Organic EL display

An organic EL display of the present invention includes a plurality of organic EL devices, each including a first electrode, an intermediate insulating film disposed on the first electrode so as to have an exposed portion in which the first electrode is partially exposed, an organic layer disposed on the exposed portion, and a second electrode disposed on the organic layer. The organic layer has, on a part disposed on the exposed portion, a thick film portion disposed around the center of the exposed portion and a thin film portion disposed around the end of the exposed portion and having a smaller thickness than the thick film portion, and the thin film portions of the adjacent organic EL devices are substantially equal in width.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2005-314820, filed Oct. 28, 2005, entitled “ORGANIC EL DISPLAY.” The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic EL display (Organic Electroluminescent Display).

2. Description of the Related Art

At present, organic EL displays, plasma displays, liquid crystal displays and so on are mainly known as thin flat displays. Of these displays, organic EL displays use self-luminous devices and have various advantages including low dependence on vision, low power consumption, low cost, and a simple manufacturing process, and thus organic EL displays are expected to become dominant as next-generation displays.

However, striped unevenness recognizable by persons may clearly appear on the display screens of the organic EL displays.

The present invention is made in view of this problem. An object of the present invention is to provide a technique for improving picture quality by reducing the occurrence of striped unevenness on a display.

SUMMARY OF THE INVENTION

An organic EL display of the present invention comprises a plurality of organic EL devices. Each of the organic EL devices includes a first electrode, an intermediate insulating film disposed on the first electrode so as to form an exposed portion in which the first electrode is partially exposed, an organic layer disposed at least on the exposed portion, and a second electrode disposed on the organic layer. The organic layer has, on a part disposed on the exposed portion, a thick film portion and a thin film portion. The thick film portion is disposed around the center of the exposed portion, and the thin film portion is disposed around the end of the exposed portion and has a smaller thickness than the thick film portion. The thin film portions of the adjacent organic EL devices are substantially equal in width.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will be discussed below in accordance with the accompanying drawings.

<Schematic Configuration of an Organic EL Display>

FIG. 1is a schematic sectional view illustrating schematic configuration of an organic EL display21according to the embodiment of the present invention. InFIG. 1and other figures, three axes of X, Y and Z orthogonal to one another or two axes of X and Y orthogonal to each other are illustrated when necessary in order to clarify the directional relationship.

The organic EL display21is, for example, a top emission type display. As shown inFIG. 1, the organic EL display21includes a substrate23made of a glass substrate or the like as a transparent substrate, an element part25formed on the substrate23, and a sealing film29formed so as to cover the entire element part25. The element part25includes a first electrode31, an organic layer33, and a second electrode35in this order from the substrate23. The organic layer33is interposed between the first and second electrodes31and35.

FIG. 2is a schematic plan view illustrating the pixel layout of the organic EL display21.

In the organic EL display21, a number of pixels UC having substantially rectangular shapes are arranged in rows and columns. As shown inFIG. 2, the long sides of the pixels UC lie along the X-axis direction and a number of the pixels UC are spaced at predetermined distances or intervals along the X direction and the Y direction. In the present embodiment, the pixel UC is a rectangle having long sides and short sides.

FIG. 3is a schematic sectional view illustrating the configuration of organic EL devices25aeach forming a pixel UC.FIG. 3shows three organic EL devices25awhich form three adjacent pixels. In order to clarify the shape of the organic layer, the second electrode35and the sealing film29sequentially formed on the organic layer are omitted inFIG. 3.

As shown inFIG. 3, a TFT layer23band a flattening film23care sequentially formed on a substrate23a, and the organic EL devices25aare formed thereon.

The organic EL device25aincludes a lower electrode31acorresponding to the first electrode31, an intermediate insulating film33b, an organic layer33a(hatched portion), and the second electrode35.

The lower electrode31ais a layer having a rectangular surface on the XY plane. The lower electrodes31aare spaced at certain distances on the flattening film23caccording to the pixel layout ofFIG. 2.

The intermediate insulating film33bis formed so as to surround the lower electrode31a. The outer edge portion of the top surface of the lower electrode31ais covered with the intermediate insulating film33b. The other portion of the lower electrode31ais not covered with the intermediate insulating film33band is exposed to the organic layer33a.

The organic layer33ais disposed on a portion Ex not covered with the intermediate insulating film33b(hereinafter, will be also referred to as an “exposed portion”) on the top surface of the lower electrode31a. The area of the exposed portion Ex, when being viewed from the front of the screen (in the Z direction), corresponds to the pixel UC.

