifically to a thin type display device that displays an image by reflection of ambient light in a light place but by self-emission in a dark place.
2. Description of the Related Art
A reflection type liquid crystal display device has been conventionally known as a thin type display device that displays an image by reflection of ambient light.
On the surfaces of the both substrates in contact with the liquid crystal 130, alignment layers 114 and 124 are respectively disposed, for predetermining an alignment direction of the liquid crystal. Also, a wiring 112 for independently controlling potential of each of the reflecting electrodes 111 is arranged among the reflecting electrodes 111 on the surface of the substrate 110. Also, the color filters 121 are constituted of three types of materials that respectively transmit red, green and blue rays at a high rate, and black matrix 122 made of a material that does not transmit any light is disposed in clearances between the three types of color filters 121. The black matrix 122 generally has a grid-patterned plane face. Further, a circular polarizing plate 140 having a function of exclusively transmitting a particular circular polarized light out of ambient incident light is disposed on a surface of the transparent substrate 120 that confronts a viewer. The circular polarizing plate 140 is usually constituted of a linear polarizing plate and a λ/4 wavelength plate layered with their light axis tilted by a certain angle from each other. Also, in the constitution shown in this drawing, a function of diffusing light is performed by a light diffusing material included in the color filter 121, and the reflecting electrodes 111 do not have a light diffusing function.
Now referring to
Secondly, in case where a voltage is applied to the reflecting electrode 111, the light that has passed through the liquid crystal 130 turns into linear polarized light, which the reflecting electrode 111 can reflect. The reflected linear polarized light turns into one circularly-polarized light upon passing through the liquid crystal 130, and therefore passes through the circular polarizing plate 140. Accordingly, this pixel displays a color determined by the color filter 121. Through such control of potential of each reflecting electrode 111, a desired color image can be displayed.
Meanwhile, an organic EL (Electro-Luminescence) display device in which an electro-luminescence effect of an organic material is utilized has conventionally been known as a thin type display device that displays an image by self-emission.
Now referring to
However, since a reflection type liquid crystal display device displays an image by reflection of ambient light, it has a problem that image quality significantly deteriorates in a dark place. An auxiliary light source has been proposed for resolving this problem, however addition of an auxiliary light source spoils a great advantage of the device of being thin. Further, in case where a front light is used as an auxiliary light source, image quality deteriorates in its normal use in which ambient light is utilized. Accordingly, a conventional reflection type liquid crystal display device has the disadvantage of deterioration of image quality when used in a dark place.
On the other hand, since an organic EL display device emits light itself, a clear display can be performed in a dark place. However in a light place contrast of a displayed image deteriorates because of reflection of ambient light. Increasing its light emitting capacity could be a measure for this problem, but it would result in deterioration of a life span of the light emitting device and increase of power consumption, therefore actually there is no other choice but to reluctantly accept an unsatisfactory image quality. Accordingly, a conventional organic EL display device has the disadvantage of deterioration of displayed image contrast in a light place.
Because of such characteristics, visibility just in one situation, whether a light place or a dark place, has to be focused with priority depending on operating principle of the display device. In other words, by a conventional thin type display device it is impossible to achieve a satisfactory visibility in both of a light place and a dark place.
With an object to solve this problem, a display device wherein an organic electro-luminescence display device 1 and a liquid crystal display device 2 are layered in this sequence from a viewer's side has been proposed (JP-A No. 2001-92390). Also, a display device has been disclosed with an object to provide a display device having a backlight capable of displaying a design pattern such as a cartoon character, in addition to surface emitting function for a transmission type display device, wherein an organic EL panel is disposed at the back of a liquid crystal display panel and an electrode is formed on the organic EL device so that a pattern can be displayed (JP-A No. 11-160704).
However, display devices according to these prior arts have a constitution in which an EL display device or EL backlight and a liquid crystal display device, which are separately manufactured, are simply layered. Therefore, still there is a problem that a transparent substrate is formed between the EL display device or EL backlight and the liquid crystal display unit, and that both of them respectively require an appropriate diving circuit.
