JP2010117468A - Display element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a display element having good display quality of high contrast and reflectivity without forming a fine contact hole in an insulation protective layer. <P>SOLUTION: This display element is formed of a pixel having a pixel electrode, a transistor, and a reflective layer, and the reflective layer is formed in an electrically floating state at a position facing the pixel electrode on the insulation protective layer. Thus, the display element having the good display quality of high contrast and reflectivity is provided without forming the fine contact hole in the insulation protective layer. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a display element, and more particularly to a display element having a reflective layer formed in an electrically floating state.

  As a display element of a device in which power saving is desired such as a cellular phone or an electronic book, there is a reflective display provided with a reflective layer. Since the reflective display does not require a backlight for display, it consumes less power and can operate for a long time even when driven by a battery. As the reflective display, for example, a single polarizing plate type TN (Twisted Nematic) liquid crystal display element (hereinafter referred to as LCD) has been put into practical use.

  On the other hand, research and development related to a method for producing an organic thin film transistor (hereinafter referred to as TFT) array by a coating method or a printing method without using high-cost photolithography or a vacuum process has been actively conducted in recent years. If members such as wiring, semiconductors and electrodes can be made into ink and a fine organic TFT array can be manufactured by an inexpensive and high-speed printing process, significant cost reduction and improvement in throughput can be expected.

  As the ink material for printing, a solution obtained by dissolving a solid material in a solvent, a dispersion of fine particles, a mixture thereof, or the like is used. It has been reported that semiconductors and electrodes function even with thin films having a thickness of about 10 nm to 200 nm and can be printed using techniques such as an ink jet method, a flexographic printing method, a microcontact printing method, and a reverse offset printing method.

  In particular, the microcontact printing method is attracting attention as a technique that can print electrodes and semiconductors with a resolution of 1 μm or less (see Non-Patent Document 1).

  As a semiconductor material suitable for printing, an organic semiconductor having solubility in a solvent is promising. However, since an organic semiconductor is easily deteriorated by oxygen, moisture, light, etc. in the atmosphere, it is necessary to form a light-shielding insulating protective layer so as to cover the organic semiconductor. The insulating protective layer is preferably formed as a thick film having a thickness of 500 nm or more, more preferably 1000 nm or more in order to satisfy the functions of gas barrier properties and light shielding properties.

Usually, in the reflective display including the above-described reflective layer, for example, as disclosed in Patent Document 1, the TFT and the reflective layer are electrically connected via a contact hole provided in the insulating protective layer. It is made with. This is shown in FIG. FIG. 6 is a cross-sectional view showing the configuration of a conventional reflective display. As can be seen from the figure, the drain electrode 115 of the TFT 111 and the reflective layer 125 are connected via a through hole 131 provided in the insulating protective layer 117. By applying a voltage between the reflective layer 125 and the common electrode 107, a voltage is applied to the display layer 105 such as a liquid crystal to perform display.
JP 2006-86502 A (Germany) National Institute of Advanced Industrial Science and Technology, press release, June 9, 2008, printing organic thin film transistor array on flexible substrate (http://www.aist.go.jp/aist_j/press_release/pr2008/pr20080609/pr20080609.pr. html) Search October 10, 2008

  However, in the method of Patent Document 1, it is necessary to form a fine contact hole in the insulating protective layer, but it is technically difficult to form a fine contact hole in the thick insulating protective layer by printing. A method of forming a fine contact hole by photolithography, laser ablation, etc. after forming a solid insulating protective layer is also conceivable, but this leads to high cost and reduced throughput.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a display element having good display quality with high contrast and reflectance without forming a fine contact hole in an insulating protective layer. .

  The object of the present invention can be achieved by the following constitution.

1. A plurality of pixels arranged in a matrix on a substrate;
The plurality of pixels are
A transparent common electrode common to all the pixels;
A display electrode provided for each pixel;
A display layer sandwiched between the common electrode and the display electrode, common to all the pixels;
In a display element that is provided for each pixel and includes a transistor that applies a voltage between the common electrode and the display electrode,
The display electrode is
An electrically floating reflective layer that reflects light incident from the common electrode;
A pixel electrode connected to the drain electrode of the transistor, at least a part of which is opposed to the reflective layer;
A display element comprising a solid layer provided on the entire surface of all the pixels and comprising an insulating protective layer that insulates between the reflective layer and the pixel electrode.

