Patent Publication Number: US-2023157041-A1

Title: Light-emitting device and electronic apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2021-0156050, filed on Nov. 12, 2021, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     One or more embodiments relate to a light-emitting device and an electronic apparatus including the same. 
     2. Description of the Related Art 
     Light-emitting devices are self-emissive devices that, as compared with devices of the related art, have wider viewing angles, higher contrast ratios, short response times, and/or more excellent or suitable characteristics in terms of luminance, driving voltage, and/or response speed. 
     In a light-emitting device, a first electrode is located on a substrate, and a hole transport region, an emission layer, an electron transport region, and a second electrode are sequentially formed on the first electrode. Holes provided from the first electrode may move toward the emission layer through the hole transport region, and electrons provided from the second electrode may move toward the emission layer through the electron transport region. Carriers, such as holes and electrons, recombine in the emission layer to produce light. 
     SUMMARY 
     An aspect according to embodiments of the present disclosure is directed toward a light-emitting device having improved lifespan. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments of the disclosure. 
     According to one or more embodiments, a light-emitting device includes 
     a first electrode, 
     a second electrode facing the first electrode, and 
     an interlayer between the first electrode and the second electrode and including an emission layer, 
     wherein the emission layer includes a first host, a first dopant, and a second dopant, and 
     wherein: 
     an absolute value of a highest occupied molecular orbital (HOMO) energy level of the first dopant (HOMO_D1) is smaller than an absolute value of a HOMO energy level of the second dopant (HOMO_D2), and a difference between a HOMO energy level of the first host (HOMO_H1) and the HOMO energy level of the first dopant (HOMO_D1) is 0.3 eV or less; or the absolute value of the HOMO energy level of the second dopant (HOMO_D2) is smaller than the absolute value of the HOMO energy level of the first dopant (HOMO_D1), and a difference between the HOMO energy level of the first host (HOMO_H1) and the HOMO energy level of the second dopant (HOMO_D2) is 0.3 eV or less. 
     According to one or more embodiments, 
     an electronic apparatus includes the light-emitting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and enhancements of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a schematic view of a light-emitting device according to an embodiment; 
         FIG.  2    is a cross-sectional view of an electronic apparatus according to an embodiment; and 
         FIG.  3    is a cross-sectional view of an electronic apparatus according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in more detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout, and duplicative descriptions thereof may not be provided. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the drawings, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Throughout the disclosure, the expression “at least one of a, b or c” indicates only a, only b, only c, both (e.g., simultaneously) a and b, both (e.g., simultaneously) a and c, both (e.g., simultaneously) b and c, all of a, b, and c, or variations thereof. 
     According to one or more embodiments of the present disclosure, a light-emitting device includes: 
     a first electrode; 
     a second electrode facing the first electrode; and 
     an interlayer located between the first electrode and the second electrode and including an emission layer, 
     wherein the emission layer may include a first host, a first dopant, and a second dopant, and 
     wherein: 
     an absolute value of a highest occupied molecular orbital (HOMO) energy level of the first dopant (HOMO_D1) may be smaller than an absolute value of a HOMO energy level of the second dopant (HOMO_D2), and a difference between a HOMO energy level of the first host (HOMO_H1) and the HOMO energy level of the first dopant (HOMO_D1) may be 0.3 eV or less; or the absolute value of the HOMO energy level of the second dopant (HOMO_D2) may be smaller than the absolute value of the HOMO energy level of the first dopant (HOMO_D1), and a difference between the HOMO energy level of the first host (HOMO_H1) and the HOMO energy level of the second dopant (HOMO_D2) may be 0.3 eV or less. 
     For example, in the emission layer of the light-emitting device according to an embodiment of the present disclosure, a difference between a HOMO energy level of a dopant having a smaller absolute value among the first dopant and the second dopant and the HOMO energy level of the first host (HOMO_H1) may be 0.3 eV or less. 
     As a result of numerous repeated experiments and insights, the inventors of the present disclosure have found that in the emission layer of the light-emitting device including the first host, the first dopant, and the second dopant, as the difference between the HOMO energy level of the dopant having the smaller absolute value among the first dopant and the second dopant and the HOMO energy level of the first host (HOMO_H1) becomes smaller, the lifespan of the light-emitting device may increase, and when the difference is 0.3 eV or less, the degree of increase in the lifespan of the light-emitting device may be significantly and qualitatively different. 
     Furthermore, the inventors have found that, likewise, as the difference between the HOMO energy level of the first dopant (HOMO_D1) and the HOMO energy level of the second dopant (HOMO_D2) becomes smaller, the lifespan of the light-emitting device may increase, and when the difference is 0.3 eV or less, the degree of increase in the lifespan of the light-emitting device may be significantly and qualitatively different. 
     In an embodiment, the first electrode may be an anode, the second electrode may be a cathode, and the interlayer may further include a hole transport region located between the first electrode and the emission layer and including a hole injection layer, a hole transport layer, an electron blocking layer, or any combination thereof. 
     In an embodiment, the first electrode may be an anode, the second electrode may be a cathode, and the interlayer may further include an electron transport region located between the second electrode and the emission layer and including a hole blocking layer, an electron transport layer, an electron injection layer, or any combination thereof. 
     In an embodiment, the emission layer may be to emit red light, green light, blue light, or white light. For example, the emission layer may be to emit blue light. 
     In an embodiment, one of the first dopant and the second dopant may be a phosphorescent dopant, and the other one of the first dopant and the second dopant may be a fluorescent dopant. For example, the first dopant may be a phosphorescent dopant, and the second dopant may be a fluorescent dopant. For example, the first dopant may be a fluorescent dopant, and the second dopant may be a phosphorescent dopant. 
     In an embodiment, in one of the first dopant and the second dopant, energy transfer may occur more actively than emission of light. The energy transfer may be, for example, a Forster transfer or a Dexter transfer. The meaning of the term “Forster transfer” or “Dexter transfer” should be known to those skilled in the art, and thus, detailed description thereof is not provided. 
     In an embodiment, one selected from the first dopant and the second dopant may be a phosphorescent dopant, and the other one selected from the first dopant and the second dopant may be a fluorescent dopant, wherein, in the phosphorescent dopant, energy transfer (the Forster transfer or Dexter transfer) may occur more actively than emission of light. 
     For example, the first dopant may be a phosphorescent dopant, wherein energy transfer (Forster or Dexter) may occur more actively than emission of light. Singlet excitons generated in the host may be transferred to the second dopant by the energy transfer (Forster or Dexter). 
     For example, about 20% to about 30% of the phosphorescent dopant, which is the first dopant, may emit light, and about 80% to about 70% of the phosphorescent dopant may cause energy transfer (Forster or Dexter). Singlet excitons generated by the first host (or in the presence of a second host, singlet excitons generated by the second host and/or excitons generated by the first host and the second host) may be transferred to the fluorescent dopant, which is the second dopant, by the energy transfer (Forster or Dexter). 
     In an embodiment, the fluorescent dopant may be a thermally activated delayed fluorescence dopant. 
     In an embodiment, a weight ratio of the first dopant to the second dopant may be from about 1:9 to about 9:1. For example, the emission layer may include the first dopant and the second dopant at a weight ratio of about 3:5 to about 5:3. When the weight ratio of the first dopant to the second dopant is within these ranges, operation of emission system through the energy transfer (Forster or Dexter) may be excellent or suitable. 
     In an embodiment, the first host may be a host capable of transporting both (e.g., simultaneously) holes and electrons. For example, the emission layer of the light-emitting device may include only the first host capable of transporting both (e.g., simultaneously) holes and electrons as a single host. 
     In an embodiment, the light-emitting device may further include a second host, the first host may be a hole-transporting host, and the second host may be an electron-transporting host. 
     In an embodiment, a weight ratio of the first host to the second host may be from about 1:9 to about 9:1. For example, the emission layer may include the first host and the second host at a weight ratio of about 3:7 to about 7:3. When the weight ratio of the first host and the second host is within these ranges, the hole transport may be in a desirable balance with the electron transport. 
     The host(s) and the dopant(s) will be described in more detail below. 
     According to one or more embodiments of the present disclosure, an electronic apparatus includes the light-emitting device. 
     In an embodiment, the electron apparatus may further include a thin-film transistor, 
     wherein the thin-film transistor may include a source electrode and a drain electrode, and 
     the first electrode of the light-emitting device may be electrically connected to the source electrode or the drain electrode of the thin-film transistor. 
     In an embodiment, the electronic apparatus may further include a color filter, a color conversion layer, a touch screen layer, a polarizing layer, or any combination thereof. 
     The term “interlayer” as used herein refers to a single layer and/or all of a plurality of layers located between the first electrode and the second electrode of the light-emitting device. 
     Description of FIG.  1   
       FIG.  1    is a schematic cross-sectional view of a light-emitting device  10  according to an embodiment of the present disclosure. The light-emitting device  10  includes a first electrode  110 , an interlayer  130 , and a second electrode  150 . 
     Hereinafter, the structure of the light-emitting device  10  according to an embodiment and a method of manufacturing the light-emitting device  10  will be described with reference to  FIG.  1   . 
     First electrode  110   
     In  FIG.  1   , a substrate may be additionally located under the first electrode  110  or on the second electrode  150 . As the substrate, a glass substrate and/or a plastic substrate may be utilized. In one or more embodiments, the substrate may be a flexible substrate, and may include plastics with excellent or suitable heat resistance and durability, such as polyimide, polyethylene terephthalate (PET), polycarbonate, polyethylene naphthalate, polyarylate (PAR), polyetherimide, or any combination thereof. 
     The first electrode  110  may be formed by, for example, depositing or sputtering a material for forming the first electrode  110  on the substrate. When the first electrode  110  is an anode, a material for forming the first electrode  110  may be a high-work function material that can suitably facilitate injection of holes. 
     The first electrode  110  may be a reflective electrode, a semi-transmissive electrode, or a transmissive electrode. When the first electrode  110  is a transmissive electrode, a material for forming the first electrode  110  may include indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO 2 ), zinc oxide (ZnO), or any combination thereof. In one or more embodiments, when the first electrode  110  is a semi-transmissive electrode or a reflective electrode, a material for forming the first electrode  110  may include magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), or any combination thereof. 
     The first electrode  110  may have a single-layered structure consisting of a single layer or a multi-layered structure including a plurality of layers. For example, the first electrode  110  may have a three-layered structure of ITO/Ag/ITO. 
     Interlayer  130   
     The interlayer  130  may be located on the first electrode  110 . The interlayer  130  may include an emission layer. 
     In an embodiment, the interlayer  130  may further include a hole transport region located between the first electrode  110  and the emission layer and an electron transport region located between the emission layer and the second electrode  150 . 
     In an embodiment, the interlayer  130  may further include, in addition to one or more suitable organic materials, a metal-containing compound, such as an organometallic compound, an inorganic material, such as a quantum dot, and/or the like. 
     In one or more embodiments, the interlayer  130  may include i) two or more emission layers sequentially stacked between the first electrode  110  and the second electrode  150 , and ii) a charge generation layer located between the two or more emission layers. When the interlayer  130  includes the two or more emission layers and the charge generation layer as described above, the light-emitting device  10  may be a tandem light-emitting device. 
     Hole Transport Region in Interlayer  130   
     The hole transport region may have i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer including (e.g., consisting of) a plurality of different materials, or iii) a multi-layered structure including a plurality of layers including different materials. 
     The hole transport region may include a hole injection layer, a hole transport layer, an emission auxiliary layer, an electron blocking layer, or any combination thereof. 
     For example, the hole transport region may have a multi-layered structure including a hole injection layer/hole transport layer structure, a hole injection layer/hole transport layer/emission auxiliary layer structure, a hole injection layer/emission auxiliary layer structure, a hole transport layer/emission auxiliary layer structure, or a hole injection layer/hole transport layer/electron blocking layer structure, wherein, in each structure, constituting layers are sequentially stacked from the first electrode  110  in the respective stated order. 
     The hole transport region may include a compound represented by Formula 201, a compound represented by Formula 202, or any combination thereof: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 201 and 202, 
     L 201  to L 204  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     L 205  may be *—O—*′, *—N(Q 201 )-*′, a C 1 -C 20  alkylene group unsubstituted or substituted with at least one R 10a , a C 2 -C 20  alkenylene group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     xa1 to xa4 may each independently be an integer from 0 to 5, 
     xa5 may be an integer from 1 to 10, 
     R 201  to R 204  and Q 201  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     R 201  and R 202  may optionally be linked to each other via a single bond, a C 1 -C 5  alkylene group unsubstituted or substituted with at least one R 10a , or a C 2 -C 5  alkenylene group unsubstituted or substituted with at least one R 10a , to form a C 8 -C 60  polycyclic group (for example, a carbazole group and/or the like) unsubstituted or substituted with at least one R 10a  (for example, see Compound HT16), 
