Abstract:
In an organic EL device, the light emission efficiency by a TADF mechanism is to be improved with an emissive layer structure that can be easily formed. An OLED has at least an emissive layer between an upper electrode and a lower electrode. The emissive layer includes: a host layer including a host material; an assistant dopant layer which is a layer adjacent to the host layer and where an assistant dopant made of a thermally activated delayed fluorescence material and the host material are intermingled within a plane; and a light-emitting dopant layer which is a layer adjacent to the assistant dopant layer and where a light-emitting dopant made of a fluorescent material emitting light by being excited by the assistant dopant and the host material are intermingled within a plane.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese application JP2015-222820 filed on Nov. 13, 2015, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an organic electroluminescence (EL) device and particularly to an improvement in light emission efficiency using a thermally activated delayed fluorescence (TADF) material. 
         [0004]    2. Description of the Related Art 
         [0005]    An organic EL device is generally referred to as OLED (organic light-emitting diode), which is a kind of light-emitting diode. In the emissive layer of the organic EL device, a light-emitting dopant is excited by the recombination of holes injected from the anode and electrons injected from the cathode, and a singlet excited state and a triplet excited state are generated at a ratio of 1:3. In the organic EL device using a fluorescent material as the light-emitting dopant, only the singlet excited state contributes to light emission and light is not emitted when the triplet excited state is deactivated. Therefore, the limit of its internal quantum efficiency is considered to be 25%. Research has been done utilizing a TADF mechanism as an organic EL light emission mechanism to solve this problem. This TADF mechanism utilizes the phenomenon of reverse intersystem crossing (RISC) from a triplet excited state with lower energy to a singlet exciton with higher energy, generated by thermal activation in a material with a small difference in energy between the singlet excited state and the triplet excited state. According to this, theoretically, the internal quantum efficiency of fluorescent emission can be increased to 100%. 
         [0006]    Recently, a TADF material which enables light emission in all of red (R), green (G), and blue (B) at room temperature has been developed.  FIG. 10  is a schematic view for explaining a fluorescent emission mechanism in an organic EL device using a TADF material as an assistant dopant (see H. Nakanotani, et al., “High efficiency organic light-emitting diodes with fluorescent emitters,” Nature Commun. 5, 4016 (2014)). To the emissive layer of this organic EL device, the TADF material is added as well as a host material and a light-emitting dopant material. In  FIG. 10 , the energy level of each material is shown. So indicates the ground state. S 1  indicates the lowest singlet excited state. T 1  indicates the lowest triplet excited state. In the illustration, the higher the position is, the higher the energy level is. The light-emitting dopants TBPe, TTPA, TBRb, and DBP emit blue, green, orange, and red fluorescent lights, respectively. The shorter the light emission wavelength of the material is, the higher the S 1  energy level is. 25% of the recombination of holes (h + ) and electrons (e − ) results in the S 1  level of TADF molecules of the assistant dopant, and 75% results in the T 1  level. Here, the TADF molecules of the T 1  level are upconverted to the S 1  level by the RISC process with thermal energy. Using the TADF molecules having a higher S 1  level than the light-emitting dopant, as the assistant dopant, energy transfer of the singlet exciton of the TADF molecules to the light-emitting dopant of each color can be performed by fluorescence resonance energy transfer (FRET), and fluorescent emission of each color can thus be achieved. 
       SUMMARY OF THE INVENTION 
       [0007]    The improvement in the light emission efficiency in the mechanism shown in  FIG. 10  requires efficient transfer of the excitation energy generated by the recombination of holes and electrons to the assistant dopant and efficient transfer of the excitation energy from the assistant dopant to the light-emitting dopant. 
         [0008]    In this respect, as the distance between the host molecules and the assistant dopant molecules and the distance between the assistant dopant molecules and the light-emitting dopant molecules increase, the probability of energy transfer drops and therefore improvement in the light emission efficiency becomes difficult. 
         [0009]    The invention is to provide an organic EL device having an emissive layer of a structure which can be formed relatively easily and in which respective materials can be brought closer to each other, thus allowing for the expectation of improved light emission efficiency. 
         [0010]    According to an aspect of the invention, an organic EL device having at least an emissive layer between a pair of electrodes made up of an anode and a cathode. The emissive layer includes: a host layer made of a host material; an assistant dopant layer which is adjacent to the host layer and where an assistant dopant made of a thermally activated delayed fluorescence material and the host material are intermingled within a plane; and a light-emitting dopant layer which is adjacent to the assistant dopant layer and where a light-emitting dopant made of a fluorescent material emitting light by being excited by the assistant dopant and the host material are intermingled within a plane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic view showing a schematic configuration of an organic EL display device according to an embodiment of the invention. 
