Patent Publication Number: US-10784460-B2

Title: Electroluminescent device and electroluminescent display device including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/836,732 filed on Dec. 8, 2017, which claims the benefit under 35 U.S.C. § 119(a) of Republic of Korea Patent Application No. 2016-0172405, filed on Dec. 16, 2016, which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an electroluminescent device, and more particularly, to an electroluminescent device having high luminous efficiency due to a soft cavity effect, and an electroluminescent display device including the same. 
     2. Discussion of the Related Art 
     With the advancement of an information society, the demand for a display device which displays an image is increasing in various forms. A flat panel display device including an electroluminescent display device, such as a liquid crystal display (LCD) device, a plasma display panel (PDP) device, or an organic light-emitting diode (OLED) display device, which is thinner and lighter in weight than a conventional cathode ray tube (CRT) display device is being actively researched and commercialized. 
     The OLED display device which is an electroluminescent display device includes an OLED as an essential component and does not need a backlight used for an LCD device which is a non-light-emitting device, and may thus be manufactured to be light in weight and thin. Furthermore, the OLED display device is advantageous in terms of power consumption, can be driven with a low voltage, and has a high response rate, when compared to the LCD device. 
     For example, the OLED includes a hole injection layer (HIL), a hole transporting layer (HTL), an emitting material layer (EML), an electron transporting layer (ETL), and an electron injection layer (EIL) which are sequentially stacked between an anode and a cathode. The HIL, the HTL, the EML, the ETL, and the EIL are formed by a deposition process. 
     However, there is a limit in manufacturing a large-sized OLED using the deposition process. Thus, a soluble OLED formed by a soluble process has been proposed. 
     For example, the soluble OLED may include an anode, a cathode facing the anode, an EML located between the anode and the cathode, a HIL located between the anode and the EML, an ETL located between the EML and the cathode. 
     However, there is a limit in achieving a cavity effect in such an OLED having a multilayered structure. Accordingly, the brightness of the OLED and an OLED display device including the same are limited. 
     SUMMARY 
     Embodiments relate to solving a problem of the brightness of an electroluminescent device and an electroluminescent display device including the same by increasing a cavity in the electroluminescent device. 
     One or more embodiments relate to an electroluminescent device including an anode and a cathode facing each other, a light compensation layer located between the anode and the cathode and having a first refractive index, and an emitting material layer located between the light compensation layer and the cathode, and having a second refractive index higher than the first refractive index. 
     One or more embodiments relate to an electroluminescent display device including a substrate, the above-described electroluminescent device located on a substrate, and a thin-film transistor located between the substrate and the electroluminescent device, and connected to the electroluminescent device. 
     Advantages and features of the disclosure will be set forth in part in the description, which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the disclosure. Other advantages and features of the embodiments herein may be realized and attained by the structures particularly pointed out in the written description and claims herein as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are explanatory, and are intended to provide further explanation of the embodiments as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the disclosure, are incorporated in and constitute a part of this specification, illustrate implementations of the disclosure and together with the description serve to explain the principles of embodiments of the disclosure. 
         FIG. 1  is a schematic cross-sectional view showing an organic light-emitting diode (OLED). 
         FIG. 2  is a diagram for explaining a cavity phenomenon occurring in the OLED of  FIG. 1 . 
         FIG. 3  is a schematic circuit diagram of an electroluminescent display device according to the present disclosure. 
         FIG. 4  is a schematic cross-sectional view of an electroluminescent display device according to the present disclosure. 
         FIG. 5  is a schematic cross-sectional view of an electroluminescent device according to a first embodiment of the present disclosure. 
         FIG. 6  is a schematic cross-sectional view of an electroluminescent device according to a second embodiment of the present disclosure. 
         FIG. 7  is a diagram for explaining a cavity phenomenon occurring in an electroluminescent device according to the present disclosure. 
         FIGS. 8A and 8B  are graphs showing characteristics of an electroluminescent device according to whether an electron transporting layer (ETL) is provided or not. 