  2. 2. The display element according to 1 above, wherein the pixel electrode and the reflective layer have substantially the same shape, and substantially the entire surfaces thereof are opposed to each other with the insulating protective layer interposed therebetween.

  3. 3. The display element according to 1 or 2, wherein the insulating protective layer is made of a polymer compound.

  4). 4. The display element according to any one of 1 to 3, wherein the insulating protective layer has a light shielding property.

  5. 5. The display element according to any one of 1 to 4, wherein the pixel electrode is made of metal fine particles.

  6). 6. The display element according to any one of 1 to 5, wherein the pixel electrode is formed thicker than the drain electrode.

  According to the present invention, a pixel having a pixel electrode, a transistor, and a reflective layer is formed, and the reflective layer is formed in an electrically floating state at a position facing the pixel electrode on the insulating protective layer, A display element having a good display quality with high contrast and reflectance can be provided without forming a fine contact hole in the insulating protective layer.

  Hereinafter, the present invention will be described based on the illustrated embodiment, but the present invention is not limited to the embodiment. In the drawings, the same or equivalent parts are denoted by the same reference numerals, and redundant description is omitted.

  First, an example of the configuration of a reflective display device according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram for explaining an example of the configuration of a reflective display device 1 according to the present invention.

  In FIG. 1, the display device 1 includes a display element 10, a control circuit 20, a signal circuit 30, a scanning circuit 40, and the like. The display element 10 includes m × n pixels 11 arranged in a two-dimensional matrix of m rows and n columns (m and n are positive integers). Each pixel 11 includes a TFT 111 and a display unit 121. The display unit 121 has a reflective configuration. Details will be described with reference to FIG.

  The control circuit 20 controls the operation of the entire display device 1, controls the operations of the signal circuit 30 and the scanning circuit 40, and supplies the common potential VCOM to the pixels 11 of the display element 10. The signal circuit 30 has n signal lines Sk (k = 1 to n), and supplies an image signal to be displayed to each pixel 11 in the row selected by the scanning circuit 40. The scanning circuit 40 has m scanning lines Gi (i = 1 to m), and sequentially selects one of the m pixel rows of the display element 10.

  The display operation will be described. For example, when the scanning line G1 in the first row of the scanning circuit 40 is selected, the TFT 111 of each pixel 11 in the first row of the display element 10 whose gate is connected to the scanning line G1 is turned on. At this time, since the scanning lines G2 to Gm are not selected, the TFTs 111 in the respective rows from the second row to the m-th row of the display element 10 are in an off state. In this state, when an image signal is supplied from each of the n signal lines S1 to Sn of the signal circuit 30, the display unit 121 of each pixel 11 in the first row of the display element 10 follows each image signal. Display is performed.

  The above-described operation is performed over m lines, and a full screen is displayed. When scanning up to m rows is completed, the operation returns to the first row and the same operation is repeated.

  Next, a first embodiment of a pixel constituting the reflective display element according to the present invention will be described with reference to FIG. 2. FIG. 2 shows a pixel 11 constituting the reflective display element 10 according to the present invention. FIG. 2A is a schematic diagram for explaining the first embodiment of FIG. 2A, FIG. 2A is a cross-sectional view of the pixel 11 along BB ′ in FIG. 2B, and FIG. It is the top view seen from the arrow A direction of a).

  In FIG. 2A, the pixel 11 includes a TFT 111 and a display unit 121. The configuration of the pixel 11 will be described from the lower layer to the upper layer in the figure. A gate electrode 112 is formed on an array substrate 101 common to all the pixels 11, and the gate electrode 112 is covered with a gate insulating layer 113 common to all the pixels 11. A source electrode 114 and a drain electrode 115 are formed on the gate insulating layer 113, and a semiconductor layer 116 is formed so as to straddle between the source electrode 114 and the drain electrode 115. Here, the TFT 111 is a bottom gate type TFT.