The second electrode35is disposed on the organic layer33a.

FIG. 4focuses on the organic EL devices25aofFIG. 3.

As shown inFIG. 4, each organic layer33ahas a flat top surface at the center portion and a thickness of the organic layer33adecreases toward the outer edge portion due to a method of forming the organic layer (will be described later). To be specific, each organic layer33aincludes a portion (hereinafter, will be referred to as a “thick film portion”)33acand a portion (hereinafter, will be referred to as a “thin film portion”)33ae. The thick film portion is disposed around the center of the exposed portion Ex, has a relatively large thickness (in the Z direction), and has a substantially flat top surface. The thin film portion is disposed around the edge of the exposed portion Ex, has a relatively small thickness, and has a top surface tilted toward the outer edge portion of the organic layer33a, immediately above the exposed portion Ex (that is, the area of the exposed portion Ex when being viewed in the Z direction).

In this figure, in the organic layer33adisposed just above the exposed portion Ex, when setting a boundary at a point having a 95% thickness relative to the average thickness of the area of a predetermined range located around the center of the exposed portion Ex, a portion having a thickness larger than 95% is referred to as33acand a portion having a thickness smaller than 95% is referred to as33ae.

<The Method of Forming the Organic Layer and the Occurrence of the Thin Film Portion>

FIG. 5is a diagram for explaining the formation of the organic layer33a. InFIG. 5, the organic layer33aofFIG. 4is omitted and thusFIG. 5shows a state before the organic layer33ais formed.

As shown inFIG. 5, a predetermined organic matter is deposited on the exposed portion Ex, so that the organic layer33aofFIG. 4is formed.

In this vapor deposition process of the organic matter, a metal mask2ofFIG. 6is used. The metal mask2has 3×4 cells. As shown inFIG. 7, each of the cells200has a number of openings201corresponding to the pixels of the organic EL display21which is an end product.

FIG. 8is a diagram schematically illustrating that the organic layer33ais formed by vapor deposition using the metal mask2. In order to avoid complexity,FIG. 8shows only one organic layer33a.

As shown inFIG. 8, by depositing the organic matter from the top surface (Z direction) in a state in which the metal mask2is placed at a predetermined distance above a layer3(formed of the substrate23, the first electrode31, and so on) on which the organic layer33ashould be deposited, so that the organic layer33ais formed. At this point, when there is a distance between the metal mask2and the layer3, the vicinity of the outer edge of the opening201has a smaller amount of vapor deposition of the organic matter as compared with the vicinity of the center of the opening201. As a result, the organic layer33atends to have a smaller thickness toward the outer edge. This tendency becomes apparent particularly when the distance between the metal mask2and the layer3is 3 μm or more.

FIGS. 9A and 9Bschematically show the enhanced shape of the organic layer33a.FIG. 9Ais a plan view of the Z direction.FIG. 9Bis a sectional view taken along line9B-9B ofFIG. 9A.

As shown inFIGS. 9A and 9B, the organic layer33ais flat at the center of the top surface (surface facing in the Z direction) and has a smaller thickness toward the outer edge. In other words, the organic layer33ahas the thick film portion33acwhich has an almost even thickness and has a flat top surface, and the thin film portion33aewhich has a smaller thickness toward the outer edge and has a tilted top surface.

<The Occurrence of Unevenness on the Screen>

FIG. 10focuses on the organic EL devices25aas inFIG. 4.FIG. 11shows the layout of the organic layer33aofFIG. 10and the layout of the thick film portions33acand the thin film portions33aeas viewed from the top.

As shown inFIG. 10, when viewing the organic EL display21from the front (Z direction), in the organic EL device25a, an area to which the exposed portion Ex is projected in the Z direction in the organic layer33acorresponds to a portion (pixel opening) CA which looks bright as the organic layer33aemits light. As shown inFIGS. 10 and 11, each pixel opening CA includes the thick film portion33acand the thin film portion33ae.

In this case, the shortest distance from the thick film portion33acto the intermediate insulating film33bin the Y direction is referred to as a width W of the thin film portion33ae,and a distance D from the center of the pixel opening CA to the boundary between the thick film portion33acand the thin film portion33aein the Y direction is used as a parameter indicating a displacement of the thick film portion33acfrom the pixel opening CA.

In the organic EL display21, it is preferable to evenly form the organic EL devices25aso as not to cause uneven brightness on the display screen.

However, when the layout and alignment of the openings201of the metal mask2are displaced from the design of the layout of the exposed portions Ex, the width W of the thin film portion33aeand the distance D vary among the organic EL devices25a.