         FIGS. 9A to 9C  are graphs showing characteristics of an electroluminescent device according to whether a light compensation layer is provided or not. 
         FIG. 10  is a schematic cross-sectional view showing an electroluminescent device according to a third embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. In the following description, when a detailed description of well-known functions or configurations related to this document is determined to unnecessarily cloud the gist of an embodiment of the disclosure, the detailed description thereof will be omitted. The progression of processing steps and/or operations described is an example; however, the sequence of steps and/or operations is not limited to that set forth herein and may be changed as is known in the art, with the exception of steps and/or operations necessarily occurring in a certain order. Like reference numerals designate like elements throughout. Names of the respective elements used in the following description are selected only for convenience in writing the specification and may be thus different from those used in actual products. 
       FIG. 1  is a schematic cross-sectional view of an organic light-emitting diode (OLED). 
     As illustrated in  FIG. 1 , an OLED  1  may include an anode  10 , a cathode  20  facing the anode  10 , and an organic emissive layer  30  which is located between the anode  10  and the cathode  20  and includes a hole injection layer (HIL)  32 , an emitting material layer (EML)  34 , and an electron transporting layer (ETL)  36 . 
     In this case, the anode  10  includes first and second transparent electrode layers  12  and  16 , and a reflective electrode layer  14  located between the first and second transparent electrode layers  12  and  16 . Light emitted from the EML  34  passes through the cathode  20  and forms an image. That is, the OLED  1  is a top-emission type. 
     However, the OLED  1  having a multilayered structure has a limit in achieving a cavity effect. That is, the EML  34  and the HIL  32  have substantially the same refractive index, and thus the cavity effect is achieved when light emitted from the EML  34  is reflected by the reflective electrode layer  14  of the anode  10 . Accordingly, the cavity effect is reduced. 
     Furthermore, since the EML  34  has a large thickness and a recombination zone RZ of a hole and an electrode is changed due to the ETL  36 , the cavity effect is reduced. 
     That is, referring to  FIG. 2  which is a diagram for explaining the cavity phenomenon occurring in the OLED of  FIG. 1 , injection of an electron is delayed, and the recombination zone RZ is shifted towards the ETL  36  due to the ETL  36  and the thick EML  34 . Thus, a distance L 1  between the recombination zone RZ and the ETL  36  for achieving the cavity effect is reduced. Accordingly, the luminous efficiency, i.e., the brightness, of the OLED  1  is reduced due to the reduction in the cavity effect. 
     In addition, carrier balance is broken due to the delay in the injection of the electrons, thus further decreasing the luminous efficiency of the OLED  1 . 
     In order to solve the above problems, the present disclosure provides an electroluminescent device including an anode and a cathode which face each other, a light compensation layer located between the anode and the cathode and having a first refractive index, and an EML located between the light compensation layer and the cathode and having a second refractive index higher than the first refractive index. 
     In the electroluminescent device according to the present disclosure, the difference between the first refractive index and the second refractive index is 0.4 or more. 
     In the electroluminescent device according to the present disclosure, the EML is in contact with the cathode. 
     The electroluminescent device according to the present disclosure further includes a HIL located between the EML and the light compensation layer or between the light compensation layer and the anode. 
     In the electroluminescent device according to the present disclosure, the HIL has a third refractive index higher than the first refractive index. 
     In the electroluminescent device according to the present disclosure, the light compensation layer has a thickness smaller than those of the EML and the HIL. 
     In the electroluminescent device according to the present disclosure, the anode includes a first transparent electrode layer. 
     In the electroluminescent device according to the present disclosure, the anode further includes a second transparent electrode layer facing the first transparent electrode layer, and a reflective electrode layer located between the first and second transparent electrode layers. 
     According to another aspect, the present disclosure provides an electroluminescent display device including a substrate, the above-described electroluminescent device located on the substrate, and a thin-film transistor (TFT) located between the substrate and the electroluminescent device and connected to the electroluminescent device. 