           [0012]      FIG. 2  is a schematic plan view of a display panel of the organic EL display device according to the embodiment of the invention. 
           [0013]      FIG. 3  is a schematic vertical cross-sectional view of the display panel, taken along shown in  FIG. 2 . 
           [0014]      FIG. 4  is a vertical cross-sectional view of the display panel, taken along IV-IV shown in  FIG. 3 , and a schematic view showing the structure of an OLED according the first embodiment of the invention. 
           [0015]      FIG. 5  is a graph showing an example of the composition rate of each component in the emissive layer shown in  FIG. 4 . 
           [0016]      FIGS. 6A to 6E  are schematic vertical cross-sectional views of the OLED part in a main process at the time of forming the emissive layer shown in  FIG. 4 . 
           [0017]      FIG. 7  is a schematic vertical cross-sectional view of the OLED, for explaining the effects of the emissive layer shown in  FIG. 6E . 
           [0018]      FIGS. 8A to 8E  are schematic vertical cross-sectional views of an OLED part in a main process at the time of forming the emissive layer of an OLED according to a second embodiment of the invention. 
           [0019]      FIG. 9  is schematic vertical cross-sectional view of an OLED having an emissive layer including a two-layer structure made up of an assistant dopant film and a light-emitting dopant film. 
           [0020]      FIG. 10  is a schematic view for explaining a fluorescent emission mechanism in an organic EL device using a TADF material as an assistant dopant. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Hereinafter, a form of embodying the invention (hereinafter referred to as an embodiment) will be described with reference to the drawings. 
         [0022]    The disclosure is only an example, and as a matter of course, any change that can be easily thought of by a person skilled in the art without departing from the spirit of the invention should be included in the scope of the invention. In order to clarify the explanation, the drawings may schematically show each part in terms of its width, thickness, shape and the like, compared with the actual configuration. However, this is simply an example and should not limit the interpretation of the invention. Also, elements similar to those described before with reference to already mentioned drawings may be denoted by the same reference signs, and detailed description of these elements may be omitted when appropriate. 
         [0023]    The embodiment below is an organic EL display device, which displays an image using an organic EL device according to the invention. The organic EL display device is an active-matrix display device and is installed in a television, personal computer, mobile terminal, mobile phone and the like. 
         [0024]    In an image display area of the display device, a plurality of pixels forming an image is arranged two-dimensionally. Here, the direction along one coordinate axis of a two-dimensional orthogonal coordinate system corresponding to the image is defined as a row direction, and the direction along the other coordinate axis is defined as a column direction. In the description below, the row direction and the column direction are basically the horizontal direction and the vertical direction of the image. However, this definition is made as a matter of convenience. For example, in a display device which can display an image, switching the vertical and horizontal sides of the image in the same image display area, the row direction and the column direction of the image display area can be the vertical direction and the horizontal direction of the image, respectively. Also, the structure of the display device can be configured in such a way that the row direction and the column direction are switched with respect to what is described below. 
         [0025]    Also, in the embodiment below, a display device which can display a color image by having a plurality of types of pixels (subpixels) with different light-emitting colors from each other arranged in an image display area will be described. The pixels in a color image correspond to a set of subpixels made up of a plurality of types of subpixels in the display device. However, in the display device, the subpixel is the structural unit, and an OLED and a pixel circuit are formed for each subpixel. Thus, in the description below, a subpixel is basically regarded as a pixel. 
       First Embodiment 
       [0026]      FIG. 1  is a schematic view showing a schematic configuration of an organic EL display device  2  according to the embodiment. The organic EL display device  2  has a pixel array unit  4  which displays an image, and a drive unit which drives the pixel array unit. In the organic EL display device  2 , a multilayer structure including a thin film transistor (TFT), OLED and the like is formed on a substrate made of glass or flexible resin film. 
         [0027]    In the pixel array unit  4 , an OLED  6  and a pixel circuit  8  are arranged in the form of a matrix corresponding pixels. The pixel circuit  8  is made up of a plurality of TFTs  10 ,  12  and a capacitor  14 . 
         [0028]    Meanwhile, the drive unit includes a scanning line drive circuit  20 , a video line drive circuit  22 , a drive power-supply circuit  24 , a reference power-supply circuit  26 , and a control device  28 . The drive unit has functions such as driving the pixel circuit  8  to control the light emission of the OLED  6 . 
