Patent Publication Number: US-2023137127-A1

Title: Organic light-emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority to Korean Patent Application No. 10-2021-0125203, filed on Sep. 17, 2021, in the Korean Intellectual Property Office, and all benefits accruing therefrom under 35 U.S.C. § 119, the content of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a composition that may satisfy certain conditions and an organic light-emitting device including the same. 
     2. Description of the Related Art 
     Organic light-emitting devices (OLEDs) are self-emissive devices that, as compared with conventional devices, have wide viewing angles, high contrast ratios, short response times, excellent brightness, driving voltage, and response speed characteristics, and OLEDs can produce full-color images. 
     OLEDs include an anode, a cathode, and an organic layer located between the anode and the cathode and including an emission layer. A hole transport region may be located between the anode and the emission layer, and an electron transport region may be located between the emission layer and the cathode. Holes provided from the anode may move toward the emission layer through the hole transport region, and electrons provided from the cathode may move toward the emission layer through the electron transport region. The holes and the electrons recombine in the emission layer to produce excitons. The excitons may transition from an excited state to a ground state, thus generating light. 
     SUMMARY 
     One or more embodiments include a composition that may satisfy certain conditions and an organic light-emitting device including the same. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments of the disclosure. 
     According to an aspect, an organic light-emitting device includes a first electrode; a second electrode; and an organic layer located between the first electrode and the second electrode and including an emission layer, wherein the emission layer includes a host, a sensitizer, and an emitter, the host, the sensitizer, and the emitter are different from each other, and the sensitizer is represented by Formula 1: 
       M 1 (L 1 )(L 2 )  Formula 1
 
     wherein, in Formula 1, 
     M 1  is Au, Ag, or Cu, 
     L 1  is a ligand represented by Formula 2-1 or Formula 2-2, and 
     L 2  is a ligand represented by one of Formulae 3-1, 3-2, or 3-3: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 2-1, 2-2, and 3-1 to 3-3, 
     A 21 , A 31 , and A 32  are each independently a monocyclic or polycyclic C 5 -C 30  carbocyclic group or a monocyclic or polycyclic C 1 -C 30  heterocyclic group, 
     X 31  is a single bond, O, S, N(R 36 ), C(R 36 )═C(R 37 ), [C(R 36 )(R 37 )] n31 , or [Si(R 36 )(R 37 )] n31 , and n31 is 1 or 2, 
     R 21 , R 22 , R 31 , and R 32  are each independently a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 7 -C 60  alkyl aryl group, a substituted or unsubstituted C 7 -C 60  aryl alkyl group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted C 2 -C 60  alkyl heteroaryl group, a substituted or unsubstituted C 2 -C 60  heteroaryl alkyl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, or a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, 
     R 23 , R 24 , and R 33  to R 37  are each independently hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 1 -C 60 alkylthio 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 7 -C 60  alkyl aryl group, a substituted or unsubstituted C 7 -C 60  aryl alkyl 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 2 -C 60  alkyl heteroaryl group, a substituted or unsubstituted C 2 -C 60  heteroaryl alkyl 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 ), —Ge(Q 1 )(Q 2 )(Q 3 ), —C(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 ), or —P(═S)(Q 1 )(Q 2 ), and R 36  and R 37  are optionally linked to each other to form a substituted or unsubstituted C 5 -C 30  carbocyclic group or a substituted or unsubstituted C 1 -C 30  heterocyclic group, and 
     b23, b33, and b34 are each independently 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, 
     wherein Q 1  to Q 3  are each independently hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, a C 7 -C 60  aryl alkyl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 2 -C 60  alkyl heteroaryl group, a C 2 -C 60  heteroaryl alkyl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each of which is unsubstituted or substituted with at least one of deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a C 6 -C 30  aryl group, or a combination thereof, and 
     each * indicates a binding site to M 1  in Formula 1. 
     According to another aspect, an organic light-emitting device includes: a first electrode; a second electrode; m emission units stacked between the first electrode and the second electrode, each including at least one emission layer; and m−1 charge generating layers located between each two adjacent emission units from among the m emission units, wherein each of the m−1 charge generating layers includes an n-type charge generating layer and a p-type charge generating layer, wherein m is an integer of 2 or greater, a maximum emission wavelength of light emitted from at least one of the m emission units may differ from a maximum emission wavelength of light emitted from at least one of the other emission units, at least one of the m emission layers includes a host, a dopant, and a sensitizer, the host, the sensitizer, and the emitter are different from each other, and the sensitizer is represented by Formula 1. 
     According to still another aspect, an organic light-emitting device includes: a first electrode; a second electrode; and m emission layers located between the first electrode and the second electrode, wherein m is an integer of 2 or greater, a maximum emission wavelength of light emitted from at least one of the m emission layers differs from a maximum emission wavelength of light emitted from at least one of the other emission layers, at least one of the m emission layers includes a host, a sensitizer, and an emitter, the host, the sensitizer, and the emitter are different from each other, and the sensitizer is represented by Formula 1. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain exemplary embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a schematic view of an organic light-emitting device  10  according to an embodiment; 
         FIG.  2    is a schematic view of an organic light-emitting device  100  according to an embodiment; and 
         FIG.  3    is a schematic view of an organic light-emitting device  200  according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in further detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the exemplary embodiments are merely described below, by referring to the figures, to explain particular aspects. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     The terminology used herein is for the purpose of describing one or more exemplary embodiments only 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. The term “or” means “and/or.” It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
     It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another element, component, region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present embodiments. 
     Exemplary embodiments are described herein with reference to cross section illustrations that are schematic illustrations of idealized embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments described herein should not be construed as limited to the particular shapes of regions as illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region illustrated or described as flat may, typically, have rough and/or nonlinear features. Moreover, sharp angles that are illustrated may be rounded. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region and are not intended to limit the scope of the present claims. 
     It will be understood that when an element is referred to as being “on” another element, it can be directly in contact with the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     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 general inventive concept belongs. It will be further understood that 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 the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     “About” or “approximately” as used herein is inclusive of the stated value and means within an acceptable range of deviation for the particular value as determined by one of ordinary skill in the art, considering the measurement in question and the error associated with measurement of the particular quantity (i.e., the limitations of the measurement system). For example, “about” can mean within one or more standard deviations, or within ±30%, 20%, 10%, 5% of the stated value. 
     Hereinafter, a work function or a highest occupied molecular orbital (HOMO) energy level is expressed as an absolute value from a vacuum level. In addition, when the work function or the HOMO energy level is referred to be “deep,” “high” or “large,” the work function or the HOMO energy level has a large absolute value based on “0 eV” of the vacuum level, while when the work function or the HOMO energy level is referred to be “shallow,” “low,” or “small,” the work function or HOMO energy level has a small absolute value based on “0 eV” of the vacuum level. 
     Description of FIG.  1   
       FIG.  1    is a schematic view of an organic light-emitting device  10  according to an exemplary embodiment. Hereinafter a structure and a method of manufacturing the organic light-emitting device  10 , according to an embodiment, will be described with reference to  FIG.  1   . 
     In  FIG.  1   , an organic light-emitting device  10  includes a first electrode  11 , a second electrode  19  facing the first electrode  11 , and an organic layer  10 A located between the first electrode  11  and the second electrode  19 . 
     In  FIG.  1   , the organic layer  10 A includes an emission layer  15 , a hole transport region  12  is located between the first electrode  11  and an emission layer  15 , and an electron transport region  17  is located between the emission layer  15  and the second electrode  19 . 
     A substrate may be additionally disposed or located under the first electrode  11  or on the second electrode  19 . The substrate may be a conventional substrate used in organic light-emitting devices, e.g., a glass substrate or a transparent plastic substrate, each having excellent mechanical strength, thermal stability, transparency, surface smoothness, ease of handling, and water repellency. 
     First Electrode  11   
     The first electrode  11  may be produced by depositing or sputtering, onto the substrate, a material for forming the first electrode  11 . The first electrode  11  may be an anode. The material for forming the first electrode  11  may be selected from materials with a high work function for easy hole injection. 
     The first electrode  11  may be a reflective electrode, a semi-transmissive electrode, or a transmissive electrode. When the first electrode  110  is a transmissive electrode, a material for forming the first electrode  110  may be indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO 2 ), zinc oxide (ZnO), or a combination thereof, but embodiments are not limited thereto. In some embodiments, when the first electrode  110  is a semi-transmissive electrode or a reflective electrode, as a material for forming the first electrode  110 , at least one of magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), or a combination thereof may be used, but embodiments are not limited thereto. 
     The first electrode  11  may have a single-layered structure or a multi-layered structure including a plurality of layers. 
     Emission Layer  15   
     The emission layer may include a host, a sensitizer, and an emitter, and the host, the sensitizer, and the emitter may be different from each other. 
     Sensitizer 
     The sensitizer is represented by Formula 1: 
       M 1 (L 1 )(L 2 )  Formula 1
 
     wherein, in Formula 1, 
     M 1  is Au, Ag, or Cu, 
     L 1  is a ligand represented by Formula 2-1 or Formula 2-2, and 
     L 2  is a ligand represented by one of Formulae 3-1, 3-2, or 3-3: 
     
       
         
         
             
             
         
       
     
     For example, in Formula 1, M 1  may be Au. 
     In Formulae 2-2, 3-2, and 3-3, A 21 , A 31 , and A 32  are each independently a monocyclic or polycyclic C 5 -C 30  carbocyclic group or a monocyclic or polycyclic C 1 -C 30  heterocyclic group. 
     For example, in Formulae 2-2, 3-2, and 3-3, A 21 , A 31 , and A 32  may each independently be a phenyl group, a naphthalene group, a phenanthrene group, a furan group, a thiophene group, a pyrrole group, a cyclopentene group, a silole group, a germole group, a benzofuran group, a benzothiophene group, an indole group, an indene group, a benzosilole group, a benzogermole group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, a fluorene group, a dibenzosilole group, a dibenzogermole group, a pyridine group, a pyrimidine group, a pyridazine group, or a pyrazine group. 
     In some embodiments, in Formula 2-2, A 21  may be a phenyl group, a naphthalene group, a pyridine group, a pyrimidine group, a pyridazine group, or a pyrazine group. 
     In some embodiments, in Formula 3-2 and 3-3, A 31  and A 32  may each independently be a phenyl group, a naphthalene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, a pyridine group, a pyrimidine group, a pyridazine group, or a pyrazine group. 
     In Formula 3-2 and 3-3, X 31  is a single bond, O, S, N(R 36 ), C(R 36 )═C(R 37 ), [C(R 36 )(R 37 )] n31 , or [Si(R 36 )(R 37 )] n31 , and n31 is 1 or 2, wherein R 36  and R 37  may respectively be understood by referring to the descriptions of R 36  and R 37  provided herein. 
     For example, in Formulae 3-2 and 3-3, X 31  may be a single bond, O, S, N(R 36 ), C(R 36 )═C(R 37 ), C(R 36 )(R 37 ), or Si(R 36 )(R 37 ). 
     In Formula 2-1, 2-2, and 3-1, R 21 , R 22 , R 31 , and R 32  are each independently a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 7 -C 60  alkyl aryl group, a substituted or unsubstituted C 7 -C 60  aryl alkyl group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted C 2 -C 60  alkyl heteroaryl group, a substituted or unsubstituted C 2 -C 60  heteroaryl alkyl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, or a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group. 
     In some embodiments, in Formulae 2-1, 2-2, and 3-1 to 3-3, R 21 , R 22 , R 31 , and R 32  may each independently be: 
     a C 1 -C 20  alkyl group unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a sulfonic acid group or a salt thereof, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 2 alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]a hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, —Si(Q 11 )(Q 12 )(Q 13 ), —Ge(Q 11 )(Q 12 )(Q 13 ), —C(Q 11 )(Q 12 )(Q 13 ), —B(Q 11 )(Q 12 ), —N(Q 11 )(Q 12 ), —P(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), —P(═S)(Q 11 )(Q 12 ), or a combination thereof, or 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, or an azadibenzothiophenyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a sulfonic acid group or a salt thereof, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), —P(═S)(Q 21 )(Q 22 ), or a combination thereof, 
     wherein Q 11  to Q 13  and Q 21  to Q 23  are each independently: 
     hydrogen, deuterium, —F, a cyano group, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 , or an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a deuterated C 1 -C 10  alkyl group, a phenyl group, or a combination thereof. 
     In some embodiments, in Formulae 2-1, 2-2, and 3-1, R 21 , R 22 , R 31 , and R 32  may each independently be: 
     a C 1 -C 20  alkyl group unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 2  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 11 )(Q 12 )(Q 13 ), —Ge(Q 11 )(Q 12 )(Q 13 ), —C(Q 11 )(Q 12 )(Q 13 ), —B(Q 11 )(Q 12 ), —N(Q 11 )(Q 12 ), —P(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), —P(═S)(Q 11 )(Q 12 ), or a combination thereof; or 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, or an azadibenzothiophenyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), —P(═S)(Q 21 )(Q 22 ), or a combination thereof; 
     wherein Q 11  to Q 13  and Q 21  to Q 23  are each independently: 
     deuterium, —F, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a deuterated C 1 -C 10  alkyl group, a phenyl group, or a combination thereof. 
     In one or more embodiments, in Formulae 2-1, 2-2, and 3-1, R 21 , R 22 , R 31 , and R 32  may each independently be —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a group represented by one of Formulae 9-1 to 9-39, a group represented by one of Formulae 9-1 to 9-39 in which at least one hydrogen is substituted with deuterium, a group represented by one of Formulae 9-1 to 9-39 in which at least one hydrogen is substituted with —F, a group represented by one of Formulae 9-201 to 9-236, a group represented by one of Formulae 9-201 to 9-236 in which at least one hydrogen is substituted with deuterium, a group represented by one of Formulae 9-201 to 9-236 in which at least one hydrogen is substituted with —F, a group represented by one of Formulae 10-1 to 10-130, a group represented by one of Formulae 10-1 to 10-130 in which at least one hydrogen is substituted with deuterium, a group represented by one of Formulae 10-1 to 10-130 in which at least one hydrogen is substituted with —F, a group represented by one of Formulae 10-201 to 10-358, a group represented by one of Formulae 10-201 to 10-358 in which at least one hydrogen is substituted with deuterium, or a group represented by one of Formulae 10-201 to 10-358 in which at least one hydrogen is substituted with —F: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 9-1 to 9-39, 9-201 to 9-236, 10-1 to 10-130, and 10-201 to 10-358, * indicates a binding site to an adjacent atom, “Ph” represents a phenyl group, “TMS” and “SiMe 3 ” each represent a trimethylsilyl group, and “TMG” and “GeMe 3 ” each represent a trimethylgermyl group. 
     In some embodiments, in Formulae 2-1, 2-2, and 3-1, R 21 , R 22 , R 31 , and R 32  may each independently be —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a group represented by one of Formulae 9-1 to 9-39, a group represented by one of Formulae 9-1 to 9-39 in which at least one hydrogen atom is substituted with a deuterium atom, a group represented by one of Formulae 9-1 to 9-39 in which at least one hydrogen atom is substituted with —F, a group represented by one of Formulae 10-1 to 10-130, a group represented by one of Formulae 10-1 to 10-130 in which at least one hydrogen atom is substituted with a deuterium atom, or a group represented by one of Formulae 10-1 to 10-130 in which at least one hydrogen atom is substituted with —F. 
     The “group represented by Formulae 9-1 to 9-39 in which at least one hydrogen is substituted with deuterium” and the “group represented by Formulae 9-201 to 9-236 in which at least one hydrogen is substituted with deuterium” may each be, for example, a group represented by one of Formulae 9-501 to 9-514 and 9-601 to 9-636: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The “group represented by Formulae 9-1 to 9-39 in which at least one hydrogen is substituted with —F” and the “group represented by Formulae 9-201 to 9-236 in which at least one hydrogen is substituted with —F” may each be, for example, a group represented by one of Formulae 9-701 to 9-710: 
     
