Patent Publication Number: US-2023147514-A1

Title: Light-emitting element, and display device

Description:
TECHNICAL FIELD 
     The present invention relates to a light-emitting element and a display device. 
     BACKGROUND ART 
     In recent years, a variety of flat panel displays have been developed, and in particular, a display device that includes a Quantum dot Light Emitting Diode (QLED) or an Organic dot Light Emitting Diode (OLED) as an electroluminescent element has attracted attention. 
     PTL 1 relates to a vertical resonance type surface emitting laser in which a light-emitting layer including quantum dots is used. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP 2006-229194 A (published on Aug. 31, 2006) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The light emission line width of one quantum dot is very narrow. On the other hand, the light emission line width of a plurality of quantum dots is wider than the light emission line width of one quantum dot due to dispersion of granularities, composition ratios, and the like. A light-emitting element including a quantum dot typically includes a plurality of quantum dots. 
     Thus, conventional light-emitting elements including quantum dots have a problem in that the light emission line width thereof is wide. 
     In PTL 1, the principles of a laser, that is, induced emission and resonance, are used in order to solve this problem. 
     The present invention has been made in view of the above problem, and an object of the present invention is to narrow a light emission line width of a light-emitting element including quantum dots by using another method. 
     Solution to Problem 
     In order to solve the problem described above, a light-emitting element according to an aspect of the present invention includes a reflective electrode, a transparent electrode, a light-emitting layer provided between the reflective electrode and the transparent electrode, the light-emitting layer including quantum dots, and a selectively reflective layer provided at an opposite side to the light-emitting layer with respect to the transparent electrode, the selectively reflective layer having a reflection band having a higher reflectivity than a reflectivity of another band, and a wavelength at a long wavelength end in the reflection band of the selectively reflective layer is longer than a wavelength at which a light emission spectrum due to electroluminescence of the quantum dots has a half value of a peak value of the light emission spectrum due to the electroluminescence of the quantum dots at a shorter wavelength side than a peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and is shorter than a wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots at a longer wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots. 
     In order to solve the problem described above, a light-emitting element according to an aspect of the present invention includes a first transparent electrode, a second transparent electrode, a light-emitting layer provided between the first transparent electrode and the second transparent electrode, the light-emitting layer including quantum dots, a first selectively reflective layer provided at an opposite side to the light-emitting layer with respect to the first transparent electrode, the first selectively reflective layer having a reflection band having a higher reflectivity than a reflectivity of another band, and a second selectively reflective layer provided at an opposite side to the light-emitting layer with respect to the second transparent electrode, the second selectively reflective layer having a reflection band having a higher reflectivity than a reflectivity of another band, a wavelength at a long wavelength end in the reflection band of the first selectively reflective layer is longer than a wavelength at which a light emission spectrum due to electroluminescence of the quantum dots has a half value of a peak value of the light emission spectrum due to the electroluminescence of the quantum dots at a shorter wavelength side than a peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and is shorter than a wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots at a longer wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and a wavelength at a long wavelength end in the reflection band of the second selectively reflective layer is longer than the wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots at the shorter wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and is shorter than the wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots at the longer wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots. 
     Advantageous Effects of Invention 
     With the light-emitting element according to the aspect of the present invention, a light emission line width of the light-emitting layer including the quantum dots can be narrowed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a flowchart illustrating an example of a manufacturing method for a display device. 
         FIG.  2    is a cross-sectional view illustrating an example of a configuration of a display region of a display device. 
         FIG.  3    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer in a display device according to a first embodiment of the present invention. 
         FIG.  4    is a schematic cross-sectional view illustrating reflection and transmission in the light-emitting element layer in a blue pixel illustrated in  FIG.  3   . 
         FIG.  5    is a diagram illustrating, on the upper side, a graph showing a light emission spectrum due to electroluminescence of a light-emitting element layer according to a comparative example, and illustrating, on the lower side, a graph showing a light emission spectrum due to electroluminescence from the example of the light-emitting element layer illustrated in  FIG.  4   . 
         FIG.  6    is a diagram illustrating, on the upper side, a graph showing a light emission spectrum due to electroluminescence of the light-emitting element layer according to the comparative example, and illustrating, on the lower side, a graph showing a light emission spectrum due to electroluminescence from another example of the light-emitting element layer  5  illustrated in  FIG.  4   . 
         FIG.  7    is a cross-sectional view illustrating a schematic configuration of an example in which a selectively reflective layer illustrated in  FIG.  3    is a dielectric multilayer film. 
         FIG.  8    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer according to a modified example of the first embodiment of the present invention. 
         FIG.  9    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer according to another modified example of the first embodiment of the present invention. 
         FIG.  10    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer in a display device according to a second embodiment of the present invention. 
         FIG.  11    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer according to a modified example of the second embodiment of the present invention. 
         FIG.  12    is a cross-sectional view illustrating another example of a configuration of a display region of the display device. 
         FIG.  13    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer in a display device according to a third embodiment of the present invention. 
         FIG.  14    is a schematic cross-sectional view illustrating reflection and transmission in the light-emitting element layer in a blue pixel illustrated in  FIG.  13   . 
         FIG.  15    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer in a display device according to a fourth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. However, shapes, dimensions, relative arrangements, and the like illustrated in the drawings are merely exemplary, and the scope of the present invention should not be construed as limiting due to these. 
     A display device  2  according to a first embodiment of the present invention is a one-sided light-emitting type. 
     Manufacturing Method of Display Device and Configuration Thereof 
     In the following description, the “same layer” means that it is formed through the same process (film formation step), the “lower layer” means that it is formed through a process before that of the compared layer, and the “upper layer” means that it is formed through a process after that of the compared layer. 
       FIG.  1    is a flowchart illustrating an example of a manufacturing method of a display device.  FIG.  2    is a schematic cross-sectional view illustrating an example of a configuration of a display region of the display device  2 . 
     In a case where a flexible display device is manufactured, as illustrated in  FIG.  1    and  FIG.  2   , first, a resin layer  12  is formed on a support substrate (a mother glass, for example) having transparency (step S 1 ). Next, a barrier layer  3  is formed (step S 2 ). Next, a thin film transistor layer (TFT layer)  4  is formed (step S 3 ). Next, a light-emitting element layer  5  of a top-emitting type is formed (step S 4 ). Next, a sealing layer  6  is formed (step S 5 ). Next, an upper face film is bonded on the sealing layer  6  (step S 6 ). 
     Next, the support substrate is peeled from the resin layer  12  due to irradiation with a laser light or the like (step S 7 ). Next, a lower face film  10  is bonded to the lower face of the resin layer  12  (step S 8 ). Next, a layered body including the lower face film  10 , the resin layer  12 , the barrier layer  3 , the thin film transistor layer  4 , the light-emitting element layer  5 , and the sealing layer  6  is divided to obtain a plurality of individual pieces (step S 9 ). Next, a function film  39  is bonded to the obtained individual piece (step S 10 ). Next, an electronic circuit board (for example, an IC chip or a Flexible Printed Circuit (FPC)) is mounted at a part (terminal portion) of a region (a non-display region or a frame region) positioned further outward than a display region where a plurality of subpixels are formed (step S 11 ). Note that steps S 1  to S 11  are executed by a display device manufacturing apparatus (including a film formation apparatus that executes the process from steps S 1  to S 5 ). 
     Examples of the material of the resin layer  12  include polyimide and the like. A portion of the resin layer  12  can be replaced with two layers of resin films (for example, polyimide films) and an inorganic insulating film sandwiched therebetween. 
     The barrier layer  3  is a layer that inhibits foreign matter such as water and oxygen from entering the thin film transistor layer  4  and the light-emitting element layer  5 . For example, the barrier layer can be constituted of a silicon oxide film, a silicon nitride film, or a silicon oxynitride film, or a layered film thereof formed by Chemical Vapor Deposition (CVD). 
     The thin film transistor layer  4  includes a semiconductor film  15 , an inorganic insulating film  16  (gate insulating film) that is an upper layer than the semiconductor film  15 , a gate electrode GE and a gate wiring line GH 1  that are upper layers than the inorganic insulating film  16 , an inorganic insulating film  18  (interlayer insulating film) that is an upper layer than the gate electrode GE and the gate wiring line GH, a capacitance electrode CE that is an upper layer than the inorganic insulating film  18 , an inorganic insulating film  20  (interlayer insulating film) that is an upper layer than the capacitance electrode CE, a source wiring line SH that is an upper layer than the inorganic insulating film  20 , and a flattening film  21  (interlayer insulating film) that is an upper layer than the source wiring line SH. 
