Patent Publication Number: US-2021167147-A1

Title: Light emitting device, display panel having the same, and method of manufacturing display panel

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a light emitting device, a display panel having the light emitting device, and a method of manufacturing the display panel. 
     2. Description of the Related Art 
     In recent years, studies for forming various electronic devices by using a printing method have been actively conducted. According to the printing method, a required amount of ink can be applied only to a required location. Therefore, compared with a vacuum vapor deposition or sputtering method of the related art, the efficiency of using a material is high. However, materials (functional materials such as light emitting materials and conductive materials) used for these electronic devices are generally very expensive. Therefore, a material loss is a major problem. On the other hand, the printing method is desirable from the viewpoint of operation energy since a film can be formed in the atmosphere. 
     Examples of electronic devices formed according to the printing method include wirings using conductive ink, transistors using semiconductor ink, and display devices using light emitting materials. Examples of the printing method include screen printing, letterpress printing, intaglio printing, and the like. 
     In recent years, an ink jet method in which there is no contact with a print target and any pattern can be formed on demand has attracted attention. Specifically, for example, development of forming a color filter, or a display device such as an organic EL display or a quantum dot display according to the ink jet method has been actively performed. 
     As a next-generation display, a display using an inorganic quantum dot material as a light emitting layer has been actively developed. The quantum dot is a special semiconductor with a very small diameter of 2 to 10 nanometers (10 to 50 atoms). Such a substance having the minute size expresses property different from that of a normal substance. For example, in the quantum dot, a size of a band gap can be accurately controlled by simply changing a particle size of the quantum dot. An emission wavelength of the quantum dot depends on the size of the band gap. Thus, the emission wavelength of the quantum dots can be adjusted with high accuracy by changing the particle size of the quantum dot. In other words, the emission wavelength of the quantum dots can be changed simply by changing the particle size of the quantum dots. For example, the emission wavelength of the quantum dot shifts to a blue side as the particle size of the quantum dot becomes smaller, and shifts to a red side as it becomes larger. The full width at half maximum of the emission wavelength of the quantum dot is very small. Specifically, an emission spectrum of the quantum dot is several tens of nanometers or less. 
     In other words, for example, when the red, blue, and green light emitting layers are formed of quantum dots, the full width at half maximum of each emission wavelength can be reduced. Thus, a light emitting layer is formed by using quantum dots, and thus a display device having high color gamut characteristics can be implemented. As a result, the performance of the display device can be considerably improved. 
     A typical quantum dot material includes a core made of an inorganic material such as cadmium-selenium, indium-phosphorus, copper-indium-sulfur system, silver-indium-sulfur system, and perovskite structure, and a layer called a shell made of a material such as zinc sulfide around the core. A ligand is formed around the shell to realize stability of ink. 
     Examples of materials of the quantum dots forming the light emitting device include a photoluminescent material that is excited by light energy to emit light, and an electroluminescent material that is excited by electric energy to emit light. For example, a quantum dot display using the photoluminescent material is used as a color filter of a micro LED display. As a quantum dot display using the electroluminescent material, there is a quantum dot display formed by thinning a quantum dot material between an anode and a cathode. 
     The above quantum dot display has much higher brightness and has more excellent outdoor visibility than those of an organic EL display. Thus, the quantum dot display is expected to be used in applications such as displays for mobile phones and in-vehicle devices and head mounted displays. It is expected that these displays will require a pixel resolution of 200 pixel per inch (ppi) or more in the future. 
     However, in a case where a display device such as a display panel is formed according to the ink jet method, it is difficult to increase a pixel resolution due to factors such as a size of a liquid droplet in ink jetting or the accuracy of a liquid droplet landing position. Thus, in forming a display device according to the ink jet method, improvement in stability of ink application to a pattern having a high pixel resolution is desired. In other words, as the pixel resolution becomes higher, a region of pixels to which ink is applied becomes smaller. Thus, in a case where the accuracy of a liquid droplet landing position in ink jetting is low, ink is printed to extrude from a pixel region. As a result, color mixing occurs between adjacent pixels. 
     Thus, in order to prevent color mixing with adjacent pixels, for example, International Publication No. WO2008/149498 (hereinafter, referred to as “Patent Literature 1”) discloses a method of manufacturing an organic EL device by using the ink jet method.  FIG. 11  is a plan view illustrating an organic EL device disclosed in Patent Literature 1. 
     As illustrated in  FIG. 11 , the organic EL device has banks  3  that are formed in a line shape, banks  3 ′ that are formed to divide a region surrounded by banks  3  into two or more pixel regions  11 , and functional layers such as hole transport layers  4  formed in the region surrounded by banks  3  on substrate  1 . Bank  3  is made of a material that has liquid repellency to functional ink for hole transport layer  4  or the like. Red material  10 R, blue material  10 B, and green material  10 G are disposed between banks  3 . 
     In the structure of the organic EL device disclosed in Patent Literature 1, the wettability of the bank with ink is low. In other words, a contact angle with respect to the ink applied in the bank is high. In a state in which the contact angle is high, it becomes difficult for ink to be applied to a sidewall surface of the bank. Even though the ink is applied to the end of the bank, the surface tension of the ink may cause a decrease in a film thickness of the ink. 
     The organic EL device includes a functional thin film such as a light emitting layer above an anode formed in the bank, and a cathode formed above the light emitting layer. Thus, when a film thickness of the light emitting layer formed at the end of the bank is small, there is concern that the anode and the cathode may be short-circuited to each other. 
     SUMMARY 
     The present disclosure provides a light emitting device capable of improving the wettability with ink at the end of a bank and thus suppressing a short circuit between an anode and a cathode, a display panel including the light emitting device, and a method for manufacturing the display panel. 
