Patent Publication Number: US-2023165136-A1

Title: Organic light-emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2021-0151643, filed on Nov. 5, 2021, in the Korean Intellectual Property Office, the content of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to an organic light-emitting device. 
     2. Description of the Related Art 
     Organic light-emitting devices (OLEDs) are self-emissive devices that, as compared with devices of the related art, have wide viewing angles, high contrast ratios, short response times, and excellent brightness, driving voltage, and response speed characteristics, and produce full-color images. 
     OLEDs include an anode, a cathode, and an organic layer between the anode and the cathode and including an emission layer. A hole transport region may be between the anode and the emission layer, and an electron transport region may be 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 
     Provided is an organic light-emitting device with improved lifespan characteristics. 
     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, a light-emitting device includes: a first electrode, 
     a second electrode facing the first electrode; and 
     an emission layer between the first electrode and the second electrode and including a host and a phosphorescent dopant, 
     wherein a photoluminescence (PL) spectrum of the emission layer includes a first peak (I max ) having a maximum intensity of and a second peak (I 2nd ) having a second highest intensity, and 
     a rate of change of an intensity of the second peak of the emission layer is greater than 0 percent (%) of an intensity of the second peak of a PL spectrum of the phosphorescent dopant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    illustrates a schematic view of an organic light-emitting device according to an exemplary embodiment; and 
         FIGS.  2 A to  2 C  each show graphs of a photoluminescence (PL) curve and a transient photoluminescence decay (trPL) curve of an emission layer according to one or more embodiments, a control emission layer, and a comparative emission layer. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain 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. 
     It will be understood that when an element is referred to as being “on” another element, it can be directly on 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 
     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 herein. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, “a,” “an,” “the,” and “at least one” do not denote a limitation of quantity, and are intended to cover both the singular and plural, unless the context clearly indicates otherwise. For example, “an element” has the same meaning as “at least one element,” unless the context clearly indicates otherwise. 
     “Or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 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. 
     Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element&#39;s relationship to another element as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower,” can therefore, encompasses both an orientation of “lower” and “upper,” depending on the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below. 
     “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% or 5% of the stated value. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure 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. 
     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. 
     The organic light-emitting device may include: a first electrode; a second electrode facing the first electrode; and an emission layer between the first electrode and the second electrode and including a host and a phosphorescent dopant, wherein a photoluminescence (PL) spectrum of the emission layer may include a first peak (I max ) having a maximum intensity of and a second peak (I 2nd ) having a second highest intensity, and a rate of change of an intensity of the second peak of the emission layer may be greater than 0 percent (%) of an intensity of the second peak of a PL spectrum of the phosphorescent dopant. 
     The first peak (I max ) of the PL spectrum indicates a peak of the maximum intensity, and the second peak (I 2nd ) indicates a peak of having the highest peak after the first peak. 
     In some embodiments, a rate of change of a full width at half maximum (FWHM) of the PL spectrum of the emission layer may be greater than 0% of a FWHM of the PL spectrum of the phosphorescent dopant. 
     In some embodiments, in a transient photoluminescence decay curve of the emission layer, an amount of change of prompt fluorescence may be greater than 30%, as compared with an amount of change of prompt fluorescence of the phosphorescent dopant. 
     In an embodiment, the emission layer may satisfy any one of Conditions 1 to 3: 
     Condition 1: a rate of change of an intensity of the second peak of the emission layer may be greater than 0 percent (%) of an intensity of the second peak of a PL spectrum of the phosphorescent dopant; 
     Condition 2: a rate of change of a full width at half maximum (FWHM) of the PL spectrum of the emission layer may be greater than 0% of a FWHM of the PL spectrum of the phosphorescent dopant; and 
     Condition 3: in a transient photoluminescence decay curve of the emission layer, an amount of change of prompt fluorescence may be greater than 30%, as compared with an amount of change of prompt fluorescence of the phosphorescent dopant. 
     For example, the emission layer may satisfy all of Condition 1 to Condition 3. 
     When the emission layer satisfies at least one of Condition 1 to Condition 3, host energy may be stabilized, thus suppressing deterioration of the emission layer and improving lifespan characteristics of the organic light-emitting device. 
     In an embodiment, the host and the phosphorescent dopant may form an exciplex. 
     Although it will be described in detail with reference to the Examples described herein, when Condition 1 to Condition 3 are all satisfied, an exciplex may be formed due to interaction of the host and the phosphorescent dopant, and host energy may be stabilized due to a low energy of the thus formed exciplex, thus improving overall lifespan characteristics of the organic light-emitting device. 
