Patent Publication Number: US-2011068330-A1

Title: Light emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No.2009-216892, filed on Sep. 18, 2009; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a light emitting device. 
     BACKGROUND 
     Recently, attention has been focused on thin light emitting sources which can be used for display backlights, illumination devices and the like. Such a light emitting device needs to satisfy certain requirements including color temperature, color rendition, or in depending on the mode of use, preference and the like. Depending on such requirements, it is possible to adopt a structure of stacking light emitting elements with different emission colors (see, e.g., JP-A-2006-155940). In such a light emitting device, emission light from the stacked light emitting elements is mixed, which makes it possible to obtain white light with predetermined color rendition, for instance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are principal schematics of a light emitting device; 
         FIGS. 2A and 2B  illustrate the function and effect of the light emitting device; 
         FIGS. 3A and 3B  are principal schematics of a light emitting device; 
         FIGS. 4A and 4B  are principal schematics of a light emitting device; and 
         FIGS. 5A and 5B  are principal schematics of a light emitting device. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments will now be described with reference to the drawings. In the embodiments, a first reflective layer configured to reflect light in a first wavelength band, a first light emitting element configured to emit light in the first wavelength band, a second reflective layer with transmittance for light in the first wavelength band being higher than transmittance for light in a second wavelength band different from the first wavelength band, and a second light emitting element configured to emit light in the second wavelength band, are stacked in this order. 
     First Embodiment 
       FIGS. 1A and 1B  are principal schematics of a light emitting device.  FIG. 1A  is a principal plan view of the light emitting device  1 , and  FIG. 1B  shows the X-Y cross section of  FIG. 1A . 
     The light emitting device  1  includes a transparent substrate  23  constituting a window member, a substrate  20  opposed to the transparent substrate  23 , light emitting elements  10 G,  10 R,  10 B stacked between the transparent substrate  23  and the substrate  20 , transparent substrates (light transmissive substrates)  21 ,  22  provided between the stacked light emitting elements, and dichroic mirrors (reflective layers)  40 R,  40 B formed on the major surface (upper surface or lower surface) of the transparent substrates  21 ,  22 , respectively. 
     The light emitting element can illustratively be any of various light emitting elements, such as an organic EL element, an inorganic EL element, or a light emitting diode primarily composed of semiconductor materials. 
     In the embodiments described below, a description is given mainly of configurations using an organic EL element as an example of the light emitting element. 
     The light emitting device  1  is a planar light emitting device and has a structure in which a plurality of light emitting elements are stacked with the major surfaces opposed to each other. The planar shape may be a square or rectangle. Alternatively, various other shapes can be used. 
     Specifically, the light emitting device  1  has a structure in which, for instance, three light emitting elements  10 G,  10 R,  10 B are stacked with the major surfaces opposed to each other. The light emitting elements  10 G,  10 R,  10 B emit green (first wavelength band), red (second wavelength band), and blue (third wavelength band), respectively. The thickness of the light emitting elements  10 G,  10 R,  10 B is illustratively 100 to 500 nm. 
     The light emitting element  10 G (first light emitting element) is provided on the upper surface of the substrate  20  having an area comparable to or larger than the area of the major surface of the light emitting element  10 G. The substrate  20  may have a monolayer structure or multilayer structure. A reflective film  30  (first reflective layer) is formed immediately above the substrate  20 . That is, this reflective film  30  is interposed between the light emitting element  10 G and the substrate  20 . 
     The light emitting element  1 OR (second light emitting element) is provided on the upper surface of the transparent substrate  21  having an area comparable to or larger than the area of the major surface of the light emitting element  10 R. The transparent substrate  21  may have a monolayer structure or multilayer structure. The dichroic mirror  40 R (second reflective layer) is provided on the lower surface side of the transparent substrate  21 . 
     This dichroic mirror  40 R reflects light with a red wavelength, and transmits light with a green wavelength emitted from the light emitting element  10 G. That is, in the dichroic mirror  40 R, the reflectance for light with the red wavelength is higher than the reflectance for green light, and the transmittance for light with the green wavelength is higher than the transmittance for red light. The dichroic mirror  40 R like this may be formed on the upper surface side of the transparent substrate  21 . 