     Hereinafter, the present disclosure will be described in detail with reference to the accompanying drawings. 
       FIG. 3  is a schematic circuit diagram of an electroluminescent display device according to the present disclosure. 
     As illustrated in  FIG. 3 , a gate line GL, a data line DL, and a power line PL intersecting one another to define a pixel region P are formed in an electroluminescent display device  100 . A switching thin-film transistor (TFT) Ts, a driving TFT Td, and a storage capacitor Cst, and an electroluminescent device D are formed in the pixel region P. 
     The switching TFT Ts is connected to the gate line GL and the data line DL. The driving TFT Td and the storage capacitor Cst are connected between the switching TFT Ts and the power line PL. The electroluminescent device D is connected to the driving TFT Td. 
     In the electroluminescent display device  100 , when the switching TFT Ts is turned on according to a gate signal supplied to the gate line GL, a data signal supplied to the data line DL is supplied to a gate electrode of the driving TFT Td and an electrode of the storage capacitor Cst via the switching TFT Ts. 
     The driving TFT Td is turned on according to the data signal supplied to the gate electrode, and thus, current which is proportional to the data signal flows from the power line PL to the electroluminescent device D via the driving TFT Td. Then, the electroluminescent device D emits light with brightness which is proportional to the current flowing via the driving TFT Td. 
     In this case, the storage capacitor Cst is charged with a voltage which is proportional to the data signal and thus a voltage of the gate electrode of the driving TFT Td is maintained constant for a period of one frame. 
     Accordingly, the electroluminescent display device  100  may display a desired image. 
       FIG. 4  is a schematic cross-sectional view of an electroluminescent display device according to the present disclosure. 
     As illustrated in  FIG. 4 , a driving TFT Td and an electroluminescent device D connected thereto are located on a substrate  150 . 
     The substrate  150  may be a glass substrate or a plastic substrate formed of a polymer such as a polyimide. 
     Although not shown, a buffer layer formed of an inorganic insulating material such as silicon oxide or silicon nitride may be formed on the substrate  150 . 
     The driving TFT Td is connected to a switching TFT (not shown), and includes a semiconductor layer  152 , a gate electrode  160 , a source electrode  170 , and a drain electrode  172 . 
     The semiconductor layer  152  may be formed on the substrate  150  which is a flexible substrate, and may be formed of an oxide semiconductor material or polycrystalline silicon. 
     When the semiconductor layer  152  is formed of the oxide semiconductor material, a light-shielding pattern (not shown) may be formed below the semiconductor layer  152 . The light-shielding pattern prevents light from being incident on the semiconductor layer  152  to prevent the semiconductor layer  152  from being degraded due to the light. Alternatively, the semiconductor layer  152  may be formed of polycrystalline silicon. In this case, opposite edge portions of the semiconductor layer  152  may be doped with impurities. 
     A gate insulating film  154  formed of an insulating material is formed on the semiconductor layer  152  to correspond to the entire flexible substrate  150 . The gate insulating film  154  may be formed of an inorganic insulating material such as silicon oxide or silicon nitride. 
     The gate electrode  160  formed of a conductive material such as a metal may be formed on the gate insulating film  154  to correspond to a center of the semiconductor layer  152 . The gate electrode  160  is connected to the switching TFT. 
     Although the gate insulating film  154  is formed over the entire flexible substrate  150 , the gate insulating film  154  may be patterned to the same shape as the gate electrode  160 . 
     An interlayer insulating film  162  formed of an insulating material is formed on the gate electrode  160  to correspond to the entire substrate  150 . The interlayer insulating film  162  may be formed of an inorganic insulating material such as silicon oxide or silicon nitride or an organic insulating material such as benzocyclobutene or photo-acryl. 
     The interlayer insulating film  162  has first and second contact holes  164  and  166  which expose opposite sides of the semiconductor layer  152 . The first and second contact holes  164  and  166  are located at opposite sides of the gate electrode  160  to be spaced apart from the gate electrode  160 . 
