Patent Publication Number: US-2022216432-A1

Title: Light-emitting device and an apparatus including same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit of Korean Patent Application No. 10-2020-0185212, filed on Dec. 28, 2020, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     Field 
     Embodiments of the invention relate generally to display devices and, more particularly, to a light-emitting device and an electronic apparatus including the same. 
     Discussion of the Background 
     Organic light-emitting devices (OLEDs) are self-emissive devices that, as compared with devices of the related art, have wide viewing angles, high contrast ratios, short response times, and excellent characteristics in terms of brightness, driving voltage, and response speed, and produce full-color images. 
     OLEDs may include a first electrode on a substrate, and a hole transport region, an emission layer, an electron transport region, and a second electrode sequentially stacked 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 excitons. The excitons may transit from an excited state to a ground state, thus generating light. 
     The above information disclosed in this Background section is only for understanding of the background of the inventive concepts, and, therefore, it may contain information that does not constitute prior art. 
     SUMMARY 
     Light-emitting devices and electronic apparatuses constructed according to principles and illustrative implementations of the invention have a long lifespan, excellent luminescence efficiency and/or driving voltage. For example, when a light-emitting device may satisfies Condition 1 disclosed herein, that is, the first compound and the second compound having a HOMO energy level difference of about 0.05 eV or greater may be present together in the hole transport layer, hole injection into the emission layer may be delayed to adjust a barrier between the hole transport layer and the emission layer, and thus, the concentration of excitons in the emission layer in the light-emitting device may be reduced. Thus, the light-emitting device may have long lifespan. 
     Additional features of the inventive concepts will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the inventive concepts. 
     According to one aspect of the invention, a light-emitting device includes: a first electrode; a second electrode; and an interlayer between the first electrode and the second electrode and including an emission layer, wherein the interlayer further includes a hole transport region between the emission layer and the first electrode, the hole transport region includes a hole transport layer, the hole transport layer includes a first compound and a second compound, the hole transport layer directly contacts the first electrode, and the first compound and the second compound satisfy Condition 1: 
       0.05 eV≤EHOMO( C 1)−EHOMO( C 2)  Condition 1
         wherein, in Condition 1,   EHOMO(C1) is a HOMO energy level of the first compound, and   EHOMO(C2) is a HOMO energy level of the second compound.       

     The first electrode may include an anode, the second electrode may include a cathode, the light-emitting device may further include an electron transport region between the emission layer and the second electrode, and the electron transport region may include a hole blocking layer, an electron transport layer, an electron injection layer, or any combination thereof. 
     The hole transport layer may directly contact the emission layer. 
     The first compound may have a content of about 50 parts by weight or greater and about 70 parts by weight or less, based on 100 parts by weight of the total weight of the first compound and the second compound. 
     The first compound and the second compound satisfy Condition 1-1: 0.05 eV≤E HOMO (C1)−E HOMO (C2)≤0.25 eV, wherein, in Condition 1-1, E HOMO  (C1) is a HOMO energy level of the first compound, and E HOMO (C2) is a HOMO energy level of the second compound. 
     The HOMO energy level of the first compound may be greater than about −5.3 eV and smaller than about −4.6 eV. 
     The HOMO energy level of the second compound may be greater than about −5.4 eV and smaller than about −4.7 eV. 
     The first compound and the second compound may each be independently a compound represented by Formula 1-1 or Formula 1-2 or a p-dopant, as defined herein. 
     The p-dopant may include a compound including quinone moiety, a compound including a cyano group, or a compound including an element EL1 and an element EL2, the element EL1 may be a metal, a metalloid, or a combination thereof, and element EL2 is at least one non-metal. 
     The compound represented by Formula 1-1 or Formula 1-2 may each be, independently from one another, are one of Formulae 1-11 to Formula 1-14, as defined herein. 
     The emission layer may be configured to emit blue light or blue-green light. 
     The emission layer may include a third compound, and the third compound may be a TADF compound, a phosphorescence luminescence compound, or any combination thereof, wherein the TADF compound satisfies Equation 1: ΔE ST =S1−T1≤0.3 eV, wherein, Equation 1, S1 represents an excited singlet energy level of the TADF compound, and T1 represents an excited triplet energy level of the TADF compound. 
     The TADF Compound may be represented by one of Formulae 4-1 to 4-9, as defined herein. 
     The phosphorescence luminescence compound may be a platinum or iridium organometallic complex. 
     The emission layer may include a host and a dopant, the host may be different from the dopant, and the amount of the host in the emission layer may be greater than the amount of the dopant in the emission layer. 
     The host and the dopant satisfy Condition 2: 0.2 eV≤|EHOMO(H)−EHOMO(D)|, wherein, in Condition 2, EHOMO(H) is a HOMO energy level of the host, and EHOMO(D) is a HOMO energy level of the dopant, when the host comprises at least two compounds, EHOMO(H) is a highest value of the HOMO energy level of the host, and when the dopant comprises at least two compounds, EHOMO(D) is a highest value of the HOMO energy level of the dopant. 
     The host may include a hole transporting compound, an electron transporting compound, a bipolar compound, or any combination thereof. 
     The emission layer may include a fourth compound, and the fourth compound may include a fluorescence luminescence compound. 
     An electronic apparatus may include: a light-emitting device as described above; and a thin-film transistor having 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. 
     The electronic apparatus may further include a color filter, a color-conversion layer, a touchscreen layer, a polarization layer, or any combination thereof. 
     It is to be understood that both the foregoing general description and the following detailed description are illustrative and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate illustrative embodiments of the invention, and together with the description serve to explain the inventive concepts. 
         FIG. 1  is a schematic cross-sectional view of an embodiment of a light-emitting device constructed according to the principles of the invention. 
         FIG. 2  is a schematic cross-sectional view of an embodiment of a light-emitting apparatus including a light-emitting device constructed according to the principles of the invention. 
         FIG. 3  is a schematic cross-sectional view of another embodiment of a light-emitting apparatus including a light-emitting device constructed according to the principles of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of various embodiments or implementations of the invention. As used herein “embodiments” and “implementations” are interchangeable words that are non-limiting examples of devices or methods employing one or more of the inventive concepts disclosed herein. It is apparent, however, that various embodiments may be practiced without these specific details or with one or more equivalent arrangements. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring various embodiments. Further, various embodiments may be different, but do not have to be exclusive. For example, specific shapes, configurations, and characteristics of an embodiment may be used or implemented in another embodiment without departing from the inventive concepts. 
     Unless otherwise specified, the illustrated embodiments are to be understood as providing illustrative features of varying detail of some ways in which the inventive concepts may be implemented in practice. Therefore, unless otherwise specified, the features, components, modules, layers, films, panels, regions, and/or aspects, etc. (hereinafter individually or collectively referred to as “elements”), of the various embodiments may be otherwise combined, separated, interchanged, and/or rearranged without departing from the inventive concepts. 
     The use of cross-hatching and/or shading in the accompanying drawings is generally provided to clarify boundaries between adjacent elements. As such, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, dimensions, proportions, commonalities between illustrated elements, and/or any other characteristic, attribute, property, etc., of the elements, unless specified. Further, in the accompanying drawings, the size and relative sizes of elements may be exaggerated for clarity and/or descriptive purposes. When an embodiment may be implemented differently, a specific process order may be performed differently from the described order. For example, two consecutively described processes may be performed substantially at the same time or performed in an order opposite to the described order. Also, like reference numerals denote like elements, and redundant explanations are omitted to avoid redundancy. 
     When an element, such as a layer, is referred to as being “on,” “connected to,” or “coupled to” another element or layer, it may be directly on, connected to, or coupled to the other element or layer or intervening elements or layers may be present. When, however, an element or layer is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. To this end, the term “connected” may refer to physical, electrical, and/or fluid connection, with or without intervening elements. Further, the D1-axis, the D2-axis, and the D3-axis are not limited to three axes of a rectangular coordinate system, such as the x, y, and z-axes, and may be interpreted in a broader sense. For example, the D1-axis, the D2-axis, and the D3-axis may be perpendicular to one another, or may represent different directions that are not perpendicular to one another. For the purposes of this disclosure, “at least one of X, Y, and Z” and “at least one selected from the group consisting of X, Y, and Z” may be construed as X only, Y only, Z only, or any combination of two or more of X, Y, and Z, such as, for instance, XYZ, XYY, YZ, and ZZ. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms “first,” “second,” etc. may be used herein to describe various types of elements, these elements should not be limited by these terms. These terms are used to distinguish one element from another element. Thus, a first element discussed below could be termed a second element without departing from the teachings of the disclosure. 
     Spatially relative terms, such as “beneath,” “below,” “under,” “lower,” “above,” “upper,” “over,” “higher,” “side” (e.g., as in “sidewall”), and the like, may be used herein for descriptive purposes, and, thereby, to describe one elements relationship to another element(s) as illustrated in the drawings. Spatially relative terms are intended to encompass different orientations of an apparatus in use, operation, and/or manufacture in addition to the orientation depicted in the drawings. For example, if the apparatus in the drawings is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. Furthermore, the apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations), and, as such, the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used herein, the singular forms, “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the terms “comprises,” “comprising,” “includes,” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It is also noted that, as used herein, the terms “substantially,” “about,” and other similar terms, are used as terms of approximation and not as terms of degree, and, as such, are utilized to account for inherent deviations in measured, calculated, and/or provided values that would be recognized by one of ordinary skill in the art. 
     Various embodiments are described herein with reference to sectional and/or exploded illustrations that are schematic illustrations of idealized embodiments and/or intermediate structures. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments disclosed herein should not necessarily be construed as limited to the particular illustrated shapes of regions, but are to include deviations in shapes that result from, for instance, manufacturing. In this manner, regions illustrated in the drawings may be schematic in nature and the shapes of these regions may not reflect actual shapes of regions of a device and, as such, are not necessarily intended to be limiting. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure is a part. Terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and should not be interpreted in an idealized or overly formal sense, unless expressly so defined herein. 
     An organic light-emitting device may include: a first electrode; a second electrode; and an interlayer between the first electrode and the second electrode and including an emission layer, wherein the interlayer may further include a hole transport region between the emission layer and the first electrode, the hole transport region may include a hole transport layer, the hole transport layer may include a first compound and a second compound, the hole transport layer may be in direct contact with the first electrode, and the first compound and the second compound may satisfy Condition 1: 
       0.05 eV≤EHOMO( C 1)−EHOMO( C 2)  Condition 1
 
     wherein, in Condition 1, 
     EHOMO(C1) represents a HOMO energy level (eV) of the first compound, and 
     EHOMO(C2) represents a HOMO energy level (eV) of the second compound. 
     The HOMO energy level (eV) may be calculated according to GAUSSIAN09 TOOL (Linux ver.). 
     As the light-emitting device may satisfy Condition 1, that is, the first compound and the second compound having a HOMO energy level difference of about 0.05 eV or greater may be present together in the hole transport layer, hole injection into the emission layer may be delayed to adjust a barrier between the hole transport layer and the emission layer, and thus, a concentration of excitons in the emission layer in the light-emitting device may be reduced. Thus, the light-emitting device may have long lifespan. 
     In addition, as the hole transport layer may be in direct contact with the first electrode in the light-emitting device, hole injection between the emission layer and the first electrode may be directly controlled, thus controlling a concentration and a distribution of excitons in the emission layer and improving lifespan of the light-emitting device. In some embodiments, the first electrode may be an anode, the second electrode may be a cathode, the light-emitting device may further include an electron transport region between the emission layer and the second electrode, and the electron transport region may include a hole blocking layer, an electron transport layer, an electron injection layer, or a combination thereof. 
     In an embodiment, the hole transport layer may be in direct contact with the emission layer, and as the hole transport layer may be in direct contact with the emission layer, a concentration and a distribution of excitons may be controlled in the emission layer by controlling holes. Further, positive triplet-polaron quenching may also be controlled effectively. In some embodiments, the content of the first compound may be about 50 parts by weight or greater and about 70 parts by weight, based on 100 parts by weight of a total weight of the first compound and the second compound. In an embodiment, the hole transport layer may be a single layer. 
     In an embodiment, the first compound and the second compound may satisfy Condition 1-1: 
       0.05 eV≤EHOMO( C 1)−EHOMO( C 2)≤0.25 eV  Condition 1-1
 
