Patent Publication Number: US-2022231263-A1

Title: Display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-005019, filed Jan. 15, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a display device. 
     BACKGROUND 
     In recent years, display devices in which organic light-emitting diodes (OLEDs) are applied as display elements have been used in practical applications. The display element comprises an organic layer between a pixel electrode and a common electrode. 
     In the field of the top-emission display devices, the application of a microcavity structure thereto is known, which uses the resonance effect of light between a reflective electrode as a pixel electrode and a semi-transmissive electrode as a common electrode. The microcavity structure is a structure configured such that, in display elements emitting red, green and blue colors, respectively, the length of the optical path between the pixel electrode and the common electrode matches the peak wavelength of the spectrum of the emitted light. With this structure, only the light of the wavelength that matches the length of the optical path resonates, thereby improving the luminance and color purity. In such a microcavity structure, it is important to weaken the light of wavelengths deviating from the length of the optical path in order to obtain the desired chromaticity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration example of a display device DSP according to one embodiment. 
         FIG. 2  is a diagram showing an example of a configuration of a display element  20 . 
         FIG. 3  is a plan view showing an example of a pixel PX shown in  FIG. 1 . 
         FIG. 4  is a cross-sectional view of the display element  20 , take along line A-B in  FIG. 3 . 
         FIG. 5  is a cross-sectional view of a comparative example of the display element  20 . 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a display device comprises an insulating substrate, a first insulating layer disposed above the insulating substrate, a lower electrode disposed on the first insulating layer, a second insulating layer disposed on the first insulating layer and comprising an opening overlapping the lower electrode, an organic layer including a light-emitting layer, disposed in the opening and covering the lower electrode, an upper electrode comprising a first end surface, which is an inclined surface, directly above the second insulating layer and stacked on the organic layer and an optical adjustment layer comprising a second end surface on an inner side with respect to the first end surface and in contact with the upper electrode. 
     Embodiments will be described hereinafter with reference to the accompanying drawings. 
     The disclosure is merely an example, and proper changes within the spirit of the invention, which are easily conceivable by a skilled person, are included in the scope of the invention as a matter of course. In addition, in some cases, in order to make the description clearer, the widths, thicknesses, shapes, etc., of the respective parts are schematically illustrated in the drawings, compared to the actual modes. However, the schematic illustration is merely an example, and adds no restrictions to the interpretation of the invention. Besides, in the specification and drawings, the same or similar elements as or to those described in connection with preceding drawings or those exhibiting similar functions are denoted by like reference numerals, and a detailed description thereof is omitted unless otherwise necessary. 
     Note that, in order to make the descriptions more easily understandable, some of the drawings illustrate an X axis, a Y axis and a Z axis orthogonal to each other. A direction along the X axis is referred to as an X direction or a first direction, a direction along the Y axis is referred to as a Y direction or a second direction and direction along the Z axis is referred to as a Z direction or a third direction. A plane defined by the X axis and the Y axis is referred to as an X-Y plane. Further, viewing towards the X-Y plane is referred to as planar view. A direction on the observer side along the third direction is referred to as an upper side, and a surface on the observer side along the third direction is referred to as an upper surface. A direction opposite to the observer along the third direction is referred to as a lower side or a bottom side, and a surface opposite to the observer along the third direction is referred to as a lower surface or a bottom surface. 
     The display device DSP of this embodiment is an organic electroluminescent display device comprising an organic light-emitting diode (OLED) as a display element, and can be mounted on televisions, personal computers, mobile terminals, cell phones and the like. Note that display element described below can be applied as a light-emitting element of an illumination device, and the display device DSP can be converted to some other electronic device such an illumination device. 
       FIG. 1  is a diagram showing an example configuration of a display device DSP of this embodiment. The display device DSP comprises a display area DA which displays images, on an insulating base  10 . The base  10  is an insulating substrate and may be glass or a flexible resin film. 
