Display device having a light emitting layer on the auxiliary layer

A display device, in which self-luminous elements are arranged, prevents a leakage current through a common layer, provided for the self-luminous elements and disposed throughout its image display area, from causing adjacent pixels to emit unintended light. A light-emitting element layer 102 includes a lower layer 102d and a light-emitting layer. The lower layer 102d has carrier mobility and includes a carrier transport layer or a carrier injection layer. The lower layer 102d is stacked on lower electrodes 100 and banks 106. The light-emitting layer is stacked on the lower layer 102d. An upper electrode 62 is disposed on the light-emitting element layer 102 and supplies carriers to the light-emitting element layer 102 together with each lower electrode. A lower layer 102d has a dividing area 112 on the bank. The dividing area 112 prevents carriers from traveling between adjacent pixels through the lower layer 102d.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2014-240887 filed on Nov. 28, 2014, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device using self-luminous elements that emit light by application of a voltage.

2. Description of the Related Art

Display devices using self-luminous elements such as organic light-emitting electroluminescent (EL) elements have been developed. The organic electroluminescent element is a type of light-emitting diode usually called an organic light-emitting diode (OLED). The OLED emits light when carriers (electrons or holes) are injected into its light-emitting layer made of organic compounds. The OLED typically has a structure including an auxiliary layer with carrier mobility, for example, between an electrode and the light-emitting layer so that carriers are efficiently injected into the light-emitting layer during application of a voltage.

For example, a hole transport layer (HTL) and a hole injection layer (HIL), each as an auxiliary layer, are disposed between the anode and the light-emitting layer. In addition, an electron transport layer (ETL) and an electron injection layer (EIL) are disposed between the cathode and the light-emitting layer. These auxiliary layers are formed in common throughout an image display area in which a plurality of pixels are arranged, for example, by chemical vapor deposition (CVD), sputtering, or vacuum evaporation.

SUMMARY OF THE INVENTION

A display device using light-emitting elements in which an auxiliary layer with carrier mobility, such as the HTL and the HIL in the OLED described above, is formed as a common layer lying continuously throughout its image display area may cause the leakage of carriers through the common layer between adjacent pixels. The leakage current that flows to adjacent pixels causes the adjacent pixels to emit unintended light. Specifically, the leakage current causes deterioration in resolution of images. Moreover, the leakage current that flows between different pixels of different luminescent colors causes reduction in color reproducibility (color purity). In particular, as the openings (or the light-emitting areas) of adjacent pixels get closer to each other with increasing definition due to smaller pixel size, these problems become more pronounced.

The present invention provides a display device that prevents a leakage current from flowing to self-luminous elements in adjacent pixels and causing the adjacent pixels to emit unintended light.

A display device according to an aspect of the present invention includes a plurality of pixels, pixel electrodes each provided in each of the pixels, a bank that is positioned in a border between the pixels and exposes part of each of the pixel electrodes, an auxiliary layer that includes at least one of a carrier transport layer and a carrier injection layer and is stacked on the pixel electrodes and the bank, a light-emitting layer stacked on the auxiliary layer, and a counter electrode that is positioned on the light-emitting layer and spreads over the pixels. In the display device, the auxiliary layer may have a dividing area on the bank, and the dividing area may have lower carrier mobility than the other area has, which is different from the dividing area, in the auxiliary layer. The auxiliary layer in a display device according to another aspect of the present invention may be divided on the top of the bank so as to correspond to the each of the plurality of pixels.

DETAILED DESCRIPTION OF THE INVENTION

The disclosure herein is merely an example, and appropriate modifications coming within the spirit of the present invention, which are easily conceived by those skilled in the art, are intended to be included within the scope of the invention as a matter of course. The accompanying drawings schematically illustrate widths, thicknesses, shapes, or other characteristics of each part for clarity of illustration, compared to actual configurations. However, such schematic illustrations are merely examples and are not intended to limit the present invention. In the present specification and drawings, some elements identical or similar to those shown previously are denoted by the same reference signs as the previously shown elements, and thus are not described in detail herein as appropriate.

A display device in each embodiment described below is an organic EL display device. The organic EL display device is an active matrix display device and is built in televisions, personal computers, handheld devices, mobile phones, and other devices.

