Patent Publication Number: US-2021184151-A1

Title: Organic light-emitting device and apparatus including the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from and the benefit of Korean Patent Application No. 10-2019-0166006, filed on Dec. 12, 2019, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     Field 
     Exemplary implementations of the invention relate generally to an organic light-emitting device and an apparatus including the same. 
     Discussion of the Background 
     Organic light-emitting devices are self-emission devices that produce full-color images, and also have wide viewing angles, high contrast ratios, short response times, as well as excellent characteristics in terms of brightness, driving voltage, and response speed. 
     An example of the organic light-emitting devices may include a first electrode located on a substrate, and a hole transport region, an emission layer, an electron transport region, and a second electrode, which are sequentially located 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. These excitons transit from an excited state to a ground state, thereby generating light. Many existing organic light-emitting devices suffer from reduced lifespans during operation. 
     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 
     Applicant discovered that when electrons are not efficiently injected from the electron transport region to the emission layer in an organic light-emitting device (OLED), charges are accumulated at the interface between the emission layer and the electron transport region, thus deteriorating the interface and reducing the lifespan of the organic light-emitting device. 
     Organic light-emitting devices and apparatuses including the same constructed according to the principles and exemplary implementations of the invention herein have improved lifespans due to certain, defined material conditions and compositions. For example, in some exemplary implementations, an intermediate layer is provided between an emission layer and an electron transport region or a hole transport region or includes an intermediate layer in an emission layer to control the concentration and distribution of excitons in an emission region. Electron injection can be adjusted by controlling the attributes and composition of the intermediate layer, thereby significantly and surprisingly increasing lifespan of the organic light-emitting device by controlling exciton concentration and distribution in the emission layer. 
     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, an organic light-emitting device includes: a first electrode; a second electrode facing the first electrode; and an organic layer disposed between the first electrode and the second electrode, wherein, the organic layer further includes an emission region, and a first region disposed between the first electrode and the emission region, and a second region disposed between the emission region and the second electrode, the second region includes a first layer and a second layer, each including an organic compound, the emission region includes a first compound, a second compound, a third compound, and a fourth compound, the first compound includes a hole transporting host compound, the second compound includes an electron transporting host compound or a bipolar host compound, the third compound includes a thermally active delayed fluorescence compound or an organometallic complex, satisfying Equation 1 below, the fourth compound includes a fluorescent dopant, and at least one intermediate layer is disposed between the first region and the second region and includes at least one of the first compound, the second compound, and the fourth compound: 
       Δ E   ST ( C 3)≤about 0.3 eV  &lt;Equation 1&gt;
 
     wherein, in Equation 1, ΔE ST (C3) is a difference between a lowest excitation singlet energy level (E S1 (C3)) and a lowest excitation triplet energy level (E T1 (C3)) of the third compound. 
     The first electrode may be an anode, the second electrode may be a cathode, the first region is a hole transport region including a hole injection layer, a hole transport layer, an emission auxiliary layer, an electron blocking layer, or any combination thereof, and the second region may be an electron transport region including a hole blocking layer, a buffer layer, an electron injection layer, or any combination thereof. 
     The intermediate layer may be disposed between the first region and the emission region and may have the first compound, the fourth compound, or a combination thereof. 
     The intermediate layer may be disposed between the second region and the emission region and may have the second compound, the fourth compound, or a combination thereof. 
     The intermediate layer may be in direct contact with an emission layer. 
     The emission region may include a first emission layer and a second emission layer, and the intermediate layer may be disposed between the first emission layer and the second emission layer and may include the first compound, the second compound, the fourth compound, or any combination thereof. 
     The first compound, the second compound, and the third compound may be different from one another. 
     The interval between a maximum peak wavelength of an absorption spectrum of the fourth compound and a maximum peak wavelength of an emission spectrum of the fourth compound may be about 35 nm or less. 
     The concentration of the fourth compound in the emission layer may be about 1/3 or less of concentration of the third compound. 
     The first layer and the second layer may include a fifth compound and a sixth compound, respectively, the fifth compound may be different from the first compound, the second compound, and the fourth compound, and the sixth compound may be different from the first compound, the second compound, and the fourth compound. 
     The first compound may be of Formula 1: 
     
       
         
         
             
             
         
       
     
     wherein, the variables are defined herein. 
     The variable A 11  may be defined herein. 
     The electron transporting host compound may include an electron withdrawing group (EWG), and the bipolar host compound may include an EWG and an electron donating group (EDG). 
     The second compound may be of one of Formulae 2-1 and 2-2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 2-1, 2A, and 2-2, the variables are defined herein. 
     The third compound may be one of Formulae 3 and 4-1 to 4-3: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 3, 3A to 3D, and 4-1 to 4-3, the variables are defined herein. 
     The third compound may be of one of Formulae 3-1 and 3-2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 3-1 and 3-2, the variables are defined herein. 
     The variables D 41  and D 42  in Formulae 4-1 to 4-3 may be each, independently from one another, defined herein. 
     The fourth compound may be of one of Formulae 5(1), 5(2), 5(3), and 501: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 5(1) to 5(3), and 501, the variables are defined herein. 
     The first compound may be a compound of Group I; the second compound may be a compound of Group II; the third compound may be a compound of Group III-1 and Group III-2; and the fourth compound may be a compound of Group IV, as disclosed herein. 
     An apparatus may include: a thin-film transistor having a source electrode, a drain electrode, and an activation layer; and the organic light-emitting device as described above; wherein the first electrode of the organic light-emitting device may be electrically connected to one of the source electrode and the drain electrode of the thin-film transistor. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary 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 exemplary embodiments of the invention, and together with the description serve to explain the inventive concepts. 
         FIG. 1A  is a schematic cross-sectional diagram of an exemplary embodiment of an organic light-emitting device constructed according to the principles of the invention. 
         FIG. 1B  is a schematic cross-sectional diagram of the organic layer between electrodes shown in  FIG. 1A . 
         FIG. 1C  is a schematic cross-sectional diagram of an emission region of the organic layer of  FIG. 1B . 
         FIG. 1D  is a schematic cross-sectional diagram of a hole transport region of the organic layer of  FIG. 1B . 
         FIG. 1E  is a schematic cross-sectional diagram of an electron transport region of the organic layer of  FIG. 1B . 
         FIG. 2  is a schematic cross-sectional diagram of another exemplary embodiment of an organic light-emitting device constructed according to the principles of the invention. 
         FIG. 3  is a schematic cross-sectional diagram of yet another exemplary embodiment of an organic light-emitting device constructed according to the principles of the invention. 
         FIG. 4  is a schematic cross-sectional diagram of still another exemplary embodiment of an organic light-emitting device constructed according to the principles of the invention. 
         FIG. 5  is a schematic diagram of an exemplary embodiment of an apparatus containing an organic light-emitting device constructed according to 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 exemplary 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 exemplary 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 exemplary embodiments. Further, various exemplary embodiments may be different, but do not have to be exclusive. For example, specific shapes, configurations, and characteristics of an exemplary embodiment may be used or implemented in another exemplary embodiment without departing from the inventive concepts. 
     Unless otherwise specified, the illustrated exemplary embodiments are to be understood as providing exemplary 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 exemplary 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. 
     When an element, such as a layer, region, or component, is referred to as being “on,” “connected to,” or “coupled to” another element, layer, region, or component, it may be directly on, connected to, or coupled to the other element, layer, region, or component or intervening elements, layers, regions, or components may be present. When, however, an element, layer, region, or component is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element, layer, region, or component there are no intervening elements, layers, regions, or components 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 exemplary 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. 
     The term “organic layer” as used herein refers to a single layer and/or a plurality of layers located between the first electrode and the second electrode of the organic light-emitting device. A material included in the “organic layer” is not limited to an organic material. 
     Various exemplary embodiments are described herein with reference to sectional and/or exploded illustrations that are schematic illustrations of idealized exemplary 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, exemplary 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. 
     According to some exemplary embodiments of the invention an organic light-emitting device includes: a first electrode; a second electrode facing the first electrode; an organic layer between the first electrode and the second electrode, wherein the organic layer further includes an emission region including at least one emission layer, a first region between the first electrode and the emission region, and a second region between the emission region and the second electrode, the second region includes a first layer and a second layer, each including an organic compound, the emission layer includes a first compound, a second compound, a third compound, and a fourth compound, the first compound is a hole transporting host compound, the second compound is an electron transporting host compound or a bipolar host compound, the third compound is a thermally active delayed fluorescence (TADF) compound or an organometallic complex, satisfying Equation 1 below, the fourth compound is a fluorescent compound, and at least one intermediate layer located between the first region and the second region and including at least one of the first compound, the second compound, and the fourth compound is further located. 
       Δ E   ST ( C 3)≤about 0.3 eV  &lt;Equation 1&gt;
 
     In Equation 1, ΔE ST (C3) is the difference between a lowest excitation singlet energy level (E S1 (C3)) and a lowest excitation triplet energy level (E T1 (C3)) of the third compound. 
     In one exemplary embodiment, the first electrode is an anode, the second electrode is a cathode, the first region may include a hole transport region including a hole injection layer, a hole transport layer, an emission auxiliary layer, an electron blocking layer, or any combination thereof, and the second region may include an electron transport region including a hole blocking layer, a buffer layer, an electron transport layer, an electron injection layer, or any combination thereof. 
     For example, the first region may include a first hole transport layer and a second hole transport layer, wherein each of the first hole transport layer and the second hole transport layer may include an organic compound. For example, the second region may include a first electron transport layer and a second electron transport layer, wherein each of the first electron transport layer and the second electron transport layer may include an organic compound. 
     In one exemplary embodiment, the first layer and the second layer may be a first electron transport layer and a second electron transport layer, respectively. 
     In one exemplary embodiment, the intermediate layer may be located between the first region and the emission region and may include the first compound, the fourth compound, or a combination thereof. 
     For example, the intermediate layer may be located between the hole transport layer and the emission layer and may include the first compound, the fourth compound, or a combination thereof. 
     In one exemplary embodiment, the intermediate layer may be located between the second region and the emission region and may include the second compound, the fourth compound, or a combination thereof. 
     For example, the intermediate layer may be located between the electron transport layer and the emission layer and may include the second compound, the fourth compound, or a combination thereof. 
     In one exemplary embodiment, the intermediate layer may be in direct contact with the emission layer. 
     For example, the emission region may include n number of emission layers, and the emission region includes n−1 number of intermediate layers, wherein the n number of emission layers and the n−1 number of intermediate layers may be in direct contact with each other (where n is an integer of 1 or more). At this time, the n number of emission layers may each include the same material. 
     In one or more exemplary embodiments, the emission region may include two or more emission layers, and the intermediate layer may be in direct contact with the two or more emission layers. 
     In one exemplary embodiment, the emission region may include a first emission layer and a second emission layer, and the intermediate layer may be located between the first emission layer and the second emission layer and may include the first compound, the second compound, the fourth compound, or a combination thereof. 
     In one or more exemplary embodiments, the emission region may further include a third emission layer between the first emission layer and the second emission layer and may include a first intermediate layer between the first emission layer and the third emission layer and a second intermediate layer between the second emission layer and the third emission layer. 
     In one exemplary embodiment, the first compound, the second compound, and third compound may be different from one another. 
     In one exemplary embodiment, the interval between a maximum peak wavelength of an absorption spectrum of the fourth compound and a maximum peak wavelength of an emission spectrum of the fourth compound is about 35 nm or less. 
     The concentration of the fourth compound in the emission layer may be about 1/3 or less of concentration of the third compound. When concentration of the fourth compound exceeds about 1/3 of the concentration of the third compound, a charge balance is maintained with difficulty due to hole trap in the emission layer and the charge transfer delayed phenomenon, and thus lifespan characteristics may be deteriorated. 
     In one exemplary embodiment, the first layer and second layer may include a fifth compound and a sixth compound, respectively, wherein the fifth compound may be different from the first compound, the second compound, and the fourth compound, and the sixth compound may be different from the first compound, the second compound, and the fourth compound. 
     For example, the fifth compound is the hole transport material, and details thereof will be described below. For example, the sixth compound is an electron transport material, and details thereof will be described below. 
     In one exemplary embodiment, the first compound may be represented by Formula 1 below: 
     
       
         
         
             
             
         
       
     