     The semiconductor film  15  is formed of low-temperature polysilicon (LTPS) or an oxide semiconductor (for example, an In—Ga—Zn—O based semiconductor), for example.  FIG.  2    illustrates the transistor that has a top gate structure, but the transistor may have a bottom gate structure. 
     The gate electrode GE, the gate wiring line GH and the capacitance electrode CE, and the source wiring line SH are each constituted by, for example, a single layer film containing at least one of aluminum, tungsten, molybdenum, tantalum, chromium, titanium, and copper, or a layered film thereof. 
     The inorganic insulating films  16 ,  18 , and  20  can be constituted by, for example, a silicon oxide (SiOx) film, a silicon nitride (SiNx) film, or a silicon oxynitride (SiNO) film formed by CVD, or a layered film thereof. The flattening film  21  can be constituted by a coatable organic material such as polyimide or acrylic. 
     The light-emitting element layer  5  includes a cathode  25  (cathode electrode, or so-called pixel electrode) provided as an upper layer than the flattening film  21 , an edge cover  23  having an insulating property and covering an edge of the cathode  25 , an active layer  24  that is an ElectroLuminescence (EL) layer provided as an upper layer than the edge cover  23 , and an anode  22  (anode electrode, or so-called common electrode) provided as an upper layer than the active layer  24 , and further includes a selectively reflective layer  40  provided as an upper layer than the anode  22 . The edge cover  23  is formed by applying an organic material such as polyimide or acrylic and then patterning the organic material by photolithography, for example. 
     For each subpixel, a light-emitting element ES (electroluminescent element) including the cathode  25  having an island shape, the active layer  24 , and the anode  22  and being a QLED is formed in the light-emitting element layer  5 , and a subpixel circuit for controlling the light-emitting element ES is formed in the thin film transistor layer  4 . 
     For example, the active layer  24  is constituted by layering an electron injection layer, an electron transport layer, a light-emitting layer including quantum dots, a hole transport layer, and a hole injection layer in this order, from the lower layer side. The light-emitting layer is formed, together with the hole transport layer, into an island shape at an opening of the edge cover  23  (for each subpixel) by photolithography. Other layers are formed in an island shape or a solid-like shape (common layer). In addition, it is also possible to adopt a configuration in which one or more layers among the electron injection layer, the electron transport layer, the hole transport layer, and the hole injection layer are not formed. 
     A material to be used for the hole injection layer is not particularly limited as long as the material is a hole injection material capable of stabilizing the injection of positive holes into the light-emitting layer. Examples of the hole injection material include conductive polymers such as arylamine derivatives, porphyrin derivatives, phthalocyanine derivatives, carbazole derivatives, polyaniline derivatives, polythiophene derivatives, and polyphenylene vinylene derivatives. Note that the material to be used for the hole injection layer is more preferably poly (3,4-ethylenedioxythiophene)-polystyrene sulfonic acid (PEDOT-PSS). The PEDOT-PSS improves the efficiency of light emission resulting from recombination of electrons and positive holes in a quantum dot light-emitting layer, and thus exhibits the effect of improving the light-emission characteristics of an electroluminescent element. 
     A constituent material of the hole transport layer is not particularly limited as long as the material is a hole transport material capable of stabilizing the transport of the positive holes into the quantum dot light-emitting layer  3 . The hole transport material preferably has high hole mobility. Furthermore, the hole transport material is preferably a material (electron blocking material) capable of preventing the penetration of electrons that have traveled from the cathode electrode. This makes it possible to increase a recombination efficiency of the holes and the electrons within the light-emitting layer. 
     Examples of materials to be used for the hole transport layer include arylamine derivatives, anthracene derivatives, carbazole derivatives, thiophene derivatives, fluorene derivatives, distyrylbenzene derivatives, and spiro compounds. Note that materials to be used for the hole transport layers  2 R and  2 G are more preferably polyvinyl carbazole (PVK) or poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)) diphenylamine)] (TFB). The PVK and the TFB improve the efficiency of light emission resulting from recombination of the electrons and the positive holes in the quantum dot light-emitting layer, and thus exhibit the effect of improving the light-emission characteristics of the electroluminescent element. 
     Examples of the method of forming the hole transport layer and the hole injection layer include vapor deposition, a printing method, an ink-jet method, a spin coating method, a casting method, a dipping method, a bar coating method, a blade coating method, a roll coating method, a gravure coating method, a flexographic printing method, a spray coating method, a photolithography method, and a self-organization method (layer-by-layer method, self-assembled monolayer method), but the method is not limited thereto. Among these, the vapor deposition, the spin coating method, the ink-jet method, or the photolithography method is preferably used. 
     The quantum dots may include one or a plurality of semiconductor materials selected from a group including Cd, S, Te, Se, Zn, In, N, P, As, Sb, Al, Ga, Pb, Si, Ge, Mg, and compounds thereof. The quantum dots may be a two-component core type, a three-component core type, a four-component core type, a core-shell type, or a core multi-shell type. Further, the quantum dots may include doped nanoparticles, or may include a compositionally graded structure. 
     A method of forming the light-emitting layer is not particularly limited as long as the method is capable of forming a fine pattern required for the electroluminescent element. Examples include vapor deposition, a printing method, an ink-jet method, a spin coating method, a casting method, a dipping method, a bar coating method, a blade coating method, a roll coating method, a gravure coating method, a flexographic printing method, a spray coating method, a photolithography method, and a self-organization method (layer-by-layer method, self-assembled monolayer method). Among these, the vapor deposition, the spin coating method, the ink-jet method, or the photolithography method is preferably used. Additionally, a thickness of the light-emitting layer is not particularly limited as long as the thickness is capable of expressing a function of providing a place for recombination between the electrons and the positive holes to achieve light emission, and can be, for example, from about 1 nm to 200 nm. 
     Examples of the vapor deposition include a vacuum vapor deposition technique, a sputtering method, and an ion plating method, and specific examples of the vacuum vapor deposition technique include resistance heating vapor deposition, flash vapor deposition, arc vapor deposition, laser vapor deposition, high frequency heating vapor deposition, and electron beam vapor deposition. 
     When the light-emitting layer is formed by application of a coating liquid such as a spin coating method or an ink-jet method, a solvent of the coating liquid is not particularly limited as long as the solvent can dissolve or disperse the constituent material of the light-emitting layer, and examples thereof include toluene, xylene, cyclohexanone, cyclohexanol, tetralin, mesitylene, methylene chloride, tetrahydrofuran, dichloroethane, and chloroform. 
     The active layer  24  may further include an intermediate layer between the light-emitting layer and the electron transport layer. 
     The cathode  25  is a reflective electrode that is constituted by layering, for example, Indium Tin Oxide (ITO) and silver (Ag) or an alloy containing Ag, or constituted by a material containing Ag or Al and that has light reflectivity. The anode  22  is a transparent electrode constituted by a thin film of Ag, Au, Pt, Ni, or Ir, a thin film of an MgAg alloy, or a conductive material having transparency such as ITO, or Indium Zinc Oxide (IZO). When the display device is not a top-emitting type display device but is a bottom-emitting type display device, the lower face film  10  and the resin layer  12  have transparency, the cathode  25  is a transparent electrode, and the anode  22  is a reflective electrode. 
     The selectively reflective layer  40  has a reflection band having a higher reflectivity than those of other bands. Details will be described below. 
     In the light-emitting element ES, the positive holes and the electrons recombine inside the light-emitting layer in response to a drive current between the anode  22  and the cathode  25 , and when excitons generated due to this recombination transition from the Lowest Unoccupied Molecular Orbital (LUMO) or the conduction band to the Highest Occupied Molecular Orbital (HOMO) or the valence band of the quantum dots, light is emitted. 
     The sealing layer  6  has transparency, and includes an inorganic sealing film  26  for covering the anode  22 , an organic buffer film  27  provided as an upper layer than the inorganic sealing film  26 , and an inorganic sealing film  28  provided as an upper layer than the organic buffer film  27 . The sealing layer  6  covering the light-emitting element layer  5  inhibits foreign matters such as water and oxygen from penetrating the light-emitting element layer  5 . 