     R 203  and R 204  may optionally be linked to each other via a single bond, a C 1 -C 5  alkylene group unsubstituted or substituted with at least one R 10a , or a C 2 -C 5  alkenylene group unsubstituted or substituted with at least one R 10a , to form a C 8 -C 60  polycyclic group unsubstituted or substituted with at least one R 10a , and 
     na1 may be an integer from 1 to 4. 
     For example, each of Formulae 201 and 202 may include at least one of the groups represented by Formulae CY201 to CY217: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     R 10b  and R 10c  in Formulae CY201 to CY217 may each independently be the same as described in connection with R 10a , ring CY 201  to ring CY 204  may each independently be a C 3 -C 20  carbocyclic group or a C 1 -C 20  heterocyclic group, and at least one hydrogen in Formulae CY 201  to CY 217  may be unsubstituted or substituted with R 10a . 
     In an embodiment, ring CY 201  to ring CY 204  in Formulae CY201 to CY217 may each independently be a benzene group, a naphthalene group, a phenanthrene group, or an anthracene group. 
     In one or more embodiments, each of Formulae 201 and 202 may include at least one of the groups represented by Formulae CY201 to CY203. 
     In one or more embodiments, Formula 201 may include at least one of the groups represented by Formulae CY201 to CY203 and at least one of the groups represented by Formulae CY204 to CY217. 
     In one or more embodiments, in Formula 201, xa1 may be 1, R 201  may be a group represented by one of Formulae CY201 to CY203, xa2 may be 0, and R 202  may be a group represented by one of Formulae CY204 to CY207. 
     In one or more embodiments, each of Formulae 201 and 202 may not include (e.g., may exclude) any of the groups represented by Formulae CY201 to CY203. 
     In one or more embodiments, each of Formulae 201 and 202 may not include (e.g., may exclude) any of the groups represented by Formulae CY201 to CY203, and may include at least one of the groups represented by Formulae CY204 to CY217. 
     In one or more embodiments, each of Formulae 201 and 202 may not include (e.g., may exclude) any of the groups represented by Formulae CY201 to CY217. 
     For example, the hole transport region may include one of Compounds HT1 to HT46, m-MTDATA, TDATA, 2-TNATA, NPB(NPD), β-NPB, TPD, Spiro-TPD, Spiro-NPB, methylated NPB, TAPC, HMTPD, 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), polyaniline/dodecylbenzenesulfonic acid (PANI/DBSA), poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), polyaniline/camphor sulfonic acid (PANI/CSA), polyaniline/poly(4-styrenesulfonate) (PANI/PSS), or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A thickness of the hole transport region may be in a range of about 50 Å to about 10,000 Å, for example, about 100 Å to about 4,000 Å. When the hole transport region includes a hole injection layer, a hole transport layer, or any combination thereof, a thickness of the hole injection layer may be in a range of about 100 Å to about 9,000 Å, for example, about 100 Å to about 1,000 Å, and a thickness of the hole transport layer may be in a range of about 50 Å to about 2,000 Å, for example, about 100 Å to about 1,500 Å. When the thicknesses of the hole transport region, the hole injection layer, and the hole transport layer are within these ranges, satisfactory hole transporting characteristics may be obtained without a substantial increase in driving voltage. 
     The emission auxiliary layer may increase light-emission efficiency by compensating for an optical resonance distance according to the wavelength of light emitted from the emission layer, and the electron blocking layer may block or reduce the leakage of electrons from the emission layer to the hole transport region. Materials that may be included in the hole transport region may be included in the emission auxiliary layer and the electron blocking layer. 
     p-Dopant 
     The hole transport region may further include, in addition to the materials as described above, a charge-generation material for improving conductive properties. The charge-generation material may be uniformly or non-uniformly dispersed in the hole transport region (for example, in the form of a single layer consisting of a charge-generation material). 
     The charge-generation material may be, for example, a p-dopant. 
     For example, a lowest unoccupied molecular orbital (LUMO) energy level (or a work function) of the p-dopant may be −3.5 eV or less. 
     In an embodiment, the p-dopant may include a quinone derivative, a cyano group-containing compound, a compound including element EL1 and element EL2 (to be described in more detail below), or any combination thereof. 
     Examples of the quinone derivative may include TCNQ, F4-TCNQ, and/or the like. 
     Examples of the cyano group-containing compound may include HAT-CN, a compound represented by Formula 221, and/or the like: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 221, 
     R 221  to R 223  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , and 
     at least one of R 221  to R 223  may each independently be a C 3 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, each substituted with a cyano group; —F; —CI; —Br; —I; a C 1 -C 20  alkyl group substituted with a cyano group, —F, —Cl, —Br, —I, or any combination thereof; or any combination thereof. 
     In the compound including element EL1 and element EL2, element EL1 may be a metal, a metalloid, or any combination thereof, and element EL2 may be a non-metal, a metalloid, or any combination thereof. 
     Examples of the metal may include: an alkali metal (for example, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and/or the like); an alkaline earth metal (for example, beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and/or the like); a transition metal (for example, titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), technetium (Tc), rhenium (Re), iron (Fe), ruthenium (Ru), osmium (Os), cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt), copper (Cu), silver (Ag), gold (Au), and/or the like); a post-transition metal (for example, zinc (Zn), indium (In), tin (Sn), and/or the like); a lanthanide metal (for example, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and/or the like); and/or the like. 
     Examples of the metalloid may include silicon (Si), antimony (Sb), tellurium (Te), and/or the like. 
     Examples of the non-metal may include oxygen (O), halogen (for example, F, Cl, Br, I, and/or the like), and/or the like. 
     For example, the compound including element EL1 and element EL2 may include a metal oxide, a metal halide (for example, a metal fluoride, a metal chloride, a metal bromide, a metal iodide, and/or the like), a metalloid halide (for example, a metalloid fluoride, a metalloid chloride, a metalloid bromide, a metalloid iodide, and/or the like), a metal telluride, or any combination thereof. 
     Examples of the metal oxide may include tungsten oxide (for example, WO, W 2 O 3 , WO 2 , WO 3 , W 2 O 5 , and/or the like), vanadium oxide (for example, VO, V 2 O 3 , VO 2 , V 2 O 5 , and/or the like), molybdenum oxide (MoO, Mo 2 O 3 , MoO 2 , MoO 3 , Mo 2 O 5 , and/or the like), rhenium oxide (for example, ReO 3  and/or the like), and/or the like. 
     Examples of the metal halide may include alkali metal halide, alkaline earth metal halide, transition metal halide, post-transition metal halide, lanthanide metal halide, and/or the like. 
     Examples of the alkali metal halide may include LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, CsI, and/or the like. 
     Examples of the alkaline earth metal halide may include BeF 2 , MgF 2 , CaF 2 , SrF 2 , BaF 2 , BeCl 2 , MgCl 2 , CaCl 2 ), SrCl 2 , BaCl 2 , BeBr 2 , MgBr 2 , CaBr 2 , SrBr 2 , BaBr 2 , BeI 2 , MgI 2 , CaI 2 , SrI 2 , BaI 2 , and/or the like. 
     Examples of the transition metal halide may include titanium halide (for example, TiF 4 , TiCl 4 , TiBr 4 , Tii 4 , and/or the like), zirconium halide (for example, ZrF 4 , ZrCl 4 , ZrBr 4 , Zri 4 , and/or the like), hafnium halide (for example HfF 4 , HfCl 4 , HfBr 4 , Hfi 4 , and/or the like), vanadium halide (for example, VF 3 , VCl 3 , VBr 3 , VI 3 , and/or the like), niobium halide (for example, NbF 3 , NbCl 3 , NbBr 3 , NbI 3 , and/or the like), tantalum halide (for example, TaF 3 , TaCl 3 , TaBr 3 , TaI 3 , and/or the like), chromium halide (for example, CrF 3 , CrCl 3 , CrBr 3 , Cr 13 , and/or the like), molybdenum halide (for example, MoF 3 , MoCl 3 , MoBr 3 , MoI 3 , and/or the like), tungsten halide (for example, WF 3 , WCl 3 , WBr 3 , WI 3 , and/or the like), manganese halide (for example, MnF 2 , MnCl 2 , MnBr 2 , MnI 2 , and/or the like), technetium halide (for example, TcF 2 , TcCl 2 , TcBr 2 , TcI 2 , and/or the like), rhenium halide (for example, ReF 2 , ReCl 2 , ReBr 2 , ReI 2 , and/or the like), iron halide (for example, FeF 2 , FeCl 2 , FeBr 2 , FeI 2 , and/or the like), ruthenium halide (for example, RuF 2 , RuCl 2 , RuBr 2 , RuI 2 , and/or the like), osmium halide (for example, OsF 2 , OsCl 2 , OsBr 2 , OsI 2 , and/or the like), cobalt halide (for example, CoF 2 , CoCl 2 , CoBr 2 , CoI 2 , and/or the like), rhodium halide (for example, RhF 2 , RhCl 2 , RhBr 2 , RhI 2 , and/or the like), iridium halide (for example, IrF 2 , IrCl 2 , IrBr 2 , IrI 2 , and/or the like), nickel halide (for example, NiF 2 , NiCl 2 , NiBr 2 , NiI 2 , and/or the like), palladium halide (for example, PdF 2 , PdC 12 , PdBr 2 , PdI 2 , and/or the like), platinum halide (for example, PtF 2 , PtC 12 , PtBr 2 , PtI 2 , and/or the like), copper halide (for example, CuF, CuCl, CuBr, CuI, and/or the like), silver halide (for example, AgF, AgCl, AgBr, AgI, and/or the like), gold halide (for example, AuF, AuCl, AuBr, AuI, and/or the like), and/or the like. 
     Examples of the post-transition metal halide may include zinc halide (for example, ZnF 2 , ZnCl 2 , ZnBr 2 , ZnI 2 , and/or the like), indium halide (for example, InI 3  and/or the like), tin halide (for example, SnI 2  and/or the like), and/or the like. 
     Examples of the lanthanide metal halide may include YbF, YbF 2 , YbF 3 , SmF 3 , YbCl, YbCl 2 , YbCl 3 , SmCl 3 , YbBr, YbBr 2 , YbBr 3 , SmBr 3 , YbI, YbI 2 , YbI 3 , SmI 3 , and/or the like. 
     Examples of the metalloid halide may include antimony halide (for example, SbCl 5  and/or the like) and/or the like. 
     Examples of the metal telluride may include alkali metal telluride (for example, Li 2 Te, Na 2 Te, K 2 Te, Rb 2 Te, Cs 2 Te, and/or the like), alkaline earth metal telluride (for example, BeTe, MgTe, CaTe, SrTe, BaTe, and/or the like), transition metal telluride (for example, TiTe 2 , ZrTe 2 , HfTe 2 , V 2 Te 3 , Nb 2 Te 3 , Ta 2 Te 3 , Cr 2 Te 3 , Mo 2 Te 3 , W 2 Te 3 , MnTe, TcTe, ReTe, FeTe, RuTe, OsTe, CoTe, RhTe, IrTe, NiTe, PdTe, PtTe, Cu 2 Te, CuTe, Ag 2 Te, AgTe, Au 2 Te, and/or the like), post-transition metal telluride (for example, ZnTe and/or the like), lanthanide metal telluride (for example, LaTe, CeTe, PrTe, NdTe, PmTe, EuTe, GdTe, TbTe, DyTe, HoTe, ErTe, TmTe, YbTe, LuTe, and/or the like), and/or the like. 
     Emission Layer in Interlayer  130   
     When the light-emitting device  10  is a full-color light-emitting device, the emission layer may be patterned into a red emission layer, a green emission layer, and/or a blue emission layer, according to a subpixel. In one or more embodiments, the emission layer may have a stacked structure of two or more layers of a red emission layer, a green emission layer, and a blue emission layer, in which the two or more layers contact each other or are separated from each other to emit white light. In one or more embodiments, the emission layer may include two or more materials of a red light-emitting material, a green light-emitting material, and a blue light-emitting material, in which the two or more materials are mixed with each other in a single layer to emit white light. 
     The emission layer may include a host and a dopant. The dopant may include a phosphorescent dopant, a fluorescent dopant, or any combination thereof. 
     An amount of the dopant in the emission layer may be from about 0.01 parts by weight to about 15 parts by weight based on 100 parts by weight of the host. 
     For example, the total amount of the first dopant and the second dopant in the emission layer may be from about 0.01 parts by weight to about 15 parts by weight, based on 100 parts by weight of the first host as a single host (e.g., when a single host is utilized), or 100 parts by weight of the first host and the second host as a mixed host (e.g., when a mixture of two hosts is utilized and 100 parts by weight of the mixture is utilized). 
     In one or more embodiments, the emission layer may include a quantum dot. 
     In some embodiments, the emission layer may include a delayed fluorescence material. The delayed fluorescence material may act as the host or the dopant in the emission layer. 
     A thickness of the emission layer may be in a range of about 100 Å to about 1,000 Å, for example, about 200 Å to about 600 Å. When the thickness of the emission layer is within these ranges, excellent or suitable light-emission characteristics may be obtained without a substantial increase in driving voltage. 
     Host 
     The hole-transporting host may be a compound having strong hole properties. The expression “a compound having strong hole properties” refers to a compound that is easy to accept holes, and such properties may be obtained by including a hole-receiving moiety (also referred to as a hole-transporting moiety). 
     The hole-receiving moiety may include, for example, a 7-electron-rich heteroaromatic compound (for example, a carbazole derivative or an indole derivative), and/or an aromatic amine compound. 
     The electron-transporting host may be a compound having strong electron properties. The expression “a compound having strong electron properties” refers to a compound that is easy to accept electrons, and such properties may be obtained by including an electron-receiving moiety (also referred to as an electron-transporting moiety). 
     The electron-receiving moiety may include, for example, a π electron-deficient heteroaromatic compound. For example, the electron-receiving moiety may include a nitrogen-containing heteroaromatic compound. 
     The emission layer of the light-emitting device according to an embodiment of the present disclosure may include only a first host (e.g., a single host material) that is capable of transporting both (e.g., simultaneously) holes and electrons. In this case, the first host may include both (e.g., simultaneously) a hole-transporting moiety and an electron-transporting moiety, and thus may have hole-transporting properties and electron-transporting properties at the same time. In this case, the hole-transporting properties and the electron-transporting properties of the first host may be of the same magnitude, the hole-transporting properties may be greater than the electron-transporting properties, or the electron-transporting properties may be greater than the hole-transporting properties. 
     The emission layer of the light-emitting device according to an embodiment of the present disclosure may further include a second host, and in this case, the first host may be a hole-transporting host, and the second host may be an electron-transporting host. 
     When a compound includes only a hole-transporting moiety or only an electron-transporting moiety, it is clear whether the nature of the compound has hole-transporting properties or electron-transporting properties. That is, it is easy to tell that a compound includes only a hole-transporting moiety has hole-transporting properties and a compound includes only an electron-transporting moiety has electron-transporting properties. 
     In an embodiment, a compound may include both (e.g., simultaneously) a hole-transporting moiety and an electron-transporting moiety. In this case, a simple comparison between the total number of the hole-transporting moieties and the total number of the electron-transporting moieties in the compound may be a criterion for predicting whether the compound is a hole-transporting compound or an electron-transporting compound, but this simple comparison cannot be utilized as an absolute criterion. One of the reasons why such a simple comparison cannot be utilized as an absolute criterion is that one hole-transporting moiety and one electron-transporting moiety may not have exactly the same ability to attract holes and electrons respectively. 
     Therefore, a relatively reliable way to determine whether a compound having a certain structure is a hole-transporting compound or an electron-transporting compound is to directly implement the compound in a device. 
     The host may include a compound represented by Formula 301: 
       [Ar 301 ] xb11 [(L 301 ) xb1 -R 301 ] xb21   Formula 301
 