     According to the present disclosure, there is provided a light emitting device including a first bank that partitions pixel regions; a second bank that is disposed above the first bank and defines the pixel regions; and a light emitting layer that is disposed in each of the pixel regions surrounded by the first bank or the second bank. The light emitting device is configured such that at least one of the first bank and the second bank has a communicator via which two or more of the pixel regions including light emitting layers with a same color communicate with each other. 
     A display panel of the present disclosure includes the light emitting device. 
     According to the present disclosure, there is provided a method of manufacturing a display panel, including a first step of forming a first bank partitioning pixel regions including light emitting layers that emit a same light emission color among pixel regions are formed on a substrate; and a second step of forming a second bank partitioning pixel regions including light emitting layers that emit a different light emission color among the pixel regions are formed. The method of manufacturing a display panel further includes a third step of forming a light emitting layer in a region surrounded by the first bank or the second bank. 
     As described above, it is possible to provide a light emitting device capable of improving the wettability with ink at the end of a bank and thus suppressing a short circuit between an anode and a cathode, a display panel including the same, and a method for manufacturing the display panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a plan view of a light emitting device according to an exemplary embodiment; 
         FIG. 1B  is a sectional view taken along line IB-IB in  FIG. 1A ; 
         FIG. 1C  is a sectional view taken along line IC-IC in  FIG. 1A ; 
         FIG. 1D  is a sectional view taken along line ID-ID in  FIG. 1A ; 
         FIG. 2A  is a plan view of the light emitting device according to the exemplary embodiment before ink applied through ink jetting is dried; 
         FIG. 2B  is a sectional view taken along line IIB-IIB in  FIG. 2A ; 
         FIG. 2C  is a sectional view taken along line IIC-IIC in  FIG. 2A ; 
         FIG. 3  is a plan view of the light emitting device described in the same exemplary embodiment; 
         FIG. 4A  is a plan view related to Example 1 of a light emitting device according to the exemplary embodiment; 
         FIG. 4B  is a sectional view taken along line IVB-IVB in  FIG. 4A ; 
         FIG. 4C  is a sectional view taken along line IVC-IVC in  FIG. 4A ; 
         FIG. 5A  is a plan view related to Example 2 of a light emitting device according to the exemplary embodiment; 
         FIG. 5B  is a sectional view taken along line VB-VB in  FIG. 5A ; 
         FIG. 5C  is a sectional view taken along line VC-VC in  FIG. 5A ; 
         FIG. 6A  is a sectional view taken along line VB-VB in  FIG. 5A , illustrating an effect of the light emitting device related to Example 2; 
         FIG. 6B  is a sectional view taken along line VB-VB in  FIG. 5A , illustrating the effect of the light emitting device related to Example 2; 
         FIG. 6C  is a sectional view taken along line VC-VC in  FIG. 5A , illustrating the effect of the light emitting device related to Example 2; 
         FIG. 7A  is a plan view related to Example 3 of a light emitting device according to the exemplary embodiment; 
         FIG. 7B  is a sectional view taken along line VIIB-VIIB in  FIG. 7A ; 
         FIG. 8A  is a plan view related to Example 4 of a light emitting device according to the exemplary embodiment; 
         FIG. 8B  is a sectional view taken along line VIIIB-VIIIB in  FIG. 8A ; 
         FIG. 8C  is a sectional view taken along line VIIIC-VIIIC in  FIG. 8A ; 
         FIG. 8D  is a sectional view taken along line VIIID-VIIID in  FIG. 8A ; 
         FIG. 9A  is a plan view related to Example 5 of a light emitting device according to the exemplary embodiment; 
         FIG. 9B  is a sectional view taken along line IXB-IXB in  FIG. 9A ; 
         FIG. 9C  is a sectional view taken along line IXC-IXC in  FIG. 9A ; 
         FIG. 10A  is a plan view related to Example 6 of a light emitting device according to the exemplary embodiment; 
         FIG. 10B  is a sectional view taken along line XB-XB in  FIG. 10A ; 
         FIG. 10C  is a sectional view taken along line XC-XC in  FIG. 10A ; and 
         FIG. 11  is a plan view illustrating a structure of an organic EL device disclosed in Patent Literature 1. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary Embodiment 
     Hereinafter, light emitting device  100  according to an exemplary embodiment of the present disclosure will be described with reference to  FIGS. 1A to 1D . 
       FIG. 1A  is a plan view of light emitting device  100  according to the exemplary embodiment.  FIG. 1B  is a sectional view taken along line IB-IB in  FIG. 1A .  FIG. 1C  is a sectional view taken along line IC-IC in  FIG. 1A .  FIG. 1D  is a sectional view taken along line ID-ID in  FIG. 1A . 
     As illustrated in  FIGS. 1A to 1D , light emitting device  100  according to the present exemplary embodiment includes substrate  101 , first banks  102 , second banks  103 , light emitting layers  104 , and the like formed on substrate  101 . Light emitting layers  104  include red light emitting layer  104 R that emits red light, green light emitting layer  104 G that emits green light, and blue light emitting layer  104 B that emits blue light. 
     As illustrated in  FIG. 1C , first banks  102  partition the same light emission color layer, for example, blue light emitting layer  104 B among light emitting layers  104 . As illustrated in  FIG. 1C , second banks  103  communicate with each other to connect two or more pixel regions in which the same light emission color layer, for example, blue light emitting layer  104 B are formed among the light emitting layers  104  via communicator  110 . As illustrated in  FIG. 1B , second banks  103  partition different light emission color layers, for example, red light emitting layer  104 R, green light emitting layer  104 G, and blue light emitting layer  104 B among light emitting layers  104 . 
     For example, a film thickness of blue light emitting layer  104 B among light emitting layers  104  is smaller (thinner) than a film thickness of first bank  102 , as illustrated in  FIG. 1C . A film thickness of each of red light emitting layer  104 R and green light emitting layer  104 G is also smaller (thinner) than the film thickness of first bank  102 . 