     Each of the inorganic sealing film  26  and the inorganic sealing film  28  is an inorganic insulating film and can be formed of, for example, a silicon oxide film, a silicon nitride film, or a silicon oxynitride film, or a layered film of these, formed by CVD. The organic buffer film  27  is a transparent organic film having a flattening effect and can be formed of a coatable organic material such as an acrylic. The organic buffer film  27  can be formed, for example, by ink-jet application, and a bank for stopping droplets may be provided in a non-display region. 
     The lower face film  10  is, for example, a PET film bonded to a lower face of the resin layer  12  after the support substrate is peeled, to achieve a display device having excellent flexibility. The function film  39  has at least one of an optical compensation function, a touch sensor function, and a protection function, for example. 
     The display device being flexible has been described above, but when the display device is manufactured as a display device being non-flexible, because formation of the resin layer, replacement of the base material and the like are typically not required, processing proceeds to step S 9  after the layering process on the glass substrate of steps S 2  to S 5  is executed, for example. Furthermore, when a display device being non-flexible is manufactured, a sealing member having transparency may be caused to adhere using a sealing adhesive instead of or in addition to forming the sealing layer  6  under a nitrogen atmosphere. The sealing member having transparency can be formed from glass, plastic, or the like, and preferably has a concave shape. 
     An embodiment of the present invention relates specifically to the light-emitting element layer  5  of the configuration of the display device described above. 
     Configuration of Light-Emitting Element Layer 
       FIG.  3    is a cross-sectional view illustrating a schematic configuration of the light-emitting element layer  5  in the display device  2  according to the first embodiment of the present invention. 
     As illustrated in  FIG.  3   , the display device according to the first embodiment of the present invention includes a red pixel Pr (light-emitting element) including a red pixel electrode PEr, a green pixel Pg (light-emitting element) including a green pixel electrode PEg, and a blue pixel Pb (light-emitting element) including a blue pixel electrode PEb. 
     The light-emitting element layer  5  according to the first embodiment of the present invention includes the cathodes  25  as the green pixel electrode PEg, the blue pixel electrode PEb, and the red pixel electrode PEr. The cathode  25  is a reflective electrode. 
     The light-emitting element layer  5  includes the edge cover  23  having an insulating property and covering the edges of the cathodes  25 . The edge cover  23  is a light-blocking body that blocks light among the red pixel Pr, the green pixel Pg, and the blue pixel Pb. 
     The light-emitting element layer  5  includes the active layer  24  that is an ElectroLuminescence (EL) layer provided as an upper layer than the edge cover  23 . 
     The active layer  24  includes an electron transport layer  33 . The electron transport layer  33  is formed covering the cathode  25 . The electron transport layer  33  may be a single layer structure or multilayer structure. The electron transport layer  33  may be separately or commonly formed for the red pixel Pr, the green pixel Pg, and the blue pixel Pb. In a case of being separately formed, the electron transport layer  33  provided in the red pixel Pr, the electron transport layer  33  provided in the green pixel Pg, and the electron transport layer  33  provided in the blue pixel Pb may have different film thicknesses and/or compositions from each other. The active layer  24  may include an electron injection layer formed between the electron transport layer  33  and the cathode  25 . 
     The active layer  24  includes a red light-emitting layer  35   r  formed in an island shape in the red pixel Pr. The red light-emitting layer  35   r  includes a plurality of red quantum dots that emit red light. A peak wavelength of the light emission spectrum due to the electroluminescence of the red quantum dots is equal to or greater than 600 nm and equal to or less than 780 nm. Note that, in order to improve the color reproduction range of the display device, the peak wavelength of the light emission spectrum of the red pixel Pr is preferably equal to or greater than 620 nm and equal to or less than 650 nm. 
     The active layer  24  includes a green light-emitting layer  35   g  formed in an island shape in the green pixel Pg. The green light-emitting layer  35   g  includes a plurality of green quantum dots that emit green light. A peak wavelength of the light emission spectrum due to the electroluminescence of the green quantum dots is equal to or greater than 500 nm and equal to or less than 600 nm. Note that, in order to improve the color reproduction range of the display device, the peak wavelength of the light emission spectrum of the green pixel Pg is preferably equal to or greater than 520 nm and equal to or less than 540 nm. 
     The active layer  24  includes the blue light-emitting layer  35   b  formed in an island shape in the blue pixel Pb. The blue light-emitting layer  35   b  includes a plurality of blue quantum dots that emit blue light. A peak wavelength of the light emission spectrum due to the electroluminescence of the blue quantum dots is equal to or greater than 400 nm and equal to or less than 500 nm. Note that, in order to improve the color reproduction range of the display device, the peak wavelength of the light emission spectrum of the blue pixel Pb is preferably equal to or greater than 440 nm and equal to or less than 460 nm. 
     The active layer  24  includes a hole transport layer  37  formed in a solid-like shape. The hole transport layer  37  is formed in the solid-like shape so as to cover the green light-emitting layer  35   g , the red light-emitting layer  35   r , and the blue light-emitting layer  35   b . The hole transport layer  37  is not limited thereto, and the hole transport layer  37  may be formed integrally with the anode  22  or may be formed in an island shape so as to individually cover each of the green light-emitting layer  35   g , the red light-emitting layer  35   r , and the blue light-emitting layer  35   b . In addition, the hole transport layer  37  may be a single layer structure or a layered structure. The active layer  24  may include a hole injection layer formed between the hole transport layer  37  and the anode  22 . 
     The light-emitting element layer  5  includes the anode  22  provided as an upper layer than the active layer  24 . The anode  22  is a transparent electrode. The anode  22  is integrally formed across the red pixel Pr, the green pixel Pg, and the blue pixel Pb. The anode  22  is not limited thereto, and may be separately formed for each of the red pixel Pr, the green pixel Pg, and the blue pixel Pb. 
     The light-emitting element layer  5  includes the selectively reflective layer  40  that is an upper layer than the anode  22 . The selectively reflective layer  40  is provided at an opposite side to the red light-emitting layer  35   r , the green light-emitting layer  35   g , and the blue light-emitting layer  35   b  with respect to the anode  22 . The selectively reflective layer  40  is integrally formed across the red pixel Pr, the green pixel Pg, and the blue pixel Pb. The selectively reflective layer  40  has a reflection band having a higher reflectivity than those of other bands. The selectively reflective layer  40  is configured so that the absorption and re-emission of light of the blue quantum dots occur when light having a wavelength included in the reflection band of the selectively reflective layer  40  is incident on the blue light-emitting layer  35   b.    
     Reflection and Transmission in Light-Emitting Element Layer 
     The reflection and transmission in the light-emitting element layer  5  in the blue pixel Pb will be described below with reference to  FIG.  4   . 
       FIG.  4    is a schematic cross-sectional view illustrating the reflection and transmission in the light-emitting element layer  5  in the blue pixel Pb illustrated in  FIG.  3   . 
     As illustrated by an arrow A in  FIG.  4   , among light emitted from the blue light-emitting layer  35   b  to the anode  22  side, light having a wavelength included in the reflection band of the selectively reflective layer  40  is reflected by the selectively reflective layer  40 . On the other hand, as illustrated by an arrow C in  FIG.  4   , among light emitted from the blue light-emitting layer  35   b  to the anode  22  side, light having a wavelength not included in the reflection band of the selectively reflective layer  40  is transmitted through the selectively reflective layer  40 . 
     As illustrated by arrows B and D in  FIG.  4   , among light emitted from the blue light-emitting layer  35   b  to the cathode  25  side is reflected by the cathode  25  regardless of a wavelength that the light has. 
     Thus, the light having a wavelength included in the reflection band of the selectively reflective layer  40  (hereinafter, referred to as “light within the reflection band”) reciprocates between the cathode  25  and the selectively reflective layer  40 , and repeatedly passes through the blue light-emitting layer  35   b . The light within the reflection band is absorbed into the blue quantum dots inside the blue light-emitting layer  40  during repeated passing. The blue quantum dots that has absorbed the light re-emits light having a wavelength equal to or lower than the wavelength of the absorbed light. Thus, finally, the light within the reflection band is converted into light having a wavelength that is longer than a wavelength at a long wavelength end of the reflection band of the selectively reflective layer  40  due to the absorption and re-emission of light by the blue quantum dots. The light having the wavelength that is longer than the wavelength at the long wavelength end of the reflection band is light having a wavelength not included in the reflection band of the selectively reflective layer  40  (hereinafter, referred to as “light outside the reflection band”). 
     Then, the light having the wavelength outside the reflection band passes through the selectively reflective layer  40  and is emitted outside the light-emitting element layer  5 . 
