1. Field of the Invention
The present invention relates to an active-driving organic EL light emission device (which may be referred to merely as an organic EL device hereinafter) having a thin film transistor (which may be referred to as a TFT). More specifically, the present invention relates to an organic EL device used suitably for display equipment and color displays for the people""s livelihood and industries, and the like.
In the present specification, the description xe2x80x9cELxe2x80x9d means xe2x80x9celectroluminescencexe2x80x9d.
2. Description of the Related Art
Conventionally, it is known a simple-driving organic EL light emission device which is simply driven by XY matrix electrodes to display an image (Japanese Patent Application Laid-Open (JP-A) No. 37385/1990, JP-A No. 233891/1991 and the like) as an organic EL light emission device (display).
However, in such a simple-driving organic EL light emission device, the so-called line sequential driving is performed. Therefore, if the number of scanning lines is several hundreds, required instantaneous brightness is several-hundred times larger than observed brightness so that the following problems arise.
(1) Since a driving voltage becomes not less than 2-3 times higher than a direct-current constant voltage, luminous efficiency drops or power consumption becomes large.
(2) Since the electrical current that passes instantaneously becomes several-hundred times larger, the organic luminous layer is apt to deteriorate.
(3) Since the electrical current is very large in the same manner as in the (2), a voltage-drop in the electrode wiring becomes large.
Thus, in order to solve the problems that simple-driving organic EL light emission devices have, various active-driving organic EL light emission devices, wherein organic EL elements are driven by TFTs (thin film transistors), are suggested (JP-A No. 122360/1995, JP-A No. 122361/1995, JP-A No. /153576/1995, JP-A No. 54836/1996, JP-A No. 111341/1995, JP-A No. 312290/1995, JP-A No. 109370/1996, JP-A No. 129359/1996, JP-A No. 241047/1996, JP-A No. 227276/1996, JP-A No. 339968/1999, and the like).
Examples of the structure of such an active-driving organic EL light emission device are shown in FIGS. 18 and 19. According to such active-driving organic EL light emission devices, it is possible to obtain advantages as follows: driving voltage is highly lowered, luminous efficiency is improved and power consumption can be reduced, as compared with simple-driving organic EL light emission devices.
However, the following problems (1)-(3) are caused even in active-driving organic EL light emission devices having advantageous as described above.
(1) The aperture ratio of their pixels becomes small.
In an active-driving organic EL light emission device, at least one TFT is fitted to each pixel on a transparent substrate and further a great deal of scanning electrode lines and signal electrode lines are disposed on the substrate to select appropriate TFTs and drive them. Accordingly, there arises a problem that when light is taken out from the side of the transparent substrate, the aperture ratio of the pixels (the ratio of portions that emits light actually in the pixels) becomes small since the TFTs and the various electrode lines shut off the light. For example, in an active-driving organic EL light emission device that has been developed recently, TFTs for driving organic EL elements at a constant current are disposed besides the above-mentioned two kinds of TFTs. Therefore, its aperture ratio becomes smaller and smaller (about 30% or less). As a result, dependently on the aperture ratio, the current density that passes through the organic luminous medium becomes large, causing a problem that the life span of the organic EL elements is shortened.
This matter will be described in more detail, referring to FIGS. 10, 11 and 18. FIG. 10 shows a diagram of a circuit for switch-driving the active-driving organic EL light emission device 100 illustrated in FIG. 18, and illustrates a state that gate lines (scanning electrode lines) 50 (108 in FIG. 18) and source lines (signal electrode lines) 51 are formed on the substrate and they are in an XY matrix form. Common electrode lines 52 are disposed in parallel to the source lines (signal electrode lines) 51. About each pixel, a first TFT 55 and a second TFT 56 are fitted to the gate lines 50 and the source lines 51. A capacitance 57 is connected between the gate of the second TFT 56 and the common electrode line 52 to hold the gate voltage at a constant value.
Therefore, an organic EL element 26 can be effectively driven by applying the voltage held by the capacitance 57 to the gate of the second TFT 56 shown in the circuit diagram of FIG. 10 and then attaining switching.