     Light emitting layer  104  is formed of ink in which an inorganic compound quantum dot material is dispersed in an organic solvent such as tetradecane having a relatively high boiling point. Specifically, in the ink, the content concentration of the quantum dot material contained in the ink for light emitting layer  104  is about 1% by weight to 5% by weight. 
     As described above, light emitting device  100  of the present exemplary embodiment is configured. 
     Hereinafter, steps of forming light emitting layer  104  of light emitting device  100  will be described. 
     First, ink in which a quantum dot material is dispersed in an organic solvent is applied on substrate  101  according to an ink jet method, and then the ink is dried. In a drying step, most of the organic solvent contained in the ink is evaporated to leave only the quantum dot material that is a solid content, and thus light emitting layer  104  is formed. 
     In this case, immediately after the ink for light emitting layer  104  is applied in a region surrounded by first banks  102  or second banks  103  according to the ink jet method, the ink having a liquid amount just before overflowing may be applied to each bank. In this case, when the ink just before overflowing from the bank is decompressed in a vacuum furnace to dry the organic solvent, the liquid amount of the ink decreases along a wall surface of each bank. Finally, light emitting layer  104  having a film thickness smaller than that of first bank  102 , for example, a film thickness of several tens of nm is formed. 
     The ink also contains an additive such as a dispersant in order to disperse the quantum dot nanoparticles in an organic solvent. Thus, depending on the additive, the surface tension of the ink becomes relatively low, for example, about 15 mN/m to 25 mN/m. In other words, the ink having a low surface tension is applied to regions surrounded by first banks  102  and second banks  103 . Consequently, a contact surface with each bank is easily wetted with the applied ink. Thus, it is possible to prevent the film thickness of light emitting layer  104  from being reduced on the sidewall surface of first bank  102  due to the low surface tension. In other words, there is no occurrence of a portion where ink cannot be applied on the anode or cathode electrode. As a result, it is possible to more reliably suppress the occurrence of a short circuit between anode and cathode electrodes. Here, in the present exemplary embodiment, for example, a highly reflective metal such as a silver-palladium-copper alloy is used as the anode. On the other hand, for example, a highly transparent material such as an indium tin oxide is used as the cathode. 
     Although not illustrated, light emitting device  100  of the present exemplary embodiment may include, in addition to light emitting layer  104 , functional thin films such as a hole injection layer, a hole transport layer, and an electron injection layer (may also be referred to as functional layers). These functional thin films are designed as appropriate to have predetermined film thicknesses in consideration of light emission efficiency or light extraction efficiency of light emitting device  100 . For example, light emitting device  100  is designed through microcavity design to efficiently extract emitted light. In order to design a microcavity, it is necessary to accurately adjust a film thickness of functional thin films. 
     The functional thin films are formed in a region defined by first banks  102 . The functional thin films are formed such that a total thickness thereof is smaller than a film thickness of first bank  102 . This is because, when the film thickness of the functional thin films is larger than the thickness of first bank  102 , the uniformity of the film thickness of the functional thin films deteriorates in a portion beyond first bank  102 . Thus, it is difficult to appropriately design the microcavity light emitting device  100 . 
     First bank  102  is made of a resin that does not contain a liquid repellent component such as fluorine. Thus, first bank  102  is relatively easily wet with the ink that forms the functional thin films. Consequently, it is possible to form uniform functional thin films in the region defined by first banks  102 . 
     Substrate  101  of light emitting device  100  may be transparent or opaque as described above, and may be made of any insulating material. In other words, substrate  101  may be made of, for example, glass or a flexible resin sheet such as polyimide. First bank  102  may be made of any insulating material. In other words, first bank  102  is made of, for example, a photosensitive resin such as acryl, epoxy, or polyimide, or an inorganic compound such as silicon oxide. Thus, first bank  102  has a property of being considerably wetted with ink. 
     On the other hand, second bank  103  may be made of any insulating material, and is made of, for example, a photosensitive resin such as acryl, epoxy, or polyimide. In the same manner as first bank  102 , second bank  103  is formed on substrate  101  provided with first bank  102  according to a photolithography method using the above material. 
     Second banks  103  are required to store ink that forms light emitting layer  104  and the like, the ink being applied according to the ink jet method. Thus, it is desirable that second bank  103  has low wettability with ink and thus has liquid repellency. Therefore, second bank  103  is made of, for example, a resin containing a functional group having fluorine atoms. Consequently, second bank  103  having the liquid repellency to the ink is formed. 
     The fluororesin is not particularly limited as long as a resin has a fluorine atom in at least some recurring units among recurring units of a polymer. Examples of the fluororesin include fluorinated polyolefin resin, fluorinated polyimide resin, and fluorinated polyacrylic resin. 
     With the above material configuration, a static contact angle of first bank  102  with respect to the ink is 5° to 30°. On the other hand, a static contact angle of second bank  103  with respect to the ink is 30° to 70°. In other words, first bank  102  is more easily wetted with ink than second bank  103 . 
     The thickness of first bank  102  is smaller (thinner) than the thickness of second bank  103 , as described above. Specifically, the thickness of second bank  103  is approximately 0.5 μm to 3.0 μm, and is preferably 0.8 μm to 1.5 μm. On the other hand, the thickness of first bank  102  is 0.1 μm to 0.5 μm, and is preferably 0.2 μm to 0.4 μm. 
     Light emitting layer  104  of light emitting device  100  of the present exemplary embodiment contains the quantum dot material as described above. Specifically, the quantum dot material contained in light emitting layer  104  is made of, for example, a material having a cadmium-selenium system, an indium-phosphorus system, a copper-indium-sulfur system, a silver-indium-sulfur system, or a perovskite structure. In the ink, the above materials are dispersed by using an organic solvent as a dispersion medium. Specifically, the ink is configured such that the concentration of the quantum dot material is 0.5% by weight to 10% by weight in the dispersion medium. 