     In Formula 1, 
     X 11  may be selected from O, S, N(R 19 ), and C(R 19 )(R 20 ), 
     R 11  to R 20  may each independently be selected from: a group represented by *-(L 11 ) a11 -A 11 , hydrogen, deuterium, a C 1 -C 60  alkyl group, a π electron-deficient nitrogen-free cyclic group, —C(Q 1 )(Q 2 )(Q 3 ), —Si(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), and —N(Q 1 )(Q 2 ); 
     a π electron-deficient nitrogen-free cyclic group substituted with at least one selected from deuterium, a C 1 -C 60  alkyl group, a π electron-deficient nitrogen-free cyclic group, —C(Q 31 )(Q 32 )(Q 33 ), —Si(Q 31 )(Q 32 )(Q 33 ), —B(Q 31 )(Q 32 ), and —N(Q 31 )(Q 32 ); and 
     a π electron-deficient nitrogen-free cyclic group, substituted with at least one of π electron-deficient nitrogen-free cyclic groups substituted with at least one selected from deuterium, a C 1 -C 60  alkyl group, a π electron-deficient nitrogen-free cyclic group, —C(Q 21 )(Q 22 )(Q 23 ), —Si(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), and —N(Q 21 )(Q 22 ), 
     L 11  may be selected from: a π electron-deficient nitrogen-free cyclic group, —C(Q 1 )(Q 2 )-, —Si(Q 1 )(Q 2 )-, —B(Q 31 )-, and —N(Q 32 )-; and 
     a π electron-deficient nitrogen-free cyclic group substituted with at least one selected from deuterium, a C 1 -C 60  alkyl group, a π electron-deficient nitrogen-free cyclic group, —C(Q 31 )(Q 32 )(Q 33 ), —Si(Q 31 )(Q 32 )(Q 33 ), —B(Q 31 )(Q 32 ), and —N(Q 31 )(Q 32 ), 
     a11 may be selected from 1, 2, and 3, 
     A 11  may be selected from: a π electron-deficient nitrogen-free cyclic group; 
     a π electron-deficient nitrogen-free cyclic group substituted with at least one selected from deuterium, a C 1 -C 60  alkyl group, a π electron-deficient nitrogen-free cyclic group, —C(Q 31 )(Q 32 )(Q 33 ), —Si(Q 31 )(Q 32 )(Q 33 ), —B(Q 31 )(Q 32 ), and —N(Q 31 )(Q 32 ); and 
     a π electron-deficient nitrogen-free cyclic group substituted with a π electron-deficient nitrogen-free cyclic group substituted with at least one selected from deuterium, a C 1 -C 60  alkyl group, a π electron-deficient nitrogen-free cyclic group, —C(Q 21 )(Q 22 )(Q 23 ), —Si(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), and —N(Q 21 )(Q 22 ), and 
     any two or more groups among R 11  to R 20  are optionally linked to each other to form one selected from: a π electron-deficient nitrogen-free cyclic group; and a π electron-deficient nitrogen-free cyclic group substituted with at least one selected from deuterium, a C 1 -C 60  alkyl group, a π electron-deficient nitrogen-free cyclic group, —C(Q 31 )(Q 32 )(Q 33 ), —Si(Q 31 )(Q 32 )(Q 33 ), —B(Q 31 )(Q 32 ), and —N(Q 31 )(Q 32 ). 
     For example, at least one of R 11  to R 19  in Formula 1 may be a group represented n by *-(L 11 ) a11 -A 11.    
     For example, R 11  to R 20  in Formula 1 may each independently be selected from: a group represented by *-(L 11 ) a11 -A 11 , hydrogen, deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —C(Q 1 )(Q 2 )(Q 3 ), —Si(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), and —N(Q 1 )(Q 2 ); 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —C(Q 31 )(Q 32 )(Q 33 ), —Si(Q 31 )(Q 32 )(Q 33 ), —B(Q 31 )(Q 32 ), and —N(Q 31 )(Q 32 ); and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group, each substituted with at least one selected from a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group that are each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —C(Q 21 )(Q 22 )(Q 23 ), —Si(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), and —N(Q 21 )(Q 22 ). 
     For example, L 11  in Formula 1 may be selected from: a benzene group, a naphthalene group, a phenalene group, an anthracene group, a fluoranthene group, a triphenylene group, a phenanthrene group, a pyrene group, a chrysene group, a perylene group, a fluorene group, a carbazole group, a dibenzofuran group, a dibenzothiophene group, —C(Q 1 )(Q 2 )-, and —Si(Q 1 )(Q 2 )-; and 
     a benzene group, a naphthalene group, a phenalene group, an anthracene group, a fluoranthene group, a triphenylene group, a phenanthrene group, a pyrene group, a chrysene group, a perylene group, a fluorene group, a carbazole group, a dibenzofuran group, and a dibenzothiophene group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —C(Q 31 )(Q 32 )(Q 33 ), and —Si(Q 31 )(Q 32 )(Q 33 ). 
     In one exemplary embodiment, L 11  in Formula 1 may be selected from: a benzene group, a fluorene group, a carbazole group, a dibenzofuran group, and a dibenzothiophene group, —C(Q 1 )(Q 2 )-, and —Si(Q 1 )(Q 2 )-; and 
     a benzene group, a fluorene group, a carbazole group, a dibenzofuran group, and a dibenzothiophene group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, —C(Q 31 )(Q 32 )(Q 33 ), and —Si(Q 31 )(Q 32 )(Q 33 ). 
     For example, a11 in Formula 1 may be 1 or 2. 
     For example, A 11  in Formula 1 may be selected from: a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —C(Q 31 )(Q 32 )(Q 33 ), —Si(Q 31 )(Q 32 )(Q 33 ), —B(Q 31 )(Q 32 ), and —N(Q 31 )(Q 32 ); and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group, each substituted with at least one selected from a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group that are each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —C(Q 21 )(Q 22 )(Q 23 ), —Si(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), and —N(Q 21 )(Q 22 ). 
     In one exemplary embodiment, A 11  in Formula 1 may be selected from: a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, —C(Q 31 )(Q 32 )(Q 33 ), and —Si(Q 31 )(Q 32 )(Q 33 ); and 
     a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, each substituted with at least one selected from a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group that are each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, —C(Q 21 )(Q 22 )(Q 23 ), and —Si(Q 21 )(Q 22 )(Q 23 ). 
     In one or more exemplary embodiments, A 11  in Formula 1 may be represented by one of Formulae 8-1 to 8-5 below: 
     
       
         
         
             
             
         
       
     
     In Formulae 8-1 to 8-5, 
     X 81  may be selected from O, S, N(R 89 ), and C(R 89 )(R 90 ), 
     R 81  to R 90  may each independently be selected from hydrogen, deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, and 
     * indicates a binding site to a neighboring atom. 
     In one exemplary embodiment, the first compound may be represented by one of Formulae 1-1 to 1-15 below: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 1-1 to 1-15, 
     L 11 , a11, A 11 , and R 11  to R 20  are each the same as defined in Formula 1, and 
     R 11a , R 11b , R 12a , R 12b , R 13a , R 13b , R 14a , R 14b , R 15a , R 15b , R 16a , R 16b , R 17a , R 17b , R 18a , and R 18b  are the same as defined in connection with R 11  to R 20  in Formula 1. 
     In one exemplary embodiment, the electron transport host compound may include an electron withdrawing group (EWG), and 
     the bipolar host compound may include an EWG and an electron donating group (EDG). 
     For example, the EWG may include at least one selected from a π electron-deficient nitrogen-containing ring, —F, —Cl, —Br, —I, a cyano group, and a C 1 -C 60  alkyl group substituted with at least one selected from —F, —Cl, —Br, —I, and a cyano group, and the EDG may include at least one selected from a carbazole group and an amine group. 
     In one exemplary embodiment, the second compound may be represented by one of Formulae 2-1 and 2-2: 
     
       
         
         
             
             
         
       
     
     In Formulae 2-1, 2A, and 2-2, 
     Yi may be selected from a single bond, —O—, —S—, —C(R 24 )(R 25 )—, —N(R 24 )—, Si(R 24 )(R 25 )—, —C(═O)—, —S(═O) 2 —, —B(R 24 )—, —P(R 24 )—, and —P(═O)(R 24 )(R 25 )—, 
     k1 may be 0 or 1, 
     CY 21  and CY 22  may each independently be a C 5 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, 
     L 21  to L 27  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     Ar 21  to Ar 27  may each independently be selected from a group represented by Formula 2A, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, 
     at least one of Ar 21  to Ar 23  may be a group represented by Formula 2A, 
     a21 to a27 may each independently be an integer from 0 to 3, 
     b21 to b27 may each independently be an integer from 1 to 8, 
     c21 and c22 may each independently be an integer from 1 to 20, 
     c23 is an integer from 1 to 5; 
     n21 and n22 may each independently be an integer from 1 to 8, 
     X 21  to X 23  may each independently be C or N, 
     when each of X 21  to X 23  is C, at least one selected from R 21  to R 23  may be —F, a cyano group, or a C 1 -C 60  alkyl group substituted with at least one selected from —F and a cyano group, 
     X 24  to X 26  may each independently be C(R 26 ) or N, 
     at least one selected from X 24  to X 26  may be N, 
     R 21  to R 26  may each independently be selected from *-(L 27 ) a27 -(Ar 27 ) b27 , hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted C 1 -C 60  heteroaryloxy group, a substituted or unsubstituted C 1 -C 60  heteroarylthio group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —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 ), and —P(═O)(Q 1 )(Q 2 ), 
     any two or more groups among Ar 21  to Ar 27  and R 21  to R 26  may optionally be linked to each other to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     * indicates a binding site to a neighboring atom, and 
     at least one substituent selected from the substituted C 5 -C 60  carbocyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 6 -C 60  aryloxy group, the substituted C 6 -C 60  arylthio group, the substituted C 1 -C 60  heteroaryl group, the substituted C 1 -C 60 heteroaryloxy group, the substituted C 1 -C 60  heteroarylthio group, the substituted monovalent non-aromatic condensed polycyclic group, and the substituted monovalent non-aromatic condensed heteropoly cyclic group may be selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic 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 ), and —P(═O)(Q 11 )(Q 12 ); 
     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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono 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 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic 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 ), and —P(═O)(Q 21 )(Q 22 ); and 
     —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 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein 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 selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono 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 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, a biphenyl group, and a terphenyl group. 
     In one exemplary embodiment, CY 21  and CY 22  in Formula 2 may each independently be selected from a benzene group, a naphthalene 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 pentaphene group, an indenoanthracene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, 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 pyrimidine group, a pyridazine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline 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 ibenzoxazole 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 azafluorene group, and an azacarbazole group. 
     In one exemplary embodiment, CY 21  and CY 22  in Formula 2 may each independently be selected from a benzene group, a naphthalene group, a fluorene group, a carbazole group, a pyridine group, a phenanthroline group, an azafluorene group, and an azacarbazole group. 
     In one exemplary embodiment, L 21  to L 27  in Formula 2 may each independently be selected from: a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group; and 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl 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 hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, —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 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     In one exemplary embodiment, Ar 21  to Ar 27  in Formula 2 may each independently be a group represented by one selected from Formulae 5-1 to 5-26 and 6-1 to 6-55, and at least one selected from Ar 21  to Ar 23 , may be a group represented by one selected from Formulae 6-1 to 6-55: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 5-1 to 5-26 and 6-1 to 6-55, 
     Y 31  and Y 32  may each independently be O, S, C(Z 34 )(Z 35 ), N(Z 34 ), or Si(Z 34 )(Z 35 ), 
     Z 31  to Z 35  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkenyl group, a C 1 -C 20  alkynyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a triperylenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group, 
     e2 may be 1 or 2, 
     e3 is an integer from 1 to 3, 
     e4 is an integer from 1 to 4, 
     e5 is an integer from 1 to 5, 
     e6 may be an integer from 1 to 6, 
     e1 may be an integer from 1 to 7, 
     e9 may be an integer from 1 to 9, and 
     * indicates a binding site to a neighboring atom. 
     In one exemplary embodiment, R 21  to R 26  in Formula 2 may each independently be selected from: *-(L 27 ) a27 -(Ar 27 ) b27 , 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 cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a spiro-fluorene-benzofluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a pyrenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, a silolyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indolyl group, an isoindolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an oxadiazolyl group, a triazinyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a carbazolyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a thiadiazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an oxazolopyridinyl group, a thiazolopyridinyl group, a benzonaphthyridinyl group, an azafluorenyl group, an azaspiro-bifluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, an azadibenzosilolyl group, a biphenyl group, and a terphenyl group; and 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a spiro-fluorene-benzofluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a pyrenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, a silolyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indolyl group, an isoindolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an oxadiazolyl group, a triazinyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a carbazolyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a thiadiazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an oxazolopyridinyl group, a thiazolopyridinyl group, a benzonaphthyridinyl group, an azafluorenyl group, an azaspiro-bifluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, an azadibenzosilolyl group, a biphenyl group, and a terphenyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a spiro-fluorene-benzofluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a pyrenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, a silolyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indolyl group, an isoindolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an oxadiazolyl group, a triazinyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a carbazolyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a thiadiazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an oxazolopyridinyl group, a thiazolopyridinyl group, a benzonaphthyridinyl group, an azafluorenyl group, an azaspiro-bifluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, an azadibenzosilolyl group, a biphenyl group, and a terphenyl group. 
     In one exemplary embodiment, the third compound may be represented by one selected from Formulae 3 and 4-1 to 4-3: 
     