A plurality of pixels for producing images are arranged two-dimensionally in the image display area of the display device. Here, the direction along one coordinate axis of a two-dimensional Cartesian coordinate system corresponding to an image is defined as the row direction, and the direction along the other coordinate axis of the coordinate system is defined as the column direction. In the following description, merely for convenience, the row direction and the column direction are basically defined as the horizontal direction and the vertical direction of the image, respectively. For example, for such a display device as can switch the orientation of an image displayed in the same image display area between portrait and landscape modes, the row direction and the column direction of the image display area may be the vertical direction and the horizontal direction of the image, respectively. The structure itself of the display device can be such that its physical orientation is changed between row and column directions relative to structures described below.

Each of the following embodiments describes a display device that can display color images. The display device has a plurality of types of sub-pixels of mutually different luminescent colors arranged in its image display area. Each pixel in the color images corresponds to a sub-pixel group constituted by a plurality of types of sub-pixels in the display device, whereas a single sub-pixel is the structural unit in the display device. For example, an OLED and a pixel circuit are formed for each sub-pixel. For this reason, in the following description, each sub-pixel is basically referred to as a pixel.

First Embodiment

FIG. 1is a schematic diagram showing a configuration of an organic EL display device2according to an embodiment. The organic EL display device2includes a pixel array unit4that displays images and a driver that drives the pixel array unit4. The organic EL display device2includes a substrate and a stack structure on the substrate. The substrate is made of, for example, a glass substrate or a flexible resin film. The stack structure includes thin film transistors (TFTs) and OLEDs.

In the pixel array unit4, OLEDs6and pixel circuits8, each corresponding to a pixel, are arranged in a matrix. Each pixel circuit8includes TFTs10and12and a capacitor14.

The driver includes a scan line driver circuit20, a data line driver circuit22, a driver power supply circuit24, a reference power supply circuit26, and a controller28. For example, the driver is responsible for driving the pixel circuits8to control the light emission of the OLEDs6.

The scan line driver circuit20is connected to scan lines30, each provided for the corresponding horizontal pixel alignment (pixel row). The scan line driver circuit20sequentially selects the scan lines30in response to timing signals input from the controller28, and applies, to each selected scan line, a voltage enough to turn on the corresponding lighting TFT10.

The data line driver circuit22is connected to data lines32, each provided for the corresponding vertical pixel alignment (pixel column). The data line driver circuit22receives image signals from the controller28. In synchronization with the selection of the scan line30by the scan line driver circuit20, the data line driver circuit22outputs voltages, which correspond to an image signal for the selected pixel row, to the data lines32. In the selected pixel row, each of the voltages is written into the corresponding capacitor14via the lighting TFT10. Each driver TFT12supplies a current, which corresponds to the written voltage, to the corresponding OLED6. Thus, the OLEDs6in the pixels corresponding to the selected scan line30emit light.

The driver power supply circuit24is connected to drive power lines34, each provided for the corresponding pixel column, and supplies a current to the OLEDs6via the drive power lines34and the driver TFTs12in the selected pixel row.

The reference power supply circuit26applies a constant potential φREFto a common electrode (not shown) constituting the cathode electrodes of the OLEDs6The potential φREFcan be set to, for example, ground potential GND (0 V).

In this embodiment, the lower electrode of the OLED6is a pixel electrode formed for each pixel, and the upper electrode of the OLED6is a counter electrode disposed to face the pixel electrode. The lower electrode is connected to the driver TFT12. In contrast, the upper electrode is constituted by the electrode common to the OLEDs6of all the pixels. In this embodiment, the lower electrode is the anode of the OLED6and the upper electrode is the cathode.

The display panel40in this embodiment displays color images. Each pixel in the color images is constituted by, for example, three types of pixels (sub-pixels) that emit light corresponding to red (R), green (G), and blue (B).

This embodiment describes an example where R pixels52r, G pixels52g, and B pixels52bare arranged in a stripe matrix in the display area42. In this arrangement, the pixels of the same type (color) are arranged in the vertical direction of the images, and the R, G, and B pixels are arranged cyclically in the horizontal direction. InFIG. 2, each of the R pixels52r, the G pixels52g, and the B pixels52bschematically represents an effective light-emitting area and structurally corresponds to a pixel opening60. The area between these pixels corresponds to a bank.