     As described above, the quantum dot material changes its light emission color depending on a particle size of a particle, and emits red light as the particle size becomes larger. In other words, red light emitting layer  104 R, green light emitting layer  104 G, and blue light emitting layer  104 B are made of quantum dot materials having different particle sizes. The electron injection layer described above is formed by using, for example, ink in which nanoparticles such as zinc oxide are dispersed in an organic solvent. 
     Light emitting layer  104  of light emitting device  100  is configured as described above. 
     Hereinafter, a configuration of second bank  103  of light emitting device  100  of the present exemplary embodiment will be described with reference to  FIGS. 2A to 3 . 
     As described above, second banks  103  communicate with each other to connect two or more pixel regions in which light emitting layers  104  having the same light emission color are formed via communicator  110 . 
       FIGS. 2A to 2C  illustrate a state before ink is dried when the ink is applied according to the ink jet method in light emitting device  100 . Specifically,  FIG. 2A  is a plan view of light emitting device  100 .  FIG. 2B  is a sectional view taken along line IIB-IIB illustrated in  FIG. 2A .  FIG. 2C  is a sectional view taken along line IIC-IIC illustrated in  FIG. 2A .  FIG. 3  is a plan view of light emitting device  100  after the ink is dried. 
     When the ink forming light emitting layer  104  is applied according to the ink jet method, liquid droplets are applied by using ink jet head  201  having plurality of nozzles  202  as illustrated in  FIG. 2A . In this case, first, an amount of ink that exceeds the height of first bank  102  is applied. The ink applied to exceed the height of first bank  102  spreads in the pixel regions of light emitting layers  104  with the same color via communicator  110  of second bank  103 . Through the ink application, a variation between volumes of the liquid droplets ejected from respective nozzles  202  is reduced. As a result, it is possible to uniformly apply ink to adjacent pixel regions with the same color. 
     Generally, a hole diameter of each nozzle  202  of ink jet head  201  varies during processing. Thus, a volume of the liquid droplets ejected from each nozzle  202  varies by a certain amount. The variation in the ejected liquid droplet causes a variation in the film thickness of light emitting layer  104 . The variation in the film thickness affects the light emission characteristics of light emitting device  100 . Thus, it is very important to control an amount of ink applied uniformly. 
     In light emitting device  100  of the exemplary embodiment, the configuration in which communicator  110  is formed only in second bank  103  has been described as an example, but is not limited thereto. For example, the communicator may be formed in either or both of first bank  102  and second bank  103 . When the communicator is formed in both of the banks, it is preferable that a size of the communicator formed in second bank  103  is larger than a size of the communicator formed in first bank  102 . Consequently, the applied ink flows in the communication direction with communicator  110  formed in second bank  103  as a main portion. First bank  102  has a function of electrically insulating adjacent pixels and optically blocking light emitted between the adjacent pixels. Thus, it is desirable that first bank  102  does not have the communicator. 
     As illustrated in  FIG. 3 , communicators  110  of second bank  103  are formed such that sectional areas of communicators  110  via which the pixel regions arranged on the outer side of substrate  101  communicate with each other are successively reduced from communicator  110  disposed at the central part of substrate  101  in the arrangement direction (long axis direction) of the light emitting layers  104  with the same color. In other words, communicator  110  is formed such that the sectional areas in a direction perpendicular to the communication direction are gradually reduced from the central part of substrate  101  toward the outer side thereof. Typically, when the ink applied to the region surrounded by second banks  103  is dried, a solute such as the light emitting material contained in the ink moves to the outer side due to convection. This is because, in the ink applied to the region surrounded by second banks  103 , the ink located on the outer side is dried faster. Due to the movement of the ink to the outer side, the film thickness of light emitting layer  104  disposed toward the outside of the bank increases. As a result, there is concern that the film thickness of light emitting layer  104  may be non-uniform depending on a disposition location thereof. 
     Thus, in the present exemplary embodiment, a sectional area of the communicator is sequentially reduced to increase the flow path resistance such that the ink does not easily move toward the outside of the bank. This suppresses the outward movement of the solute in the ink during drying. As a result, it is possible to prevent the formation of a non-uniform film thickness of a light emitting layer and thus to form a light emitting layer having a more uniform film thickness. 
     With the device structure described above, it is possible to implement a light emitting device in which the film thickness of the light emitting layer is highly uniform by using the ink jet method. Consequently, it is possible to manufacture a display panel provided with the light emitting device having excellent light emission characteristics at low cost. 
     Method of Manufacturing Light Emitting Device of Exemplary Embodiment 
     Hereinafter, a method of manufacturing light emitting device  100  of the present exemplary embodiment will be described with reference to  FIGS. 1A to 3 . 
     The method of manufacturing light emitting device  100  of the present exemplary embodiment includes at least three steps as described below. 
     The first step is a step of forming, on substrate  101 , first banks  102  that partition pixel regions including light emitting layers that emit the same light emission color among the pixel regions are formed. The second step is a step of forming second banks  103  that partition pixel regions including light emitting layers including that emit different light emission colors among the pixel regions are formed. The third step is a step of applying ink containing a quantum dot material to the region surrounded by first banks  102  or second banks  103  to form light emitting layer  104 . 
     Hereinafter, each step will be described individually. 
     First Step 
     Hereinafter, the first step will be specifically described. 
     First, a photosensitive resin that is cured by exposure to ultraviolet light is applied onto substrate  101  by using an application method such as spin coating or slit coating. In this case, conditions for applying the photosensitive resin are adjusted depending on a required film thickness, such as a rotation speed in the spin coating and a scanning speed in the slit coating. 