       
         
         
             
             
         
       
     
     In Formulae 3, 3A to 3D, and 4-1 to 4-3, 
     M 31  may be selected from transition metals of Period 1, Period 2, and Period 3 of the Periodic Table of Elements, 
     L 31  may be a ligand represented by one selected from Formulae 3A to 3D, 
     L 32  may be selected from a monodentate ligand, a bidentate ligand, and a tridentate ligand, 
     n31 may be 1 or 2, 
     n32 may be selected from 0, 1, 2, 3, and 4, 
     A 31  to A 34  may each independently be selected from a C 5 -C 30  carbocyclic group, and a C 1 -C 30  heterocyclic group, 
     T 31  to T 34  may each independently be selected from 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 )—*′, and *—P(R 35 )—*′, 
     k31 to k34 may each independently be selected from 1, 2, and 3, 
     Y 31  to Y 34  may each independently be selected from a single bond, *—O—*′, *—S—*′, *—C(R 37 )(R 38 )—*′, *—Si(R 37 )(R 38 )—*′, *—B(R 37 )—*′, *—N(R 37 )—*′, and *—P(R 37 )—*′, 
     * 1 , * 2 , * 3 , and * 4  each indicate a binding site to M 31 , 
     R 31  to R 38  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —C(Q 1 )(Q 2 )(Q 3 ), —Si(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), —N(Q 1 )(Q 2 ), —P(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O)(Q 1 ), —S(═O) 2 (Q 1 ), —P(═O)(Q 1 )(Q 2 ), and —P(═S)(Q 1 )(Q 2 ), wherein R 31  to R 38  may optionally be linked to each other to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     b31 to b34 may each independently be an integer from 0 to 10, 
     A 41  and A 42  may each independently be selected from: a π electron-deficient nitrogen-containing cyclic group, —Si(R 41 )(R 42 )(R 43 ), —B(R 41 )(R 42 ), and —N(R 41 )(R 42 ); 
     a π electron-deficient nitrogen-containing cyclic group substituted with at least one selected from deuterium, a C 1 -C 60  alkyl group, π electron-deficient nitrogen-containing cyclic group, —C(Q 31 )(Q 32 )(Q 33 ), —Si(Q 31 )(Q 32 )(Q 33 ), —B(Q 31 )(Q 32 ), and —N(Q 31 )(Q 32 ); and 
     an π electron-deficient nitrogen-containing cyclic group, substituted with at least one selected from % electron-deficient nitrogen-containing cyclic groups, each substituted with at least one selected from deuterium, a C 1 -C 60  alkyl group, a % electron-deficient nitrogen-containing cyclic group, —C(Q 21 )(Q 22 )(Q 23 ), —Si(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), and —N(Q 21 )(Q 22 ), 
     R 41  to R 43  may each independently be optionally linked to L 41  or L 42  via *-(z)-*′ to form a C 5 -C 30  carbocyclic group and a C 1 -C 30  heterocyclic group, 
     z may be selected from a single bond, O, S, N, Se, Si(R 44 )(R 45 ), and C(R 44 )(R 45 ), 
     R 41  and R 45  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid or a salt thereof, a sulfonic acid or a salt thereof, a phosphoric acid or a salt thereof, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10 heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —C(Q 1 )(Q 2 )(Q 3 ), —Si(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), —N(Q 1 )(Q 2 ), —P(QI)(Q 2 ), —C(═O)(Q 1 ), —S(═O)(Q 1 ), —S(═O) 2 (Q 1 ), —P(═O)(Q 1 )(Q 2 ), and —P(═S)(Q 1 )(Q 2 ), 
     m41 and m42 may each independently be selected from 1, 2, and 3, 
     D 41  and D 42  may each independently be selected from: —F, a cyano group, a π electron-deficient nitrogen-free cyclic group, a C(═O)-containing group, a P(═O)-containing group, and a P(═S)-containing group; 
     a π electron-deficient nitrogen-free cyclic group, a C(═O)-containing group, a P(═O)-containing group, and a P(═S)-containing group, each substituted with at least one selected from deuterium, —F, a cyano group, a C 1 -C 60  alkyl group, and a π electron-deficient nitrogen-free cyclic group; 
     a C 1 -C 60  alkyl group and a π electron-deficient nitrogen-free cyclic group, each substituted with at least one selected from —F, a cyano group, and a π electron-deficient nitrogen free-cyclic group; 
     a π electron-deficient nitrogen-free cyclic group, a C(═O)-containing group, a P(═O)-containing group, and a P(═S)-containing group, each substituted with at least one selected from a C 1 -C 60  alkyl group and a π electron-deficient nitrogen-free cyclic group, each substituted with at least one selected from deuterium, —F, a cyano group, a C 1 -C 60  alkyl group, and a π electron-deficient nitrogen-free cyclic group; 
     a C 1 -C 60  alkyl group and a π electron-deficient nitrogen-free cyclic group, each substituted with at least one selected from π electron-deficient nitrogen-free cyclic groups, each substituted with at least one selected from deuterium, —F, a cyano group, a C 1 -C 60  alkyl group, and a π electron-deficient nitrogen-free cyclic group; and 
     a C 1 -C 60  alkyl group and a π electron-deficient nitrogen-free cyclic group, each substituted with at least one selected from a C 1 -C 60  alkyl group and a π electron-deficient nitrogen-free cyclic group, each substituted with at least one selected from —F, a cyano group, and a π electron-deficient nitrogen-free cyclic group, 
     n41 and n42 may each independently be selected from 1, 2, and 3, 
     L 41  and L 42  may each independently be a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     a41 and a42 may each independently be selected from 0, 1, 2, and 3, and 
     at least one substituent of the substituted C 5 -C 60  carbocyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10 heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 6 -C 60  aryloxy group, the substituted C 6 -C 60  arylthio group, the substituted C 1 -C 60  heteroaryl group, the substituted monovalent non-aromatic condensed polycyclic group, the substituted monovalent non-aromatic condensed heteropolycyclic group, the substituted C 5 -C 60  carbocyclic group, and the substituted C 1 -C 60  heterocyclic group may be selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic 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 ), and —P(═O)(Q 11 )(Q 12 ); 
     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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono 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 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic 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 ), and —P(═O)(Q 21 )(Q 22 ); and 
     —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 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein 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 selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono 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 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, a biphenyl group, and a terphenyl group. 
     For example, M 31  in Formula 3 may be selected from platinum (Pt), palladium (Pd), copper (Cu), silver (Ag), gold (Au), rhodium (Rh), iridium (Ir), ruthenium (Ru), osmium (Os), titanium (Ti), zirconium (Zr), hafnium (Hf), europium (Eu), terbium (Tb), and thulium (Tm). 
     In one exemplary embodiment, M 31  in Formula 3 may be selected from Pt and Ir. 
     For example, A 31  to A 34  in Formulae 3A to 3D may each independently be i) a first ring, ii) a second ring, iii) a condensed ring in which two or more first rings are condensed with each other, iv) a condensed ring in which two or more second rings are condensed with each other, or v) a condensed ring in which one or more first rings and one or more second rings are condensed with each other, 
     wherein the first ring is selected from a cyclopentane group, a cyclopentene group, a cyclopentadiene group, a furan group, a thiophene group, a pyrrole group, a borole group, a phosphole group, a silole group, a germole group, a selenophene group, an oxazole group, a dihydroxazole group, an isoxazole group, a dihydroisoxazole group, an oxadiazole group, a dihydroxaddiazole group, an isozadiazole group, a dihydroisozadiazole group, an oxatriazole group, a dihydroxatriazole group, an isoxatriazole group, a dihydroisoxatriazole group, a thiazole group, a dihydrothiazole group, an isothiazole group, a dihydroisothiazole group, a thiadiazole group, a dihydrothiadiazole group, an isothiadiazole group, a dihydroisothiadiazole group, a thiatriazole group, a dihydrothiatriazole group, an isothiatriazole group, a dihydroisothiatriazole group, a pyrazole group, a dihydropyrazole group, an imidazole group, a dihydroimidazole group, a triazole group, a dihydrotriazole group, a tetrazole group, a dihydrotetrazole group, an azasilole group, a diazasilole group, and a triazasilole group, and 
     the second ring may be selected from a cyclohexane group, a cyclohexene group, a cyclohexadiene group, an admantane group, a norbomane group, a norbornene group, a benzene group, a pyridine group, a dihydropyridine group, a tetrahydropyridine group, a pyrimidine group, a dihydropyrimidine group, a tetrahydropyrimidine group, a pyrazine group, a dihydropyrazine group, a tetrahydropyrazine group, a pyridazine group, a dihydropyridazine group, a tetrahydropyridazine group, and a triazine group. 
     In one exemplary embodiment, A 31  to A 34  in Formulae 3A to 3D may each independently be selected from a benzene group, a naphthalene group, an anthracene group, a phenanthrene group, a triphenylene group, a pyrene group, a chrysene group, a furan group, a thiophene group, a silole group, an indene group, a fluorene group, a benzofuran group, a dibenzofuran group, a benzothiophene group, a dibenzothiophene group, a benzosilole group, a dibenzosilole group, an indole group, a carbazole group, an indenopyridine group, an indolopyridine group, a benzofuropyridine group, a benzothienopyridine group, a ibenzosilolopyridine group, an indenopyrimidine group, an indolopyrimidine group, a benzofuropyrimidine group, a benzothienopyrimidine group, a benzosilolopyrimidine group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a quinoline group, an isoquinoline group, a quinoxaline group, a quinazoline group, a cinnoline group, a phthalazine group, a phenanthroline group, a pyrrole group, a pyrazole group, an imidazole group, a dihydroimidazole group, a triazole group, a dihydrotriazole group, an oxazole group, a dihydroxazole group, an isoxazole group, a thiazole group, a dihydrothiazole group, an isothiazole group, an oxadiazole group, a dihydroxaddiazole group, a thiadiazole group, a dihydrothiadiazole group, a benzopyrazole group, a benzimidazole group, a dihydrobenzimidazole group, an imidazopyridine group, an imidazopyrimidine group, an imidazopyrazine group, a benzoxazole group, a dihydrobenzoxazole group, a benzothiazole group, a dihydrobenzothiazole group, a benzoxadiazole group, a dihydrobenzoxadiazole group, a benzothiadiazole group, and a dihydrobenzothiadiazole group. 
     For example, T 31  to T 34  in Formulae 3A to 3D may each independently be selected from a single bond, a double bond, *—O—*′, *—S—*′, *—C(R 35 )(R 36 )—*′, and *—N(R 35 )—*′. 
     For example, Y 31  to Y 34  in Formulae 3A to 3D may each independently be selected from a single bond, *—O—*′, and *—S—*′. 
     For example, R 31  to R 38  in Formulae 3A to 3D may each independently be selected from: hydrogen, deuterium, —F, —Cl, —Br, —I, a cyano group, a C 1 -C 20  alkyl group, and a C 1 -C 10  alkoxy group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, a diazafluorenyl group, a diazacarbazolyl group, a diazadibenzofuranyl group, and a diazadibenzothiophenyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, a diazafluorenyl group, a diazacarbazolyl group, a diazadibenzofuranyl group, and a diazadibenzothiophenyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, a diazafluorenyl group, a diazacarbazolyl group, a diazadibenzofuranyl group, and a diazadibenzothiophenyl group; and 
     —B(Q 1 )(Q 2 ) and —N(Q 1 )(Q 2 ), and 
     Q 1  and Q 2  may each independently be selected from: hydrogen, deuterium, and a C 1 -C 20  alkyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, a diazafluorenyl group, a diazacarbazolyl group, a diazadibenzofuranyl group, and a diazadibenzothiophenyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, a diazafluorenyl group, a diazacarbazolyl group, a diazadibenzofuranyl group, and a diazadibenzothiophenyl group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, a diazafluorenyl group, a diazacarbazolyl group, a diazadibenzofuranyl group, and a diazadibenzothiophenyl group. 
     In one exemplary embodiment, R 31  to R 38  in Formulae 3A to 3D may each independently be selected from: hydrogen, deuterium, —F, —Cl, —Br, —I, a cyano group, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a methoxy group, an ethoxy group, a propoxy group and butoxy group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a cyano group, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a methoxy group, an ethoxy group, a propoxy group, a butoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; and 
     —B(Q 1 )(Q 2 ) and —N(Q 1 )(Q 2 ), and 
     Q 1  and Q 2  may each independently be selected from: hydrogen, deuterium, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, each substituted with at least one selected from deuterium, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group. 
     In one exemplary embodiment, the third compound may be represented by one selected from Formulae 3-1 and 3-2 below: 
     