     In an embodiment, the host included in the emission layer include a compound represented by Formula 2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 2 and A, 
     CY 21  and CY 22  may each independently be a C 5 -C 30  carbocyclic group or a C 1 -C 30  heterocyclic group, 
     L 13 , L 14 , and L 21  may each independently be a substituted or unsubstituted C 1 -C 20  alkylene group, a substituted or unsubstituted C 2 -C 20  alkenylene group, a substituted or unsubstituted C 5 -C 60  carbocyclic group, or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     a13, a14, and a21 may each independently be an integer from 0 to 5, 
     when a13 is 0, L 13  may not be present, and a silicon (Si) atom may be directly bound to a triazine moiety in Formula 2, when a14 is 0, L 14  may not be present, and a Si atom may be directly bound to a Si atom, and when a21 is 0, L 21  may not be present, a nitrogen (N) atom may be directly bound to a carbon (C) atom in a triazine group, 
     T 21  may be a group represented by Formula A, 
     c21 may be 1 or 2, 
     n may be an integer from 0 to 3, 
     R 21  to R 23  and R 51  to R 55  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 substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —N(Q 1 )(Q 2 ), —Ge(Q 3 )(Q 4 )(Q 5 ), —B(Q 6 )(Q 7 ), —P(═O)(Q 8 )(Q 9 ), or —P(Q 8 )(Q 9 ), 
     b21 to b22 may each independently be an integer from 0 to 10, 
     b23 may be 0 or 1, 
     b51 to b55 may each independently be an integer from 0 to 5, 
     a substituent of the substituted C 5 -C 60  carbocyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 1 -C 20  alkylene group, the substituted C 2 -C 20  alkenylene group, the substituted C 1 -C 20  alkylene group, the substituted C 2 -C 20  alkenylene group, the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 6 -C 60  aryloxy group, the substituted C 6 -C 60  arylthio group, the substituted C 1 -C 60  heteroaryl group, the substituted monovalent non-aromatic condensed polycyclic group, and the substituted monovalent non-aromatic condensed heteropolycyclic group may be: 
     deuterium, —F, —Cl, —Br, —I, —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, or a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, or a C 1 -C 60  alkoxy group, each substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CF H 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 11 )(Q 12 ), —Ge(Q 13 )(Q 14 )(Q 15 ), —B(Q 16 )(Q 17 ), —P(═O)(Q 18 )(Q 19 ), —P(Q 18 )(Q 19 ), or any 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, —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 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 21 )(Q 22 ), —Ge(Q 23 )(Q 24 )(Q 25 ), —B(Q 26 )(Q 27 ), —P(═O)(Q 28 )(Q 29 ), —P(Q 28 )(Q 29 ), or any combination thereof; 
     —N(Q 31 )(Q 32 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), or —P(Q 38 )(Q 39 ), or 
     any combination thereof, 
     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; 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 unsubstituted or substituted with deuterium, a C 1 -C 60  alkyl group, a C 6 -C 60  aryl group, or any combination thereof; a C 2 -C 60  alkenyl group; a C 2 -C 60  alkynyl group; a C 1 -C 60  alkoxy group; a C 3 -C 10  cycloalkyl group; a C 1 -C 10  heterocycloalkyl group; a C 3 -C 10  cycloalkenyl group; a C 1 -C 10  heterocycloalkenyl group; a C 6 -C 60  aryl group unsubstituted or substituted with deuterium, a C 1 -C 60  alkyl group, a C 6 -C 60  aryl group, or any combination thereof; a C 6 -C 60  aryloxy group; a C 6 -C 60  arylthio group; a C 1 -C 60  heteroaryl group; a monovalent non-aromatic condensed polycyclic group; or a monovalent non-aromatic condensed heteropolycyclic group. 
     In an embodiment, the emission layer may include one type of host compound, and the host compound may be represented by Formula 2. 
     For example, the emission layer may consist of one type of host compound and a dopant compound, e.g., a phosphorescent dopant compound or a phosphorescent dopant compound and a fluorescent dopant compound. For example, the emission layer may include one type of host compound in a content greater than about 50 wt % and less than 100 wt %, based on the total weight of the emission layer. 
     In an embodiment, CY 21  and CY 22  may each independently be a benzene group, a naphthalene group, an anthracene group, a phenanthrene group, a pyrene group, a furan group, a thiophene group, a pyrrole group, a silole group, a benzofuran group, a benzothiophene group, an indole group, an indene group, a benzosilole group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, a fluorene group, a dibenzosilole group, an azabenzofuran group, an azabenzothiophene group, an azaindole group, an azaindene group, an azabenzosilole group, an azadibenzofuran group, an azadibenzothiophene group, an azacarbazole group, an azafluorene group, or an azadibenzosilole group. 
     For example, CY 21  and CY 22  may each independently be a benzene group, a benzofuran group, a benzothiophene group, an indole group, a benzosilole group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, a fluorene group, or a dibenzosilole group. 
     L 13 , L 14 , and L 21  may each independently be: 
     a C 1 -C 20  alkylene group or a C 2 -C 20  alkenylene group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, or a combination thereof; or 
     a phenylene group, a pentalenylene group, a naphthylene group, an azulenylene group, a phenalenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, or a naphthacenylene group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a pentalenyl group, a naphthyl group, an azulenyl 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, —N(Q 31 )(Q 32 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), —P(Q 38 )(Q 39 ) or a combination thereof, 
     wherein Q 31  to Q 39  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group. 
     In some embodiments, L 13  and L 14  may each independently be: 
     a phenylene group, a naphthylene group, a phenanthrenylene group, or an anthracenylene group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a naphthyl group, a phenanthrenyl group, an anthracenyl group, —N(Q 31 )(Q 32 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), —P(Q 38 )(Q 39 ), or a combination thereof, wherein Q 31  to Q 39  may each independently be a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group. 
     In some embodiments, L 21  may be a phenylene group, a naphthylene group, a phenanthrenylene group, or an anthracenylene group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a naphthyl group, a phenanthrenyl group, an anthracenyl group, —N(Q 31 )(Q 32 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), —P(Q 38 )(Q 39 ), or a combination thereof, 
     wherein Q 31  to Q 39  may each independently be a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group. 
     In an embodiment, a13 may be 0 or 1, and n may be 0. 
     In an embodiment, a21 may be 0. 
     In an embodiment, c21 may be 1, and b23 may be 1, or c21 may be 2, and b23 may be 0. 
     In some embodiments, R 21  to R 23  and R 51  to R 55  may each independently be: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, a C 1 -C 60  alkyl group, or a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group or a C 1 -C 60  alkoxy group, each substituted with at least one of deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group(CN), a nitro group, an amino group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl 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 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 dibenzoselenophenyl group, a dibenzosilole group, or a carbazolyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, a C 1 -C 60  alkyl group, a C 1 -C 60  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl 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 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 carbazolyl group, —N(Q 31 )(Q 32 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), —P(Q 38 )(Q 39 ), or a combination thereof; or 
     —N(Q 1 )(Q 2 ), —Ge(Q 3 )(Q 4 )(Q 5 ), —B(Q 6 )(Q 7 ), —P(═O)(Q 8 )(Q 9 ), or —P(Q 8 )(Q 9 ). 
     In some embodiments, R 21  to R 23  and R 51  to R 55  may each independently be: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, or a combination thereof; 
     a phenyl group, a biphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzoselenophenyl group, a dibenzosilole group, a triazinyl group, or a carbazolyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, a C 1 -C 20  alkyl group, and a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzoselenophenyl group, a dibenzosilole group, a triazinyl group, a carbazolyl group, —N(Q 31 )(Q 32 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), —P(Q 38 )(Q 39 ), or any combination thereof; or 
     N(Q 1 )(Q 2 ), —Ge(Q 3 )(Q 4 )(Q 5 ), —B(Q 6 )(Q 7 ), —P(═O)(Q 8 )(Q 9 ), or —P(Q 8 )(Q 9 ). 
