JPH04254887A - El display device - Google Patents

Abstract

PURPOSE:To improve a yield and reliability, and make high performance. CONSTITUTION:A transparent electrode 2A, composed of a metal layer 2a and a transparent electrode layer 2b, is stripedly formed on the upper surface of a glass substrate 1, and an EL luminous element 3 is formed on the upper surface of the glass substrate 1 including the transparent electrode 2A. A counter electrode 4 is formed stripedly and crossly to the transparent electrode 2A on the upper surface of the EL luminous element 3. The transparent electrode 2A itself can be made low resistance by making the metal layer 2a, composing the transparent electrode 2A, low resistance compared with the transparent electrode layer 2b, consequently the transparent electrode 2A itself can be made more thinner compared with a former transparent electrode.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an EL display device having an EL layer that emits light upon application of an electric field.

0002

[Prior Art] In recent years, in order to make display devices such as displays thinner, liquid crystals (LCDs), which have the property of changing the molecular arrangement and light transmittance when heat or voltage is applied, are used. LCD TVs equipped with this technology are becoming popular. In addition, by applying voltage or current, E
EL display devices equipped with L light emitting elements are also being developed. By the way, when such EL light emitting elements are classified according to the material of the EL layer, they are divided into inorganic and organic. Research on EL in organic compounds began with the discovery of an EL emission phenomenon due to carrier injection in a single crystal such as anthracene, which has strong fluorescence, and has been developed into thin film type devices.

Recently, a multilayer organic thin film E in which a hole transport layer and an electron transport layer are inserted between a light emitting layer and an electrode has been developed.
It has been reported that the L element emits light with high brightness of 10,000 cd/m2 or more and operates at a driving voltage of 10 V or less. FIG. 4 shows the structure of an EL display device using such an organic thin film EL element.
Thin film transparent electrodes 2, 2, made of nO2, ITO, etc.
... are formed in stripes. These transparent electrodes 2
An EL light emitting element 3 consisting of a hole transport layer 3a and an organic EL layer 3b is formed on the upper surface of the glass substrate 1 containing the above. On the upper surface of the EL light emitting element 3, counter electrodes 4, 4,...
are formed in a stripe shape so as to cross each of the transparent electrodes 2 described above. The crossing portions of each transparent electrode 2 and each opposing electrode 4 are defined as pixels, and when a voltage is applied to the crossing portions, the EL light emitting elements 3 in those portions emit light. Although not shown, there may be a structure in which an organic EL layer (fluorescent layer) and an electron transport layer are stacked between a transparent electrode and a counter electrode, or a structure in which a hole transport layer, an organic EL layer, and an electron transport layer are stacked. Things are also being considered.

0004

[Problem to be solved by the invention] By the way, the conventional AC
In the case of a type electroluminescent element, a liquid crystal display element, and a plasma display element, a driving voltage is supplied through a transparent electrode similarly to the above-mentioned EL light emitting element. However, these elements can be driven satisfactorily even when the driving current density is lower than that of EL light emitting elements. Therefore, the voltage drop due to the transparent electrodes for driving these elements is reduced, and the unevenness in brightness is also reduced, especially in an XY matrix type display panel. Therefore, in such an X-Y matrix form,
Constant voltage drive is sufficient.

On the other hand, in organic EL devices, the required current density is said to be an order of magnitude higher than that of the above-mentioned AC type electroluminescent devices, liquid crystal display devices, and plasma display devices. Furthermore, the applied voltage is set to a small value of 10-odd volts. Therefore, in the case of constant voltage driving, due to the resistance of the transparent electrode,
A pixel located far from the drive voltage supply end has a larger voltage drop than a pixel located closer. Therefore, the voltage drop causes uneven brightness.

Further, in the above-described EL display device, as shown in FIG. 4, for example, each transparent electrode 2 formed on the upper surface of the glass substrate 1 has a rectangular cross section. Furthermore, the region that contributes to light emission in the EL light emitting element 3 formed on the upper surface of the glass substrate 1 including these transparent electrodes 2 is said to extend to a height of several hundred angstroms from the interface between the two types of organic layers 3a and 3b. ing. In other words, even if the thickness is greater than this, the EL light emitting element 3 acts as a resistor, resulting in a decrease in luminous efficiency. Therefore, when forming the EL light emitting element 3, it is desirable to make it as thin as possible.

As described above, in order to improve the luminous efficiency, it is desirable to make the transparent electrode thinner to eliminate the step difference and to make the EL light emitting element thinner.On the other hand, in order to prevent uneven brightness, it is desirable to make the transparent electrode thinner and eliminate the step difference. It is desirable to increase the thickness and reduce the resistance. However, if the transparent electrode is made thinner, uneven brightness will occur, and if the transparent electrode is made thicker, the EL layer will be damaged due to the step difference, causing short circuits.