The invention has been made in view of the foregoing problems, with an object to provide a thin type display device that offers an excellent visibility both in a light place and in a dark place, with an additional advantage of a simplified constitution and driving circuit.
A display device according to the first invention comprises a first substrate on which a plurality of light emitting devices including a reflecting electrode is disposed; a second substrate made of a transparent material provided with a color filter and a transparent electrode; a liquid crystal layer placed between the first substrate and the second substrate disposed in such a manner that a face with the light emitting devices and a face with the transparent electrode confront each other; and a driving circuit for controlling a voltage to be applied to the liquid crystal layer, and selecting either a reflection display mode of displaying an image by reflecting ambient light with the reflecting electrode of the light emitting device or a self-emission display mode of displaying an image by light emission of the light emitting device.
A display device according to the second invention comprises a first substrate on which a plurality of light emitting devices including a reflecting electrode is disposed and a color filter is provided over them; a second substrate made of a transparent material provided with a transparent electrode; a liquid crystal layer provided between the first substrate and the second substrate disposed in such a manner that a face with the color filter and a face with the transparent electrode confront each other; and a driving circuit for controlling a voltage to be applied to the liquid crystal layer, and selecting either a reflection display mode of displaying an image by reflecting ambient light with the reflecting electrode of the light emitting device or a self-emission display mode of displaying an image by light emission of the light emitting device.
In these display devices, the light emitting device may, for example, further comprise a light emitting material formed on the reflecting electrode and a transparent electrode formed on the light emitting material.
Also, the driving circuit may, for example, further control optical characteristic of the liquid crystal through control of a voltage applied between the transparent electrode of the light emitting device and the transparent electrode of the second substrate, and control light emission by the light emitting device through control of a voltage applied between the transparent electrode of the light emitting device and the reflecting electrode.
Also, in these display devices, for example, each of the light emitting devices and the color filter corresponding to each light emitting device may constitute a pixel, and the driving circuit may comprise a switching transistor for selecting each of such pixels, light emission amount control circuit for controlling a current amount of the light emitting device and a switching control circuit for switching to either of the reflection display mode or the self-emission display mode.
Further, the switching control circuit may, for example, electrically connect the reflecting electrode of the light emitting device with the transparent electrode of light emitting device as well as the switching transistor with the reflecting electrode of the light emitting device, in the reflection display mode.
Further, the switching control circuit may, for example, connect the light emitting device with the light emission amount control circuit as well as the switching transistor with the light emission amount control circuit, in the self-emission display mode.
Also, these display devices may, for example, further comprise a circular polarizing plate on a surface of the second substrate on a face not confronting the liquid crystal layer, so that the driving circuit adjusts a reflection factor of ambient light by switching a polarization status of light passing through the liquid crystal layer, in the reflection display mode.
Also, these display devices may, for example, further comprise a circular polarizing plate on a surface of the second substrate on a face not confronting the liquid crystal layer, so that the driving circuit maintains a constant polarization status of light passing through the liquid crystal layer, in the self-emission display mode.
The first substrate may, for example, comprise a protection layer covering the light emitting device and a first alignment layer formed on the protection layer, and the second substrate may comprise a second alignment layer formed on the transparent electrode. In this case, the protection layer may be provided with a projection or a strut that reaches the second substrate, between the first substrate and the second substrate and in a region where the light emitting devices are not located.
The first substrate may, for example, comprise a protection layer formed between the color filter and the light emitting device so as to cover the light emitting device and a first alignment layer formed on the color filter, and the second substrate may comprise a second alignment layer formed on the transparent electrode.