     The light emitting element  10 B (third light emitting element) is provided on the upper surface of the transparent substrate  22  having an area comparable to or larger than the area of the major surface of the light emitting element  10 B. The transparent substrate  22  may have a monolayer structure or multilayer structure. The dichroic mirror  40 B (third reflective layer) is provided on the lower surface side of the transparent substrate  22 . 
     This dichroic mirror  40 B reflects light with a blue wavelength, and transmits green light emitted from the light emitting element  10 G and red light emitted from the light emitting element  10 R. Alternatively, the dichroic mirror  40 B may transmit light except the blue wavelength. That is, in the dichroic mirror  40 B, the reflectance for light with the blue wavelength is higher than the reflectance for green light and red light, and the transmittance for light with the green and red wavelength is higher than the transmittance for blue light. Then, in the third reflective layer (dichroic mirror  40 B), the transmittance for the light in the third wavelength band (blue wavelength) is less than the transmittance for the light in the first wavelength band (green wavelength) and the transmittance for the light in the second wavelength band (red wavelength). The dichroic mirror  40 B like this may be formed on the upper surface side of the transparent substrate  22 . 
     The transparent substrate  23  having an area comparable to or larger than the area of the major surface of the light emitting element  10 B is provided above the light emitting element  10 B. Furthermore, a resin member  50  with light transmissivity and insulating property is provided between the transparent substrate  23  and the transparent substrate  22 , between the dichroic mirror  40 B and the transparent substrate  21 , and between the dichroic mirror  40 R and the reflective film  30 . The resin member  50  is interposed also between the transparent substrate  23  and the light emitting element  10 B, between the dichroic mirror  40 B and the light emitting element  10 R, and between the dichroic mirror  40 R and the light emitting element  10 G. Furthermore, the upper surface of the transparent substrate  23  serves as a light extraction surface  23   s  of the light emitting device  1 . 
     The material of the substrate  20  is illustratively a glass, transparent resin, or metal. The material of the transparent substrates  21 ,  22 ,  23  is illustratively a glass or transparent resin. The thickness of the substrate  20  and the transparent substrates  21 ,  22 ,  23  is 0.1 to 1.0 mm. The substrate  20  and the transparent substrates  21 ,  22 ,  23  have light transmissivity. 
     The material of the reflective film  30  is illustratively silver (Ag) or aluminum (Al). 
     The material of the resin member  50  is illustratively epoxy resin. 
     The dichroic mirror  40 B,  40 R illustratively has a stacked structure in which a multilayer film of dielectric (e.g., SiO 2 , Al 2 O 3 , and TiO 2 ) and the like is provided on a translucent substrate. Specifically, for instance, by alternately stacking two kinds of different dielectric layers and adjusting the refractive index and thickness of these dielectric layers, a dichroic mirror having a specific reflection/transmission spectrum can be formed like a so-called DBR (distributed Bragg reflector). Here, in the case where the substrate  20  is made of a material having high reflectance such as stainless steel and aluminum (Al), the reflective film  30  may be omitted. 
     Each light emitting element  10 G,  10 R,  10 B illustratively has an anode  10   a  and a cathode  10   c . The light emitting element  10 G,  10 R,  10 B has light transmissivity. In the case where the light emitting element  10 G,  10 R,  10 B is an organic EL element, a stacked film  10   gl ,  10   rl ,  10   bl  composed of a hole transport layer, a light emitting layer, and an electron transport layer in this order from the anode  10   a  toward the cathode  10   c  is provided between the anode  10   a  and the cathode  10   c . When holes and electrons are injected from the anode  10   a  and the cathode  10   c , respectively, into the light emitting layer, light is emitted by recombination of holes and electrons. 
     The anode  10   a  is illustratively a transparent electrode made of a material such as indium in oxide (ITO), in oxide (SnO 2 ), and aluminum-containing zinc oxide (ZnO:Al). 
     The cathode  10   c  is a light transmissive electrode illustratively made of an ultrathin metal material such as a magnesium silver alloy (MgAg), magnesium indium alloy (MgIn), and aluminum (Al). The cathode  10   c  may be covered with a transparent conductive material layer such as an ITO layer. 