The plan view shown in FIG. 11 is a view obtained by seeing, along the plane direction, through switch portions and the like according to the circuit diagram shown in FIG. 10.
Thus, the active-driving organic EL light emission device 100 has a problem that when EL light is taken out from the side of lower electrodes (ITO, indium tin oxide) 102 side, that is, the side of a substrate 104 side, a TFT 106, a gate line 108, a source line (not illustrated) and the like shut off EL light so that the aperture ratio of pixels becomes small.
In an active-driving organic EL light emission device 204, as shown in FIG. 19, wherein a TFT 200 and an organic EL element 202 are arranged on the same plane, the TFT 200 and the like never block off EL light. However, its aperture ratio of pixels is further lowered, as compared with the active-driving organic EL light emission device 100 shown in FIG. 18.
(2) The sheet resistivity of upper electrodes is large.
In the case that light is taken out from the side opposite to the substrate, that is, the side of upper electrodes, the TFTs and the like do not shut off the light to keep the aperture ratio large. As a result, a high-brightness image can be obtained. However, when EL light is taken out from the upper electrode side, in order to take out the EL light effectively to the outside, it is necessary to form the upper electrodes from transparent conductive material. For this reason, the sheet resistivity of the upper electrodes exceeds, for example, 20 xcexa9/xe2x96xa1, resulting in a serious problem at the time of using large-area display.
In the case that light is emitted, for example, at a brightness of 300 nit from the entire surface of an EL light emission device having a diagonal size of 20 inches (the ratio of length to breadth, 3:4), it is necessary to send a large current having a current of 3600 mA to the upper electrodes even if an organic luminous material having a high luminous efficiency of 10 cd/A (luminous power per unit amperage) is used in the organic luminous medium.
More specifically, the value of a voltage-drop based on the resistances of the upper electrodes is represented by xcexa3nir and calculated on the following formula.
xcexa3nir=xc2xdxc3x97N(N+1)ir
N: (the total number of pixels in the longitudinal direction)xc3x97xc2xd,
r: the ohmic value (xcexa9) of the upper electrode in each pixel, and
i: a constant current value (A) that flows through each pixel.
Therefore, if luminous efficiency, luminous brightness, the shape of the pixels and the sheet resistivity of the upper electrodes are set to, for example, 10 cd/A, 300 nit, 200xc3x97600 xcexcm square, and 20 xcexa9/xe2x96xa1, respectively, the pixel current value is 3.6xc3x9710xe2x88x926 A. If the total number of the pixels in the longitudinal direction is set to 2000, drop-voltage in the longitudinal direction is 12V (xc2xdxc3x971000xc3x971000xc3x973.6xc3x9710xe2x88x926xc3x9720xc3x97⅓). This exceeds an allowable voltage range (10 V) for driving circuits which are driven at a constant current. Thus, it is difficult to emit light under the above-mentioned conditions.
In short, if the sheet resistivity of the upper electrodes is large, voltage-drop, particularly at the center of the screen, becomes large accordingly. As a result, a problem that brightness is remarkably lowered becomes apparent. Incidentally, the following is also attempted: amendment is made by using a circuit to make a current value (brightness) constant for each pixel. However, this attempt is insufficient.
(3) From the viewpoint of production, it is difficult to control the ohmic value of the upper electrodes.
It is known that in order to set the resistivity of the upper electrodes of an active-driving organic EL light emission device having a diagonal size of several inches to 10 inches to a low value, for example, 1xc3x9710xe2x88x923 xcexa9xc2x7cm or less by using an ordinary material such as ITO or ZnO, it is necessary to set heating temperature to 200xc2x0 C. or higher. However, heat-resistance of ordinary organic luminous media is 200xc2x0 C. or lower. Thus, it is necessary to set the heating temperature to 200xc2x0 C. or lower. Accordingly, the value of the resistivity of the upper electrodes cannot be controlled so that the value may exceed 1xc3x9710xe2x88x923 xcexa9xc2x7cm. As a result, a problem that the sheet resistivity becomes a high value over 20 xcexa9/xe2x96xa1 occurs. In the case that plasma is used for sputtering at the time of forming an oxide such as ITO or IZO on the organic luminous medium to form the upper electrodes, a problem that the organic luminous medium is damaged by the plasma also arises.