     wherein, in Formula 301, 
     Ar 301  and L 301  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     xb11 may be 1, 2, or 3, 
     xb1 may be an integer from 0 to 5, 
     R 301  may be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 60  alkyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkenyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkynyl group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  alkoxy group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a ,—Si(Q 301 )(Q 302 )(Q 303 ), —N(Q 301 )(Q 302 ), —B(Q 301 )(Q 302 ), —C(═O)(Q 301 ), —S(═O) 2 (Q 301 ), or —P(═O)(Q 301 )(Q 302 ), 
     xb21 may be an integer from 1 to 5, and 
     Q 301  to Q 303  may each independently be the same as described in connection with Q 1 . 
     For example, when xb11 in Formula 301 is 2 or more, two or more of Ar 301 (s) may be linked to each other via a single bond. 
     In one or more embodiments, the host may include a compound represented by Formula 301-1, a compound represented by Formula 301-2, or any combination thereof: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 301-1 and 301-2, 
     ring A 301  to ring A 304  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     X 301  may be O, S, N-[(L 304 ) xb4 -R 304 ], C(R 304 )(R 305 ), or Si(R 304 )(R 305 ), 
     xb22 and xb23 may each independently be 0, 1, or 2, 
     L 301 , xb1 7  and R 301  may each independently be the same as respectively described in the present specification, 
     L 302  to L 304  may each independently be the same as described in connection with L 301 , 
     xb2 to xb4 may each independently be the same as described in connection with xb1, and 
     R 302  to R 305  and R 311  to R 314  may each independently be the same as described in connection with R 301 . 
     In one or more embodiments, the host may include an alkaline earth-metal complex. For example, the host may include a Be complex (for example, Compound H55), an Mg complex, a Zn complex, or any combination thereof. 
     In one or more embodiments, the host may include one of Compounds H1 to H124, HT-01, HT-02, HT-03, ET-01, ET-02, 9,10-di(2-naphthyl)anthracene (ADN), 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN), 9,10-di-(2-naphthyl)-2-t-butyl-anthracene (TBADN), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), 1,3-di-9-carbazolylbenzene (mCP), 1,3,5-tri(carbazol-9-yl)benzene (TCP), or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Phosphorescent Dopant 
     The phosphorescent dopant may include at least one transition metal as a center metal (e.g., a central metal atom). 
     The phosphorescent dopant may include a monodentate ligand, a bidentate ligand, a tridentate ligand, a tetradentate ligand, a pentadentate ligand, a hexadentate ligand, or any combination thereof. 
     The phosphorescent dopant may be electrically neutral. 
     For example, the phosphorescent dopant may include an organometallic compound represented by Formula 401: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 401 and 402, 
     M may be a transition metal (for example, iridium (Ir), platinum (Pt), palladium (Pd), osmium (Os), titanium (Ti), gold (Au), hafnium (Hf), europium (Eu), terbium (Tb), rhodium (Rh), rhenium (Re), or thulium (Tm)), 
     L 401  may be a ligand represented by Formula 402, and xc1 may be 1, 2, or 3, wherein when xc1 is two or more, two or more of L 401 (s) may be identical to or different from each other, 
     L 402  may be an organic ligand, and xc2 may be 0, 1, 2, 3, or 4, wherein, when xc2 is 2 or more, two or more of L 402 (s) may be identical to or different from each other, 
     X 401  and X 402  may each independently be nitrogen or carbon, 
     ring A 401  and ring A 402  may each independently be a C 3 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, 
     T 401  may be a single bond, —O—, —S—, —C(═O)—, —N(Q 411 )-, —C(Q 411 )(Q 412 )-, —C(Q 411 )═C(Q 412 )-, —C(Q 411 )═, or ═C═, 
     X 403  and X 404  may each independently be a chemical bond (for example, a covalent bond or a coordination bond), O, S, N(Q 413 ), B(Q 413 ), P(Q 413 ), C(Q 413 )(Q 414 ), or Si(Q 413 )(Q 414 ), 
     Q 411  to Q 414  may each independently be the same as described in connection with Q 1  (to be described in more detail below), 
     R 401  and R 402  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group unsubstituted or substituted with at least one R 10a , a C 1 -C 20  alkoxy group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , —Si(Q 401 )(Q 402 )(Q 403 ), —N(Q 401 )(Q 402 ), —B(Q 401 )(Q 402 ), —C(═O)(Q 401 ), —S(═O) 2 (Q 401 ), or —P(═O)(Q 401 )(Q 402 ), 
     Q 401  to Q 403  may each independently be the same as described in connection with Q 1 , 
     xc11 and xc12 may each independently be an integer from 0 to 10, and 
     * and * 1  in Formula 402 each indicate a binding site to M in Formula 401. 
     For example, in Formula 402, i) X 401  may be nitrogen, and X 402  may be carbon, or ii) each of X 401  and X 402  may be nitrogen. 
     In one or more embodiments, when xc1 in Formula 401 is 2 or more, two ring A 401 (s) in two or more of L 401  (s) may be optionally linked to each other via T 402 , which is a linking group, and two ring A 402 (s) in two or more of L 401  (s) may be optionally linked to each other via T 403 , which is a linking group (see Compounds PD1 to PD4 and PD7). T 402  and T 403  may each independently be the same as described in connection with T 401 . 
     L 402  in Formula 401 may be an organic ligand. For example, L 402  may include a halogen group, a diketone group (for example, an acetylacetonate group), a carboxylic acid group (for example, a picolinate group), —C(═O), an isonitrile group, a —CN group, a phosphorus group (for example, a phosphine group, a phosphite group, and/or the like), or any combination thereof. 
     The phosphorescent dopant may include, for example, one of Compounds PD1 to PD39 or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Fluorescent Dopant 
     The fluorescent dopant may include an amine group-containing compound, a styryl group-containing compound, or any combination thereof. 
     For example, the fluorescent dopant may include a compound represented by Formula 501: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 501, 
     Ar 501 , L 501  to L 503 , R 501 , and R 502  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     xd1 to xd3 may each independently be 0, 1, 2, or 3, and 
     xd4 may be 1, 2, 3, 4, 5, or 6. 
     For example, Ar 501  in Formula 501 may be a condensed cyclic group (for example, an anthracene group, a chrysene group, a pyrene group, and/or the like) in which three or more monocyclic groups are condensed together. 
     In one or more embodiments, xd4 in Formula 501 may be 2. 
     For example, the fluorescent dopant may include one of Compounds FD1 to FD36, DPVBi, DPAVBi, or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Delayed Fluorescence Material 
     The emission layer may include a delayed fluorescence material. 
     In the present specification, the delayed fluorescence material may be selected from compounds capable of emitting delayed fluorescence by a delayed fluorescence emission mechanism. 
     The delayed fluorescence material included in the emission layer may act (e.g., serve) as a host or a dopant, depending on the type or kind of other materials included in the emission layer. 
     In an embodiment, a difference between a triplet energy level (eV) of the delayed fluorescence material and a singlet energy level (eV) of the delayed fluorescence material may be equal to or greater than 0 eV and equal to or less than 0.5 eV. When the difference between the triplet energy level (eV) of the delayed fluorescence material and the singlet energy level (eV) of the delayed fluorescence material satisfies the above-described range, up-conversion from the triplet state to the singlet state of the delayed fluorescence materials may effectively occur, and thus, the luminescence efficiency of the light-emitting device  10  may be improved. 
     For example, the delayed fluorescence material may include i) a material including at least one electron donor (for example, a π electron-rich C 3 -C 60  cyclic group such as a carbazole group and/or the like) and at least one electron acceptor (for example, a sulfoxide group, a cyano group, a π electron-deficient nitrogen-containing C 1 -C 60  cyclic group, and/or the like), ii) a material including a C 8 -C 60  polycyclic group including at least two cyclic groups condensed to each other and sharing boron (B), and/or the like. 
     Examples of the delayed fluorescence material may include at least one of Compounds DF1 to DF12: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Electron Transport Region in Interlayer  130   
     The electron transport region may have i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer including (e.g., consisting of) a plurality of different materials, or iii) a multi-layered structure including a plurality of layers including different materials. 
     The electron transport region may include a hole blocking layer, an electron transport layer, an electron injection layer, or any combination thereof. 
     For example, the electron transport region may have an electron transport layer/electron injection layer structure or a hole blocking layer/electron transport layer/electron injection layer structure, wherein, in each structure, constituting layers are sequentially stacked from the emission layer in the respective stated order. 
     The electron transport region (for example, the hole blocking layer or the electron transport layer in the electron transport region) may include a metal-free compound including at least one π electron-deficient nitrogen-containing C 1 -C 60  cyclic group. 
     For example, the electron transport region may include a compound represented by Formula 601: 
       [Ar 601 ] xe11 -[(L 601 ) xe1 -R 601 ] xe21   Formula 601
 