The third invention provides driving method of a display device comprising a first substrate on which a plurality of light emitting devices including a reflecting electrode is disposed; a second substrate made of a transparent material provided with a color filter and a transparent electrode; a liquid crystal layer provided between the first substrate and the second substrate disposed in such a manner that a face with the light emitting devices and a face with the transparent electrode confront each other; comprising the steps of controlling a voltage to be applied to the liquid crystal layer, and selecting either a reflection display mode of displaying an image by reflecting ambient light with the reflecting electrode of the light emitting device or a self-emission display mode of displaying an image by light emission of the light emitting device.
The fourth invention provides driving method of a display device comprising a first substrate on which a plurality of light emitting devices including a reflecting electrode is disposed and a color filter is provided over them; a second substrate made of a transparent material provided with a transparent electrode; a liquid crystal layer provided between the first substrate and the second substrate disposed in such a manner that a face with the color filter and a face with the transparent electrode confront each other; comprising the steps of controlling a voltage to be applied to the liquid crystal layer, and selecting either a reflection display mode of displaying an image by reflecting ambient light with the reflecting electrode of the light emitting device or a self-emission display mode of displaying an image by light emission of the light emitting device.
In the foregoing method, the reflecting electrode and the transparent electrode of the light emitting device formed with the light emitting material disposed therebetween may be electrically connected in the reflection display mode.
Also, in the self-emission display mode, a light emission amount control circuit for controlling a light emission amount of the light emitting device may be connected with the light emitting device.
Referring to the accompanying drawings, embodiments of the present invention shall be described hereunder.
The light emitting device 20 is provided with a lower electrode 21 formed on the substrate 10, a light emitting layer 22 formed on the lower electrode 21 and a transparent upper electrode 23 formed on the light emitting layer 22, in such a manner that the light emitting layer 22 is disposed between the lower electrode 21 and the upper electrode 23. The lower electrode 21 and the upper electrode 23 are made of a light reflecting material and a light transmitting material respectively. Therefore, light passes through the upper electrode 23 and is emitted toward outside. Also, a material for constituting the light emitting device is to be selected so that white light is emitted when power is supplied to the light emitting device 20.
Also, a wiring 30 for electrically controlling the light emitting device and the liquid crystal 70 is arranged among each light emitting device 20 on the surface of the substrate 10. Further, the substrate 10 is provided with various circuit factors on its surface in addition to the light emitting devices 20 though they are not shown in
Meanwhile referring to the transparent substrate 60, on a face that is confronting the substrate 10 a transparent electrode 63 is formed so as to cover the color filter 61, and on a face of the transparent electrode 63 in contact with the liquid crystal 70 an alignment layer 64 for predetermining an alignment direction of the liquid crystal is provided. With respect to the alignment of the liquid crystal, materials of the liquid crystal 70 and the alignment layers 50 and 64 are to be selected so that the liquid crystal molecules are horizontally aligned (homogeneous alignment) along the substrate.
Also, the color filter 61 comprises three types of materials that respectively transmit red, green and blue rays at a high rate, and black matrix 62 made of a material that does not transmit any light is disposed in clearances between the three types of color filters 121 forming a grid pattern in a plan view. Further, in this embodiment a function of diffusing light is performed by a light diffusing material included in the color filter 61.
Further, a circular polarizing plate 80 having a function of exclusively transmitting a particular circular polarized light out of ambient incident light is disposed on a surface of the transparent substrate 60 that confronts a viewer. The circular polarizing plate 80 is usually constituted of a linear polarizing plate and a λ/4 wavelength plate layered with their light axis tilted by a certain angle from each other.