     In an embodiment, the second compound may be represented by Formula 2-1 or 2-2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 2-2 and 2-2, 
     CY 21 , CY 22 , L 21 , a21, R 21 , R 22 , b21, b22, and R 23  may respectively be understood by referring to the descriptions of CY 21 , CY 22 , L 21 , a21, R 21 , R 22 , b21, b22, and R 23  provided herein, and 
     T 21a  and T 21b  may each be understood by referring to the description of T 21  provided herein. 
     In some embodiments, in Formulae 2-2 and 2-2, 
     CY 21  and CY 22  may each be a benzene group, 
     a21 may be 0, 
     R 23 , R 21 , and R 22  may each independently be: hydrogen, a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, an n-hexyl group, an iso-hexyl group, a sec-hexyl group, a tert-hexyl group, an n-heptyl group, an iso-heptyl group, a sec-heptyl group, a tert-heptyl group, an n-octyl group, an iso-octyl group, a sec-octyl group, a tert-octyl group, an n-nonyl group, an iso-nonyl group, a sec-nonyl group, a tert-nonyl group, an n-decyl group, an iso-decyl group, a sec-decyl group, or a tert-decyl group; or 
     a phenyl group, a biphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzoselenophenyl group, a dibenzosilole group, a triazinyl group, or a carbazolyl group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, an n-hexyl group, an iso-hexyl group, a sec-hexyl group, a tert-hexyl group, an n-heptyl group, an iso-heptyl group, a sec-heptyl group, a tert-heptyl group, an n-octyl group, an iso-octyl group, a sec-octyl group, a tert-octyl group, an n-nonyl group, an iso-nonyl group, a sec-nonyl group, a tert-nonyl group, an n-decyl group, an iso-decyl group, a sec-decyl group, a tert-decyl group, or a combination thereof. 
     In an embodiment, the second compound may be selected from Compounds E1 to E17: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In an embodiment, the phosphorescent dopant included in the emission layer may include an organometallic compound represented by Formula 3 or Formula 5. 
     For example, the phosphorescent dopant may be a blue phosphorescence-emitting dopant. For example, the third compound may be a blue platinum dopant. 
     In one or more embodiments, the phosphorescent dopant may be a sensitizer compound that may be used with a fluorescent dopant described herein, e.g., a fluorescent dopant represented by Formula 4, to thereby transfer excitons to a fluorescent dopant. 
     In an embodiment, the phosphorescent dopant may include a transition metal and a tetradentate ligand. In one or more embodiments, the phosphorescent dopant may include a transition metal and at least one of monodentate ligand, bidentate ligand, tridentate ligand, or a combination thereof. 
     In an embodiment, the phosphorescent dopant may include an organometallic compound represented by Formula 3 or Formula 5. 
     In an embodiment, the phosphorescent dopant may include an organometallic compound represented by Formula 3: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 3, 
     M 31  may be a transition metal, 
     X 11  to X 14  may each independently be C or N, 
     two bonds of a bond between X 11  and M 31 , a bond between X 12  and M 31 , a bond between X 13  and M 31 , and a bond between X 14  and M 31  may each be a coordinate bond, and the other two bonds may each be a covalent bond, 
     ring CY 31  to ring CY 34  may each independently be a C 5 -C 30  carbocyclic group or a C 1 -C 30  heterocyclic group, 
     T 31  may be a single bond, a double bond, *—N(R 35a )—*′, *—B(R 35a )—*′, *—P(R 35a )—*′, *—C(R 35a )(R 35b )—*′, *—Si(R 35a )(R 35b )—*′, *—Ge(R 35a )(R 35b )—*′, *—S—*′, *—Se—*′, *—(O)—*′, *—C(═O)—*′, *—S(═O)—*′, *—S(═O) 2 —*′, *—C(R 35a )═*′, *═C(R 35a )—*′, *—C(R 35a )═C(R 35b )—*′, *—C(═S)—*′, *—C≡C—*′, a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a , or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     T 32  may be a single bond, a double bond, *—N(R 36a )—*′, *—B(R 36a )—*′, *—P(R 36a )—*′, *—C(R 36a )(R 36b )—*′, *—Si(R 36a )(R 36b )—*′, *—Ge(R 36a )(R 36b )—*′, *—S—*′, *—Se—*′, *—(O)—*′, *—C(═O)—*′, *—S(═O)—*′, *—S(═O) 2 —*′, *—C(R 36a )═*′, *═C(R 36a )—*′, *—C(R 36a )═C(R 36b )—*′, *—C(═S)—*′, *—C≡C—*′, a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a , or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     T 33  may be a single bond, a double bond, *—N(R 37a )—*′, *—B(R 37a )—*′, *—P(R 37a )—*′, *—C(R 37a )(R 37b )—*′, *—Si(R 37a )(R 37b )—*′, *—Ge(R 37a )(R 37b )—*′, *—S—*′, *—Se—*′, *—(O)—*′, *—C(═O)—*′, *—S(═O)—*′, *—S(═O) 2 —*′, *—C(R 37a )═*′, *═C(R 37a )—*′, *—C(R 37a )═C(R 37b )—*′, *—C(═S)—*′, *—C≡C—*′, a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a , or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     T 34  may be a single bond, a double bond, *—N(R 38a )—*′, *—B(R 38a )—*′, *—P(R 38a )—*′, *—C(R 38a )(R 38b )—*′, *—Si(R 38a )(R 38b )—*′, *—Ge(R 38a )(R 38b )—*′, *—S—*′, *—Se—*″, *—O—*′, *—C(═O)—*′, *—S(═O)—*′, *—S(═O) 2 *′, *—C(R 38a )═*′, *═C(R 38a )—*′, *—C(R 38a )═C(R 38b )—*′, *—C(═S)—*′, *—C≡C—*′, a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a , or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     n31 to n34 may each independently be an integer from 0 to 5, and three of n31 to n34 may each independently be an integer from 1 to 5, 
     when n31 is 0, T 31  may not be present, when n32 is 0, T 32  may not be present, when n33 is 0, T 33  may not be present, and when n34 is 0, T 34  may not be present, 
     when n31 is 2 or greater, at least two T 31 (s) may be identical to or different from each other, when n32 is 2 or greater, at least two T 32 (s) may be identical to or different from each other, when n33 is 2 or greater, at least two T 33 (s) may be identical to or different from each other, and when n34 is 2 or greater, at least two T 34 (s) may be identical to or different from each other, 
     R 31  to R 34 , R 35a , R 35b , R 36a , R 36b , R 37a , R 37b , R 38a , and R 38b  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —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 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  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  heteroaryl alkyl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —N(Q 1 )(Q 2 ), —Si(Q 3 )(Q 4 )(Q 5 ), —Ge(Q 3 )(Q 4 )(Q 5 ), —B(Q 6 )(Q 7 ), —P(═O)(Q 8 )(Q 9 ), or —P(Q 8 )(Q 9 ), 
     a31 to a34 may each independently be an integer from 0 to 20, 