In light of the above-mentioned problems, the present invention has been made. Its object is to provide an organic active EL light emission device making it possible to increase the aperture ratio of respective pixels even if TFTs are disposed to drive organic EL elements, reduce the sheet resistivity of upper electrodes even if luminescence is taken out from the side of the upper electrodes, and display an image having a high brightness and a homogeneous brightness; and a method for manufacturing such an organic active EL light emission device effectively.
[1] The present invention is an active-driving organic EL light emission device comprising an organic EL element comprising an organic luminous medium between an upper electrode and a lower electrode, and a thin film transistor for driving this organic EL element, wherein light emitted from the organic EL element (EL light) is taken out from the side of the upper electrode, and the upper electrode comprises a main electrode formed of a transparent conductive material (embracing a transparent semiconductor material), and an auxiliary electrode formed of a low-resistance material.
Such a structure makes it possible to make a numerical aperture large even if a TFT is set up and make the sheet resistivity of the upper electrode reduced even if luminescence is taken out from the side of the upper electrode.
It is also possible to improve brightness and further prolong the life span of the organic luminous medium remarkably because of a reduction in the density of electric current passing through the organic luminous medium.
[2] The active-driving organic EL light emission device of the present invention preferably comprises an electric switch comprising the thin film transistor and a transistor for selecting a pixel, and a signal electrode line and a scanning electrode line for driving the electric switch.
Namely, it is preferred to comprise a scanning electrode line and a signal electrode line arranged, for example, in an XY matrix form, and an electric switch composed of a TFT connected electrically to these electrode lines and a transistor for selecting a pixel.
Such a structure makes it possible to drive the organic EL element effectively by selecting any pixel, applying a scanning signal pulse and a signal pulse through the scanning electrode line and the signal electrode line and thus performing switching-operation of the electric switch comprising the TFT.
[3] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the transparent conductive material is at least one material selected from the group consisting of a conductive oxide, a light-transmissible metal film, a non-degeneracy semiconductor, an organic conductor, and a semiconductive carbon compound.
Namely, the sheet resistivity of the upper electrode can be reduced. It is therefore possible to use, in the main electrode, not only transparent conductive material that has been conventionally used but also transparent conductive material other than it. Thus, the above-mentioned transparent conductive material has also been able to be used.
It is possible to use a non-degeneracy semiconductor and the like, for example, which can be made into a film at a low temperature, preferably 200xc2x0 C. or lower and more preferably 100xc2x0 C. or lower. It is therefore possible to make heat damage of any organic layer at the time of film-making small. Vapor deposition at low temperature or wet coating can be attained by using the organic conductor, the semiconductive carbon compound and the like.
[4] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that a plurality of the auxiliary electrodes are regularly placed in a plane.
For example, the resistance of the upper electrode can be uniformly and effectively made low by arranging the auxiliary electrode in a matrix, stripe and the like form.
[5] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that a sectional shape of the auxiliary electrode is an overhang form.
Such a structure makes it possible to connect certainly the auxiliary electrode electrically to the upper electrode, using a site positioned below the overhanging upper portion (embracing a conversely-tapered portion and the like) even if an insulating organic layer is deposited on the auxiliary electrode.
[6] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the auxiliary electrode comprises a lower auxiliary electrode and an upper auxiliary electrode.
Such a structure of the auxiliary electrode makes it possible to easily connect the auxiliary electrode electrically to the main element, using the lower auxiliary electrode or the upper auxiliary electrode. Since the assistant is separated into the lower auxiliary electrode and the upper auxiliary electrode as described above, the overhanging form can easily be made.
[7] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the lower auxiliary electrode and the upper auxiliary electrode in the auxiliary electrode comprise constituent materials having different etching rates.
Such a structure makes it possible to form the overhang shape easily by etching.
[8] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the lower auxiliary electrode and the upper auxiliary electrode in the auxiliary electrode, or one thereof is electrically connected to the main electrode.