     The display area DA comprises a plurality of pixels PX arranged in a matrix along the first direction X and the second direction Y in the display area DA. The pixels PX each comprises a plurality of subpixels SP 1 , SP 2  and SP 3 . For example, each pixel PX comprises a red subpixel SP 1 , a green subpixel SP 2  and a blue subpixel SP 3 . In place of the three subpixels of the three colors, the pixel PX may contain four or more subpixels of other colors, including white. 
     A configuration example of one subpixel SP contained in a pixel PX will be briefly described. 
     That is, the subpixel SP comprises a pixel circuit  1  and a display element  20  that is driven and controlled by the pixel circuit  1 . The pixel circuit  1  comprises a pixel switch  2 , a drive transistor  3  and a capacitor  4 . The pixel switch  2  and the drive transistor  3  are switch elements constituted by thin-film transistors, for example. 
     In the pixel switch  2 , a gate electrode thereof is connected to a scanning line GL, a source electrode is connected to a signal line SL and a drain electrode is connected to one of the electrodes constituting the capacitor  4  and the gate electrode of the drive transistor  3 . In the drive transistor  3 , a source electrode thereof is connected to the other electrode of the capacitor  4  and a power line PL, and a drain electrode is connected to an anode of the display element  20 . A cathode of the display element  20  is connected to a power feed line FL. Note that the configuration of the pixel circuit  1  is not limited to that of the example shown in the figure. 
     The display element  20  is an organic light-emitting diode (OLED), which is a light-emitting element. For example, a subpixel SP 1  comprises a display element that emits light corresponding to the red wavelength, a subpixel SP 2  comprises a display element that emits light corresponding to the green wavelength, and a subpixel SP 3  comprises a display element that emits light corresponding to the blue wavelength. The pixel PX comprises multiple subpixels SP 1 , SP 2  and SP 3  of display colors different from each other, and with this configuration, multi-color display can be realized. 
     Note that the display element  20  may be configured so that the subpixels SP 1 , SP 2  and SP 3  emit light of the same color. Thus, monochromatic display can be realized. 
     Here, when the display elements  20  of the subpixels SP 1 , SP 2  and SP 3  are configured to emit white light, a color filter may be disposed to oppose the display elements  20 . For example, the subpixel SP 1  comprises a red color filter opposing the respective display element  20 , the subpixel SP 2  comprises a green color filter opposing the respective display element  20 , and the subpixel SP 3  comprises a blue color filter opposing the respective display element  20 . With this structure, it is possible to realize multi-color display. 
     Alternatively, when the display elements  20  of the subpixels SP 1 , SP 2  and SP 3  are configured to emit ultraviolet light, a photo conversion layer is provided to oppose the display elements  20 , and thus the multi-color display can be realized. 
       FIG. 2  is a diagram showing an example of the configuration of each display element  20 . 
     The display element  20  comprises a lower electrode (first electrode) E 1 , an organic layer OR and an upper electrode (second electrode) E 2 . The organic layer OR includes a carrier adjustment layer CA 1 , a light-emitting layer EL, and a carrier adjustment layer CA 2 . The carrier adjustment layer CA 1  is located between a lower electrode E 1  and the light-emitting layer EL, and the carrier adjustment layer CA 2  is located between the light-emitting layer EL and an upper electrode E 2 . The carrier adjustment layers CA 1  and CA 2  each contain a plurality of functional layers. 
     Here, the case where the lower electrode E 1  corresponds to an anode and the upper electrode E 2  corresponds to a cathode will be described as an example. 
     The carrier adjustment layer CA 1  includes a hole injection layer F 11 , a hole transport layer F 12 , an electron blocking layer F 13  and the like, as functional layers. The hole injection layer F 11  is disposed on the lower electrode E 1 , the hole transport layer F 12  is disposed on the hole injection layer F 11 , the electron blocking layer F 13  is disposed on the hole transport layer F 12  and the light-emitting layer EL is disposed on the electron blocking layer F 13 . 
     The carrier adjustment layer CA 2  includes a hole blocking layer F 21 , an electron transport layer F 22 , an electron injection layer F 23  and the like, as functional layers. The hole blocking layer F 21  is disposed on the light-emitting layer EL, the electron transport layer F 22  is disposed on the hole blocking layer F 21 , the electron injection layer F 23  is disposed on the electron transport layer F 22 , and the upper electrode E 2  is disposed on the electron injection layer F 23 . 