The present invention has been made in response to the above-mentioned circumstances, and an object of the present invention is to provide an EL display device that can improve yield, reliability, and performance.

[0009]

[Means for solving the problem] The invention according to claim 1 includes:
A transparent electrode consisting of a metal layer and a transparent electrode layer is formed in a stripe shape on the upper surface of a glass substrate, an EL light emitting element is formed on the upper surface of the glass substrate including these transparent electrodes, and a stripe shape is formed on the upper surface of this EL light emitting element. Further, a counter electrode is formed to cross the transparent electrode. In the invention according to claim 2, transparent electrodes are formed in a stripe shape on the upper surface of a glass substrate, an EL light emitting element is formed on the upper surface of the glass substrate including these transparent electrodes, and a stripe shape is formed on the upper surface of the EL light emitting element. Further, in an EL display device in which a counter electrode is formed to cross the transparent electrode, a drive voltage supply means for supplying a drive voltage to a pixel at a portion where the transparent electrode and the counter electrode cross, and a drive voltage supplying means for supplying the drive voltage. In this case, taking into account the voltage drop across the transparent electrode, the voltage supplied to the pixels located far from the supply end of the drive voltage is set higher than that of the pixels located close to the supply end of the drive voltage, and the voltage drop between the transparent electrode and the counter electrode is increased. The present invention is characterized by comprising a drive voltage control means for controlling drive voltages to all pixels in the crossing portion to be constant.

0010

[Operation] In the invention as claimed in claim 1, transparent electrodes consisting of a metal layer and a transparent electrode layer are formed in stripes on the upper surface of the glass substrate, and the upper surface of the glass substrate containing these transparent electrodes is
An L light emitting element was formed, and a counter electrode was formed on the top surface of this EL light emitting element in the form of a stripe and crossing the transparent electrode. Therefore, by making the metal layer constituting the transparent electrode have a lower resistance than the transparent electrode layer, the transparent electrode itself can have a low resistance. As a result, the transparent electrode itself can be made thinner than conventional transparent electrodes. In the invention according to claim 2, the driving voltage control means takes into account the voltage drop of the transparent electrode and increases the voltage supplied to the pixels located far from the supply end of the driving voltage relative to the pixels located near the supply end of the driving voltage. The drive voltage to all pixels at the intersection of the electrode and the counter electrode is controlled to be constant. As a result, the brightness of all pixels at the intersection of the transparent electrode and the counter electrode is made constant.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, details of embodiments of the present invention will be explained based on the drawings. In the figures described below, parts common to those in FIG. 4 are designated by the same reference numerals, and redundant explanation will be omitted. FIG. 1 shows an embodiment of the EL display device of the present invention, in which a plurality of transparent electrodes 2A are formed in stripes on the upper surface of a glass substrate 1. As shown in FIG. Here, each transparent electrode 2
A is made up of a metal layer 2a made of Al or the like and a thin film-like transparent electrode layer 2b made of ITO or the like. Further, the thickness of the metal layer 2a is set to be 50 angstroms to 200 angstroms, and the thickness of the transparent electrode layer 2b is set to be 100 angstroms to 900 angstroms.

On the upper surface of the glass substrate 1 including the transparent electrode 2A, an E layer consisting of a hole transport layer 3a and an organic EL layer 3b is provided.
An L light emitting element 3 is formed. On the upper surface of the EL light emitting element 3, counter electrodes 4, 4, . The EL display device having such a configuration is manufactured as follows. First, a metal layer 2a such as Al and a thin film transparent electrode 2b such as ITO are sequentially formed on the upper surface of the glass substrate 1 by vapor deposition or sputtering, and then a striped electrode pattern is formed by etching.

The thickness of each layer at this time is within the range of 50 angstroms to 200 angstroms and 100 angstroms to 900 angstroms, respectively, as described above. Next, the hole transport layer 3a and the organic EL layer 3b are laminated in order to form the EL light emitting element 3, and then each counter electrode 4 is formed in a stripe shape and each transparent electrode 2A is formed on the upper surface of the EL light emitting element 3 by etching or the like. Form into a cross.