     wherein, in Condition 1-1, 
     EHOMO(C1) and EHOMO(C2) may respectively be understood by referring to the descriptions of EHOMO(C1) and EHOMO(C2) provided herein. In some embodiments, the highest occupied molecular orbital (HOMO) energy level of the first compound may be greater than about −5.3 eV and smaller than about −4.6 eV. In some embodiments, the HOMO energy level of the second compound may be greater than about −5.4 eV and smaller than about −4.7 eV. In some embodiments, the HOMO energy level of the second compound may be about −5.3 eV or greater and smaller than about −4.7 eV. 
     In some embodiments, the first compound and the second compound may each independently be a compound represented by Formula 1-1 or Formula 1-2 or a p-dopant, as discussed in further detail below: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 1-1 and 1-2, 
     L 1  to L 4  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 5  may be *—O—*′, *—S—*′, *—N(R 5 )—*′, 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 , 
     a1 to a4 may each independently be an integer selected from 0 to 5, 
     a5 may be an integer selected from 1 to 10, 
     n1 may be an integer selected from 0 to 6, 
     R 1  to R 5  may each independently 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 , a C 6 -C 60  aryloxy group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  arylthio group unsubstituted or substituted with at least one R 10a , —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), 
     b1 to b4 may each independently be an integer selected from 1 to 10, 
     R 1  and R 2  may optionally be bound 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 
     R 3  and R 4  may optionally be bound 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 . 
     In some embodiments, the p-dopant may include a quinone derivative, a compound containing a cyano group, a compound containing element EL1 and element EL2, or any combination thereof. 
     Examples of the quinone derivative may include tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ), and the like. 
     Examples of the cyano group-containing compound include 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN), a compound represented by Formula 221, and 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, substituted with a cyano group; —F; —Cl; —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 containing element EL1 and element EL2, element EL1 may be a metal, a metalloid, or a combination thereof, and element EL2 may be a non-metal, a metalloid, or a combination thereof. 
     Examples of the compound containing element EL1 and element EL2 may include Compound P1: 
     
       
         
         
             
             
         
       
     
     Examples of the metal may include: an alkali metal (e.g., lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), or the like); an alkaline earth metal (e.g., beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), or the like); a transition metal (e.g., 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), or the like); a post-transition metal (e.g., zinc (Zn), indium (In), tin (Sn), or the like); a lanthanide metal (e.g., 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), or the like); and the like. 
     Examples of the metalloid may include silicon (Si), antimony (Sb), tellurium (Te), and the like. Examples of the non-metal may include oxygen (O), a halogen (e.g., F, Cl, Br, I, and the like), and the like. Metalloids are now often referred to as semiconductors. As used herein, the non-metal excludes one or more metalloids or semiconductors. 
     For example, the compound containing element EL1 and element EL2 may include a metal oxide, a metal halide (e.g., a metal fluoride, a metal chloride, a metal bromide, a metal iodide, and the like), a metalloid halide (e.g., a metalloid fluoride, a metalloid chloride, a metalloid bromide, a metalloid iodide, and the like), a metal telluride, or any combination thereof. 
     Examples of the metal oxide may include a tungsten oxide (e.g., WO, W 2 O 3 , WO 2 , O 2 , WO 3 , W 2 O 5 , and the like), a vanadium oxide (e.g., VO, V 2 O 3 , VO 2 , V 2 O 5 , and the like), a molybdenum oxide (MoO, Mo 2 O 3 , MoO 2 , MoO 3 , Mo 2 O 5 , and the like), a rhenium oxide (e.g., ReO 3  and the like), and the like. Examples of the metal halide may include an alkali metal halide, an alkaline earth metal halide, a transition metal halide, a post-transition metal halide, a lanthanide metal halide, and 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 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 the like. 
     Examples of the transition metal halide may include a titanium halide (e.g., TiF 4 , TiCl 4 , TiBr 4 , TiI 4 , and the like), a zirconium halide (e.g., ZrF 4 , ZrCl 4 , ZrBr 4 , ZrI 4 , and the like), a hafnium halide (e.g., HfF 4 , HfCl 4 , HfBr 4 , HfI 4 , and the like), a vanadium halide (e.g., VF 3 , VCl 3 , VBr 3 , VI 3 , and the like), a niobium halide (e.g., NbF 3 , NbCl 3 , NbBr 3 , NbI 3 , and the like), a tantalum halide (e.g., TaF 3 , TaCl 3 , TaBr 3 , TaI 3 , and the like), a chromium halide (e.g., CrF 3 , CrCl 3 , CrBr 3 , CrI 3 , and the like), a molybdenum halide (e.g., MoF 3 , MoCl 3 , MoBr 3 , MoI 3 , and the like), a tungsten halide (e.g., WF 3 , WCl 3 , WBr 3 , WI 3 , and the like), a manganese halide (e.g., MnF 2 , MnCl 2 , MnBr 2 , MnI 2 , and the like), a technetium halide (e.g., TcF 2 , TcCl 2 , TcBr 2 , TcI 2 , and the like), a rhenium halide (e.g., ReF 2 , ReCl 2 , ReBr 2 , ReI 2 , and the like), an iron halide (e.g., FeF 2 , FeCl 2 , FeBr 2 , FeI 2 , and the like), a ruthenium halide (e.g., RuF 2 , RuCl 2 , RuBr 2 , RuI 2 , and the like), an osmium halide (e.g., OsF 2 , OsCl 2 , OsBr 2 , OsI 2 , and the like), a cobalt halide (e.g., CoF 2 , CoCl 2 , CoBr 2 , CoI 2 , and the like), a rhodium halide (e.g., RhF 2 , RhCl 2 , RhBr 2 , RhI 2 , and the like), an iridium halide (e.g., IrF 2 , IrCl 2 , IrBr 2 , IrI 2 , and the like), a nickel halide (e.g., NiF 2 , NiCl 2 , NiBr 2 , NiI 2 , and the like), a palladium halide (e.g., PdF 2 , PdCl 2 , PdBr 2 , PdI 2 , and the like), a platinum halide (e.g., PtF 2 , PtCl 2 , PtBr 2 , PtI 2 , and the like), a copper halide (e.g., CuF, CuCl, CuBr, CuI, and the like), a silver halide (e.g., AgF, AgCl, AgBr, AgI, and the like), a gold halide (e.g., AuF, AuCl, AuBr, AuI, and the like), and the like. 
     Examples of the post-transition metal halide may include a zinc halide (e.g., ZnF 2 , ZnCl 2 , ZnBr 2 , ZnI 2 , and the like), an indium halide (e.g., InI 3  and the like), a tin halide (e.g., SnI 2  and the like), and 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 the like. Examples of the metalloid halide may include an antimony halide (e.g., SbCl 5  and the like) and the like. 
     Examples of the metal telluride may include an alkali metal telluride (e.g., Li 2 Te, Na 2 Te, K 2 Te, Rb 2 Te, Cs 2 Te, and the like), an alkaline earth metal telluride (e.g., BeTe, MgTe, CaTe, SrTe, BaTe, and the like), a transition metal telluride (e.g., 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 the like), a post-transition metal telluride (e.g., ZnTe and the like), a lanthanide metal telluride (e.g., LaTe, CeTe, PrTe, NdTe, PmTe, EuTe, GdTe, TbTe, DyTe, HoTe, ErTe, TmTe, YbTe, LuTe, and the like), and the like. 
     In some embodiments, the compound represented by Formula 1-1 or Formula 1-2 may each independently be represented by one of Formulae 1-11 to Formula 1-14: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 1-11 to 1-14, 
     X 11  may be O, S, N(E 11 ), or C(E 11 )(E 12 ), 
     X 12  may be O, S, N(E 13 ), or C(E 13 )(E 14 ), 
     E 11  may be *-(L 11 ) a11 -(R 11 ) b11 , 
     E 11  may be *-(L 12 ) a12 -(R 12 ) b12 , 
     E 13  may be *-(L 13 ) a13 -(R 13 ) b13 , 
     E 14  may be *-(L 14 ) a14 -(R 14 ) b14 , 
     * indicates a binding site to an adjacent atom, 
     L 11  to L 17  may each independently be a single bond, *—N(R 18 )—*′, a carbocyclic group unsubstituted or substituted with at least one R 10a  or a heterocyclic group unsubstituted or substituted with at least one R 10a , 
     a11 to a17 may each independently be an integer selected from 1 to 5, 
     R 11  to R 18  may each be understood by referring to the description of R 1  provided herein, 
     b11 to b17 may each independently be an integer selected from 1 to 8, 
     R 11  and R 12  may optionally be bound 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 
     R 13  and R 14  may optionally be bound 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 . 
     In an embodiment, the first compound and the second compound may each independently be one of Compounds HT1 to HT57, 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA), 1-N, 1-N-bis[4-(diphenylamino)phenyl]-4-N,4-N-diphenylbenzene-1,4-diamine (TDATA), 4,4′,4″-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA), bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB or NPD), N4,N4′-di(naphthalen-2-yl)-N4,N4′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (β-NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-9,9-spirobifluorene-2,7-diamine (spiro-TPD), N2,N7-di-1-naphthalenyl-N2,N7-diphenyl-9,9′-spirobi[9H-fluorene]-2,7-diamine (spiro-NPB), N,N′-di(1-naphthyl)-N,N′-diphenyl-2,2′-dimethyl-(1, 1 ‘-biphenyl)-4,4’-diamine (methylated-NPB), 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC), N,N,N′,N′-tetrakis(3-methylphenyl)-3,3′-dimethylbenzidine (HMTPD), 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), polyaniline/dodecylbenzenesulfonic acid (PANI/DBSA), poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), polyaniline/camphorsulfonic acid (PANI/CSA), polyaniline/poly(4-styrenesulfonate (PANI/PSS), or any combination thereof, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the emission layer may emit blue light or blue-green light. In some embodiments, the emission layer may emit blue light or blue-green light having a maximum emission wavelength in a range of about 400 nanometers (nm) to about 500 nm. In some embodiments, the emission layer may include a third compound, and the third compound may be a thermal activated delayed fluorescence (TADF) compound, a phosphorescence luminescence compound, or any combination thereof: 
     
       
         
           
             
               
                 
                   ΔEST 
                   = 
                   
                     
                       
                         S 
                         ⁢ 
                         1 
                       
                       - 
                       
                         T 
                         ⁢ 
                         1 
                       
                     
                     ≤ 
                     
                       0.3 
                          
                       eV 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                      
                   1 
                 
               
             
           
         