     Here, the first and second contact holes  164  and  166  are also formed inside the gate insulating film  154 . Alternatively, when the gate insulating film  154  is patterned to the same shape as the gate electrode  160 , the first and second contact holes  164  and  166  may be formed only inside the interlayer insulating film  162 . 
     The source electrode  170  and the drain electrode  172  formed of a conductive material such as a metal are formed on the interlayer insulating film  162 . 
     The source electrode  170  and the drain electrode  172  are spaced apart from each other with respect to the gate electrode  160 , and are in contact with the opposite sides of the semiconductor layer  152  via the first and second contact holes  164  and  166 . The source electrode  170  is connected to the power line PL of  FIG. 3 . 
     The semiconductor layer  152 , the gate electrode  160 , the source electrode  170 , and the drain electrode  172  form the driving TFT Td. The driving TFT Td has a coplanar structure, in which the gate electrode  160 , the source electrode  170 , and the drain electrode  172  are located on the semiconductor layer  152 . 
     Alternatively, the driving TFT Td may have an inverted staggered structure, in which the gate electrode  160  is located below the semiconductor layer  152  and the source electrode  170  and the drain electrode  172  are located on the semiconductor layer  152 . In this case, the semiconductor layer  152  may be formed of amorphous silicon. 
     The switching TFT (not shown) may have substantially the same structure as the driving TFT Td. 
     A protective layer  174  having a drain contact hole  176  exposing the drain electrode  172  of the driving TFT Td is formed covering the driving TFT Td. 
     An anode  110  connected to the drain electrode  172  of the driving TFT Td via the drain contact hole  176  is formed on the protective layer  174  for each pixel region such that anodes  110  in the pixel regions are separated from each other. 
     The anode  110  may be formed of a conductive material having a relatively high work function. For example, the anode  110  may be formed of a transparent conductive material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO). 
     When an electroluminescent display device according to the present disclosure is a top-emission type, a reflective electrode or a reflective layer may be further formed below the anode  110 . For example, the anode  110  may have a triple-layer structure including a first transparent electrode layer, a reflective electrode layer formed of an aluminum-palladium-copper (APC) alloy, and a second transparent electrode layer. 
     A bank layer  115  is formed on the protective layer  174  to cover edges of the anode  110 . The bank layer  115  exposes a center of the anode  110  corresponding to the pixel region. 
     An organic emissive layer  120  is formed on the anode  110 . A structure of the organic emissive layer  120  will be described in detail below. 
     A cathode  130  is formed on the substrate  150  on which the organic emissive layer  120  is formed. The cathode  130  is formed of a conductive material having a relatively small work function to cover an entire display area. For example, the cathode  130  may be formed of one selected from among aluminum (Al), magnesium (Mg), and an Al—Mg alloy. 
     The anode  110 , the organic emissive layer  120 , and the cathode  130  form the electroluminescent device D. 
       FIG. 5  is a schematic cross-sectional view of electroluminescent device according to a first embodiment of the present disclosure. 
     As illustrated in  FIG. 5 , an electroluminescent device D according to a first embodiment of the present disclosure may include an anode  110  including a first transparent electrode layer  112 , a reflective electrode layer  114 , and a second transparent electrode layer  110 , a cathode  130  facing the anode  110 , and an organic emissive layer  120  which is located between the anode  110  and the cathode  130  and includes a light compensation layer  122 , a HIL  124 , an EML  126 , and an ETL  128 . 
     The light compensation layer  122 , the HIL  124 , the EML  126 , and the ETL  128  are sequentially stacked on the anode  110 . That is, the EML  126  is located between the anode  110  and the cathode  130 , and the light compensation layer  122  is located between the anode  110  and the EML  126 . The HIL  124  is located between the light compensation layer  122  and the EML  126 . The ETL  128  is located between the EML  126  and the cathode  130 . 