         [0029]    The scanning line drive circuit  20  is connected to a scanning signal line  30  provided for each horizontal line of pixels (pixel row). The scanning line drive circuit  20  sequentially selects a scanning signal line  30  in response to a timing signal inputted from the control device  28 , and applies a voltage to switch on the lighting TFT  10 , to the selected scanning signal line  30 . 
         [0030]    The video line drive circuit  22  is connected to a video signal line  32  provided for each vertical line of pixels (pixel column). The video line drive circuit  22  has a video signal inputted from the control device  28 , and outputs a voltage corresponding to the video signal for the selected pixel row to each video signal line  32 , simultaneously with the selection of the scanning signal line  30  by the scanning line drive circuit  20 . This voltage is written in the capacitor  14  via the lighting TFT  10 , in the selected pixel row. The drive TFT  12  supplies a current corresponding to the written voltage to the OLED  6 , and this causes the OLED  6  of the pixel corresponding to the selected scanning signal line  30  to emit light. 
         [0031]    The drive power-supply circuit  24  is connected to a drive power-supply line  34  provided for each pixel column, and supplies a current to the OLED  6  via the drive power-supply line  34  and the drive TFT  12  in the selected pixel row. 
         [0032]    The reference power-supply circuit  26  provides a constant potential φ REF  to a common electrode (not illustrated) forming the cathode electrode of the OLED  6 . φ REF  can be set to ground potential GND (0 V), for example. 
         [0033]    In this embodiment, the lower electrode of the OLED  6  is a pixel electrode formed for each pixel, and the upper electrode of the OLED  6  is a counter electrode arranged opposite the pixel electrode. The lower electrode is connected to the drive TFT  12 . Meanwhile, the upper electrode is formed by an electrode common to the OLEDs  6  of all the pixels. In this embodiment, the lower electrode is the anode of the OLED  6 , and the upper electrode is the cathode. 
         [0034]      FIG. 2  is a schematic plan view of a display panel  40  of the organic EL display device  2 . The pixel array unit  4  shown in  FIG. 1  is provided in a display area  42  of the display panel  40 , and the OLEDs are arrayed in the pixel array unit  4  as described above. A component mounting area  46  is provided on one side of the rectangular display panel  40 , and a wiring connected to the display area  42  is arranged in the component mounting area  46 . Moreover, in the component mounting area  46 , a driver IC  48  forming the drive unit is installed and an FPC  50  is connected. The FPC  50  is connected to the control device  28  and the other circuits  20 ,  22 ,  24 ,  26  and the like, and has an IC installed thereon. 
         [0035]    The display panel  40  in this embodiment displays a color image. The pixels in the color image are made up of pixels (subpixels) which emit light corresponding to red (R), green (G), and blue (B), for example. 
         [0036]    In this embodiment, an example in which an R pixel  52   r , a G pixel  52   g , and a B pixel  52   b  are arranged in stripes in the display area is described. In this arrangement, pixels of the same type (color) are arrayed in the vertical direction of the image, and RGB are arrayed cyclically in the horizontal direction. In  FIG. 2 , each of the R pixel  52   r , the G pixel  52   g , and the B pixel  52   b  schematically shows an effective light-emitting area. In terms of structure, these pixels correspond to pixel apertures  60 , and the areas between these pixels correspond to banks  106 . 
         [0037]    The display panel  40  has a structure in which a TFT substrate and a counter substrate are bonded together with filler held between these substrates, for example. A circuit formed by a TFT  72  or the like, and the OLED  6  or the like are formed on the TFT substrate. A polarizer and a touch panel can be provided on the counter substrate. 
         [0038]      FIG. 3  is schematic vertical cross-sectional view of the display panel  40 , taken along shown in  FIG. 2 .  FIG. 3  shows the cross-sectional structure of the TFT substrate but does not show the structure of the filler layer and the counter substrate formed thereon. In this embodiment, the pixel array unit  4  is a top emission type, and the light generated by the OLED  6  formed on the TFT substrate is emitted from the counter substrate. That is, in  FIG. 3 , the light of the OLED  6  is emitted upward. 
         [0039]    The structure of the TFT substrate is formed by stacking and patterning various layers on a substrate  70  made of glass or resin film. 