       
     
     wherein, Equation 1, S1 represents an excited singlet energy level (eV) of the TADF compound, and T1 represents an excited triplet energy level (eV) of the TADF compound. S1 and T1 may be evaluated according to photoluminescence spectrum analysis or Gaussian according to DFT method. 
     In some embodiments, the TADF compound may be represented by one of Formulae 4-1 to 4-9: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 4-1, 
     A 41  may 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 41  to L 43  may each independently be a single bond, 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 , 
     a41 to a43 may each independently be an integer selected from 0 to 3, 
     Ar 41  and Ar 42  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 , 
     Ar 41  and Ar 42  may optionally be bound 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 , 
     m41 may be an integer selected from 1 to 6, 
     wherein, in Formulae 4-2 to 4-5, 
     X 41  to X 45  may each independently be a single bond, O, S, N(R 46 ), B(R 46 ), C(R 46 )(R 47 ), or Si(R 46 )(R 47 ), 
     n41 and n42 may each independently be 0, 1, or 2, and when n41 is 0, A 41  and A 42  may not be bound to each other, and when n42 is 0, A 44  and A 45  may not be bound to each other, 
     Y 41  and Y 42  may each independently be N, B, or P, 
     Z 41  and Z 42  may each independently be N, C(R 48 ) or Si(R 48 ), 
     A 41  to A 45  may each independently be a C 5 -C 30  carbocyclic group or a C 1 -C 30  heterocyclic group, 
     R 41  to R 48  may each independently 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 , a C 6 -C 60  aryloxy group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  arylthio group unsubstituted or substituted with at least one R 10a , —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), 
     c41 to c45 may each independently be an integer selected from 1 to 8, 
     wherein, in Formulae 4-6 to 4-9, 
     “EDG” represents an electron donating group, and “EWG” represent an electron withdrawing group, 
     b41, b411, b412, t42, t421, and t422 may each independently be an integer selected from 1, 2, and 3, 
     L 44  and L 45  may each independently be a single bond, 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 , 
     a44 and a45 may each independently be an integer selected from 0 to 3, and 
     s41 and s42 may each independently be an integer selected from 1 to 3. 
     In an embodiment, the TADF compound may be selected from Group III-1, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the phosphorescence luminescence compound may be a platinum (Pt) or iridium (Ir) organometallic complex. In an embodiment, the phosphorescence luminescence compound may be represented by Formula 3: 
       M 31 (L 31 ) n31 (L 32 ) n32   Formula 3
 
     wherein, in Formula 3, 
     M 31  may be platinum (Pt) or iridium (Ir), 
     L 31  may be a ligand represented by one of Formulae 3A to 3D, 
     n31 may be 1 or 2, 
     L 32  may be an organic ligand, and 
     n32 may be 0, 1, 2, 3, or 4: 
     
       
         
         
             
             
         
       
     
     wherein in Formulae 3A to 3D, 
     A 31  to A 34  may each independently be a C 3 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, 
     T 31  to T 34  may each independently be a single bond, a double bond, *—O—*′, *—S—*′, *—C(═O)—*′, *—S(═O)—*′, *—C(R 35 )(R 36 )—*′, *—C(R 35 )═C(R 36 )—*′, *—C(R 35 )═*′, *—Si(R 35 )(R 36 )—*′, *—B(R 35 )—*′, *—N(R 35 )—*′, or *—P(R 35 )—*′, 
     k31 to k34 may each independently be 1, 2, or 3, 
     Y 31  to Y 34  may each independently be a single bond, *—O—*′, *—S—*′, *—C(R 37 )(R 38 )—*′, *—Si(R 37 )(R 38 )—*′, *—B(R 37 )—*′, *—N(R 37 )—*′, or *—P(R 37 )—*′, 
     R 31  to R 38  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 ), 
     R 31  to R 38  may optionally be bound to each other to form a C 5 -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 , 
     b31 to b34 may each independently be an integer selected from 0 to 10, 
     *1, *2, *3, and *4 each indicate a binding site to M 31 , and 
     Q 401  to Q 403  may each be understood by referring to the description of Q 1  provided herein. 
     In an embodiment, the phosphorescence luminescence compound may be represented by Formula 3-1 or Formula 3-2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 3-1 and 3-2, 
     M 31 , n31, L 32 , n32, A 31  to A 34 , T 31  to T 33 , k31 to k33, Y 31  to Y 34 , R 31  to R 34 , and b31 to b34 may be each independently understood by referring to the descriptions of M 31 , n31, L 32 , n32, A 31  to A 34 , T 31  to T 33 , k31 to k33, Y 31  to Y 34 , R 31  to R 34 , and b31 to b34, and X 31  to X 40  may each independently be N or C. 
     In an embodiment, the phosphorescence luminescence compound may be selected from Group 111-2, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In an embodiment, the emission layer may include a host and a dopant, the host may be different from the dopant, and the content of the host may be greater than the content of the dopant. In an embodiment, the third compound may be the dopant. 
     In an embodiment, the host and the dopant may satisfy Condition 2: 
       0.2 eV≤|EHOMO( H )−EHOMO( D )|  Condition 2
 