       
         
         
             
             
         
       
     
     The “group represented by Formulae 10-1 to 10-130 in which at least one hydrogen is substituted with a deuterium” and the “group represented by Formulae 10-201 to 10-358 in which at least one hydrogen is substituted with deuterium” may each be, for example, a group represented by one of Formulae 10-501 to 576: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The “group represented by Formulae 10-1 to 10-130 in which at least one hydrogen is substituted with —F” and the “group represented by Formulae 10-201 to 10-358 in which at least one hydrogen is substituted with —F” may each be, for example, a group represented by one of Formulae 10-601 to 617: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Formulae 2-1, 2-2, and 3-1 to 3-3, R 23 , R 24 , and R 33  to R 37  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, a substituted or unsubstituted C 7 -C 60  aryl alkyl 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 2 -C 60  alkyl heteroaryl group, a substituted or unsubstituted C 2 -C 60  heteroaryl alkyl 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 ), —Ge(Q 1 )(Q 2 )(Q 3 ), —C(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 ), or —P(═S)(Q 1 )(Q 2 ), 
     R 36  and R 37  may optionally be linked to each other to form a substituted or unsubstituted C 5 -C 30  carbocyclic group or a substituted or unsubstituted C 1 -C 30  heterocyclic group, 
     wherein Q 1  to Q 3  are each independently hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, a C 7 -C 60  aryl alkyl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 2 -C 60  alkyl heteroaryl group, a C 2 -C 60  heteroaryl alkyl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each of which is unsubstituted or substituted with at least one of deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a C 6 -C 30  aryl group, or a combination thereof. 
     For example, in Formulae 2-1, 2-2, and 3-1 to 3-3, R 23 , R 24 , and R 33  to R 37  may each independently be: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , 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, or a phosphoric acid group or a salt thereof; 
     a C 1 -C 20  alkyl group, a C 1 -C 20  alkenyl group, a C 1 -C 20  alkoxy group, or a C 1 -C 20  alkylthio group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a sulfonic acid group or a salt thereof, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]a hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, —Si(Q 11 )(Q 12 )(Q 13 ), —Ge(Q 11 )(Q 12 )(Q 13 ), —C(Q 11 )(Q 12 )(Q 13 ), —B(Q 11 )(Q 12 ), —N(Q 11 )(Q 12 ), —P(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), —P(═S)(Q 11 )(Q 12 ), or a combination thereof, or 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, or an azadibenzothiophenyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a sulfonic acid group or a salt thereof, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), —P(═S)(Q 21 )(Q 22 ), or a combination thereof; or 
     —Si(Q 1 )(Q 2 )(Q 3 ), —Ge(Q 1 )(Q 2 )(Q 3 ), —C(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 ), or —P(═S)(Q 1 )(Q 2 ), 
     wherein Q 1  to Q 3 , Q 11  to Q 13 , and Q 21  to Q 23  may each independently be: 
     hydrogen, deuterium, —F, a cyano group, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a deuterated C 1 -C 10  alkyl group, a phenyl group, or a combination thereof. 
     In some embodiments, in Formulae 2-1, 2-2, and 3-1 to 3-3, R 23 , R 24 , and R 33  to R 37  may each independently be: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, or a nitro group; 
     a C 1 -C 20  alkyl group unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 11 )(Q 12 )(Q 13 ), —Ge(Q 11 )(Q 12 )(Q 13 ), —C(Q 11 )(Q 12 )(Q 13 ), —B(Q 11 )(Q 12 ), —N(Q 11 )(Q 12 ), —P(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), —P(═S)(Q 11 )(Q 12 ), or a combination thereof, 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, or an azadibenzothiophenyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), —P(═S)(Q 21 )(Q 22 ), or a combination thereof; or 
     —Si(Q 1 )(Q 2 )(Q 3 ), —Ge(Q 1 )(Q 2 )(Q 3 ), —C(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), or —N(Q 1 )(Q 2 ), and 
     R 36  and R 37  may optionally be linked to each other to form a substituted or unsubstituted C 5 -C 30  carbocyclic group or a substituted or unsubstituted C 1 -C 30  heterocyclic group, 
     wherein Q 1  to Q 3 , Q 11  to Q 13 , and Q 21  to Q 23  may each independently be: 
     deuterium, —F, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a deuterated C 1 -C 10  alkyl group, a phenyl group, or a combination thereof. 
     In some embodiments, in Formulae 2-1, 2-2, and 3-1 to 3-3, R 23 , R 24 , and R 33  to R 37  may each independently be: 
     hydrogen, deuterium, —F, a cyano group, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a group represented by one of Formulae 9-1 to 9-39, a group represented by one of Formulae 9-1 to 9-39 in which at least one hydrogen is substituted with deuterium, a group represented by one of Formulae 9-1 to 9-39 in which at least one hydrogen is substituted with —F, a group represented by one of Formulae 9-201 to 9-236, a group represented by one of Formulae 9-201 to 9-236 in which at least one hydrogen is substituted with deuterium, a group represented by one of Formulae 9-201 to 9-236 in which at least one hydrogen is substituted with —F, a group represented by one of Formulae 10-1 to 10-130, a group represented by one of Formulae 10-1 to 10-130 in which at least one hydrogen is substituted with deuterium, a group represented by one of Formulae 10-1 to 10-130 in which at least one hydrogen is substituted with —F, a group represented by one of Formulae 10-201 to 10-358, a group represented by one of Formulae 10-201 to 10-358 in which at least one hydrogen is substituted with deuterium, a group represented by one of Formulae 10-201 to 10-358 in which at least one hydrogen is substituted with —F, —Si(Q 1 )(Q 2 )(Q 3 ), —Ge(Q 1 )(Q 2 )(Q 3 ), or —C(Q 1 )(Q 2 )(Q 3 ), 
     R 36  and R 37  may optionally be linked to each other to form a substituted or unsubstituted C 5 -C 30  carbocyclic group or a substituted or unsubstituted C 1 -C 30  heterocyclic group, 
     wherein Q 1  to Q 3  may be each independently: 
     deuterium, —F, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 , or 
     an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a phenyl group, or a combination thereof. 
     In some embodiments, in Formulae 2-1, 2-2, and 3-1 to 3-3, R 23 , R 24 , and R 33  to R 37  may each independently be hydrogen, deuterium, —F, a cyano group, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a group represented by one of Formulae 9-1 to 9-39, a group represented by one of Formulae 9-1 to 9-39 in which at least one hydrogen atom is substituted with a deuterium atom, a group represented by one of Formulae 9-1 to 9-39 in which at least one hydrogen atom is substituted with —F, a group represented by one of Formulae 10-1 to 10-130, a group represented by one of Formulae 10-1 to 10-130 in which at least one hydrogen atom is substituted with a deuterium atom, a group represented by one of Formulae 10-1 to 10-130 in which at least one hydrogen atom is substituted with —F, —Si(Q 1 )(Q 2 )(Q 3 ), or —Ge(Q 1 )(Q 2 )(Q 3 ), 
     R 36  and R 37  may optionally be linked to each other to form a substituted or unsubstituted C 5 -C 30  carbocyclic group or a substituted or unsubstituted C 1 -C 30  heterocyclic group, 
     wherein Q 1  to Q 3  may be each independently: 
     —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 ; or 
     a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a deuterated C 1 -C 10  alkyl group, a phenyl group, or a combination thereof. 
     In Formulae 2-2, 3-2, and 3-3, b23, b33, and b34 may respectively indicate the number of substitution of R 23 , R 33 , and R 34 , and b23, b33, and b34 may each independently be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10. 
     In Formulae 2-1, 2-2, and 3-1 to 3-3, * indicates a binding site to M 1  in Formula 1. 
     In some embodiments, in Formula 1, L 1  may be a ligand represented by Formula 2-11 or Formula 2-12. In Formula 1, L 2  may be a ligand represented by one of Formulae 3-11 to 3-17: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Formulae 2-11, 2-12, and 3-11 to 3-17, 
     X 31 , R 23 , R 24 , R 34 , and R 35  are respectively understood by referring to the descriptions of X 31 , R 23 , R 24 , R 34 , and R 35  in Formula 1, and 
     R 21a  to R 21e , R 22a  to R 22e , R 23a  to R 23d , R 31a  to R 31e , R 32a  to R 32e , R 33a  to R 33d , and R 34a  to R 34d  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a sulfonic acid group or a salt thereof, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), or —P(═S)(Q 21 )(Q 22 ), 
     wherein Q 21  to Q 23  may each independently be: 
     hydrogen, deuterium, —F, a cyano group, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 , or 
     an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a deuterated C 1 -C 10  alkyl group, a phenyl group, or a combination thereof, and 
     * indicates a binding site to M 1  in Formula 1. 
     For example, in Formula 3-13, at least one of R 33a  to R 33d  and R 34a  to R 34d  may be a cyano group, but embodiments are not limited thereto. 
     In some embodiments, in Formula 3-13, one of R 33a  to R 33d  and R 34a  to R 34d  may be a cyano group, and in some embodiments, R 34a , R 34b , R 34c , or R 34d  may be a cyano group. 
     In some embodiments, in Formula 1, L 1  may be a ligand represented by Formula 2-11 or Formula 2-12, and L 2  may be a ligand represented by Formula 3-13. 
     In some embodiments, in Formula 1, L 1  may be a ligand represented by one of Formulae 2-21 to 2-26: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 2-21 to 2-26, 
     R 23 , R 24 , and R 23a  to R 23d  may each independently be: hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, or a nitro group; 
     a C 1 -C 20  alkyl group unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 11 )(Q 12 )(Q 13 ), —Ge(Q 11 )(Q 12 )(Q 13 ), —C(Q 11 )(Q 12 )(Q 13 ), —B(Q 11 )(Q 12 ), —N(Q 11 )(Q 12 ), —P(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), —P(═S)(Q 11 )(Q 12 ), or combination thereof, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, or an azadibenzothiophenyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), —P(═S)(Q 21 )(Q 22 ), or a combination thereof; or 
     —Si(Q 1 )(Q 2 )(Q 3 ), —Ge(Q 1 )(Q 2 )(Q 3 ), —C(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), or —N(Q 1 )(Q 2 ), and 
     R 21a , R 21b , R 21c , R 21d , R 21e , R 22a , R 22b , R 22c , R 22d , and R 22e  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a sulfonic acid group or a salt thereof, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), or —P(═S)(Q 21 )(Q 22 ), 
     wherein Q 1  to Q 3 , Q 11  to Q 13 , and Q 21  to Q 23  may each independently be: 
     hydrogen, deuterium, —F, a cyano group, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 , or 
     an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a deuterated C 1 -C 10  alkyl group, a phenyl group, or a combination thereof, and 
     * indicates a binding site to M 1  in Formula 1. 
     In some embodiments, in Formulae 2-21, 2-22, 2-24, and 2-25, at least one of R 21a , R 21b , R 22a , and R 21b  may not be hydrogen. 
     In some embodiments, in Formula 1, L 2  may be a ligand represented by one of Formulae 3-21 to 3-26: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 3-21 to 3-26, 
     R 36  and R 37  may each independently be: hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, or a nitro group; 
     a C 1 -C 20  alkyl group unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 11 )(Q 12 )(Q 13 ), —Ge(Q 11 )(Q 12 )(Q 13 ), —C(Q 11 )(Q 12 )(Q 13 ), —B(Q 11 )(Q 12 ), —N(Q 11 )(Q 12 ), —P(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), —P(═S)(Q 11 )(Q 12 ), or a combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, or an azadibenzothiophenyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), —P(═S)(Q 21 )(Q 22 ), or a combination thereof; or 
     —Si(Q 1 )(Q 2 )(Q 3 ), —Ge(Q 1 )(Q 2 )(Q 3 ), —C(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), or —N(Q 1 )(Q 2 ), and 
     R 33a  to R 33d  and R 34a  to R 34d  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a sulfonic acid group or a salt thereof, a C 1 -C 20  alkyl group, a deuterated C 2 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a (C 1 -C 20  alkyl)cyclopentyl group, a (C 1 -C 20  alkyl)cyclohexyl group, a (C 1 -C 20  alkyl)cycloheptyl group, a (C 1 -C 20  alkyl)cyclooctyl group, a (C 1 -C 20  alkyl)adamantanyl group, a (C 1 -C 20  alkyl)norbornanyl group, a (C 1 -C 20  alkyl)norbornenyl group, a (C 1 -C 20  alkyl)cyclopentenyl group, a (C 1 -C 20  alkyl)cyclohexenyl group, a (C 1 -C 20  alkyl)cycloheptenyl group, a (C 1 -C 20  alkyl)bicyclo[1.1.1]pentyl group, a (C 1 -C 20  alkyl)bicyclo[2.1.1]hexyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.1]heptyl group, a (C 1 -C 20  alkyl)bicyclo[2.2.2]octyl group, a phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl 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 isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl 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 benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —Ge(Q 21 )(Q 22 )(Q 23 ), —C(Q 21 )(Q 22 )(Q 23 ), —B(Q 21 )(Q 22 ), —N(Q 21 )(Q 22 ), —P(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), or —P(═S)(Q 21 )(Q 22 ), 
     wherein Q 1  to Q 3 , Q 11  to Q 13 , and Q 21  to Q 23  may each independently be: 
     hydrogen, deuterium, —F, a cyano group, —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, —CD 2 CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —CH 2 CF 3 , —CH 2 CF 2 H, —CH 2 CFH 2 , —CHFCH 3 , —CHFCF 2 H, —CHFCFH 2 , —CHFCF 3 , —CF 2 CF 3 , —CF 2 CF 2 H, or —CF 2 CFH 2 , or 
     an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, a phenyl group, a biphenyl group, or a naphthyl group, each unsubstituted or substituted with at least one of deuterium, —F, a C 1 -C 10  alkyl group, a deuterated C 1 -C 10  alkyl group, a benzene group, or a combination thereof, and 
     * indicates a binding site to M 1  in Formula 1. 
     For example, in Formula 3-21, at least one of R 33a  to R 33d  and R 34a  to R 34d  may be a cyano group, but embodiments are not limited thereto. 
     In some embodiments, in Formula 3-21, one of R 33a  to R 33d  and R 34a  to R 34d  may be a cyano group, and in some embodiments, R 34c  or R 34d  may be a cyano group. 
     In some embodiments, the sensitizer may be a compound represented by Group I, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Group 1, “Ph” represents a phenyl group. 
     A content (i.e., an amount) of the sensitizer in the emission layer may be in a range of about 1 weight (wt %) to about 50 wt %, or for example, about 10 wt % to about 20 wt %. When the content of the sensitizer in the emission layer is within this range, energy transfer in the emission layer may be effectively achieved. Thus, the organic light-emitting device may have high efficiency and long lifespan. 
     A difference (in electron volts, eV) between a lowest excited singlet energy level (S 1 ) of the sensitizer and a lowest excited triplet energy level (T 1 ) of the sensitizer may be about 0.3 eV or less, but embodiments are not limited thereto. For example, the Si and T 1  energy levels may be evaluated by density functional theory (DFT) using the Gaussian 09 program with the molecular structure optimization obtained at the B3LYP-basis level. When the difference is within this range, the reverse intersystem crossing of the sensitizer may be facilitated, thus providing improved efficiency and colorimetric purity of an organic light-emitting device. 
     In one or more embodiments, the sensitizer does not emit light. For example, the sensitizer may not emit light. 
     Host 
     In one or more embodiments, the host does not include a metal atom. For example, the host may not include a metal atom. 
     In some embodiments, the host may include at least one of a hole transporting host, an electron transporting host, or a bipolar host. 
     When the host includes a first compound and a second compound, the first compound and the second compound may respectively include a hole transporting host, an electron transporting host, or a bipolar host. 
     In some embodiments, the first compound may include a hole transporting host, and the second compound may include an electron transporting host; the first compound may include an electron transporting host, and the second compound may include a hole transporting host; the first compound and the second compound may respectively include a bipolar host; the first compound may include a hole transporting host, and the second compound may include a bipolar host; the first compound may include an electron transporting host, and the second compound may include a bipolar host; the first compound may include a bipolar host, and the second compound may include a hole transporting host; or the first compound may include a bipolar host, and the second compound may include an electron transporting host. 
     The electron transporting host may include at least one electron transporting moiety. The hole transporting host may include a hole transporting moiety and may not include an electron transporting moiety. The bipolar host may include at least one electron transporting moiety and at least one hole transporting moiety. 
     The electron transporting moiety may be a cyano group, —F, —CFH 2 , —CF 2 H, —CF 3 , a π electron-depleted nitrogen-containing C 1 -C 60  cyclic group, or a group represented by one of Formulae ET-moiety: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae ET-moiety, *, *′, and *″ may each indicate a binding site to an adjacent atom. 
     The hole transporting moiety may be a π electron-rich C 3 -C 60  cyclic group or a group represented by Formula HT-moiety: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula HT-moiety, *, *′, *″, and *′″ each indicate a binding site to an adjacent atom. 
     In an embodiment, an electron transporting host may include at least one of a cyano group or a π electron-depleted nitrogen-containing C 1 -C 60  cyclic group. 
     In one or more embodiments, the electron transporting host may include at least one cyano group. 
     In one or more embodiments, an electron transporting host may include a cyano group and at least one π electron-depleted nitrogen-containing C 1 -C 60  cyclic group. 
     In some embodiments, the hole transporting host may include at least one r electron-depleted nitrogen-free C 1 -C 60  cyclic group, and does not include an electron transporting moiety. 
     The term “π electron-depleted nitrogen-containing C 1 -C 60  cyclic group” as used herein refers to a cyclic group having 1 to 60 carbon atoms and including at least one *—N═*′ as a ring-forming moiety (wherein * and *′ each indicate a binding site to an adjacent atom). For example, the π electron-depleted nitrogen-containing C 1 -C 60  cyclic group may be a) a first ring, b) a condensed ring in which at least two first rings are condensed, or c) a condensed ring in which at least one first ring and at least one second ring are condensed. 
     The “first ring” as used herein may be an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, or a thiadiazole group. 
     The “second ring” as used herein may be a phenyl group, a cyclopentadiene group, a pyrrole group, a furan group, a thiophene group, or a silole group. 
     In some embodiments, the π electron-depleted nitrogen-containing C 1 -C 60  cyclic group may be an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a benzoisoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline group, a benzoquinoxaline group, a quinazoline group, a cinnoline group, a phenanthridine group, an acridine group, a phenanthroline group, a phenazine group, a benzimidazole group, an isobenzothiazole group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a thiadiazole group, an imidazopyridine group, an imidazopyrimidine group, an azacarbazole group, an azadibenzofuran group, an azadibenzothiophene group, an azadibenzosilole group, an acridine group, or a pyridopyrazine group: 
     The term “π electron-rich C 3 -C 60  cyclic group” as used herein refers to a cyclic group having 3 to 60 carbon atoms and not including *—N═*′ as a ring-forming moiety (wherein * and *′ each indicate a binding site to an adjacent atom). For example, the π electron-rich C 3 -C 60  cyclic group may be a) a second ring or b) a condensed ring in which at least two second rings are condensed. 
     In some embodiments, the π electron-rich C 3 -C 60  cyclic group may be a phenyl group, a heptalene group, an indene group, a naphthalene group, an azulene group, an indacene group, an acenaphthylene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a naphthacene group, a picene group, a perylene group, a pentacene group, a hexacene group, a pentaphene group, a rubicene group, a coronene group, an ovalene group, a pyrrole group, a furan group, a thiophene group, an isoindole group, an indole group, an indene group, a benzofuran group, a benzothiophene group, a benzosilole group, a naphthopyrrole group, a naphthofuran group, a naphthothiophene group, a naphthosilole group, a benzocarbazole group, a dibenzocarbazole group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, a dibenzosilole group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, a benzothienocarbazole group, a benzosilolocarbazole group, a triindolobenzene group, a pyrrolophenanthrene group, a furanophenanthrene group, a thienophenanthrene group, a benzonaphthofuran group, a benzonaphthothiophene group, an (indolo)phenanthrene group, a (benzofurano)phenanthrene group, or a (benzothieno)phenanthrene group. 
     In some embodiments, the electron transporting host may be a compound represented by Formula E-1, and the hole transporting host may be a compound represented by Formula H-1, but embodiments are not limited thereto: 
       [Ar 301 ] xb11 -[(L 301 ) xb1 -R 301 ] xb21   Formula E-1
 