  The drain electrode 115 is connected to a pixel electrode 123 formed so as to spread over almost the entire area of the pixel 11. On the source electrode 114, the drain electrode 115, the semiconductor layer 116, and the pixel electrode 123, a solid insulating protection layer 117 common to all the pixels 11 is formed. At least the insulating protective layer 117 on the TFT 111 needs to have a light shielding property in order to prevent malfunction and deterioration due to external light of the TFT 111.

  A reflective layer 125 that reflects light incident on the pixel 11 from the direction of arrow A in the figure, which is the viewing direction of the display element 10, is formed at a position on the insulating protective layer 117 facing the pixel electrode 123. The pixel electrode 123, the insulating protective layer 117, and the reflective layer 125 function as display electrodes in the present invention.

  The reflective layer 125 is in an electrically floating state that is not connected to any potential, that is, a so-called floating state. The reflective layer 125 is desirably conductive. On the reflective layer 125, a display layer 105 common to all the pixels 11 is formed. The display layer 105 is a voltage-driven display element such as a liquid crystal. On the display layer 105, a transparent counter substrate 103 common to all the pixels 11 is disposed, on which the transparent common electrode 107 common to all the pixels 11 is formed on the side in contact with the display layer 105.

  The TFT 111 described above includes a gate electrode 112, a gate insulating layer 113, a source electrode 114, a drain electrode 115, a semiconductor layer 116, an insulating protective layer 117, and the like. In addition, the display unit 121 includes a pixel electrode 123, an insulating protective layer 117, a reflective layer 125, a display layer 105, a common electrode 107, and the like.

  FIG. 2B shows only each electrode and the semiconductor layer 116 that constitute the pixel 11. In FIG. 2B, the gate electrode 112 is connected to the above-described scanning line Gi, and a semiconductor layer 116 is formed on the gate electrode 112 with a gate insulating layer 113 (not shown) interposed therebetween.

  The source electrode 114 is connected to the signal line Sk described above and is in contact with the semiconductor layer 116 on the gate electrode 112 and the gate insulating layer 113. The drain electrode 115 is connected to the pixel electrode 123, faces the source electrode 114, and is in contact with the semiconductor layer 116 on the gate electrode 112 and the gate insulating layer 113.

The pixel electrode 123 and the reflective layer 125 have substantially the same shape, and are disposed substantially opposite to each other across an insulating protective layer 117 (not shown). This makes it possible to increase the capacitance C _L between the reflective layer 125 and the pixel electrode 123. Details will be described with reference to FIG.

  As the array substrate 101 and the counter substrate 103, for example, glass, resin plates or sheets of polyimide, polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyethersulfone, or the like can be used.

  Each electrode is formed by a vacuum deposition method, sputtering method, spin coating method, bar coating method, slit coating method, roller coating method, capillary coating method, spray coating method, flexographic printing method or micro contact printing method. It can be formed by using a method such as a relief printing method such as gravure printing, an intaglio printing method such as a gravure printing method, a screen printing method, or an ink jet method. The electrode layer may be formed as a solid film, and an electrode pattern may be formed by a photolithography method or the like.

  Examples of the electrode material include inorganic materials such as Au, Ag, Cu, Cr, Al, W, Ta, ITO (indium tin oxide), PEDOT / PSS (polyethylenedioxythiophene / polystyrene sulfonic acid), polyaniline, and polypyrrole. Such a conductive polymer material can be used. When an electrode is formed by coating, a solution or a dispersion of fine particles is used as necessary.

  The gate insulating layer 113 can also be formed using a method similar to that for electrodes. The material of the gate insulating layer 113 includes inorganic films such as silicon oxide, silicon nitride, and aluminum oxide, polyimide, polyamide, polyester, polyacrylate, photo-curing polymerization photopolymerization resin, and acrylonitrile component. An organic compound such as a copolymer, polyvinyl phenol, polyvinyl alcohol, novolac resin, epoxy resin, fluororesin, or cyanoethyl pullulan can be used.

  In addition, inorganic oxide particles such as silicon oxide, aluminum oxide, tantalum oxide, and titanium oxide, and inorganic nitride fine particles such as silicon nitride and aluminum nitride may be dispersed in the organic compound. Moreover, you may have the multilayered structure which laminated | stacked them.