     at least two of R 31 (s) in the number of a31 may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     at least two of R 32 (s) in the number of a32 may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     at least two of R 33 (s) in the number of a33 may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     at least two of R 34 (s) in the number of a34 may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     at least two of R 31  to R 34 , R 35a , R 35b , R 36a , R 36b , R 37a , R 37b , R 38a , and R 38b  may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     R 10a  may be understood by referring to the description of R 31  provided herein, 
     * and *′ each indicate a binding site to an adjacent atom, and 
     a substituent of the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 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  heteroaryl alkyl 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, —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, or a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, or a C 1 -C 60  alkoxy group, each substituted with at least one deuterium, —F, —Cl, —Br, —I, —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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 11 )(Q 12 ), —Si(Q 13 )(Q 14 )(Q 15 ), —Ge(Q 13 )(Q 14 )(Q 15 ), —B(Q 16 )(Q 17 ), —P(═O)(Q 18 )(Q 19 ), —P(Q 18 )(Q 19 ), or any 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CF H 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 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 21 )(Q 22 ), —Si(Q 23 )(Q 24 )(Q 25 ), —Ge(Q 23 )(Q 24 )(Q 25 ), —B(Q 26 )(Q 27 ), —P(═O)(Q 28 )(Q 29 ), —P(Q 28 )(Q 29 ), or any combination thereof; 
     —N(Q 31 )(Q 32 ), —Si(Q 33 )(Q 34 )(Q 35 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), or —P(Q 38 )(Q 39 ); or 
     any combination thereof, 
     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; 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 unsubstituted or substituted with deuterium, a C 1 -C 60  alkyl group, a C 6 -C 60  aryl group, or any combination thereof; a C 2 -C 60  alkenyl group; a C 2 -C 60  alkynyl group; a C 1 -C 60  alkoxy group; a C 3 -C 10  cycloalkyl group; a C 1 -C 10  heterocycloalkyl group; a C 3 -C 10  cycloalkenyl group; a C 1 -C 10  heterocycloalkenyl group; a C 6 -C 60  aryl group unsubstituted or substituted with deuterium, a C 1 -C 60  alkyl group, a C 6 -C 60  aryl group, or any combination thereof; a C 6 -C 60  aryloxy group; a C 6 -C 60  arylthio group; a C 1 -C 60  heteroaryl group; a monovalent non-aromatic condensed polycyclic group; or a monovalent non-aromatic condensed heteropolycyclic group. 
     In an embodiment, M in Formula 3 may be Pt, Pd, or Au. 
     In an embodiment, in Formula 3, a bond between X 11  and M 31  may be a coordinate bond. 
     In one or more embodiments, in Formula 3, X 11  may be C, and a bond between X 11  and M 31  may be a coordinate bond. That is, in Formula 3, X 11  may be C in a carbene moiety. 
     In an embodiment, in Formula 3, ring CY 31  to ring CY 34  may each independently be i) a first ring, ii) a second ring, iii) a condensed ring in which at least two first rings are condensed, iv) a condensed ring in which at least two second rings are condensed, or v) a condensed ring in which at least one first ring is condensed with at least one second ring, 
     the first ring may be a cyclopentadiene group, a furan group, a thiophene group, a pyrrole group, a silole group, an oxazole group, an isoxazole group, an oxadiazole group, an isooxadiazole group, an oxatriazole group, an isooxatriazole group, a thiazole group, an isothiazole group, a thiadiazole group, an isothiadiazole group, a thiatriazole group, an isothiatriazole group, a pyrazole group, an imidazole group, a triazole group, a tetrazole group, an azasilole group, a diazasilole group, or a triazasilole group, and 
     the second ring may be an adamantane group, a norbornane group, a norbornene group, a cyclohexane group, a cyclohexene group, a benzene group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, an oxazine group, a thiazine group, a dihydropyrazine group, a dihydropyridine group, or a dihydroazasilane group. 
     In some embodiments, in Formula 3, R 31  to R 34 , R 35a , R 35b , R 36a , R 36b , R 37a , R 37b , R 38a , and R 38b  may each independently be: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group (CN), a nitro group, an amino group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with at least one of deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group (CN), a nitro group, an amino group, a phenyl group, or a combination thereof; or 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, or an anthracenyl group, each unsubstituted or substituted with at least one of deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group (CN), a nitro group, an amino group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, or a combination thereof. 
     In an embodiment, the phosphorescent dopant may include an organometallic compound represented by Formula 3-1 or Formula 3-2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 3-1 and 3-2, 
     M 31 , CY 32 , CY 33 , CY 34 , X 12 , X 13 , X 14 , T 31 , T 32 , T 33 , n31, n32, n33, R 32 , R 33 , R 34 , a32, a33, and a34 may respectively be understood by referring to the descriptions of M 31 , CY 32 , CY 33 , CY 34 , X 12 , X 13 , X 14 , T 31 , T 32 , T 33 , n31, n32, n33, R 32 , R 33 , R 34 , a32, a33, and a34 provided herein, and 
     R 311  to R 317  may each be understood by referring to the description of R 31  provided herein. 
     In an embodiment, in Formulae 3-1 and 3-2, 
     R 311  to R 317  may each independently be: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —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, or a phosphoric acid group or a salt thereof; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —SF 5 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, or any combination thereof; 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 6 -C 60  aryl group, a C 7 -C 60  arylalkyl group, C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a substituted or unsubstituted C 2 -C 60  heteroaryl alkyl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , —SF 5 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, or any combination thereof; 
     —N(Q 1 )(Q 2 ), —Si(Q 3 )(Q 4 )(Q 5 ), —Ge(Q 3 )(Q 4 )(Q 5 ), —B(Q 6 )(Q 7 ), —P(═O)(Q 5 )(Q 9 ), or —P(C 25 )(Q 9 ). 