     The hole transport layer is composed of an organic-containing layer. Its material can illustratively be N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD), or bis(ditolylaminophenyl)cyclohexane (TPAC). The ionization energy of the hole transport layer lies between the work function of the anode  10   a  and the ionization energy of the light emitting layer. 
     The light emitting layer is an organic-containing layer, including a host material and a dopant. The host material can illustratively be tris(8-hydroxyquinolinate) aluminum (Alq 3 ) or the like. The dopant is any of various materials (such as coumarin, quinacridone, styrylamine, and perlyne) developing blue, red, and green. 
     The light emitting layer is not limited to these materials, but can illustratively be made of Be-benzoquinolinol (BeBq 2 ), benzothiazole-based, benzimidazole-based, benzoxazole-based and other fluorescent brightening agents, metal chelated oxonium compounds, styrylbenzene-based compounds, distyrylpyrazine derivatives, aromatic dimethylidine-based compounds, naphthalimide derivatives, perylene derivatives, oxadiazole derivatives, aldazine derivatives, cyclopentadiene derivatives, styrylamine derivatives, coumarin-based derivatives, and aromatic dimethylidine derivatives or the like. 
     The light emitting layer can be made of polymer organic materials as well as low-molecular organic materials. Furthermore, phosphorescent materials such as Btp 2 Ir(acac), Ir(ppy) 3 , and Flrpic may be used for light emitting materials. 
     The electron transport layer is composed of an organic-containing layer. Its material can illustratively be Alq 3 , oxadiazole derivatives (tBu-PBD), and 1,3,4-oxazole derivatives (OXD). The electron affinity of the electron transport layer lies between the electron affinity of the light emitting layer and the work function of the cathode  10   c.    
     The light emitting elements  10 G,  10 R,  10 B in the light emitting device  1  can be independently controlled by power supplies  60 G,  60 R,  60 B, respectively. Alternatively, light emission of these light emitting elements  10 G,  10 R,  10 B may each be controlled by a single power supply through suitable adjustment circuits. By adjusting the driving voltage of each light emitting element  10 G,  10 R,  10 B, an arbitrary color can be provided from the light emitting device  1 . 
     In the case where the light emitting element is an inorganic EL element, the aforementioned light emitting layer is replaced by a stacked film of a thin-film insulating layer (lower layer), light emitting layer, and thin-film insulating layer (upper layer). In this case, AC power supplies are used as the power supplies  60 G,  60 R,  60 B. The thin-film insulating layer is illustratively made of Y 2 O 3 , Ta 2 O 5 , Al 2 O 3 , and transparent dielectrics such as Si 3 N 4 , BaTiO 3 , and SrTiO 3 . The light emitting layer is a phosphor thin film illustratively made of ZnS:Mn or the like. 
     In the case where the light emitting element is a light emitting diode composed of semiconductor materials, it is primarily composed of, for instance, silicon (Si), gallium arsenide (GaAs), gallium nitride (GaN), and indium gallium nitride (InGaN) or the like. 
     Next, the function and effect of the light emitting device  1  are described. 
       FIGS. 2A and 2B  illustrate the function and effect of the light emitting device.  FIG. 2A  shows the light emitting device  1  according to this embodiment, and  FIG. 2B  shows a light emitting device  100  of a comparative example. The light emitting device  100  has a structure in which the dichroic mirrors  40 B,  40 R are removed from the light emitting device  1 . The light emitting device  100  has the same planar size as the light emitting device  1 . 
     First, the function and effect of the light emitting device  1  are described. 
     The power supply  60 G is used to drive the light emitting element  10 G located at the bottom of the light emitting device  1 . Green light  70   g  is emitted upward and downward from the light emitting element  10 G. Here, the reflective film  30  is provided on the substrate  20 . Hence, the light  70   g  directed downward from the light emitting element  10 G is reflected by the reflective film  30 . Thus, the light  70   g  generated from the light emitting element  10 G is emitted above the substrate  20 . 
     This light  70   g  reaches the dichroic mirror  40 R. However, the dichroic mirror  40 R transmits light except red. Thus, the light  70   g  passes through the dichroic mirror  40 R and further through the transparent substrate  21 , and reaches the position of the light emitting element  10 R. 