     wherein, in Formula E-1, 
     Ar 301  may be a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     xb11 may be 1, 2, or 3, 
     L 301  may each independently be a single bond, a substituted or unsubstituted C 5 -C 60  carbocyclic group, a substituted or unsubstituted C 1 -C 60  heterocyclic group, or a group represented by one of following Formulae, wherein in the following Formulae, *, *′, and *″ may each indicate a binding site to an adjacent atom, 
     
       
         
         
             
             
         
       
     
     xb1 may be an integer from 1 to 5, 
     R 301  may be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, a substituted or unsubstituted C 7 -C 60  aryl alkyl 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 2 -C 60  alkyl heteroaryl group, a substituted or unsubstituted C 2 -C 60  heteroaryl alkyl 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 301 )(Q 302 )(Q 303 ), —N(Q 301 )(Q 302 ), —B(Q 301 )(Q 302 ), —C(═O)(Q 301 ), —S(═O) 2 (Q 301 ), —S(═O)(Q 301 ), —P(═O)(Q 301 )(Q 302 ), or —P(═S)(Q 301 )(Q 302 ), 
     xb21 may be an integer from 1 to 5, 
     wherein Q 301  to Q 303  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a C 1 -C 10  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group, and 
     at least one of Conditions A, B, and C may be satisfied: 
     Condition A 
     wherein, at least one of Ar 301 , L 301 , and R 301  in Formula E-1 may each independently include a π electron-depleted nitrogen-containing C 1 -C 60  cyclic group, 
     Condition B 
     wherein, in Formula E-1, L 301  may be a group represented by one of the following Formulae, and 
     
       
         
         
             
             
         
       
     
     Condition C 
     wherein, in Formula E-1, R 301  may be a cyano group, —S(═O) 2 (Q 301 ), —S(═O)(Q 301 ), —P(═O)(Q 301 )(Q 302 ), or —P(═S)(Q 301 )(Q 302 ) 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae H-1, 11, and 12, 
     L 401  may be:
         a single bond, or   a phenyl group, a heptalene group, an indene group, a naphthalene group, an azulene 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 pentaphene group, a rubicene group, a corozene 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 benzofurocarbazole group, a benzothienocarbazole group, or a triindolobenzene group, each unsubstituted or substituted with at least one of deuterium, a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a C 1 -C 10  alkylthio group, a phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a triphenylenyl group, a biphenyl group, a terphenyl group, a tetraphenyl group, or —Si(Q 401 )(Q 402 )(Q 403 ),       

     xd1 may be an integer from 1 to 10; and when xd1 is 2 or greater, at least two L 401  groups may be identical to or different from each other, 
     Ar 401  may be a group represented by Formulae 11 or 12, 
     Ar 402  may be:
         a group represented by Formulae 11 or 12, a phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a biphenyl group, a terphenyl group, or a triphenylenyl group; or   a phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a biphenyl group, a terphenyl group, and a triphenylenyl group, each substituted with at least one of deuterium, a hydroxyl 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20 alkylthio group, a phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a biphenyl group, a terphenyl group, a triphenylenyl group, or a combination thereof;       