     Next, a coating film of the photosensitive resin is pre-baked by using a hot plate or the like, and thus a solvent component in the photosensitive resin is evaporated such that the coating film is dried. Thereafter, the dried coating film is exposed to ultraviolet light through a photomask on which a desired pattern (corresponding to first bank  102 ) is formed. In this case, the photosensitive resin includes a negative type material in which an exposed portion irradiated with ultraviolet light is cured and a positive type material in which an unexposed portion to ultraviolet light is cured. 
     Thus, next, an uncured portion of the photosensitive resin is removed by using an appropriate developing solution according to the type of photosensitive resin material to be used. 
     Next, the photosensitive resin pattern remaining after the removal is post-baked in a curing furnace or the like. 
     First banks  102  are formed through the above first step. 
     Second Step 
     Next, the second step will be specifically described. 
     The second step is a step of forming second bank  103  on the outer side (outer periphery) of first bank  102 . 
     Specifically, similarly to first bank  102 , second bank  103  is formed through a photolithography process by using a photosensitive resin. In this case, a film thickness of second bank  103  is formed to be larger than a film thickness of first bank  102  (including a film thickness to the same degree). 
     In the above description, a description has been made of an example in which first bank  102  is formed in the first step and second bank  103  is formed in the second step, that is, the first bank and the second bank are formed in different steps, but the present disclosure is not limited thereto. First bank  102  and second bank  103  may be formed simultaneously in a single step, for example. 
     Specifically, the transmittance of, for example, a photomask with respect to ultraviolet light is locally changed, and the photosensitive resin applied to substrate  101  is half-etched. Consequently, patterns of first bank  102  and second bank  103  having different film thicknesses can be simultaneously formed. 
     For example, in a case where a photosensitive resin of a negative type material is used, an amount of transmitted ultraviolet light is reduced in a portion of which a film thickness is desired to be small. Consequently, since the degree of curing is reduced in a portion where an exposure amount is small, a large amount of the photosensitive resin is etched by a developing solution. 
     Through the above method, first bank  102  and second bank  103  having different film thicknesses can be simultaneously formed in a single step. As a result, productivity can be improved. 
     Third Step 
     Next, the third step will be specifically described. 
     First, ink in which a quantum dot material is dispersed in a solvent at a predetermined concentration is applied to the region surrounded by second banks  103  according to an ink jet method. In this case, an amount of the ejected ink from nozzle  202  of ink jet head  201  is determined such that a film thickness of the applied ink after drying is a predetermined film thickness. 
     Next, the ink applied on substrate  101  is dried under reduced pressure in a drying furnace. Specifically, the internal pressure of the drying furnace is reduced by a vacuum pump such that the ink is dried. As a result, the evaporation of the solvent in the ink is promoted, and the ink is dried. Typically, in the ink ejected from ink jet head  201 , a solvent having a high boiling point is often used in order to suppress drying of the solvent when the ink is held in nozzle  202 . Thus, the solvent in the ink is hardly dried. Therefore, decompression drying is used when the coating film is dried. Consequently, the solvent having a high boiling point contained in the ink of the coating film can be efficiently evaporated. 
     Conditions for the decompression drying are, for example, an ultimate vacuum degree of several Pa and a holding time of several tens of minutes. However, the ultimate vacuum degree and holding time conditions differ depending on the boiling point of the solvent contained in the ink. Thus, the above-described decompression drying conditions are only examples, and the present disclosure is not limited to the conditions. 
     In a case of ink in which a quantum dot material is dispersed only in an ultraviolet curable resin instead of a solvent being contained in the ejected ink, the solvent may not be dried through decompression drying. 
     Next, substrate  101  on which a coating film of the ink dried under reduced pressure is formed is placed on, for example, a hot plate. The coating film is pre-baked by the hot plate under conditions of, for example, 100° C. and 5 minutes. 
     Next, the pre-baked coating film is irradiated with ultraviolet light having a wavelength of 365 nm, and thus the coating film is exposed and cured. In this case, an irradiation amount of the ultraviolet light is, for example, 200 mJ/cm 2  to 1000 mJ/cm 2 . 
     Next, the coating film that has been exposed to and cured by the ultraviolet light is post-baked in a curing furnace under the conditions of, for example, 150° C. and about 20 minutes. Consequently, light emitting layer  104  is formed. 
     As described above, light emitting device  100  of the present exemplary embodiment is manufactured. 
     Example 1 
     Hereinafter, light emitting device  100   a  related to Example 1 of light emitting device  100  of the present exemplary embodiment will be described with reference to  FIGS. 4A to 4C . 
       FIG. 4A  is a plan view of light emitting device  100   a  related to Example 1.  FIG. 4B  is a sectional view taken along line IVB-IVB in  FIG. 4A .  FIG. 4C  is a sectional view taken along line IVC-IVC in  FIG. 4A . 
     As illustrated in  FIGS. 4A to 4C , light emitting device  100   a  related to Example 1 includes reflective anode  120  formed on substrate  101  such as glass. 
     Hereinafter, a method for manufacturing light emitting device  100   a  will be described. 
     First, reflective anode  120  is formed on substrate  101 . Specifically, for example, a silver-palladium-copper alloy having a high reflectance is formed on substrate  101  according to a sputtering method. Thereafter, reflective anode  120  is formed through patterning in accordance with a pixel region by using a photolithography method. 
     Next, first banks  102  are formed to partition light emitting layers  104  with the same color. Here, light emitting layers  104  with the same color include red light emitting layer  104 R that emits red light, green light emitting layer  104 G that emits green light, and blue light emitting layer  104 B that emits blue light. In this case, it is desirable that first banks  102  are formed of a photosensitive resin such as an acrylic resin that is easily wetted with ink forming a film such as light emitting layer  104  in first bank  102  and does not contain a liquid repellent component such as fluorine. In other words, first banks  102  are formed by patterning the above material according to a photolithography method. 