     For example, in Formulae 3-1 and 3-2, 
     at least one of R 311  to R 317  may be 
     a C 1 -C 20  alkyl group, a C 6 -C 60  aryl group, or a C 7 -C 60  arylalkyl group, each unsubstituted or substituted with at least one of a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a phenyl group, a cumyl group, or a combination thereof. 
     In one or more embodiments, the phosphorescent dopant may be a phosphorescent dopant compound represented by Formula 5: 
       M 51 (L 51 ) n51 (L 52 ) n52    Formula 5
 
     wherein, in Formula 5, M may be a transition metal. 
     In some embodiments, M 51  may be a first-row transition metal, a second-row transition metal, or a third-row transition metal. 
     In some embodiments, M 51  may be iridium (Ir), platinum (Pt), osmium (Os), titanium (Ti), zirconium (Zr), hafnium (Hf), europium (Eu), terbium (Tb), thulium (Tm), or rhodium (Rh). 
     In some embodiments, M 51  may be Ir, Pt, Os, or Rh. 
     In Formula 5, L 51  may be a ligand represented by Formula 5A, and L 52  may be a ligand represented by Formula 5B: 
     
       
         
         
             
             
         
       
     
     Formulae 5A and 5B may respectively be understood by referring to the descriptions of Formulae 5A and 5B provided herein. 
     In Formula 5, n51 may be 1, 2, or 3, and when n51 is 2 or greater, at least two L 5 i(s) may be identical to or different from each other. 
     In Formula 5, n52 may be 0, 1, or 2, and when n52 is 2, two L 52 (s) may be identical to or different from each other. 
     In Formula 5, a sum of n51 and n52 may be 2 or 3. For example, a sum of n51 and n52 may be 3. 
     In an embodiment, in Formula 5, i) M may be Ir, and n51+n52=3; or ii) M may be Pt, and n51+n52=2. 
     In one or more embodiments, in Formula 5, M may be Ir, and i) n51 may be 1, and n52 may be 2, or ii) n51 may be 2, and n52 may be 1. 
     L 51  and L 52  in Formula 5 may be different from each other. 
     In Formulae 5A and 5B, Y 51  to Y 54  may each independently be C or N. In some embodiments, Y 51  and Y 53  may each be N, and Y 52  and Y 54  may each be C. 
     In Formulae 5A and 5B, ring CY 51  to ring CY 54  may each independently be a C 5 -C 30  carbocyclic group or a C 1 -C 30  heterocyclic group. 
     In some embodiments, ring CY 51  to ring CY 54  may each independently include i) a third ring, ii) a fourth ring, iii) a condensed ring in which at least two third rings are condensed, iv) a condensed ring in which at least two fourth rings are condensed, or v) a condensed ring in which at least one third ring is condensed with at least one fourth ring, 
     the third ring may be a cyclopentane group, a cyclopentene group, a furan group, a thiophene group, a pyrrole group, a silole group, a borole group, a phosphole group, a germole group, a selenophene group, an oxazole group, an oxadiazole group, an oxatriazole group, a thiazole group, a thiadiazole group, a thiatriazole group, a pyrazole group, an imidazole group, a triazole group, a tetrazole group, or an azasilole group, and 
     the fourth ring may be an adamantane group, a norbornane group, a norbornene group, a cyclohexane group, a cyclohexene group, a benzene group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, or a triazine group. 
     In some embodiments, in Formulae 5A and 5B, ring CY 1  to ring CY 4  may each independently be a cyclopentane group, a cyclohexane group, a cyclohexene group, a benzene group, a naphthalene group, an anthracene group, a phenanthrene group, a triphenylene group, a pyrene group, a chrysene group, a 1,2,3,4-tetrahydronaphthalene group, a cyclopentadiene group, a pyrrole group, a furan group, a thiophene group, a silole group, a borole group, a phosphole group, a germ ole group, a selenophene group, an indene group, an indole group, a benzofuran group, a benzothiophene group, a benzosilole group, a benzoborole group, a benzophosphole group, a benzogermole group, a benzoselenophene group, a fluorene group, a carbazole group, a dibenzofuran group, a dibenzothiophene group, a dibenzosilole group, a dibenzoborole group, a dibenzophosphole group, a dibenzogermole group, a dibenzoselenophene group, a benzofluorene group, a benzocarbazole group, a naphthobenzofuran group, a naphthobenzothiophene group, a naphthobenzosilole group, a naphthobenzoborole group, a naphthobenzophosphole group, a naphthobenzogermole group, a naphthobenzoselenophene group, a dibenzofluorene group, a dibenzocarbazole group, a dinaphthofuran group, a dinaphthothiophene group, a dinaphthosilole group, a dinaphthoborole group, a dinaphthophosphole group, a dinaphthogermole group, a dinaphthoselenophene group, an indenophenanthrene group, an indolophenanthrene group, a phenanthrobenzofuran group, a phenanthrobenzothiophene group, a phenanthrobenzosilole group, a phenanthrobenzoborole group, a phenanthrobenzophosphole group, a phenanthrobenzogermole group, a phenanthrobenzoselenophene group, a dibenzothiophene 5-oxide group, a 9H-fluorene-9-one group, a dibenzothiophene 5,5-dioxide group, an azaindene group, an azaindole group, an azabenzofuran group, an azabenzothiophene group, an azabenzosilole group, an azabenzoborole group, an azabenzophosphole group, an azabenzogermole group, an azabenzoselenophene group, an azafluorene group, an azacarbazole group, an azadibenzofuran group, an azadibenzothiophene group, an azadibenzosilole group, an azadibenzoborole group, an azadibenzophosphole group, an azadibenzogermole group, an azadibenzoselenophene group, an azabenzofluorene group, an azabenzocarbazole group, an azanaphthobenzofuran group, an azanaphthobenzothiophene group, an azanaphthobenzosilole group, an azanaphthobenzoborole group, an azanaphthobenzophosphole group, an azanaphthobenzogermole group, an azanaphthobenzoselenophene group, an azadibenzofluorene group, an azadibenzocarbazole group, an azadinaphthofuran group, an azadinaphthothiophene group, an azadinaphthosilole group, an azadinaphthoborole group, an azadinaphthophosphole group, an azadinaphthogermole group, an azadinaphthoselenophene group, an azaindenophenanthrene group, an azaindolophenanthrene group, an azaphenanthrobenzofuran group, an azaphenanthrobenzothiophene group, an azaphenanthrobenzosilole group, an azaphenanthrobenzoborole group, an azaphenanthrobenzophosphole group, an azaphenanthrobenzogermole group, an azaphenanthrobenzoselenophene group, an azadibenzothiophene 