     Alternatively, the light compensation layer  122  may be located between the HIL  124  and the EML  126 . 
     Although not shown, an HTL may be located between the HIL  124  and the EML  126 . 
     Although only one pixel region is illustrated in  FIG. 5 , the electroluminescent device D may include red, green, and blue pixel regions, and the organic emissive layer  120  included in the electroluminescent device D in each of the red, green, and blue pixel regions may have a different thickness. For example, the organic emissive layer  120  in the green pixel region may have a thickness smaller than that of the organic emissive layer  120  in the red pixel region and greater than that of the organic emissive layer  120  in the blue pixel region. 
     In the anode  110 , the first and second transparent electrode layers  112  and  110  may be each formed of a transparent conductive material such as ITO or IZO, and the reflective electrode layer  114  may be formed of a highly reflective material such as Al or an APC alloy. 
     The cathode  130  may be formed of one selected from among Al, Mg, and an Al—Mg alloy, and have a thin thickness to transmit light therethrough 
     The EML  126  may include an inorganic emitting material such as quantum dots or an organic emitting material. That is, the electroluminescent display device  100  according to the present disclosure may be an OLED display device or a quantum-dot light-emitting diode (QLED) display device. 
     The light compensation layer  122  has a refractive index lower than those of the HIL  124  and the EML  126 . The light compensation layer  122  may have a first refractive index, and the HIL  124  and the EML  126  may respectively have second and third refractive indices. The difference between the first and second refractive indices and the difference between the first and third indices may be 0.4 or more. 
     For example, the first refractive index may be about 1.2 to 1.6. The second and third refractive indices may be each about 1.6 to 2.0. The second refractive index and the third refractive index may be substantially the same. 
     The light compensation layer  122  has a thickness smaller than those of the HIL  124  and the EML  126 . For example, the thickness of the light compensation layer  122  may be about ⅖ to 1/10 times that of the HIL  124  and may be about ⅕ to 1/20 times that of the EML  126 . 
     For example, the light compensation layer  122  may be formed of an amine compound. 
     In the electroluminescent device D described above, a hole from the anode  110  and an electron from the cathode  130  combine with each other in a recombination zone RZ located in the EML  126  and thus light is emitted. 
     A part of light emitted from the EML  126  is reflected at a boundary between the HIL  124  and the light compensation layer  122  toward the cathode  130 . The remaining light is reflected from the reflective electrode layer  114  toward the cathode  130 . 
     That is, in the present disclosure, light is reflected within the organic emissive layer  120 , and the reflection of the light within the organic emissive layer  120  will be referred to as a soft cavity effect. 
     In other words, as the light compensation layer  122  having a low refractive index is arranged below the HIL  124  or between the HIL  124  and the EML  126 , the cavity phenomenon occurs in the organic emissive layer  120 . Accordingly, the luminous efficiencies, i.e., the brightnesses, of the electroluminescent device D and the electroluminescent display device  100  are improved. 
       FIG. 6  is a schematic cross-sectional view of an electroluminescent device D according to a second embodiment of the present disclosure. 
     As illustrated in  FIG. 6 , an electroluminescent device D according to the second embodiment of the present disclosure may include an anode  210  including a first transparent electrode layer  212 , a reflective electrode layer  214 , and a second transparent electrode layer  216 ; a cathode  230  facing the anode  110 ; and an organic emissive layer  220  located between the anode  210  and the cathode  230  and including a light compensation layer  222 , a HIL  224 , and an EML  226 . 
     The light compensation layer  222 , the HIL  224 , and the EML  226  are sequentially stacked on the anode  210 . That is, the EML  226  is located between the anode  210  and the cathode  230  and is in contact with the cathode  230 . The light compensation layer  222  is located between the anode  210  and the EML  226 . The HIL  224  is located between the light compensation layer  222  and the EML  226 . 
     Alternatively, the light compensation layer  222  may be located between the HIL  224  and the EML  226 . 