     wherein, in Condition 2, EHOMO(H) represents a HOMO energy level of the host, and EHOMO(D) represents a HOMO energy level of the dopant, 
     when the host includes at least two compound, EHOMO(H) represents a highest value of the HOMO energy level of the at least two compounds, and 
     when the dopant includes at least two compound, EHOMO(D) represents a highest value of the HOMO energy level of the at least two compounds. 
     EHOMO(H) and EHOMO(D) may each be measured according to a method of measuring HOMO energy level. In an embodiment, the host may be a hole transporting compound, an electron transporting compound, a bipolar compound, or any combination thereof. The electron transporting compound may include at least one electron withdrawing group, the hole transporting compound may include at least one electron donating group, and the bipolar compound refers to a compound including at least one electron accepting group and at least one electron donating group. 
     For example, the electron accepting group may be: 
     —F, —CFH 2 , —CF 2 H, —CF 3 , —CN, or —NO 2 ; 
     a C 1 -C 60  alkyl group substituted with —F, —CFH 2 , —CF 2 H, —CF 3 , —CN, —NO 2 , or any combination thereof; 
     —B(Ar 1 )(Ar 2 ); or 
     a π electron-depleted nitrogen-containing C 1 -C 60  cyclic group unsubstituted or substituted with at least one R 10a , and 
     the Ar 1  and Ar 2  may each independently be a carbocyclic group unsubstituted or substituted with at least one R 10a  or a heterocyclic group unsubstituted or substituted with at least one R 10a . 
     For example, the electron accepting group may be a π electron-rich C 3 -C 60  cyclic group unsubstituted or substituted with at least one R 20a , —N(Ar 3 )(Ar 4 ), or —Si(Ar 3 )(Ar 4 )(Ar 5 ), and 
     Ar 3  to Ar 5  may each independently be a π electron-rich C 3 -C 60  cyclic group unsubstituted or substituted with at least one R 20a . 
     In some embodiments, the π electron-depleted nitrogen-containing C 1 -C 60  cyclic group as used herein may be a) a first ring, b) a condensed ring in which at least two first rings are condensed, or c) a condensed ring in which at least one first ring and at least one second ring are condensed, the π electron-rich C 3 -C 60  cyclic group may be a) second ring or b) a condensed ring in which at least two second rings are condensed, the first ring may be an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, or a thiadiazole group, and the second ring may be a benzene group, a cyclopentadiene group, a pyrrole group, a furan group, a thiophene group, or a silole group. 
     In some embodiments, the π electron-depleted nitrogen-containing C 1 -C 60  cyclic group may be an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a benzoisoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline group, a benzoquinoxaline group, a quinazoline group, a cinnoline group, a phenanthridine group, an acridine group, a phenanthroline group, a phenazine group, a benzimidazole group, an isobenzothiazole group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a thiadiazole group, an imidazopyridine group, an imidazopyrimidine group, an azacarbazole group, an azadibenzofuran group, an azadibenzothiophene group, an azadibenzosilole group, an acridine group, or a pyridopyrazine group. 
     In one or more embodiments, the π electron-rich C 3 -C 60  cyclic group may be a benzene group, a heptalene group, an indene group, a naphthalene group, an azulene group, an indacene group, an acenaphthylene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a naphthacene group, a picene group, a perylene group, a pentacene group, a hexacene group, a pentaphene group, a rubicene group, a coronene group, an ovalene group, a pyrrole group, a furan group, a thiophene group, an isoindole group, an indole group, an indene group, a benzofuran group, a benzothiophene group, a benzosilole group, a naphthopyrrole group, a naphthofuran group, a naphthothiophene group, a naphthosilole group, a benzocarbazole group, a thienodicarbazole group, a benzothienopyrrolecarbazole group, a dibenzocarbazole group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, a dibenzosilole group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, a benzothienocarbazole group, a benzosilolocarbazole group, a triindolobenzene group, a pyrrolophenanthrene group, a furanophenanthrene group, a thienophenanthrene group, a benzonaphthofuran group, a benzonapthothiophene group, an (indolo)phenanthrene group, a (benzofurano)phenanthrene group, or a (benzothieno)phenanthrene group. 
     In an embodiment, the host may include a first host and a second host, the first host may be a hole transporting compound, and the second host may be an electron transporting compound or a bipolar compound. 
     In an embodiment, the first host may be represented by Formula 2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 2, 
     X 21  may be O, S, N(E 23 ), or C(E 23 )(E 24 ), 
     A 21  and A 22  may each independently be a π electron-rich C 3 -C 60  cyclic group, 
     E 21  to E 24  may each independently be *-(L 21 ) a21 -(R 21 ) b21 , 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 , a C 6 -C 60  aryloxy group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  arylthio group unsubstituted or substituted with at least one R 10a , —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), 
     d21 and d22 may each independently be an integer selected from 1 to 10, 
     L 21  may be a single bond, a π electron-rich C 3 -C 60  cyclic group unsubstituted or substituted with at least one R 20a , *—C(Ar 21 )(Ar 22 )—*′, *—Si(Ar 21 )(Ar 22 )—*′, *—B(Ar 21 )—*′, or *—N(Ar 21 )—′, 
     a21 may be an integer selected from 1 to 5, 
     R 21  may be a C 1 -C 60  alkyl group unsubstituted or substituted at least one R 20a , a π electron-rich C 3 -C 60  cyclic group unsubstituted or substituted at least one R 20a , —C(Ar 23 )(Ar 24 )(Ar 25 ), —Si(Ar 23 )(Ar 24 )(Ar 25 ), —N(Ar 23 )(Ar 24 ), or —B(Ar 23 )(Ar 24 ), 
     b21 may be an integer selected from 1 to 10, and 
     Ar 21  to Ar 25  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted at least one R 20a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted at least one R 20a . 
     In an embodiment, the first host may be selected from Group I, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In an embodiment, the second host may be represented by Formula 6-1 or Formula 6-2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 6-1, 6A, and 6-2, 
     Y 61  may be a single bond, *—O—*′, *—S—*′, *—C(R 61a )(R 61b )—*′, *—N(R 61a )—*′, *—Si(R 61a )(R 61b )—*′, *—C(═O)—*′, *—S(═O) 2 —*′, *—B(R 61a )—*′, *—P(R 61a )—*′, or *—P(═O)(R 61a )(R 61b )—*′, 
     k61 may be 0 or 1, 
     A 61  and A 62  may each independently be a C 5 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, 
     L 61  to L 66  may each independently be a carbocyclic group unsubstituted or substituted with at least one R 10a  or a heterocyclic group unsubstituted or substituted with at least one R 10a , 
     Ar 61  to Ar 66  may each independently be the group represented by Formula 2A, a carbocyclic group unsubstituted or substituted with at least one R 10a , or a heterocyclic group unsubstituted or substituted with at least one R 10a , 
     at least one of Ar 61  to Ar 63  may be a group represented by Formula 6A, 
     a61 to a66 may each independently be an integer selected from 0 to 3, 
     b61 to b66 may each independently be an integer selected from 1 to 8, 
     c61 and c62 may each independently be an integer selected from 1 to 20, 
     c63 may be an integer selected from 1 to 5, 
     n61 and n62 may each independently be an integer selected from 1 to 8, 
     X 61  to X 63  may each independently be C or N, 
     when X 61  to X 63  are each C, at least one of R 61  to R 63  may be: —F; a cyano group; a C 1 -C 60  alkyl group substituted with at least one of —F and a cyano group; or a π electron-depleted nitrogen-containing C 1 -C 60  cyclic group unsubstituted or substituted with at least one of R 10a , 
     X 64  may be C(R 64 ) or N, 
     X 65  may be C(R 65 ) or N, 
     X 66  may be C(R 66 ) or N, 
     at least one of X 64  to X 66  may be N, 
     R 61a , R 61b , and R 61  to R 66  may each independently be *-(L 67 ) a67 -(Ar 67 ) b67 , 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, a C 6 -C 60  aryloxy group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  arylthio group unsubstituted or substituted with at least one R 10a , —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), 
     L 67  may each independently be the group represented by Formula 2A, a carbocyclic group unsubstituted or substituted with at least one R 10a , or a heterocyclic group unsubstituted or substituted with at least one R 10a , 
     a67 may be an integer selected from 0 to 3, 
     Ar 67  may each independently be the group represented by Formula 2A, a carbocyclic group unsubstituted or substituted with at least one R 10a , or a heterocyclic group unsubstituted or substituted with at least one R 10a , 
     b67 may be an integer selected from 1 to 8, 
     at least two adjacent groups of Ar 61  to Ar 67 , R 61a , R 61b , and R 61  to R 66  may optionally be bound to each other to form a C 5 -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 
     * indicates a binding site to an adjacent atom. 
     In an embodiment, the second compound may be selected from Group II, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In an embodiment, the emission layer may include a fourth compound. In an embodiment, the fourth compound may be a fluorescence luminescence compound. In an embodiment, the fourth compound may be represented by one of Formulae 5-1 to 5-3: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 5-1 to 5-3, 
     Y 51  and Y 52  may each independently be N, B, or P, 
     X 51  to X 55  may each independently be a single bond, O, S, N(R 56 ), C(R 56 )(R 57 ), or Si(R 56 )(R 57 ), 
     n51 and n54 may each independently be 0, 1, or 2, when n51 is 0, A 51  and A 52  may not be bound to each other, and when n54 is 0, A 54  and A 55  may not be bound to each other, 
     A 51  to A 55  may each independently be a C 5 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, 
     A 56  may be a C 5 -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 , 
     D 51  may be a π electron-rich C 3 -C 60  cyclic group unsubstituted or substituted with at least one R 10a  or —N(E 52 )(E 53 ), 
     E 52  may be *-(L 52 ) a52 -(Ar 52 ) b52 , 
     E 53  may be *-(L 53 ) a53 -(Ar 53 ) b53 , 
     R 51  to R 57 , Ar 52 , and Ar 53  may each independently 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 , a C 6 -C 60  aryloxy group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  arylthio group unsubstituted or substituted with at least one R 10a , —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), 
     b52 to b53 may each independently be an integer selected from 0 to 3, 
     c51 to c55 may each independently be an integer selected from 0 to 3, d51 may each independently be an integer selected from 1 to 3, 
     L 51  to L 53  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 , 
     a51 to a53 may each independently be an integer selected from 0 to 3, and 
     m51 may be an integer selected from 1 to 6. 
     Q 1  to Q 3  may have, independently from one another, the same meaning as defined for Q 11  to Q 13 , Q 21  to Q 23 , and Q 31  to Q 33  as defined for R 10a  below. 
     In an embodiment, the fourth compound may be selected from Group IV, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     R 10a  as used herein may be: 
     deuterium (-D), —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, —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, or a C 6 -C 60  arylthio 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, —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 ). 
     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; —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; or a C 3 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, each 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. 
     R 20a  as used herein may be: 
     deuterium (-D), a hydroxyl 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, a hydroxyl group, a nitro group, a π electron-rich C 3 -C 60  cyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, —Si(Q 41 )(Q 42 )(Q 43 ), —N(Q 41 )(Q 42 ), —B(Q 41 )(Q 42 ), or any combination thereof; 
     a π electron-rich C 3 -C 60  cyclic group, a C 6 -C 60  aryloxy group, or a C 6 -C 60  arylthio group, each unsubstituted or substituted with deuterium, a hydroxyl 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 π electron-rich C 3 -C 60  cyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, —Si(Q 51 )(Q 52 )(Q 53 ), —N(Q 51 )(Q 52 ), —B(Q 51 )(Q 52 ), or any combination thereof; or 
     —Si(Q 61 )(Q 62 )(Q 63 ), —N(Q 61 )(Q 62 ), or —B(Q 61 )(Q 62 ), 
     wherein Q 41  to Q 43 , Q 51  to Q 53  and Q 61  to Q 63  may each independently be: hydrogen; deuterium; a hydroxyl 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; or a π electron-rich C 3 -C 60  cyclic 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 expression that a “(hole transport layer) including a first compound” may be construed as an “(interlayer) including a first compound that may satisfy Condition 1 or two different types of a first compound that may satisfy Condition 1”. 
     For example, the interlayer may include Compound 1 only as the first compound. In this embodiment, Compound 1 may be included in the hole transport layer of the light-emitting device. In some embodiments, Compounds 1 and 2 may be included in the interlayer as the first compounds. In this embodiment, Compounds 1 and 2 may be included in the same layer (for example, both Compounds 1 and 2 may be included in a hole transport layer) or in different layers (for example, Compound 1 may be included in an emission layer, and Compound 2 may be included in an hole transport layer). 
     According to one or more embodiments, an electronic apparatus may include the light-emitting device. The electronic apparatus may further include a thin-film transistor. 
     In some embodiments, the electronic apparatus may further include a thin-film transistor including a source electrode and drain electrode, and a first electrode of the light-emitting device may be electrically connected to the source electrode or the drain electrode. 
     In some embodiments, the electronic apparatus may further include a color filter, a color-conversion layer, a touchscreen layer, a polarization layer, or any combination thereof. In some embodiments, the electronic apparatus may be a substantially flat panel display device, but embodiments are not limited thereto. 
     The electronic apparatus may be understood by referring to the description of the electronic apparatus provided herein. 
     Description of FIG.  1   
       FIG. 1  is a schematic cross-sectional view of an embodiment of a light-emitting device constructed according to the principles of the invention. 
     The light-emitting device  10  may include 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 an illustrative method of manufacturing the light-emitting device  10  will be described in connection with  FIG. 1 . 
     First Electrode  110   
     In  FIG. 1 , a substrate may be additionally located under the first electrode  110  or above the second electrode  150 . The substrate may be a glass substrate or a plastic substrate. The substrate may be a flexible substrate including a plastic having excellent heat resistance and durability, for example, a polyimide, a polyethylene terephthalate (PET), a polycarbonate, a polyethylene naphthalate, a polyarylate (PAR), a polyetherimide, or any combination thereof. 
     The first electrode  110  may be formed by depositing or sputtering, on the substrate, a material for forming the first electrode  110 . When the first electrode  110  is an anode, a high work function material that may easily inject holes may be used as a material for a first electrode. 
     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 be an indium tin oxide (ITO), an indium zinc oxide (IZO), a tin oxide (SnO 2 ), a zinc oxide (ZnO), or any combinations thereof. In some embodiments, when the first electrode  110  is a semi-transmissive electrode or a reflective electrode, magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), or any combination thereof may be used as a material for forming the first electrode  110 . The first electrode  110  may have a single-layered structure consisting of a single layer or a multi-layered structure including two or more layers. In some embodiments, the first electrode  110  may have a triple-layered structure of an ITO/Ag/ITO. 
     Interlayer  130   
     The interlayer  130  may be on the first electrode  110 . The interlayer  130  may include an emission layer. The interlayer  130  may further include a hole transport region between the first electrode  110  and the emission layer and an electron transport region between the emission layer and the second electrode  150 . The interlayer  130  may further include metal-containing compounds such as organometallic compounds, inorganic materials such as quantum dots, and the like, in addition to various organic materials. 
     The interlayer  130  may include: i) at least two emitting units sequentially stacked between the first electrode  110  and the second electrode  150 ; and ii) a charge-generation layer located between the at least two emitting units. When the interlayer  130  includes the at least two emitting units and a charge generation layer, 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 a plurality of different materials, or iii) a multi-layered structure having a plurality of layers including a plurality of 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 a combination thereof. 
     For example, the hole transport region may have a multi-layered structure, e.g., 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 layers of each structure are sequentially stacked on the first electrode  110  in each stated order. 
     The hole transport region may include the compound represented by Formula 201, the 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—*′, *—S—*′, *—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 selected from 0 to 5, 
     xa5 may be an integer selected 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 bound 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 (e.g., a carbazole group or the like) unsubstituted or substituted with at least one R 10a  (e.g., Compound HT16 described herein), 
     R 203  and R 204  may optionally be bound 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 selected from 1 to 4. 
     In some embodiments, Formulae 201 and 202 may each include at least one of groups represented by Formulae CY201 to CY217: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Formulae CY201 to CY217, R 10b  and R 10c  may each be understood by referring to the descriptions of 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 CY201 to CY217 may be unsubstituted or substituted with R 10a . 
     In some embodiments, in Formulae CY201 to CY217, ring CY 201  to ring CY 204  may each independently be a benzene group, a naphthalene group, a phenanthrene group, or an anthracene group. In one or more embodiments, Formulae 201 and 202 may each include at least one of groups represented by Formulae CY201 to CY203. In one or more embodiments, Formula 201 may include at least one of groups represented by Formulae CY201 to CY203 and at least one of 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 any one of Formulae CY201 to CY203, xa2 may be 0, and R 202  may be a group represented by Formulae CY204 to CY207. In one or more embodiments, Formulae 201 and 202 may each not include groups represented by Formulae CY201 to CY203. In one or more embodiments, Formulae 201 and 202 may each not include groups represented by Formulae CY201 to CY203, and include at least one of groups represented by Formulae CY204 to CY217. In one or more embodiments, Formulae 201 and 202 may each not include groups represented by Formulae CY201 to CY217. 