     CY 401  and CY 402  may each independently be a phenyl group, a naphthalene group, a fluorene group, a carbazole group, a benzocarbazole group, an indolocarbazole group, a dibenzofuran group, a dibenzothiophene group, a dibenzosilole group, a benzonaphthofuran group, a benzonaphthothiophene group, or a benzonaphthosilole group, 
     A 21  may be a single bond, O, S, N(R 51 ), C(R 51 )(R 52 ), or Si(R 51 )(R 52 ), 
     A 22  may be a single bond, O, S, N(R 53 ), C(R 53 )(R 54 ), or Si(R 53 )(R 54 ), 
     at least one of A 21  and A 22  in Formula 12 may not be a single bond, 
     R 51  to R 54 , R 60 , and R 70  may each independently be: 
     hydrogen, deuterium, a hydroxyl 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, or a C 1 -C 20 alkylthio group; 
     a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, or a C 1 -C 30  alkylthio group, each substituted with at least one of deuterium, a hydroxyl 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 phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, or a combination thereof; 
     a π electron-depleted nitrogen-free C 1 -C 60  cyclic group (e.g., a phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a biphenyl group, a terphenyl group, and a triphenylenyl group); 
     a π electron-depleted nitrogen-free C 1 -C 60  cyclic group (e.g., a phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a biphenyl group, a terphenyl group, and a triphenylenyl group) substituted with at least one of deuterium, a hydroxyl 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a biphenyl group, or a combination thereof, or 
     —Si(Q 404 )(Q 405 )(Q 406 ), 
     e1 and e2 may each independently be an integer from 0 to 10, 
     wherein Q 401  to Q 406  may each independently be hydrogen, deuterium, a hydroxyl 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 phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a biphenyl group, a terphenyl group, or a triphenylenyl group, and 
     * indicates a binding site to an adjacent atom. 
     In some embodiments, in Formula E-1, Ar 301  and L 301  may each independently be a phenyl 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, 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, or an azacarbazole group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 2  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a cyano group-containing phenyl group, a cyano group-containing biphenyl group, a cyano group-containing terphenyl group, a cyano group-containing naphthyl group, a pyridinyl group, a phenylpyridinyl group, a diphenylpyridinyl group, a biphenylpyridinyl group, a di(biphenyl)pyridinyl group, a pyrazinyl group, a phenylpyrazinyl group, a diphenylpyrazinyl group, a biphenylpyrazinyl group, a di(biphenyl)pyrazinyl group, a pyridazinyl group, a phenylpyridazinyl group, a diphenylpyridazinyl group, a biphenylpyridazinyl group, a di(biphenyl)pyridazinyl group, a pyrimidinyl group, a phenylpyrimidinyl group, a diphenylpyrimidinyl group, a biphenylpyrimidinyl group, a di(biphenyl)pyrimidinyl group, a triazinyl group, a phenyltriazinyl group, a diphenyltriazinyl group, a biphenyltriazinyl group, a di(biphenyl)triazinyl 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 ), —P(═O)(Q 31 )(Q 32 ), or a combination thereof, 
     at least one of L 301  in the number of xb1 may each independently be an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a 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, or an azacarbazole group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a cyano group-containing phenyl group, a cyano group-containing biphenyl group, a cyano group-containing terphenyl group, a cyano group-containing naphthyl group, a pyridinyl group, a phenylpyridinyl group, a diphenylpyridinyl group, a biphenylpyridinyl group, a di(biphenyl)pyridinyl group, a pyrazinyl group, a phenylpyrazinyl group, a diphenylpyrazinyl group, a biphenylpyrazinyl group, a di(biphenyl)pyrazinyl group, a pyridazinyl group, a phenylpyridazinyl group, a diphenylpyridazinyl group, a biphenylpyridazinyl group, a di(biphenyl)pyridazinyl group, a pyrimidinyl group, a phenylpyrimidinyl group, a diphenylpyrimidinyl group, a biphenylpyrimidinyl group, a di(biphenyl)pyrimidinyl group, a triazinyl group, a phenyltriazinyl group, a diphenyltriazinyl group, a biphenyltriazinyl group, a di(biphenyl)triazinyl 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 ), —P(═O)(Q 31 )(Q 32 ), or a combination thereof, and 
     R 301  may be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, a tetraphenyl group, a naphthyl group, a cyano group-containing phenyl group, a cyano group-containing biphenyl group, a cyano group-containing terphenyl group, a cyano group-containing tetraphenyl group, a cyano group-containing naphthyl group, a pyridinyl group, a phenylpyridinyl group, a diphenylpyridinyl group, a biphenylpyridinyl group, a di(biphenyl)pyridinyl group, a pyrazinyl group, a phenylpyrazinyl group, a diphenylpyrazinyl group, a biphenylpyrazinyl group, a di(biphenyl)pyrazinyl group, a pyridazinyl group, a phenylpyridazinyl group, a diphenylpyridazinyl group, a biphenylpyridazinyl group, a di(biphenyl)pyridazinyl group, a pyrimidinyl group, a phenylpyrimidinyl group, a diphenylpyrimidinyl group, a biphenylpyrimidinyl group, a di(biphenyl)pyrimidinyl group, a triazinyl group, a phenyltriazinyl group, a diphenyltriazinyl group, a biphenyltriazinyl group, a di(biphenyl)triazinyl 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 ), or —P(═O)(Q 31 )(Q 32 ), 
     wherein Q 31  to Q 33  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a C 1 -C 10  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group, but embodiments are not limited thereto. 
     According to another embodiment, Ar 301  may be a phenyl 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, or a dibenzothiophene group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 10  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenyl group substituted with at least one cyano group, a biphenyl group substituted with at least one cyano group, a terphenyl group substituted with at least one cyano group, a naphthyl group substituted with at least one cyano group, a pyridinyl group, a phenylpyridinyl group, a diphenylpyridinyl group, a biphenylpyridinyl group, a di(biphenyl)pyridinyl group, a pyrazinyl group, a phenylpyrazinyl group, a diphenylpyrazinyl group, a biphenylpyrazinyl group, a di(biphenyl)pyrazinyl group, a pyridazinyl group, a phenylpyridazinyl group, a diphenylpyridazinyl group, a biphenylpyridazinyl group, a di(biphenyl)pyridazinyl group, a pyrimidinyl group, a phenylpyrimidinyl group, a diphenylpyrimidinyl group, a biphenylpyrimidinyl group, a di(biphenyl)pyrimidinyl group, a triazinyl group, a phenyltriazinyl group, a diphenyltriazinyl group, a biphenyltriazinyl group, a di(biphenyl)triazinyl 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 ), or —P(═O)(Q 31 )(Q 32 ), or 
     a group represented by one of Formulae 5-1 to 5-3 or 6-1 to 6-33, and 
     L 301  may be a group represented by one of Formulae 5-1 to 5-3 or 6-1 to 6-33: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Formulae 5-1 to 5-3 and 6-1 to 6-33, 
     Z 1  may be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenyl group substituted with at least one cyano group, a biphenyl group substituted with at least one cyano group, a terphenyl group substituted with at least one cyano group, a naphthyl group substituted with at least one cyano group, a pyridinyl group, a phenylpyridinyl group, a diphenylpyridinyl group, a biphenylpyridinyl group, a di(biphenyl)pyridinyl group, a pyrazinyl group, a phenylpyrazinyl group, a diphenylpyrazinyl group, a biphenylpyrazinyl group, a di(biphenyl)pyrazinyl group, a pyridazinyl group, a phenylpyridazinyl group, a diphenylpyridazinyl group, a biphenylpyridazinyl group, a di(biphenyl)pyridazinyl group, a pyrimidinyl group, a phenylpyrimidinyl group, a diphenylpyrimidinyl group, a biphenylpyrimidinyl group, a di(biphenyl)pyrimidinyl group, a triazinyl group, a phenyltriazinyl group, a diphenyltriazinyl group, a biphenyltriazinyl group, a di(biphenyl)triazinyl 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 ), or —P(═O)(Q 31 )(Q 32 ), 
     d4 may be 0, 1, 2, 3, or 4, 
     d3 may be 0, 1, 2, or 3, 
     d2 may be 0, 1, or 2, and 
     * and *′ each indicate a binding site to an adjacent atom, 
     wherein Q 31  to Q 33  may respectively be understood by referring to the descriptions of Q 31  to Q 33  provided herein. 
     In one or more embodiments, L 301  may be a group represented by one of Formulae 5-2, 5-3, or 6-8 to 6-33. 
     In one or more embodiments, R 301  may be a cyano group or a group represented by one of Formulae 7-1 to 7-18, and at least one of Ar 402  in the number of xd11 may be a group represented by one of Formulae 7-1 to 7-18, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Formulae 7-1 to 7-18, 
     xb41 to xb44 may each be 0, 1, or 2, provided that xb41 in Formula 7-10 may not be 0, xb41+xb42 in Formulae 7-11 to 7-13 may not be 0, xb41+xb42+xb43 in Formulae 7-14 to 7-16 may not be 0, xb41+xb42+xb43+xb44 in Formulae 7-17 and 7-18 may not be 0, and * indicates a binding site to an adjacent atom. 
     In Formula E-1, at least two Ar 301 (s) may be identical to or different from each other, and at least two L 301 (s) may be identical to or different from each other. In Formula H-1, at least two L 401 (s) may be identical to or different from each other, and at least two Ar 402 (s) may be identical to or different from each other. 
     The electron transporting host may be, for example, a compound represented by one of Groups HE1 to HE7, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the electron transporting host may include DPEPO, TSPO1, or Compound ET host: 
     
       
         
         
             
             
         
       
     
     In some embodiments, the hole transporting host may be a compound represented by one of Group HH1, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the hole transporting host may include o-CBP, mCP or Compound HT host: 
     
       
         
         
             
             
         
       
     
     In some embodiments, the bipolar host may be a compound represented by one of Group HEH1, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein in Compounds 1 to 432, 
     “Ph” represents a phenyl group. 
     A content (i.e., an amount) of the host in the emission layer may be in a range of about 30 wt % to about 90 wt %. When the content of the host in the emission layer is within this range, energy transfer in the emission layer may be effectively occurred. Thus, the organic light-emitting device may have high efficiency and long lifespan. 
     In some embodiments, when the host includes a first compound and a second compound, the first compound and the second compound may form an exciplex, but embodiments are not limited thereto. When an exciplex is formed, the exciplex may be understood by referring to the description of the host provided herein. 
     When the host includes a first compound and a second compound, a weight ratio of the first compound to the second compound may be in a range of about 1:9 to about 9:1, for example, about 2:8 to about 8:2, for example, about 4:6 to 6:4, or for example, about 5:5. 
     In one or more embodiments, the host does not emit light. For example, the host may not emit light. 
     Emitter 
     In one or more embodiments, the emitter emits light. For example, the emitter may emit light. 
     In some embodiments, the emitter may be a fluorescent dopant, a fluorescent dopant that may emit delayed fluorescence, a delayed fluorescent dopant, or a combination thereof. Accordingly, a decay time of the emitter (T decay (E)) may be less than 100 microseconds (μs). 
     T decay (E) may be measured from a time-resolved photoluminescence (TRPL) spectrum at room temperature of a film having a thickness of 40 nanometers (nm) formed by vacuum-depositing the host and the emitter at a weight ratio of 90:10 included in the emission layer on a quartz substrate at a vacuum degree of 10 −7  torr. 
     The emitter may be a condensed polycyclic compound or a styryl-containing compound. 
     In some embodiments, the emitter may include a naphthalene-containing core, a fluorene-containing core, a spiro-bifluorene-containing core, a benzofluorene-containing core, a dibenzofluorene-containing core, a phenanthrene-containing core, an anthracene-containing core, a fluoranthene-containing core, a triphenylene-containing core, a pyrene-containing core, a chrysene-containing core, a picene-containing core, a perylene-containing core, a pentacene-containing core, an indenoanthracene-containing core, a tetracene-containing core, a bisanthracene-containing core, or a core represented by one of Formulae 501-1 to 501-20: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the emitter may be represented by Formula 51: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 51, 
     Ar 51  may be a naphthalene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a picene group, a perylene group, a pentacene group, an indenoanthracene group, a tetracene group, a bisanthracene group, or a group represented by one of Formulae 501-1 to 501-20, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a sulfonic acid group or a salt thereof, 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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, a C 7 -C 60  aryl alkyl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 2 -C 60  alkyl heteroaryl group, a C 2 -C 60  heteroaryl alkyl 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 511 )(Q 512 )(Q 513 ), —Ge(Q 511 )(Q 512 )(Q 513 ), —C(Q 511 )(Q 512 )(Q 513 ), —B(Q 511 )(Q 512 ), —N(Q 511 )(Q 512 ), —P(Q 511 )(Q 512 ), —C(═O)(Q 511 ), —S(═O)(Q 511 ), —S(═O) 2 (Q 511 ), —P(═O)(Q 511 )(Q 512 ), —P(═S)(Q 511 )(Q 512 ), or a combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     L 511  to L 514  may each independently be 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, or a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     a511 to a514 may each independently be 0, 1, 2, or 3, 
     R 511  to R 513  may each independently be a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 7 -C 60  alkyl aryl group, a substituted or unsubstituted C 7 -C 60  aryl alkyl group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted C 2 -C 60  alkyl heteroaryl group, a substituted or unsubstituted C 2 -C 60  heteroaryl alkyl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, or a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, 
     wherein Q 511  to Q 513  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, a C 7 -C 60  aryl alkyl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 2 -C 60  alkyl heteroaryl group, a C 2 -C 60  heteroaryl alkyl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each unsubstituted or substituted with at least one of deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, and a C 6 -C 30  aryl group, a C 6 -C 60  aryl group, or a combination thereof, and 
     n511 and n512 may each independently 0, 1, 2, 3, 4, 5, or 6. 
     In some embodiments, in Formula 51, a sum of n511 and n512 may be 1 or greater, but embodiments are not limited thereto. 
     In some embodiments, in Formula 51, R 511  and R 512  may each independently be 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 pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazole group, a triazinyl group, a dibenzofuranyl group, or a dibenzothiophenyl group; or 
     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 pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a triazinyl group, a dibenzofuranyl group, or a dibenzothiophenyl group, each substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio 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 pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a triazinyl group, a dibenzofuranyl group, a dibenzothiophenyl group, or a combination thereof. 
     In some embodiments, the emitter may be a compound represented by one of Group FD1: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The maximum emission wavelength (λ max ) of an emission spectrum of the emitter may be about 400 nm or greater and about 650 nm or lower. In some embodiments, the maximum emission wavelength of an emission spectrum of the fluorescence emitter may be about 400 nm or greater and about 550 nm or lower, about 400 nm or greater and about 495 nm or lower, or about 450 nm or greater and about 495 nm or lower, but embodiments are not limited thereto. The emitter may emit blue light to green light, for example, blue light, but embodiments are not limited thereto. The “maximum emission wavelength” as used herein refers to a wavelength of which the emission intensity is greatest. In other words, the “maximum emission wavelength” may be referred to as “peak emission wavelength”. 
     The emission layer may have an emitter content (i.e., an amount) in a range of about 0.01 wt % to about 15 wt %, but embodiments are not limited thereto. 
     In the emission layer, regarding a content of the host, a content of the sensitizer, and a content of the emitter, the content of the host may be greatest, and the content of the emitter may be smallest, but embodiments are not limited thereto. 
     The organic light-emitting device may satisfy Condition 1: 
         S   1 ( H )&gt; S   1 ( S )≥ S   1 ( E )  Condition 1
 