  The semiconductor layer 116 can also be formed using a method similar to that of the electrode. As the semiconductor material, generally known organic semiconductor materials such as pentacene and derivatives thereof, polythiophene, and oligothiophene can be used.

  In the present invention, it is desirable to have a bottom-gate TFT structure in which the gate electrode 112 and the gate insulating layer 113 are formed before the semiconductor layer 116, the source electrode 114, and the drain electrode 115. Although the present invention is effective in a top gate type TFT configuration, in the case of the top gate type, the gate insulating layer 113 is interposed between the pixel electrode 123 and the reflective layer 125, so that the driving voltage is reduced. Invite.

  Regarding the positional relationship between the semiconductor layer 116 and the source electrode 114 and the drain electrode 115, even if the source electrode 114 and the drain electrode 115 are a bottom contact type in which the gate electrode 113 is closer to the semiconductor layer 116, the gate insulating layer 113 is used. Either of the top contact type on the opposite side and the configuration may be used.

  The insulating protective layer 117 can also be formed using a method similar to that for electrodes. As the material of the insulating protective layer 117, a material similar to the material of the gate insulating layer 113 described above and added with, for example, a black pigment for imparting light shielding properties can be used. As the pigment, carbon, graphite, metal, metal oxide, and the like can be used. In particular, an insulating material such as titanium oxynitride called titanium black is preferable.

  Since the insulating protective layer 117 is also formed on the above-described bus lines such as the signal lines Sk and the scanning lines Gi and the TFT 111, it also has a function of suppressing the scattering of light from these members and improving the contrast. In addition, since the insulating protective layer 117 covers all the electrodes of the TFT 111, it also has a function of reducing a leakage current between the source electrode 114 and the drain electrode 115.

  In the present invention, at least a part of the reflective layer 125 needs to be formed to face the pixel electrode 123. In order to increase the aperture ratio of the pixel 11, it is desirable to make the reflective layer 125 as large as possible.

  However, if the reflective layer 125 is formed on the TFT 111, the threshold voltage of the TFT 111 is affected. Further, when the reflective layer 125 is formed on the bus lines such as the signal lines Sk and the scanning lines Gi, the image quality is deteriorated due to an increase in parasitic capacitance. Therefore, it is preferable to design as large a size as possible so as not to overlap with the bus line or the TFT 111.

  Here, the relationship between the reflective layer 125 and the pixel electrode 123 will be described with reference to FIG. FIG. 3 is an equivalent circuit diagram of the pixel 11 for explaining the relationship between the reflective layer 125 and the pixel electrode 123.

As shown in FIG.
The capacitance between the pixel electrode 123 and the reflective layer 125 is represented by C _L ,
The capacitance between the reflective layer 125 and the common electrode 107 is represented by C _P ,
The voltage applied between the pixel electrode 123 and the common electrode 107 is V _S ,
The voltage between the reflective layer 125 and the common electrode 107 as _{V P,}
When calculated as a series of two capacitors,
V _P = V _S × C _L / (C _P + C _L ) (1 formula)
There is a relationship.

Usually, it is desirable to drive the display element 11 with as low a voltage as possible. Therefore, by increasing the C _L in the expression (1), V _P approaches the V _S, it is possible to increase the voltage applied to the display layer 105 by the same driving voltage. To increase the C _L is a method of reducing the thickness of the insulating protective layer 117, a method of using a high dielectric constant material, the area opposed to the pixel electrode 123 and the reflective layer 125 to increase as the material of the insulating protective layer 117 A method etc. can be considered.

  As described above, according to the first embodiment of the present invention, the reflective layer is formed in an electrically floating state at a position facing the pixel electrode on the insulating protective layer, so that the insulating protection is achieved. A display element having good display quality with high contrast and reflectance can be provided without forming a fine contact hole in the layer.

  Next, a second embodiment of the pixel constituting the reflective display element in the present invention will be described with reference to FIG. FIG. 4 is a schematic diagram for explaining a second embodiment of the pixel 11 constituting the reflective display element 10 in the present invention, and is a cross-sectional view of the pixel 11 similar to FIG.

  In FIG. 4, the second embodiment is different from the first embodiment in FIG. 2A in that two pixel electrodes 123 are stacked and formed thick. Other points are the same as those of the first embodiment in FIG.