     In some embodiments, the hole transport region may include one of Compounds HT1 to HT57 and m-MTDATA, TDATA, 2-TNATA, NPB (NPD), TPD, spiro-TPD, spiro-NPB, methylated-NPB, TAPC, HMTPD, TCTA, PANI/DBSA, PEDOT/PSS, PANI/CSA, PANI/PSS, or any combination thereof, as discussed above for the first and second compounds. 
     The thickness of the hole transport region may be in a range of about 50 Angstroms (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, and any combination thereof, the 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, the 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 any of these ranges, excellent hole transport 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 by an emission layer. The electron blocking layer may prevent leakage of electrons to a hole transport region from the emission layer. Materials that may be included in the hole transport region may also be included in an emission auxiliary layer and an electron blocking layer. 
     p-Dopant 
     The hole transport region may include a charge generating material as well as the aforementioned materials to improve conductive properties of the hole transport region. The charge generating material may be substantially homogeneously or non-homogeneously dispersed (for example, as a single layer consisting of charge generating material) in the hole transport region. 
     The charge generating material may include, for example, a p-dopant. In some embodiments, a lowest unoccupied molecular orbital (LUMO) energy level of the p-dopant may be about −3.5 eV or less. In some embodiments, the p-dopant may include a quinone derivative, a compound containing a cyano group, a compound containing element EL1 and element EL2, or any combination thereof. Examples of the quinone derivative may include TCNQ, F 4 -TCNQ, and the like, as discussed above. Examples of the cyano group-containing compound include HAT-CN, a compound represented by Formula 221, and the like, as discussed above. In the compound containing element EL1 and element EL2, elements EL1 and EL2 may be the same as described above. 
     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 sub-pixel. In one or more embodiments, the emission layer may have a stacked structure. The stacked structure may include two or more layers selected from a red emission layer, a green emission layer, and a blue emission layer. The two or more layers may be in direct contact with each other. In some embodiments, the two or more layers may be separated from each other. In one or more embodiments, the emission layer may include two or more materials. The two or more materials may include a red light-emitting material, a green light-emitting material, or a blue light-emitting material. The two or more materials may be mixed with each other in a single layer. The two or more materials mixed with each other in the single layer may emit white light. 
     The emission layer may include a host and a dopant. The dopant may be a phosphorescent dopant, a fluorescent dopant, or any combination thereof. The amount of the dopant in the emission layer may be in a range of about 0.01 parts to about 15 parts by weight based on 100 parts by weight of the host. In some embodiments, the emission layer may include a quantum dot. The emission layer may include a delayed fluorescence material. The delayed fluorescence material may serve as a host or a dopant in the emission layer. 
     The thickness of the emission layer may be in a range of about 100 Å to about 1,000 Å, and in some embodiments, about 200 Å to about 600 Å. When the thickness of the emission layer is within any of these ranges, improved luminescence characteristics may be obtained without a substantial increase in driving voltage. The emission layer may include a first emission layer and a second emission layer. The first emission layer and the second emission layer may respectively be understood by referring to the descriptions of the emission layer described herein. 
     Host 
     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 selected 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 selected from 1 to 5, and 
     wherein Q 301  to Q 303  may each be understood by referring to the description of Q 1  provided herein. 
     In some embodiments, when xb11 in Formula 301 is 2 or greater, at least two Ar 301 (s) may be bound via a single bond. 
     In some 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 to 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, and R 301  may respectively be understood by referring to the descriptions of L 301 , xb1, and R 301  provided herein, 
     L 302  to L 304  may each be understood by referring to the description of L 301  provided herein, 
     xb2 to xb4 may each be understood by referring to the descriptions of xb1 provided herein, and 
     R 302  to R 305  and R 311  to R 314  may each be understood by referring to the descriptions of R 301  provided herein. 
     In some embodiments, the host may include an alkaline earth-metal complex, a post-transitional metal complex, or any combination thereof. For example, the host may include a Be complex (e.g., Compound H55), a Mg complex, a Zn complex, or any combination thereof. 
     In some embodiments, the host may include one of Compounds H1 to H124, 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. 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. In some embodiments, the phosphorescent dopant may include an organometallic complex represented by Formula 401: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 401 and 402, 
     M may be a transition metal (e.g., 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, and when xc1 is 2 or greater, at least two L 401 (s) may be identical to or different from each other, 
     L 402  may be an organic ligand, and xc2 may be an integer selected from 0 to 4, and when xc2 is 2 or greater, at least two 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(Q 411 )═*′, 
     X 403  and X 404  may each independently be a chemical bond (e.g., a covalent bond or a coordinate 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 be understood by referring to the description of Q 1  provided herein, 
     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 be understood by referring to the description of Q 1  provided herein, 
     xc11 and xc12 may each independently be an integer selected from 0 to 10, and 
     * and *′ in Formula 402 each indicate a binding site to M in Formula 401. 
     In one or more embodiments, in Formula 402, i) X 401  may be nitrogen, and X 402  may be carbon, or ii) X 401  and X 402  may both be nitrogen. 
     In one or more embodiments, when xc1 in Formula 402 is 2 or greater, two ring A 401 (s) of at least two L 401 (s) may optionally be bound via T 402  as a linking group, or two ring A 402 (s) may optionally be bound via T 403  as a linking group (see Compounds PD1 to PD4 and PD7). The variables T 402  and T 403  may each be understood by referring to the description of T 401  provided herein. 
     The variable L 402  in Formula 401 may be any suitable organic ligand. For example, L 402  may be a halogen group, a diketone group (e.g., an acetylacetonate group), a carboxylic acid group (e.g., a picolinate group), a —C(═O) group, an isonitrile group, a —CN group, or a phosphorus group (e.g., a phosphine group or a phosphite group). 
     The phosphorescent dopant may be, for example, one of Compounds PD1 to PD25 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. In some embodiments, 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. 
     In some embodiments, in Formula 501, Ar 501  may include a condensed ring group (e.g., an anthracene group, a chrysene group, or a pyrene group) in which at least three monocyclic groups are condensed. In some embodiments, xd4 in Formula 501 may be 2. 
     In some embodiments, 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. The delayed fluorescence material described herein may be any suitable compound that may emit delayed fluorescence according to a delayed fluorescence emission mechanism. The delayed fluorescence material included in the emission layer may serve as a host or a dopant, depending on types of other materials included in the emission layer. 
     In some embodiments, the difference between a triplet energy level (eV) of the delayed fluorescence material and the singlet energy level (eV) of the delayed fluorescence material may be about 0 eV or greater and about 0.5 eV or less. 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 is within this range, up-conversion from the triplet state to the singlet state in the delayed fluorescence material may effectively occur, thus improving luminescence efficiency and the like of the light-emitting device  10 . 
     In some embodiments, the delayed fluorescence material may include: i) a material including at least one electron donor (e.g., a π electron-rich C 3 -C 60  cyclic group such as a carbazole group and the like) and at least one electron acceptor (e.g., a sulfoxide group, a cyano group, a π electron-deficient nitrogen-containing C 1 -C 60  cyclic group, and 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 the like. 
     Examples of the delayed fluorescence material may include at least one of Compounds DF1 to DF9: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Quantum Dots 
     The emission layer may include quantum dots. The diameter of the quantum dot may be, for example, in a range of about 1 nm to about 10 nm. Quantum dots may be synthesized by a wet chemical process, an organic metal chemical vapor deposition process, a molecular beam epitaxy process, or any similar process. 
     The wet chemical process is a method of growing a quantum dot particle crystal by mixing a precursor material with an organic solvent. When the crystal grows, the organic solvent may naturally serve as a dispersant coordinated on the surface of the quantum dot crystal and control the growth of the crystal. Thus, the wet chemical method may be easier to perform than the vapor deposition process such a metal organic chemical vapor deposition (MOCVD) or a molecular beam epitaxy (MBE) process. Further, the growth of quantum dot particles may be controlled with a lower manufacturing cost. 
     The quantum dot may include a semiconductor compound of Groups II-VI; a semiconductor compound of Groups III-V; a semiconductor compound of Groups III-VI; a semiconductor compound of Groups I, III, and VI; a semiconductor compound of Groups IV-VI; an element or a compound of Group IV; or any combination thereof. 
     Examples of the semiconductor compound of Groups II-VI may include a binary compound such as CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, or MgS; a ternary compound such as CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, or MgZnS; a quaternary compound such as CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, or HgZnSTe; or any combination thereof. 
     Examples of the semiconductor compound of Groups III-V may include a binary compound such as GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, or InSb; a ternary compound such as GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InGaP, InNP, InAIP, InNAs, InNSb, InPAs, or InPSb; a quaternary compound such as GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, or InAlPSb; or any combination thereof. In some embodiments, the semiconductor compound of Groups III-V may further include a Group II element. Examples of the semiconductor compound of Groups III-V further including the Group II element may include InZnP, InGaZnP, InAlZnP, and the like. 
     Examples of the semiconductor compound of Groups III-VI may include a binary compound such as GaS, GaSe, Ga 2 Se 3 , GaTe, InS, InSe, In 2 S 3 , In 2 Se 3 , InTe, and the like; a ternary compound such as InGaS 3 , InGaSe 3 , and the like; or any combination thereof. Examples of the semiconductor compound of Groups I, III, and VI may include a ternary compound such as AgInS, AgInS 2 , CuInS, CuInS 2 , CuGaO 2 , AgGaO 2 , AgAlO 2 , or any combination thereof. 
     Examples of the semiconductor compound of Groups IV-VI may include a binary compound such as SnS, SnSe, SnTe, PbS, PbSe, or PbTe; a ternary compound such as SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, or SnPbTe; a quaternary compound such as SnPbSSe, SnPbSeTe, or SnPbSTe; or any combination thereof. 
     The element or compound of Group IV may be a single element compound such as Si or Ge; a binary compound such as SiC or SiGe; or any combination thereof. Individual elements included in the multi-element compound, such as a binary compound, a ternary compound, and a quaternary compound, may be present in a particle thereof at a uniform or non-uniform concentration. The quantum dot may have a single structure in which the concentration of each element included in the quantum dot is uniform or a core-shell double structure. In some embodiments, materials included in the core may be different from materials included in the shell. 
     The shell of the quantum dot may serve as a protective layer for preventing chemical denaturation of the core to maintain semiconductor characteristics and/or as a charging layer for imparting electrophoretic characteristics to the quantum dot. The shell may be a monolayer or a multilayer. The interface between a core and a shell may have a concentration gradient where a concentration of elements present in the shell decreases toward the core. 
     Examples of the shell of the quantum dot include metal, metalloid, or nonmetal oxide, a semiconductor compound, or a combination thereof. Examples of the metal oxide, metalloid, or nonmetal oxide may include: a binary compound such as SiO 2 , Al 2 O 3 , TiO 2 , ZnO, MnO, Mn 2 O 3 , Mn 3 O 4 , CuO, FeO, Fe 2 O 3 , Fe 3 O 4 , CoO, Co 3 O 4 , or NiO; a ternary compound such as MgAl 2 O 4 , CoFe 2 O 4 , NiFe 2 O 4 , or CoMn 2 O 4 ; and any combination thereof. Examples of the semiconductor compound may include a semiconductor compound of Groups II-VI; a semiconductor compound of Groups III-V; a semiconductor compound of Groups III-VI; a semiconductor compound of Groups I, III, and VI; a semiconductor compound of Groups IV-VI; or any combination thereof. In some embodiments, the semiconductor compound may be CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnSeS, ZnTeS, GaAs, GaP, GaSb, HgS, HgSe, HgTe, InAs, InP, InGaP, InSb, AlAs, AlP, AlSb, or any combination thereof. 
     The quantum dot may have a full width of half maximum (FWHM) of a spectrum of an emission wavelength of about 45 nm or less, about 40 nm or less, or about 30 nm or less. When the FWHM of the quantum dot is within this range, color purity or color reproducibility may be improved. In addition, because light emitted through the quantum dot is emitted in all directions, an optical viewing angle may be improved. 
     In addition, the quantum dot may be specifically, a generally spherical, a generally pyramidal, a generally multi-armed, or a generally cubic nanoparticle, a nanotube-shaped particle, a nanowire-shaped particle, a nanofiber-shaped particle, or a nanoplate-shaped particle. 
     By adjusting the size of the quantum dot, the energy band gap may also be adjusted, thereby obtaining light of various wavelengths in the quantum dot emission layer. By using quantum dots of various sizes, a light-emitting device that may emit light of various wavelengths may be realized. In some embodiments, the size of the quantum dot may be selected such that the quantum dot may emit red, green, and/or blue light. In addition, the size of the quantum dot may be selected such that the quantum dot may emit white light by combining various light colors. 
     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 a plurality of different materials, or iii) a multi-layered structure having a plurality of layers including a plurality of different materials. 
     The electron transport region may include a buffer layer, a hole blocking layer, an electron control layer, an electron transport layer, or an electron injection layer. In some embodiments, the electron transport region may have an electron transport layer/electron injection layer structure, a hole blocking layer/electron transport layer/electron injection layer structure, an electron control layer/electron transport layer/electron injection layer structure, or a buffer layer/electron transport layer/electron injection layer structure, wherein layers of each structure are sequentially stacked on the emission layer in each stated order. The electron transport region (e.g., a buffer layer, a hole blocking layer, an electron control layer, or an 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. 
     In some embodiments, 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 be understood by referring to the description of Q 1  provided herein, 
     xe21 may be 1, 2, 3, 4, or 5, and 
     at least one of Ar 601 , L 601 , and 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 104 . 
     In some embodiments, when xe11 in Formula 601 is 2 or greater, at least two Ar 601 (s) may be bound via a single bond. In some embodiments, in Formula 601, Ar 601  may be a substituted or unsubstituted anthracene group. In some 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 ), at least one selected from X 614  to X 616  may be N, 
     L 611  to L 613  may each be understood by referring to the description of L 601  provided herein, 
     xe611 to xe613 may each be understood by referring to the description of xe1 provided herein, 
     R 611  to R 613  may each be understood by referring to the description of R 601  provided herein, 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 ET47, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (Bphen), tris-(8-hydroxyquinoline)aluminum (Alq 3 ), bis(2-methyl-8-quinolinolato-N1,08)-(1,1′-biphenyl-4-olato)aluminum (BAlq), 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ), 4-(naphthalen-1-yl)-3,5-diphenyl-4H-1,2,4-triazole (NTAZ), or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The thickness of the electron transport region may be in a range of about 100 Angstroms (Å) to about 5,000 Å, for example, about 160 Å to about 4,000 Å. When the electron transport region includes the buffer layer, the hole blocking layer, the electron control layer, the electron transport layer, or any combination thereof, the thicknesses of the buffer layer, the hole blocking layer, or the electron control layer may each independently be in a range of about 20 Å to about 1,000 Å, for example, about 30 Å to about 300 Å, and the thickness of the electron transport layer may be in a range of about 100 Å to about 1,000 Å, for example, about 150 Å to about 500 Å. When the thicknesses of the buffer layer, the hole blocking layer, the electron control layer, the electron transport layer, and/or the electron transport layer are each within these ranges, excellent electron transport 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 lithium (Li) ion, a sodium (Na) ion, a potassium (K) ion, a rubidium (Rb) ion, or a cesium (Cs) ion. A metal ion of the alkaline earth metal complex may be a beryllium (Be) ion, a magnesium (Mg) ion, a calcium (Ca) ion, a strontium (Sr) ion, or a barium (Ba) ion. Each ligand coordinated with the metal ion of the alkali metal complex and the alkaline earth metal complex may independently be a hydroxyquinoline, a hydroxyisoquinoline, a hydroxybenzoquinoline, a hydroxyacridine, a hydroxyphenanthridine, a hydroxyphenyloxazole, a hydroxyphenylthiazole, a hydroxyphenyloxadiazole, a hydroxyphenylthiadiazole, a hydroxyphenylpyridine, a hydroxyphenylbenzimidazole, a hydroxyphenylbenzothiazole, a bipyridine, a phenanthroline, a cyclopentadiene, or any combination thereof. 
     For example, the metal-containing material may include a Li complex. The Li complex may include, e.g., Compound ET-D1 (lithium quinolate, LiQ) or Compound ET-D2: 
     