Here, the thickness of the metal layer 2a is, for example, 10
If it is 0 angstrom, its area resistivity is 3Ω.
/cm2, and has a visible light transmittance of about 50%. Metal layer 2a and transparent electrode layer 2 having such characteristics
By forming the transparent electrode 2A in combination with b, it is possible to form the transparent electrode 2A with low resistance even when the thickness of the transparent electrode layer 2b is reduced. Note that the transparent electrode layer 2b on the metal layer 2a is an essential element for increasing the efficiency of current injection into the organic EL layer 3b of the EL light emitting element 3. Further, the material of the metal layer 2a of the transparent electrode 2A has a resistivity of 1×10 −5 Ω·cm or less,
Any material may be used as long as it has good adhesion to the glass substrate 1, and it is not limited to a single metal but may be an alloy. Therefore, for example, it is possible to further reduce the resistance of the metal layer 2a made of Al by increasing its thickness, but on the other hand, the light transmittance decreases, so if the metal layer 2a is made of Al, It is preferable to keep it within this range. Furthermore, since the purpose of the thickness of the transparent electrode layer 2b is to be thinner than the thickness of a conventional transparent electrode, which is 1000 angstroms, the upper limit is set to 900 angstroms, and the lower limit is set to 100 angstroms. In other words,
This is because if the thickness is less than the lower limit of 100 angstroms, there is a strong possibility that a uniform film will not be formed and will be formed piecemeal.

[0015] Next, the brightness unevenness caused by the transparent electrode 2A composed of the metal layer 2a and the transparent electrode layer 2b will be considered. First, in order to obtain a certain level of brightness, 10
00 cd/m2 or more is required, and conventional EL
In the display device, the driving voltage at this time was about 9 V, and the current flowing per unit area of the transparent electrode was 30 mA. Therefore, as shown in FIG. 2, one side of the display panel is L, the width of the transparent electrode 2A is d, and its sheet resistivity is ρs.
shall be. Further, while the width of the counter electrode 4 is d,
Its area resistivity is very small compared to that of the transparent electrode 2A, so it is ignored. In addition, consider the case where a pixel (2) located at a distance of approximately φ from the voltage supply end A of the transparent electrode 2A and a pixel (2) located at a distance of approximately L from the voltage supply end A of the transparent electrode 2A are driven. When pixel ■ is driven, the resistance component added in series to that part is:
It is approximately proportional to the distance of φ, so that A-Ba
By applying 9V between them, a brightness of 1000 cd can be obtained.

On the other hand, in pixel (2), it is considered that a resistance of (L/d)·ρs is added in series to that portion. Here, for example, L=200mm, d=0.5mm, ρ
If s = 30Ω/cm2, (L/d)・ρs
=12KΩ. The area of pixel ■ is 0.5 x 0.5 m
Since m=1/400 mm, in order to obtain a current density of 30 mA/cm2, a current of 0.075 mA is supplied from the voltage supply end A of the transparent electrode 2A. At this time, the voltage drop due to the transparent electrode 2A is 0.075mA×12K=
It becomes 0.9V. In other words, pixel ■ requires a 10% higher drive voltage than pixel ■, so in order to obtain the same brightness of 1000 cd/m2 as pixel ■, 9.9V is required.
An applied voltage of

Furthermore, when driven at 9V, which is the same as pixel ■, the current flows only about 10-odd mA, so the brightness at that time is only about 500 cd/m2, which is about half of the target, resulting in low cost. Constant voltage driving, which is the driving method used in the above, causes uneven brightness on the panel surface. On the other hand, conventional inorganic material EL, plasma display, etc. require a high voltage to drive, ranging from 100 to over 100 volts, but the required current density is several mA/m2, which is much higher than that of organic EL. small. Here, when performing the same calculation as above, assuming a current density of 3 mA/m2 and assuming that the applied voltage at pixel ■ is 100 V, the required voltage at pixel ■ is 10
0V+12KΩ×(1/400cm2)×3mA/c
m2 = 100.09V, the voltage increase is only 1%
It is. Therefore, in these devices, there has not been a great demand for lower resistance of transparent electrodes.

FIG. 3 shows the configuration of the drive circuit in the EL display device of the present invention. As shown in the figure, the drive circuit includes a common electrode C1 corresponding to the transparent electrode layer 2b.
~Cn and line electrode L1 corresponding to counter electrode 4 ~
Ln is provided. A controller 5a is connected to the common electrodes C1 to Cn via switches SI1 to SIn.
, a constant voltage supply circuit E0, and a variable voltage supply circuit Ec are connected. A controller 5b, a variable voltage supply circuit Ec, and a constant voltage supply circuit E0 are connected to the line electrodes L1 to Ln via switches SM1 to SMn. The on/off operations of the switches SI1 to SIn and the switches SM1 to SMn are controlled by the controllers 5a and 5b, respectively. In addition, each switch SI1 to SIn and switch SM1
The drive voltage supplied through SMn is such that in addition to the supply voltage from the constant voltage supply circuit E0, a variable voltage is supplied from the variable voltage supply circuit Ec controlled by the controller 5b.