Since the thickness of the insulating protective layer 117 between the pixel electrode 123 and the reflection layer 125 is reduced by increasing the thickness of the two pixel electrodes 123, the reflection from the pixel electrode 123 in the above (1) is reflected. it is possible to increase the capacitance C _L between the layer 125. Thereby, the voltage applied to the display layer 105 can be increased, and the driving voltage of the display element 11 can be reduced. The two layers of the pixel electrode 123 are simply a method, and in short, the pixel electrode 123 may be thickened.

  As described above, according to the second embodiment, in addition to the effects of the first embodiment, the pixel electrode 123 is made thicker, and the film thickness of the insulating protective layer 117 is reduced accordingly. The drive voltage of the display element 11 can be reduced.

  In addition to the single polarizing plate type reflective TN-LCD described above, the display having a reflective layer includes a TFD drive type reflective TN-LCD, a GH (Guest Host) type-LCD, and a PD (polymer dispersion) -LCD. Etc. These displays are described in detail in, for example, reflective color liquid crystal display technology (supervised by Tatsuo Uchida, CMC Publishing).

  The display element to which the present invention can be applied is not limited to the LCD, and any system may be used as long as it is a voltage-driven display element having a reflective layer. In addition to a reflective display that modulates external light and displays an image on the element, it can also be applied as a spatial light modulation element for a camera viewfinder or projector. In a PD-LCD, a reflective layer is not essential, but it is desirable to form a reflective layer in order to obtain good reflectivity in a liquid crystal cell having a narrow cell gap that can be driven.

  Hereinafter, detailed examples of the embodiments of the present invention will be described, but the present invention is not limited to these examples.

Example 1
Example 1 is an example of the first embodiment described above. A polyethylene terephthalate substrate was used as the array substrate 101, and a TFT array in which TFTs 111 having a bottom gate and bottom contact structure were two-dimensionally arranged was produced using a microcontact printing method, which is a relief printing method using a fine processing technique. Nano silver ink was used for the material of each electrode, P3HT (poly-3-hexylthiophene) ink was used for the material of the semiconductor layer 116, and PDMS (polydimethylsiloxane) was used for the plate material of the microcontact print.

  The gate insulating layer 113 was applied with polyimide by a spin coater. The drain electrode 115 and the pixel electrode 123 are integrally formed as shown in FIG. As an ink for the insulating protective layer 117, an ink in which titanium black nanoparticles were dispersed in a polyimide precursor solution was used, and the insulating protective layer 117 was formed on the TFT array using a flexographic printing method. The insulating protective layer 117 was not formed with a hole such as a through hole, so that the film thickness was almost uniform as a whole. The TFT array on which the insulating protective layer 117 was formed was baked at 180 ° C. in an oven to imidize the insulating protective layer 117.

  On the insulating protective layer 117, a nano-silver reflective layer 125 having substantially the same shape as the pixel electrode 123 was printed by a microcontact printing method. The area ratio of the reflective layer 125 in the display unit 121 of the pixel 11 was 60%.

  Using the TFT array thus prepared as a backplane, a polyethylene terephthalate substrate with a transparent conductive film as the counter substrate 103, and using a general liquid crystal panel manufacturing process by vacuum injection, a polymer dispersed LCD is manufactured. did. In the completed polymer dispersed LCD, the pixel electrode 123 had a thickness of 200 nm, the insulating protective layer 117 had a thickness of 1 μm, and the liquid crystal layer as the display layer 105 had a thickness of 10 μm.

  When the drive test of the produced polymer dispersed LCD was performed, the reflectance of the LCD panel to which no voltage was applied was 13%, and the reflectance of the LCD panel in the state where the voltage was applied was 3%. The driving voltage of the LCD was 7.5V.

(Example 2)
Example 2 is an example of the second embodiment described above. In addition to Example 1, an electrode having a thickness of 500 nm was applied over the pixel electrode 123 by a microcontact printing method. Others were produced in the same manner as in Example 1. Therefore, the pixel electrode 123 of the completed LCD had a thickness of 700 nm, the insulating protective layer 117 on the pixel electrode 123 had a thickness of 500 nm, and the liquid crystal layer as the display layer 105 had a thickness of 10 μm. As a result, the driving voltage of the LCD was 6.8 V, and it could be driven at a lower voltage than that of Example 1.