         [0040]    Specifically, a polysilicon (p-Si) film is formed via an underlying layer  80  made of an inorganic insulating material such as silicon nitride (SiN y ) or silicon oxide (SiO x ) on the substrate  70 , and this p-Si film is patterned and selectively left at a part used for a circuit layer. For example, a semiconductor area  82  that forms a channel part and source and drain parts of a top gate-type TFT  72  is formed using the p-Si film. On the channel part of the TFT  72 , a gate electrode  86  is arranged via a gate insulating film  84 . The gate electrode  86  is formed by patterning a metal film formed by sputtering or the like. Subsequently, an interlayer insulating film  88  covering the gate electrode  86  is stacked. An impurity is introduced by ion injection into the p-Si that forms the source part and the drain part of the TFT  72 , and a source electrode  90   a  and a drain electrode  90   b  that are electrically connected to these parts are formed. After the TFT  72  is thus formed, an interlayer insulating film  92  is stacked. On the surface of the interlayer insulating film  92 , a wiring  94  or the like formed by patterning a metal film formed by sputtering can be formed. This metal film, and the metal film used to form the gate electrode  86 , the source electrode  90   a  and the drain electrode  90   b  can form, for example, the scanning signal line  30 , the video signal line  32 , and the drive power-supply line  34  shown in  FIG. 1 , as a multilayer wiring structure. For example, an organic material such as an acrylic resin is stacked thereon to forma flattening film  96 , and the OLED  6  is formed on the surface of the display area  42  thus flattened. A sealing film  108  is formed on the OLED  6 . The sealing film  108  has the function of preventing moisture or the like from passing through and thus protecting the OLED  6 . 
         [0041]    The OLED  6  is made up of a lower electrode  100 , a light-emitting element layer  102 , and an upper electrode  62 . The lower electrode  100 , the light-emitting element layer  102 , and the upper electrode  62  are stacked in order from the side of the substrate  70 . 
         [0042]    If the TFT  72  shown in  FIG. 3  is the drive TFT  12  having an n-channel, the lower electrode  100  is connected to the source electrode  90   a  of the TFT  72 . Specifically, after the flattening film  96  is formed, a contact hole  104  for connecting the lower electrode  100  to the TFT  72  is formed, and a conductive film formed on the surface of the flattening film  96  and inside the contact hole  104  is patterned, thus forming the lower electrode  100  connected to the TFT  72  separately for each pixel. 
         [0043]    For example, the lower electrode  100  is formed of ITO, IZO or the like. Also, since this embodiment is a top emission type, the lower electrode  100  can be formed as a structure in which a transparent conductive film is stacked on a reflection layer formed of a material with high light reflectance. For example, the reflection layer can be formed of aluminum (Al), silver (Ag) or the like, thus reflecting the light from the emissive layer toward the display surface, that is, toward the upper electrode  62 . 
         [0044]    As described above, the drive TFT  12  controls the current flowing to the OLED  6  in accordance with the video signal of each pixel, and the lower electrode  100  supplies carriers in an amount corresponding to the video signal of each pixel, to the light-emitting element layer  102 . Specifically, in this embodiment, the lower electrode  100  is the anode, and holes as carriers are supplied from the lower electrode  100  to the light-emitting element layer  102 . 
         [0045]      FIG. 4  is a schematic view showing the structure of the OLED  6  in the display panel  40 , and shows a vertical cross section taken along IV-IV in  FIG. 3 . 
         [0046]    The light-emitting element layer  102  has an emissive layer (EML)  110  made of an organic compound and emits light as carriers (electrons and holes) are injected into the emissive layer  110 . The light-emitting element layer  102  also has an auxiliary layer for efficiently injecting carriers into the emissive layer  110  when applying a voltage to the OLED  6 . Specifically, a hole transport layer (HTL) and a hole injection layer (HIL) are provided between the anode and the emissive layer. An electron transport layer (ETL) and an electron injection layer (EIL) are provided between the cathode and the emissive layer. For example, in  FIG. 4 , an HTL/HIL layer  112  provided between the lower electrode  100  as the anode and the emissive layer  110  is made up of an HIL layer provided toward the lower electrode  100  and an HTL layer provided toward the emissive layer  110 . An ETL/EIL layer  114  between the upper electrode  62  as the cathode and the emissive layer  110  is made up of an EIL layer provided toward the upper electrode  62  and an ETL layer provided toward the emissive layer  110 . 
         [0047]    The emissive layer  110  includes a host layer  120 , an assistant dopant layer  122 , and a light-emitting dopant layer  124 . The host layer  120  is formed of a host material. The host material is an organic substance responsible for transporting carriers. For example, the host material is mCBP (3,3′-di(9H-carbazol-9-yl) biphenyl), mCP (1,3-bis(carbazol-9-yl)benzene), DPEPO (Bis(2-[(oxo)diphenylphosphino]phenyl)ether) described in the reference literature, or the like. 