For example, the display panel40has a structure including a TFT substrate and a counter substrate bonded to each other with a filler sandwiched between these substrates. The TFT substrate has the OLEDs6and a circuit including, for example, TFTs72formed on it. The counter substrate can be provided with a polarizing plate and a touch screen. When the display panel40produces color images by using a color filter, for example, the counter substrate has the color filter formed on it, and white light generated by the OLEDs6passes through the color filter to provide each color of RGB.

FIG. 3is a schematic vertical cross-sectional view of the display panel40taken along line III-III shown inFIG. 2.FIG. 3shows the cross-sectional structure of the above TFT substrate, whereas a filler layer formed on this and the structure of the counter substrate are not shown. The pixel array unit4in this embodiment is a top-emitting unit and emits light, which is generated by the OLEDs6formed on the TFT substrate, through the counter substrate. That is, the light from the OLEDs6are emitted upward inFIG. 3.

The structure of the TFT substrate is formed by stacking and patterning various types of layers on a substrate70made of glass or a resin film.

Specifically, a polysilicon (p-Si) film is formed on the substrate70via an underlayer80made of an inorganic insulating material, such as silicon nitride (SiNy) or silicon oxide (SiOx), and then the p-Si film is patterned so that p-Si films to be regions used in a circuit layer are selectively left. For example, each p-Si film forms a semiconductor region82to be the channel, the source, and the drain of the top-gate TFT72. A gate electrode86is disposed on the channel of the TFT72via a gate insulating film84. The gate electrode86is formed by patterning a metal film formed, for example, by sputtering. Then, an interlayer insulating film88covering the gate electrode86is stacked. The p-Si to be the source and the drain of the TFT72is doped with a dopant by ion implantation. Then, a source electrode90aand a drain electrode90b, each electrically connected to the p-Si, are formed. After the TFT72is thus formed, an interlayer insulating film92is stacked. On the surface of the interlayer insulating film92, interconnections94and other interconnections can be formed by patterning a metal film formed, for example, by sputtering. This metal film can constitute, for example, a multilayer interconnection structure including the scan lines30, the data lines32, and the drive power lines34shown inFIG. 1, together with the metal film used to form the gate electrodes86, the source electrodes90a, and the drain electrodes90b. On this structure, for example, a planarization film96is formed by stacking an organic material, such as an acrylic resin. On the surface of the display area42thus planarized, the OLEDs6are formed.

Each OLED6is constituted by a lower electrode100, a light-emitting element layer102, and the upper electrode62, which are stacked in this order from the substrate70.

Assuming that the TFT72shown inFIG. 3is the driver TFT12having an n channel, the lower electrode100is connected to the source electrode90aof the TFT72. Specifically, after the above planarization film96is formed, contact holes104, each for coupling the lower electrode100to the corresponding TFT72, are formed. Then, the lower electrode100connected to the TFT72is formed separately for each pixel by patterning a conductive film formed on the surface of the planarization film96and in the contact holes104.

For example, the lower electrode100is formed of ITO or IZO. The organic EL display device2in this embodiment is a top-emitting device, so that the lower electrode100can have a structure in which a transparent conductive film is stacked on a reflective layer formed of a highly light-reflective material. For example, the reflective layer can be formed of aluminum (Al) or silver (Ag), and reflects light form the light-emitting layer toward the display surface, that is, toward the upper electrode62.

As described above, in each pixel, the driver TFT12controls a current flowing through the OLED6in response to an image signal for the pixel, and the lower electrode100supplies as many carriers as correspond to the image signal for the pixel to the light-emitting element layer102. Specifically, in this embodiment, the lower electrode100is the anode, and holes as carriers are supplied from the lower electrode100to the light-emitting element layer102.

FIGS. 4A to 4Eare each a partial cross-sectional views of the TFT substrate in a main step of forming the OLEDs6. These figures show a schematic process flow of a manufacturing process of the display panel40after formation of the lower electrodes100. The following describes how the OLEDs6are formed with reference toFIGS. 4A to 4E.