Now referring to
Specifically, the control signal Vgate is input to a gate of the switching transistor Tp, and the control signal Vdata is input to its source. The transistor Tcc, light emitting device C
Also, referring to the cross-sectional drawing of
The liquid crystal 70 is a two-terminal device controlled by a potential given to the transparent electrode 63 and the transparent upper electrode 23 of the light emitting device 20, and is denoted as C
A constitution of the transistors (Tp, T1, T2, etc.) in
Further, the capacitor Cs in
Some specific materials and numerical values are given below. It is desirable that the protection layer 40 has a thickness of not less than approx. 1 μm in order to secure a sufficient protecting effect. Silicon oxide nitride (SiON) or various organic materials are used as protection layer 40, and a refraction factor of these materials is in a range of approx. 1.4 to 1.7. Refraction factor of the liquid crystal 70 varies depending on whether ordinary light or extraordinary light, and it is desirable to adopt a liquid crystal that has a higher refraction factor against ordinary light than the protection layer. Preferable thickness of the liquid crystal layer is in a range of approx. 2 to 6 μm. For example, refraction factors of a liquid crystal BDH-TL213 manufactured by Merck Japan, Ltd. against ordinary light and extraordinary light is 1.52 and 1.76 respectively. The alignment layer may be formed of a polyimide family material in a thickness of approx. 100 nm, and through a rubbing process thereon the liquid crystal can be horizontally aligned. Also, an indium tin oxide (ITO) may be used as a material for the transparent upper electrode 23, transparent electrode 50 and transparent electrode 64, and its refraction factor is approx. 1.8 to 1.9. Thickness of these layers is preferably approx. 100 nm, which is thinner than a light wavelength. The light emitting layer 22 may be formed of a material popularly used in an organic EL display having a function of emitting light by electro-luminescence effect. An example of such materials is Alq (aluminum quinolinolato complex) etc. In
Manufacturing method of the display device shown in
Now, operation of the display device of this embodiment shall be described. Firstly, operation in the reflection mode of this display device shall be described referring to
In a frame denoted as “LED-reset frame” in
Then Vmode is set at a level L in subsequent frames after the frame, denoted as “1st frame” and “2nd frame” in
Referring to
In this case, designing parameters like liquid crystal material or liquid crystal layer thickness etc. are to be selected so that, for example, the polarization status is not changed while a voltage is not applied, and in case where a voltage is applied the circular polarized light is converted into a linear polarized light.
Firstly, in case where a voltage is not applied to the transparent upper electrode 23, the one circularly-polarized light of a particular wavelength incident on the liquid crystal 70 remains as it is and passes through the alignment layer 50, protection layer 40, transparent upper electrode 23 and light emitting layer 22 in turn to reach the reflecting lower electrode 21. This one circularly-polarized light turns into other circularly-polarized light upon reflecting from the reflecting lower electrode 21, and passes through the mentioned factors in a reverse sequence to reach the liquid crystal 70. Polarization status is not changed when the light passes upward from a lower direction either, therefore the other circularly-polarized light passes through the alignment layer 64, transparent electrode 63, color filter 61 and the transparent substrate 60 in turn to finally reach the circular polarizing plate 80. Since the other circularly-polarized light is absorbed by the circular polarizing plate 80, the light does not leak outside. Accordingly, this pixel displays black.
Secondly, in case where a voltage is applied also to the reflecting lower electrode 21, the same potential is applied to the transparent upper electrode 23. Because of an alignment status of the liquid crystal, the light that has passed through the liquid crystal 70 turns into linear polarized light, and this linear polarized light reaches the reflecting lower electrode 21. The linear polarized light that has reflected from the reflecting lower electrode 21 turns into one circularly-polarized light upon passing through the liquid crystal 70, and passes through the circular polarizing plate 80. Accordingly, this pixel displays a color determined by the color filter 61. Through such control of potential of each reflecting lower electrode 21, a desired color image can be displayed.
What is unique in the foregoing constitution and operation is that the transparent upper electrode 23 of the light emitting device 20 is given a function as an electrode for applying a voltage to the liquid crystal, and that the reflecting lower electrode 21 of the light emitting device 20 is serving as a reflecting plate of the light.
Further, the black matrix 62 can reduce an amount of incident light into the color filter of an adjacent pixel, originating from the light that has reflected from one of the reflecting lower electrodes. Therefore, deterioration of image quality due to mixing of colors can be prevented.