     wherein, in Formula 601, 
     Ar 601  and L 601  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     xe11 may be 1, 2, or 3, 
     xe1 may be 0, 1, 2, 3, 4, or 5, 
     R 601  may be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , —Si(Q 601 )(Q 602 )(Q 603 ), —C(═O)(Q 601 ), —S(═O) 2 (Q 601 ), or —P(═O)(Q 601 )(Q 602 ), 
     Q 601  to Q 603  may each independently be the same as described in connection with Q 1 , 
     xe21 may be 1, 2, 3, 4, or 5, and 
     at least one of Arm, L 601 , or R 601  may each independently be a π electron-deficient nitrogen-containing C 1 -C 60  cyclic group unsubstituted or substituted with at least one R 10a . 
     For example, when xe11 in Formula 601 is 2 or more, two or more of Ar 601 (s) may be linked to each other via a single bond. 
     In one or more embodiments, Ar 601  in Formula 601 may be a substituted or unsubstituted anthracene group. 
     In one or more embodiments, the electron transport region may include a compound represented by Formula 601-1: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 601-1, 
     X 614  may be N or C(R 614 ), X 615  may be N or C(R 615 ), X 616  may be N or 
     C(R 616 ), and at least one of X 614  to X 616  may be N, 
     L 611  to L 613  may each independently be the same as described in connection with L 601 , 
     xe611 to xe613 may each independently be the same as described in connection with xe1, 
     R 611  to R 613  may each independently be the same as described in connection with Root, and 
     R 614  to R 616  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a . 
     For example, xe1 and xe611 to xe613 in Formulae 601 and 601-1 may each independently be 0, 1, or 2. 
     The electron transport region may include one of Compounds ET1 to ET45, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (Bphen), Alq3, BAlq, TAZ, NTAZ, or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A thickness of the electron transport region may be in a range of about 100 Å to about 5,000 Å, for example, about 160 Å to about 4,000 Å. When the electron transport region includes the hole blocking layer, the electron transport layer, or any combination thereof, a thickness of the hole blocking layer or electron transport layer may each independently be from about 20 Å to about 1,000 Å, for example, about 30 Å to about 300 Å, and a thickness of the electron transport layer may be from about 100 Å to about 1,000 Å, for example, about 150 Å to about 500 Å. When the thicknesses of the hole blocking layer and/or the electron transport layer are within these ranges, satisfactory electron transporting characteristics may be obtained without a substantial increase in driving voltage. 
     The electron transport region (for example, the electron transport layer in the electron transport region) may further include, in addition to the materials described above, a metal-containing material. 
     The metal-containing material may include an alkali metal complex, an alkaline earth metal complex, or any combination thereof. A metal ion of the alkali metal complex may be a Li ion, a Na ion, a K ion, a Rb ion, or a Cs ion, and a metal ion of the alkaline earth metal complex may be a Be ion, a Mg ion, a Ca ion, a Sr ion, or a Ba ion. A ligand coordinated with the metal ion of the alkali metal complex and the alkaline earth metal complex may each independently be hydroxyquinoline, hydroxyisoquinoline, hydroxybenzoquinoline, hydroxyacridine, hydroxyphenanthridine, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxyphenyloxadiazole, hydroxyphenylthiadiazole, hydroxyphenylpyridine, hydroxyphenylbenzimidazole, hydroxyphenylbenzothiazole, bipyridine, phenanthroline, cyclopentadiene, or any combination thereof. 
     For example, the metal-containing material may include a Li complex. The Li complex may include, for example, Compound ET-D1 (LiQ) or ET-D2: 
     
       
         
         
             
             
         
       
     