     Note that in addition to the functional layers described above, the carrier adjustment layers CA 1  and CA 2  may include other functional layers including a carrier generation layer as needed, or at least one of the functional layers described above may be omitted in the carrier adjustment layers CA 1  and CA 2 . 
       FIG. 3  is a plan view showing an example of the pixel PX shown in  FIG. 1 . 
     The subpixels SP 1 , SP 2  and SP 3  that constitute one pixel PX are each formed into a rectangular shape elongated along the second direction Y, and are aligned in the first direction X. The outer shape of the subpixels corresponds to the outer shape of the light-emitting area EA in the display element  20 , but it is shown in a simplified way and therefore it does not necessarily reflect the actual shape. Here, it is assumed that the light-emitting area EA is formed into a rectangular shape with short sides extending along the first direction X and long sides extending along the second direction Y. 
     The insulating layer  12 , which will be described in detail later, is formed into a grid pattern extending along the first and second directions X and Y in planar view, which surrounds each of the subpixels SP 1 , SP 2  and SP 3 , or the display element  20  of each subpixel. The insulating layer  12  with such a configuration may be referred to as ribs, partitions, banks, etc. The light-emitting area EA is formed in each opening OP of the insulating layer  12  and corresponds to the region where the organic layer OR is interposed between the lower electrode E 1  and the upper electrode E 2 . 
     The upper electrode E 2  of the display element  20  overlaps the light-emitting area EA, as indicated by the single-dot chain line. To the upper electrode E 2 , a predetermined potential is supplied from a power feed line, which will not be described in detail. 
       FIG. 4  is a cross-sectional view of the display element  20  taken along line A-B in  FIG. 3 . 
     The pixel circuit  1  shown in  FIG. 1  is disposed on the base  10  and covered by an insulating layer  11 .  FIG. 4  illustrates a drive transistor  3  contained in the pixel circuit  1  in a simplified way. The insulating layer (first insulating layer)  11  corresponds to an underlying layer of the display element  20 . The insulating layer (second insulating layer)  12  is disposed on the insulating layer  11 . The insulating layers  11  and  12  are, for example, organic insulating layers. 
     The lower electrode E 1  is disposed on the insulating layer  11 . The lower electrode E 1  is an electrode provided for each subpixel or each display element, and is electrically connected to the drive transistor  3 . The lower electrode E 1  with such a configuration may be referred to as a pixel electrode, anode or the like. 
     The lower electrode E 1  is, for example, a metal electrode formed of a metal material such as silver or aluminum. Note that the lower electrode E 1  may be, for example, a transparent electrode formed of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO). Further, the lower electrode E 1  may as well be a stacked body of a transparent electrode and a metal electrode. For example, the lower electrode E 1  may be configured as a stacked body consisting of a transparent electrode, a metal electrode and a transparent electrode stacked one on another in this order, or may be configured as a stacked body consisting of three or more layers. The lower electrode E 1  may be configured to be different from other regions in a partial region. In the display device  20  of a top emission type, the lower electrode E 1  includes a metal electrode as a reflective electrode. 
     The insulating layer  12  includes an opening OP, sloping surfaces S 1  and S 2  and upper surfaces U 1  and U 2 . The opening OP is a through-hole formed in the region overlapping the lower electrode E 1  and penetrating the insulating layer  12  to the lower electrode E 1 . The peripheral portion of the lower electrode E 1  is covered by the insulating layer  12 , and the central portion of the lower electrode E 1  is exposed from the insulating layer  12  in the opening OP. 
     The sloping surfaces S 1  and S 2  face the opening OP. The upper surface U 1  is connected to the sloping surface S 1 . The upper surface U 2  is connected to the sloping surface S 2 . The upper surfaces U 1  and U 2  and the sloping surfaces S 1  and S 2  are, for example, flat surfaces, but they may as well be curved surfaces. The angle between the sloping surface S 1  and the upper surface U 1 , and the angle between the sloping surface S 2  and the upper surface U 2  are preferably both obtuse angles. The angle between the sloping surface S 1  and the lower electrode E 1 , and the angle between the sloping surface S 2  and the lower electrode E 1  are preferably both obtuse angles. 