     Although not shown, an HTL may be located between the HIL  224  and the EML  226 . 
     In the anode  210 , the first and second transparent electrode layers  212  and  216  may be each formed of a transparent conductive material such as ITO or IZO, and the reflective electrode layer  214  may be formed of a highly reflective material such as Al or an APC alloy. 
     The cathode  230  may be formed of one selected from among Al, Mg, and an Al—Mg alloy, and have a thin thickness to transmit light therethrough 
     The EML  226  may include an inorganic emitting material such as quantum dots or an organic emitting material. That is, the electroluminescent display device  100  according to the present disclosure may be an OLED display device or a QLED display device. 
     The light compensation layer  222  has a refractive index lower than those of the HIL  224  and the EML  226 . The light compensation layer  222  may have a first refractive index. The HIL  224  and the EML  226  may respectively have second and third refractive indices. The difference between the first and second refractive indices and the difference between the first and third indices may be 0.4 or more. 
     For example, the first refractive index may be about 1.2 to 1.6. The second and third refractive indices may be each about 1.6 to 2.0. The second refractive index and the third refractive index may be substantially the same. 
     The light compensation layer  222  has a thickness smaller than those of the HIL  224  and the EML  226 . For example, the thickness of the light compensation layer  222  may be about ⅖ to 1/10 times that of the HIL  224 , and may be ⅕ to 1/20 times that of the EML  226 . 
     In the electroluminescent device D, a hole from the anode  210  and an electron from the cathode  230  combine with each other in a recombination zone RZ located in the EML  226 , and thus light is emitted. 
     A part of light emitted from the EML  226  is reflected at a boundary between the HIL  224  and the light compensation layer  222  toward the cathode  230 . The remaining light is reflected from the reflective electrode layer  214  toward the cathode  230 . 
     That is, in the present disclosure, light is reflected within the organic emissive layer  220 , and the reflection of the light within the organic emissive layer  220  is referred to as the soft cavity effect. 
     In other words, as the light compensation layer  222  having a low refractive index is arranged below the HIL  224  or between the HIL  224  and the EML  226 , the cavity phenomenon occurs in the organic emissive layer  220 . Accordingly, the luminous efficiencies, i.e., the brightnesses, of the electroluminescent device D and the electroluminescent display device  100  are improved 
     Additionally, the recombination zone RZ is located at a center of the EML  226 , thus increasing the soft cavity effect of the electroluminescent device D and the electroluminescent display device  100 . 
     That is, referring to  FIG. 7  which is a diagram for explaining the cavity phenomenon occurring in an electroluminescent device according to the present disclosure, when the EML  226  is located on the cathode  230  without the ETL  128  of  FIG. 5  to be in contact with the cathode  230 , injection of electrons is not delayed and thus a recombination zone RZ moves to a center of the EML  226 . Accordingly, a distance L 2  between the recombination zone RZ and the cathode  230  for achieving a cavity is secured, thus increasing the soft cavity effect. Therefore, the luminous efficiencies, i.e., the brightnesses, of the electroluminescent device D and the electroluminescent display device  100  are greatly improved 
       FIGS. 8A and 8B  are graphs showing characteristics of an electroluminescent device according to whether an ETL is provided or not. 
     As illustrated in  FIGS. 8A and 8B , the brightness of an electroluminescent device (without an ETL) in which an EML is in contact with a cathode without an ETL is improved and a driving voltage thereof is low, when compared to an electroluminescent device (with an ETL) including an ETL. 
       FIGS. 9A to 9C  are graphs showing characteristics of an electroluminescent device according to whether a light compensation layer is provided or not. 
     As illustrated in  FIG. 9A , the brightness of an electroluminescent device including a light compensation layer (refractive index=1.8 (Ex 1 ), 1.6 (Ex 2 ), 1.4 (Ex 3 ), or 1.2 (Ex 4 )), a HTL (refractive index=1.8), and an EML (refractive index=1.8) is higher. When the difference between refractive indices of the light compensation layer, the HTL, and the ELM is high, the soft cavity effect increases and thus the brightness of the electroluminescent device increases. 