       
         
         
             
             
         
       
     
     The electron transport region may include an electron injection layer that facilitates 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 a plurality of different materials, or iii) a multi-layered structure having a plurality of layers including a plurality of 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 be Li, Na, K, Rb, Cs or any combination thereof. The alkaline earth metal may be Mg, Ca, Sr, Ba, or any combination thereof. The rare earth metal may be 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 respectively be oxides, halides (e.g., fluorides, chlorides, bromides, or iodides), tellurides, or any combination thereof of each of the alkali metal, the alkaline earth metal, and the rare earth metal. 
     The alkali metal-containing compound may be alkali metal oxides such as Li 2 O, Cs 2 O, or K 2 O, alkali metal halides such as LiF, NaF, CsF, KF, LiI, NaI, CsI, or KI, or any combination thereof. The alkaline earth-metal-containing compound may include alkaline earth-metal compounds, such as BaO, SrO, CaO, Ba x Sr 1−x O (wherein x is a real number satisfying 0&lt;x&lt;1), or Ba x Ca 1−x O (wherein x is a real number satisfying 0&lt;x&lt;1). The rare earth metal-containing compound may include YbF 3 , ScF 3 , Sc 2 O 3 , Y 2 O 3 , Ce 2 O 3 , GdF 3 , TbF 3 , YbI 3 , ScI 3 , TbI 3 , or any combination thereof. In some embodiments, the rare earth metal-containing compound may include a 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 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, alkaline earth metal, and rare earth metal described above and ii) a ligand bond to the metal ion, e.g., a hydroxyquinoline, a hydroxyisoquinoline, a hydroxybenzoquinoline, a hydroxyacridine, a hydroxyphenanthridine, a hydroxyphenyloxazole, a hydroxyphenylthiazole, a hydroxyphenyloxadiazole, a hydroxyphenylthiadiazole, a hydroxyphenylpyridine, a hydroxyphenylbenzimidazole, a hydroxyphenylbenzothiazole, a bipyridine, a phenanthroline, a cyclopentadiene, or any combination thereof. 
     The electron injection layer may 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 some embodiments, the electron injection layer may further include an organic material (e.g., a compound represented by Formula 601). 
     In some embodiments, the electron injection layer may consist of i) an alkali metal-containing compound (e.g., an alkali metal halide), or ii) a) an alkali metal-containing compound (e.g., an alkali metal halide); and b) an alkali metal, an alkaline earth metal, a rare earth metal, or any combination thereof. In some embodiments, the electron injection layer may be a KI:Yb co-deposition layer, a RbI:Yb co-deposition layer, and 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 homogeneously or non-homogeneously dispersed in a matrix including the organic material. 
     The thickness of the electron injection layer may be in a range of about 1 Å to about 100 Å, and in some embodiments, about 3 Å to about 90 Å. When the thickness of the electron injection layer is within any of these ranges, excellent electron injection characteristics may be obtained without a substantial increase in driving voltage. 
     Second Electrode  150   
     The second electrode  150  may be on the interlayer  130 . In an embodiment, the second electrode  150  may be a cathode that is an electron injection electrode. In this embodiment, a material for forming the second electrode  150  may be a material having a low work function, for example, a metal, an alloy, an electrically conductive compound, or any combination thereof. 
     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), an ITO, an 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 two or more layers. 
     Capping Layer 
     A first capping layer may be located outside the first electrode  110 , and/or a second capping layer may be located outside the second electrode  150 . In some embodiments, 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 this 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 this 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 this stated order. 
     In the light-emitting device  10 , light emitted from the emission layer in the interlayer  130  may pass through the first electrode  110  (which may be a semi-transmissive electrode or a transmissive electrode) and through the first capping layer to the outside. In the light-emitting device  10 , light emitted from the emission layer in the interlayer  130  may pass through the second electrode  150  (which may be a semi-transmissive electrode or a transmissive electrode) and through the second capping layer to the outside. 
     Although not wanting to be bound by theory, the first capping layer and the second capping layer may improve the external luminescence efficiency based on the principle of constructive interference. Accordingly, the optical extraction efficiency of the light-emitting device  10  may be increased, thus improving the luminescence efficiency of the light-emitting device  10 . The first capping layer and the second capping layer may each include a material having a refractive index of about 1.6 or higher (at 589 nm). The first capping layer and the second capping layer may each independently be a 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 and the second capping layer may each independently include carbocyclic compounds, heterocyclic compounds, amine group-containing compounds, porphine derivatives, phthalocyanine derivatives, naphthalocyanine derivatives, alkali metal complexes, alkaline earth metal complexes, or any combination thereof. The carbocyclic compound, the heterocyclic compound, and the amine group-containing compound may optionally be substituted with a substituent of O, N, S, Se, Si, F, Cl, Br, I, or any combination thereof. In some embodiments, at least one of the first capping layer and the second capping layer may each independently include an amine group-containing compound. 
     In some embodiments, at least one of the first capping layer and the second capping layer may each independently include the compound represented by Formula 201, the compound represented by Formula 202, or any combination thereof. In one or more embodiments, at least one of the first capping layer and the second capping layer may each independently include one of Compounds HT28 to HT33, one of Compounds CP1 to CP6, N4,N4′-di(naphthalen-2-yl)-N4,N4′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (β-NPB), or any combination thereof: 
     
       
         
         
             
             
         
       
     