     This light  70   g  reaches the dichroic mirror  40 B. However, the dichroic mirror  40 B transmits light except blue. Thus, the light  70   g  passes through the dichroic mirror  40 B and further through the transparent substrate  22 , and reaches the position of the light emitting element  10 B. Then, this light  70   g  passes through the transparent substrate  23  and is emitted from a light extraction surface  23   s.    
     On the other hand, the power supply  60 R is used to drive the light emitting element  1 OR located at the midpoint of the light emitting device  1 . Red light  70   r  is emitted upward and downward from the light emitting element  10 R. Here, the dichroic mirror  40 R is provided below the transparent substrate  21 . The dichroic mirror  40 R selectively reflects red light. Hence, the light  70   r  directed downward from the light emitting element  1 OR is reflected by the dichroic mirror  40 R. Thus, the light  70   r  generated from the light emitting element  1 OR is emitted above the dichroic mirror  40 R. 
     This light  70   r  reaches the dichroic mirror  40 B. However, the dichroic mirror  40 B transmits light except blue. Thus, the light  70   r  passes through the dichroic mirror  40 B and further through the transparent substrate  22 , and reaches the position of the light emitting element  10 B. Then, this light  70   r  passes through the transparent substrate  23  and is emitted from the light extraction surface  23   s.    
     Furthermore, the power supply  60 B is used to drive the light emitting element  10 B located at the top of the light emitting device  1 . Blue light  70   b  is emitted upward and downward from the light emitting element  10 B. Here, the dichroic mirror  40 B is provided below the transparent substrate  22 . The dichroic mirror  40 B selectively reflects blue light. Hence, the light  70   b  directed downward from the light emitting element  10 B is reflected by the dichroic mirror  40 B. Thus, the light  70   b  generated from the light emitting element  10 B is emitted above the dichroic mirror  40 B. Then, the light  70   b  passes through the transparent substrate  23  and is emitted from the light extraction surface  23   s.    
     Thus, the light emitting device  1  includes the substrate  20  with a plurality of light emitting elements stacked thereon, and dichroic mirrors (reflective layers) each provided between the light emitting elements. Each dichroic mirror reflects light emitted from the closest light emitting element located on the side opposite to the substrate  20 , and transmits light emitted from the light emitting elements located therebelow. 
     For instance, the dichroic mirror  40 R,  40 B reflects light emitted from the light emitting element neighboring on the transparent substrate (window member)  23  side, and transmits light except this light. Thus, all the green, red, and blue light are efficiently emitted from the light extraction surface  23   s . Furthermore, the intensity of each of green, red, and blue light can be independently adjusted. 
     In contrast, the light emitting device  100  shown in  FIG. 2B  does not include the dichroic mirrors  40 B,  40 R. 
     In the light emitting device  100  like this, the blue light  70   b  emitted from the light emitting element  10 B passes through the transparent substrate  22 , which is the foundation of the light emitting element  10 B, and reaches the position of the light emitting element  10 R. Furthermore, this light  70   b  may pass through the transparent substrate  21 , which is the foundation of the light emitting element  10 R, and reach the position of the light emitting element  10 G. 
     The light  70   b  transmitted through the transparent substrate  22  is absorbed inside the light emitting device  100  or scattered at the layer interface inside the light emitting device  100 . Furthermore, the light  70   b  may be multiply reflected in a particular layer. Such phenomena may occur also for the red light  70   r  emitted from the light emitting element  10 R. 
     That is, in the light emitting device  100 , the light  70   b  is diffused not only above the light emitting element  10 B but also therebelow. Likewise, the light  70   r  is diffused not only above the light emitting element  1 OR but also therebelow. This causes loss for the component of the light  70   b ,  70   r  diffused downward. Consequently, the light emitting device  100  has a lower light extraction efficiency or light emission efficiency (the amount of light which can be extracted from the light extraction surface  23   s ) than the light emitting device  1 . 
     In contrast, the light emitting device  1  includes dichroic mirrors  40 B,  40 R on the lower surface side of the light emitting element  10 B,  10 R, respectively. Such a structure can suppress the downward diffusion of the light  70   b ,  70   r . Thus, the light emitting device  1  has a higher light extraction efficiency or light emission efficiency than the light emitting device  100 . 