In this respect, the present inventors found that striped unevenness recognizable by a person clearly appears on the display screen of the organic EL display21when the width W of the thin film portion33aeand the distance D greatly vary among the organic EL devices25a. Further, the inventors found that, in the organic EL display21, variations in the width W along the short intervals of the pixel layout (Y direction) affect the occurrence of striped unevenness more than variations in the width of the thin film portion33aealong the long intervals of the pixel layout (X direction).

To be specific, when a variation (difference) ΔW in the width W between the adjacent organic EL devices25ais larger than 5% of the average width WAof the exposed portions corresponding to the adjacent organic EL devices25a, that is, when ΔW/WA×100>5 is established, striped unevenness appears. When the variation (difference) ΔW is 5% or less, the striped unevenness becomes invisible. The width of the exposed portion and the width of the thin film portion are defined in the same direction. When the widths W of the thin film portions are compared between the adjacent organic EL devices25ato calculate ΔW, the widths W of the thin film portions on the same side are used (inFIG. 11, the thin film portions on the right sides).

The following will show a specific example of the relationship between variations in the width W and the occurrence of striped unevenness.

SPECIFIC EXAMPLE

FIGS. 12,14and16show variations in the width W of the thin film portion33ae.The horizontal axis represents a position in a direction along which the organic EL devices25aare placed at short intervals (for example, the Y direction inFIG. 11) and the vertical axis represents a displacement from a reference value (a predetermined design value). The interval between the organic EL devices is represented as a distance between the centers of the adjacent organic EL devices.

FIGS. 13,15and17show the states of the display screen when the width W varies as shown inFIGS. 12,14and16.FIG. 13Ashows an image obtained by directly photographing the display screen.FIG. 13Bis a conceptual rendering of the display screen which clearly shows the areas of striped unevenness inFIG. 13Awith lines.FIG. 15is a schematic drawing illustrating the areas of striped unevenness appearing on the display screen.FIG. 17shows an image obtained by directly photographing the display screen.

As shown inFIG. 12, the presence of portions DW1to DW6(six in total) where ΔW/WAis larger than 0.05 between adjacent organic EL devices25acauses, as shown inFIG. 13, striped unevenness on the display screen according to variations in the width W.

As shown inFIG. 14, when the number of portions where ΔW/WAis larger than 0.05 between adjacent organic EL devices25ais larger than that ofFIG. 12, a number of uneven stripes appear as shown inFIG. 15on the display screen so as to correspond to the number of portions where the width W greatly varies.

As shown inFIG. 16, when ΔW/WAis 0.05 or less, and particularly when the maximum value is not more than 0.02 between adjacent organic EL devices25a, no striped unevenness appears on the display screen as shown inFIG. 17.

<The Reduction of Unevenness on the Screen>

As described in the specific example above, in the presence of portions where ΔW/WAis larger than 0.05 between adjacent organic EL devices25a, striped unevenness corresponding to the portions appears on the display screen. Thus, the occurrence of striped unevenness on the display screen can be reduced by adjusting ΔW/WAto 0.05 or less between adjacent organic EL devices25a.

Further, it is possible to adjust ΔW/WAbetween adjacent organic EL devices25ato about 0.05 or less by adjusting the widths W of the organic EL devices25ato be substantially equal.

As above, reduction of variations in the widths W between the organic EL devices25ais attainable by elaborating so as to improve various accuracies in the manufacturing process, for example, in the steps of preparing metal mask2and vapor deposition of the organic layer33.

The following will describe ingenuity in the manufacturing of the metal mask2and the vapor deposition of the organic layer33.

<The Method of Manufacturing the Mask>

The metal mask2is fabricated by the steps of (1) fabricating a master photomask, (2) forming a resist pattern, (3) forming a metal mask on a base material, (4) fixing the metal mask on a frame, and (5) removing the base material.

The steps (1) to (5) will be specifically described below.

A metal (Cr) having been deposited over a glass substrate is coated with a photoresist, and the photoresist is exposed by laser lithography. And then, a portion having been radiated in the photoresist with a laser is removed by development. After that, the exposed Cr is dissolved by dipping the glass substrate into a predetermined etching solution, and then the remaining photoresist is removed, so that the master photomask, in which radiated light passes through the portion having been removed by dissolving Cr, is completed.