         [0048]    The assistant dopant layer  122  is formed of an assistant dopant and the host material. An area made up of the assistant dopant and an area made up of the host material are intermingled within the plane of this layer. The assistant dopant is made of a TADF material. For example, the assistant dopant is ACRSA (10-phenyl-10H,10′H-spiro[acridine-9,9′-anthracen]-10′-one), ACRXTN (3-(9,9-dimethylacridin-10(9H)-yl)-9H-xanthen-9-one), PXZ-TRZ (2-phenoxazine-4,6-diphenyl-1,3,5-triazine), tri-PXZ-TRZ (2,4,6-tri(4-(10H-phenoxazin-10H-yl)phenyl)-1,3,5-triazine) described in the reference literature, or the like. 
         [0049]    The light-emitting dopant layer  124  is formed of a light-emitting dopant and the host material. An area made up of the light-emitting dopant and an area made up of the host material are intermingled within the plane of this layer. The light-emitting dopant layer  124  can also include the assistant dopant. In this case, an area made up of the assistant dopant and the areas of the other two materials are intermingled within the plane of the light-emitting dopant layer  124 . For example, the light-emitting dopant is as described in the reference literature. Specifically, TBPe (2,5,8,11-tetra-tert-butylperylene) for blue light emission, TTPA (9,10-bis[N,N-di-(p-tolyl)-amino] anthracene for green light emission, and DBP (tetraphenyldibenzoperiflanthene) for red light emission can be used. 
         [0050]    The host material, the assistant dopant, and the light-emitting dopant used to form the emissive layer  110 , that is, the host layer  120 , the assistant dopant layer  122 , and the light-emitting dopant layer  124 , are combined in such a way that their respective energy levels satisfy the relation shown in  FIG. 10 . That is, the T 1  level of the host material, the S 1  level of the assistant dopant, and the S 1  level of the light-emitting dopant are given in order from the highest energy level. 
         [0051]    Inside the emissive layer  110 , the assistant dopant layer  122  is in contact with the host layer  120 , and the light-emitting dopant layer  124  is in contact with the assistant dopant layer  122 . For example, as shown in  FIG. 4 , the emissive layer  110  can be a multilayer structure in which the assistant dopant layer  122  is stacked on both sides of the light-emitting dopant layer  124  and in which the host layer  120  is stacked on the surface opposite to the surface in contact with the light-emitting dopant layer  124 , of each assistant dopant layer  122 . The upper electrode  62  and the lower electrode  100  are in contact with these host layers  120 . 
         [0052]      FIG. 5  is a graph showing an example of the composition ratio of the respective components of the emissive layer  110  shown in  FIG. 4 . A z-axis corresponding to the direction of the film thickness of the emissive layer  110  is set in the vertical direction, and an x-axis corresponding to the material ratio is set in the horizontal direction. The origin of the z-axis is the boundary between the emissive layer  110  and the lower electrode  100 . The center position of the light-emitting dopant layer  124  is z 1 . The boundary position between the emissive layer  110  and the upper electrode  62  is z 2 . That is, the thickness of the emissive layer  110  is z 2 . Also, the thickness of the light-emitting dopant layer  124  is expressed as 2×r 1 , and the thickness of each assistant dopant layer  122  is expressed as r 2 . In  FIG. 5 , a solid line  150  indicates the ratio of the light-emitting dopant. A chain-dotted line  152  indicates the ratio of the assistant dopant. A dotted line  154  indicates the ratio of the host material. 
         [0053]    In the example shown in  FIG. 5 , the light-emitting dopant layer  124  is made up of three components, that is, the light-emitting dopant, the assistant dopant, and the host material. For example, the volume ratio of the respective components in the light-emitting dopant layer  124  is even, that is, x=⅓ for each component. 
         [0054]    In the assistant dopant layer  122 , the volume ratio of the respective components can be made even, that is, x=½ for each of the assistant dopant and the host material. However, in the assistant dopant layer  122 , the light-emitting dopant may be present in a very small amount compared with the other two materials. 
         [0055]    The host layer  120  is substantially made up of the host material only, of the three components forming the emissive layer  110 . However, in the host layer  120 , the light-emitting dopant and the assistant dopant may be present in a very small amount compared with the host material. 
         [0056]    For example, the thickness z 2  of the emissive layer  110  can be 30 nanometers (nm), and r 1 ≦2.5 nm and r 2 ≦10 nm can hold. 
         [0057]    Next, the manufacturing method of the emissive layer  110  shown in  FIG. 4  will be described. The emissive layer  110  is formed by vapor deposition after stacking the HTL/HIL layer  112  on the lower electrode  100 . The vapor deposition process for forming the emissive layer  110  includes a plurality of vapor deposition processes corresponding to the layers with different compositions provided in the emissive layer  110 , that is, the host layer  120 , the assistant dopant layer  122 , and the light-emitting dopant layer  124 . 