     Furthermore, with the increase in light emission efficiency, the light emitting device  1  achieves blue intensity and red intensity comparable to those of the light emitting device  100  by lower power than that inputted to the light emitting element  10 B,  1 OR of the light emitting device  100 . Thus, the light emitting device  1  has a longer lifetime than the light emitting device  100 . 
     Furthermore, in the light emitting device  1 , the dichroic mirror  40 R,  40 B is attached to the major surface of the transparent substrate  21 ,  22 . This increases the thermal volume of the foundation of the light emitting element  10 B,  10 R, and enhances the heat dissipation effect of the light emitting element  10 B,  10 R. That is, the light emitting elements acting as heat sources are spaced from each other, and thereby the increase in temperature can be suppressed. Consequently, the lifetime of the light emitting element  10 B,  1 OR is further extended. 
     Thus, the light emission efficiency and lifetime of the light emitting device  1  are significantly increased. 
     From the viewpoint of light extraction efficiency, it is preferable to determine the stacking order of the light emitting elements by taking into consideration the absorption of light by other light emitting elements. For instance, the amount of absorption (loss) incurred when the light  70   b  emitted from the light emitting element  10 B passes through the light emitting element  1 OR is compared with the amount of absorption (loss) incurred when the light  70   r  emitted from the light emitting element  1 OR passes through the light emitting element  10 B. If the former is larger, then as illustrated in  FIGS. 1A to 2B , it is preferable that the light emitting element  10 B be placed above the light emitting element  10 R. That is, with regard to the light emitting element  1 OR and the light emitting element  10 B, preferably, the light emitting element  10 B is placed closer to the light extraction surface  23   s.    
     This also applies to the vertical relationship between the light emitting element  10 B and the light emitting element  10 G, and the vertical relationship between the light emitting element  1 OR and the light emitting element  10 G. 
     Next, variations of the light emitting device are described. In the figures illustrated below, the same members as those in the light emitting device  1  are labeled with like reference numerals, and the detailed description of like members and like configurations is omitted as appropriate. Also in the variations illustrated below, a description is given by taking an organic EL element as an example of the light emitting element. 
     Second Embodiment 
       FIGS. 3A and 3B  are principal schematics of a light emitting device.  FIG. 3A  is a principal plan view of the light emitting device  2 , and  FIG. 3B  shows the X-Y cross section of  FIG. 3A . 
     In the light emitting device  2 , the aforementioned substrate  20  is replaced by a transparent substrate  24 . 
     For instance, the light emitting element  10 G is provided on the upper surface of the transparent substrate  24  having an area comparable to or larger than the area of the major surface of the light emitting element  10 G. The transparent substrate  24  may have a monolayer structure or multilayer structure. The light emitting device  2  does not include the aforementioned reflective film  30 , but a dichroic mirror  40 G (fourth reflective layer) is provided on the lower surface side of the transparent substrate  24 . The material of the transparent substrate  24  is illustratively a glass or transparent resin. The dichroic mirror  40 G illustratively has a stacked structure in which a multilayer film of dielectric (e.g., SiO 2 , Al 2 O 3 , and TiO 2 ) and the like is provided on a mirror surface. 
     This dichroic mirror  40 G reflects light with a green wavelength, and transmits light except the green wavelength. That is, in the dichroic mirror  40 G, the reflectance for light with the green wavelength is higher than the reflectance for red light and blue light, and the transmittance for light with the red and blue wavelength is higher than the transmittance for green light. Then, in the fourth reflective layer (dichroic mirror  40 G), the transmittance for the light in the first wavelength band is less than the transmittance for the light in the second wavelength band and the transmittance for the light in the third wavelength band. The dichroic mirror  40 G like this may be formed on the upper surface side of the transparent substrate  24 . The upper surface of the transparent substrate  23  serves as the light extraction surface  23   s  of the light emitting device  2 . 
     That is, the light emitting device  2  has a structure in which a unit  80 G including the dichroic mirror  40 G, the transparent substrate  24 , and the light emitting element  10 G, a unit  80 R including the dichroic mirror  40 R, the transparent substrate  21 , and the light emitting element  10 R, and a unit  80 B including the dichroic mirror  40 B, the transparent substrate  22 , and the light emitting element  10 B, are stacked. 