     Electronic Apparatus 
     The light-emitting device  10  may be included in various electronic apparatuses. In some embodiments, an electronic apparatus including the light-emitting device may be an emission apparatus or an authentication apparatus. 
     The electronic apparatus (e.g., an emission apparatus) may further include, in addition to the light-emitting device  10 , 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 disposed on at least one traveling direction of light emitted from the light-emitting device  10 . For example, light emitted from the light-emitting device  10  may be blue light or white light. The light-emitting device  10  may be understood by referring to the descriptions provided herein. In some embodiments, the color-conversion layer may include quantum dots. The quantum dot may be, for example, the quantum dot described herein. The electronic apparatus may include a first substrate. The first substrate may include a plurality of sub-pixel areas, the color filter may include a plurality of color filter areas respectively corresponding to the plurality of sub-pixel areas, and the color-conversion layer may include a plurality of color-conversion areas respectively corresponding to the plurality of sub-pixel areas. A pixel defining film may be located between the plurality of sub-pixel areas to define each sub-pixel area. 
     The color filter may further include a plurality of color filter areas and light-blocking patterns between the plurality of color filter areas, and the color-conversion layer may further include a plurality of color-conversion areas and light-blocking patterns between the plurality of color-conversion areas. 
     The plurality of color filter areas (or a plurality of color-conversion areas) may include: a first area emitting first color light; a second area emitting second color light; and/or a third area emitting third color light, and the first color light, the second color light, and/or the third color light may have different maximum emission wavelengths. In some embodiments, 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 some embodiments, the plurality of color filter areas (or the plurality of color-conversion areas) may each include quantum dots. In some embodiments, the first area may include red quantum dots, the second area may include green quantum dots, and the third area may not include a quantum dot. The quantum dot may be understood by referring to the description of the quantum dot described herein. The first area, the second area, and/or the third area may each further include an emitter. 
     In some embodiments, the light-emitting device  10  may emit first light, the first area may absorb the first light to emit  1 - 1  color light, the second area may absorb the first light to emit  2 - 1  color light, and the third area may absorb the first light to emit  3 - 1  color light. In this embodiment, the  1 - 1  color light, the  2 - 1  color light, and the  3 - 1  color light may each have a different maximum emission wavelength. In some embodiments, the first light may be blue light, the  1 - 1  color light may be red light, the  2 - 1  color light may be green light, and the  3 - 1  light may be blue light. 
     The electronic apparatus may further include a thin-film transistor, in addition to the light-emitting device  10 . The thin-film transistor may include a source electrode, a drain electrode, and an active layer, wherein one of the source electrode and the drain electrode may be electrically connected to one of the first electrode and the second electrode of the light-emitting device  10 . 
     The thin-film transistor may further include a gate electrode, a gate insulating film, or the like. The active layer may include a crystalline silicon, an amorphous silicon, an organic semiconductor, and an oxide semiconductor. 
     The electronic apparatus may further include an encapsulation unit for sealing the light-emitting device  10 . The encapsulation unit may be located between the color filter and/or the color-conversion layer and the light-emitting device  10 . The encapsulation unit may allow light to pass to the outside from the light-emitting device  10  and prevent the air and moisture to permeate to the light-emitting device  10  at the same time. The encapsulation unit may be a sealing substrate including a transparent glass or a plastic substrate. The encapsulation unit may be a thin-film encapsulating layer including at least one of an organic layer and/or an inorganic layer. When the encapsulation unit is a thin film encapsulating layer, the electronic apparatus may be flexible. 
     In addition to the color filter and/or the color-conversion layer, various functional layers may be disposed on the encapsulation unit depending on the use of an electronic apparatus. Examples of the functional layer may include a touch screen layer, a polarization layer, or the like. The touch screen layer may be a resistive touch screen layer, a capacitive touch screen layer, or an infrared beam touch screen layer. The authentication apparatus may be, for example, a biometric authentication apparatus that identifies an individual according to biometric information (e.g., a fingertip, a pupil, or the like). 
     The authentication apparatus may further include a biometric information collecting unit, in addition to the light-emitting device  10  described above. The electronic apparatus may take the form of or be applicable to various displays, an optical source, lighting, a personal computer (e.g., a mobile personal computer), a cellphone, a digital camera, an electronic note, an electronic dictionary, an electronic game console, a medical device (e.g., an electronic thermometer, a blood pressure meter, a glucometer, a pulse measuring device, a pulse wave measuring device, an electrocardiograph recorder, an ultrasonic diagnosis device, or an endoscope display device), a fish finder, various measurement devices, gauges (e.g., gauges of an automobile, an airplane, or a ship), and a projector. 
     Descriptions of FIGS.  2  and  3   
       FIG. 2  is a schematic cross-sectional view of an embodiment of a light-emitting apparatus including a light-emitting device constructed according to the principles of the invention. 
     An emission apparatus  180  in  FIG. 2  may include a substrate  100 , a thin-film transistor  200 , a light-emitting device  10 , and an encapsulation unit  300  sealing the light-emitting device  10 . 
     The substrate  100  may be a flexible substrate, a glass substrate, or a metal substrate. A buffer layer  210  may be on the substrate  100 . The buffer layer  210  may prevent penetration of impurities through the substrate  100  and provide a substantially flat surface on the substrate  100 . 
     The thin-film transistor  200  may be on the buffer layer  210 . The thin-film transistor  200  may include an active layer  220 , a gate electrode  240 , a source electrode  260 , and a drain electrode  270 . The active layer  220  may include an inorganic semiconductor such as a silicon or a polysilicon, an organic semiconductor, or an oxide semiconductor and include a source area, a drain area, and a channel area. 
     A gate insulating film  230  for insulating the active layer  220  and the gate electrode  240  may be on the active layer  220 , and the gate electrode  240  may be on the gate insulating film  230 . An interlayer insulating film  250  may be on the gate electrode  240 . The interlayer insulating film  250  may be between the gate electrode  240  and the source electrode  260  and between the gate electrode  240  and the drain electrode  270  to provide insulation therebetween. 
     The source electrode  260  and the drain electrode  270  may be on the interlayer insulating film  250 . The interlayer insulating film  250  and the gate insulating film  230  may be formed to expose the source area and the drain area of the active layer  220 , and the source electrode  260  and the drain electrode  270  may be adjacent to the exposed source area and the exposed drain area of the active layer  220 . 
     Such a thin-film transistor  200  may be electrically connected to a light-emitting device  10  to drive the light-emitting device  10  and may be protected by a passivation layer  280 . The passivation layer  280  may include an inorganic insulating film, an organic insulating film, or a combination thereof. The light-emitting device  10  may be on the passivation layer  280 . The light-emitting device  10  may include a first electrode  110 , an interlayer  130 , and a second electrode  150 . 
     The first electrode  110  may be on the passivation layer  280 . The passivation layer  280  may not fully cover the drain electrode  270  and expose a specific area of the drain electrode  270 , and the first electrode  110  may be disposed to connect to the exposed area of the drain electrode  270 . 
     A pixel-defining film  290  may be on the first electrode  110 . The pixel-defining film  290  may expose a specific area of the first electrode  110 , and the interlayer  130  may be formed in the exposed area. The pixel-defining film  290  may be a polyimide or a polyacryl organic film. Some higher layers of the interlayer  130  may extend to the upper portion of the pixel-defining film  290  and may be disposed in the form of a common layer. The second electrode  150  may be 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 unit  300  may be on the capping layer  170 . The encapsulation unit  300  may be on the light-emitting device  10  to protect it from moisture or oxygen. The encapsulation unit  300  may include: an inorganic film including a silicon nitride (SiN x ), a silicon oxide (SiO x ), an indium tin oxide, an indium zinc oxide, or any combination thereof; an organic film including a polyethylene terephthalate (PET), a polyethylene naphthalate, a polycarbonate, a polyimide, a polyethylene sulfonate, a polyoxy methylene, a polyaryllate, a hexamethyl disiloxane, an acrylic resin (e.g., a polymethyl methacrylate, a polyacrylic acid, and the like), an epoxy resin (e.g., an aliphatic glycidyl ether (AGE) and the like), or any combination thereof; or a combination of the inorganic film and the organic film. 
       FIG. 3  is a schematic cross-sectional view of another embodiment of a light-emitting apparatus including a light-emitting device constructed according to the principles of the invention. 
     The emission apparatus  190  shown in  FIG. 3  may be substantially identical to the emission apparatus  180  shown in  FIG. 2 , except that a light-shielding pattern  500  and a functional area  400  are additionally located on the encapsulation unit  300 . The functional area  400  may be i) a color filter area, ii) a color-conversion area, or iii) a combination of a color filter area and a color-conversion area. In some embodiments, the light-emitting device  10  shown in  FIG. 3  included in the emission apparatus  190  may be a tandem light-emitting device. 
     Manufacturing Method 
     The layers constituting the hole transport region, the emission layer, and the layers constituting the electron transport region may be formed in a specific region by using one or more suitable methods such as vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB) deposition, ink-jet printing, laser printing, and laser-induced thermal imaging. 
     When layers constituting the hole transport region, an emission layer, and layers constituting the electron transport region are each independently formed by vacuum-deposition, the vacuum-deposition may be performed at a deposition temperature in a range of about 100° C. to about 500° C., at a vacuum degree in a range of about 10 −8  torr to about 10 −3  torr, and at a deposition rate in a range of about 0.01 Angstroms per second (Å/sec) to about 100 Å/sec, depending on the material to be included in each layer and the structure of each layer to be formed. 
     General Definitions of Terms 
     The term “interlayer” as used herein refers to a single layer and/or a plurality of all layers located between a first electrode and a second electrode in a light-emitting device. 
     The term “quantum dot” as used herein refers to a crystal of a semiconductor compound and may include any suitable material capable of emitting emission wavelengths of various lengths according to the size of the crystal. 
     As used herein, the term “energy level” may be abbreviated “eV” and the term “thermal activated delayed fluorescence” may be abbreviated “TADF”. 
     A quantum dot as used herein refers to a crystal of a semiconductor compound, and may include any material capable of emitting light of various emission wavelengths according to the size of the crystal. 
     As used herein, the term “atom” may mean an element or its corresponding radical bonded to one or more other atoms. 
     The terms “hydrogen” and “deuterium” refer to their respective atoms and corresponding radicals with the deuterium radical abbreviated “-D”, and the terms “—F, —Cl, —Br, and —I” are radicals of, respectively, fluorine, chlorine, bromine, and iodine. 
     The term “C 3 -C 60  carbocyclic group” as used herein refers to a cyclic group consisting of carbon atoms only and having 3 to 60 carbon atoms as ring-forming atoms. The term “C 1 -C 60  heterocyclic group” as used herein refers to a cyclic group having 1 to 60 carbon atoms in addition to a heteroatom as ring-forming atoms other than carbon atoms. 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 at least two rings are fused. For example, the number of ring-forming atoms in the C 1 -C 60  heterocyclic group may be in a range of 3 to 61. 
     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” refers to a cyclic group having 3 to 60 carbon atoms and not including *—N═*′ as a ring-forming moiety. The term “π electron-deficient nitrogen-containing C 1 -C 60  cyclic group” as used herein refers to a heterocyclic group having 1 to 60 carbon atoms and *—N═*′ as a ring-forming moiety. 
     For example, the C 3 -C 60  carbocyclic group may be i) a TG1 group or ii) a group in which at least two TG1 groups are fused, for example, 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 TG2 group, ii) a group in which at least two TG2 groups are fused, or iii) a group in which at least one TG2 group is fused with at least one TG1 group, for example, 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 benzonapthothiophene 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 the like. 
     The π electron-rich C 3 -C 60  cyclic group may be i) a TG1 group, ii) a fused group in which at least two TG1 groups are fused, iii) a TG3 group, iv) a fused group in which at least two TG3 groups are fused, or v) a fused group in which at least one TG3 group is fused with at least one TG1 group, for example, a 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 benzonapthothiophene group, a benzonaphthosilole group, a benzofurodibenzofuran group, a benzofurodibenzothiophene group, a benzothienodibenzothiophene group, and the like. 
     The π electron-deficient nitrogen-containing C 1 -C 60  cyclic group may be i) a TG4 group, ii) a group in which at least two TG4 groups are fused, iii) a group in which at least one TG4 group is fused with at least one TG1 group, iv) a group in which at least one TG4 group is fused with at least one TG3 group, or v) a group in which at least one TG4 group, at least one TG1 group, and at least one TG3 group are fused, for example, 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 the like. 
     The TG1 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 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 TG2 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 TG3 group may be a furan group, a thiophene group, a 1H-pyrrole group, a silole group, or a borole group. 
     The TG4 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 term “cyclic group”, “C 3 -C 60  carbocyclic group”, “C 1 -C 60  heterocyclic group”, “π electron-rich C 3 -C 60  cyclic group”, or “π electron-deficient nitrogen-containing C 1 -C 60  cyclic group” as used herein may be a group fused with any suitable cyclic group, a monovalent group, or a polyvalent group (e.g., a divalent group, a trivalent group, a quadvalent group, or the like), depending on the structure of the formula to which the term is applied. For example, a “benzene group” may be a benzo group, a phenyl group, a phenylene group, or the like, and this may be understood by one of ordinary skill in the art, depending on the structure of the 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 fused polycyclic group, and a monovalent non-aromatic fused heteropolycyclic group. Examples of the divalent C 3 -C 60  carbocyclic group and the monovalent 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 fused polycyclic group, and a substituted or unsubstituted divalent non-aromatic fused 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 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, and a tert-decyl group. The term “C 1 -C 60  alkylene group” as used herein refers to a divalent group having a structure corresponding to the C 1 -C 60  alkyl group. 
     The term “C 2 -C 60  alkenyl group” as used herein refers to a hydrocarbon group having at least one carbon-carbon double bond in the middle or at the terminus of the C 2 -C 60  alkyl group. Examples thereof include an ethenyl group, a propenyl group, and a butenyl group. The term “C 2 -C 60  alkenylene group” as used herein refers to a divalent group having a structure corresponding to 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 the terminus of the C 2 -C 60  alkyl group. Examples thereof include an ethynyl group and a propynyl group. The term “C 2 -C 60  alkynylene group” as used herein refers to a divalent group having a structure corresponding to 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 a C 1 -C 60  alkyl group). Examples thereof include a methoxy group, an ethoxy group, and an isopropyloxy group. 
     The term “C 3 -C 10  cycloalkyl group” as used herein refers to a monovalent saturated hydrocarbon monocyclic group including 3 to 10 carbon atoms. Examples of the C 3 -C 10  cycloalkyl group as used herein include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl (bicyclo[2.2.1]heptyl) group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, or a bicyclo[2.2.2]octyl group. The term “C 3 -C 10  cycloalkylene group” as used herein refers to a divalent group having a structure corresponding to the C 3 -C 10  cycloalkyl group. 
     The term “C 1 -C 10  heterocycloalkyl group” as used herein refers to a monovalent cyclic group including at least one heteroatom other than carbon atoms as a ring-forming atom and having 1 to 10 carbon atoms. Examples thereof include a 1,2,3,4-oxatriazolidinyl group, a tetrahydrofuranyl group, and a tetrahydrothienyl group. The term “C 1 -C 10  heterocycloalkylene group” as used herein refers to a divalent group having a structure corresponding to 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 its ring, and is not aromatic. Examples thereof include a cyclopentenyl group, a cyclohexenyl group, and a cycloheptenyl group. The term “C 3 -C 10  cycloalkenylene group” as used herein refers to a divalent group having a structure corresponding to the C 3 -C 10  cycloalkenyl group. 
     The term “C 1 -C 10  heterocycloalkenyl group” as used herein refers to a monovalent cyclic group including at least one heteroatom other than carbon atoms as a ring-forming atom, 1 to 10 carbon atoms, and at least one double bond in its ring. Examples of the C 1 -C 10  heterocycloalkenyl group include a 4,5-dihydro-1,2,3,4-oxatriazolyl group, a 2,3-dihydrofuranyl group, and a 2,3-dihydrothienyl group. The term “C 1 -C 10  heterocycloalkylene group” as used herein refers to a divalent group having a structure corresponding to the C 1 -C 10  heterocycloalkyl group. 
     The term “C 6 -C 60  aryl group” as used herein refers to a monovalent group having a carbocyclic aromatic system having 6 to 60 carbon atoms. The term “C 6 -C 60  arylene group” as used herein refers to a divalent group having a carbocyclic aromatic system having 6 to 60 carbon atoms. Examples of the C 6 -C 60  aryl group 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, and an ovalenyl group. When the C 6 -C 60  aryl group and the C 6 -C 60  arylene group each independently include two or more rings, the respective rings may be fused. 
     The term “C 1 -C 60  heteroaryl group” as used herein refers to a monovalent group having a heterocyclic aromatic system further including at least one heteroatom other than carbon atoms as a ring-forming atom and 1 to 60 carbon atoms. The term “C 1 -C 60  heteroarylene group” as used herein refers to a divalent group having a heterocyclic aromatic system further including at least one heteroatom other than carbon atoms as a ring-forming atom and 1 to 60 carbon atoms. Examples of the C 1 -C 60  heteroaryl group 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, and a naphthyridinyl group. When the C 1 -C 60  heteroaryl group and the C 1 -C 60  heteroarylene group each independently include two or more rings, the respective rings may be fused. 
     The term “monovalent non-aromatic fused polycyclic group” as used herein refers to a monovalent group that has two or more rings fused and only carbon atoms (e.g., 8 to 60 carbon atoms) as ring forming atoms, wherein the molecular structure when considered as a whole is non-aromatic. Examples of the monovalent non-aromatic fused polycyclic group include an indenyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, an indenophenanthrenyl group, and an indenoanthracenyl group. The term “divalent non-aromatic fused polycyclic group” as used herein refers to a divalent group having a structure corresponding to the monovalent non-aromatic fused polycyclic group. 
     The term “monovalent non-aromatic fused heteropolycyclic group” as used herein refers to a monovalent group that has two or more fused rings and at least one heteroatom other than carbon atoms (e.g., 1 to 60 carbon atoms), as a ring-forming atom, wherein the molecular structure when considered as a whole is non-aromatic. Examples of the monovalent non-aromatic fused heteropolycyclic group include a pyrrolyl group, a thienyl 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 benzothienyl group, a benzofuranyl group, a carbazolyl group, a dibenzosilolyl group, a dibenzothienyl group, a dibenzofuranyl group, an azacarbazolyl group, an azafluorenyl group, an azadibenzosilolyl group, an azadibenzothienyl 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 benzooxadiazolyl group, a benzothiadiazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an imidazotriazinyl group, an imidazopyrazinyl group, an imidazopyridazinyl group, an indenocarbazolyl group, an indolocarbazolyl group, a benzofurocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, a benzoindolocarbazolyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothienyl group, a benzonaphthosilolyl group, a benzofurodibenzofuranyl group, a benzofurodibenzothienyl group, and a benzothienodibenzothienyl group. The term “divalent non-aromatic fused heteropolycyclic group” as used herein refers to a divalent group having a structure corresponding to the monovalent non-aromatic fused heteropolycyclic group. 
     The term “C 6 -C 60  aryloxy group” as used herein indicates —OA 102  (wherein A 102  is a C 6 -C 60  aryl group), and a C 6 -C 60  arylthio group as used herein indicates —SA 103  (wherein A 103  is a C 6 -C 60  aryl group). 
     The term “C 7 -C 60  aryl alkyl group” used herein refers to -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  heteroaryl alkyl group” used herein refers to -A 106 A 107  (where A 106  may be a C 1 -C 59  alkylene group, and A 107  may be a C 1 -C 59  heteroaryl group). 
     The term “R 10a ” as used herein may be: 
     deuterium (-D), —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  aryl alkyl group, a C 2 -C 60  heteroaryl alkyl 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  aryl alkyl group, or a C 2 -C 60  heteroaryl alkyl 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  aryl alkyl group, a C 2 -C 60  heteroaryl alkyl 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 ). 
     Q 11  to Q 13 , Q 21  to Q 23  and Q 31  to Q 33  may each independently be: hydrogen; 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 or a C 1 -C 60  heterocyclic group, each 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; a C 7 -C 60  aryl alkyl group; or a C 2 -C 60  heteroaryl alkyl group. 
     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. 
     A third-row transition metal as used herein may include hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au). 
     As used herein, “Ph” represents a phenyl group, “Me” represents a methyl group, “Et” represents an ethyl group, “ter-Bu” or “But” represents a tert-butyl group, and “OMe” represents a methoxy group. 
     The term “biphenyl group” as used herein refers to a phenyl group substituted with a phenyl group. 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. The “terphenyl group” belongs to “a substituted phenyl group” having a “C 6 -C 60  aryl group substituted with a C 6 -C 60  aryl group” as a substituent. 
     The symbols * and *′ as used herein, unless defined otherwise, refer to a binding site to an adjacent atom in a corresponding formula or moiety. 
     Hereinafter, compounds and a light-emitting device according to one or more embodiments will be described in more detail with reference to Synthesis Examples and Examples. The wording “B was used instead of A” used in describing Synthesis Examples means that an amount of B used was identical to an amount of A used in terms of molar equivalents. 
     EXAMPLES 
     Evaluation Example 1: Evaluation of HOMO Energy Level 
     HOMO energy level (EHOMO) of the following compounds are measured. The results thereof are shown in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Compound 
                 E HOMO  (eV) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 HT13 
                 −5.249 
               