5-oxide group, an aza9H-fluorene-9-one group, an azadibenzothiophene 5,5-dioxide group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a quinoline group, an isoquinoline group, a quinoxaline group, a quinazoline group, a benzoquinoline group, a benzoisoquinoline group, a benzoquinoxaline group, a benzoquinazoline group, a phenanthroline group, a phenanthridine group, a pyrrole group, a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isooxazole group, a thiazole group, an isothiazole group, an oxadiazole group, a thiadiazole group, an azasilole group, an azaborole group, an azaphosphole group, an azagermole group, an azaselenophene group, a benzopyrrole group, a benzopyrazole group, a benzimidazole group, a benzoxazole group, a benzisoxazole group, a benzothiazole group, a benzisothiazole group, a benzoxadiazole group, a benzothiadiazole group, a pyridinopyrrole group, a pyridinopyrazole group, a pyridinoimidazole group, a pyridinooxazole group, a pyridinoisoxazole group, a pyridinothiazole group, a pyridinoisothiazole group, a pyridinooxadiazole group, a pyridinothiadiazole group, a pyrimidinopyrrole group, a pyrimidinopyrazole group, a pyrimidinoimidazole group, a pyrimidinooxazole group, a pyrimidinoisoxazole group, a pyrimidinothiazole group, a pyrimidinoisothiazole group, a pyrimidinooxadiazole group, a pyrimidinothiadiazole group, a 5,6,7,8-tetrahydroisoquinoline group, a 5,6,7,8-tetrahydroquinoline group, an adamantane group, a norbornane group, a norbornene group, a benzene group condensed with a cyclohexane group, a benzene group condensed with a norbornane group, a pyridine group condensed with a cyclohexane group, or a pyridine group condensed with a norbornane group. 
     In some embodiments, in Formulae 5A and 5B, ring CY 51  and ring CY 53  may be different from each other. 
     In some embodiments, in Formulae 5A and 5B, ring CY 52  and ring CY 54  may be different from each other. 
     In some embodiments, in Formulae 5A and 5B, ring CY 51  to ring CY 54  may each be different from one another. 
     In Formulae 5A and 5B, R 51  to R 54  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 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 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —N(Q 51 )(Q 52 ), —Si(Q 53 )(Q 54 )(Q 55 ), —Ge(Q 53 )(Q 54 )(Q 55 ), —B(Q 56 )(Q 57 ), —P(═O)(Q 58 )(Q 59 ), or —P(Q 58 )(Q 59 ). Q 51  to Q 59  may respectively be understood by referring to the descriptions of Q 1  to Q 9  provided herein. 
     In an embodiment, in Formulae 5A and 5B, R 51  to R 54  may each independently be: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, —SF 5 , 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 20  alkylthio group, each substituted with deuterium, —F, —Cl, —Br, —I, —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 10  alkyl 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[2.2.2]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[2.2.2]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 pyrimidinyl group, or any 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[2.2.2]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 benzoisothiazolyl group, a benzoxazolyl group, an benzoisoxazolyl 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 deuterium, —F, —Cl, —Br, —I, —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 10  alkyl group, a phenyl C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy 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[2.2.2]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[2.2.2]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 benzoisothiazolyl group, a benzoxazolyl group, an benzoisoxazolyl 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, or any combination thereof; or 
     —N(Q 51 )(Q 52 ), —Ge(Q 53 )(Q 54 )(Q 55 ), —B(Q 56 )(Q 57 ), —P(═O)(Q 58 )(Q 59 ), or —P(Q 58 )(Q 59 ), wherein Q 51  to Q 59  may each independently be: 
     —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, or —CD 2 CDH 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 deuterium, a C 1 -C 20  alkyl group, a phenyl group, or any combination thereof. 
     In one or more embodiments, R 51  to R 54  may each independently be: 
     hydrogen, deuterium, —F, or a cyano group; 
     a C 1 -C 20  alkyl group unsubstituted or substituted with deuterium, a cyano group, a C 3 -C 10  cycloalkyl group, a deuterated C 3 -C 10  cycloalkyl group, a fluorinated C 3 -C 10  cycloalkyl group, a (C 1 -C 20  alkyl)C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a deuterated heterocycloalkyl group, a fluorinated C 1 -C 10  heterocycloalkyl group, a (C 1 -C 20  heterocycloalkyl group, a phenyl group, a deuterated phenyl group, a fluorinated phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a deuterated biphenyl group, a fluorinated biphenyl group, a (C 1 -C 20  alkyl)biphenyl group, a dibenzofuranyl group, a deuterated dibenzofuranyl group, a fluorinated dibenzofuranyl group, a (C 1 -C 20  alkyl)dibenzofuranyl group, a dibenzothiophenyl group, a deuterated dibenzothiophenyl group, a fluorinated dibenzothiophenyl group, a (C 1 -C 20  alkyl)dibenzothiophenyl group, or any combination thereof; 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a phenyl group, or a biphenyl group, each unsubstituted or substituted with deuterium, a cyano group, a C 1 -C 20  alkyl group, a deuterated C 1 -C 20  alkyl group, a fluorinated C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a deuterated alkoxy group, a fluorinated C 1 -C 20  alkoxy group, a C 3 -C 10  cycloalkyl group, a deuterated C 3 -C 10  cycloalkyl group, a fluorinated C 3 -C 10  cycloalkyl group, a (C 1 -C 20  alkyl)C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a deuterated heterocycloalkyl group, a fluorinated C 1 -C 10  heterocycloalkyl group, a (C 1 -C 20  heterocycloalkyl group, a phenyl group, a deuterated phenyl group, a fluorinated phenyl group, a (C 1 -C 20  alkyl)phenyl group, a biphenyl group, a deuterated biphenyl group, a fluorinated biphenyl group, a (C 1 -C 20  alkyl)biphenyl group, a dibenzofuranyl group, a deuterated dibenzofuranyl group, a fluorinated dibenzofuranyl group, a (C 1 -C 20  alkyl)dibenzofuranyl group, a dibenzothiophenyl group, a deuterated dibenzothiophenyl group, a fluorinated dibenzothiophenyl group, a (C 1 -C 20  alkyl)dibenzothiophenyl group, or any combination thereof; or 
     —Si(Q 53 )(Q 54 )(Q 55 ) or —Ge(Q 53 )(Q 54 )(Q 55 ). 