Also, the color filter 61 includes a light diffusing material. The purpose is to prevent an ambient image from being displayed (invasion of an ambient image) caused by direct reflection of ambient incident light because of specular reflection by the reflecting lower electrode 21. Forming an uneven profiles distributed in various slope angles on the reflecting plate, as generally adopted in a conventional reflection type liquid crystal display device, can prevent such invasion of an ambient image. However, a height of the uneven profiles formed on the reflecting plate is approx. 1 μm while a thickness of the light emitting layer 22 is approx. 100 nm, therefore the reflecting lower electrode and the transparent upper electrode are prone to cause a short circuit. Consequently, it is preferable to utilize a light diffusing material, though it depends on a material for forming the light emitting layer. Location to provide the light diffusing material is not limited to the color filter 61, but may be provided in the protection layer 40.
Now, operation of the display device in the self-emission mode according to this embodiment shall be described, referring to
A first frame (a portion denoted as “LED-reset frame” in
In subsequent frames, the control signal Vmode is set at a level H, to turn T1 and T3 conductive and T2 and T4 nonconductive. A circuit at this moment is equivalent to
Firstly, in case where a potential written in Cs is of the level H, the light emitting device is turned off. This is the same as in the first frame. Accordingly, a pixel in which the level H is written displays black.
Meanwhile, in case where a potential of a certain value is written in Cs, conductivity of Tcc in
According to the foregoing description, a desired color image can be displayed by respectively writing a potential corresponding to the desired image signal in Cs of all the pixels.
Now, since the light emitted by the light emitting layer 22 is isotropic, such light emitting device 20 by itself has a broad directionality. However, direction of emitted light may be restricted to a certain angle range depending on a refraction factor of materials used for the protection layer 40, alignment layer 50, liquid crystal 70, etc., by which a light emitting efficiency may be lowered resulting in a reduced luminance of the display device. It takes place because, when light emitted in a broad angle range is incident to a substance of a lower refraction factor from a substance of a higher refraction factor, a portion of light emitted in a broader angle than a critical angle determined by the two refraction factors is detained in the substance of a higher refraction factor. From the viewpoint of preventing such phenomenon, selection of materials for the protection layer 40 and liquid crystal 70, which have a thickness of not less than approx. 1 μm, is particularly important. A maximum refraction factor applied to the light passing through the liquid crystal is the refraction factor of the liquid crystal against extraordinary light, and the value, for example, of the liquid crystal BDH-TL213 manufactured by Merck Japan, Ltd. is 1.76. Therefore, by using a material having a refraction factor of approx. 1.5 to 1.7 (for instance, SiON) for the protection layer 40, the light can be prevented from being detained in the protection layer 40.
Also, even in case where ambient light is incident from an upper direction of the circular polarizing plate 80, contrast is not deteriorated by reflection of the ambient light. It is because, as described with respect to the displaying operation in the reflection mode, a “normally black” display is adopted that displays black when a voltage is not applied to the liquid crystal.
Now a variation of the foregoing embodiment shall be described. Firstly, alignment method of the liquid crystal shall be focused on. A function required from the liquid crystal layer constituted according to the invention is to control whether to reflect ambient light or not, by switching whether to change a polarizing status of light being transmitted or not. Therefore, as long as the liquid crystal can accomplish such function, different liquid crystal alignment method from the horizontal alignment employed in the foregoing embodiment can be adopted. For example, a hybrid alignment (wherein the liquid crystal molecules are vertically aligned on one substrate side and horizontally aligned on the other substrate side) may be adopted. Such alignment methods of the liquid crystal are popularly used as an ECB (Electrically Controlled Birefringence) mode in a conventional liquid crystal display device.