(Example 3)
The third embodiment is a modification of the first embodiment. The configuration of the pixel 11 of Example 3 is shown in FIG. As can be seen from FIG. 5, in Example 3, the reflective layer 125 of Example 1 was extended onto the TFT 111. By increasing the area of the reflective layer 125, the reflectance of the LCD panel to which no voltage was applied was improved to 15%. However, since the capacitance _{C P} between the reflective layer 125 and the common electrode 107 is increased, the driving voltage of the LCD is increased to 8V. In addition, since the reflective layer 125 was formed on the TFT 111, the charge charged in the reflective layer 125 affected the threshold voltage of the TFT 111, and a decrease in the ON / OFF ratio of the TFT 111 was observed.

(Comparative Example 1)
As a comparative example, the conventional pixel structure shown in FIG. 6 was produced. The comparative example is different from the first embodiment in that the pixel electrode 123 is not provided, and the drain electrode 115 and the reflective layer 125 are connected through a through hole 131 provided in the insulating protective layer 117. Others are the same as the first embodiment.

  As shown in FIG. 6, a contact hole 131 was formed in the insulating protective layer 117 by photolithography, and the drain electrode 115 and the reflective layer 125 were electrically connected. The driving voltage of the LCD was 6.2V, which was reduced. However, the manufacturing cost has increased significantly due to the photolithography process. Moreover, the chemical solution used in photolithography has an adverse effect on the characteristics of the organic TFT.

  As described above, according to the present invention, the pixel includes a pixel electrode, a transistor, and a reflective layer, and the reflective layer is electrically floated at a position facing the pixel electrode on the insulating protective layer. By forming in a state, it is possible to provide a display element having good display quality with high contrast and reflectance without forming a fine contact hole in the insulating protective layer.

  Note that the detailed configuration and detailed operation of each component constituting the display element according to the present invention can be changed as appropriate without departing from the spirit of the present invention.

It is a block diagram for demonstrating an example of a structure of the reflection type display apparatus in this invention. It is a schematic diagram for explaining a first embodiment of a pixel constituting a reflective display element in the present invention. It is an equivalent circuit diagram of a pixel for explaining a relation between a reflective layer and a pixel electrode. It is a schematic diagram for demonstrating 2nd Embodiment of the pixel which comprises the reflective display element in this invention. 6 is a cross-sectional view illustrating a configuration of a pixel according to Example 3. FIG. It is sectional drawing which shows the structure of the conventional reflection type display.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Display apparatus 10 Display element 11 Pixel 20 Control circuit 30 Signal circuit 40 Scan circuit 101 Array substrate 103 Opposite substrate 105 Display layer 107 Common electrode 111 TFT
112 Gate electrode 113 Gate insulating layer 114 Source electrode 115 Drain electrode 116 Semiconductor layer 117 Insulating protective layer 121 Display unit 123 Pixel electrode 125 Reflective layer 131 Through hole

Claims (6)

A plurality of pixels arranged in a matrix on a substrate;
The plurality of pixels are
A transparent common electrode common to all the pixels;
A display electrode provided for each pixel;
A display layer sandwiched between the common electrode and the display electrode, common to all the pixels;
In a display element that is provided for each pixel and includes a transistor that applies a voltage between the common electrode and the display electrode,
The display electrode is
An electrically floating reflective layer that reflects light incident from the common electrode;
A pixel electrode connected to the drain electrode of the transistor, at least a part of which is opposed to the reflective layer;
A display element comprising a solid layer provided on the entire surface of all the pixels and comprising an insulating protective layer that insulates between the reflective layer and the pixel electrode. The display element according to claim 1, wherein the pixel electrode and the reflective layer have substantially the same shape, and substantially the entire surfaces of the pixel electrode and the reflective layer face each other with the insulating protective layer interposed therebetween. The display element according to claim 1, wherein the insulating protective layer is made of a polymer compound. The display element according to claim 1, wherein the insulating protective layer has a light shielding property. The display element according to claim 1, wherein the pixel electrode is made of metal fine particles. The display element according to claim 1, wherein the pixel electrode is formed thicker than the drain electrode.

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