After formation of the lower electrodes100, a bank106is formed (FIG. 4A). The bank106is formed in the pixel border, for example, by patterning a photosensitive resin, such as acryl or polyimide, using photolithography or ink-jet printing so as to electrically separate the lower electrodes100from each other. The bank106can be formed of an inorganic insulating material, such as SiNyor SiOx. The areas enclosed by the bank106correspond to the pixel openings60shown inFIG. 2, and the lower electrodes100are exposed through the areas.

After the bank106is formed, layers constituting the light-emitting element layer102are sequentially stacked on the lower electrodes100. The light-emitting element layer102includes a light-emitting layer (EML) and an auxiliary layer. The light-emitting layer emits light in response to the injection of carriers. The auxiliary layer is responsible for efficiently supplying the carriers to the light-emitting layer. The light-emitting element layer102includes at least one of a HIL and a HTL as the auxiliary layer.

For example, each OLED6is an OLED that emits single-color light corresponding to the luminescent color of one of the R, G, and B pixels, and has a structure in which the HIL, the HTL, the EML, and an ETL are stacked in this order from the lower electrode100. In this structure, the HIL, the HTL, and the ETL are each the auxiliary layer. Among these, the HIL and the HTL are formed between the EML and the anode (lower electrode100), which supplies holes.FIG. 3distinguishably shows a lower layer102dincluding the HIL and the HTL, and an upper layer102uincluding the EML and the ETL, which constitute the light-emitting element layer102.

After formation of the bank106, the lower layer102dis formed first. For example, the HTL and HIL constituting the lower layer102dis formed of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) or other conductive organic materials. Throughout the surface of the display area42in which the bank106is formed, a HTL/HIL layer110is deposited, for example, by sputtering or CVD (FIG. 4B).

Subsequently, a dividing area112that divides the HTL/HIL layer110is formed on the bank106(FIG. 4C), and the HTL/HIL layer110in the remaining area becomes the lower layer102dof the light-emitting element layer102(FIGS. 4D and 4E). The dividing area112is responsible for preventing carries from traveling between adjacent pixels through the HTL/HIL layer110. As shown in the figures, the bank106has more than enough width to have the dividing area112on it, and thus the edge of the lower layer102dcan overlap with the surface of the bank106.

In this embodiment, the dividing area112is the area where no HTL/HIL layer110is formed. For example, the dividing area112is formed by removing part of the HTL/HIL layer110by patterning with photolithography. Specifically, a photoresist film is formed on the surface of the HTL/HIL layer110, and a mask116having an opening114in the area to be the dividing area112is formed by using the photoresist film. Then, by using the mask116, HTL/HIL layer110is etched away from the opening114(FIG. 4C).

As the lower layer102d, the HTL/HIL layer110can be formed to have a pattern with the dividing area112from the beginning, for example, by a printing method.

Subsequently, on the lower layer102d, the EML and the ETL are formed as the upper layer102uthroughout the display area42(FIG. 4D). For example, the upper layer102uis formed by vapor deposition. For the OLEDs each of which emits single-color light, the EML is formed of different organic light-emitting materials for different luminescent colors. In this case, the EML can be formed, for example, by ink-jet printing.

On the light-emitting element layer102including the lower layer102dand the upper layer102u, the upper electrode62is deposited, for example, by sputtering (FIG. 4E). The upper electrode62is basically formed in common throughout the display area42.

In this way, the OLEDs6are formed. On the surface of the upper electrode62, a sealing film108is formed, as shown inFIG. 3. The sealing film108prevents moisture from entering and is responsible for protecting the OLEDs6. As the sealing film108, for example, a SiNyfilm is deposited by CVD.

The display panel40may have a structure in which the TFT substrate and the counter substrate are not bonded to each other. In this case, a protective film can be formed over or under the sealing film108, or both over and under the sealing film108to increase the mechanical strength of the surface of the TFT substrate. When a protective film is formed under the sealing film108, the protective film may be formed to effectively compensate for the irregularities due to the bank106and thus to reduce the surface irregularities of the sealing film108. This can reduce the internal stress of the sealing film108and make the sealing film108less likely to peel off.