     The organic layer OR includes a plurality of functional layers in addition to the light-emitting layer EL, as explained with reference to  FIG. 2 . The organic layer OR is disposed in the opening OP and covers the lower electrode E 1 . In the example shown in  FIG. 4 , the organic layer OR is disposed on the sloping surfaces S 1  and S 2 , and further on a part of the upper surface U 1  and a part of the upper surface U 2 . 
     The upper electrode E 2  is stacked on the organic layer OR. The upper electrode E 2  includes a first end surface SS 1  located directly above the insulating layer  12 . In other words, the first end surface SS 1  is located outside the opening OP. In the example shown in  FIG. 3 , the first end surface SS 1  is located above the organic layer OR. In other words, the upper electrode E 2  exposes the peripheral portion of the organic layer OR. Note that the upper electrode E 2  may cover the entire organic layer OR including the peripheral portion of the organic layer OR. In this case, the first end surface SS 1  is located directly on the upper surfaces U 1  and U 2  of the insulating layer  12 . 
     The first end surface SS 1  is an inclined surface. The angle between the first end surface SS 1  and the organic layer OR, the angle between the first end surface SS 1  and the upper surface U 1 , and the angle between the first end surface SS 1  and the upper surface U 2  are preferably obtuse angles. That is, the thickness of the upper electrode E 2  along the third direction Z decreases as the location is further on an outer side (further from the opening OP). The region of the upper electrode E 2 , which overlaps the first end surface SS 1  is referred to as a tapered portion TP. The upper electrode E 2  with such a configuration may be referred to as a common electrode, counter electrode, cathode or the like. 
     The upper electrode E 2  is a semi-transmissive metal electrode formed of, for example, a metal material such as magnesium or silver. The upper electrode E 2  may be a transparent electrode formed of a transparent conductive material such as ITO or IZO. The upper electrode E 2  may as well be a stacked body of a transparent electrode and a metal electrode. The upper electrode E 2  may be configured to be different from other regions in a partial region. The upper electrode E 2  is electrically connected to a power feed line disposed in the display area DA or a power feed line disposed on an outer side of the display area DA. 
     The portion of the organic layer OR, which is located between the lower electrode E 1  and the upper electrode E 2  without interposing the insulating layer  12  therebetween, can form a light-emitting region of the display element  20 . For example, the thickness of the organic layer OR along the third direction Z is set such that the peak wavelength of the spectrum of the emitted light in the light-emitting layer EL matches the effective length of the optical path between the lower electrode E 1  and the upper electrode E 2 . Thus, a microcavity structure to achieve the resonance effect can be realized. 
     The optical adjustment layer  40  is in contact with the upper electrode E 2 . The optical adjustment layer  40  includes a second end surface SS 2  located on an inner side with respect to the first end surface SS 1  (or the tapered portion TP). In other words, the optical adjustment layer  40  exposes the peripheral portion of the upper electrode E 2  including the tapered portion TP. The second end surface SS 2  is located on an outer side with respect to the opening OP. That is, the optical adjustment layer  40  is disposed to be in contact with the entire surface of the upper electrode E 2  at least in the opening OP. Thus, the second end surface SS 2  is located on an outer side of the opening OP and an inner side of the tapered portion TP (or the first end surface SS 1 ). 
     The second end surface SS 2  is, for example, an inclined surface. That is, the thickness of the optical adjustment layer  40  along the third direction Z decreases as the location is further on an outer side (further from the opening OP). For example, the inclination of the second end surface SS 2  is greater than that of the first end surface SS 1 . In other words, the first end surface SS 1  is a gentle slope with a relatively small inclination (a relatively small angle with respect to the bottom surface), while the second end surface SS 2  is a steep slope with a relatively large inclination. 