       
         
         
             
             
         
       
     
     In Formulae 3-1 and 3-2, 
     X 31  to X 40  may each independently be selected from N and C, and 
     the remaining components may each be understood by referring to the corresponding descriptions thereof provided herein. 
     In Formulae 3-1 and 3-2, X 31  and X 32  may each independently be a ring member of A 31 , and X 33  to X 40  may be also understood by referring to descriptions provided in connection with Formulae 3-1 and 3-2, X 31 , and X 32.    
     For example, D 41  and D 42  in Formulae 4-1 to 4-3 may each independently be a group represented by Formula 12: 
     
       
         
         
             
             
         
       
     
     In Formula 12, 
     X 121  may be selected from O, S, N(R 123 ), and C(R 123 )(R 124 ), 
     X 122  may be selected from a single bond, O, S, N(R 125 ), and C(R 125 )(R 126 ), 
     A 121  and A 122  may be a π electron-deficient nitrogen-free cyclic group, 
     R 121  to R 126  may each independently be selected from: a binding site, hydrogen, deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —B(Q 31 )(Q 32 ), and —N(Q 31 )(Q 32 ); and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), and —N(Q 21 )(Q 22 ), wherein R 123  and R 124  may be optionally linked to each other to form a π electron-deficient nitrogen-free cyclic group, R 125  and R 126  may be optionally linked to each other to form a π electron-deficient nitrogen-free cyclic group, and at least one selected from R 121  to R 126  is a binding site, and 
     b122 and b122 may each independently be selected from 1, 2, 3, 4, 5, and 6, 
     wherein Q 21  to Q 23  and Q 31  to Q 33  are the same as described above. 
     For example, A 41  and A 42  in Formulae 4-1 to 4-3 may each independently be selected from: —F, a cyano group, a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isoxazole group, an oxadiazole group, an isozadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, an isothiadiazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, a triazine group, a quinoline group, an isoquinoline group, a naphthyridine group, a quinoxaline group, a quinazoline group, —B(R 41 )(R 42 ), and groups represented by Formulae 13-1 to 13-2; 
     a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isoxazole group, an oxadiazole group, an isozadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, an isothiadiazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, a triazine group, a quinoline group, an isoquinoline group, a naphthyridine group, a quinoxaline group, and a quinazoline group, each substituted with at least one selected from deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazolyl group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group; 
     a C 1 -C 20  alkyl group, a benzene group, a biphenyl group, a terphenyl group, a naphthalene group, a phenanthrene group, a triphenylene group, a chrysene group, a fluoranthene group, a fluorene group, a carbazole group, a dibenzofuran group, a dibenzothiophene group, a benzofluorene group, a benzocarbazole group, a benzonaphthofuran group, a benzonaphthothiophene group, a dibenzofluorene group, a dibenzocarbazole group, a dinaphthofuran group, and a dinaphthothiophene group, each substituted with at least one selected from —F, a cyano group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazolyl group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group; 
     a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isoxazole group, an oxadiazole group, an isozadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, an isothiadiazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, a triazine group, a quinoline group, an isoquinoline group, a naphthyridine group, a quinoxaline group, and a quinazoline group, each substituted with at least one selected from a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazolyl group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group that are each substituted with at least one selected from deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazolyl group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group; 
     a C 1 -C 20  alkyl group, a benzene group, a biphenyl group, a terphenyl group, a naphthalene group, a phenanthrene group, a triphenylene group, a chrysene group, a fluoranthene group, a fluorene group, a carbazole group, a dibenzofuran group, a dibenzothiophene group, a benzofluorene group, a benzocarbazole group, a benzonaphthofuran group, a benzonaphthothiophene group, a dibenzofluorene group, a dibenzocarbazole group, a dinaphthofuran group, and a dinaphthothiophene group, each substituted with at least one selected from a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazolyl group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group that are each substituted with at least one selected from deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazolyl group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group; and 
     a C 1 -C 20  alkyl group, a benzene group, a biphenyl group, a terphenyl group, a naphthalene group, a phenanthrene group, a triphenylene group, a chrysene group, a fluoranthene group, a fluorene group, a carbazole group, a dibenzofuran group, a dibenzothiophene group, a benzofluorene group, a benzocarbazole group, a benzonaphthofuran group, a benzonaphthothiophene group, a dibenzofluorene group, a dibenzocarbazole group, a dinaphthofuran group, and a dinaphthothiophene group, each substituted with at least one selected from a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, and a dinaphthothiophenyl group that are each substituted with at least one selected from —F, a cyano group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazolyl group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group: 
     
       
         
         
             
             
         
       
     
     In Formulae 13-1 to 13-2, 
     A 41  to A 43  may each independently be selected from a C 5 -C 30  carbocyclic group and a C 1 -C 30  heterocyclic group, 
     R 42  and R 441  to R 443  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid or a salt thereof, a sulfonic acid or a salt thereof, a phosphoric acid or a salt thereof, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10 heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —C(Q 1 )(Q 2 )(Q 3 ), —Si(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), —N(Q 1 )(Q 2 ), —P(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O)(Q 1 ), —S(═O) 2 (Q 1 ), —P(═O)(Q 1 )(Q 2 ), and —P(═S)(Q 1 )(Q 2 ), 
     b441 to b443 may each independently be an integer from 0 to 10, 
     Z 41  and Z 42  are the same as described in connection with z, and 
     * indicates a binding site to a neighboring group. 
     For example, A 41  to A 43  in Formulae 13-1 to 13-2 may each independently be selected from a benzene group, a naphthalene group, a phenalene group, an anthracene group, a fluoranthene group, a triphenylene group, a phenanthrene group, a pyrene group, a chrysene group, a perylene group, a fluorene group, a carbazole group, a dibenzofuran group, and a dibenzothiophene group. 
     For example, R 42  and R 441  to R 443  in Formulae 13-1 to 13-2 may each independently be selected from: hydrogen, deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazole group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group; and 
     a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazole group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group, each substituted with at least one selected from deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, an oxazolyl group, an isoxazolyl group, an oxadiazolyl group, an isoxadiazole group, a thiazolyl group, an isothiazolyl group, a thiadiazolyl group, an isothiadiazolyl group, a pyridinyl group, a pyrazinyl group, a pyridazinyl group, a pyrimidinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a naphthyridinyl group, a quinoxalinyl group, and a quinazolinyl group, and 
     b441 to b443 may each independently be selected from 1, 2, 3, 4, 5, and 6. 
     For example, L 41  and L 42  in Formulae 4-1 to 4-3 may each independently be selected from: a benzene group, a naphthalene group, a phenalene group, an anthracene group, a fluoranthene group, a triphenylene group, a phenanthrene group, a pyrene group, a chrysene group, a perylene group, a fluorene group, a carbazole group, a dibenzofuran group, a dibenzothiophene group, —C(Q 1 )(Q 2 )-, and —Si(Q 1 )(Q 2 )-; and 
     a benzene group, a naphthalene group, a phenalene group, an anthracene group, a fluoranthene group, a triphenylene group, a phenanthrene group, a pyrene group, a chrysene group, a perylene group, a fluorene group, a carbazole group, a dibenzofuran group, and a dibenzothiophene group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a triphenylenyl group, a chrysenyl group, a fluoranthenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzofluorenyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, —C(Q 31 )(Q 32 )(Q 33 ), and —Si(Q 31 )(Q 32 )(Q 33 ), 
     wherein Q 1 , Q 2 , and Q 31  to Q 33  are the same as described above. 
     In one exemplary embodiment, L 41  and L 42  in Formulae 4-1 to 4-3 may each independently be selected from: a benzene group, a fluorene group, a carbazole group, a dibenzofuran group, and a dibenzothiophene group, —C(Q 1 )(Q 2 )-, and —Si(Q 1 )(Q 2 )-; and 
     a benzene group, a fluorene group, a carbazole group, a dibenzofuran group, and a dibenzothiophene group, each substituted with at least one selected from deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, —C(Q 31 )(Q 32 )(Q 33 ), and —Si(Q 31 )(Q 32 )(Q 33 ), 
     wherein Q 1 , Q 2 , and Q 31  to Q 33  are the same as described above. 
     In one exemplary embodiment, the fourth compound may be represented by one of Formulae 5(1), 5(2), 5(3), and 501: 
     
       
         
         
             
             
         
       
     