     In Formulae 5A and 5B, b51 to b54 may each be the number of R 51 (s) to R 54 (s), respectively, and b51 to b54 may each independently be an integer from 0 to 20. When b51 is 2 or greater, at least two R 51 (s) may be identical to or different from each other, when b52 is 2 or greater, at least two R 52 (s) may be identical to or different from each other, when b53 is 2 or greater, at least two R 53 (s) may be identical to or different from each other, and when b54 is 2 or greater, at least two R 54 (s) may be identical to or different from each other. For example, b51 to b54 may each independently be an integer from 0 to 8. 
     In an embodiment, the third compound may be at least one of Compounds P1 to P54: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     As the phosphorescent dopant is at least one of Compounds P1 to P54, formation of exciplex with the host compound represented by Formula 2 may be facilitated. For example, when the phosphorescent dopant includes a bulky substituent (e.g., a tert-butyl group or a cumyl group), formation of exciplex may be facilitated by having an energy level similar to that of the host. In the case of the phosphorescent dopant, a gap between a lowest occupied molecular orbital (LUMO) level of an electron transporting (ET) host and a highest occupied molecular orbital (HOMO) level of a phosphorescent dopant may be reduced, thus facilitating formation of an exciplex. 
     In an embodiment, the emission layer may further include a fluorescent dopant. For example, the fluorescent dopant may be a thermally activated delayed fluorescence dopant and a blue dopant. 
     In an embodiment, the fluorescent dopant may be a luminescence emitter that may emit light by receiving excitons from the exciplex of the host and the phosphorescent dopant according to one or more embodiments so that the received excitons transition to a ground state. 
     In an embodiment, the fluorescent dopant may be a polycyclic compound represented by Formula 4: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 4, 
     Z may be B or N, 
     ring CY 41  to ring CY 43  may each independently be a C 5 -C 30  carbocyclic group or a C 1 -C 30  heterocyclic group, 
     Y 41  may be *—N(R 44 )—*′, *—B(R 44 )—*′, *—P(R 44 )—*′, *—C(R 44 )(R 45 )—*′, *—Si(R 44 )(R 45 )—*′, *—Ge(R 44 )(R 45 )—*′, *—O—′, *—S—*′, *—Se—*′, *—C(═O)—*′, or *—S(═O) 2 —*′, 
     Y 42  may be *—N(R 46 )—*′, *—B(R 46 )—*′, *—P(R 46 )—*′, *—C(R 46 )(R 47 )—*′, *—Si(R 46 )(R 47 )—*′, *—Ge(R 46 )(R 47 )—*′, *—O—*′, *—S—*′, *—Se—*′, *—C(═O)—*′, or *—S(═O) 2 —*′, 
     Y 43  may be *—N(R 48 )—*′, *—B(R 48 )—*′, *—P(R 48 )—*′, *—C(R 48 )(R 49 )—*′, *—Si(R 48 )(R 49 )—*′, *—Ge(R 48 )(R 49 )—*′, *—O—*′, *—S—*′, *—Se—*′, *—C(═O)—*′, or *—S(═O) 2 —*′, 
     b41 to b43 may each independently be 0 or 1, 
     when b41 is 0, Y 41  may not be present, when b42 is 0, Y 42  may not be present, and when b43 is 0, Y 43  may not be present, 
     R 41  to R 49  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 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  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  heteroaryl alkyl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —N(Q 1 )(Q 2 ), —Si(Q 3 )(Q 4 )(Q 5 ), —Ge(Q 3 )(Q 4 )(Q 5 ), —B(Q 6 )(Q 7 ), —P(═O)(Q 8 )(Q 9 ), or —P(Q 8 )(Q 9 ), 
     a41 to a43 may each independently be an integer from 0 to 20, 
     at least two of R 41 (S) in the number of a41 may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10b , 
     at least two of R 42 (s) in the number of a42 may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10b , 
     at least two of R 43 (s) in the number of a43 may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10b , 
     at least two of R 41  to R 49  may optionally be bound to each other to form a C 5 -C 30  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 30  heterocyclic group unsubstituted or substituted with at least one R 10b , 
     R 10b  may be understood by referring to the description of R 41  provided herein, 
     * and *′ each indicate a binding site to an adjacent atom, and 
     a substituent of the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 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  heteroaryl alkyl 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, —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, or a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, or a C 1 -C 60  alkoxy group, each substituted with deuterium, —F, —Cl, —Br, —I, —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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 11 )(Q 12 ), —Si(C 213 )(Q 14 )(Q 15 ), —Ge(Q 13 )(Q 14 )(Q 15 ), —B(Q 16 )(Q 17 ), —P(═O)(Q 18 )(Q 19 ), —P(Q 18 )(Q 19 ), or any 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 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, —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 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 21 )(Q 22 ), —Si(Q 23 )(Q 24 )(Q 25 ), —Ge(Q 23 )(Q 24 )(Q 25 ), —B(Q 26 )(Q 27 ), —P(═O)(Q 28 )(Q 29 ), —P(Q 28 )(Q 29 ), or any combination thereof; 
     —N(Q 31 )(Q 32 ), —Si(Q 33 )(Q 34 )(Q 35 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), or —P(Q 38 )(Q 39 ); or 
     any combination thereof, 
     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; —CI; —Br; —I; 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 unsubstituted or substituted with deuterium, a C 1 -C 60  alkyl group, a C 6 -C 60  aryl group, or any combination thereof; a C 2 -C 60  alkenyl group; a C 2 -C 60  alkynyl group; a C 1 -C 60  alkoxy group; a C 3 -C 10  cycloalkyl group; a C 1 -C 10  heterocycloalkyl group; a C 3 -C 10  cycloalkenyl group; a C 1 -C 10  heterocycloalkenyl group; a C 6 -C 60  aryl group unsubstituted or substituted with deuterium, a C 1 -C 60  alkyl group, a C 6 -C 60  aryl group, or any combination thereof; a C 6 -C 60  aryloxy group; a C 6 -C 60  arylthio group; a C 1 -C 60  heteroaryl group; a monovalent non-aromatic condensed polycyclic group; or a monovalent non-aromatic condensed heteropolycyclic group. 