     The electron transport region may include an electron injection layer that facilitates the injection of electrons from the second electrode  150 . The electron injection layer may be in direct contact with the second electrode  150 . 
     The electron injection layer may have i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer including (e.g., consisting of) a plurality of different materials, or iii) a multi-layered structure including a plurality of layers including different materials. 
     The electron injection layer may include an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal-containing compound, an alkaline earth metal-containing compound, a rare earth metal-containing compound, an alkali metal complex, an alkaline earth metal complex, a rare earth metal complex, or any combination thereof. 
     The alkali metal may include Li, Na, K, Rb, Cs, or any combination thereof. The alkaline earth metal may include Mg, Ca, Sr, Ba, or any combination thereof. The rare earth metal may include Sc, Y, Ce, Tb, Yb, Gd, or any combination thereof. 
     The alkali metal-containing compound, the alkaline earth metal-containing compound, and the rare earth metal-containing compound may be one or more oxides, halides (for example, fluorides, chlorides, bromides, iodides, and/or the like), tellurides of the alkali metal, the alkaline earth metal, and the rare earth metal, or any combination thereof. 
     The alkali metal-containing compound may include one or more alkali metal oxides, such as Li 2 O, Cs 2 O, and/or K 2 O, alkali metal halides, such as LiF, NaF, CsF, KF, LiI, NaI, CsI, and/or KI, or any combination thereof. The alkaline earth metal-containing compound may include an alkaline earth metal compound, such as BaO, SrO, CaO, Ba x Sr 1-x O (wherein x is a real number satisfying the condition of 0&lt;x&lt;1), Ba x Ca 1-x O (wherein x is a real number satisfying the condition of 0&lt;x&lt;1), and/or the like. The rare earth metal-containing compound may include YbF 3 , ScF 3 , Sc 203 , Y 2 O 3 , Ce 2 O 3 , GdF 3 , TbF 3 , YbI 3 , ScI 3 , TbI 3 , or any combination thereof. In one or more embodiments, the rare earth metal-containing compound may include lanthanide metal telluride. Examples of the lanthanide metal telluride may include LaTe, CeTe, PrTe, NdTe, PmTe, SmTe, EuTe, GdTe, TbTe, DyTe, HoTe, ErTe, TmTe, YbTe, LuTe, La 2 Te 3 , Ce 2 Te 3 , Pr 2 Te 3 , Nd 2 Te 3 , Pm 2 Te 3 , Sm 2 Te 3 , Eu 2 Te 3 , Gd 2 Te 3 , Tb 2 Te 3 , Dy 2 Te 3 , Ho 2 Te 3 , Er 2 Te 3 , Tm 2 Te 3 , Yb 2 Te 3 , Lu 2 Te 3 , and/or the like. 
     The alkali metal complex, the alkaline earth metal complex, and the rare earth metal complex may include i) one of ions of the alkali metal, the alkaline earth metal, and the rare earth metal described above and ii) a ligand bond to the metal ion, for example, hydroxyquinoline, hydroxyisoquinoline, hydroxybenzoquinoline, hydroxyacridine, hydroxyphenanthridine, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxyphenyloxadiazole, hydroxyphenylthiadiazole, hydroxyphenylpyridine, hydroxyphenylbenzimidazole, hydroxyphenylbenzothiazole, bipyridine, phenanthroline, cyclopentadiene, or any combination thereof. 
     The electron injection layer may include (e.g., consist of) an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal-containing compound, an alkaline earth metal-containing compound, a rare earth metal-containing compound, an alkali metal complex, an alkaline earth metal complex, a rare earth metal complex, or any combination thereof, as described above. In one or more embodiments, the electron injection layer may further include an organic material (for example, a compound represented by Formula 601). 
     In an embodiment, the electron injection layer may include (e.g., consist of) i) an alkali metal-containing compound (for example, an alkali metal halide), ii) a) an alkali metal-containing compound (for example, an alkali metal halide); and b) an alkali metal, an alkaline earth metal, a rare earth metal, or any combination thereof. For example, the electron injection layer may be a KI:Yb co-deposited layer, an RbI:Yb co-deposited layer, and/or the like. 
     When the electron injection layer further includes an organic material, the alkali metal, the alkaline earth metal, the rare earth metal, the alkali metal-containing compound, the alkaline earth metal-containing compound, the rare earth metal-containing compound, the alkali metal complex, the alkaline earth-metal complex, the rare earth metal complex, or any combination thereof may be uniformly or non-uniformly dispersed in a matrix including the organic material. 
     A thickness of the electron injection layer may be in a range of about 1 Å to about 100 Å, for example, about 3 Å to about 90 Å. When the thickness of the electron injection layer is within these ranges, satisfactory electron injection characteristics may be obtained without a substantial increase in driving voltage. 
     Second Electrode  150   
     The second electrode  150  may be located on the interlayer  130  having a structure as described above. The second electrode  150  may be a cathode, which is an electron injection electrode, and a material for forming the second electrode  150  may include a metal, an alloy, an electrically conductive compound, or any combination thereof, each having a low-work function. 
     The second electrode  150  may include lithium (Li), silver (Ag), magnesium (Mg), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), ytterbium (Yb), silver-ytterbium (Ag—Yb), ITO, IZO, or any combination thereof. The second electrode  150  may be a transmissive electrode, a semi-transmissive electrode, or a reflective electrode. 
     The second electrode  150  may have a single-layered structure or a multi-layered structure including a plurality of layers. 
     Capping Layer 
     A first capping layer may be located outside the first electrode  110  (e.g., on the side opposite to the second electrode  150 ), and/or a second capping layer may be located outside the second electrode  150  (e.g., on the side opposite to the first electrode  110 ). In one embodiment. In one embodiment, the light-emitting device  10  may have a structure in which the first capping layer, the first electrode  110 , the interlayer  130 , and the second electrode  150  are sequentially stacked in the stated order, a structure in which the first electrode  110 , the interlayer  130 , the second electrode  150 , and the second capping layer are sequentially stacked in the stated order, or a structure in which the first capping layer, the first electrode  110 , the interlayer  130 , the second electrode  150 , and the second capping layer are sequentially stacked in the stated order. 
     In some embodiments, light generated in an emission layer of the interlayer  130  of the light-emitting device  10  may be emitted or extracted toward the outside through the first electrode  110  which is a semi-transmissive electrode or a transmissive electrode, and the first capping layer. In some embodiments, light generated in an emission layer of the interlayer  130  of the light-emitting device  10  may be emitted or extracted toward the outside through the second electrode  150  which is a semi-transmissive electrode or a transmissive electrode, and the second capping layer. 
     The first capping layer and the second capping layer may increase external luminescence efficiency according to the principle of constructive interference. Accordingly, the light extraction efficiency of the light-emitting device  10  may be increased, so that the luminescence efficiency of the light-emitting device  10  may be improved. 
     Each of the first capping layer and the second capping layer may include a material having a refractive index of 1.6 or more (at 589 nm). 
     The first capping layer and the second capping layer may each independently be an organic capping layer including an organic material, an inorganic capping layer including an inorganic material, or an organic-inorganic composite capping layer including an organic material and an inorganic material. 
     At least one of the first capping layer or the second capping layer may each independently include a carbocyclic compound, a heterocyclic compound, an amine group-containing compound, a porphyrin derivative, a phthalocyanine derivative, a naphthalocyanine derivative, an alkali metal complex, an alkaline earth metal complex, or any combination thereof. The carbocyclic compound, the heterocyclic compound, and the amine group-containing compound may optionally be substituted with a substituent containing O, N, S, Se, Si, F, Cl, Br, I, or any combination thereof. In an embodiment, at least one of the first capping layer or the second capping layer may each independently include an amine group-containing compound. 
     For example, at least one of the first capping layer or the second capping layer may each independently include a compound represented by Formula 201, a compound represented by Formula 202, or any combination thereof. 
     In one or more embodiments, at least one of the first capping layer or the second capping layer may each independently include one of Compounds HT28 to HT33, one of Compounds CP1 to CP6, β-NPB, or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Electronic Apparatus 
     The light-emitting device may be included in one or more suitable electronic apparatuses. For example, the electronic apparatus including the light-emitting device may be a light-emitting apparatus, an authentication apparatus, and/or the like. 
     The electronic apparatus (for example, a light-emitting apparatus) may further include, in addition to the light-emitting device, i) a color filter, ii) a color conversion layer, or iii) a color filter and a color conversion layer. The color filter and/or the color conversion layer may be located in at least one traveling direction of light emitted from the light-emitting device. For example, the light emitted from the light-emitting device may be blue light. For details on the light-emitting device, related description provided above may be referred to. In an embodiment, the color conversion layer may include a quantum dot. 
     The electronic apparatus may include a first substrate. The first substrate may include a plurality of subpixel areas, the color filter may include a plurality of color filter areas respectively corresponding to the plurality of subpixel areas, and the color conversion layer may include a plurality of color conversion areas respectively corresponding to the plurality of subpixel areas. 
     A pixel-defining film may be located among the subpixel areas to define each of the subpixel areas. 
     The color filter may further include a plurality of color filter areas and light-shielding patterns located among the color filter areas, and the color conversion layer may further include a plurality of color conversion areas and light-shielding patterns located among the color conversion areas. 
     The plurality of color filter areas (or the plurality of color conversion areas) may include a first area emitting a first color light, a second area emitting a second color light, and/or a third area emitting a third color light, wherein the first color light, the second color light, and/or the third color light may have different maximum emission wavelengths from one another. In an embodiment, the first color light may be red light, the second color light may be green light, and the third color light may be blue light. In an embodiment, the plurality of color filter areas (or the plurality of color conversion areas) may include quantum dots. In an embodiment, the first area may include a red quantum dot, the second area may include a green quantum dot, and the third area may not include(e.g., may exclude) a quantum dot. For details on the quantum dot, related descriptions provided herein may be referred to. The first area, the second area, and/or the third area may each further include a scatter. 
     In an embodiment, the light-emitting device may be to emit a first light, the first area may be to absorb the first light to emit a first-first color light, the second area may be to absorb the first light to emit a second-first color light, and the third area may be to absorb the first light to emit a third-first color light. In this regard, the first-first color light, the second-first color light, and the third-first color light may have different maximum emission wavelengths. In an embodiment, the first light may be blue light, the first-first color light may be red light, the second-first color light may be green light, and the third-first color light may be blue light. 
     The electronic apparatus may further include a thin-film transistor, in addition to the light-emitting device as described above. The thin-film transistor may include a source electrode, a drain electrode, and an activation layer, wherein one of the source electrode or the drain electrode may be electrically connected to one of the first electrode or the second electrode of the organic light-emitting device. 
     The thin-film transistor may further include a gate electrode, a gate insulating film, and/or the like. 
     The activation layer may include crystalline silicon, amorphous silicon, an organic semiconductor, an oxide semiconductor, and/or the like. 
     The electronic apparatus may further include a sealing portion for sealing the light-emitting device. The sealing portion may be located between the color conversion layer and/or color filter and the light-emitting device. The sealing portion allows light from the light-emitting device to be extracted to the outside, and concurrently (e.g., simultaneously) prevents or substantially prevents ambient air and/or moisture from penetrating into the light-emitting device. The sealing portion may be a sealing substrate including a transparent glass substrate and/or a plastic substrate. The sealing portion may be a thin-film encapsulation layer including at least one of an organic layer or an inorganic layer. When the sealing portion is a thin-film encapsulation layer, the electronic apparatus may be flexible. 
     Various suitable functional layers may be additionally located on the sealing portion, in addition to the color filter and/or the color conversion layer, according to the usage of the electronic apparatus. Examples of the functional layers may include a touch screen layer, a polarizing layer, and/or the like. The touch screen layer may be a pressure-sensitive touch screen layer, a capacitive touch screen layer, and/or an infrared touch screen layer. The authentication apparatus may be, for example, a biometric authentication apparatus that authenticates an individual by utilizing biometric information of a living body (for example, fingertips, pupils, and/or the like). 
     The authentication apparatus may further include, in addition to the light-emitting device as described above, a biometric information collector. 