     wherein, in Condition 1, 
     S 1 (H) indicates a lowest excited singlet energy level of the host, 
     S 1 (S) indicates a lowest excited singlet energy level of the sensitizer, and 
     S 1 (E) indicates a lowest excited singlet energy level of the emitter. 
     S 1 (H), S 1 (S), and S 1 (E) may respectively be obtained by calculation from photoluminescence (PL) spectrum obtained from a film having a thickness of 40 nm formed by vacuum-depositing the respective host, sensitizer, or emitter each on a quartz substrate at a vacuum degree of 10 −7  torr. 
     When Condition 1 is satisfied, the emitter may emit light, and the organic light-emitting device may have improved efficiency. 
     For example, when Condition 1 is satisfied, the emission ratio from the emitter in the organic light-emitting device may be about 85% or greater. When the above-described ratio is satisfied, in the organic light-emitting device, the emitter may substantially emit light only, and the exciplex and the sensitizer may not substantially emit light. 
     The singlet and/or triplet excitons formed in the host may be transitioned to the sensitizer, and triplet excitons may be transferred to singlet excitons through reverse intersystem crossing (RISC) in the sensitizer, and then the singlet excitons may be transferred to the emitter through Förster energy transfer (FRET). As singlet excitons and triplet excitons of the host may all be transferred to the emitter, the organic light-emitting device may have significantly improved lifespan and efficiency. 
     The host and the sensitizer may satisfy Condition 2: 
         T   1 ( H )≥ T   1 ( S )  Condition 2
 
     wherein, in Condition 2, 
     T 1 (H) indicates a lowest excited triplet energy level of the host, and 
     T 1 (S) indicates a lowest excited triplet energy level of the sensitizer. 
     The thickness of the emission layer may be in a range of about 100 angstrom (Å) to about 1,000 Å, and in some embodiments, about 200 Å to about 600 Å. When the thickness of the emission layer is within any of these ranges, improved luminescence characteristics may be obtained without a substantial increase in driving voltage. 
     Hole Transport Region  12   
     In the organic light-emitting device  10 , the hole transport region  12  may be located between the first electrode  11  and the emission layer  15 . 
     The hole transport region  12  may have a single-layered structure or a multi-layered structure. 
     For example, the hole transport region  12  may have a structure of hole injection layer, a structure of hole transport layer, a structure of hole injection layer/hole transport layer, a structure of hole injection layer/first hole transport layer/second hole transport layer, a structure of hole transport layer/intermediate layer, a structure of hole injection layer/hole transport layer/intermediate layer, a structure of hole transport layer/electron blocking layer, or a structure of hole injection layer/hole transport layer/electron blocking layer, but embodiments are not limited thereto. 
     The hole transport region  12  may include a compound having hole transport characteristics. 
     For example, the hole transport region  12  may include an amine-containing compound. 
     In an embodiment, the hole transport region  12  may include at least one compound represented by one of Formulae 201 to 205, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 201 to 205, 
     L 201  to L 209  may each independently be *—O—*′, *—S—*′, a substituted or unsubstituted C 5 -C 60  carbocyclic group, or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     xa1 to xa9 may each independently be an integer from 0 to 5, 
     R 201  to R 206  may each independently be 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 7 -C 60  alkyl 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 2 -C 60  alkyl 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, or a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, and two adjacent groups from R 201  to R 206  may optionally be linked together via a single bond, a dimethyl-methylene group, or a diphenyl-methylene group. 
     In some embodiments, 
     L 201  to L 209  may be a phenyl 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 pentaphene group, a rubicene group, a corozene 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 benzofurocarbazole group, a benzothienocarbazole group, or a triindolobenzene group, each unsubstituted or substituted with at least one of deuterium, a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a C 1 -C 10  alkylthio group, a phenyl group, a naphthyl group, a fluorenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a triphenylenyl group, a biphenyl group, a terphenyl group, a tetraphenyl group, —Si(Q 11 )(Q 12 )(Q 13 ), or a combination thereof, 
     xa1 to xa9 may each independently be 0, 1, or 2, and 
     R 201  to R 206  may each independently be 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, a pyridinyl group, an indenocarbazolyl group, an indolocarbazolyl group, a benzofurocarbazolyl group, or a benzothienocarbazolyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio 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 ), —N(Q 31 )(Q 32 ), or a combination thereof, 
     wherein Q 11  to Q 13  and Q 31  to Q 33  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a C 1 -C 10  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group. 
     According to an embodiment, the hole transport region  12  may include a carbazole-containing amine-containing compound. 
     In one or more embodiments, the hole transport region  12  may include a carbazole-containing amine-containing compound and a carbazole-free amine-containing compound. 
     The carbazole-containing amine-containing compound may be, for example, a compound represented by Formula 201 including a carbazole group and further including at least one of a dibenzofuran group, a dibenzothiophene group, a fluorene group, a spirofluorene group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, or a benzothienocarbazole group. 
     The carbazole-free amine-containing compound may be, for example, a compound represented by Formula 201 not including a carbazole group and including at least one of a dibenzofuran group, a dibenzothiophene group, a fluorene group, a spirofluorene group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, or a benzothienocarbazole group. 
     In one or more embodiments, the hole transport region  12  may include at least one compound represented by one of Formulae 201 or 202. 
     In an embodiment, the hole transport region  12  may include at least one compound represented by one of Formulae 201-1, 202-1, or 201-2, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     wherein in Formulae 201-1, 202-1, and 201-2, L 201  to L 203 , L 205 , xa1 to xa3, xa5, R 201 , and R 202  may each be understood by referring to the descriptions for those provided herein, and R 211  to R 213  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio 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 dimethylfluorenyl group, a diphenylfluorenyl group, a triphenylenyl 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, or a pyridinyl group. 
     In some embodiments, the hole transport region  12  may include at least one compound represented by HT1 to HT39, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The hole transport region  12  of the organic light-emitting device  10  may further include a p-dopant. When the hole transport region  12  further includes a p-dopant, the hole transport region  12  may have a structure including a matrix (for example, at least one compound represented by Formulae 201 to 205) and a p-dopant included in the matrix. The p-dopant may be homogeneously or non-homogeneously doped in the hole transport region  12 . 
     In some embodiments, a lowest unoccupied molecular orbital (LUMO) energy level of the p-dopant may be 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 embodiments are not limited thereto. 
     In some embodiments, the p-dopant may include: 
     a quinone derivative such as tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), or F6-TCNNQ; 
     a metal oxide such as tungsten oxide or molybdenum oxide; 
     1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN); or 
     a compound represented by Formula 221, 
     but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 221, 
     R 221  to R 223  may each independently be 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 7 -C 60  alkyl aryl group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted C 2 -C 60  alkyl heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, or a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, provided that at least one from R 221  to R 223  may include at least one substituent that is a cyano group, —F, —Cl, —Br, —I, —SF 5 , 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, or a C 1 -C 20  alkyl group substituted with —I. 
     A thickness of the hole transport region  12  may be in a range of about 100 Å to about 10,000 Å, e.g., about 400 Å to about 2,000 Å, and a thickness of the emission layer  15  may be in a range of about 100 Å to about 3,000 Å, e.g., about 300 Å to about 1,000 Å. When the thicknesses of the hole transport region  12  and the emission layer  15  are within any of these ranges, satisfactory hole transporting characteristics and/or luminescence characteristics may be obtained without a substantial increase in driving voltage. 
     Electron Transport Region  17   
     In the organic light-emitting device  10 , the electron transport region  17  may be located between the emission layer  15  and the second electrode  19 . 
     The electron transport region  17  may have a single-layered structure or a multi-layered structure. 
     For example, the electron transport region  17  may have a structure of electron transport layer, a structure of electron transport layer/electron injection layer, a structure of buffer layer/electron transport layer, a structure of hole blocking layer/electron transport layer, a structure of buffer layer/electron transport layer/electron injection layer, or a structure of hole blocking layer/electron transport layer/electron injection layer, but embodiments are not limited thereto. The electron transport region  17  may include an electron control layer. 
     The electron transport region  17  may include a known electron transport material. 
     The electron transport region  17  (for example, the buffer layer, the hole blocking layer, the electron control layer, or the electron transport layer in the electron transport region  17 ) may include a metal-free compound including at least one π electron-depleted nitrogen-containing C 1 -C 60  cyclic group. The π electron-depleted nitrogen-containing C 1 -C 60  cyclic group may be understood by referring to the description for those provided herein. 
     In some embodiments, the electron transport region may include a compound represented by Formula 601: 
       [Ar 601 ] xe11 -[(L 601 ) xe1 -R 601 ] xe21   Formula 601
 
     wherein, in Formula 601, 
     Ar 601  and L 601  may each independently be a 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, 
     xe1 may be an integer from 0 to 5, 
     R 601  may be 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 7 -C 60  alkyl 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 2 -C 60  alkyl 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 601 )(Q 602 )(Q 603 ), —C(═O)(Q 601 ), —S(═O) 2 (Q 601 ), or —P(═O)(Q 601 )(Q 602 ), 
     wherein Q 601  to Q 603  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a C 1 -C 10  alkylthio 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 embodiment, at least one Ar 601 (s) in the number of xe11 and R 601 (s) in the number of xe21 may include the π electron-depleted nitrogen-containing C 1 -C 60  cyclic group. 
     In an embodiment, ring Ar 601  and L 601  in Formula 601 may be a phenyl 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, or an azacarbazole group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio 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 ), —P(═O)(Q 31 )(Q 32 ), or a combination thereof, 
     wherein Q 31  to Q 33  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a C 1 -C 10  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group. 
     When xe11 in Formula 601 is 2 or greater, at least two Ar 601 (s) may be linked to each other via a single bond. 
     In one or more embodiments, Ar 601  in Formula 601 may be an anthracene group. 
     In one or more embodiments, a compound represented by Formula 601 may be represented by Formula 601-1: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 601-1, 
     X 614  may be N or C(R 614 ), X 615  may be N or C(R 615 ), X 616  may be N or C(R 616 ), and at least one of X 614  to X 616  may be N, 
     L 611  to L 613  may each independently be understood by referring to the description of L 601  provided herein, 
     xe611 to xe613 may each independently be understood by referring to the description of xe1 provided herein, 
     R 611  to R 613  may each independently be understood by referring to the description of R 601  provided herein, and 
     R 614  to R 616  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group. 
     In one or more embodiments, xe1 and xe611 to xe613 in Formulae 601 and 601-1 may each independently be 0, 1, or 2. 
     In one or more embodiments, in Formulae 601 and 601-1, R 601  and R 611  to R 613  may each independently be 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, or an azacarbazolyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , a hydroxyl group, a cyano group, a nitro 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 C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 1 -C 20  alkylthio 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, or a combination thereof, or 
     —S(═O) 2 (Q 601 ) or —P(═O)(Q 601 )(Q 602 ), 
     wherein Q 601  and Q 602  may respectively be understood by referring to the descriptions of Q 601  and Q 602  provided herein. 
     The electron transport region may include at least one compound represented by one of Compounds ET1 to ET36, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the electron transport region 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), Alq 3 , BAlq, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ), and NTAZ: 
     
       
         
         
             
             
         
       
     
     The thicknesses of the buffer layer, the hole blocking layer, or the electron control layer may each independently be in a range of about 20 Å to about 1,000 Å, and in some embodiments, about 30 Å to about 300 Å. When the thicknesses of the buffer layer, the hole blocking layer or the electron control layer are within any of these ranges, excellent hole blocking characteristics or excellent electron controlling characteristics may be obtained without a substantial increase in driving voltage. 
     The thickness of the electron transport layer may be in a range of about 100 Å to about 1,000 Å, and in some embodiments, about 150 Å to about 500 Å. When the thickness of the electron transport layer is within any of these ranges, excellent electron transport characteristics may be obtained without a substantial increase in driving voltage. 
     The electron transport region  17  (e.g., the electron transport layer in the electron transport region  17 ) may further include, in addition to the materials described above, a material including metal. 
     The metal-containing material may include at least one of an alkali metal complex or an alkaline earth metal complex. The alkali metal complex may include a metal ion that is a lithium (Li) ion, a sodium (Na) ion, a potassium (K) ion, a rubidium (Rb) ion, or a cesium (Cs) ion. The alkaline earth metal complex may include a metal ion that is a beryllium (Be) ion, a magnesium (Mg) ion, a calcium (Ca) ion, a strontium (Sr) ion, or a barium (Ba) ion. Each ligand coordinated with the metal ion of the alkali metal complex and the alkaline earth metal complex may independently be hydroxyquinoline, hydroxyisoquinoline, hydroxybenzoquinoline, hydroxyacridine, hydroxyphenanthridine, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxyphenyloxadiazole, hydroxyphenylthiadiazole, hydroxyphenylpyridine, hydroxyphenylbenzimidazole, hydroxyphenylbenzothiazole, bipyridine, phenanthroline, or cyclopentadiene, but embodiments are not limited thereto. 
     For example, the metal-containing material may include a Li complex. The Li complex may include, e.g., Compound ET-D1 (LiQ) or Compound ET-D2: 
     
       
         
         
             
             
         
       
     