     Specifically, first, the acrylic resin is applied onto substrate  101  through slit coating, and is pre-baked at 80° C. for 30 minutes on a hot plate. Thereafter, the acrylic resin is cured by applying ultraviolet light having a wavelength of 365 nm. In this case, an exposure amount is 500 mJ/cm 2 . 
     Next, the acrylic resin cured by the ultraviolet light is developed. The development is performed through spray coating for 60 seconds by using, for example, a developing solution such as Na 2 CO 3  of 1% by weight. 
     Next, the developed acrylic resin is post-baked at 150° C. for 60 minutes by using a heating furnace. 
     Next, second banks  103  are formed to partition pixel regions with different light emission colors. Second banks  103  are linearly formed to include two or more pixel regions in which the light emitting layers with the same color partitioned by first banks  102  are formed. In this case, second bank  103  is formed by using a fluorine-containing acrylic resin containing fluorine. 
     Specifically, second bank  103  is formed according to the photolithography method in the same manner as first bank  102 . In this case, a material having a feature that fluorine is unevenly distributed on a surface thereof through exposure is used as the fluorine-containing acrylic resin. Consequently, second bank  103  having a lyophilic side surface and a lyophobic top is formed. In this case, a static contact angle of second bank  103  with respect to the ink is about 50°. A film thickness of first bank  102  is 0.3 μm, and a film thickness of second bank  103  is 1.0 μm. 
     Next, hole injection layer  130  is formed on reflective anode  120  in a pixel region formed by first bank  102  and second bank  103 . Ink forming hole injection layer  130  is formed by dissolving 2.0% by weight of a solid content such as polyethylenedioxythiophene/polystyrenesulfonic acid (PEDOT/PSS) in an alcohol solvent. The ink formed as described above is applied to the pixel region according to an ink jet method. In this case, the ink is applied from the nozzle in an ejection amount such that a film thickness of the solvent contained in the ink after drying is 50 nm. The solvent is dried through vacuum drying in which a furnace is depressurized with a vacuum pump. The vacuum drying is performed, for example, at the vacuum degree of several Pa for a holding time of 15 minutes. 
     Next, red light emitting layer  104 R, green light emitting layer  104 G, and blue light emitting layer  104 B forming light emitting layers  104  are formed on hole injection layer  130 . Specifically, as the ink forming light emitting layer  104 , ink in which a cadmium-selenium-based quantum dot material is dispersed in a linear aliphatic organic solvent in a concentration of 2.5% by weight is used. A quantum dot material having a particle size of 10 to 30 nm is used. The ink is applied to the region surrounded by second banks  103  according to an ink jet method. In this case, the ink is applied to cover first banks  102 . Thus, when the applied ink is wet, first bank  102  is also coated with the ink. In other words, in the formation of light emitting layer  104 , in order to apply the ink in the above-described form, it is desirable that first bank  102  has a contact angle as low as possible and is easily wetted with the ink, and is further lyophilic. 
     As illustrated in  FIG. 4A , the ink is printed in a direction perpendicular to the long axis direction of second bank  103  in which the pixels of red light emitting layer  104 R, green light emitting layer  104 G, and blue light emitting layer  104 B are arranged. In other words, nozzles  202  of ink jet head  201  are provided to be arranged in a direction in which pixels including light emitting layers  104  with the same light emission color are arranged. Thus, regarding an ink landing position during ejection, the ink can be relatively easily landed at a predetermined position by adjusting an ejection timing in the printing direction. 
     However, it is hard to correct an ink landing position in the arrangement direction of nozzles  202  (long axis direction), and, thus, as described above, the ink landing position depends on the processing accuracy (in particular, a hole diameter) of nozzle  202 . Therefore, a region where ink can be landed is widened in the arrangement direction of the nozzles  202 . Consequently, it is possible to increase an allowed fluctuation range of an ink landing position. 
     A plurality of nozzles  202  are disposed in the ink application region. Consequently, even though a certain nozzle  202  cannot eject ink due to clogging of foreign substances or the like, it is possible to supplement the ink with ink ejected from the adjacent nozzle  202 . 
     A pixel region including two or more light emitting layers  104  with the same color is defined by second banks  103 . Thus, ink forming light emitting layer  104  such as red light emitting layer  104 R can be applied within second banks  103  by using plurality of nozzles  202 . Consequently, it is possible to average variations in volumes of liquid droplets of ink ejected from nozzles  202  of ink jet head  201 . 
     Next, the solvent in the ink forming light emitting layers  104  is dried through vacuum drying. In this case, the vacuum drying was performed under the conditions that the vacuum degree was several Pa and the drying time was 20 minutes. Consequently, the solvent in the ink is dried after vacuum drying, and a film thickness of the ink becomes smaller than that of first bank  102 . Thus, the ink covering first bank  102  also disappears immediately after the above-described application. 
     Next, electron injection layer  140  is formed after light emitting layers  104  are formed. For example, ink in which zinc oxide nanoparticles are dispersed in an alcohol-based organic solvent is used to form electron injection layer  140 . The nanoparticles dispersed in the organic solvent have a particle size of 5 nm to 20 nm, and the concentration of nanoparticles in the ink is 3.0% by weight. The ink formed in the above-described way is applied onto red light emitting layer  104 R, green light emitting layer  104 G, and blue light emitting layer  104 B according to an ink jet method. 
     Next, after the ink is applied, the ink is dried through vacuum drying such that the solvent in the ink is dried in the same manner as in hole injection layer  130  and light emitting layer  104 . 
     Finally, as illustrated in  FIGS. 4B and 4C , transparent electrode  150  made of, for example, indium tin oxide is formed on the entire surface of substrate  101  on which the functional films (functional layers) are formed. 
     In this case, it is desirable that second bank  103  is formed in a shape with a smoothly rounded corner or a shape with a low taper angle. Consequently, the coverage of transparent electrode  150  on second bank  103  can be improved. 