     Reflection Band of Selectively Reflective Layer 
       FIG.  5    is a diagram illustrating, on the upper side, a graph showing a light emission spectrum due to electroluminescence of a light-emitting element layer according to a comparative example in which the selectively reflective layer  40  is removed from the light-emitting element layer  5  illustrated in  FIG.  4   , and illustrating, on the lower side, a graph showing a light emission spectrum due to electroluminescence from the example of the light-emitting element layer  5  illustrated in  FIG.  4   .  FIG.  6    is a diagram illustrating, on the upper side, a graph showing a light emission spectrum due to electroluminescence of the light-emitting element layer according to the comparative example in which the selectively reflective layer  40  is removed from the light-emitting element layer  5  illustrated in  FIG.  4   , and illustrating, on the lower side, a graph showing a light emission spectrum due to electroluminescence from another example of the light-emitting element layer  5  illustrated in  FIG.  4   . In  FIG.  5    and  FIG.  6   , a vertical axis indicates a light emission intensity (without a unit) standardized with a light emission intensity at each peak wavelength as one unit, and a horizontal axis indicates a wavelength (nm). 
     As described above, an absorption rate in the reflection band of the light-emitting element layer  5  excluding the blue light-emitting layer  35   b  is so small as to be negligible. Also, an absorption rate of the light-emitting element layer  5  outside the reflection band is so small as to be negligible. Thus, the light emission spectrum due to the electroluminescence of the light-emitting element layer according to the comparative example (on the upper side in  FIG.  5    and  FIG.  6   ) is substantially the same as the light emission spectrum due to the electroluminescence of the blue quantum dots. 
     The spectra shown in  FIG.  5    and  FIG.  6    are defined as follows.
         λ 0 : A peak wavelength of the light emission spectrum due to the electroluminescence of the blue quantum dots.   δλ: A full width at half maximum of the light emission spectrum due to the electroluminescence of the blue quantum dots.   λ 1 : A wavelength at which the light emission spectrum due to the electroluminescence of the blue quantum dots has a half value of a peak value of the light emission spectrum due to the electroluminescence of the blue quantum dots at the shorter wavelength side than the peak wavelength λ 0 .   λ 2 : A wavelength at which the light emission spectrum due to the electroluminescence of the blue quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the blue quantum dots at the longer wavelength side than the peak wavelength λ 0 .   λ 3 : A wavelength that is shorter than the wavelength λ 1  by δλ.   λ 4 : A wavelength that is longer than the wavelength λ 1  by δλ.   λ S0 : A peak wavelength of the light emission spectrum of the light-emitting element layer  5 .   λ S1 : A wavelength at which the light emission spectrum of the light-emitting element layer  5  has a half value of a peak value of the light emission spectrum of the light-emitting element layer  5  at the shorter wavelength side than the peak wavelength λ S0 .   λ S2 : A wavelength at which the light emission spectrum of the light-emitting element layer  5  has the half value of the peak value of the light emission spectrum of the light-emitting element layer  5  at the longer wavelength side than the peak wavelength λ S0 .   λ t1 : A wavelength at a short wavelength end in the reflection band of the selectively reflective layer  40 .   λ t2 : A wavelength at a long wavelength end in the reflection band of the selectively reflective layer  40 .       

     As described above, light within the reflection band is converted into light having a wavelength that is longer than the wavelength at the long wavelength end in the reflection band, and then emitted out of the light-emitting element layer  5 . Thus, as shown in  FIG.  5    and  FIG.  6   , the light emission spectrum of the light-emitting element layer  5  is made narrower in bandwidth at the longer wavelength side than the light emission spectrum due to the electroluminescence of the blue quantum dots. 
     In order to facilitate such narrowing in bandwidth, it is preferable that the probability of the occurrence of the absorption and re-emission of light by the blue quantum dots be high. Thus, it is preferable that the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  be close to the peak wavelength λ 0  of the light emission spectrum due to the electroluminescence of the blue quantum dots. Specifically, the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  is (i) preferably longer than the wavelength λ 1  having the half value of the peak value of the light emission spectrum due to the electroluminescence of the blue quantum dots at the shorter wavelength side than the peak wavelength λ 0  of the light emission spectrum due to the electroluminescence of the blue quantum dots, and (ii) preferably shorter than the wavelength λ 2  having the half value of the peak value of the light emission spectrum due to the electroluminescence of the blue quantum dots at the longer wavelength side than the peak wavelength λ 0  of the light emission spectrum due to the electroluminescence of the blue quantum dots. In other words, it is preferable to satisfy λ 1 &lt;λ t2 &lt;λ 2 . When the relationships of λ 1 =λ 0 −δλ/2, and λ 2 =λ 0 +δλ/2 hold, it is preferable to satisfy λ 0 −δλ/2&lt;λ t2 &lt;λ 0 +δλ/2. 
     Additionally, in order to facilitate such narrowing in bandwidth, a range in which the reflection band of the selectively reflective layer  40  overlaps the tail at the shorter wavelength side of the light emission spectrum due to the electroluminescence of the blue quantum dots is preferably wide. Specifically, the wavelength λ t1  at the short wavelength end in the reflection band of the selectively reflective layer  40  is preferably equivalent to or shorter than the wavelength λ 3 . Thus, it is preferable to satisfy λ t1 ≤λ 3 . 
     Furthermore, in order to facilitate such narrowing in bandwidth, it is preferable that the reflectivity of the selectively reflective layer  40  in the reflection band (being equal to or greater than λ t1  and equal to or less than λ t2 ) of the selectively reflective layer  40  be high. Specifically, the reflectivity is preferably equal to or greater than 95%. At the same time, it is preferable that an absorption rate of the selectively reflective layer  40  in the peripheral band of the peak wavelength λ 0  of the light emission spectrum due to the electroluminescence of the blue quantum dots be low. Specifically, the absorption rate in the wavelength range being equal to or greater than the wavelength λ 3  and equal to or less than the wavelength λ 4  is preferably equal to or less than 1%. Further, the selectively reflective layer  40  is formed across the red pixel Pr and the green pixel Pg. Thus, it is preferable that an absorption rate of the selectively reflective layer  40  in a peripheral band of a peak wavelength of a light emission spectrum due to electroluminescence of each of the red quantum dots and the green quantum dots be also low. 
     Furthermore, as shown in  FIG.  6   , the peak wavelength λ S0  of the light emission spectrum of the light-emitting element layer  5  can be shifted to a longer wavelength side than the peak wavelength λ 0  of the light emission spectrum due to the electroluminescence of the blue quantum dots. Such a shift to the longer wavelength side may be achieved by the fact that the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  is longer than the peak wavelength λ 0  of the light emission spectrum due to the electroluminescence of the blue quantum dots. Thus, it is preferable to satisfy λ 0 &lt;λ t2 &lt;λ 2 , that is, λ 0 &lt;λ t2 &lt;λ 0 +δλ/2. 
     Such a shift to the longer wavelength side has a beneficial effect when the peak wavelength of the blue quantum dots is too short, compared to the peak wavelength of the blue pixel Pb to be targeted (for example, equal to or greater than 440 nm and equal to or greater than 460 nm). The shift to the longer wavelength side allows an actual peak wavelength of the blue pixel Pb to be closer to the peak wavelength to be targeted from the peak wavelength of the blue quantum dots. This can improve the color reproduction range of the display device. 
     Dielectric Multilayer Film 
     The selectively reflective layer  40  may have any configuration as long as the selectively reflective layer  40  functions as a bandpass filter having a high reflectivity in the reflection band as described above. The selectively reflective layer  40  is, for example, a dielectric multilayer film. 
       FIG.  7    is a cross-sectional view illustrating a schematic configuration of an example in a case where the selectively reflective layer  40  illustrated in  FIG.  3    is a dielectric multilayer film. 
     As illustrated in  FIG.  7   , when the selectively reflective layer  40  is a dielectric multilayer film, the selectively reflective layer  40  is preferably a layered body in which a first dielectric film  41  and a second dielectric film  42  having different dielectric constants from each other are alternately layered. Note that the first dielectric film  41  has a higher refractive index than that of the second dielectric film, and a dielectric film closest to the anode  22  is the second dielectric film  42 . 
     A thickness of the first dielectric film  41  is preferably equal to or greater than 189 nm and equal to or less than 246 nm, and a thickness of the second dielectric film  42  is preferably equal to or greater than 291 nm and equal to or less than 378 nm. A sum of the number of layers of the first dielectric film  41  and the number of layers of the second dielectric film  42  that are included in the selectively reflective layer  40  is equal to or greater than three. 