     The electron transport region  17  may include an electron injection layer that facilitates injection of electrons from the second electrode  19 . The electron injection layer may be in direct contact with the second electrode  19 . 
     The electron injection layer may have i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer including a plurality of different materials, or iii) a multi-layered structure having a plurality of layers, each including a plurality of different materials. 
     The electron injection layer may include an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal 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 a combination thereof. 
     The alkali metal may be Li, Na, K, Rb, or Cs. In some embodiments, the alkali metal may be Li, Na, or Cs. In one or more embodiments, the alkaline metal may be Li or Cs, but is not limited thereto. 
     The alkaline earth metal may be Mg, Ca, Sr, or Ba. 
     The rare earth metal may be Sc, Y, Ce, Tb, Yb, or Gd. 
     The alkali metal compound, the alkaline earth metal compound, and the rare earth metal compound may each independently be oxides or halides (e.g., fluorides, chlorides, bromides, or iodines) of the alkali metal, the alkaline earth metal, or the rare earth metal, respectively. 
     The alkali metal compound may be an alkali metal oxide, such as Li 2 O, Cs 2 O, or K 2 O, or an alkali metal halide, such as LiF, NaF, CsF, KF, LiI, NaI, CsI, or KI. In an embodiment, the alkali metal compound may be LiF, Li 2 O, NaF, LiI, NaI, CsI, or KI, but embodiments are not limited thereto. 
     The alkaline earth metal compound may be an alkaline earth metal compound such as BaO, SrO, CaO, BaxSr 1-x O (wherein 0&lt;x&lt;1), or Ba x Ca 1-x O (wherein 0&lt;x&lt;1). In an embodiment, the alkaline earth metal compound may BaO, SrO, or CaO, but embodiments are not limited thereto. 
     The rare earth metal compound may be YbF 3 , ScF 3 , ScO 3 , Y 2 O 3 , Ce 2 O 3 , GdF 3 , or TbF 3 . In some embodiments, the rare earth metal compound may be YbF 3 , ScF 3 , TbF 3 , Ybl 3 , SCl 3 , or Tbl 3 , but embodiments are not limited thereto. 
     The alkali metal complex, the alkaline earth metal complex, and the rare earth metal complex may each include ions of the above-described alkali metal, alkaline earth metal, and/or rare earth metal. Each ligand coordinated with the metal ion of the alkali metal complex, the alkaline earth metal complex, and/or the rare earth metal complex may independently be hydroxyquinoline, hydroxyisoquinoline, hydroxybenzoquinoline, hydroxyacridine, hydroxyphenanthridine, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxyphenyloxadiazole, hydroxyphenylthiadiazole, hydroxyphenylpyridine, hydroxyphenylbenzimidazole, hydroxyphenylbenzothiazole, bipyridine, phenanthroline, or cyclopentadiene, but embodiments are not limited thereto. 
     The electron injection layer may comprise, consist essentially of, or 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 a combination thereof, as described above. In some embodiments, the electron injection layer may further include an organic material. When the electron injection layer further includes an organic material, the alkali metal, the alkaline earth metal, the rare earth metal, the alkali metal compound, the alkaline earth metal compound, the rare earth metal compound, the alkali metal complex, the alkaline earth metal complex, the rare earth metal complex, or a combination thereof may be homogeneously or non-homogeneously dispersed in a matrix including the organic material. 
     The thickness of the electron injection layer may be in a range of about 1 Å to about 100 Å, and in some embodiments, about 3 Å to about 90 Å. When the thickness of the electron injection layer is within any of these ranges, excellent electron injection characteristics may be obtained without a substantial increase in driving voltage. 
     Second Electrode  19   
     The second electrode  19  may be on the organic layer  10 A. In an embodiment, the second electrode  19  may be a cathode that is an electron injection electrode. In this embodiment, a material for forming the second electrode  19  may be a material having a low work function, for example, a metal, an alloy, an electrically conductive compound, or a combination thereof. 
     The second electrode  19  may include at least one of lithium (Li), silver (Ag), magnesium (Mg), aluminum (Al), silver (Ag), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), ITO, or IZO, but embodiments are not limited thereto. The second electrode  19  may be a transmissive electrode, a semi-transmissive electrode, or a reflective electrode. 
     The second electrode  19  may have a single-layered structure, or a multi-layered structure including two or more layers. 
     Hereinbefore the organic light-emitting device  10  has been described with reference to  FIG.  1   , but embodiments are not limited thereto. 
     Description of FIG.  2   
       FIG.  2    is a schematic view of an organic light-emitting device  100  according to an embodiment. 
     The organic light-emitting device  100  in  FIG.  2    may include a first electrode  110 , a second electrode  190  facing the first electrode  110 , and a first light-emitting unit  151  and a second light-emitting unit  152  located between the first electrode  100  and the second electrode  190 . A charge generating layer  141  may be located between the first light-emitting unit  151  and the second light-emitting unit  152 , and the charge generating layer  141  may include an n-type charge generating layer  141 -N and a p-type charge generating layer  141 -P. The charge generating layer  141  is a layer serving to generate charges and to supply the generated charges to the adjacent light-emitting unit, and may include a known material in the art. 
     The first light-emitting unit  151  may include a first emission layer  151 -EM, and the second light-emitting unit  152  may include a second emission layer  152 -EM. A maximum emission wavelength of light emitted by the first light-emitting unit  151  may be different from a maximum emission wavelength of light emitted by the second light-emitting unit  152 . For example, mixed light of the light emitted by the first light-emitting unit  151  and the light emitted by the second light-emitting unit  152  may be white light, but embodiments are not limited thereto. 
     A hole transport region  120  may be located between the first light-emitting unit  151  and the first electrode  110 , and the second light-emitting unit  152  may include a first hole transport region  121  located towards or most adjacent to the first electrode  110 . 
     An electron transport region  170  may be located between the second light-emitting unit  152  and the second electrode  190 , and the first light-emitting unit  151  may include a first electron transport region  171  located between the charge generating layer  141  and the first emission layer  151 -EM. 
     The first emission layer  151 -EM may include a host, a sensitizer, and an emitter, the host, the sensitizer, and the emitter may be different from each other, and the sensitizer may be represented by Formula 1. 
     The second emission layer  152 -EM may include a host, a sensitizer, and an emitter, the host, the sensitizer, and the emitter may be different from each other, and the sensitizer may be represented by Formula 1. 
     In  FIG.  2   , the first electrode  110  and the second electrode  190  may each be understood by referring to the descriptions for the first electrode  11  and the second electrode  19  in  FIG.  1   , respectively. 
     In  FIG.  2    the first emission layer  151 -EM and the second emission layer  152 -EM may each be understood by referring to the descriptions for the emission layer  15  in  FIG.  1   . 
     In  FIG.  2   , the hole transport region  120  and the first hole transport region  121  may each be understood by referring to the descriptions for the hole transport region  12  in  FIG.  1   . 
     In  FIG.  2   , the electron transport region  170  and the first electron transport region  171  may each be understood by referring to the descriptions for the electron transport region  17  in  FIG.  1   . 
     Hereinbefore, by referring to  FIG.  2   , the first light-emitting unit  151  and the second light-emitting unit  152  has been described as being included in an organic light-emitting device including the host, the sensitizer, and the emitter. However, the organic light-emitting device  100  in  FIG.  2    may be subjected to various modifications, for example, at least one of the first light-emitting unit  151  and the second light-emitting unit  152  of the organic light-emitting device  100  in  FIG.  2    may be replaced by any suitable light-emitting unit including those available in the art, or three or more light-emitting units may be included together. 
     Description of FIG.  3   
       FIG.  3    is a schematic view of an organic light-emitting device  200  according to another embodiment. 
     The organic light-emitting device  100  in  FIG.  4    includes a first electrode  210 , a second electrode  290  facing the first electrode  210 , and a first emission layer  251  and a second emission layer  252  located between the first electrode  210  and the second electrode  290 . 
     A maximum emission wavelength of light emitted by the first emission layer  251  may be different from a maximum emission wavelength of light emitted by the second emission layer  252 . For example, mixed light of the light emitted by the first emission layer  251  and the light emitted by the second emission layer  252  may be white light, but embodiments are not limited thereto. 
     A hole transport region  220  may be located between the first emission layer  251  and the first electrode  210 , and an electron transport region  270  may be located between the second emission layer  252  and the second electrode  290 . 
     The first emission layer  251  may include a host, a sensitizer, and an emitter, the host, the sensitizer, and the emitter may be different from each other, and the sensitizer may be represented by Formula 1. 
     The second emission layer  252  may include a host, a sensitizer, and an emitter, the host, the sensitizer, and the emitter may be different from each other, and the sensitizer may be represented by Formula 1. 
     In  FIG.  3   , the first electrode  210 , the hole transport region  220 , and the second electrode  290  may each be understood by referring to the descriptions for the first electrode  11 , the hole transport region  12 , and the second electrode  19  in  FIG.  1   , respectively. 
     In  FIG.  3   , the first emission layer  251  and the second emission layer  252  may each be understood by referring to the descriptions for the emission layer  15  in  FIG.  1   . 
     In  FIG.  3   , the electron transport region  170  may be understood by referring to the descriptions for the electron transport region  17  in  FIG.  1   . 
     Hereinbefore, by referring to  FIG.  3   , the first emission layer  251  and the second emission layer  252  has been described as being included in an organic light-emitting device including the sensitizer and the emitter described herein. However, the organic light-emitting device in  FIG.  3    may be subjected to various modifications, for example, one of the first emission layer  251  and the second emission layer  252  may be replaced by any suitable known layer, three or more layers may be included, or an interlayer may be further located between neighboring emission layers. 
     General Definitions of Terms 
     The term “C 1 -C 60  alkyl group” as used herein refers to a linear or branched saturated aliphatic hydrocarbon monovalent group having 1 to 60 carbon atoms. Examples thereof include a methyl group, an ethyl group, a propyl group, an iso-butyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an iso-amyl group, and a hexyl group. The term “C 1 -C 60  alkylene group” as used herein refers to a divalent group having substantially the same structure as the C 1 -C 60  alkyl group. 
     The term “C 1 -C 60  alkoxy group” as used herein refers to a monovalent group represented by —OA 101  (wherein A 101  is a C 1 -C 60  alkyl group). Examples thereof include a methoxy group, an ethoxy group, and an isopropyloxy group. 
     The term “C 1 -C 60  alkylthio group” as used herein refers to a monovalent group represented by —SA 101′  (wherein A 101′  is a C 1 -C 60  alkyl group). 
     The term “C 2 -C 60  alkenyl group” as used herein refers to a group formed by including at least one carbon-carbon double bond in the middle or at the terminus of the C 2 -C 60  alkyl group. Examples thereof include an ethenyl group, a propenyl group, and a butenyl group. The term “C 2 -C 60  alkenylene group” as used herein refers to a divalent group having substantially the same structure as the C 2 -C 60  alkenyl group. 