     In the present exemplary embodiment, the microcavity design improves the light emission characteristics of the light emitting device by utilizing the microcavity effect of efficiently extracting light emitted from light emitting layer  104  to the outside. Here, the microcavity effect is an effect of enhancing a light emission color by adjusting a film thickness of light emitting layer  104 , hole injection layer  130 , or the like to resonate and emphasize light with a specific wavelength. 
     The above-described microcavity design is performed by controlling film thicknesses of functional films such as hole injection layer  130 , light emitting layer  104 , and electron injection layer  140 , and thus the uniformity of these film thicknesses is considerably important. Thus, in the microcavity design, a total thickness of laminated films of these functional layers is preferably smaller than a thickness of first bank  102 . This is because, when these laminated films are formed to exceed the thickness of first bank  102 , a film shape becomes non-uniform in the exceeded portion. As a result, it is difficult to control film thicknesses of these laminated films. 
     Light emitting device  100   a  having the structure and manufactured according to the manufacturing method and the display panel including the same have high film thickness uniformity. Consequently, it is possible to implement a display panel having excellent light emission characteristics. 
     Example 2 
     Hereinafter, light emitting device  100   b  related to Example 2 of light emitting device  100  of the present exemplary embodiment will be described with reference to  FIGS. 5A to 5C . 
       FIG. 5A  is a plan view of light emitting device  100   b  related to Example 2.  FIGS. 5B and 5C  are respectively sectional views taken along lines VB-VB and VC-VC in  FIG. 5A . 
     As illustrated in  FIGS. 5A to 5C , a structure of light emitting device  100   b  related to Example 2 is different from the structure of light emitting device  100   a  of Example 1 in that unevenness  160  including one of a protrusion step and a depression step is formed on second banks  103  that partition the pixel regions with different light emission colors. 
     In this case, unevenness  160  is formed in a stepped shape and includes protrusion step  160   a  illustrated in  FIG. 5B  or depression step  160   b  illustrated in  FIG. 5C . A height of protrusion step  160   a  or a depth of depression step  160   b  is, for example, about 100 nm to 200 nm. 
     Protrusion step  160   a  and depression step  160   b  are formed according to a photolithography method by using, for example, photomasks having different transmittances. 
     As described above, ink in which the nanoparticles such as a quantum dot material are dispersed is added with a dispersant such as a surfactant for dispersing the nanoparticles, or various additives for improving the dispersion stability. A bank may have high wettability with such ink. Specifically, ink forming red light emitting layer  104 R, green light emitting layer  104 G, and blue light emitting layer  104 B using the quantum dot material has 5° to 20° as a receding contact angle with respect to second bank  103 . In contrast, ink in which a polymer is dissolved, for example, ink forming a light emitting layer of organic EL has about 25° to 40° as a receding contact angle. In other words, the receding contact angle of the ink in which the nanoparticles are dispersed is lower than the receding contact angle of the ink in which the polymer is dissolved. However, in a case where the receding contact angle is low, for example, when ink for red light emitting layer  104 R is applied to a region surrounded by second banks  103  and then a solvent that is a dispersion medium is dried, the ink may remain on the top of second bank  103 . Depending on the type of ink, the dispersion medium may be a photosensitive resin. In the case of this ink, after the ink is exposed to light to be cured and contracted, the ink may remain on the top of second bank  103 . 
     Hereinafter, with reference to  FIGS. 6A to 6C , a description will be made of a residue of ink after the ink is applied and dried. 
       FIG. 6A  is a sectional view illustrating a state before ink is dried immediately after ink for red light emitting layer  104 R, ink for green light emitting layer  104 G, and ink for blue light emitting layer  104 B are applied to regions surrounded by second banks  103  in light emitting device  100   b  of Example 2.  FIG. 6B  is a sectional view illustrating a state after the ink illustrated in  FIG. 6A  is dried. 
     As illustrated in  FIG. 6A , the applied ink is applied separately by using protrusion step  160   a  disposed on second bank  103  as a boundary. 
     When the applied ink is dried through vacuum drying, the state illustrated in  FIG. 6B  is obtained. In this case, since the receding contact angle of the ink is low, residues of the ink are present on second bank  103  as illustrated in  FIG. 6B . Specifically, the residues include residue  104 R′ of the ink for red light emitting layer  104 R, residue  104 G′ of the ink for green light emitting layer  104 G, and residue  104 B′ of the ink for blue light emitting layer  104 B. 
     In a case where the ink remains on the top of second bank  103 , when ink with a different color is applied, for example, when the ink for green light emitting layer  104 G is applied to the pixel region adjacent to red light emitting layer  104 R, color mixing may occur due to the remaining ink on second bank  103 . In other words, for example, in  FIG. 6 , in a case where red ink is applied and then green ink is applied to the adjacent pixel region, when there is the residue of the red ink on second bank  103 , wettability increases in a wet residue of the ink. Thus, when the green ink is applied, the repellency (liquid repellency) to the green ink becomes weak, and thus the green ink may flow into the red pixel region to cause color mixing. 
     Therefore, in light emitting device  100   b  of Example 2, stepped unevenness  160  is provided on second bank  103 . Consequently, in adjacent pixel regions, color mixing due to ink for light emitting layers with different colors can be suppressed more reliably. 
     As illustrated in  FIG. 6C , when the depression step  160   b  is provided on second bank  103 , ink applied on second bank  103  stops spreading at the edge of depression step  160   b  due to surface tension. Thus, it is possible to prevent color mixing of ink in adjacent pixel regions. 
     As described above, with the structure of light emitting device  100   b  of Example 2, it is possible to apply ink in which the quantum dot material having high film thickness uniformity and high wettability is dispersed without causing color mixing. Consequently, it is possible to provide a display panel having excellent light emission characteristics. 