     Also in the light emitting device  2  like this, the green light  70   g  is emitted upward and downward from the light emitting element  10 G. Here, the dichroic mirror  40 G is provided below the transparent substrate  24 . The dichroic mirror  40 G selectively reflects green light. Hence, the light  70   g  directed downward from the light emitting element  10 G is reflected by the dichroic mirror  40 G. Thus, the light  70   g  generated from the light emitting element  10 G is emitted above the dichroic mirror  40 R. That is, the light emitting device  2  also achieves an effect similar to that of the light emitting device  1   
     Furthermore, in the light emitting device  2 , the transparent substrate  24 , which is transparent, and the dichroic mirror  40 G transmitting light except green are used as the foundation of the light emitting element  10 G. Thus, an additional light emitting unit can be provided below the unit  80 G. This increases the design flexibility of the light emitting device  2 . 
     In particular, if at least one of the unit  80 R and the unit  80 B is additionally provided below the unit  80 G, the light  70   r  or light  70   b  can be emitted with higher intensity. 
     Third Embodiment 
       FIGS. 4A and 4B  are principal schematics of a light emitting device.  FIG. 4A  is a principal plan view of the light emitting device  3 , and  FIG. 4B  shows the X-Y cross section of  FIG. 4A .  FIG. 4B  shows cross sections before and after assembly of the light emitting device. 
     The light emitting device  3  has a structure in which each light emitting element  10 G,  10 R,  10 B is sealed in between opposed substrates. For instance, the resin member  50  is sealed in between the reflective film  30  and a transparent substrate  25 , between the transparent substrate  21  and a transparent substrate  26 , and between the transparent substrate  22  and the transparent substrate  23 . Furthermore, the resin member  50  is filled also between each light emitting element and the transparent substrate  23 ,  25 ,  26 , and the light emitting element  10 G,  10 R,  10 B is sealed with the resin member  50 . The material of the transparent substrate  25 ,  26  is illustratively a glass or transparent resin. The transparent substrate  25 ,  26  has light transmissivity. 
     Here, the unit  80 G (first unit) in which the substrate  20 , the reflective film  30 , the light emitting element  10 G, and the transparent substrate  25  are stacked in this order, the unit  80 R (second unit) in which the dichroic mirror  40 R, the transparent substrate  21 , the light emitting element  10 R, and the transparent substrate  26  are stacked in this order, and the unit  80 B (third unit) in which the dichroic mirror  40 B, the transparent substrate  22 , the light emitting element  10 B, and the transparent substrate  23  are stacked in this order, are manufactured independently (see the left side of  FIG. 4B ). Then, these units  80 G,  80 R,  80 B are stacked via a transparent resin member  51  in the direction of arrow A (see the right side of  FIG. 4B ). The resin member  51  is illustratively made of a cohesive material, adhesive material or the like. The resin member  51  has light transmissivity. 
     Thus, in the light emitting device  3 , the adjacent units including the dichroic mirror and the light emitting element can be separated from each other. The light emitting device  3  like this also achieves an effect similar to that of the light emitting device  1 . 
     Furthermore, in the light emitting device  3 , because the units  80 G,  80 R,  80 B are independently prepared, the combination of the units can be easily changed depending on the purpose of the light emitting device. 
     Furthermore, by using a cohesive material as the resin member  51 , the units can be easily stuck to and separated from each other. This facilitates unit replacement. 
     Fourth Embodiment 
       FIGS. 5A and 5B  are principal schematics of a light emitting device.  FIG. 5A  is a principal plan view of the light emitting device  4 , and  FIG. 5B  shows the X-Y cross section of  FIG. 5A . 
     The light emitting device  4  has a structure in which the aforementioned transparent substrates  21 ,  22  are removed. 
     The light emitting element  10 G of the light emitting device  4  is provided on the upper surface of the substrate  20  having an area comparable to or larger than the area of the major surface of the light emitting element  10 G. A reflective film  30  is formed immediately on the substrate  20 . That is, this reflective film  30  is interposed between the light emitting element  10 G and the substrate  20 . The upper surface and the side surface of the light emitting element  10 G are covered with a buffer layer  52 . The dichroic mirror  40 R is provided on the buffer layer  52 . A buffer layer  53  is provided on the dichroic mirror  40 R. The dichroic mirror  40 R is sandwiched between the buffer layer  52  and the buffer layer  53 . 