As described above, when the organic EL display device2is driven, the OLED6of each pixel, to which carriers corresponding to an image signal are supplied, emits light. InFIG. 5, arrows schematically indicate the flow of holes that are carriers supplied from the lower electrode100to the light-emitting element layer102when the organic EL display device2is driven. A potential lower than that of the lower electrode100is applied to the upper electrode62. The holes supplied from the lower electrode100of each pixel to the lower layer102dof the light-emitting element layer102are drawn to the upper layer102uof the light-emitting element layer102and are injected into the EML of the pixel, as indicated by arrows120, basically by an electric field between the lower electrode100and the upper electrode62disposed to face the lower electrode100.

On the other hand, some of the holes supplied from the lower electrode100to the lower layer102dcan travel toward the outside of the pixel opening60through the lower layer102das indicated by arrows122. If the travel of the carriers becomes a leakage current that flows to the adjacent pixels, the above-mentioned problem arises. In this regard, in the organic EL display device2, the lower layer102dof the light-emitting element layer102, which has the dividing area112formed in the pixel border, can prevent the leakage current from flowing to the adjacent pixels and causing the adjacent pixels to emit light. Thus, deterioration in resolution of images is prevented. Disposing the dividing area112in the borders between pixels of different luminescent colors can prevent color crosstalk due to the leakage current, provide high color purity, and thus achieve desirable color reproducibility.

In many cases, the layers constituting the lower layer102d, among the auxiliary layers, are formed relatively thicker than the other auxiliary layers. Accordingly, a larger amount of carriers travel through the lower layer102dto be the leakage current. In this regard, in this embodiment, providing the dividing area112for the lower layer102dcan well prevent the leakage current between pixels.

Alternatively, the OLEDs6may be white light-emitting OLEDs. For example, the OLED6that emits white light can have a tandem structure in which a plurality of OLEDs of different luminescent colors are electrically and serially connected via light-transmissive intermediate layers. In the tandem structure, a charge generation layer (CGL) disposed as an intermediate layer is also an auxiliary layer with carrier mobility, like the HIL and the HTL. The present invention can also be applied to the organic EL display device2having the tandem-structured OLEDs6. Also in this case, like the above OLEDs6each of which emits single-color light, the dividing area112, which is provided for the HTL/HIL layer110stacked on the lower electrodes100, can well prevent the leakage current between pixels.

The dividing area112can be disposed along the entire circumference of the pixel border enclosing each pixel. That is, the HTL/HIL layer110can be divided into units of pixels by the dividing area112.

As described above, disposing the dividing area112along the borders between adjacent pixels of mutually different luminescent colors can achieve desirable color reproducibility. For example, in this embodiment, the R, G, and B pixels are arranged in a stripe matrix, and a pair of pixels adjacent to each other in the row direction emit light mutually different in color. In this case, disposing the dividing area112along the pixel borders extending in the column direction can prevent color crosstalk between adjacent pixels.

The dividing area112may be formed not along all of the borders between adjacent pixels of mutually different luminescent colors but along only some portions of them.

In manufacture of the display panels40, a method for forming a plurality of display panels40at once on a large piece of substrate70is adopted to increase the manufacturing efficiency. In this manufacturing method, during a process for forming TFTs on the substrate, the plurality of display panels40are processed together. On the other hand, during an OLED formation process, the plurality of display panels40are processed separately. That is, this manufacturing method is divided into the former and latter processes. In the former process, the plurality of display panels40are processed with all connected. In the latter process, the plurality of display panels40are divided into individual panels and the remaining processing is then applied to them. Here, the former process is referred to as the TFT process, and the latter process is referred to as the OLED process.

The TFT process basically includes steps that can be performed by using semiconductor manufacturing processes for manufacturing, for example, integrated circuits. The TFT process has a relatively high degree of flexibility in process conditions. For instance, a manufacturing process can be designed to use a high-temperature process at several hundred degrees Celsius. On the other hand, in the OLED process, the ambient temperature can be limited to several tens of degrees Celsius to prevent deterioration of the light-emitting element layer made of an organic material. That is, in the OLED process, processing the plurality of display panels40individually increases the number of steps, and the process conditions need to be controlled more accurately than the TFT process. Thus, by increasing the proportion of the TFT process in the process for manufacturing the display panels40and by reducing the proportion of the OLED process, the display panels40can be manufactured at lower costs and in shorter time periods.