In laser lithography used for fabricating the master photomask, adjustments such as the presence or absence of radiated light can be more accurately realized in the scanning direction. Thus in this case, laser lithography was performed by scanning in a direction along which pixels are arranged at relatively short intervals. This is because variations in accuracy of position affect the occurrence of striped unevenness more greatly in a direction along which pixels are arranged at relatively short intervals (Y direction) than in a direction along which pixels are arranged at relatively long intervals (X direction) in the organic EL display21.

And then, a photoresist film is applied on a base material such as stainless steel and development is performed thereon after light exposure using the master photomask, so that the resist pattern is formed as a base of the metal mask. After that, a nickel alloy or the like is electroformed on the base material where the resist pattern has been formed, so that a metal mask is formed on the base material. And then, the mask frame is fixed on the metal mask and the base material is removed, so that the metal mask2can be fabricated.

As described above, the metal mask2formed on the base material is fixed on the metal frame without being exfoliated from the base material, and then the base material is removed. This method can improve the accuracy of position of the openings201on the metal mask2.

In this method, main tension applied to the fabricated metal mask2is limited to membrane stress which is applied when the metal mask2is fabricated on the base material. Thus unlike a conventional method in which a metal mask is exfoliated from a base material and then is bonded and fixed to a metal frame while the ends of the metal mask are pulled to apply tension to the metal mask, the positions of the openings201can be evenly controlled with high accuracy.

Although the accuracy of position can be improved as above for the opening201of the metal mask2, when the organic film is deposited, it is necessary to accurately align the position of the metal mask2relative to the substrate in order to improve the accuracy of vapor deposition of the organic film.

As shown inFIG. 2, a number of pixels are two-dimensionally arranged in two directions (X direction and Y direction) substantially perpendicular to each other in the organic EL display21. Focusing on this point, when aligning the position of the metal mask2relative to the substrate, the accuracy of positioning is increased in a direction (Y direction) along which pixels are arranged at relatively short intervals as compared with a direction (X direction) along which pixels are arranged at relatively long intervals (that is, displacement from the design position is reduced), so that the position of the metal mask2can be accurately aligned relative to the substrate.

The following will discuss a reason why such alignment is performed.

When it is assumed that the position of the metal mask2is aligned with high accuracy in all the directions including not only the Y direction but also the X direction, a long time and a number of positioning operations are necessary to adjust the positional relationship between the metal mask2and the substrate within a set range of accuracy. As a result, there is an increased risk that retention in the manufacturing process may deteriorate the manufacturing tact and the metal mask2brought into contact with the substrate in a number of positioning operations may scratch the substrate.

Therefore, in consideration of the balance among the display quality of the screen, the yield, the productivity, and the like, it is preferable in the alignment of the metal mask2relative to the substrate to increase the accuracy of alignment in a direction (Y direction) along which pixels are arranged at relatively short intervals as compared with a direction (X direction) along which pixels are arranged at relatively long intervals.

Further, the widths W of the thin film portions33aeare adjusted so as to be substantially equal between the organic EL devices25a. The width W is a distance from the thick film portion33acto the intermediate insulating film33balong a predetermined direction (Y direction in the above description) in the organic EL device25a. As a result, it is possible to prevent striped unevenness from appearing on the display.

The above explanation described the embodiment of the present invention. The present invention, however, is not limited to the above description.

For example, as described above, by keeping ΔW/WAat 0.05 or less between adjacent organic EL devices25a, it is possible to reduce the occurrence of striped unevenness on the display screen, whereas as shown inFIGS. 16 and 17, by keeping ΔW/WAat 0.02 or less between adjacent organic EL devices25a, it is possible to more positively reduce the occurrence of striped unevenness on the display screen.

In the present embodiment, for all the organic EL devices25aincluded in the organic EL display21, that is, for an area including all the organic EL devices25a(display area), variations in the width W were adjusted between adjacent organic EL devices25a. The present invention is not limited to such an adjustment. For example, in order to prevent the occurrence of striped unevenness in a partial area (for example, a square area of 0.2 mm×0.2 mm) of a predetermined size included in the display screen of the organic EL display21, variations in ΔW/WAmay be adjusted between adjacent organic EL devices25ain the partial area.

In the present embodiment, the width W is adjusted in view of variations in the width W of the thin film portion. The distance D from the center of the pixel opening CA to the boundary between the thick film portion33acand the thin film portion33aein a predetermined direction (for example, Y direction) may be adjusted. To be specific, the distances D may be evenly set between the organic EL devices in a predetermine direction, or ΔD/WAobtained by dividing a variation ΔD of the distance D by the average width WAof the exposed portions may be set at 0.05 or less between adjacent organic EL devices.