     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 55 ), C(R 55 )(R 56 ), or Si(R 55 )(R 56 ), 
     X 56  is N, C(R 55 ), or Si(R 55 ); 
     m51 and m54 may each independently be 0, 1, or 2, wherein, when m51 is 0, A 51  and A 52  are not linked to each other, and when m54 is 0, A 53  and A 54  are not linked 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, 
     a51 to a54 and a60 may each independently be an integer from 0 to 10; 
     R 51  to R 60  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —N(Q 1 )(Q 2 ), —P(═O)(Q 1 )(Q 2 ), and —S(═O) 2 (Q 1 )(Q 2 ), 
     in Formula 501, 
     Ar 501  may be a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     L 501  to L 503  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     xd1 to xd3 may each independently be an integer from 0 to 3, 
     R 501  and R 502  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, 
     xd4 may be an integer from 1 to 6, and 
     at least one substituent of the substituted C 5 -C 60  carbocyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 6 -C 60  aryloxy group, the substituted C 6 -C 60  arylthio group, the substituted C 1 -C 60  heteroaryl group, the substituted C 1 -C 60  heteroaryloxy group, the substituted C 1 -C 60  heteroarylthio group, the substituted monovalent non-aromatic condensed polycyclic group, the substituted monovalent non-aromatic condensed heteropolycyclic group, the substituted C 3 -C 10  cycloalkylene group, the substituted C 1 -C 10  heterocycloalkylene group, the substituted C 3 -C 10  cycloalkenylene group, the substituted C 1 -C 10  heterocycloalkenylene group, the substituted C 6 -C 60  arylene group, the substituted C 1 -C 60  heteroarylene group, a substituted divalent non-aromatic condensed polycyclic group, and the substituted divalent non-aromatic condensed heteropoly cyclic group may be selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 11 )(Q 12 )(Q 13 ), —N(Q 11 )(Q 12 ), —B(QU)(Q 12 ), —C(═O)(Q 11 ), —S(═O) 2 (Q 11 ), and —P(═O)(Q 11 )(Q 12 ); 
     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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono 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 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic 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 ), and —P(═O)(Q 21 )(Q 22 ); and 
     —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 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein 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 selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono 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 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, a biphenyl group, and a terphenyl group. 
     In one exemplary embodiment, the first compound may be selected from compounds of Group I, 
     the second compound may be selected from compounds of Group II, 
     the third compound is selected from compounds of Group III-1 and Group III-2, and 
     the fourth compound may be selected from compounds of Group IV: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The first compound and the second compound may not substantially emit light. 
     For example, the third compound and the fourth compound may emit light. 
     For example, the third compound may be a thermally activated delayed fluorescence emitter (TADF) compound and may be a TADF emitter. 
     For example, the third compound may be a phosphorescence metal complex and may be a phosphorescence emitter. 
     The third compound may have a maximum emission wavelength of about 450 nm to about 550 nm, but the exemplary embodiments are not limited thereto. 
     In detail, the third compound in the emission layer may receive energy from the formed exciton and emit blue delayed fluorescence or blue phosphorescence, without directly participating in forming excitons. 
     By including the above-described middle layer between the hole transport region and the emission layer, between the electron transport region and the emission layer, or inside the emission layer, exciton delocalization in which exciton is dispersed is achieved, resulting in increased efficiency and longer lifespan. 
     Furthermore, since the electron transport region may further include a two or more-layered organic layer, the electron moving speed can be easily adjusted to make the balance between holes and electrons, thereby reducing driving current and increasing driving lifespan. 
     In addition, since the emission layer simultaneously contains a first compound (hole transport compound), a second compound (electron transport compound), a third compound (organic compound or organometallic complex, which is ΔEst≤0.3), and a fourth compound (fluorescent dopant), the energy transfer to the dopant may be easily achieved, and due to hyper fluorescence mechanism, efficiency may be increased and triplet exciton bondage effects may be obtained. 
     In general, when electrons are not efficiently injected from the electron transport region to the emission layer, charges are accumulated at the interface between the emission layer and the electron transport region, thus deteriorating the interface. On the contrary, when holes are not efficiently injected from the hole transport region to the emission layer, charges are accumulated at the interface between the emission layer and the hole transport region, thus deteriorating the interface. Thus, the lifespan of the organic light-emitting device is lowered. 
     Since the second compound is a compound essentially including an electron transport group, the second compound may be easily used to adjust the electron transporting characteristics of the organic light-emitting device. Since the first compound is a compound essentially including a hole transport group, the first compound may be easily used to adjust the hole transporting characteristics of the organic light-emitting device. In this manner, it is possible to optimize a charge balance in the emission layer of the organic light-emitting device. 
     An amount of the first compound in the emission layer is in a range of about 10 wt % to about 90 wt % based on the total weight of the emission layer. 
     An amount of the second compound in the emission layer may be in a range of about 10 wt % to about 90 wt % based on the total weight of the emission layer. 
     An amount of the third compound in the emission layer may be smaller than or equal to the amount of the first compound and the amount of the second compound. 
     An amount of the fourth compound in the emission layer may be smaller than or equal to the amount of the first compound and the amount of the second compound. 
     The amount of the third compound in the emission layer may be greater than or equal to the amount of the fourth compound. 
     The amount of the fourth compound in the emission layer may be in a range of about 0.1 wt % to about 5 wt % based on the total weight of the emission layer. 
     The amount of the fourth compound may be in a range of about 0.01 parts by weight to about 30 parts by weight based on 100 parts by weight of the sum of the amount of the first compound and the amount of the second compound. 
     When the first compound, the second compound, the third compound, and the fourth compound are within these ranges, the organic light-emitting device having both improved efficiency and improved lifespan may be provided. 
     In one exemplary embodiment, the emission layer may include the first compound, the second compound, the third compound, and the fourth compound. 
     Description of  FIGS. 1A-E   
       FIG. 1A  is a schematic cross-sectional diagram of an exemplary embodiment of an organic light-emitting device constructed according to the principles of the invention.  FIG. 1B  is a schematic cross-sectional diagram of the organic layer between electrodes shown in  FIG. 1A .  FIG. 1C  is a schematic cross-sectional diagram of an emission region of the organic layer of  FIG. 1B .  FIG. 1D  is a schematic cross-sectional diagram of a hole transport region of the organic layer of  FIG. 1B .  FIG. 1E  is a schematic cross-sectional diagram of an electron transport region of the organic layer of  FIG. 1B . 
       FIG. 1A  is a schematic view of an organic light-emitting device  10  according to some exemplary embodiments. The organic light-emitting device  10  includes a first electrode  110 , an organic layer  150 , and a second electrode  200 . 
     Hereinafter, the structure of the organic light-emitting device  10  according to some exemplary embodiments and a method of manufacturing the organic 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  200 . The substrate may be a glass substrate or a plastic substrate, each having excellent mechanical strength, thermal stability, transparency, surface smoothness, ease of handling, and water resistance. 
     The first electrode  110  may be formed by depositing or sputtering a material for forming the first electrode  110  on the substrate. When the first electrode  110  is an anode, the material for forming the first electrode  110  may be selected from materials with a high work function to facilitate hole injection. 
     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 a first electrode  110  may be selected from an indium tin oxide (ITO), an indium zinc oxide (IZO), a tin oxide (SnO 2 ), a zinc oxide (ZnO), and any combinations thereof, but the exemplary embodiments are not limited thereto. In one or more exemplary embodiments, when the first electrode  110  is a semi-transmissive electrode or a reflective electrode, a material for forming a first electrode  110  may be selected from magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), and any combinations thereof, but the exemplary embodiments are not limited thereto. 
     The first electrode  110  may have a single-layered structure or a multi-layered structure including two or more layers. For example, the first electrode  110  may have a three-layered structure of ITO/Ag/ITO, but the structure of the first electrode  110  is not limited thereto. 
     Organic Layer  150   
     The organic layer  150  is located on the first electrode  110 . The organic layer  150  may include an emission region  160 . 
     The organic layer  150  may further include a first region  140 , for example, a hole transport region  140 , between the first electrode  110  and the emission region  160  and a second region  180 , for example, an electron transport region  180 , between the emission region  160  and the second electrode  200 . 
     Hole Transport Region  140  in the Organic Layer  150 ] 
     The hole transport region  140  may have i) a single-layered structure including a single layer including a single material, ii) a single-layered structure including 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  140  may include at least one layer selected from a hole injection layer  142 , a hole transport layer  144 , an emission auxiliary layer  146 , and an electron blocking layer  148 . 
     For example, the hole transport region  140  may have a single-layered structure including a single layer including a plurality of different materials or a multi-layered structure having 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 for each structure, constituting layers are sequentially stacked from the first electrode  110  in this stated order, but the structure of the hole transport region  140  is not limited thereto. 
     The hole transport region  140  may include at least one selected from 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), N,N′-di(l-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB orNPD), 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,N′-di(l-naphthyl)-N,N′-2,2′dimethyldiphenyl-(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/camphor sulfonic acid (PANI/CSA), polyaniline/poly(4-styrenesulfonate) (PANI/PSS), a compound represented by Formula 201, and a compound represented by Formula 202: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 201 and 202, 
     L 201  to L 204  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     L 205  may be selected from *—O—*′, *—S—*′, *—N(Q 201 )—*′, a substituted or unsubstituted C 1 -C 20  alkylene group, a substituted or unsubstituted C 2 -C 20  alkenylene group, a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     xa1 to xa4 may each independently be an integer from 0 to 3, 
     xa5 may be an integer from 1 to 10, and 
     R 201  to R 204  and Q 201  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group. 
     For example, in Formula 202, R 201  and R 202  may optionally be linked to each other via a single bond, a dimethyl-methylene group, or a diphenyl-methylene group, and R 203  and R 204  may optionally be linked to each other via a single bond, a dimethyl-methylene group, or a diphenyl-methylene group. 
     In one exemplary embodiment, in Formulae 201 and 202, 
     L 201  to L 205  may each independently be selected from: 
     a phenylene group, a pentalenylene group, an indenylene group, a naphthylene group, an azulenylene group, a heptalenylene group, an indacenylene group, an acenaphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenalenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a naphthacenylene group, a picenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a rubicenylene group, a coronenylene group, an ovalenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, and a pyridinylene group; and 
     a phenylene group, a pentalenylene group, an indenylene group, a naphthylene group, an azulenylene group, a heptalenylene group, an indacenylene group, an acenaphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenalenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a naphthacenylene group, a picenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a rubicenylene group, a coronenylene group, an ovalenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, and a pyridinylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), and —N(Q 31 )(Q 32 ), 
     wherein Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     In one or more exemplary embodiments, xa1 to xa4 may each independently be 0, 1, or 2. 
     In one or more exemplary embodiments, xa5 may be 1, 2, 3, or 4. 
     In one or more exemplary embodiments, R 201  to R 204  and Q 201  may each independently be selected from: a phenyl group, a biphenyl group, a terphenyl group, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, and a pyridinyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, and a pyridinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 10  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), and —N(Q 31 )(Q 32 ), 
     wherein Q 31  to Q 33  are the same as described above. 
     In one or more exemplary embodiments, at least one selected from R 201  to R 203  in Formula 201 may each independently be selected from: 
     a fluorenyl group, a spiro-bifluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; and 
     a fluorenyl group, a spiro-bifluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20 alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, but the exemplary embodiments are not limited thereto. 
     In one or more exemplary embodiments, in Formula 202, i) R 201  and R 202  may be linked to each other via a single bond, and/or ii) R 203  and R 204  may be linked to each other via a single bond. 
     In one or more exemplary embodiments, R 201  to R 204  in Formula 202 may each independently be selected from: 
     a carbazolyl group; and 
     a carbazolyl group substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 10  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, but the exemplary embodiments are not limited thereto. 
     In one or more exemplary embodiments, the compound represented by Formula 201 may be represented by Formula 201A below: 
     
       
         
         
             
             
         
       
     
     In one or more exemplary embodiments, the compound represented by Formula 201 may be represented by Formula 201A(1) below, but the exemplary embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In one or more exemplary embodiments, the compound represented by Formula 201 may be represented by Formula 201A-1 below, but the exemplary embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In one or more exemplary embodiments, the compound represented by Formula 202 may be represented by Formula 202A below: 
     
       
         
         
             
             
         
       
     
     In one or more exemplary embodiments, the compound represented by Formula 202 may be represented by Formula 202A-1 below: 
     
       
         
         
             
             
         
       
     