     The first dielectric film  41  preferably has a vacuum dielectric constant being equal to or greater than 4.8 and equal to or less than 6.0. For example, the first dielectric film  41  is preferably configured to include at least one of titanium oxide, niobium pentoxide, and tantalum pentoxide. 
     The second dielectric film  42  preferably has a vacuum dielectric constant being equal to or greater than 1.9 and equal to or less than 3.3. For example, the second dielectric film  42  is preferably configured to include at least one of silicon oxide, magnesium fluoride, and aluminum oxide. 
     First Modified Example 
       FIG.  8    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer  5  according to a modified example of the first embodiment. 
     As illustrated in  FIG.  8   , the light-emitting element layer  5  may further include a photoluminescence layer  45  formed between the selectively reflective layer  40  and the anode  22 . The photoluminescence layer  45  may be formed across the red pixel Pr, the green pixel Pg, and the blue pixel Pb, as illustrated in  FIG.  8   , but may be formed only in the blue pixel Pb although not illustrated. 
     The photoluminescence layer  45  is configured to emit light with the same color as light emitted by the blue light-emitting layer  35   b  by being excited by the light emitted by the blue light-emitting layer  35   b . The wavelength of the light emitted by the photoluminescence layer  45  is shorter than the wavelength of the light emitted by the blue light-emitting layer  35   b.    
     The light emitted by the photoluminescence layer  45  is preferably transmitted through the selectively reflective layer  40 . Thus, it is preferable that a peak wavelength λ u0  of a light emission spectrum of the photoluminescence layer  45  be longer than the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40 . Furthermore, a wavelength λ U1  at which the light emission spectrum of the photoluminescence layer  45  has a half value of a peak value of the light emission spectrum of the photoluminescence layer  45  at the shorter wavelength side than the peak wavelength λ u0  is preferably longer than the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40 . Thus, it is preferable to satisfy λ t2 &lt;λ U0 , and is more preferable to satisfy λ t2 &lt;λ U1 . 
     This modification is applicable to second to fourth embodiments, which will be described below. 
     Second Modified Example 
       FIG.  9    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer  5  according to another modified example of the first embodiment. 
     As illustrated in  FIG.  9   , the selectively reflective layer  40  may be formed only in the blue pixel Pb. 
     Second Embodiment 
     A display device  2  according to a second embodiment of the present invention is a one-sided light-emitting type as illustrated in  FIG.  2   . 
       FIG.  10    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer  5  in the display device according to the second embodiment of the present invention. 
     As illustrated in  FIG.  10   , the light-emitting element layer  5  according to the second embodiment has the same configuration as the light-emitting element layer  5  according to the first embodiment described above, except for the following two points. One point thereof is that the selectively reflective layer  40  is constituted by a red selectively reflective layer  40   r  formed only in the red pixel Pr, a green selectively reflective layer  40   g  formed only in the green pixel Pg, and a blue selectively reflective layer  40   b  formed only in the blue pixel Pb. The other point is that the edge cover  23  is highly formed such that the upper face of the edge cover  23  has a height being higher than or equal to those of the upper surfaces of the red selectively reflective layer  40   r , the green selectively reflective layer  40   g , and the blue selectively reflective layer  40   b.    
     The red selectively reflective layer  40   r  is configured such that the absorption and re-emission of light of the red quantum dots occur when light having a wavelength included in the reflection band of the red selectively reflective layer  40   r  is incident on the red light-emitting layer  35   r . Thus, similarly to the selectively reflective layer  40  for the blue pixel Pb in the first embodiment described above, the red selectively reflective layer  40   r  according to the second embodiment causes narrowing in bandwidth of the light emission spectrum of the red pixel Pr. 
     A wavelength at the short wavelength end in the reflection band of the red selectively reflective layer  40   r  preferably satisfies a condition that the wavelength λ t1  at the short wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots, with the condition for the blue quantum dots read as the condition for the red quantum dots. A wavelength at the long wavelength end in the reflection band of the red selectively reflective layer  40   r  preferably satisfies a condition that the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots, with the condition for the blue quantum dots read as the condition for the red quantum dots. 
     The red selectively reflective layer  40   r  may have any configuration as long as the red selectively reflective layer  40   r  functions as a bandpass filter having a high reflectivity in the reflection band as described above, or may be a dielectric multilayer film. For example, when the red selectively reflective layer  40   r  is the dielectric multilayer film illustrated in  FIG.  7   , a thickness of the first dielectric film  41  is preferably equal to or greater than 126 nm and equal to or less than 157 nm, and a thickness of the second dielectric film  42  is preferably equal to or greater than 194 nm and equal to or less than 242 nm. 
     The green selectively reflective layer  40   g  is configured such that the absorption and re-emission of light of the green quantum dots occur when light having a wavelength included in the reflection band of the green selectively reflective layer  40   g  is incident on the green light-emitting layer  35   g . Thus, similarly to the selectively reflective layer  40  for the blue pixel Pb in the first embodiment described above, the green selectively reflective layer  40   g  according to the second embodiment causes narrowing in bandwidth of the light emission spectrum of the green pixel Pg. 
     A wavelength at the short wavelength end in the reflection band of the green selectively reflective layer  40   g  preferably satisfies a condition that the wavelength λ t1  at the short wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots, with the condition for the blue quantum dots read as the condition for the green quantum dots. A wavelength at the long wavelength end in the reflection band of the green selectively reflective layer  40   g  preferably satisfies a condition that the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots, with the condition for the blue quantum dots read as the condition for the green quantum dots. 
     The green selectively reflective layer  40   g  may be any configuration as long as the green selectively reflective layer functions as a bandpass filter having a high reflectivity in the reflection band as described above, or may be a dielectric multilayer film. 
     For example, when the green selectively reflective layer  40   g  is the dielectric multilayer film illustrated in  FIG.  7   , a thickness of the first dielectric film  41  is preferably equal to or greater than 157 nm and equal to or less than 189 nm, and a thickness of the second dielectric film  42  is preferably equal to or greater than 242 nm and equal to or less than 291 nm. 
     The blue selectively reflective layer  40   b  is configured such that the absorption and re-emission of light of the blue quantum dots occur when light having a wavelength included in the reflection band of the blue selectively reflective layer  40   b  is incident on the blue light-emitting layer  35   b . Thus, similarly to the selectively reflective layer  40  for the blue pixel Pb in the first embodiment described above, the blue selectively reflective layer  40   b  according to the second embodiment causes narrowing in bandwidth of the light emission spectrum of the blue pixel Pb. 
     The wavelength at the short wavelength end in the reflection band of the blue selectively reflective layer  40   b  preferably satisfies, in a similar manner, the condition that the wavelength λ t1  at the short wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots. The wavelength at the long wavelength end in the reflection band of the blue selectively reflective layer  40   b  preferably satisfies, in a similar manner, the condition that the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots. 
     The blue selectively reflective layer  40   b  may be any configuration as long as the blue selectively reflective layer  40   b  functions as a bandpass filter having a high reflectivity in the reflection band as described above, or may be a dielectric multilayer film. For example, when the blue selectively reflective layer  40   b  is the dielectric multilayer film illustrated in  FIG.  7   , a thickness of the first dielectric film  41  is equal to or greater than 189 nm and equal to or less than 246 nm, and a thickness of the second dielectric film  42  is preferably equal to or greater than 291 nm and equal to or less than 378 nm. 
     As the height of the edge cover  23  increases, the anode  22  is individually formed for each of the red pixel Pr, the green pixel Pg, and the blue pixel Pb. Additionally, the anode  22  of the red pixel Pr is surrounded by the edge cover  23 , and the anode  22  of the green pixel Pg and the anode  22  of the blue pixel Pb are also surrounded by the edge cover  23 . Thus, light reflected by the selectively reflective layer  40  is prevented from leaking through the cathode  25  to the adjacent pixels. Additionally, the red selectively reflective layer  40   r  of the red pixel Pr is surrounded by the edge cover  23 , and the green selectively reflective layer  40   g  of the green pixel Pg and the blue selectively reflective layer  40   b  of the blue pixel Pb are also surrounded by the edge cover  23 . Thus, light reflected by the selectively reflective layer  40  is prevented from leaking to the adjacent pixels through the selectively reflective layer  40 . 
     Third Modified Example 
       FIG.  11    is a cross-sectional view illustrating a schematic configuration of the light-emitting element layer  5  according to a modified example of the second embodiment. 