     The optical adjustment layer  40  with such a configuration is provided for the purpose of improving the extraction efficiency of the light from the display element  20 . The thickness and refractive index of the optical adjustment layer  40  are selected according to the intensity and wavelength of the light emitted from the emission layer EL. For example, the optical adjustment layer  40  is a multilayer film. The thin film which constitutes the optical adjustment layer  40  may be formed from a conductive material, inorganic material or organic material. 
     The display element  20  and the optical adjustment layer  40  are covered by a sealing layer  50 . In the example shown in  FIG. 4 , the sealing layer  50  is in contact with the first end surface SS 1  and also with the insulating layer  12  on an outer side of the organic layer OR. The sealing layer  50  is provided to prevent moisture and the like from entering the organic layer OR from outside as one of its functions. For example, the sealing layer  50  is formed of an inorganic material such as silicon nitride or silicon oxide. The sealing layer  50  may also include a thin film formed of an organic material. 
     As explained above, the optical adjustment layer  40  is in contact with the upper electrode E 2  that constitutes the display element  20 , thereby making it possible to improve the extraction efficiency of the light generated by the display element  20 . Further, the optical adjustment layer  40  is provided in the light-emitting area EA of the display element  20 , that is, the region overlapping the opening OP of the insulating layer  12 . Therefore, of the light generated by the display element  20 , the light having a predetermined wavelength, on which the resonance effect has been exerted, is extracted, and the luminance and color purity of the displayed light can be improved. 
     Incidentally, when light is emitted by recombination of carriers leaking from the organic layer OR on the insulating layer  12 , the light is not subjected to the resonance effect of the microcavity structure, and therefore it has a wavelength different from the predetermined wavelength. Therefore, if light of an undesired wavelength is extracted separately from the light of the predetermined wavelength during low-gradation display, the ratio of the light of the undesired wavelength occupying in the displayed light increases, and the desired chromaticity may not be obtained (that is, causing degradation in the color purity). 
     In order to avoid this, the optical adjustment layer  40  is provided in such a way that it does not overlap the first end surface SS 1  of the upper electrode E 2 . With this configuration, even if undesired light emission occurs in the vicinity of the first end surface SS 1 , the light is not transmitted through the optical adjustment layer  40  and does not substantially contribute to the display. Therefore, even in the case of low gradation display, the ratio of light having an undesired wavelength occupying in the display light is reduced, and the desired chromaticity can be obtained. 
       FIG. 5  is a cross-sectional view of a comparative example of the display element  20 . 
     The example shown in  FIG. 5  is different from that of  FIG. 4  in that the optical adjustment layer  40  covers the first end surface SS 1  of the upper electrode E 2 . In other words, the second end surface SS 2  of the optical adjustment layer  40  is located on an outer side with respect to the first end surface SS 1 . 
     In the comparative example with such a configuration, if an undesired light emission occurs in the vicinity of the first end surface SS 1 , the light is extracted by the optical adjustment layer  40  and contributes to the display. Therefore, as compared to the example shown in  FIG. 4 , the ratio of light with an undesired wavelength occupying in the display light increases during low-gradation display. Accordingly, the color purity obtained in the comparative example is lower than the color purity obtained in the example shown in  FIG. 4 . 
     According to the embodiments provided above, it is possible to provide a display device which can obtain a desired chromaticity. 
     Based on the display device which has been described in the above-described embodiments, a person having ordinary skill in the art may achieve a display device with an arbitral design change; however, as long as they fall within the scope and spirit of the present invention, such a display device is encompassed by the scope of the present invention. 
     A skilled person would conceive various changes and modifications of the present invention within the scope of the technical concept of the invention, and naturally, such changes and modifications are encompassed by the scope of the present invention. For example, if a skilled person adds/deletes/alters a structural element or design to/from/in the above-described embodiments, or adds/deletes/alters a step or a condition to/from/in the above-described embodiment, as long as they fall within the scope and spirit of the present invention, such addition, deletion, and altercation are encompassed by the scope of the present invention. 
     Furthermore, regarding the present embodiments, any advantage and effect those will be obvious from the description of the specification or arbitrarily conceived by a skilled person are naturally considered achievable by the present invention.