     Furthermore, as illustrated in  FIG. 9B , the brightness of an electroluminescent device Ex including a light compensation layer is higher than that of an electroluminescent device Ref not including the light compensation layer. 
     As illustrated in  FIG. 9C , luminous characteristics (a light-emitting wavelength) are not changed due to the light compensation layer. That is, luminous efficiency is improved due to the light compensation layer without degradation of luminous characteristics. 
       FIG. 10  is a schematic cross-sectional view showing an electroluminescent device according to a third embodiment of the present disclosure. 
     As illustrated in  FIG. 10 , an electroluminescent device D according to a third embodiment of the present disclosure, may include an anode  310 , a cathode  330  facing the anode  310 , and an organic emissive layer  320  located between the anode  310  and the cathode  330  and including a light compensation layer  322  and an EML  326 . 
     The light compensation layer  322  and the EML  326  are sequentially stacked on the anode  310 . That is, the EML  326  is located between the anode  310  and the cathode  330  and is in contact with the cathode  330 . Additionally, the light compensation layer  322  is located between the anode  310  and the EML  326 , and is in contact with the anode  310  and the EML  326 . 
     The light compensation layer  322  serves as a HIL. 
     Although not shown, a HTL may be located between the light compensation layer  322  and the EML  326   
     The anode  310  may be formed of a transparent conductive material such as ITO or IZO. The cathode  330  is formed of one selected from among Al, Mg, and an Al—Mg alloy, and serves as a reflective electrode. 
     The EML  326  may include an inorganic emitting material such as quantum dots or an organic emitting material. That is, an electroluminescent display device according to the present disclosure may be an OLED display device or a QLED display device. 
     The light compensation layer  322  has a refractive index lower than that of the EML  326 . The light compensation layer  322  may have a first refractive index, and the EML  326  may have a second refractive index. The difference between the first and second refractive indices may be 0.4 or more. 
     For example, the first refractive index may be about 1.2 to 1.6. The second refractive index may be about 1.6 to 2.0. 
     Furthermore, the light compensation layer  322  has a thickness smaller than that of the EML  326 . For example, the thickness of the light compensation layer  322  may be about ⅕ to 1/20 times that of the EML  326 . 
     In the electroluminescent device D described above, a hole from the anode  310  and an electron from the cathode  230  combine with each other in a recombination zone RZ located in the EML  326  and thus light is emitted 
     Light emitted from the EML  326  is reflected at a boundary between the ELM  326  and the light compensation layer  322  and from the cathode  330 . Thus, luminous efficiency is improved due to the soft cavity effect. 
     In other words, as the light compensation layer  322  having a low refractive index is arranged between the EML  326  and the anode  310 , the cavity phenomenon occurs in the organic emissive layer  320 . Accordingly, the luminous efficiencies, i.e., the brightnesses, of the electroluminescent device D and the electroluminescent display device  100  are improved. 
     Furthermore, since the EML  326  is in contact with the cathode  330  without the ETL  128  of  FIG. 5 , the recombination zone RZ is located at a center of the EML  326 , thus increasing the soft cavity effect of the electroluminescent device D and the electroluminescent display device  100 . 
     Therefore, the luminous efficiencies, i.e., the brightnesses, of the electroluminescent device D and the electroluminescent display device  100  are greatly improved. 
     According to the present disclosure, an emissive layer located between an anode and a cathode includes a light compensation layer having low-refractive index characteristics. Thus, the soft cavity effect occurs in the emissive layer. 
     Accordingly, the luminous efficiencies of an electroluminescent device and an electroluminescent display device are improved. 
     Additionally, since an EML is in direct contact with a cathode without an EIL, the soft cavity effect is improved and thus the luminous efficiencies of the electroluminescent device and the electroluminescent display device are further improved. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.