     As illustrated in  FIG.  11   , only the blue selectively reflective layer  40   b  may be formed, and the red selectively reflective layer  40   r  and the green selectively reflective layer  40   g  do not need to be formed. In this case, the edge cover  23  positioned between the red pixel Pr and the green pixel Pg may be formed low in height such that the upper face of the edge cover  23  has a height being equal to or lower than the lower face of the cathode  25 . 
     Third Embodiment 
     The display device  2  according to a third embodiment of the present invention is a both-sided light-emitting type. 
       FIG.  12    is a schematic cross-sectional view illustrating another example of a configuration of the display region of the display device  2 . 
     Although the display device of the one-sided light-emitting type has been described in the first embodiment described above, when a display device of a both-sided light-emitting type is manufactured, both the cathode  25  (first transparent electrode) and the anode  22  (second transparent electrode) are transparent electrodes, and the lower face film  10  and the resin layer  12  have transparency. In addition, as illustrated in  FIG.  12   , the light-emitting element layer  5  includes the cathode  25 , the edge cover  23 , the active layer  24 , and the anode  22 , and further includes a first selectively reflective layer  44  provided as a lower layer than the cathode  25  and a second selectively reflective layer  46  provided as an upper layer than the anode  22 . The first selectively reflective layer  44  and the second selectively reflective layer  46  have a reflection band having a higher reflectivity than those of other bands. Details will be described below. 
       FIG.  13    is a cross-sectional view illustrating a schematic configuration of the light-emitting element layer  5  in the display device according to the third embodiment of the present invention. 
     As illustrated in  FIG.  13   , the light-emitting element layer  5  according to the third embodiment has the same configuration as the light-emitting element layer  5  according to the first embodiment described above, except for the following two points. One point is that both the cathode  25  (first transparent electrode) and the anode  22  (second transparent electrode) are transparent electrodes. The other point is that the first selectively reflective layer  44  provided as a lower layer than the cathode  25  and the second selectively reflective layer  46  provided as an upper layer than the anode  22  are included. 
     The optical characteristics of the first selectively reflective layer  44  and the second selectively reflective layer  46  are preferably equivalent so that the light-emission characteristics of the display device are equivalent on both sides. The optical characteristics include the wavelength at the short wavelength end in the reflection band and the wavelength at the long wavelength end. 
     The first selectively reflective layer  44  is provided at the side opposite to the red light-emitting layer  35   r , the green light-emitting layer  35   g , and the blue light-emitting layer  35   b  with respect to the cathode  25 . The first selectively reflective layer  44  is integrally formed across the red pixel Pr, the green pixel Pg, and the blue pixel Pb. The first selectively reflective layer  44  has a reflection band having a higher reflectivity than those of other bands. The first selectively reflective layer  44  is configured so that the absorption and re-emission of light of the blue quantum dots occur when light having a wavelength included in the reflection band of the first selectively reflective layer  44  is incident on the blue light-emitting layer  35   b.    
     The second selectively reflective layer  46  is provided at the opposite side to the red light-emitting layer  35   r , the green light-emitting layer  35   g , and the blue light-emitting layer  35   b  with respect to the anode  22 . The second selectively reflective layer  46  is integrally formed across the red pixel Pr, the green pixel Pg, and the blue pixel Pb. The second selectively reflective layer  46  has a reflection band having a higher reflectivity than those of other bands. The second selectively reflective layer  46  is configured so that the absorption and re-emission of light of the blue quantum dots occur when light having a wavelength included in the reflection band of the second selectively reflective layer  46  is incident on the blue light-emitting layer  35   b.    
     Thus, similarly to the selectively reflective layer  40  for the blue pixel Pb in the first embodiment described above, the first selectively reflective layer  44  and the second selectively reflective layer  46  according to the third embodiment cause narrowing in bandwidth of the light emission spectrum of the blue pixel Pb. 
     A wavelength at the short wavelength end in the reflection band of the first selectively reflective layer  44  preferably satisfies, in a similar manner, the condition that the wavelength λ t1  at the short wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots. A wavelength at the long wavelength end in the reflection band of the first selectively reflective layer  44  preferably satisfies, in a similar manner, the condition that the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots. 
     A wavelength at the short wavelength end in the reflection band of the second selectively reflective layer  46  preferably satisfies, in a similar manner, the condition that the wavelength λ t1  at the short wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots. A wavelength at the long wavelength end in the reflection band of the second selectively reflective layer  46  preferably satisfies, in a similar manner, the condition that the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots. 
     The first selectively reflective layer  44  and the second selectively reflective layer  46  may have any configuration as long as the first selectively reflective layer  44  and the second selectively reflective layer  46  function as a bandpass filter having a high reflectivity in the reflection band as described above, or may be dielectric multilayer films. 
     Reflection and Transmission in Light-Emitting Element Layer Hereinafter, when the optical characteristics of the first selectively reflective layer  44  and the second selectively reflective layer  46  are identical, the reflection and transmission in the light-emitting element layer  5  in the blue pixel Pb will be described with reference to  FIG.  14   . 
       FIG.  14    is a schematic cross-sectional view illustrating the reflection and transmission in the light-emitting element layer  5  in the blue pixel Pb illustrated in  FIG.  13   . 
     As indicated by an arrow A in  FIG.  14   , light having a wavelength included in the reflection band of the second selectively reflective layer  46 , of light emitted from the blue light-emitting layer  35   b  to the anode  22  side, is reflected by the second selectively reflective layer  46 . On the other hand, as indicated by an arrow C in  FIG.  4   , light having a wavelength that is not included in the reflection band of the second selectively reflective layer  46 , of the light emitted from the blue light-emitting layer  35   b  to the anode  22  side, is transmitted through the second selectively reflective layer  46 . 
     As indicated by an arrow B in  FIG.  14   , light having a wavelength included in the reflection band of the first selectively reflective layer  44 , of light emitted from the blue light-emitting layer  35   b  to the cathode  25  side, is reflected by the first selectively reflective layer  44 . On the other hand, as indicated by an arrow D in  FIG.  4   , light having a wavelength that is not included in the reflection band of the first selectively reflective layer  44 , of the light emitted from the blue light-emitting layer  35   b  to the cathode  25  side, is transmitted through the first selectively reflective layer  44 . 
     Thus, light having a wavelength included in the reflection bands of the first selectively reflective layer  44  and the second selectively reflective layer  46  (hereinafter, referred to as light “in the reflection bands”) reciprocates between the first selectively reflective layer  44  and the second selectively reflective layer  46 , and repeatedly passes through the blue light-emitting layer  35   b . The light having the wavelength in the reflection bands is absorbed into the blue quantum dots provided thereinside during repeated passing. The blue quantum dots into which the light has been absorbed re-emits light having a wavelength equal to or less than the wavelength of the absorbed light. Thus, finally, the light having the wavelength in the reflection band is converted into light having a wavelength being longer than that at the long wavelength end in the reflection band through the absorption and re-emission of light by the blue quantum dots. The light having the wavelength being longer than that at the long wavelength end in the reflection band is light having a wavelength that is not included in the reflection bands of the first selectively reflective layer  44  and the second selectively reflective layer  46  (hereinafter, referred to as “light outside the reflection bands”). 
     Then, the light having the wavelength outside the reflection bands passes through the selectively reflective layer  40  and is emitted outside the light-emitting element layer  5 . 
     Fourth Embodiment 
     A display device  2  according to a fourth embodiment of the present invention is a both-sided light-emitting type as illustrated in  FIG.  12   . 
       FIG.  15    is a cross-sectional view illustrating a schematic configuration of a light-emitting element layer  5  in the display device according to the fourth embodiment of the present invention. 
     As illustrated in  FIG.  13   , the light-emitting element layer  5  according to the fourth embodiment has the same configuration as the light-emitting element layer  5  according to the second embodiment, except for the following two points. One point is that both the cathode  25  (first transparent electrode) and the anode  22  (second transparent electrode) are transparent electrodes. The other point is that the first selectively reflective layer  44  provided as a lower layer than the cathode  25  and the second selectively reflective layer  46  provided as an upper layer than the anode  22  are included. 
     The first selectively reflective layer  44  includes a red first selectively reflective layer  44   r  formed only in the red pixel Pr, a green first selectively reflective layer  44   g  formed only in the green pixel Pg, and a blue first selectively reflective layer  44   b  formed only in the blue pixel Pb. 