     In light emitting device  100   b  of Example 2, although not particularly illustrated in order to describe the effect, when a display panel is manufactured by using light emitting device  100   b,  in the same manner as in Example 1, needless to say, a reflective anode, a hole injection layer, an electron injection layer, a transparent electrode, and the like are formed. 
     Example 3 
     Hereinafter, light emitting device  100   c  related to Example 3 of light emitting device  100  of the present exemplary embodiment will be described with reference to  FIGS. 7A and 7B . 
       FIG. 7A  is a plan view of light emitting device  100   c  related to Example 3.  FIG. 7B  is a sectional view taken along line VIIB-VIIB in  FIG. 7A . 
     As illustrated in  FIGS. 7A and 7B , light emitting device  100   c  related to Example 3 is different from light emitting device  100   b  related to Example 2 in that fine uneven structure  160   c  is provided on the top of second bank  103 . 
     Uneven structure  160   c  has a height in the order of nanometers, specifically, a height of several nanometers to several tens of nanometers. By forming uneven structure  160   c  with such a surface shape, a phenomenon called super water repellency appears between a liquid and a solid. Consequently, a contact angle of the liquid is increased. 
     In other words, in a configuration of the related art, the receding contact angle can be increased even with ink having a low receding contact angle on second bank  103 . Consequently, it is possible to suppress a residue of the ink on second bank  103  and thus to more reliably prevent color mixing between different colors applied to adjacent pixel regions. 
     Example 4 
     Hereinafter, light emitting device  100   d  related to Example 4 of light emitting device  100  of the present exemplary embodiment will be described with reference to  FIGS. 8A to 8D . 
       FIG. 8A  is a plan view of light emitting device  100   d  related to Example 4.  FIG. 8B  is a sectional view taken along line VIIIB-VIIIB in  FIG. 8A .  FIG. 8C  is a sectional view taken along line VIIIC-VIIIC in  FIG. 8A .  FIG. 8D  is a sectional view taken along line VIIID-VIIID in  FIG. 8A . 
     As illustrated in  FIGS. 8A to 8D , light emitting device  100   d  related to Example 4 is different from light emitting device  100   a  related to Example 1 in that first banks  102  communicate with each other to connect two or more pixel regions of light emitting layers with the same color via communicator  110 . Light emitting device  100   d  related to Example 4 is different from light emitting device  100   a  related to Example 1 in that second banks  103  are formed to partition pixel regions of light emitting layers with the same color and pixel regions of light emitting layers with different colors. In other words, the pixel regions of the light emitting layers with the same color are not connected to each other via second bank  103 , and the pixel regions of the light emitting layers with different colors are not connected with each other via second bank  103  either. 
     With the configuration of light emitting device  100   d,  ink applied according to an ink jet method spreads in pixel regions of light emitting layers with the same color via only communicator  110  of first bank  102 . Consequently, it is possible to improve the film thickness uniformity of light emitting layers in the same manner as in Example 1. 
     Example 5 
     Hereinafter, light emitting device  100   e  related to Example 5 of light emitting device  100  of the present exemplary embodiment will be described with reference to  FIGS. 9A to 9C . 
       FIG. 9A  is a plan view of light emitting device  100   e  related to Example 5.  FIG. 9B  is a sectional view taken along line IXB-IXB in  FIG. 9A .  FIG. 9C  is a sectional view taken along line IXC-IXC in  FIG. 9A . 
     As illustrated in  FIGS. 9A to 9C , light emitting device  100   e  related to Example 5 is different from light emitting device  100   a  related to Example 1 in that first banks  102  are disposed in not only a direction in which pixel regions of light emitting layers with the same color are arranged but also a direction in which pixel regions of light emitting layers with different colors are arranged. 
     With the configuration of light emitting device  100   e,  the entire circumference of each light emitting region of light emitting layers with the same color and different colors is surrounded by first banks  102 . Consequently, it is possible to improve the film thickness uniformity of light emitting layers in the same manner as in Example 1. 
     Example 6 
     Hereinafter, light emitting device  100   f  related to Example 6 of light emitting device  100  of the present exemplary embodiment will be described with reference to  FIGS. 10A to 10C . 
       FIG. 10A  is a plan view of light emitting device  100   f  related to Example 6.  FIG. 10B  is a sectional view taken along line XB-XB in  FIG. 10A .  FIG. 10C  is a sectional view taken along line XC-XC in  FIG. 10A . 
     As illustrated in  FIGS. 10A to 10C , light emitting device  100   f  related to Example 6 is different from light emitting device  100   e  of Example 5 that is made of a light emitting material that emits light through photoexcitation in that red light emitting layer  104 R, green light emitting layer  104 G, and blue light emitting layer  104 B are made of a material that emits light through electric field excitation. 
     In the case of a light emitting material that emits light through photoexcitation, light emitting layer  104  is formed to have a film thickness of about 5 μm to 10 μm. The reason is that, in a case of a light emitting device using a photoexcitation material, a blue LED may be used as a photoexcitation light source. However, the efficiency of converting blue as a light emission color into red or green is low. Therefore, light emitting layer  104  is made thicker such that the light emission efficiency is ensured. 
     A composition of ink forming light emitting layer  104  is a photosensitive acrylic resin or an epoxy resin that is cured by light instead of that of ink in which quantum dots are dispersed in an organic solvent as in Examples 1 to 4. In this case, the ink contains not only a light emitting material such as the quantum dots but also a scattering agent having a light scattering effect. Specifically, the scattering agent is, for example, particles of titanium oxide. 
     Light emitting device  100   f  made of the above material functions as a color conversion device. Thus, light emitting device  100   f  can be used as, for example, a color filter of a micro LED display. 
     In this case, light emitting device  100   f  is used by being bonded to a substrate on which blue LEDs are arranged. Consequently, it is possible to improve the film thickness uniformity of light emitting layers in the same manner as in Examples 1 to 5.