         [0058]      FIGS. 6A to 6E  are schematic vertical cross-sectional views of the OLED part in the main process at the time of forming the emissive layer  110  shown in  FIG. 4 . After the HTL/HIL layer  112  is formed, the host material is deposited to form the first host layer  120  (first host layer forming process,  FIG. 6A ). 
         [0059]    After the first host layer  120  is formed, the assistant dopant and the host material are co-deposited. The ratio of the amounts of deposition of these materials can be ½ each, corresponding to the composition ratio of the assistant dopant layer  122  shown in  FIG. 5 . The thickness of the film formed in this process is set to such an extent that, for example, the film of the assistant dopant is formed in the shape of an island or a plurality of islands on the surface of the first host layer  120 . For example, the average film thickness within the vapor deposition target surface can be several nm. In this process, an assistant dopant film  200   a  or a host material film (host film)  202   a  is formed on substantially the entire surface of the first host layer  120 , thus substantially forming the first assistant dopant layer  122  (first assistant dopant layer forming process,  FIG. 6B ). 
         [0060]    Next, the light-emitting dopant, the assistant dopant, and the host material are co-deposited. The ratio of the amounts of deposition of these materials can be ⅓ each, corresponding to the composition ratio of the light-emitting dopant layer  124  shown in  FIG. 5 . The thickness of the film formed in this process can be set to such an extent that the film of the light-emitting dopant and the film of the assistant dopant are formed in the shape of an island or a plurality of islands on the vapor deposition target surface. For example, the average film thickness within the vapor deposition target surface can be several nm. In this process, any of a light-emitting dopant film  204 , an assistant dopant film  200   b , and a host film  202   b  is formed on substantially the entire vapor deposition target surface, thus substantially forming the light-emitting dopant layer  124  (light-emitting dopant layer forming process,  FIG. 6C ). A part of the material deposited in this light-emitting dopant layer forming process fills a recess that can be present on the vapor deposition target surface after the first assistant dopant layer forming process, and thus becomes a part of the first assistant dopant layer  122 . 
         [0061]    Next, the assistant dopant and the host material are co-deposited again. The ratio of the amounts of deposition of these materials can be ½ each, corresponding to the composition ratio of the assistant dopant layer  122  shown in  FIG. 5 . The thickness of the film formed in this process can be set to such an extent that the film of the assistant dopant is formed in the shape of an island or a plurality of islands on the vapor deposition target surface. For example, the average film thickness within the vapor deposition target surface can be several nm. In this process, an assistant dopant film  200   c  or a host film  202   c  is formed on substantially the entire vapor deposition target surface, thus substantially forming the second assistant dopant layer  122  (second assistant dopant layer forming process,  FIG. 6D ). Apart of the material deposited in this second assistant dopant layer forming process fills a recess that can be present on the vapor deposition target surface after the light-emitting dopant layer forming process, and thus becomes a part of the light-emitting dopant layer  124 . 
         [0062]    The host material is deposited thereon to form the second host layer  120  (second host layer forming process). Thus, the multilayer structure of the emissive layer  110  is completed ( FIG. 6E ). A part of the material deposited in the second host layer forming processing fills a recess that can be present on the vapor deposition target surface after the second assistant dopant layer forming process, and thus becomes a part of the light-emitting dopant layer  124 . 
         [0063]    On the surface of this emissive layer  110 , the ETL/EIL layer  114  and the upper electrode  62  are stacked in order, thus providing the OLED shown in  FIG. 4 . 
         [0064]      FIG. 7  is a schematic vertical cross-sectional view of the OLED for explaining the effects of the emissive layer  110  shown in  FIG. 6E . By mixing a TADF as an assistant dopant into the emissive layer, improved light emission efficiency can be achieved in the TADF mechanism shown in  FIG. 10 . In order for the TADF mechanism to function effectively, it is necessary that a charge transport path of the host should be formed in such a way as to secure the probability of recombination in the host, of holes injected into the emissive layer from the anode and electrons injected into the emissive layer from the cathode, that the triplet state of excitons generated by the charge recombination should be smoothly transferred to the TADF molecules, and that excitons converted to the singlet state with the TADF molecules should be smoothly transferred to the light-emitting dopant molecules. In this respect, a structure in which particle bodies, each made up of a very small light-emitting dopant  230  surrounded by a thin film of an assistant dopant  232 , are scattered inside a host material  234 , is preferable, as shown in  FIG. 7 . For example, it is conceivable that such very small particle bodies are achieved by a micelle-like structure. 