     The second selectively reflective layer  46  includes a red second selectively reflective layer  46   r  formed only in the red pixel Pr, a green second selectively reflective layer  46   g  formed only in the green pixel Pg, and a blue second selectively reflective layer  46   b  formed only in the blue pixel Pb. 
     The red first selectively reflective layer  44   r  has a reflection band having a higher reflectivity than those of other bands. The red first selectively reflective layer  44   r  is configured so that the absorption and re-emission of light of the red quantum dots occur when light having a wavelength included in the reflection band of the red first selectively reflective layer  44   r  is incident on the red light-emitting layer  35   r . The red second selectively reflective layer  46   r  has a reflection band having a higher reflectivity than those of other bands. The red second selectively reflective layer  46   r  is configured so that the absorption and re-emission of light of the red quantum dots occur when light having a wavelength included in the reflection band of the red second selectively reflective layer  46   r  is incident on the red light-emitting layer  35   r . Thus, similarly to the first selectively reflective layer  44  and the second selectively reflective layer  46  for the blue pixel Pb in the third embodiment, the red first selectively reflective layer  44   r  and the red second selectively reflective layer  46   r  according to the fourth embodiment cause narrowing in bandwidth of the light emission spectrum of the red pixel Pr. 
     The green first selectively reflective layer  44   g  has a reflection band having a higher reflectivity than those of other bands. The green first selectively reflective layer  44   g  is configured so that the absorption and re-emission of light of the green quantum dots occur when light having a wavelength included in the reflection band of the green first selectively reflective layer  44   g  is incident on the green light-emitting layer  35   g . The green second selectively reflective layer  46   g  has a reflection band having a higher reflectivity than those of other bands. The green second selectively reflective layer  46   g  is configured so that the absorption and re-emission of light of the green quantum dots occur when light having a wavelength included in the reflection band of the green second selectively reflective layer  46   g  is incident on the green light-emitting layer  35   g . Thus, similarly to the first selectively reflective layer  44  and the second selectively reflective layer  46  for the blue pixel Pb in the third embodiment, the green first selectively reflective layer  44   g  and the green second selectively reflective layer  46   g  according to the fourth embodiment cause narrowing in bandwidth of the light emission spectrum of the green pixel Pg. 
     The blue first selectively reflective layer  44   b  has a reflection band having a higher reflectivity than those of other bands. The blue first selectively reflective layer  44   b  is configured so that the absorption and re-emission of light of the blue quantum dots occur when light having a wavelength included in the reflection band of the blue first selectively reflective layer  44   b  is incident on the blue light-emitting layer  35   b . The blue second selectively reflective layer  46   b  has a reflection band having a higher reflectivity than those of other bands. The blue second selectively reflective layer  46   b  is configured so that the absorption and re-emission of light of the blue quantum dots occur when light having a wavelength included in the reflection band of the blue second selectively reflective layer  46   b  is incident on the blue light-emitting layer  35   b . Thus, similarly to the first selectively reflective layer  44  and the second selectively reflective layer  46  for the blue pixel Pb in the third embodiment, the blue first selectively reflective layer  44   b  and the blue second selectively reflective layer  46   b  according to the fourth embodiment cause narrowing in bandwidth of the light emission spectrum of the blue pixel Pb. 
     Thus, the optical characteristics of the red first selectively reflective layer  44   r  and the red second selectively reflective layer  46   r  are preferably equivalent. A wavelength at the short wavelength end in the reflection band of each of the red first selectively reflective layer  44   r  and the red second selectively reflective layer  46   r  preferably satisfies, in a similar manner, the condition that the wavelength at the short wavelength end in the reflection band of the red selectively reflective layer  40   r  according to the second embodiment described above preferably satisfies for the red quantum dots. A wavelength at the long wavelength end in the reflection band of each of the red first selectively reflective layer  44   r  and the red second selectively reflective layer  46   r  preferably satisfies, in a similar manner, the condition that the wavelength at the long wavelength end in the reflection band of the red selectively reflective layer  40   r  according to the second embodiment described above preferably satisfies for the red quantum dots. 
     Furthermore, the optical characteristics of the green first selectively reflective layer  44   g  and the green second selectively reflective layer  46   g  are preferably equivalent. A wavelength at the short wavelength end in the reflection band of each of the green first selectively reflective layer  44   g  and the green second selectively reflective layer  46   g  preferably satisfies, in a similar manner, the condition that the wavelength at the short wavelength end in the reflection band of the green selectively reflective layer  40   g  according to the second embodiment described above preferably satisfies for the green quantum dots. A wavelength at the long wavelength end in the reflection band of each of the green first selectively reflective layer  44   g  and the green second selectively reflective layer  46   g  preferably satisfies, in a similar manner, the condition that the wavelength at the long wavelength end in the reflection band of the green selectively reflective layer  40   g  according to the second embodiment described above preferably satisfies for the green quantum dots. 
     Furthermore, the optical characteristics of the blue first selectively reflective layer  44   b  and the blue second selectively reflective layer  46   b  are preferably equivalent. A wavelength at the short wavelength end in the reflection band of each of the blue first selectively reflective layer  44   b  and the blue second selectively reflective layer  46   b  preferably satisfies, in a similar manner, the condition that the wavelength λ t1  at the short wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots. A wavelength at the long wavelength end in the reflection band of each of the blue first selectively reflective layer  44   b  and the blue second selectively reflective layer  46   b  preferably satisfies, in a similar manner, the condition that the wavelength λ t2  at the long wavelength end in the reflection band of the selectively reflective layer  40  according to the first embodiment described above preferably satisfies for the blue quantum dots. 
     The red first selectively reflective layer  44   r , the red second selectively reflective layer  46   r , the green first selectively reflective layer  44   g , the green second selectively reflective layer  46   g , the blue first selectively reflective layer  44   b , and the blue second selectively reflective layer  46   b  may have any configuration as long as they function as a bandpass filter having a high reflectivity in the reflection band as described above, and may be a dielectric multilayer film. 
     Supplement 
     A light-emitting element according to a first aspect of the present invention includes a reflective electrode, a transparent electrode, a light-emitting layer provided between the reflective electrode and the transparent electrode, the light-emitting layer including quantum dots, and a selectively reflective layer provided at an opposite side to the light-emitting layer with respect to the transparent electrode, the selectively reflective layer having a reflection band having a higher reflectivity than a reflectivity of another band, and a wavelength at a long wavelength end in the reflection band of the selectively reflective layer is longer than a wavelength at which a light emission spectrum due to electroluminescence of the quantum dots has a half value of a peak value of the light emission spectrum due to the electroluminescence of the quantum dots at a shorter wavelength side than a peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and is shorter than a wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots at a longer wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots. 
     The light-emitting element according to a second aspect of the present invention may have, in the configuration according to the first aspect, a configuration satisfying that the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots is λ 0 , a full width at half maximum is δλ, the reflection band of the selectively reflective layer is from a wavelength λ t1  to a wavelength λ t2 , λ t1 &lt;λ t2 , and λ 0 −δλ/2&lt;λ t2 &lt;λ 0 +δλ/2. 
     The light-emitting element according to a third aspect of the present invention may have, in the configuration according to the first or second aspect described above, a configuration in which the wavelength at the long wavelength end in the reflection band of the selectively reflective layer is longer than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots. 
     The light-emitting element according to a fourth aspect of the present invention may have, in the configuration according to any one of the first to third aspects described above, a configuration in which the wavelength at the short wavelength end in the reflection band of the selectively reflective layer is shorter, by a length being larger than or equal to a full width at half maximum of the light emission spectrum due to the electroluminescence of the quantum dots, than the wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots, at the shorter wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots. 
     The light-emitting element according to a fifth aspect of the present invention may have, in the configuration according to any one of the first to fourth aspects described above, a configuration in which the selectively reflective layer has a reflectivity being equal to or larger than 95% in the reflection band. 
     The light-emitting element according to a sixth aspect of the present invention may have, in the configuration according to any one of the first to fifth aspects described above, a configuration in which the selectively reflective layer has an absorption rate being equal to or less than 1% between (i) a wavelength being shorter, by a full width at half maximum of the light emission spectrum due to the electroluminescence of the quantum dots, than the wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots, at the shorter wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and (ii) a wavelength being longer, by the full width at half maximum of the light emission spectrum due to the electroluminescence of the quantum dots, than the wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots, at the longer wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots. 
     The light-emitting element according to a seventh aspect of the present invention may have, in the configuration according to any one of the first to sixth aspects described above, a configuration in which the selectively reflective layer is a dielectric multilayer film, and the dielectric multilayer film is a layered body of a first dielectric film and a second dielectric film having a dielectric constant different from a dielectric constant of the first dielectric film. 