Secondly, a guest host (GH) mode liquid crystal may be adopted, which is also generally known in a conventional reflection type liquid crystal display device, instead of the ECB mode. The GH mode is based on a principle of mixing several percent of bicolor coloring matter molecules (guest) in the liquid crystal (host), and adjusting an extent of optical absorption by controlling an alignment of the bicolor coloring matter molecules with an alignment of the liquid crystal molecules. The GH mode has various constitutions, among which in a phase transition type, polymer-dispersed liquid crystal (PDLC) type, etc. especially, the reflecting electrode disposed on the substrate surface is in contact with the liquid crystal. Accordingly, by forming the light emitting device shown in
Thirdly, the normally black mode wherein black is displayed when a voltage is not applied is adopted in the foregoing embodiments, while a normally white mode may be adopted wherein black is displayed when a voltage is applied, as is popular in a conventional reflection type liquid crystal display device.
Fourthly, the light emitting device of the invention is not limited to one that emits white light. Specifically, for example, light emitting devices that emit red, green and blue light may be disposed so as to confront a color filter that transmits the respective emitted wavelengths. Such constitution has the disadvantage of an increase of production cost since three types of light emitting devices are required, but on the other hand it has the advantage that a brighter display is achieved in the self-emission mode because less light is absorbed by the color filter.
As described above, it is possible for those of ordinary skill in the art to select different liquid crystal modes and control system of the liquid crystal, as well as to replace various constitutional factors without departing from the spirit of the invention. Therefore, it is to be understood that such variations are within the scope of the invention.
In case where a displaying area of a display device is considerably large, it is difficult to maintain a constant thickness of the liquid crystal layer. The reason is as follows. Normally, objects (spacers) for defining a thickness of the liquid crystal layer are mixed in a sealing material of the liquid crystal and disposed along a perimeter of the displaying area, so that the liquid crystal layer thickness is maintained at a constant level. Accordingly, in case that the displaying area becomes larger, it becomes difficult to control the liquid crystal layer thickness at a portion distant from the objects sealed in the liquid crystal (i.e. a central portion of the displaying area).
This problem has been well known in a conventional liquid crystal display device and, as a solution, in general similar spacers are dispersed over the liquid crystal layer. Otherwise, such method is also known that columns are provided instead of spacers at positions where the reflecting electrodes are not disposed, so that the liquid crystal layer thickness is maintained constant at a height of the column. Either of these solutions can be applied to the display device of the invention.
However, in case of applying method of dispersing spacers to the display device of the invention, the light emitting devices are prone to be damaged by the spacers unless manufacturing conditions for the process of combining and fixing two substrates are optimum. The reason is that generally the spacer is made of glass having a substantially high hardness, which causes a load to be imposed on the light emitting device through the protection layer. Depending on a material for constituting the light emitting device its load resistance may be insufficient, in which case the light emitting device may be destroyed even merely by a finger press on the display panel during an operation. Therefore, an embodiment wherein a column that eliminates such risk is adopted shall be described hereunder.
In
Such projection 41 can be formed by lithography and etching after forming a thick protection layer 40b. In this case, the protection layer 41b and the projection 41 are formed of an identical material. Otherwise, a different material may be layered in a sufficient thickness over the protection layer 40 formed in an originally specified thickness, so that lithography and etching may be performed to form the projection 41. Further, a projection may be formed on the transparent substrate 60 instead of the substrate 10, so that a constitution shown in
Consequently, it is to be understood that according to the invention a display device performs as a reflection type liquid crystal display device in a light place and as a self-emission type display device in a dark place, since each pixel is provided with a light emitting device having a function of reflecting light, a liquid crystal layer and a color filter. In other words, in the reflection mode the cathode of the light emitting device serves as a reflecting plate of ambient light, and ON/OFF of the ambient light reflection is switched based on a combination of alignment control of the liquid crystal and a circular polarizing plate. Also, in the self-emission mode ON/OFF of light emission is switched by a current control circuit of the pixel, and reflection of ambient light is shielded by the circular polarizing plate. Therefore by appropriately selecting these display modes according to an ambient light intensity, a clear image can be displayed both in a light place and in a dark place.
Further, the constitution shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 129411/2002 | Apr 2002 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10424925 | Apr 2003 | US |
| Child | 12057172 | US |