               
                   
                 HT16 
                 −5.025 
               
               
                   
                 HT18 
                 −5.201 
               
               
                   
                 HT28 
                 −4.787 
               
               
                   
                 HT3 
                 −4.813 
               
               
                   
                 A 
                 −5.350 
               
               
                   
                 B 
                 −5.065 
               
               
                   
                 F4-TCNQ 
                 −8.300 
               
               
                   
                 C 
                 −5.504 
               
               
                   
                   
               
            
           
         
       
     
     Example 1 
     As an anode, a substrate on which ITO were deposited was cut to a size of 50 millimeters (mm)×50 mm×0.5 mm, sonicated in isopropyl alcohol and pure water for 15 minutes in each solvent, cleaned with ultraviolet rays for 30 minutes, and then ozone, and mounted on a vacuum deposition apparatus. 
     The compounds HT13 and HT16 were co-deposited at a weight ratio of 1:1 on the ITO substrate to form a hole transport layer having a thickness of 1, 100 Å. 
     Compound 1-3 was deposited on the hole transport layer to form a first emission layer having a thickness of 5 nm, and Compound 1-3, Compound 2-16, Compound 3-11, and Compound 4-8 were co-deposited at a weight ratio of 7:3:1:0.1 on the first emission layer to form a second emission layer having a thickness of 30 nm. 
     Compound ET46 was deposited on the second emission layer to form a first buffer layer having a thickness of 5 nm. Compounds ET47 and lithium quinolate (Liq) were co-deposited at a weight ratio of 5:5 on the first buffer layer to form a second buffer layer having a thickness of 20 nm. Liq was deposited on the second buffer layer to form an electron transport layer having a thickness of 1 nm. 
     Elements Mg and Al were co-deposited at a weight ratio of 5:5 on the electron transport layer to form a cathode having a thickness of 10 nm, thereby completing the manufacture of a light-emitting device. 
     
       
         
         
             
             
         
       
     
     Example 2 and Comparative Examples 1 to 6 
     Light-emitting devices were manufactured in substantially the same manner as in Example 1, except that compounds, weight ratios, and thicknesses shown in Table 2 were used to form a hole transport layer. 
     Evaluation Example 2 
     The driving voltage of each of the light-emitting devices of Example 1, 2 and Comparative Examples 1 to 6 at a current density of 10 mA/cm 2  was measured to evaluate characteristics of the light-emitting devices. The driving voltages of the light-emitting devices were measured by using a source-measure unit sold under the trade designation Keithley Instrument 2400 series by Tektronix, Inc., of Beaverton, Oreg., based on 100% of the value of the light-emitting device of Comparative Example 2. The lifespan represents the time elapsed for the luminance to decrease to 95% of the initial luminance of 1,000 nit, based on 100% of the light-emitting device of Comparative Example 2. The evaluation results of the testing of the light-emitting devices are shown in Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 HTL 1 
                 HTL 2 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Thickness 
                   
                 Thickness 
                   
                 Driving 
               
               
                   
                 Compound 
                 (nm) 
                 Compound 
                 (nm) 
                 Lifespan 
                 voltage 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 HT13 + HT16 
                 1100 
                 — 
                 134% 
                 101% 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 (1:1) 
                   
                   
                   
                   
                   
               
               
                 Example 2 
                 HT13 + F4- 
                  100 
                 Compound 
                 1000 
                  97% 
                 105% 
               
               
                   
                 TCNQ 
                   
                 C 
                   
                   
                   
               
               
                   
                 (1:0.3) 
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 HT16 
                  550 
                 HT13 
                  550 
                 122% 
                 110% 
               
               
                 Example 1 
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Comparative 
                 HT16 
                 1100 
                 — 
                 100% 
                 100% 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 2 
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Comparative 
                 HT13 
                 1100 
                 — 
                 102% 
                 120% 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 3 
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 HT28 
                  550 
                 HT3 
                  550 
                  80% 
                  65% 
               
               
                 Example 4 
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 HAT-CN 
                  550 
                 Compound 
                  550 
                  37% 
                 213% 
               
               
                 Example 5 
                   
                   
                 A + B 
                   
                   
                   
               
               
                   
                   
                   
                 (1:1) 
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Comparative 
                 HT13 + HT18 
                 1100 
                 — 
                  94% 
                 105% 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 6 
                 (1:1) 
                   
                   
                   
                   
                   
               
               
                   
               
               
                                   
  
               
               
                                   
  
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
            
           
         
       
     
     Referring to the results of Table 2, the light-emitting device of Example 1 and 2 was found to have significant and unexpectedly excellent or equivalent luminescence efficiency and/or driving voltage, as compared with the light-emitting devices of Comparative Examples 1 to 6. 
     Although certain embodiments and implementations have been described herein, other embodiments and modifications will be apparent from this description. Accordingly, the inventive concepts are not limited to such embodiments, but rather to the broader scope of the appended claims and various obvious modifications and equivalent arrangements as would be apparent to a person of ordinary skill in the art.