     In Formulae 201A, 201A(1), 201A-1, 202A, and 202A-1, 
     L 201  to L 203 , xa1 to xa3, xa5, and R 202  to R 204  are the same as described above, 
     R 211  and R 212  are the same as described in connection with R 203 , and 
     R 213  to R 217  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, and a pyridinyl group. 
     The hole transport region  140  may include at least one compound selected from Compounds HT1 to HT39, but compounds to be included in the hole transport region are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The thickness of the hole transport region  140  may be in a range of about 100 Å to about 10,000 Å, for example, about 100 Å to about 1,000 Å. When the hole transport region  140  includes at least one selected from the hole injection layer  142  and the hole transport layer  144 , the thickness of the hole injection layer  142  may be in a range of about 100 Å to about 9,000 Å, for example, about 100 Å to about 1,000 Å, and the thickness of the hole transport layer  144  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  140 , the hole injection layer  142 , and the hole transport layer  144  are within these ranges, satisfactory hole transporting characteristics may be obtained without a substantial increase in driving voltage. 
     The emission auxiliary layer  146  may increase light-emission efficiency by compensating for an optical resonance distance according to the wavelength of light emitted by the emission region  160 , and the electron blocking layer  148  may block the flow of electrons from an electron transport region  180 . The emission auxiliary layer  146  and the electron blocking layer  148  may include the materials as described above, p-dopant 
     The hole transport region  140  may further include, in addition to these materials, a charge-generation material for improvement of conductive properties. The charge-generation material may be homogeneously or non-homogeneously dispersed in the hole transport region  140 . 
     The charge-generation material may be, for example, a p-dopant. 
     In one exemplary embodiment, the p-dopant may have a lowest unoccupied molecular orbital (LUMO) energy level of about −3.5 eV or less. 
     The p-dopant may include at least one selected from a quinone derivative, a metal oxide, and a cyano group-containing compound, but the exemplary embodiments are not limited thereto. 
     For example, the p-dopant may include at least one selected from: a quinone derivative, such as tetracyanoquinodimethane (TCNQ) or F4-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ); 
     a metal oxide, such as tungsten oxide or molybdenum oxide; 
     1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN); and 
     a compound represented by Formula 221 below, 
     but the exemplary embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In Formula 221, 
     R 221  to R 223  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, wherein at least one selected from R 221  to R 223  may have at least one substituent selected from a cyano group, —F, —Cl, —Br, —I, a C 1 -C 20  alkyl group substituted with —F, a C 1 -C 20  alkyl group substituted with —Cl, a C 1 -C 20  alkyl group substituted with —Br, and a C 1 -C 20  alkyl group substituted with —I. 
     Emission Region  160  in the Organic Layer  150   
     When the organic light-emitting device  10  is a full-color organic light-emitting device, the emission region  160  may be patterned into a red emission layer, a green emission layer, or a blue emission layer, according to a sub-pixel. In one or more exemplary embodiments, the emission region  160  may have a stacked structure of two or more layers selected from the red emission layer, the green emission layer, and the blue emission layer, in which the two or more layers contact each other or are separated from each other. In one or more exemplary embodiments, the emission region  160  may include two or more materials selected from a red light-emitting material, a green light-emitting material, and a blue light-emitting material, in which the two or more materials are mixed with each other in a single layer to emit white light. 
     Referring to  FIG. 1C , in one exemplary embodiment, the emission region  160  may include a first emission layer  162  and a second emission layer  166 , and the intermediate layer  164  may be located between the first emission layer  162  and the second emission layer  166  and may include the first compound, the second compound, the fourth compound, or a combination thereof, as described above. In some exemplary embodiments, the intermediate layer may be located between the first region  140  and the emission region  160  and may include the first compound, the fourth compound, or a combination thereof. For example, the intermediate layer may be located between the hole transport layer  144  and the emission region  160  and may include the first compound, the fourth compound, or a combination thereof. In yet other exemplary embodiments, the intermediate layer may be located between the second region  180  and the emission region  160  and may include the second compound, the fourth compound, or a combination thereof. 
     The thickness of the emission region  160  may be in a range of about 100 Å to about 1,000 Å, for example, about 200 Å to about 600 Å. When the thickness of the emission region  160  is within this range, excellent light-emission characteristics may be obtained without a substantial increase in driving voltage. 
     Electron Transport Region  180  in the Organic Layer  150   
     The electron transport region  180  may have i) a single-layered structure including a single layer including a single material, ii) a single-layered structure including 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  180  may include at least one layer selected from a buffer layer  182 , a hole blocking layer  184 , an electron control layer  186 , an electron transport layer  188 , and an electron injection layer  194 , but the exemplary embodiments are not limited thereto. In the illustrated embodiment, the electron transport region  188  may further include a first layer  190  and a second layer  192 , each including an organic compound as described above. However, in other exemplary embodiments each of the first and second layers  190  and  192  may be, independently, the buffer layer  182 , hole blocking layer  184 , electron control layer  186 , electron transport layer  188 , and electron injection layer  194 , or be sub-layers in one or more of the buffer layer  182 , hole blocking layer  184 , electron control layer  186 , electron transport layer  188 , and electron injection layer  194 . 
     For example, the electron transport region  180  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 for each structure, constituting layers are sequentially stacked from the emission region  160 . However, the exemplary embodiments of the structure of the electron transport region  180  are not limited thereto. 
     The electron transport region  180  (for example, the buffer layer  182 , the hole blocking layer  184 , the electron control layer  186 , or the electron transport layer  188  in the electron transport region  180 ) may include a metal-free compound containing at least one π electron-deficient nitrogen-containing ring. 
     The “π electron-deficient nitrogen-containing ring” indicates a C 1 -C 60  heterocyclic group having at least one *—N═*′ moiety as a ring-forming moiety. 
     For example, the “π electron-deficient nitrogen-containing ring” may be i) a 5-membered to 7-membered heteromonocyclic group having at least one *—N═*′ moiety, ii) a heteropolycyclic group in which two or more 5-membered to 7-membered heteromonocyclic groups each having at least one *—N═*′ moiety are condensed with each other, or iii) a heteropolycyclic group in which at least one of 5-membered to 7-membered heteromonocyclic groups, each having at least one *—N═*′ moiety, is condensed with at least one C 5 -C 60  carbocyclic group. 
     Examples of the π electron-deficient nitrogen-containing ring include an imidazole ring, a pyrazole ring, a thiazole ring, an isothiazole ring, an oxazole ring, an isoxazole ring, a pyridine ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, an indazole ring, a purine ring, a quinoline ring, an isoquinoline ring, a benzoquinoline ring, a phthalazine ring, a naphthyridine ring, a quinoxaline ring, a quinazoline ring, a cinnoline ring, a phenanthridine ring, an acridine ring, a phenanthroline ring, a phenazine ring, a benzimidazole ring, an isobenzothiazole ring, a benzoxazole ring, an isobenzoxazole ring, a triazole ring, a tetrazole ring, an oxadiazole ring, a triazine ring, a thiadiazole ring, an imidazopyridine ring, an imidazopyrimidine ring, and an azacarbazole ring, but the exemplary embodiments are not limited thereto. 
     For example, the electron transport region may include a compound represented by Formula 601 below: 
       [Ar 601 ] xe11 -[(L 601 ) xe1 -R 601 ] xe21   &lt;Formula 601&gt;
 
     In Formula 601, 
     Ar 601  may be a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     xe11 may be 1, 2, or 3, 
     L 601  may be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     xe1 may be an integer from 0 to 5, 
     R 601  may be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 601 )(Q 602 )(Q 603 ), —C(═O)(Q 601 ), —S(═O) 2 (Q 601 ), and —P(═O)(Q 601 )(Q 602 ), 
     Q 601  to Q 603  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group, and 
     xe21 may be an integer from 1 to 5. 
     In one exemplary embodiment, at least one of Ar 601 (s) in the number of xe11 and R 601 (S) in the number of xe21 may include the π electron-deficient nitrogen-containing ring. 
     In one exemplary embodiment, Ar 601  in Formula 601 may be selected from: 
     a benzene group, a naphthalene 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 pentaphene group, an indenoanthracene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, 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 pyrimidine group, a pyridazine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline 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, and an azacarbazole group; and 
     a benzene group, a naphthalene 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 pentaphene group, an indenoanthracene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, 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 pyrimidine group, a pyridazine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline 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, and an azacarbazole group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     When xe11 in Formula 601 is 2 or more, two or more Ar 601 (s) may be linked to each other via a single bond. 
     In one or more exemplary embodiments, Ar 601  in Formula 601 may be an anthracene group. 
     In one or more exemplary embodiments, a compound represented by Formula 601 may be represented by Formula 601-1 below: 
     
       
         
         
             
             
         
       
     
     In Formula 601-1, 
     X 614  may be N or C(R 614 ), X 615  may be N or C(R 615 ), X 616  may be N or C(R 616 ), and at least one selected from X 614  to X 616  may be N, 
     L 611  to L 613  may each independently be the same as described in connection with L 601 , 
     xe611 to xe613 may each independently be the same as described in connection with xe1, 
     R 611  to R 613  may each independently be the same as described in connection with R 601 , and 
     R 614  to R 616  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     In one exemplary embodiment, L 601  and L 611  to L 613  in Formulae 601 and 601-1 may each independently be selected from: 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group; and 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl 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 hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group, but the exemplary embodiments are not limited thereto. 
     In one or more exemplary embodiments, xe1 and xe611 to xe613 in Formulae 601 and 601-1 may each independently be 0, 1, or 2. 
     In one or more exemplary embodiments, R 601  and R 611  to R 613  in Formulae 601 and 601-1 may each independently be selected from: 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl 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 hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl 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 hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl 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 hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group; and 
       —S(═O) 2 (Q 601 ) and —P(═O)(Q 601 )(Q 602 ),
 
     wherein Q 601  and Q 602  are the same as described above. 
     The electron transport region may include at least one compound selected from Compounds ET1 to ET36, but the exemplary embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In one or more exemplary embodiments, the electron transport region  180  may include at least one compound selected from 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (Bphen), tris-(8-hydroxyquinoline)aluminum (Alq 3 ), bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)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-1,2,4-triazole (NTAZ), and diphenyl(4-(triphenylsilyl)phenyl)-phosphine oxide (TSPO1): 
     
       
         
         
             
             
         
       
     
     Thicknesses of the buffer layer  182 , the hole blocking layer  184 , and the electron control layer  186  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. When the thicknesses of the buffer layer  182 , the hole blocking layer  184 , and the electron control layer  186  are within these ranges, excellent hole blocking characteristics or excellent electron control characteristics may be obtained without a substantial increase in driving voltage. 
     The thickness of the electron transport region  180  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 thickness of the electron transport region  180  is within the range described above, the electron transport region  180  may have satisfactory electron transport characteristics without a substantial increase in driving voltage. 
     The electron transport region  180  (for example, the electron transport layer  188  in the electron transport region  180 ) may further include, in addition to the materials described above, a metal-containing material. 
     The metal-containing material may include at least one selected from an alkali metal complex and an alkaline earth-metal complex. The alkali metal complex may include a metal ion selected from a Li ion, a Na ion, a K ion, an Rb ion, and a Cs ion, and the alkaline earth-metal complex may include a metal ion selected from a Be ion, a Mg ion, a Ca ion, a Sr ion, and a Ba ion. A ligand coordinated with the metal ion of the alkali metal complex or the alkaline earth-metal complex may be selected from a hydroxy quinoline, a hydroxy isoquinoline, a hydroxy benzoquinoline, a hydroxy acridine, a hydroxy phenanthridine, a hydroxy phenyloxazole, a hydroxy phenylthiazole, a hydroxy diphenyloxadiazole, a hydroxy diphenylthiadiazole, a hydroxy phenylpyridine, a hydroxy phenylbenzimidazole, a hydroxy phenylbenzothiazole, a bipyridine, a phenanthroline, and a cyclopentadiene, but the exemplary embodiments are not limited thereto. 
     For example, the metal-containing material may include a Li complex. The Li complex may include, for example, Compound ET-D1 (lithium quinolate, LiQ) or ET-D2: 
     
       
         
         
             
             
         
       
     