     The electronic apparatus may be applied to one or more suitable displays, light sources, lighting (e.g., lighting apparatuses), personal computers (for example, a mobile personal computer), mobile phones, digital cameras, electronic organizers, electronic dictionaries, electronic game machines, medical instruments (for example, electronic thermometers, sphygmomanometers, blood glucose meters, pulse measurement devices, pulse wave measurement devices, electrocardiogram displays, ultrasonic diagnostic devices, and/or endoscope displays), fish finders, one or more suitable measuring instruments, meters (for example, meters for a vehicle, an aircraft, and/or a vessel), projectors, and/or the like. 
     Description of FIGS.  2  and  3   
       FIG.  2    is a cross-sectional view of an electronic apparatus according to an embodiment of the present disclosure. 
     The electronic apparatus of  FIG.  2    includes a substrate  100 , a thin-film transistor (TFT), a light-emitting device, and an encapsulation portion  300  that seals the light-emitting device. 
     The substrate  100  may be a flexible substrate (e.g., a plastic substrate), a glass substrate, and/or a metal substrate. A buffer layer  210  may be located on the substrate  100 . The buffer layer  210  may prevent or reduce penetration of impurities through the substrate  100  and may provide a flat surface on the substrate  100 . 
     A TFT may be located on the buffer layer  210 . The TFT may include an activation layer  220 , a gate electrode  240 , a source electrode  260 , and a drain electrode  270 . 
     The activation layer  220  may include an inorganic semiconductor such as silicon and/or polysilicon, an organic semiconductor, and/or an oxide semiconductor, and may include a source region, a drain region, and a channel region. 
     A gate insulating film  230  for insulating the activation layer  220  from the gate electrode  240  may be located on the activation layer  220 , and the gate electrode  240  may be located on the gate insulating film  230 . 
     An interlayer insulating film  250  may be located on the gate electrode  240 . The interlayer insulating film  250  may be located between the gate electrode  240  and the source electrode  260  to insulate the gate electrode  240  from the source electrode  260  and between the gate electrode  240  and the drain electrode  270  to insulate the gate electrode  240  from the drain electrode  270 . 
     The source electrode  260  and the drain electrode  270  may be located on the interlayer insulating film  250 . The interlayer insulating film  250  and the gate insulating film  230  may be formed to expose the source region and the drain region of the activation layer  220 , and the source electrode  260  and the drain electrode  270  may be located in contact with the exposed portions of the source region and the drain region of the activation layer  220 . 
     The TFT may be electrically connected to a light-emitting device to drive the light-emitting device, and may be covered and protected by a passivation layer  280 . The passivation layer  280  may include an inorganic insulating film, an organic insulating film, or any combination thereof. A light-emitting device may be provided on the passivation layer  280 . The light-emitting device may include a first electrode  110 , an interlayer  130 , and a second electrode  150 . 
     The first electrode  110  may be located on the passivation layer  280 . The passivation layer  280  may not completely cover the drain electrode  270  and may expose a portion of the drain electrode  270 , and the first electrode  110  may be located or arranged to be connected to the exposed portion of the drain electrode  270 . 
     A pixel defining layer  290  including an insulating material may be located on the first electrode  110 . The pixel defining layer  290  may expose a certain region of the first electrode  110 , and an interlayer  130  may be formed in the exposed region of the first electrode  110 . The pixel defining layer  290  may be a polyimide or polyacrylic organic film. In one or more embodiments, one or more layers of the interlayer  130  may extend beyond the upper portion of the pixel defining layer  290  to be located in the form of a common layer. 
     The second electrode  150  may be located on the interlayer  130 , and a capping layer  170  may be additionally formed on the second electrode  150 . The capping layer  170  may be formed to cover the second electrode  150 . 
     The encapsulation portion  300  may be located on the capping layer  170 . The encapsulation portion  300  may be located on a light-emitting device to protect the light-emitting device from moisture or oxygen. The encapsulation portion  300  may include: an inorganic film including silicon nitride (SiNx), silicon oxide (SiOx), indium tin oxide, indium zinc oxide, or any combination thereof; an organic film including polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, polyethylene sulfonate, polyoxymethylene, polyarylate, hexamethyldisiloxane, an acrylic resin (for example, polymethyl methacrylate, polyacrylic acid, and/or the like), an epoxy-based resin (for example, aliphatic glycidyl ether (AGE), and/or the like), or any combination thereof; or any combination of the inorganic films and the organic films. 
       FIG.  3    is a cross-sectional view of an electronic apparatus according to another embodiment of the present disclosure. 
     The electronic apparatus of  FIG.  3    is the same as the electronic apparatus of  FIG.  2   , except that a light-shielding pattern  500  and a functional region  400  are additionally located on the encapsulation portion  300 . The functional region  400  may be i) a color filter area, ii) a color conversion area, or iii) a combination of the color filter area and the color conversion area. In an embodiment, the light-emitting device included in the light-emitting apparatus of  FIG.  3    may be a tandem light-emitting device. 
     Manufacturing Method 
     Respective layers included in the hole transport region, the emission layer, and respective layers included in the electron transport region may be formed in a certain region by utilizing one or more suitable methods selected from vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB) deposition, ink-jet printing, laser-printing, laser-induced thermal imaging, and/or the like. 
     When layers constituting the hole transport region, the emission layer, and layers constituting the electron transport region are formed by vacuum deposition, the deposition may be performed at a deposition temperature of about 100° C. to about 500° C., a vacuum degree of about 10 −8  torr to about 10 −3  torr, and a deposition speed of about 0.01 Å/sec to about 100 Å/sec, depending on a material to be included in the layer to be formed and the structure of the layer to be formed. 
     When layers constituting the hole transport region, the emission layer, and layers constituting the electron transport region are formed by spin coating, the spin coating may be performed at a coating speed of about 2,000 rpm to about 5,000 rpm and at a heat treatment temperature of about 80° C. to about 200° C. by taking into account a material to be included in the layer to be formed and the structure of the layer to be formed. 
     General Definition of Substituents 
     The term “C 3 -C 60  carbocyclic group” as used herein refers to a cyclic group consisting of only carbon atoms as ring-forming atoms and having 3 to 60 carbon atoms, and the term “C 1 -C 60  heterocyclic group” as used herein refers to a cyclic group that has, in addition to 1 to 60 carbon atoms, a heteroatom as a ring-forming atom. The C 3 -C 60  carbocyclic group and the C 1 -C 60  heterocyclic group may each be a monocyclic group consisting of one ring or a polycyclic group in which two or more rings are condensed with each other. For example, the C 1 -C 60  heterocyclic group has 3 to 61 ring-forming atoms. 
     The term “cyclic group” as used herein may include the C 3 -C 60  carbocyclic group, and the C 1 -C 60  heterocyclic group. 
     The term “π electron-rich C 3 -C 60  cyclic group” as used herein refers to a cyclic group (e.g., a carbocyclic group or a heterocyclic group) that has 3 to 60 carbon atoms and does not include *—N═*′ as a ring-forming moiety, and the term “π electron-deficient nitrogen-containing C 1 -C 60  cyclic group” as used herein refers to a heterocyclic group that has 1 to 60 carbon atoms and further includes *—N═*′ as a ring-form ing moiety. 
     For example, 
     the C 3 -C 60  carbocyclic group may be i) a T 1  group or ii) a group in which at least two T1 groups are condensed with each other (for example, the C 3 -C 60  carbocyclic group may be a cyclopentadiene group, an adamantane group, a norbornane group, a benzene group, a pentalene group, a naphthalene group, an azulene group, an indacene group, an acenaphthylene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a perylene group, a pentaphene group, a heptalene group, a naphthacene group, a picene group, a hexacene group, a pentacene group, a rubicene group, a coronene group, an ovalene group, an indene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, an indenophenanthrene group, or an indenoanthracene group), 
     the C 1 -C 60  heterocyclic group may be i) a T2 group, ii) a group in which at least two T2 groups are condensed with each other, or iii) a group in which at least one T2 group is condensed with at least one T1 group (for example, the C 1 -C 60  heterocyclic group may be a pyrrole group, a thiophene group, a furan group, an indole group, a benzoindole group, a naphthoindole group, an isoindole group, a benzoisoindole group, a naphthoisoindole group, a benzosilole group, a benzothiophene group, a benzofuran group, a carbazole group, a dibenzosilole group, a dibenzothiophene group, a dibenzofuran group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, a benzothienocarbazole group, a benzosilolocarbazole group, a benzoindolocarbazole group, a benzocarbazole group, a benzonaphthofuran group, a benzonaphthothiophene group, a benzonaphthosilole group, a benzofurodibenzofuran group, a benzofurodibenzothiophene group, a benzothienodibenzothiophene group, a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isoxazole group, an oxadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, a benzopyrazole group, a benzimidazole group, a benzoxazole group, a benzoisoxazole group, a benzothiazole group, a benzoisothiazole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a benzoisoquinoline group, a quinoxaline group, a benzoquinoxaline group, a quinazoline group, a benzoquinazoline group, a phenanthroline group, a cinnoline group, a phthalazine group, a naphthyridine group, an imidazopyridine group, an imidazopyrimidine group, an imidazotriazine group, an imidazopyrazine group, an imidazopyridazine group, an azacarbazole group, an azafluorene group, an azadibenzosilole group, an azadibenzothiophene group, an azadibenzofuran group, and/or the like), 
     the π electron-rich C 3 -C 60  cyclic group may be i) a T1 group, ii) a condensed cyclic group in which at least two T1 groups are condensed with each other, iii) a T3 group, iv) a condensed cyclic group in which at least two T3 groups are condensed with each other, or v) a condensed cyclic group in which at least one T3 group and at least one T1 group are condensed with each other (for example, the π electron-rich C 3 -C 60  cyclic group may be the C 3 -C 60  carbocyclic group, a 1H-pyrrole group, a silole group, a borole group, a 2H-pyrrole group, a 3H-pyrrole group, a thiophene group, a furan group, an indole group, a benzoindole group, a naphthoindole group, an isoindole group, a benzoisoindole group, a naphthoisoindole group, a benzosilole group, a benzothiophene group, a benzofuran group, a carbazole group, a dibenzosilole group, a dibenzothiophene group, a dibenzofuran group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, a benzothienocarbazole group, a benzosilolocarbazole group, a benzoindolocarbazole group, a benzocarbazole group, a benzonaphthofuran group, a benzonaphthothiophene group, a benzonaphthosilole group, a benzofurodibenzofuran group, a benzofurodibenzothiophene group, a benzothienodibenzothiophene group, and/or the like), 
     the π electron-deficient nitrogen-containing C 1 -C 60  cyclic group may be i) a T4 group, ii) a group in which at least two T4 groups are condensed with each other, iii) a group in which at least one T4 group is condensed with at least one T1 group, iv) a group in which at least one T4 group is condensed with at least one T3 group, or v) a group in which at least one T4 group, at least one T1 group, and at least one T3 group are condensed with each other (for example, the π electron-deficient nitrogen-containing C 1 -C 60  cyclic group may be a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isoxazole group, an oxadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, a benzopyrazole group, a benzimidazole group, a benzoxazole group, a benzoisoxazole group, a benzothiazole group, a benzoisothiazole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a benzoisoquinoline group, a quinoxaline group, a benzoquinoxaline group, a quinazoline group, a benzoquinazoline group, a phenanthroline group, a cinnoline group, a phthalazine group, a naphthyridine group, an imidazopyridine group, an imidazopyrimidine group, an imidazotriazine group, an imidazopyrazine group, an imidazopyridazine group, an azacarbazole group, an azafluorene group, an azadibenzosilole group, an azadibenzothiophene group, an azadibenzofuran group, and/or the like), 
     wherein the T1 group may be a cyclopropane group, a cyclobutane group, a cyclopentane group, a cyclohexane group, a cycloheptane group, a cyclooctane group, a cyclobutene group, a cyclopentene group, a cyclopentadiene group, a cyclohexene group, a cyclohexadiene group, a cycloheptene group, an adamantane group, a norbornane (or a bicyclo[2.2.1]heptane) group, a norbornene group, a bicyclo[1.1.1]pentane group, a bicyclo[2.1.1]hexane group, a bicyclo[2.2.2]octane group, or a benzene group, 
     the T2 group may be a furan group, a thiophene group, a 1H-pyrrole group, a silole group, a borole group, a 2H-pyrrole group, a 3H-pyrrole group, an imidazole group, a pyrazole group, a triazole group, a tetrazole group, an oxazole group, an isoxazole group, an oxadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, an azasilole group, an azaborole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a tetrazine group, a pyrrolidine group, an imidazolidine group, a dihydropyrrole group, a piperidine group, a tetrahydropyridine group, a dihydropyridine group, a hexahydropyrimidine group, a tetrahydropyrimidine group, a dihydropyrimidine group, a piperazine group, a tetrahydropyrazine group, a dihydropyrazine group, a tetrahydropyridazine group, or a dihydropyridazine group, 