Such a structure makes it possible to connect easily and certainly the auxiliary electrode electrically to the main electrode so that the resistance of the upper electrode can be made low.
[9] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the auxiliary electrode is formed on an interlayer insulating film for forming the organic EL element, on the electrically insulating film for insulating electrically the lower electrode, or on the electrically insulating film for insulating electrically the TFT.
Such a structure makes it possible to make the numerical aperture in pixels wide.
[10] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that an active layer of the TFT is made of polysilicon.
Such a structure makes it possible to produce an active-driving organic EL light emission device whose TFT has high endurance since the active layer made of polysilicon has preferable resistance against the amount of electricity.
[11] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that an interlayer insulating film is formed on the TFT, the lower electrode of the organic EL element is deposited on the interlayer insulating film, and the TFT and the lower electrode are electrically connected to each other through a via hole made in the interlayer insulating film.
Such a structure makes it possible to obtain superior electrical insulation between the TFT and the organic EL element.
[12] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that charges are injected from the auxiliary electrode to the main electrode and transported in parallel to a main surface of a substrate, and subsequently the charges are injected to the organic luminous medium.
Such a structure makes it possible to adopt a non-metal compound for the main electrode so that the transparency of the main electrode can be improved. The non-metal compound herein means, for example, a non-degenerate semiconductor, an organic conductor, or a semiconductive carbon compound that will be described later.
[13] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the sheet resistivity of the main electrode is set to a value within the range of 1 K to 10 Mxcexa9/xe2x96xa1. In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the sheet resistivity of the auxiliary electrode is set to a value within the range of 0.01 to 10 xcexa9/xe2x96xa1.
Adoption of such a structure for the respective electrodes makes it possible to send electrical current giving a high luminous brightness and cause a certain drop in the sheet resistivity of the upper electrode.
[14] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that a color filter for color-converting the taken-out light and a fluorescent film, or one thereof is arranged on the side of the upper electrode.
Such a structure makes it possible to color-convert luminescence taken out from the upper electrode in the color filter or the fluorescent film so that full-color display can be performed.
[15] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that a black matrix is formed on a part of the color filter or the fluorescent film, and the black matrix and the auxiliary electrode overlap with each other in a vertical direction.
Such a structure makes it possible to suppress reflection of outdoor daylight on the auxiliary electrode effectively by the black matrix and make numerical aperture wide.
[16] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the auxiliary electrode is formed on the main electrode, and an area of the auxiliary electrode is smaller than that of the main electrode.
Such a structure makes it possible to form the auxiliary electrode after the main electrode is formed. Therefore, it is easier to form the auxiliary electrode.
[17] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the auxiliary electrode is embedded in a sealing member surrounding a periphery thereof.
Such a structure does not cause the thickness of the organic EL light emission device to be excessively large on the basis of the thickness of the auxiliary electrode. Since the auxiliary electrode can be formed beforehand in the sealing member, sealing based on the sealing member and electrical connection between the auxiliary electrode and the main electrode can be performed at the same time.
[18] In the structure of the active-driving organic EL light emission device of the present invention, it is preferred that the auxiliary electrode is closely arranged between the sealing member and the main electrode.
Such a structure makes it possible to perform sealing based on the sealing member and electrical connection between the auxiliary electrode and the main electrode at the same time.
[19] According to another embodiment of the present invention, when an active-driving organic EL light emission device is made, there is used a method for manufacturing an active-driving organic EL light emission device comprising an organic EL element having an organic luminous medium between an upper electrode and a lower electrode, and a thin film transistor for driving the organic EL element, the method comprising the steps of forming the organic EL element and forming the thin film transistor, wherein during the step of forming the organic EL element, the lower electrode and the organic luminous medium are formed and subsequently a main electrode is formed from a transparent conductive material (embracing a transparent semiconductor material) and the upper electrode is formed by forming an electrically auxiliary electrode formed from a low-resistance material.
According to such an embodiment, it is possible to provide an active-driving organic EL light emission device wherein numerical aperture is large even if the TFT is disposed and further the sheet resistivity of the upper electrode is low even if luminescence is taken out from the side of the upper electrode.