     The electron transport region  180  may include the electron injection layer  194  that facilitates electron injection from the second electrode  200 . The electron injection layer  194  may directly contact the second electrode  200 . 
     The electron injection layer  194  may have i) a single-layered structure including a single layer including a single material, ii) a single-layered structure including 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  194  may include an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth-metal compound, a rare earth metal compound, an alkali metal complex, an alkaline earth-metal complex, a rare earth metal complex, or any combinations thereof. 
     The alkali metal may be selected from Li, Na, K, Rb, and Cs. In one exemplary embodiment, the alkali metal may be Li, Na, or Cs. In one or more exemplary embodiments, the alkali metal may be Li or Cs, but the exemplary embodiments are not limited thereto. 
     The alkaline earth metal may be selected from Mg, Ca, Sr, and Ba. 
     The rare earth metal may be selected from Sc, Y, Ce, Tb, Yb, and Gd. 
     The alkali metal compound, the alkaline earth-metal compound, and the rare earth metal compound may be selected from oxides and halides (for example, fluorides, chlorides, bromides, or iodides) of the alkali metal, the alkaline earth-metal, and the rare earth metal. 
     The alkali metal compound may be selected from alkali metal oxides, such as Li 2 O, Cs 2 O, or K 2 O, and alkali metal halides, such as LiF, NaF, CsF, KF, LiI, NaI, CsI, or KI. In one exemplary embodiment, the alkali metal compound may be selected from LiF, Li 2 O, NaF, LiI, NaI, CsI, and KI, but the exemplary embodiments are not limited thereto. 
     The alkaline earth-metal compound may be selected from alkaline earth-metal oxides, such as BaO, SrO, CaO, BaxSr 1-x O (0&lt;x&lt;1), or Ba x Ca 1-x O (0&lt;x&lt;1). In one exemplary embodiment, the alkaline earth-metal compound may be selected from BaO, SrO, and CaO, but the exemplary embodiments are not limited thereto. 
     The rare earth metal compound may be selected from YbF 3 , ScF 3 , ScO 3 , Sc 2 O 3 , Y 2 O 3 , Ce 2 O 3 , GdF 3 , and TbF 3 . In one exemplary embodiment, the rare earth metal compound may be selected from YbF 3 , ScF 3 , TbF 3 , YbI 3 , ScI 3 , and TbI 3 , but the exemplary embodiments are not limited thereto. 
     The alkali metal complex, the alkaline earth-metal complex, and the rare earth metal complex may include an ion of alkali metal, alkaline earth-metal, and rare earth metal as described above, and a ligand coordinated with a metal ion of the alkali metal complex, the alkaline earth-metal complex, or the rare earth metal complex may be selected from hydroxy quinoline, hydroxy isoquinoline, hydroxy benzoquinoline, hydroxy acridine, hydroxy phenanthridine, hydroxy phenyloxazole, hydroxy phenylthiazole, hydroxy diphenyloxadiazole, hydroxy diphenylthiadiazole, hydroxy phenylpyridine, hydroxy phenylbenzimidazole, hydroxy phenylbenzothiazole, bipyridine, phenanthroline, and cyclopentadiene, but the exemplary embodiments are not limited thereto. 
     The electron injection layer  194  may consist of an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth-metal compound, a rare earth metal compound, an alkali metal complex, an alkaline earth-metal complex, a rare earth metal complex, or any combinations thereof, as described above. In one or more exemplary embodiments, the electron injection layer  194  may further include an organic material. When the electron injection layer  194  further includes an organic material, an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth-metal compound, a rare earth metal compound, an alkali metal complex, an alkaline earth-metal complex, a rare earth metal complex, or any combinations thereof may be homogeneously or non-homogeneously dispersed in a matrix including the organic material. 
     The thickness of the electron injection layer  194  may be in a range of about 1 Å to about 100 Å, for example, about 3 Å to about 90 Å. When the thickness of the electron injection layer  194  is within the range described above, the electron injection layer  194  may have satisfactory electron injection characteristics without a substantial increase in driving voltage. 
     Second Electrode  200   
     The second electrode  200  may be located on the organic layer  150  having such a structure. The second electrode  200  may be a cathode which is an electron injection electrode, and in this regard, a material for forming the second electrode  200  may be selected from a metal, an alloy, an electrically conductive compound, and a combination thereof, which have a relatively low work function. 
     The second electrode  200  may include at least one selected from lithium (Li), silver (Ag), magnesium (Mg), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), ITO, and IZO, but the exemplary embodiments are not limited thereto. The second electrode  200  may be a transmissive electrode, a semi-transmissive electrode, or a reflective electrode. 
     The second electrode  200  may have a single-layered structure or a multi-layered structure including two or more layers. 
     Description of  FIGS. 2 to 4   
       FIG. 2  is a schematic cross-sectional diagram of another exemplary embodiment of an organic light-emitting device constructed according to the principles of the invention.  FIG. 3  is a schematic cross-sectional diagram of yet another exemplary embodiment of an organic light-emitting device constructed according to the principles of the invention.  FIG. 4  is a schematic cross-sectional diagram of still another exemplary embodiment of an organic light-emitting device constructed according to the principles of the invention. 
     Referring to  FIG. 2 , the organic light-emitting device  20  includes a first capping layer  210 , the first electrode  110 , the organic layer  150 , and the second electrode  200 , which are sequentially stacked in this stated order. Referring to  FIG. 3 , an organic light-emitting device  30  includes the first electrode  110 , the organic layer  150 , the second electrode  200 , and a second capping layer  220 , which are sequentially stacked in this stated order. Referring to  FIG. 4 , an organic light-emitting device  40  includes the first capping layer  210 , the first electrode  110 , the organic layer  150 , the second electrode  200 , and the second capping layer  220 , which are sequentially stacked in this stated order. 
     Regarding  FIGS. 2 to 4 , the first electrode  110 , the organic layer  150 , and the second electrode  200  may be understood by referring to the description presented in connection with  FIG. 1 . 
     In the organic layer  150  of each of the organic light-emitting devices  20  and  40 , light generated in the emission region  160  may pass through the first electrode  110  and the first capping layer  210  toward the outside, wherein the first electrode  110  may be a semi-transmissive electrode or a transmissive electrode. In the organic layer  150  of each of the organic light-emitting devices  30  and  40 , light generated in the emission region  160  may pass through the second electrode  200  and the second capping layer  220  toward the outside, wherein the second electrode  200  may be a semi-transmissive electrode or a transmissive electrode. 
     The first capping layer  210  and the second capping layer  220  may increase external luminescence efficiency according to the principle of constructive interference. 
     The first capping layer  210  and the second capping layer  220  may each independently be an organic capping layer including an organic material, an inorganic capping layer including an inorganic material, or a composite capping layer including an organic material and an inorganic material. 
     At least one selected from the first capping layer  210  and the second capping layer  220  may each independently include at least one material selected from a carbocyclic compound, a heterocyclic compound, an amine-based compound, a porphine derivative, a phthalocyanine derivative, a naphthalocyanine derivative, an alkali metal complex, and an alkaline earth-metal complex. The carbocyclic compound, the heterocyclic compound, and the amine-based compound may be optionally substituted with a substituent containing at least one element selected from O, N, S, Se, Si, F, Cl, Br, and I. In one exemplary embodiment, at least one selected from the first capping layer  210  and the second capping layer  220  may each independently include an amine-based compound. 
     In one exemplary embodiment, at least one selected from the first capping layer  210  and the second capping layer  220  may each independently include the compound represented by Formula 201 or the compound represented by Formula 202. 
     In one or more exemplary embodiments, at least one selected from the first capping layer  210  and the second capping layer  220  may each independently include a compound selected from Compounds HT28 to HT33 and Compounds CP1 to CP5, but the exemplary embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     Hereinbefore, the organic light-emitting device according to an embodiment has been described in connection with  FIGS. 1 to 4 , but the exemplary embodiments are not limited thereto. 
     Layers constituting the hole transport region  140 , the emission region  160 , and the electron transport region  180  may be formed in a certain region by using one or more suitable methods selected from 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  140 , the emission region  160 , and the electron transport region  180  are formed by vacuum deposition, the deposition may be performed at a deposition temperature of about 100° C. to about 500° C., a vacuum degree of about 10 −8  torr to about 10 −3  torr, and a deposition speed of about 0.01 Å/sec to about 100 Å/sec by taking into account a material to be included in a layer to be formed and the structure of a layer to be formed. 
     When layers constituting the hole transport region  140 , the emission region  160 , and the electron transport region  180  are formed by spin coating, the spin coating may be performed at a coating speed of about 2,000 rpm to about 5,000 rpm and at a heat treatment temperature of about 80° C. to 200° C. by taking into account a material to be included in a layer to be formed and the structure of a layer to be formed. 
     Apparatus 
       FIG. 5  is a schematic diagram of an exemplary embodiment of an apparatus containing an organic light-emitting device constructed according to principles of the invention. 
     Referring to  FIG. 5 , an apparatus  300  includes a thin-film transistor  310  having a source electrode  320 , a drain electrode  340 , and an activation layer  330 . In the illustrated embodiment, the first electrode  110  of the organic light-emitting device  10  is in electrical connection with one of the source electrode  320  or the drain electrode  340 . In other exemplary embodiments, other devices, such as devices  20 ,  30  and/or  40  may be used instead of the device  10 . 
     The organic light-emitting device may be included in various apparatuses. 
     The thin-film transistor  310  may further include a gate electrode, a gate insulation layer, or the like. 
     The activation layer  330  may include a crystalline silicon, an amorphous silicon, an organic semiconductor, an oxide semiconductor, or the like, but the exemplary embodiments are not limited thereto. 
     The apparatus  300  may further include a sealing part for sealing the organic light-emitting device  10 . The sealing part may allow an image from the organic light-emitting device  10  to be implemented and may block outside air and moisture from penetrating into the organic light-emitting device  10 . The sealing part may be a sealing substrate including a transparent glass or a plastic substrate. The sealing part may be a thin film encapsulation layer including a plurality of organic layers and/or a plurality of inorganic layers. When the sealing part is a thin film encapsulation layer, the entire apparatus may be flexible. 
     For example, the apparatus  300  may be a light-emitting apparatus, an authentication apparatus, or an electronic apparatus. 
     The light-emitting apparatus may be used as various displays, light sources, and the like. 
     The authentication apparatus may be, for example, a biometric authentication apparatus for authenticating an individual by using biometric information of a biometric body (for example, a finger tip, a pupil, or the like). The authentication apparatus may further include, in addition to the organic light-emitting device, a biometric information collector. 
     The electronic apparatus may be applied to personal computers (for example, a mobile personal computer), mobile phones, digital cameras, electronic organizers, electronic dictionaries, electronic game machines, medical instruments (for example, electronic thermometers, sphygmomanometers, blood glucose meters, pulse measurement devices, pulse wave measurement devices, electrocardiogram (ECG) displays, ultrasonic diagnostic devices, or endoscope displays), fish finders, various measuring instruments, meters (for example, meters for a vehicle, an aircraft, and a vessel), projectors, and the like, but the exemplary embodiments are not limited thereto. 
     General Definition of Substituents 
     The term “π electron-deficient nitrogen-containing cyclic group” as used herein refers to a cyclic group having at least one *—N═*′ moiety, and non-limiting examples thereof include 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 phthalazine group, a naphthyridine group, a quinoxaline 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. 
     The π electron-deficient nitrogen-free cyclic group may be i) a *—N═*′ moiety-free, 5-membered or 7-membered cyclic or heterocyclic group, ii) a cyclic or heteropolycyclic group in which two or more of a *—N═*′ moiety-free, 5-membered to 7-membered cyclic or heterocyclic group are fused with each other, or iii) a cyclic or heteropolycyclic group in which at least one of a *—N═*′ moiety-free, 5-membered to 7-membered cyclic or heteromonocyclic group is fused with at least one C 5 -C 60  carbocyclic group. For example, the π electron-deficient nitrogen-free cyclic group may be selected from a benzene group, a heptalene group, an indene group, a naphthalene group, an azulene group, a heptalene group, an indacene group, 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 pentacene group, a rubicene group, a corogen group, an ovalene group, a pyrrole group, an isoindole group, an indole group, a furan group, a thiophene group, a benzofuran group, a benzothiophene group, a benzocarbazole group, a dibenzocarbazole group, a dibenzofuran group, a dibenzothiophene group, a dibenzothiophene sulfone group, a carbazole group, a dibenzosilole group, an indenocarbazole group, an indolocarbazole group, a benzofuro carbazole group, a benzothienocarbazole group, a thienodiacarbazole group, a triindolobenzene group, and groups represented by Formulae 13-1 to 13-2, but the exemplary embodiments are not limited thereto. 
     The term “transition metal of Period 1 of the Periodic Table of Elements” as used herein refers to an element of Period 4 and the d-block of the Periodic Table of Elements, and non-limiting examples thereof include scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn). 
     The term “transition metal of Period 2 of the Periodic Table of Elements” as used herein refers to an element of Period 5 and the d-block of the Periodic Table of Elements, and non-limiting examples thereof include yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), and cadmium (Cd). 
     The term “transition metal of Period 3 of the Periodic Table of Elements” as used herein refers to an element of Period 6 and the d-block and the f-block of the Periodic Table of Elements, and non-limiting examples thereof include lanthanum (La), samarium (Sm), europium (Eu), terbium (Tb), thulium (Tm), ytterbium (Yb), lutetium (Lu), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pr), gold (Au), and mercury (Hg). 
     The term “C 1 -C 60  alkyl group” as used herein refers to a linear or branched aliphatic saturated hydrocarbon monovalent group having 1 to 60 carbon atoms, and examples thereof include a methyl group, an ethyl group, a propyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an isoamyl group, and a hexyl 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, and 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 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, and 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 the C 1 -C 60  alkyl group), and 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 having 3 to 10 carbon atoms, and examples thereof include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and a cycloheptyl 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 monocyclic group having at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom and 1 to 10 carbon atoms, and examples thereof include a 1,2,3,4-oxatriazolidinyl group, a tetrahydrofuranyl group, and a tetrahydrothiophenyl 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 used herein refers to a monovalent monocyclic group that has 3 to 10 carbon atoms and at least one double bond in the ring thereof and no aromaticity, and 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 monocyclic group that has at least one heteroatom selected from N, O, Si, P, and S 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-dihydrothiophenyl group. The term “C 1 -C 10  heterocycloalkenylene group” as used herein refers to a divalent group having a structure corresponding to the C 1 -C 10  heterocycloalkenyl group. 
     The term “C 6 -C 60  aryl group” as used herein refers to a monovalent group having a carbocyclic aromatic system having 6 to 60 carbon atoms, and a C 6 -C 60  arylene group 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 naphthyl group, an anthracenyl group, a phenanthrenyl group, a pyrenyl group, and a chrysenyl group. When the C 6 -C 60  aryl group and the C 6 -C 60  arylene group each include two or more rings, the two or more rings may be fused to each other. 
     The term “C 1 -C 60  heteroaryl group” as used herein refers to a monovalent group having a heterocyclic aromatic system that has at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, in addition to 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 that has at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, in addition to 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, and an isoquinolinyl group. When the C 1 -C 60  heteroaryl group and the C 1 -C 60  heteroarylene group each include two or more rings, the two or more rings may be fused with each other. 
     The term “C 6 -C 60  aryloxy group” as used herein refers to —OA 102  (wherein A 102  is the C 6 -C 60  aryl group), and a C 6 -C 60  arylthio group used herein refers to —SA 103  (wherein A 103  is the C 6 -C 60  aryl group). 
     The term “C 1 -C 60  heteroaryloxy group” as used herein refers to —OA 104  (wherein A 104  is the C 1 -C 60  heteroaryl group), and a C 1 -C 60  heteroarylthio group used herein refers to —SA 105  (wherein A 105  is the C 1 -C 60  heteroaryl group). 
     The term “monovalent non-aromatic fused polycyclic group” as used herein refers to a monovalent group (for example, having 8 to 60 carbon atoms) having two or more rings fused with each other, only carbon atoms as ring-forming atoms, and no aromaticity in its entire molecular structure. A detailed example of the monovalent non-aromatic fused polycyclic group is a fluorenyl 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 (for example, having 1 to 60 carbon atoms) having two or more rings fused to each other, at least one heteroatom selected from N, O, Si, P, and S, other than carbon atoms, as a ring-forming atom, and no aromaticity in its entire molecular structure. A detailed example of the monovalent non-aromatic fused heteropolycyclic group is a carbazolyl 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 5 -C 60  carbocyclic group” as used herein refers to a monocyclic or polycyclic group that includes only carbon as a ring-forming atom and consists of 5 to 60 carbon atoms. The C 5 -C 60  carbocyclic group may be an aromatic carbocyclic group or a non-aromatic carbocyclic group. The C 5 -C 60  carbocyclic group may be a ring, such as benzene, a monovalent group, such as a phenyl group, or a divalent group, such as a phenylene group. In one or more exemplary embodiments, depending on the number of substituents connected to the C 5 -C 60  carbocyclic group, the C 5 -C 60  carbocyclic group may be a trivalent group or a quadrivalent group. 
     The term “C 1 -C 60  heterocyclic group” as used herein refers to a group having a structure corresponding to the C 5 -C 60  carbocyclic group, except that as a ring-forming atom, at least one heteroatom selected from N, O, Si, P, and S is used in addition to carbon (the number of carbon atoms may be in a range of 1 to 60). 
     As used herein, at least one substituent selected from the substituted C 5 -C 60  carbocyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 3 -C 10  cycloalkylene group, the substituted C 1 -C 10  heterocycloalkylene group, the substituted C 3 -C 10  cycloalkenylene group, the substituted C 1 -C 10  heterocycloalkenylene group, the substituted C 6 -C 60  arylene group, the substituted C 1 -C 60  heteroarylene group, the substituted divalent non-aromatic fused polycyclic group, the substituted divalent non-aromatic fused heteropolycyclic group, the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 6 -C 60  aryloxy group, the substituted C 6 -C 60  arylthio group, the substituted C 1 -C 60  heteroaryl group, the substituted C 1 -C 60  heteroaryloxy group, the substituted C 1 -C 60  heteroarylthio group, the substituted monovalent non-aromatic fused polycyclic group, and the substituted monovalent non-aromatic fused heteropolycyclic group may be selected from: 
     deuterium (-D), —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic fused polycyclic group, a monovalent non-aromatic fused heteropolycyclic 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 ), and —P(═O)(Q 11 )(Q 12 ); 
     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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic fused polycyclic group, and a monovalent non-aromatic fused heteropolycyclic group; 
     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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic fused polycyclic group, and a monovalent non-aromatic fused heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono 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 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a G-C 60  heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic fused polycyclic group, a monovalent non-aromatic fused heteropolycyclic 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 ), and —P(═O)(Q 21 )(Q 22 ); and 
     —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 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein Q 11  to Q 13 , Q 21  to Q 23 , and Q 31  to Q 33  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 1 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, 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 C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic fused polycyclic group, a monovalent non-aromatic fused heteropolycyclic group, a C 1 -C 60  alkyl group substituted with at least one selected from deuterium, —F, and a cyano group, a C 6 -C 60  aryl group substituted with at least one selected from deuterium, —F, and a cyano group, a biphenyl group, and a terphenyl group. 
     The term “Ph” as used herein refers to a phenyl group, the term “Me” as used herein refers to a methyl group, the term “Et” as used herein refers to an ethyl group, the term “ter-Bu” or “Bu t ” as used herein refers to a tert-butyl group, and the term “OMe” as used herein refers to a methoxy group. 
     The term “biphenyl group” as used herein refers to “a phenyl group substituted with a phenyl group”. In other words, the “biphenyl group” is a substituted phenyl group having a C 6 -C 60  aryl group as a substituent. 
     The term “terphenyl group” as used herein refers to “a phenyl group substituted with a biphenyl group”. In other words, the “terphenyl group” is a phenyl group having, as a substituent, a C 6 -C 60  aryl group substituted with a C 6 -C 60  aryl group. 
     The terms “hydrogen” and “deuterium” refer to their respective atoms and corresponding radicals, and the terms “—F, —Cl, —Br, and —I” are radicals of, respectively, fluorine, chlorine, bromine, and iodine. 
     As used herein, the term “atom” may mean an element or its corresponding radical bonded to one or more other atoms. 
     As used herein, a substituent for a monovalent group, e.g., alkyl, may also be, independently, a substituent for a corresponding divalent group, e.g., alkylene. 
     * and *′ used herein, unless defined otherwise, each refer to a binding site to a neighboring atom in a corresponding formula. 
     Hereinafter, a compound according to embodiments and an organic light-emitting device according to embodiments will be described in detail with reference to Synthesis Examples and Examples. The wording “B was used instead of A” used in describing Synthesis Examples refers to that an identical molar equivalent of B was used in place of A. 
     EXAMPLES 
     Example 1 
     As an anode, an ITO substrate was cut to a size of 50 mm×50 mm×0.5 mm, sonicated with acetone, isopropyl alcohol, and pure water, each for 15 minutes, and then cleaned by exposure to ultraviolet rays and ozone for 30 minutes. Then, the ITO substrate was provided to a vacuum deposition apparatus. 
     Compound HT1 was deposited on the ITO substrate to form a hole injection layer having a thickness of 120 nm, Compound 1-3 was vacuum-deposited on the hole injection layer to form a hole transport layer having a thickness of 5 nm, Compound 1-3 was deposited on the hole transport layer to form an intermediate layer having a thickness of 5 nm, and Compounds 1-3, 2-16, 3-11, and 4-1 were co-deposited at a weight ratio of 62.6:26.9:10:0.5 on the intermediate layer to form an emission layer having a thickness of 30 nm. Compound ET1 was deposited on the emission layer to form a first electron transport layer having a thickness of 5 nm, and Compound ET2 and LiQ were co-deposited at a weight ratio of 5:5 on the first electron transport layer to form a second electron transport layer having a thickness of 20 nm. Yb was deposited on the second electron transport layer to form an electron injection layer having a thickness of 1 nm. Mg and Ag were co-deposited at a weight ratio of 5:5 on the electron injection layer to form a cathode having a thickness of 10 nm, thereby completing the manufacture of an organic light-emitting device. 
     