The drive circuit configured as described above operates as follows. First, when the switch SM1 is turned on by the controller 5b, the pixel on the line electrode L1 is enabled to be driven, and the controller 5a turns on any of the switches SI1 to SIn that should drive that pixel. , line electrode L1 and common electrode C
Pixels at the intersections of 1 to Cn emit light. Subsequently, when the switch SM2 is turned on after the switch SM1 is turned off, the pixel on the line electrode L2 is enabled to be driven, and the controller 5a selects one of the switches SI1 to SIn to drive that pixel. is turned on, line electrode L2 and common electrodes C1 to C
The pixel at the intersection with n emits light. By sequentially performing such control up to switch SMn, display for one screen is completed. Then, the screen is continuously displayed by repeating the same control as above from switch SM1 to switch SMn. Note that in the case of normal interlaced scanning, after the pixels on the line electrode L1 are set to a drivable state, the pixels on the line electrodes L3, L5,
L7...The upper pixel is enabled to be driven. When this is completed, the pixels on the line electrodes L2, L4, L6, . . . are enabled to be driven.

At this time, when the controller 5b causes the pixels located at the intersections of the line electrode L1 and the common electrodes C1 to Cn to emit light, the voltage of the transparent electrode 2A corresponding to the distance S1 from the voltage supply end of the transparent electrode is adjusted. Voltage is supplied to the pixels to be driven taking into account the voltage drop. In other words, the voltage supplied to pixels located far from the voltage supply end is set to be higher than that of pixels located close to the voltage supply end, and the driving voltage to the pixels on each line electrode L1 to Ln is controlled to be constant. . As a result, line electrodes L1 to L
The brightness of all pixels located at the intersections of n and the common electrodes C1 to Cn becomes constant.

As described above, in this example, transparent electrodes consisting of a metal layer and a transparent electrode layer are formed in stripes on the upper surface of a glass substrate, and EL light emitting elements are formed on the upper surface of the glass substrate including these transparent electrodes. Then, a counter electrode was formed in a stripe shape on the upper surface of this EL light emitting element, crossing the transparent electrode. Therefore, by making the metal layer constituting the transparent electrode have a lower resistance than the transparent electrode layer, the transparent electrode itself can have a low resistance. As a result, the transparent electrode itself can be made thinner than conventional transparent electrodes. In addition, in this example, in consideration of the voltage drop due to the transparent electrode, the voltage supplied to the pixels located far from the drive voltage supply end is set higher than that of the pixels located near the supply end, and the voltage drop between the transparent electrode and the counter electrode is set higher. Since the drive voltage to all pixels in the crossing portion was controlled to be constant, the brightness of all pixels in the crossing portion of the transparent electrode and the counter electrode was constant.

[0022]

[Effects of the Invention] As explained above, according to the EL display device of the present invention, the transparent electrode itself can be made thinner than the conventional transparent electrode. Since short circuits with the counter electrode are avoided, product defects are reduced, yields can be improved, and reliability can also be improved. In addition, in consideration of the voltage drop of the transparent electrode, the voltage supplied to pixels located far from the drive voltage supply end is set higher than that of pixels located close to the drive voltage supply end, and all parts where the transparent electrode and the counter electrode cross Since the drive voltage to the pixels is controlled to be constant, the brightness of all pixels at the intersection of the transparent electrode and the counter electrode is kept constant, so performance can be improved.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a display section according to an embodiment of an EL display device of the present invention.

FIG. 2 is a diagram for explaining a voltage drop due to a transparent electrode of the display section in FIG. 1;

FIG. 3 is a diagram showing a drive circuit for driving the display section of FIG. 1;

FIG. 4 is a perspective view showing a conventional EL display device.

[Explanation of symbols]

1 Glass substrate 2a Metal layer 2b, 2A Transparent electrode layer 3a Hole transport layer 3b Organic EL layer 4 Counter electrode 5a, 5b Controller

Claims (3)

[Claims] 1. A transparent electrode consisting of a metal layer and a transparent electrode layer is formed in a stripe shape on the upper surface of a glass substrate, and an EL light emitting element is formed on the upper surface of the glass substrate including these transparent electrodes. An EL display device characterized in that a counter electrode is formed on the upper surface in a stripe shape and crosses the transparent electrode. 2. A transparent electrode is formed in a stripe shape on the upper surface of a glass substrate, an EL light emitting element is formed on the upper surface of the glass substrate including these transparent electrodes, and the transparent electrode is formed in a stripe shape on the upper surface of the EL light emitting element. In an EL display device in which a counter electrode is formed to cross an electrode, a drive voltage supply means for supplying a drive voltage to a pixel at a portion where the transparent electrode and the counter electrode intersect; , taking into account the voltage drop of the transparent electrode, the voltage supplied to the pixels located far from the supply end of the drive voltage is set higher than that of the pixels located close to the supply end of the drive voltage, and the portion where the transparent electrode and the counter electrode cross. 1. An EL display device comprising: drive voltage control means for controlling drive voltages to all pixels of the EL display device to be constant. 3. An EL display device, wherein the transparent electrode comprises a metal layer and a transparent electrode layer.