         [0065]    However, it is not easy to obtain a material having properties for forming a micelle-like structure, and to prepare the foregoing particle structure directly in that form. The invention is to achieve improvement in light emission efficiency in the TADF mechanism by realizing the foregoing particle structure in a simulative manner. That is, with the structure of the emissive layer  110  according to the embodiment described with reference to  FIGS. 6A to 6E , the charge transport path of the host can be secured and the respective materials can be brought closer to each other. Therefore, the upconversion and transfer of excitons are carried out smoothly. In forming the structure of the emissive layer  110 , its material requires no special characteristics such as forming a micelle-like structure. Therefore, a high degree of freedom in material selection is achieved and this structure can be formed relatively easily. 
         [0066]    In the embodiment, in order to provide a structure similar to the particle bodies shown in  FIG. 7 , the assistant dopant layer  122  and the host layer  120  are stacked in order, vertically symmetrically as viewed from the light-emitting dopant layer  124  in the example shown in  FIG. 4  or  FIGS. 6A to 6E . However, improvement in light emission efficiency can also be achieved with other arrangements of the host layer  120 , the assistant dopant layer  122 , and the light-emitting dopant layer  124  in the multilayer structure of the emissive layer  110 , as long as the distance between the host material and the assistant dopant and the distance between the assistant dopant and the light-emitting dopant are shortened. For example, the emissive layer  110  can be a multilayer structure in which the host layer  120 , the assistant dopant layer  122 , the light-emitting dopant layer  124 , and the host layer  120  are stacked in this order or in the reverse order from the side of the lower electrode  100 , which is simpler than the structure shown in  FIG. 4 . 
         [0067]    Also, if a multilayer body made up of the assistant dopant layer  122  and the light-emitting dopant layer  124  only, specifically, the multilayer body in which the light-emitting dopant layer  124  is held between the two assistant dopant layers  122 , present in the emissive layer  110  shown in  FIG. 4 , or a simpler multilayer body made up of the assistant dopant layer  122  and the light-emitting dopant layer  124 , is called a dopant multilayer body, the emissive layer  110  can be a structure including a plurality of dopant multilayer bodies. This can further improve light emission efficiency. Inside the emissive layer  110 , the plurality of dopant multilayer bodies is stacked, holding the host layer  120  between the respective dopant multilayer bodies. 
       Second Embodiment 
       [0068]    An organic EL display device according to a second embodiment of the invention is different from the first embodiment in the structure of the emissive layer of the OLED and the manufacturing method, but is basically the same as the first embodiment in the other features. Therefore, hereinafter, the same components as those in the first embodiment are denoted by the same reference signs, without further explanation, and mainly the differences are described. 
         [0069]      FIGS. 1 to 3  are incorporated by reference in this embodiment.  FIGS. 8A to 8E  are schematic vertical cross-sectional views of the OLED part in the main process at the time of forming an emissive layer  110   b  in this embodiment. The emissive layer  110   b  is formed by vapor deposition after the HTL/HIL layer  112  is stacked on the lower electrode  100 . The vapor deposition process for forming the emissive layer  110   b  includes a plurality of vapor deposition processes corresponding to the layers with different compositions provided within the emissive layer  110   b.    
         [0070]    First, after the HTL/HIL layer  112  is formed, the host material is deposited on the entire surface thereof. A host film  300   a  is thus formed ( FIG. 8A ). 
         [0071]    After the host film  300   a  is formed, the assistant dopant is deposited. The thickness of the film formed in this process is set to such an extent that the film of the assistant dopant is formed in the shape of an island on the vapor deposition target surface. For example, the average film thickness within the vapor deposition target surface can be several nm. In this process, an assistant dopant film  302   a  is formed in patches on the surface of the host film  300   a  ( FIG. 8B ). 
         [0072]    Next, the light-emitting dopant is deposited. The thickness of the film formed in this process can be set to such an extent that the film of the light-emitting dopant is formed in the shape of an island on the vapor deposition target surface. For example, the average film thickness within the vapor deposition target surface can be several nm. In this process, a light-emitting dopant film  304  is formed in patches in the vapor deposition target surface ( FIG. 8C ). In the light-emitting dopant film  304  deposited in this process, there can be a part stacked on the surface of the assistant dopant film  302   a  and apart stacked on the surface of the host film  300   a.    