     The term “C 2 -C 6  alkynyl group” as used herein refers to a group formed by including at least one carbon-carbon triple bond in the middle or at the terminus of the C 2 -C 60  alkyl group. Examples thereof include an ethenyl group and a propenyl group. The term “C 2 -C 60  alkynylene group” as used herein refers to a divalent group having substantially the same structure as the C 2 -C 60  alkynyl group. 
     The term “C 3 -C 10  cycloalkyl group” as used herein refers to a monovalent monocyclic saturated hydrocarbon group including 3 to 10 carbon atoms. 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 substantially the same structure as the C 3 -C 10  cycloalkyl group. 
     The term “C 1 -C 10  heterocycloalkyl group” as used herein refers to a monovalent monocyclic group including at least one heteroatom selected from N, O, P, Si, Ge, Se, and S as a ring-forming atom and 1 to 10 carbon atoms. Examples thereof include a tetrahydrofuranyl group and a tetrahydrothiophenyl group. The term “C 1 -C 10  heterocycloalkylene group” as used herein refers to a divalent group having substantially the same structure as the C 1 -C 10  heterocycloalkyl group. 
     The term “C 3 -C 10  cycloalkenyl group” as used herein refers to a monovalent monocyclic group including 3 to 10 carbon atoms and at least one carbon-carbon double bond in its ring, wherein the molecular structure as a whole is non-aromatic. Examples thereof include a cyclopentenyl group, a cyclohexenyl group, and a cycloheptenyl group. The term “C 3 -C 10  cycloalkenylene group” as used herein refers to a divalent group having substantially the same structure as the C 3 -C 10  cycloalkenyl group. 
     The term “C 1 -C 10  heterocycloalkenyl group” as used herein refers to a monovalent monocyclic group including at least one heteroatom selected from N, O, P, Si, Ge, Se, 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 2,3-dihydrofuranyl group and a 2,3-dihydrothiophenyl group. The term “C 1 -C 10  heterocycloalkylene group” as used herein refers to a divalent group having substantially the same structure as the C 1 -C 10  heterocycloalkenyl group. 
     The term “C 6 -C 60  aryl group” as used herein refers to a monovalent group having a carbocyclic aromatic system having 6 to 60 carbon atoms. The term “C 6 -C 60  arylene group” as used herein refers to a divalent group having a carbocyclic aromatic system having 6 to 60 carbon atoms. Examples of the C 6 -C 60  aryl group include a phenyl group, a naphthyl group, an anthracenyl group, a phenanthrenyl group, a pyrenyl group, and a chrysenyl group. When the C 6 -C 60  aryl group and a C 6 -C 60  arylene group each include at least two rings, the at least two rings may be fused. 
     The term “C 7 -C 60  alkyl aryl group” as used herein refers to a monovalent group having a carbocyclic aromatic system substituted with an alkyl group, wherein the alkyl aryl group has 7 to 60 carbon atoms. Examples of the C 7 -C 60  alkyl aryl group include a methylphenyl group. 
     The term “C 7 -C 60  aryl alkyl group” a used herein refers to a monovalent group having an alkyl group substituted with a group having a carbocyclic aromatic system, wherein the aryl alkyl group has 7 to 60 carbon atoms. 
     The term “C 1 -C 60  heteroaryl group” as used herein refers to a monovalent group having a heterocyclic aromatic system having at least one heteroatom selected from N, O, P, Si, and S as a ring-forming atom and 1 to 60 carbon atoms. The term “C 1 -C 60  heteroarylene group” as used herein refers to a divalent group having a heterocyclic aromatic system having at least one heteroatom selected from N, O, P, Si, and S as a ring-forming atom and 1 to 60 carbon atoms. Examples of the C 1 -C 60  heteroaryl group include a pyridinyl group, a pyrimidinyl group, a pyrazinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, and an isoquinolinyl group. When the C 1 -C 60  heteroaryl group and the C 1 -C 60  heteroarylene group each include at least two rings, the at least two rings may be fused. 
     The term “C 2 -C 60  alkyl heteroaryl group” as used herein refers to a monovalent group having a heterocyclic aromatic system substituted with an alkyl group and having at least one heteroatom selected from N, O, P, Si, and S as a ring-forming atom and 1 to 60 carbon atoms. Examples of the C 2 -C 60  alkyl heteroaryl group include a methylpyridinyl group. 
     The term “C 2 -C 60  heteroaryl alkyl group” as used herein refers to a monovalent group having an alkyl group substituted with a heterocyclic aromatic system, and having at least one heteroatom selected from N, O, P, Si, and S as a ring-forming atom and 1 to 60 carbon atoms. 
     The term “C 6 -C 60  aryloxy group” as used herein refers to a group represented by —OA 102  (where A 102  is a C 6 -C 60  aryl group). The term “C 6 -C 60  arylthio group” as used herein refers to a group represented by —SA 103  (where A 103  is a C 6 -C 60  aryl group). 
     The term “C 1 -C 6  heteroaryloxy group” as used herein refers to a group represented by —OA 102′  (where A 102 , is a C 1 -C 60  heteroaryl group). The term “C 1 -C 6  heteroarylthio group” as used herein refers to a group represented by —SA 103 , (where A 103 , is a C 1 -C 60  heteroaryl group). 
     The term “monovalent non-aromatic condensed polycyclic group” as used herein refers to a monovalent group having two or more rings condensed and only carbon atoms (for example, the number of carbon atoms may be in a range of 8 to 60) as ring-forming atoms, wherein the molecular structure as a whole is non-aromatic. Examples of the non-aromatic condensed polycyclic group include a fluorenyl group. The term “divalent non-aromatic condensed polycyclic group” as used herein refers to a divalent group having substantially the same structure as the monovalent non-aromatic condensed polycyclic group. 
     The term “monovalent non-aromatic condensed heteropolycyclic group” as used herein refers to a monovalent group having at least two rings condensed and a heteroatom selected from N, O, P, Si, Ge, Se, and S as well as carbon atoms (for example, the number of carbon atoms may be in a range of 1 to 60) as ring-forming atoms, wherein the molecular structure as a whole is non-aromatic. Examples of the monovalent non-aromatic condensed heteropolycyclic group include a carbazolyl group. The term “divalent non-aromatic condensed heteropolycyclic group” as used herein refers to a divalent group having substantially the same structure as the monovalent non-aromatic condensed heteropolycyclic group. 
     The term “C 5 -C 30  carbocyclic group” as used herein refers to a saturated or unsaturated cyclic group including 5 to 30 carbon atoms only as ring-forming atoms. The C 5 -C 30  carbocyclic group may be a monocyclic group or a polycyclic group. Depending on formula structure, the C 5 -C 30  carbocyclic group may be monovalent, divalent, trivalent, quadrivalent, pentavalent, or hexavalent. 
     The term “C 1 -C 30  heterocyclic group” as used herein refers to saturated or unsaturated cyclic group including 1 to 30 carbon atoms and at least one heteroatom selected from N, O, P, Si, Ge, Se, and S as ring-forming atoms. The C 1 -C 30  heterocyclic group may be a monocyclic group or a polycyclic group. Depending on formula structure, the C 5 -C 30  heterocyclic group may be monovalent, divalent, trivalent, quadrivalent, pentavalent, or hexavalent. 
     In the present specification, at least one substituent of the substituted C 5 -C 30  carbocyclic group, the substituted C 1 -C 30  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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, the substituted C 7 -C 60  aryl alkyl 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 2 -C 60  alkyl heteroaryl group, the substituted C 2 -C 60  alkyl heteroaryl group, the substituted C 2 -C 60  heteroaryl alkyl group, the substituted C 1 -C 60  heteroaryloxy group, the substituted C 1 -C 60  heteroarylthio group, the substituted monovalent aromatic condensed polycyclic group, the substituted monovalent aromatic condensed heteropolycyclic group, the substituted monovalent non-aromatic condensed polycyclic group, and the substituted monovalent non-aromatic condensed heteropolycyclic group may be: 
     deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , 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 C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, or a C 1 -C 60  alkylthio group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, or a C 1 -C 60  alkylthio group, each substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , 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 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 7 -C 60  alkyl 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 2 -C 60  alkyl heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent aromatic condensed polycyclic group, a monovalent aromatic condensed heteropolycyclic group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 11 )(Q 12 ), —Si(Q 13 )(Q 14 )(Q 15 ), —B(Q 16 )(Q 17 ), —P(═O)(Q 18 )(Q 19 ), or a combination thereof; 
     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 7 -C 60  alkyl 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 2 -C 60  alkyl heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent aromatic condensed polycyclic group, a monovalent aromatic condensed heteropolycyclic group, a monovalent non-aromatic condensed polycyclic group, or 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 7 -C 60  alkyl 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 2 -C 60  alkyl heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent aromatic condensed polycyclic group, a monovalent aromatic condensed heteropolycyclic group, a monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one of deuterium, —F, —Cl, —Br, —I, —SF 5 , —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , 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 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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, a C 7 -C 60  aryl alkyl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 2 -C 60  alkyl heteroaryl group, a C 2 -C 60  heteroaryl alkyl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent aromatic condensed polycyclic group, a monovalent aromatic condensed heteropolycyclic group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 21 )(Q 22 ), —Si(Q 23 )(Q 24 )(Q 25 ), —B(Q 26 )(Q 27 ), —P(═O)(Q 28 )(Q 29 ), or a combination thereof; or 
     —N(Q 31 )(Q 32 ), —Si(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), or —P(═O)(Q 38 )(Q 39 ), 
     wherein Q 1  to Q 9 , Q 11  to Q 19 , Q 21  to Q 29 , and Q 31  to Q 39  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —SF 5 , 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 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 1 -C 60  alkylthio 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 7 -C 60  alkyl aryl group, a C 7 -C 60  aryl alkyl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a C 2 -C 60  heteroaryl alkyl group, a C 2 -C 60  alkyl heteroaryl group, a C 1 -C 60  heteroaryloxy group, a C 1 -C 60  heteroarylthio group, a monovalent aromatic condensed polycyclic group, a monovalent aromatic condensed heteropolycyclic group, a monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each of which is unsubstituted or substituted with at least one of deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a C 6 -C 30  aryl group, or a combination thereof. 
     The term “room temperature” as used herein refers to a temperature of about 25° C. 
     The terms “a biphenyl group, a terphenyl group, and a tetraphenyl group” as used herein each refer to a monovalent group having two, three, and four phenyl groups linked together via a single bond, respectively. 
     The terms “a phenyl group substituted with at least one cyano group, a biphenyl group substituted with at least one cyano group, a terphenyl group substituted with at least one cyano group, and a tetraphenyl group substituted with at least one cyano group” as used herein each refer to a phenyl group, a biphenyl group, a terphenyl group, and a tetraphenyl group, each substituted with at least one cyano group. In “the phenyl group substituted with at least one cyano group, the biphenyl group substituted with at least one cyano group, the terphenyl group substituted with at least one cyano group, and the tetraphenyl group substituted with at least one cyano group”, a cyano group may be substituted at any position of the ring or rings, and “the phenyl group substituted with at least one cyano group, the biphenyl group substituted with at least one cyano group, the terphenyl group substituted with at least one cyano group, and the tetraphenyl group substituted with at least one cyano group” may further include a substituent in addition to a cyano group. 
     Hereinafter, an organic light-emitting device, according to an embodiment, will be described in more detail with reference to Synthesis Examples and Examples; however, the present disclosure is not limited thereto. The wording “B was used instead of A” used in describing Synthesis Examples means that an identical molar equivalent of B was used in place of A. 
     EXAMPLES 
     Synthesis Example 
     