     the T3 group may be a furan group, a thiophene group, a 1H-pyrrole group, a silole group, or a borole group, and 
     the T4 group may be a 2H-pyrrole group, a 3H-pyrrole group, an imidazole group, a pyrazole group, a triazole group, a tetrazole group, an oxazole group, an isoxazole group, an oxadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, an azasilole group, an azaborole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, or a tetrazine group. 
     The terms “the cyclic group,” “the C 3 -C 60  carbocyclic group,” “the C 1 -C 60  heterocyclic group,” “the π electron-rich C 3 -C 60  cyclic group,” or “the π electron-deficient nitrogen-containing C 1 -C 60  cyclic group” as used herein refer to a group condensed to any cyclic group, a monovalent group, or a polyvalent group (for example, a divalent group, a trivalent group, a tetravalent group, etc.) according to the structure of a formula for which the corresponding term is used. For example, the “benzene group” may be a benzo group, a phenyl group, a phenylene group, and/or the like, which may be easily understood by one of ordinary skill in the art according to the structure of a formula including the “benzene group.” 
     Examples of the monovalent C 3 -C 60  carbocyclic group and the monovalent C 1 -C 60  heterocyclic group may include a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group. Examples of the divalent C 3 -C 60  carbocyclic group and the divalent C 1 -C 60  heterocyclic group may include a C 3 -C 10  cycloalkylene group, a C 1 -C 10  heterocycloalkylene group, a C 3 -C 10  cycloalkenylene group, a C 1 -C 10  heterocycloalkenylene group, a C 6 -C 60  arylene group, a C 1 -C 60  heteroarylene group, a divalent non-aromatic condensed polycyclic group, and a divalent non-aromatic condensed heteropolycyclic group. 
     The term “C 1 -C 60  alkyl group” as used herein refers to a linear or branched aliphatic hydrocarbon monovalent group having 1 to 60 carbon atoms, and examples thereof may include a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, an n-hexyl group, an iso-hexyl group, a sec-hexyl group, a tert-hexyl group, an n-heptyl group, an iso-heptyl group, a sec-heptyl group, a tert-heptyl group, an n-octyl group, an iso-octyl group, a sec-octyl group, a tert-octyl group, an n-nonyl group, an iso-nonyl group, a sec-nonyl group, a tert-nonyl group, an n-decyl group, an iso-decyl group, a sec-decyl group, a tert-decyl group, and/or the like. The term “C 1 -C 60  alkylene group” as used herein refers to a divalent group having the same structure as the C 1 -C 60  alkyl group. 
     The term “C 2 -C 60  alkenyl group” as used herein refers to a monovalent hydrocarbon group having at least one carbon-carbon double bond in the middle or at a terminal end (e.g., the terminus) of the C 2 -C 60  alkyl group, and examples thereof may include an ethenyl group, a propenyl group, a butenyl group, and/or the like. The term “C 2 -C 60  alkenylene group” as used herein refers to a divalent group having the same structure as the C 2 -C 60  alkenyl group. 
     The term “C 2 -C 60  alkynyl group” as used herein refers to a monovalent hydrocarbon group having at least one carbon-carbon triple bond in the middle or at a terminal end (e.g., the terminus) of the C 2 -C 60  alkyl group, and examples thereof may include an ethynyl group, a propynyl group, and/or the like. The term “C 2 -C 60  alkynylene group” as used herein refers to a divalent group having the same structure as the C 2 -C 60  alkynyl group. 
     The term “C 1 -C 60  alkoxy group” as used herein refers to a monovalent group represented by —OA 101  (wherein A 101  is the C 1 -C 60  alkyl group), and examples thereof may include a methoxy group, an ethoxy group, an isopropyloxy group, and/or the like. 
     The term “C 3 -C 10  cycloalkyl group” as used herein refers to a monovalent saturated hydrocarbon cyclic group having 3 to 10 carbon atoms, and examples thereof may include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group (or a bicyclo[2.2.1]heptyl group), a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.2]octyl group, and/or the like. The term “C 3 -C 10  cycloalkylene group” as used herein refers to a divalent group having the same structure as the C 3 -C 10  cycloalkyl group. 
     The term “C 1 -C 10  heterocycloalkyl group” as used herein refers to a monovalent cyclic group that has, in addition to 1 to 10 carbon atoms, at least one heteroatom as ring-forming atoms, and examples thereof may include a 1,2,3,4-oxatriazolidinyl group, a tetrahydrofuranyl group, a tetrahydrothiophenyl group, and/or the like. The term “C 1 -C 10  heterocycloalkylene group” as used herein refers to a divalent group having the same structure as the C 1 -C 10  heterocycloalkyl group. 
     The term “C 3 -C 10  cycloalkenyl group” as used herein refers to a monovalent cyclic group that has 3 to 10 carbon atoms and at least one carbon-carbon double bond in the ring thereof and no aromaticity, and examples thereof may include a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, and/or the like. The term “C 3 -C 10  cycloalkenylene group” as used herein refers to a divalent group having the same structure as the C 3 -C 10  cycloalkenyl group. 
     The term “C 1 -C 10  heterocycloalkenyl group” as used herein refers to a monovalent cyclic group that has, in addition to 1 to 10 carbon atoms, at least one heteroatom as ring-forming atoms, and having at least one carbon-carbon double bond in the cyclic structure thereof. Examples of the C 1 -C 10  heterocycloalkenyl group may include a 4,5-dihydro-1,2,3,4-oxatriazolylgroup, a 2,3-dihydrofuranyl group, a 2,3-dihydrothiophenyl group, and/or the like. The term “C 1 -C 10  heterocycloalkenylene group” as used herein refers to a divalent group having the same structure as the C 1 -C 10  heterocycloalkenyl group. 
     The term “C 6 -C 60  aryl group” as used herein refers to a monovalent group having a carbocyclic aromatic system of 6 to 60 carbon atoms, and the term “C 6 -C 60  arylene group” as used herein refers to a divalent group having a carbocyclic aromatic system of 6 to 60 carbon atoms. Examples of the C 6 -C 60  aryl group may include a phenyl group, a pentalenyl group, a naphthyl group, an azulenyl group, an indacenyl group, an acenaphthyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a heptalenyl group, a naphthacenyl group, a picenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a fluorenyl group, and/or the like. When the C 6 -C 60  aryl group and the C 6 -C 60  arylene group each include two or more rings, the rings may be condensed with each other. 
     The term “C 1 -C 60  heteroaryl group” as used herein refers to a monovalent group having a heterocyclic aromatic system that has, in addition to 1 to 60 carbon atoms, at least one heteroatom as ring-forming atoms. The term “C 1 -C 60  heteroarylene group” as used herein refers to a divalent group having a heterocyclic aromatic system that has, in addition to 1 to 60 carbon atoms, at least one heteroatom as ring-forming atoms. Examples of the C 1 -C 60  heteroaryl group may include a pyridinyl group, a pyrimidinyl group, a pyrazinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, a benzoquinolinyl group, an isoquinolinyl group, a benzoisoquinolinyl group, a quinoxalinyl group, a benzoquinoxalinyl group, a quinazolinyl group, a benzoquinazolinyl group, a cinnolinyl group, a phenanthrolinyl group, a phthalazinyl group, a naphthyridinyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiofuranyl group, and/or the like. When the C 1 -C 60  heteroaryl group and the C 1 -C 60  heteroarylene group each include two or more rings, the two or more rings may be condensed with each other. 
     The term “monovalent non-aromatic condensed polycyclic group” as used herein refers to a monovalent group having two or more rings condensed to each other, only carbon atoms (for example, having 8 to 60 carbon atoms) as ring-forming atoms, and no aromaticity in its molecular structure when considered as a whole (e.g., the entire molecular structure is not aromatic). Examples of the monovalent non-aromatic condensed polycyclic group may include an indenyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, an indenophenanthrenyl group, an indenoanthracenyl group, an adamantyl group, and/or the like. The term “divalent non-aromatic condensed polycyclic group” as used herein refers to a divalent group having the same structure as the monovalent non-aromatic condensed polycyclic group described above. 
     The term “monovalent non-aromatic condensed heteropolycyclic group” as used herein refers to a monovalent group having two or more rings condensed to each other, at least one heteroatom other than carbon atoms (for example, 1 to 60 carbon atoms) as ring-forming atoms, and no aromaticity in its molecular structure when considered as a whole (e.g., the entire molecular structure is not aromatic). Examples of the monovalent non-aromatic condensed heteropolycyclic group may include a pyrrolyl group, a thiophenyl group, a furanyl group, an indolyl group, a benzoindolyl group, a naphthoindolyl group, an isoindolyl group, a benzoisoindolyl group, a naphthoisoindolyl group, a benzosilolyl group, a benzothiophenyl group, a benzofuranyl group, a carbazolyl group, a dibenzosilolyl group, a dibenzothiophenyl group, a dibenzofuranyl group, an azacarbazolyl group, an azafluorenyl group, an azadibenzosilolyl group, an azadibenzothiophenyl group, an azadibenzofuranyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, a tetrazolyl group, an oxazolyl group, an isoxazolyl group, a thiazolyl group, an isothiazolyl group, an oxadiazolyl group, a thiadiazolyl group, a benzopyrazolyl group, a benzimidazolyl group, a benzoxazolyl group, a benzothiazolyl group, a benzoxadiazolyl group, a benzothiadiazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an imidazotriazinyl group, an imidazopyrazinyl group, an imidazopyridazinyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofurocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, a benzoindolocarbazolyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a benzonaphthosilolyl group, a benzofurodibenzofuranyl group, a benzofurodibenzothiophenyl group, a benzothienodibenzothiophenyl group, an azaadamantyl group, and/or the like. The term “divalent non-aromatic condensed heteropolycyclic group” as used herein refers to a divalent group having the same structure as the monovalent non-aromatic condensed heteropolycyclic group described above. 
     The term “C 6 -C 60  aryloxy group” as used herein refers to a monovalent group represented by —OA 102  (wherein A 102  is the C 6 -C 60  aryl group), and the term “C 6 -C 60  arylthio group” as used herein refers to a monovalent group represented by —SA 103  (wherein A 103  is the C 6 -C 60  aryl group). 
     The term “C 7 -C 60  arylalkyl group” as used herein refers to a monovalent group represented by—A 104 A 105  (where A 104  may be a C 1 -C 54  alkylene group, and A 105  may be a C 6 -C 59  aryl group), and the term “C 2 -C 60  heteroarylalkyl group” as used herein refers to a monovalent group represented by —A 106 A 107  (where A 106  may be a C 1 -C 59  alkylene group, and A 107  may be a C 1 -059 heteroaryl group). 
     The term “R 10a ” as used herein refers to: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, or a nitro group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, or a C 1 -C 60  alkoxy group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 7 -C 60  arylalkyl group, a C 2 -C 60  heteroarylalkyl group, —Si(Q 11 )(Q 12 )(Q 13 ), —N(Q 11 )(Q 12 ), —B(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), or any combination thereof; 
     a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 7 -C 60  arylalkyl group, or a C 2 -C 60  heteroarylalkyl group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 7 -C 60  arylalkyl group, a C 2 -C 60  heteroarylalkyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —N(Q 21 )(Q 22 ), —B(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), or any combination thereof; or 
     —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), or —P(═O)(Q 31 )(Q 32 ). 
     In the present specification, Q 1  to Q 3 , Q 11  to Q 13 , Q 21  to Q 23 , and Q 31  to Q 33  may each independently be: hydrogen; deuterium; —F; —CI; —Br; —I; a hydroxyl group; a cyano group; a nitro group; a C 1 -C 60  alkyl group; a C 2 -C 60  alkenyl group; a C 2 -C 60  alkynyl group; a C 1 -C 60  alkoxy group; a C 3 -C 60  carbocyclic group; a C 1 -C 60  heterocyclic group; a C 7 -C 60  arylalkyl group; or a C 2 -C 60  heteroarylalkyl group unsubstituted or substituted with deuterium, —F, a cyano group, a C 1 -C 60  alkyl group, a C 1 -C 60  alkoxy group, a phenyl group, a biphenyl group, or any combination thereof. 
     The term “heteroatom” as used herein refers to any atom other than a carbon atom. Examples of the heteroatom may include O, S, N, P, Si, B, Ge, Se, or any combination thereof. 
     The term “the third-row transition metal” as used herein includes hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), and/or the like. 
     The term “Ph” as used herein refers to a phenyl group, the term “Me” as used herein refers to a methyl group, the term “Et” as used herein refers to an ethyl group, the term “ter-Bu” or “But” as used herein refers to a tert-butyl group, and the term “OMe” as used herein refers to a methoxy group. 
     The term “biphenyl group” as used herein refers to “a phenyl group substituted with a phenyl group”. In other words, the “biphenyl group” belongs to “a substituted phenyl group having a C 6 -C 60  aryl group as a substituent”. 
     The term “terphenyl group” as used herein refers to “a phenyl group substituted with a biphenyl group”. In other words, the “terphenyl group” belongs to “a substituted phenyl group having, as a substituent, a C 6 -C 60  aryl group substituted with a C 6 -C 60  aryl group”. 
     The maximum number of carbon atoms in this substituent definition section is an example only. For example, the maximum carbon number of 60 in the C 1 -C 60  alkyl group is an example, and the definition of the alkyl group equally applies to a C 1 -C 20  alkyl group. The same also applies to other cases. 
     * and * 1  as used herein, unless defined otherwise, each refer to a binding site to a neighboring atom in a corresponding formula. 
     Hereinafter, a compound and a light-emitting device according to embodiments will be described in more detail with reference to Examples. 
     EXAMPLES 
     HOMO Energy Level Value 
     HOMO energy levels of the following compounds were measured by cyclic voltammetry utilizing ZIVE LAB SP2, and the results are shown in Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Compound 
                 HOMO energy level (eV) 
               