         [0073]    Next, the assistant dopant is deposited again. The thickness of the film formed in this process can be set to such an extent that the film of the assistant dopant is formed in the shape of an island on the vapor deposition target surface. For example, the average film thickness within the vapor deposition target surface can be several nm. In this process, an assistant dopant film  302   b  is formed in patches in the vapor deposition target surface ( FIG. 8D ). In the assistant dopant film  302   b  deposited in this process, there can be a part stacked on the surface of the light-emitting dopant film  304 , a part stacked on the surface of the assistant dopant film  302   a , and a part stacked on the surface of the host film  300   a.    
         [0074]    The host material is deposited thereon to form a host film  300   b . The multilayer structure of the emissive layer  110   b  is thus completed ( FIG. 8E ). In the host film  300   b  deposited in this process, there can be a part stacked on the surface of the assistant dopant film  302   b , a part stacked on the surface of the light-emitting dopant film  304 , a part stacked on the surface of the assistant dopant film  302   a , and a part stacked on the surface of the host film  300   a.    
         [0075]    The ETL/EIL layer  114  and the upper electrode  62  are stacked in order on the surface of this emissive layer  110   b , thus forming the OLED  6 . 
         [0076]    Here, the assistant dopant films  302   a ,  302   b  and the light-emitting dopant film  304  are formed in such a way as not to completely cover the host film  300   a , that is, in such a way that the host film  300   a  is exposed in a certain area in the state where the assistant dopant film  302   b  is formed. Thus, the host film  300   b  stacked thereon and the exposed part of the host film  300   a  contact each other, thus forming the charge transport path in the host. That is, the probability of recombination in the host, of holes injected into the emissive layer  110   b  from the anode and electrons injected into the emissive layer  110   b  from the cathode, is secured. 
         [0077]    Also, the assistant dopant films  302   a ,  302   b  and the light-emitting dopant film  304  are formed in such a way that the overlapping part of the assistant dopant films  302   a ,  302   b  and the light-emitting dopant film  304  has a large area, while leaving a gap to form the charge transport path in the host. At this overlapping part of the assistant dopant films  302   a ,  302   b  and the light-emitting dopant film  304  formed in the host, the TADF mechanism functions, thus improving light emission efficiency. 
         [0078]    Incidentally, if the emissive layer  110   b  shown in  FIG. 8E  is applied to the multilayer structure made up of the host layer  120 , the assistant dopant layer  122 , and the light-emitting dopant layer  124  described in the first embodiment, the emissive layer  110   b  can be seen as a structure in which the host layer  120 , the light-emitting dopant layer  124 , the assistant dopant layer  122 , and the host layer  120  are stacked in order from the side of the lower electrode  100 , as shown in  FIG. 8E . 
         [0079]    In the structure shown in  FIG. 8E , a three-layer structure part in which the assistant dopant films  302   a ,  302   b  are stacked on both sides of the light-emitting dopant film  304  is formed. This part can be seen as a structure similar to the particle bodies made up of the light-emitting dopant  230  and the assistant dopant  232  shown in  FIG. 7 . 
         [0080]    Meanwhile, the TADF mechanism functions even with a two-layer structure made up of the light-emitting dopant and the assistant dopant placed in the host. Therefore, for example, in the manufacturing method shown in  FIGS. 8A to 8E , the process of forming the assistant dopant film  302   a  may be omitted, and the light-emitting dopant film  304 , the assistant dopant film  302   b , and the host film  300   b  may be stacked in order on the host film  300   a , thus forming an OLED having a two-layer structure made up of the light-emitting dopant and the assistant dopant.  FIG. 9  is a schematic vertical cross-sectional view of an OLED having an emissive layer  110   c  including this two-layer structure. 
         [0081]    Incidentally, if the emissive layer  110   c  shown in  FIG. 9  is applied to the multilayer structure of the host layer  120 , the assistant dopant layer  122 , and the light-emitting dopant layer  124  described in the first embodiment, the emissive layer  110   c  can be seen as a structure in which the host layer  120 , the light-emitting dopant layer  124 , the assistant dopant layer  122 , and the host layer  120  are stacked in order from the side of the lower electrode  100 . 
         [0082]    A person skilled in the art can readily think of various changes and modifications within the scope of the technical idea of the invention, and such changes and modifications should be understood as falling within the scope of the invention. For example, the addition or deletion of a component, or a design change suitably made to the foregoing embodiments by a person skilled in the art, or the addition or omission of a process, or a condition change in the embodiments is included in the scope of the invention as long as such change or the like includes the spirit of the invention. 
         [0083]    Also, as a matter of course, other advantageous effects that may be achieved by the configurations described in the embodiment should be understood as being achieved by the invention if those effects are clear from the specification or can be readily thought of by a person skilled in the art. 
         [0084]    While there have been described what are at present considered to be certain embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.