       
         
         
             
             
         
       
     
     Synthesis Example 1: Synthesis of Compound 7 
     1) Synthesis of Intermediate 7-(1) 
     A magnetic stir bar was put into a 250 milliliter (mL) two necked round-bottom flask and connected to a condenser. While maintaining the connection, residual water was removed therefrom by flame drying. 6.70 grams (g) (28.4 (millimoles) mmol) of 1,2-dibromobenzene was added to the cooled flask, 20.1 g (114 mmol) of 2,6-diisopropyl aniline, and 10.9 g (114 mmol) of sodium tertiary-butoxide (NaO t Bu) were added thereto and purged with argon gas. While stirring using a heating stirrer, 100 mL of anhydrous toluene was injected using a glass syringe, followed by stirring at room temperature for 10 minutes. 1.45 g (2.84 mmol) of bis(tri-tert-butylphosphine)palladium(0) (Pd(t-Bu 3 P) 2 ) was added to a vial, followed by adding 5 mL of anhydrous toluene, while argon was injected to form a Pd(tBu 3 P) 2  solution. The Pd(tBu 3 P) 2  solution was added to the reaction flask using a glass syringe. The temperature was slowly raised to 118° C., and the mixture was stirred and heated at reflux for 3 days. After stopping the stirring and heating, Celite™ was placed on the filter, and the reaction solution was filtered through a 500 mL recovery flask. The reaction solvent was evaporated from the filtered solution using a rotary evaporator, and a separation process was performed through column chromatography (dichloromethane:hexanes 1:49). (thin layer chromatography (TLC): R f  value acetate ethyl:hexane (1:99)=0.25) yield: 76% (9.30 g, yellowish brown powder). 
     Proton nuclear magnetic resonance ( 1 H NMR) spectroscopy (300 megahertz (MHz), CD 2 Cl 2 ) chemical shift (δ, parts per million (ppm)): 1.14 (d, J=6.9 Hz, 12H), 1.22 (d, J=6.9 Hz, 12H), 3.18 (sept, J=6.9 Hz, 4H), 5.23 (s, 2H) 6.22 (dd, J=6.3 Hz, 3.0 Hz, 2H), 6.60 (dd, J=6.3 Hz, 3.0 Hz, 2H), 7.23-7.29 (m, 6H). 
     Carbon-13 nuclear magnetic resonance  13 C{ 1 H} spectroscopy (126 MHz, CD 2 Cl 2 ) δ (ppm): 23.45, 25.00, 28.77, 114.89, 120.39, 124.35, 126.69, 137.45, 137.45, 145.98. 
     High resolution mass spectrometry using fast atom bombardment (HR MS (FAB)): Calcd for C 30 H 40 N 2  ([M] + ), 428.3191. Found: 428.3194. 
     Elemental analysis (C,H,N): Anal. Calcd for C 30 H 40 N 2 : C, 84.06; H, 9.41; N, 6.54. Found: C, 84.12; H, 9.43; N, 6.35%. 
     2) Synthesis of Intermediate 7-(2) 
     A magnetic stir bar and 9.00 g (21.0 mmol) of Intermediate 7-(1) were placed in a 500 mL two-necked round-bottom flask, and argon was injected thereto, while the flask was connected to the condenser and Vigreux column. While stirring using a heat stirrer, 200 mL of triethylorthoformate was added to the reaction flask using a glass syringe. While heating at reflux at 145° C. for 4 hours, ethanol, a by-product, was removed from the reaction mixture. After lowering the temperature, pressure was reduced in the flask using a vacuum pump to completely remove the remaining ethanol. The temperature was raised to 50° C. using a heat stirrer, and after injection of 43 mL of trimethylsilyl chloride (excess), the mixture was stirred for an additional 8 hours at 50° C. After cooling the reaction solution by lowering the temperature, the reaction solution was transferred to a 500 mL recovery flask, and then the reaction solvent was evaporated using a rotary evaporator. Ether was added to the reaction solution concentrated to a high concentration in dichloromethane for precipitation. The precipitated solution was poured into a glass filter to obtain only the precipitate filtered through the filter. Yield: 45% (4.20 g, grayish dark green powder). 
       1 H NMR (300 MHz, CD 2 Cl 2 ) δ (ppm): 1.16 (d, J=6.9 Hz, 12H), 1.30 (d, J=6.9 Hz, 12H), 2.27 (sept, J=6.9 Hz, 4H), 7.37 (dd, J=6.3 Hz, 3.0 Hz, 2H), 7.49 (d, J=7.8 Hz 4H), 7.70 (dd, J=6.3 Hz, 3.0 Hz, 2H), 7.72 (t, J=7.8 Hz, 2H). 
       13 C{ 1 H} NMR (126 MHz, CD 2 Cl 2 ) δ (ppm): 23.57, 25.19, 29.89, 113.94, 125.48, 127.94, 129.06, 132.73, 133.31, 146.54, 147.07, 147.21. 
     HR MS (FAB): Calcd for C 30 H 40 N 2  ([M] + ), 439.3108; Found: 439.3112. 
     Elemental analysis (C,H,N): Anal. Calcd for C 31 H 39 ClN 2 : C, 78.37; H, 8.27; N, 5.90. Found: C, 74.25; H, 8.50; N, 5.44%. 
     3) Synthesis of Intermediate 7-(3) 
     A magnetic stir bar and 4.00 g (8.42 mmol) of Intermediate 7-(2) were placed in a 100 mL two-necked round-bottom flask, and argon was injected thereto. While stirring using a stirrer, 30 mL of anhydrous tetrahydrofuran (THF) was injected using a glass syringe, followed by stirring for 5 minutes at 25° C. 9.26 mL (9.26 mmol) of 1.0 molar (M) potassium bis(trimethylsilyl)amide in THE was injected into a reaction flask by using a glass syringe, while the flask was purged with argon. The mixture was stirred for 1 hour at a temperature of 25° C. 2.48 g (8.42 mmol) of chloro(dimethylsulfide)gold(I) (AuClS(CH 3 ) 2 ) was placed in a vial and dissolved in 10 mL of anhydrous THE while the vial was purged with argon. After injection using a glass syringe in the reaction flask, the mixture was stirred at 25° C. After stopping the stirring, Celite™ was placed on the filter, and the reaction solution was filtered through a 500 mL recovery flask. The reaction solvent was evaporated from the filtered solution using a rotary evaporator. Hexanes were added to the reaction solution and concentrated to a high concentration in dichloromethane as a solvent for precipitation. The precipitated solution was poured into a glass filter to obtain the precipitate filtered through the filter. Yield: 45% (2.58 g, grayish purple powder). 
       1 H NMR (300 MHz, CD 2 Cl 2 ) δ (ppm): 1.10 (d, J=6.9 Hz, 12H), 1.32 (d, J=6.9 Hz, 12H), 2.40 (sept, J=6.9 Hz, 4H), 7.11 (dd, J=6.2 Hz, 3.0 Hz, 2H), 7.44 (dd, J=6.2 Hz, 3.0 Hz, 2H), 7.44 (d, J=7.8 Hz 4H), 7.66 (t, J=7.8 Hz, 2H). 
       13 C{ 1 H} NMR (126 MHz, CD 2 Cl 2 ) δ (ppm): 24.16, 24.92, 29.48, 112.62, 125.25, 126.09, 131.60, 131.81, 135.08, 147.15, 182.07. 
     Elemental analysis (C,H,N): Anal. Calcd for C 31 H 38 AuClN 2 : C, 55.48; H, 5.71; N, 4.17. Found: C, 55.86; H, 5.76; N, 4.12%. 
     4) Synthesis of Compound 7 
     A magnetic stir bar, 0.44 g (1.97 mmol) of 1,3,6,8-tetramethyl-9H-carbazole, 0.2 g (1.79 mmol) of Intermediate 7-(3) 1, and 0.21 g (2.15 mmol) of NaO t Bu were added to a 100 mL two necked round-bottom flask, and the flask was purged with argon. While stirring using a stirrer, 40 mL of anhydrous THE was injected using a glass syringe, followed by stirring over night at 25° C. After stopping the stirring, Celite™ was placed on the filter, and the reaction solution was filtered through a 500 mL recovery flask. The reaction solvent was evaporated from the filtered solution using a rotary evaporator. Hexane was added to the reaction solution concentrated to a high concentration in dichloromethane as a solvent for precipitation. The precipitated solution was poured into a glass filter to obtain only the precipitate filtered through the filter. Yield: 90% (1.38 g, beige powder). 
       1 H NMR (300 MHz, CD 2 Cl 2 ) δ (ppm): 1.09 (d, J=6.9 Hz, 12H), 1.30 (d, J=6.9 Hz, 12H), 2.00 (s, 6H), 2.37 (s, 6H), 2.53 (sept, J=6.9 Hz, 4H), 6.67 (s, 2H), 7.07 (dd, J=6.2 Hz, 3.0 Hz, 2H), 7.42 (dd, J=6.2 Hz, 3.0 Hz, 2H), 7.51 (s, 2H), 7.51 (d, J=7.8 Hz 4H), 7.74 (t, J=7.8 Hz, 2H). 
       13 C{ 1 H} NMR (126 MHz, CD 2 Cl 2 ) δ (ppm): 21.17, 21.37, 24.28, 24.71, 29.55, 112.55, 116.88, 122.75, 125.15, 125.22, 125.77, 125.78, 127.37, 131.83, 132.54, 135.43, 147.06, 147.33, 184.14. 
     HR MS (FAB): Calcd for C 43 H 46 AuN 3 , 857.3983; Found: 857.3973. 
     Elemental analysis (C,H,N): Anal. Calcd for C 47 H 54 AuN 3 : C, 65.80; H, 6.34; N, 4.90. Found: C, 65.96; H, 6.31; N, 4.80%. 
     Synthesis Example 2: Synthesis of Compound 8 
     A magnetic stir bar, 0.33 g (1.97 mmol) of carbazole, 1.2 g (1.79 mmol) of Intermediate 7-(3), and 0.21 g (2.15 mmol) of NaO t Bu were added to a 100 mL two necked round-bottom flask, and the flask was purged with argon. While stirring using a stirrer, 40 mL of anhydrous THE was injected using a glass syringe, followed by stirring over night at 25° C. After stopping the stirring, Celite™ was placed on the filter, and the reaction solution was filtered through a 500 mL recovery flask. The reaction solvent was evaporated from the filtered solution using a rotary evaporator. Hexane was added to the reaction solution concentrated to a high concentration in dichloromethane as a solvent for precipitation. The precipitated solution was poured into a glass filter to obtain only the precipitate filtered through the filter. Yield: 88% (1.27 g, light purple powder). 
       1 H NMR (300 MHz, CD 2 Cl 2 ) δ (ppm): 1.16 (d, J=6.9 Hz, 12H), 1.35 (d, J=6.9 Hz, 12H), 2.56 (sept, J=6.9 Hz, 4H), 6.69 (ddd, J=7.8 Hz, 1.2 Hz, 6 Hz, 2H), 6.87 (td, J=7.2 Hz, 1.2 Hz 3.0 Hz, 2H), 7.04 (td, J=7.2 Hz, 1.2 Hz 3.0 Hz, 2H), 7.24 (dd, J=6.2 Hz, 3.0 Hz, 2H), 7.49 (dd, J=6.2 Hz, 3.0 Hz, 2H), 7.53 (d, J=7.8 Hz 4H), 7.76 (t, J=7.8 Hz, 2H), 6.69 (ddd, J=7.8 Hz, 1.2 Hz, 6 Hz, 2H). 
       13 C{ 1 H} NMR (126 MHz, CD 2 Cl 2 ) δ (ppm): 24.37, 24.89, 29.66, 112.51, 114.01, 116.32, 119.66, 123.96, 124.09, 125.22, 125.90, 131.53, 132.19, 135.51, 147.62, 149.90, 185.93. 
     HR MS (FAB): Calcd for C 43 H 46 AuN 3 , 801.3357; Found: 801.3362. 
     Elemental analysis (C,H,N): Anal. Calcd for C 43 H 46 AuN 3 : C, 64.41; H, 5.78; N, 5.24. Found: C, 64.49; H, 5.75; N, 5.14%. 
     Example 1-1 
     As a glass substrate, ITO electrode-patterned having a thickness of 50 nm was sonicated in acetone, isopropyl alcohol, and deionized (DI) water for about 15 minutes each, and cleaned by exposure to ultraviolet (UV) rays and ozone for 30 minutes. 
     Subsequently, NDP-9 was deposited on the ITO electrode (anode) the glass substrate to a thickness of 10 nm, HT211 was deposited thereon to a thickness of 40 nm, and 2,2′-di(9H-carbazol-9-yl)biphenyl (o-CBP) was deposited thereon to a thickness of 10 nm. 
     Then, HT host (as a first compound), ET host (as a second compound), Compound 7 (as a sensitizer), and BD1-8 were co-deposited thereon in the ratio shown in Table 1 to form an emission layer having a thickness of 30 nm. 
     9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3,6-dicarbonitrile (mCP-2CN) was deposited on the emission layer to a thickness of 10 nm, DPEPO and LiQ were co-deposited thereon (at a weight ratio of 1:1) to a thickness of 30 nm, LiQ was deposited thereon to a thickness of 1 nm, and Al was deposited thereon to a thickness of 100 nm, thereby completing the manufacture of an organic light-emitting device having a structure of ITO (50 nm)/NDP-9 (10 nm)/HT211 (40 nm)/o-CBP (10 nm)/emission layer (30 nm)/mCP-2CN (10 nm)/DPEPO:LiQ (1:1) (30 nm)/LiQ (1 nm)/AI (100 nm). 
     Example 1-2 and Comparative Examples 1-1 to 1-3 and Examples 2-1 and 2-2, Comparative Examples 2-1 and 2-2 
     Organic light-emitting devices were manufactured in substantially the same manner as in Example 1, except that the host, the sensitizer, and the emitter were used in the emission layer as shown in Table 1. The amounts of sensitizer and emitter are each based on based on the total weight of the emission layer. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Weight ratio: 
                   
                 Sensitizer 
                   
                 Emitter 
               
               
                   
                 First 
                 Second 
                 First compound to 
                   
                 content 
                   
                 content 
               
               
                   
                 compound 
                 compound 
                 second compound 
                 Sensitizer 
                 (wt %) 
                 Emitter 
                 (wt %) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 1-1 
                 HT host 
                 ET host 
                 6:4 
                 Compound 7 
                 20 
                 BD1-8 
                 3 
               
               
                 Example 1-2 
                 HT host 
                 ET host 
                 6:4 
                 Compound 8 
                 20 
                 BD1-5 
                 3 
               
               
                 Comparative 
                 HT host 
                 ET host 
                 6:4 
                 Compound 7 
                 20 
                 — 
                 — 
               
               
                 Example 1-1 
               
               
                 Comparative 
                 HT host 
                 ET host 
                 6:4 
                 Compound 8 
                 20 
                 — 
                 — 
               
               
                 Example 1-2 
               
               
                 Comparative 
                 HT host 
                 ET host 
                 6:4 
                 BD1-9 
                 20 
                 — 
                 — 
               
               
                 Example 1-3 
               
               
                 Example 2-1 
                 HT host 
                 ET host 
                 6:4 
                 Compound 7 
                 20 
                 BD1-8 
                 1.5 
               
               
                 Example 2-2 
                 HT host 
                 ET host 
                 6:4 
                 Compound 8 
                 20 
                 BD1-5 
                 1.5 
               
               
                 Comparative 
                 HT host 
                 ET host 
                 6:4 
                 BD1-9 
                 20 
                 BD1-8 
                 1.5 
               
               
                 Example 2-1 
               
               
                 Comparative 
                 HT host 
                 ET host 
                 6:4 
                 BD1-9 
                 20 
                 BD1-5 
                 1.5 
               
               
                 Example 2-2 
               
               
                   
               
            
           
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Evaluation Example 1 
     The driving voltage (volts, V), current efficiency (candela per ampere, cd/A), maximum external quantum efficiency (EQE, %), maximum emission wavelength (Amax, nm), full width at half maximum (FWHM, nm), lifespan (T 95 , hours), and lifespan (Tso, hours) of the organic light-emitting devices manufactured in Examples 1-1 and 1-2, Examples 2-1 and 2-2, Comparative Examples 1-1 to 1-3, and Comparative Example 2-1 and 2-2 were evaluated. The results thereof are shown in Table 2. The lifespan (T 95 ) indicates a time (hour) for the luminance at 1,000 cd/m 2  of each organic light-emitting device to decline to 95% of its initial luminance. The lifespan (Tso) indicates a time (hour) for the luminance at 1,000 cd/m 2  of each organic light-emitting device to decline to 80% of its initial luminance. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Driving 
                 Current 
                 Maximum 
                   
                   
                   
                   
               
               
                   
                 voltage 
                 efficiency 
                 EQE 
                 λ max   
                 FWHM 
                 T 95   
                 T 80   
               
               
                 Structure 
                 (V) 
                 (cd/A) 
                 (%) 
                 (nm) 
                 (nm) 
                 (hours) 
                 (hours) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 1-1 
                 4.68 
                 44.09 
                 21.9 
                 496 
                 31 
                 8.83 
                 80.60 
               
               
                 Example 1-2 
                 4.47 
                 25.50 
                 30.2 
                 475 
                 20 
                 1.43 
                 11.10 
               
               
                 Comparative 
                 4.49 
                 40.84 
                 14.8 
                 519 
                 90 
                 2.66 
                 26.85 
               
               
                 Example 1-1 
               
               
                 Comparative 
                 5.01 
                 12.24 
                 7.9 
                 492 
                 79 
                 0.43 
                 2.58 
               
               
                 Example 1-2 
               
               
                 Comparative 
                 6.64 
                 2.84 
                 13.4 
                 519 
                 31 
                 0.16 
                 0.75 
               
               
                 Example 1-3 
               
               
                 Example 2-1 
                 4.61 
                 43.89 
                 22.8 
                 494 
                 29 
                 9.19 
                 79.36 
               
               
                 Example 2-2 
                 4.53 
                 24.78 
                 29.1 
                 474 
                 19 
                 2.12 
                 13.34 
               
               
                 Comparative 
                 4.85 
                 22.76 
                 21.4 
                 519 
                 28 
                 2.95 
                 15.61 
               
               
                 Example 2-1 
               
               
                 Comparative 
                 6.15 
                 4.22 
                 14.8 
                 519 
                 26 
                 0.16 
                 0.90 
               
               
                 Example 2-2 
               
               
                   
               
            
           
         
       
     
     Referring to the results of Table 2, the organic light-emitting devices of Examples 1-1 and 1-2 were found to have improved luminescence efficiency, lifespan, and/or colorimetric purity, as compared with the organic light-emitting devices of Comparative Examples 1-1, 1-2, and 1-3. Accordingly, it was found that an organic light-emitting device using the compound represented by Formula 1 as a sensitizer was found to have improved luminescence efficiency, lifespan, and/or colorimetric purity, as compared with an organic light-emitting device using the compound represented by Formula 1 as an emitter. 
     In addition, referring to the results of Table 2, the organic light-emitting devices of Examples 2-1 and 2-2 were found to have improved lifespan and/or colorimetric purity, as compared with the organic light-emitting devices of Comparative Examples 2-1 and 2-2. Accordingly, it was found that an organic light-emitting device using the compound represented by Formula 1 as a sensitizer was found to have improved luminescence efficiency, lifespan, and/or colorimetric purity, as compared with an organic light-emitting device using another known organic compound as a sensitizer. 
     As apparent from the foregoing description, the organic light-emitting device may have a long lifespan. 
     It should be understood that embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. While one or more embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.