               
                   
               
             
            
               
                   
                 100 
                 −5.55 
               
               
                   
                 mCBP-2CN 
                 −5.61 
               
               
                   
                 HT-01 
                 −5.51 
               
               
                   
                 HT-02 
                 −5.41 
               
               
                   
                 ET-01 
                 −6.36 
               
               
                   
                 PtON7-dtb 
                 −5.00 
               
               
                   
                 PD38 
                 −5.24 
               
               
                   
                 v-DBNA 
                 −5.60 
               
               
                   
                 DF10 
                 −5.12 
               
               
                   
                 DF11 
                 −5.21 
               
               
                   
                 DF12 
                 −5.29 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                 
               
            
           
         
       
     
     Manufacture of Light-Emitting Device 
     Comparative Example 1 
     A glass substrate (anode, ITO 300 Å/Ag 50 Å/ITO 300 Å) was cut to a size of 50 mm×50 mm×0.7 mm, cleaned by sonication with isopropyl alcohol and pure water each for 5 minutes, cleaned by irradiation of ultraviolet rays and exposure of ozone thereto for 30 minutes, and then loaded into a vacuum deposition apparatus. 
     HAT-CN was vacuum-deposited on the substrate to form a hole injection layer having a thickness of 150 Å. Then, NPB as a hole transporting compound was vacuum-deposited thereon to form a hole transport layer having a thickness of 1,200 Å. 
     Compound  100  as a first host, ET-01 as a second host, PD-38 as a first dopant, and DF10 as a second dopant were deposited (e.g., co-deposited) on the hole transport layer to form an emission layer having a thickness of 300 Å (wherein a weight ratio of the first host to the second host was 5:5, doping ratios of the first dopant and the second dopant were 10 wt % and 1.5 wt %, respectively, based on 100 parts by weight of the sum of the first host and the second host). 
     TPM-TAZ and LiQ were deposited (e.g., co-deposited) at a weight ratio of 5:5 on the emission layer to form an electron transport layer having a thickness of 300 Å. 
     Yb was vacuum-deposited on the electron transport layer to a thickness of 10 Å, and subsequently, AgMg was vacuum-deposited thereon to a thickness of 100 Å (wherein a doping ratio of Mg in Ag was 5 wt %) to form a cathode. Then, CP1 was deposited thereon to form a capping layer having a thickness of 700 Å, thereby completing the manufacture of a light-emitting device. 
     Comparative Example 2 
     A light-emitting device was manufactured in substantially the same manner as in Comparative Example 1, except that HT-01 as a first host, mCBP-2CN as a second host, PtON7-dtb as a first dopant, and v-DABNA a second dopant were deposited (e.g., co-deposited) on the hole transport layer to form an emission layer having a thickness of 300 Å (wherein a weight ratio of the first host to the second host was 5:5, and doping ratios of the first dopant and the second dopant were 10 wt % and 1.5 wt %, respectively, based on 100 parts by weight of the sum of the first host and the second host). 
     Example 1 
     A light-emitting device was manufactured in substantially the same manner as in Comparative Example 1, except that HT-01 as a first host, ET-01 as a second host, PD38 as a first dopant, and DF11 as a second dopant were deposited (e.g., co-deposited) on the hole transport layer to form an emission layer having a thickness of 300 Å. 
     Example 2 
     A light-emitting device was manufactured in substantially the same manner as in Comparative Example 1, except that HT-01 as a first host, ET-01 as a second host, PD38 as a first dopant, and DF12 as a second dopant were deposited (e.g., co-deposited) on the hole transport layer to form an emission layer having a thickness of 300 Å. 
     Example 3 
     A light-emitting device was manufactured in substantially the same manner as in Comparative Example 1, except that HT-02 as a first host, ET-01 as a second host, PD38 as a first dopant, and DF10 as a second dopant were deposited (e.g., co-deposited) on the hole transport layer to form an emission layer having a thickness of 300 Å. 
     Example 4 
     A light-emitting device was manufactured in substantially the same manner as in Comparative Example 1, except that HT-02 as a first host, ET-01 as a second host, PD38 as a first dopant, and DF11 as a second dopant were deposited (e.g., co-deposited) on the hole transport layer to form an emission layer having a thickness of 300 Å. 
     Example 5 
     A light-emitting device was manufactured in substantially the same manner as in Comparative Example 1, except that HT-02 as a first host, ET-01 as a second host, PD38 as a first dopant, and DF12 as a second dopant were deposited on the hole transport layer to form an emission layer having a thickness of 300 Å. 
     To evaluate characteristics of the light-emitting devices manufactured according to Comparative Examples 1 and 2 and Examples 1 to 5, the driving voltage, efficiency, and lifespan thereof at a current density of 10 mA/cm 2  were measured. The measurement results and differences in the HOMO energies of the hosts and dopants of Comparative Examples 1 and 2 and Examples 1 to 5 are shown in Table 2. 
     The efficiency (EQE) of each of the light-emitting devices was measured utilizing a measurement device C9920-2-12 manufactured by Hamamatsu Photonics Inc. 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 ΔHOMO 
                   
                   
               
               
                   
                   
                   
                   
                 (first 
               
               
                   
                   
                   
                   
                 host and 
               
               
                   
                   
                   
                   
                 dopant) 1)   
               
               
                   
                   
                   
                   
                 ΔHOMO 
                 EQE 
               
               
                   
                 First 
                 First 
                 Second 
                 (first 
                 (%) 
                 Lifespan 
               
               
                   
                 host 
                 dopant 
                 dopant 
                 dopant and 
                 @ 
                 (hr) 
               
               
                   
                 [HOMO 
                 [HOMO 
                 [HOMO 
                 second 
                 1000 
                 @ 
               
               
                   
                 energy] 
                 energy] 
                 energy] 
                 dopant) 
                 nit 
                 LT95 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Comparative 
                 100 
                 PD-38 
                 DF10 
                 0.43 eV 
                 16.5 
                 45 
               
               
                 Example 1 
                 [−5.55 eV] 
                 [−5.24 eV] 
                 [−5.12 eV] 
                 0.12 eV 
               
               
                 Comparative 
                 HT-01 
                 PtON7-dtb 
                 v-DABNA 
                 0.51 eV 
                 15.3 
                 22 
               
               
                 Example 2 
                 [−5.51 eV] 
                 [−5.00 eV] 
                 [−5.60 eV] 
                 0.60 eV 
               
               
                 Example1 
                 HT-01 
                 PD-38 
                 DF11 
                 0.30 eV 
                 17.9 
                 64 
               
               
                   
                 [−5.51 eV] 
                 [−5.24 eV] 
                 [−5.21 eV] 
                 0.03 eV 
               
               
                 Example 2 
                 HT-01 
                 PD-38 
                 DF12 
                 0.27 eV 
                 18.7 
                 70 
               
               
                   
                 [−5.51 eV] 
                 [−5.24 eV] 
                 [−5.29 eV] 
                 0.05 eV 
               
               
                 Example 3 
                 HT-02 
                 PD-38 
                 DF10 
                 0.29 eV 
                 18.5 
                 72 
               
               
                   
                 [−5.41 eV] 
                 [−5.24 eV] 
                 [−5.12 eV] 
                 0.12 eV 
               
               
                 Example 4 
                 HT-02 
                 PD-38 
                 DF11 
                 0.20 eV 
                 19.6 
                 89 
               
               
                   
                 [−5.41 eV] 
                 [−5.24 eV] 
                 [−5.21 eV] 
                 0.03 eV 
               
               
                 Example 5 
                 HT-02 
                 PD-38 
                 DF12 
                 0.17 eV 
                 19.8 
                 91 
               
               
                   
                 [−5.41 eV] 
                 [−5.24 eV] 
                 [−5.29 eV] 
                 0.05 eV 
               
               
                   
               
               
                   1) indicates the difference between the HOMO energy level of a dopant having a smaller absolute value of HOMO energy level among a first dopant and a second dopant and the HOMO energy level of a first host. 
               
            
           
         
       
     
     Referring to Table 2, it was confirmed that the light-emitting devices of Examples 1 to 5 each had excellent or suitable efficiency and lifespan compared to the light-emitting devices of Comparative Examples 1 and 2. 
     Referring to the results of Examples 1 to 5, it was confirmed that, as the difference between the HOMO energy level of the dopant with the smaller absolute value among the first dopant and the second dopant and the HOMO energy level of the first host became smaller, the efficiency and lifespan of each of the light-emitting devices increased. 
     As described above, according to the one or more embodiments, a light-emitting device may exhibit improved lifespan as compared with devices in the related art. 
     The apparatus and/or any other relevant devices or components according to embodiments of the present invention described herein may be implemented utilizing any suitable hardware, firmware (e.g. an application-specific integrated circuit), software, or a combination of software, firmware, and hardware. For example, the various components of the apparatus may be formed on one integrated circuit (IC) chip or on separate IC chips. Further, the various components of the [device] may be implemented on a flexible printed circuit film, a tape carrier package (TCP), a printed circuit board (PCB), or formed on one substrate. Further, the various components of the apparatus may be a process or thread, running on one or more processors, in one or more computing devices, executing computer program instructions and interacting with other system components for performing the various functionalities described herein. The computer program instructions are stored in a memory which may be implemented in a computing device using a standard memory device, such as, for example, a random access memory (RAM). The computer program instructions may also be stored in other non-transitory computer readable media such as, for example, a CD-ROM, flash drive, or the like. Also, a person of skill in the art should recognize that the functionality of various computing devices may be combined or integrated into a single computing device, or the functionality of a particular computing device may be distributed across one or more other computing devices without departing from the scope of the embodiments. 
     It should be understood that embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. While one or more embodiments have been described with reference to the drawings, it will be understood by those of ordinary skill in the art that one or more suitable changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims and equivalents thereof.