     Furthermore, the light emitting element  1 OR is provided on the buffer layer  53  above the major surface of the light emitting element  10 G. The upper surface and the side surface of the light emitting element  1 OR are covered with a buffer layer  54 . The dichroic mirror  40 B is provided on the buffer layer  54 . A buffer layer  55  is provided on the dichroic mirror  40 B. The dichroic mirror  40 B is sandwiched between the buffer layer  54  and the buffer layer  55 . 
     Furthermore, a resin member  50  is provided between the transparent substrate  23  and the buffer layer  55 . The resin member  50  is interposed also between the transparent substrate  23  and the light emitting element  10 B. 
     The material of the buffer layer  52 ,  53 ,  54 ,  55  is illustratively silicon oxide (SiO 2 ), alumina (Al 2 O 3 ) and the like. The buffer layer  52 ,  53 ,  54 ,  55  has light transmissivity. By providing the buffer layer  52 ,  53 ,  54 ,  55  between the dichroic mirror and the light emitting element, impurity diffusion from the dichroic mirror into the light emitting element is suppressed, and the adhesion strength between the dichroic mirror and the light emitting element is increased. Here, at least one of the buffer layers  52 ,  53 ,  54 ,  55  may be omitted as necessary. For instance, this embodiment also includes the configuration in which the light emitting elements  10 R,  10 B are provided immediately on the dichroic mirrors  40 R,  40 B, respectively. 
     Thus, in the light emitting device  4 , the transparent substrates  21 ,  22  are removed, and the light emitting elements  10 R,  10 B are provided above the dichroic mirrors  40 R,  40 B. 
     The light emitting device  4  like this also achieves an effect similar to that of the light emitting device  1 . Furthermore, in the light emitting device  4 , because the transparent substrates  21 ,  22  are removed, loss due to absorption and scattering of the light  70   b ,  70   g ,  70   r  is further reduced. This further increases the light emission efficiency. 
     The embodiments have been described with reference to examples. The light emitting devices described above achieve sufficient characteristics in terms of color temperature, color rendition and the like depending on the use environment. That is, the light emitting device of the present embodiments can provide sufficient brightness as a light source and realize long lifetime. Furthermore, the light emission color can be suitably adjusted as necessary. That is, the light emitting device of the present embodiments can provide an arbitrary color more efficiently and easily. 
     The embodiments are not limited to the above examples. That is, these examples can be suitably modified by those skilled in the art, and such modifications are also encompassed within the scope of the embodiments as long as they include the features of the embodiments. For instance, the components of the above examples and the layout, material, condition, shape, size and the like thereof are not limited to those illustrated, but can be suitably modified. 
     For instance, in the examples of the light emitting elements, the number of them is not limited to three, such as the light emitting elements  10 G,  10 R,  10 B. Furthermore, the order of stacking the light emitting elements  10 G,  10 R,  10 B illustrated in the figures is merely an example, and not limited to this stacking order. For instance, as shown in  FIG. 2A , the light emitting element located on the upper layer side undergoes less absorption by the light emitting elements and reflective layers located thereabove (see the length of the arrows). Thus, the light emitting element located on the upper layer side can be driven by a lower voltage (or current), and its lifetime can be extended. Hence, preferably, among the light emitting elements  10 G,  10 R,  10 B, the light emitting element (e.g., blue light emitting element) having a relatively short lifetime (the lifetime for operation under the rated condition) is located on the upper layer side. 
     Furthermore, the components of the above embodiments can be combined with each other as long as technically feasible, and such combinations are also encompassed within the scope of the embodiments as long as they include the features of the embodiments. 
     Furthermore, those skilled in the art can conceive various modifications and variations within the spirit of the embodiments, and it is understood that such modifications and variations are also encompassed within the scope of the embodiments. For instance, the light emitting device in the present embodiments is applicable also to a display device including a plurality of light emitting elements, and such a display device is encompassed within the scope of the embodiments. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel devices and methods described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the devices and methods described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.