     The light-emitting element according to an eighth aspect of the present invention may have, in the configuration according to the seventh aspect described above, a configuration in which the first dielectric film contains at least one of titanium oxide, niobium pentoxide, and tantalum pentoxide. 
     The light-emitting element according to a ninth aspect of the present invention may have, in the configuration according to the seventh or eighth aspect described above, a configuration in which the dielectric constant of the first dielectric film is equal to or greater than 4.8 and equal to or less than 6.0. 
     The light-emitting element according to a tenth aspect of the present invention may have, in the configuration according to any one of the seventh to ninth aspects described above, a configuration in which the second dielectric film contains at least one of silicon oxide, magnesium fluoride, and aluminum oxide. 
     The light-emitting element according to an eleventh aspect of the present invention may have, in the configuration according to any one of the seventh to tenth aspects described above, a configuration in which the dielectric constant of the first dielectric film is equal to or greater than 1.9 and equal to or less than 3.3. 
     The light-emitting element according to a twelfth aspect of the present invention may have, in the configuration according to any one of the seventh to eleventh aspects described above, a configuration in which a dielectric film closest to the transparent electrode among dielectric films included in the dielectric multilayer film is the second dielectric film. 
     The light-emitting element according to a thirteenth aspect of the present invention may have, in the configuration according to any one of the seventh to twelfth aspects described above, a configuration in which the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots is equal to or greater than 400 nm and equal to or less than 500 nm, a thickness of the first dielectric film is equal to or greater than 126 nm and equal to or less than 157 nm, and a thickness of the second dielectric film is equal to or greater than 194 nm and equal to or less than 242 nm. 
     The light-emitting element according to a fourteenth aspect of the present invention may have, in the configuration according to any one of the seventh to twelfth aspects described above, a configuration in which the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots is equal to or greater than 500 nm and equal to or less than 600 nm, a thickness of the first dielectric film is equal to or greater than 157 nm and equal to or less than 189 nm, and a thickness of the second dielectric film is equal to or greater than 242 nm and equal to or less than 291 nm. 
     The light-emitting element according to a fifteenth aspect of the present invention may have, in the configuration according to any one of the seventh to twelfth aspects described above, a configuration in which the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots is equal to or greater than 600 nm and equal to or less than 780 nm, a thickness of the first dielectric film is equal to or greater than 189 nm and equal to or less than 246 nm, and a thickness of the second dielectric film is equal to or greater than 291 nm and equal to or less than 378 nm. 
     The light-emitting element according to a sixteenth aspect of the present invention may have, in the configuration according to any one of the seventh to fifteenth aspects, a configuration according to any one of claims  7  to  15 , in which a sum of the number of layers of the first dielectric film and the number of layers of the second dielectric film included in the dielectric multilayer film is equal to or greater than three. 
     The light-emitting element according to a seventeenth aspect of the present invention may have, in the configuration according to any one of the first to sixteenth aspects, a configuration in which the light-emitting element further includes a photoluminescence layer provided between the selectively reflective layer and the transparent electrode, and the photoluminescence layer is configured to be excited by light emitted by the light-emitting layer and configured to emit light of a color identical to a color of the light emitted by the light-emitting layer. 
     A display device according to an eighteenth aspect of the present invention includes a light-emitting element including a transparent electrode, a reflective electrode, and a light-emitting layer, the light-emitting element serving as a red pixel, a light-emitting element including a transparent electrode, a reflective electrode, and a light-emitting layer, the light-emitting element serving as a green pixel, and the light-emitting element having the configuration according to any one of the first to seventeenth aspects described above, the light-emitting element serving as a blue pixel, and the selectively reflective layer of the blue pixel is formed across the red pixel, the green pixel, and the blue pixel. 
     A display device according to a nineteenth aspect of the present invention includes a light-emitting element including a transparent electrode, a reflective electrode, and a light-emitting layer, the light-emitting element serving as a red pixel, a light-emitting element including a transparent electrode, a reflective electrode, and a light-emitting layer, the light-emitting element serving as a green pixel, and the light-emitting element having the configuration according to any one of the first to seventeenth aspects described above, the light-emitting element serving as a blue pixel, and the selectively reflective layer of the blue pixel is formed only in the blue pixel. 
     A display device according to a twentieth aspect of the present invention includes the light-emitting element having the configuration according to any one of the first to seventeenth aspects describe above, the light-emitting element serving as a blue pixel, the light-emitting element having the configuration according to any one of the first to seventeenth aspects describe above, the light-emitting element serving as a red pixel, and the light-emitting element having the configuration according to any one of the first to seventeenth aspects describe above, the light-emitting element serving as a green pixel. 
     The display device according to a twenty-first aspect of the present invention may have, in the configuration according to the eighteenth or nineteenth aspect described above, a configuration in which the transparent electrode of the blue pixel is formed integrally with the transparent electrodes of the red pixel and the green pixel. 
     The display device according to a twenty-second aspect of the present invention may have, in the configuration according to the nineteenth or twentieth aspect described above, a configuration in which the transparent electrode of the blue pixel is formed separately from the transparent electrodes of the red pixel and the green pixel. 
     The display device according to a twenty-third aspect of the present invention may have, in the configuration according to the twenty-second aspect described above, a configuration in which the transparent electrode of the blue pixel is surrounded by a light-blocking body configured to block light of the blue pixel. 
     The display device according to a twenty-fourth aspect of the present invention may have, in the configuration according to the twenty-third aspect described above, a configuration in which the selectively reflective layer of the blue pixel is surrounded by the light-blocking body. 
     A light-emitting element according to a twenty-fifth aspect of the present invention includes a first transparent electrode, a second transparent electrode, a light-emitting layer provided between the first transparent electrode and the second transparent electrode, the light-emitting layer including a quantum dots, a first selectively reflective layer provided at an opposite side to the light-emitting layer with respect to the first transparent electrode, the first selectively reflective layer having a reflection band having a higher reflectivity than a reflectivity of another band, and a second selectively reflective layer provided at an opposite side to the light-emitting layer with respect to the second transparent electrode, the second selectively reflective layer having a reflection band having a higher reflectivity than a reflectivity of another band, a wavelength at a long wavelength end in the reflection band of the first selectively reflective layer is longer than a wavelength at which a light emission spectrum due to electroluminescence of the quantum dots has a half value of a peak value of the light emission spectrum due to the electroluminescence of the quantum dots at a shorter wavelength side than a peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and is shorter than a wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots at a longer wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and a wavelength at a long wavelength end in the reflection band of the second selectively reflective layer is longer than the wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots at the shorter wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots, and is shorter than the wavelength at which the light emission spectrum due to the electroluminescence of the quantum dots has the half value of the peak value of the light emission spectrum due to the electroluminescence of the quantum dots at the longer wavelength side than the peak wavelength of the light emission spectrum due to the electroluminescence of the quantum dots. 
     The present invention is not limited to each of the embodiments described above, and various modifications may be made within the scope of the claims. Embodiments obtained by appropriately combining technical approaches disclosed in each of the different embodiments also fall within the technical scope of the present invention. Furthermore, novel technical features can be formed by combining the technical approaches disclosed in each of the embodiments. 
     REFERENCE SIGNS LIST 
     
         
           22  Anode (transparent electrode, second transparent electrode) 
           23  Edge cover (light-blocking body) 
           25  Cathode (reflective electrode, first transparent electrode) 
           35   b  Blue light-emitting layer 
           35   g  Green light-emitting layer 
           35   r  Red light-emitting layer 
           40  Selectively reflective layer (dielectric multilayer film) 
           40   b  Blue selectively reflective layer (dielectric multilayer film) 
           40   g  Green selectively reflective layer (dielectric multilayer film) 
           40   r  Red selectively reflective layer (dielectric multilayer film) 
           41  First dielectric film 
           42  Second dielectric film 
         Pr Red pixel (light-emitting element) 
         Pg Green pixel (light-emitting element) 
         Pb Blue pixel (light-emitting element) 
           44  First selectively reflective layer 
           44   b  Blue first selectively reflective layer 
           44   g  Green first selectively reflective layer 
           44   r  Red first selectively reflective layer 
           45  Photoluminescence layer 
           46  Second selectively reflective layer 
           46   b  Blue second selectively reflective layer 
           46   g  Green second selectively reflective layer 
           46   r  Red second selectively reflective layer