In this embodiment, the TFT process includes a step of forming the structure shown inFIG. 4A. That is, a circuit including the TFTs72is formed on the substrate70. Then, on the circuit, the planarization film96is stacked, and the lower electrodes100and the bank106are formed.

In this embodiment, the TFT process further includes a step of forming the lower layer102dof the light-emitting element layer102, and thus can effectively reduce the above manufacturing costs and shorten the above manufacturing time periods. That is, after the bank106is formed in the border area, further in the TFT process, the HTL/HIL layer110is formed throughout the display area42(FIG. 4B), the mask116is formed on the HTL/HIL layer110, and the dividing area112is formed by using this mask116(FIG. 4C).

The OLED process includes a step of forming the upper layer102uof the light-emitting element layer102(FIG. 4D), a step of forming the upper electrode62(FIG. 4E), and subsequent steps.

Second Embodiment

The following describes an organic EL display device2baccording to a second embodiment of the present invention. This embodiment differs from the above first embodiment in the structure and the formation of the dividing area112, whereas the other respects are essentially the same between these embodiments. The following describes the second embodiment mainly on differences from the first embodiment.

A schematic plan view of the display panel40of the organic EL display device2bis essentially the same as that shown inFIG. 2for the first embodiment. Thus, the second embodiment also refers toFIG. 2.FIG. 5is a schematic vertical cross-sectional view of the display panel40in this embodiment taken along line III-III shown inFIG. 2. The dividing area112in the first embodiment is the area where the light-emitting element layer102has no lower layer102d, which is an auxiliary layer with carrier mobility such as the HTL/HIL layer110. In contrast, the dividing area112in this embodiment is the area where the material of the lower layer102dis deteriorated or modified so as to lose carrier mobility. InFIG. 5, the dividing area112on the bank106has an altered layer200being the lower layer102dthat has lost carrier mobility.

For example, as in the first embodiment, the lower layer102dand the mask116having the opening114in the area to be the dividing area112are formed (FIGS. 4B and 4C). In this embodiment, for example, ion implantation or energy ray irradiation, by using the mask116, causes a chemical change in or molecular structure damage to the lower layer102d(HTL/HIL layer110), and thus causes the lower layer102dto lose conductivity and become the altered layer200.

After formation of the altered layer200, the organic EL display device2bin this embodiment is completed through essentially the same steps as those for the organic EL display device2in the first embodiment. For example, after formation of the altered layer200, the light-emitting element layer102is formed by stacking the upper layer102u(FIG. 4D), and then the OLEDs6are formed by forming the upper electrode62on the light-emitting element layer102(FIG. 4E).

The above embodiments describe how the dividing area112prevents the leakage current in the organic EL display device2(2b) in which the R, G, and B pixels are arranged in a stripe matrix, whereas such a structure for preventing the leakage current can be applied to other pixel arrangements.

The above embodiments and modifications describe cases where the lower electrode100is the anode of the OLED6and the upper electrode62is the cathode of the OLED6. However, the present invention can also be applied to a case where the lower electrode100is the cathode of the OLED6and the upper electrode62is the anode of the OLED6. In that case, the layers in the light-emitting element layer102are stacked in the reverse order of the above structure. For example, the light-emitting element layer102has a structure in which the EIL, the ETL, the EML, the HTL, and the HIL are stacked in this order from the lower electrode100used as the cathode.

Those skilled in the art can appropriately modify the design of the organic EL display devices2and2bdescribed above as embodiments of the present invention and implement other organic EL display devices, and all such organic EL display devices also fall within the scope of the invention as long as they come within the spirit of the invention. Display devices other than the organic EL display devices, for example, quantum dot display devices that employs quantum dot elements as their light-emitting layer, also fall within the scope of the invention.

Various other variations and modifications can be conceived by those skilled in the art within the spirit of the present invention, and it will be understood that all such variations and modifications also fall within the scope of the invention. For example, those skilled in the art can appropriately modify the above embodiments by addition, deletion, or design change of components, or by addition, omission, or condition change of steps, and all such modifications also fall within the scope of the invention as long as they come within the spirit of the invention.

It will also be understood that other effects produced by an aspect of the embodiment, which are apparent from the description herein or can be appropriately conceived by those skilled in the art, are produced by the present invention as a matter of course.