       
         
         
             
             
         
       
     
     Example 2 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that, Compound 2-16 was deposited on the emission layer to form an intermediate layer having a thickness of 5 nm, instead of forming an intermediate layer on the hole transport layer, 
     Example 3 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that Compounds 1-3, 2-16, 3-11, and 4-1 were co-deposited at a weight ratio of 62.6:26.9:10:0.5 on the hole transport layer to form a first emission layer having a thickness of 13 nm, instead of forming an intermediate layer on the hole transport layer, Compound 1-3 was deposited on the first emission layer to form an intermediate layer having a thickness of 4 nm, and Compounds 1-3, 2-16, 3-11, and 4-1 were co-deposited at a weight ratio of 62.6:26.9:10:0.5 on the intermediate layer to form a second emission layer having a thickness of 13 nm. 
     Example 4 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that Compound 4-12 was used instead of Compound 1-3 in forming an intermediate layer. 
     Example 5 
     An organic light-emitting device was manufactured in the same manner as in Example 2, except that Compound 4-12 was used instead of Compound 2-16 in forming an intermediate layer. 
     Example 6 
     An organic light-emitting device was manufactured in the same manner as in Example 3, except that Compound 4-12 was used instead of Compound 1-3 in forming an intermediate layer. 
     Comparative Example 1 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that an intermediate layer was not formed. 
     Comparative Example 2 
     An organic light-emitting device was manufactured in the same manner as in Example 3, except that 2-methyl-9,10-di(2-naphthyl)anthracene (MADN), TADF dopant(2-(4-(11,11-dimethylindeno[1,2-b]carbazl-5(11H)-yl)phenyl)benzo[d]thiazole), and 2,5, 8, 11-tetra-butylperylene (TBP) as a fluorescent dopant were co-deposited at a weight ratio of 70:27:3 on a first emission layer to form a middle layer having a thickness of 20 nm, instead of forming the intermediate layer of Example 3. 
     MADN: HOMO −5.9 eV, LUMO −2.56 eV, E S1 -3.34 eV, E T1 -2.52 eV 
     TADF dopant: HOMO −5.7 eV, LUMO −2.99 eV, E S1 -2.71 eV, E T1 -2.66 eV 
     fluorescent dopant: HOMO 5.44 eV, LUMO 2.74 eV, E S1 -2.7 eV, E T1 -2.6 eV, FWHM 30 nm. 
     Comparative Example 3 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that Compound 2-16 was deposited on the emission layer to form an intermediate layer having a thickness of 5 nm, instead of forming an intermediate layer on the hole transport layer, and a first electron transport layer and a second electron transport layer were not formed on the intermediate layer. 
     Comparative Example 4 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that Compound 2-16 was deposited on the emission layer to form an intermediate layer having a thickness of 5 nm, instead of forming an intermediate layer on the hole transport layer, and Compounds 1-3, 2-16, and 3-11 were co-deposited at a weight ratio of 63:27:10 on the hole transport layer to form an emission layer having a thickness of 30 nm in forming the emission layer. 
     Evaluation Example 1 
     Lifespan and driving voltage of the organic light-emitting devices manufactured according to Examples 1 to 6 and Comparative Example 1 were measured at a current density of 10 mA/cm 2  by using a source-measure unit sold under the trade designation Keithley SMU 236 by Tektronix, Inc., of Beaverton, Oreg. by Tektronix, Inc., of Beaverton, Oreg. and a luminance meter sold under the trade designation PR650 from Konica Minolta, Inc. of Tokyo, Japan, and results thereof are shown in Table 1. The lifespan indicates an amount of time that lapsed when luminance was 90% of initial luminance (100%). The lifespan was measured based on Comparative Example 1 as 100%. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Lifespan 
                 Driving Voltage 
               
               
                   
                   
               
             
            
               
                   
                 Comparative 
                 100% 
                 5.0 V 
               
               
                   
                 Example 1 
                   
                   
               
               
                   
                 Comparative 
                  95% 
                 5.2 V 
               
               
                   
                 Example 2 
                   
                   
               
               
                   
                 Comparative 
                 105% 
                 4.8 V 
               
               
                   
                 Example 3 
                   
                   
               
               
                   
                 Comparative 
                 110% 
                 5.1 V 
               
               
                   
                 Example 4 
                   
                   
               
               
                   
                 Example 1 
                 120% 
                 4.7 V 
               
               
                   
                 Example 2 
                 135% 
                 4.5 V 
               
               
                   
                 Example 3 
                 130% 
                 4.5 V 
               
               
                   
                 Example 4 
                 120% 
                 5.2 V 
               
               
                   
                 Example 5 
                 130% 
                 5.3 V 
               
               
                   
                 Example 6 
                 125% 
                 5.0 V 
               
               
                   
                   
               
            
           
         
       
     
     Referring to Table 1, it is confirmed that organic light-emitting devices manufactured according to Examples 1 to 6 have surprisingly and significantly improved lifespans compared to organic light-emitting devices manufactured according to Comparative Examples 1 to 4, and organic light-emitting devices manufactured according to Examples 1 to 3 have lower driving voltage compared to organic light-emitting devices manufactured according to Comparative Examples 1 to 4. 
     When an organic light-emitting device includes an intermediate layer between an emission layer and an electron transport region or a hole transport region or includes an intermediate layer in an emission layer in accordance with the principles and exemplary implementations of the invention, the organic light-emitting device may have long significantly and surprising improved lifespan by controlling concentration and distribution of excitons in an emission layer. 
     Although certain exemplary 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.