     In some embodiments, R 41  to R 49  may each independently be: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, a C 1 -C 60  alkyl group, or a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group or a C 1 -C 60  alkoxy group, each substituted with at least one of deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, a phenyl group, a biphenyl 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, or a combination thereof; or 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, a 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 phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a 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, or a carbazolyl group, each unsubstituted or substituted with at least on of deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group (CN), a nitro group, an amino group, a C 1 -C 60  alkyl group, a C 1 -C 60  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl 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 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 carbazolyl group, or a combination thereof. 
     In an embodiment, Formula 4 may be selected from Formulae 4-1 to 4-8: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Formulae 4-1 to 4-8, 
     Z 1  and Z 2  may each be understood by referring to the description of Z provided herein, 
     Y 41  and Y 42  may respectively be understood by referring to the descriptions of Y 41  and Y 42  provided herein, 
     Y 44  and Y 45  may respectively be understood by referring to the descriptions of Y 41  and Y 42  provided herein, 
     R 411  may be understood by referring to the description of R 41  provided herein, R 421  may be understood by referring to the description of R 42  provided herein, R 431  and R 432  may each be understood by referring to the description of R 43  provided herein, R 441  may be understood by referring to the description of R 41  provided herein, R 451  may be understood by referring to the description of R 42  provided herein, and R 461  may be understood by referring to the description of R 43  provided herein, 
     a411 may be an integer from 0 to 4, 
     a421 may be an integer from 0 to 3, 
     a431 may be an integer from 0 to 4, 
     a441 may be an integer from 0 to 4, 
     a451 may be an integer from 0 to 3, and 
     a461 may be an integer from 0 to 4. 
     In an embodiment, the fourth compound may be selected from Compounds D1 to D30: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In an embodiment, the fourth compound may be included in the emission layer in a range of about 0 wt % to about 5 wt %. 
     Organic Light-Emitting Device 
     The organic light-emitting device according to an aspect may include: a first electrode; a second electrode facing the first electrode; and an emission layer between the first electrode and the second electrode and including a host and a phosphorescent dopant, wherein a photoluminescence (PL) spectrum of the emission layer may include a first peak (I max ) having a maximum intensity of and a second peak (I 2nd ) having a second highest intensity, and a rate of change of an intensity of the second peak of the emission layer may be greater than 0 percent (%) of an intensity of the second peak of a PL spectrum of the phosphorescent dopant. 
     The organic light-emitting device according to another aspect may include: a first electrode; a second electrode facing the first electrode; and an emission layer between the first electrode and the second electrode and including a host and a phosphorescent dopant, wherein a rate of change of a full width at half maximum (FWHM) of the PL spectrum of the emission layer may be greater than 0% of a FWHM of the PL spectrum of the phosphorescent dopant. 
     The organic light-emitting device according to another aspect may include: a first electrode; a second electrode facing the first electrode; and an emission layer between the first electrode and the second electrode and including a host and a phosphorescent dopant, wherein, in a transient photoluminescence decay curve of the emission layer, an amount of change of prompt fluorescence may be greater than 30%, as compared with an amount of change of prompt fluorescence of the phosphorescent dopant. 
     The organic light-emitting device according to another aspect may include: a first electrode; a second electrode facing the first electrode; and an emission layer between the first electrode and the second electrode and including a host and a phosphorescent dopant, wherein Condition 1 to Condition 3 may all be satisfied: 
     Condition 1: a rate of change of an intensity of the second peak of the emission layer may be greater than 0 percent (%) of an intensity of the second peak of a PL spectrum of the phosphorescent dopant; 
     Condition 2: a rate of change of a full width at half maximum (FWHM) of the PL spectrum of the emission layer may be greater than 0% of a FWHM of the PL spectrum of the phosphorescent dopant; and 
     Condition 3: in a transient photoluminescence decay curve of the emission layer, an amount of change of prompt fluorescence may be greater than 30%, as compared with an amount of change of prompt fluorescence of the phosphorescent dopant. 
     The organic light-emitting device according to another aspect may include: a first electrode; a second electrode facing the first electrode; and an emission layer between the first electrode and the second electrode and including a host and a phosphorescent dopant, wherein the phosphorescent dopant may include an organometallic compound represented by Formula 3 or Formula 5, and the host and the phosphorescent dopant may form an exciplex. 
     The organic light-emitting device according to another aspect may include: a first electrode; a second electrode facing the first electrode; and an emission layer between the first electrode and the second electrode and including a host, a phosphorescent dopant, and a fluorescent dopant, wherein the phosphorescent dopant may include an organometallic compound represented by Formula 3 or Formula 5, the fluorescent dopant may be a polycyclic compound represented by Formula 4, and the host and the phosphorescent dopant may form an exciplex. 
     In an embodiment, the emission layer may emit blue light. 
     The first electrode may be an anode, which is a hole injection electrode, and the second electrode may be a cathode, which is an electron injection electrode. In some embodiments, the first electrode may be a cathode, which is an electron injection electrode, and the second electrode may be an anode, which is a hole injection electrode. 
     For example, in the organic light-emitting device, the first electrode may be an anode, the second electrode may be a cathode, and the organic layer may further include a hole transport region between the first electrode and the emission layer and an electron transport region between the emission layer and the second electrode, wherein the hole transport region may include a hole injection layer, a hole transport layer, an electron blocking layer, an auxiliary layer, or a combination thereof, and the electron transport region may include a buffer layer, a hole blocking layer, an electron transport layer, an electron injection layer, or a combination thereof. 
     The term “organic layer” as used herein refers to a single and/or a plurality of layers between the first electrode and the second electrode in an organic light-emitting device. The “organic layer” may include not only organic compounds but also organometallic complexes including metals. 
     Description of FIG.  1   
       FIG.  1    illustrates a schematic cross-sectional view of an organic light-emitting device  10  according to an exemplary embodiment. Hereinafter, a structure of an organic light-emitting device according to one or more embodiments and a method of manufacturing the organic light-emitting device 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 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 between the first electrode  11  and an emission layer  15 , and an electron transport region  17  is between the emission layer  15  and the second electrode  19 . 
     A substrate may be additionally disposed 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 include a material 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  11  is a transmissive electrode, a material for forming the first electrode  11  may include indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO 2 ), zinc oxide (ZnO), or any combinations thereof. In some embodiments, when the first electrode  11  is a semi-transmissive electrode or a reflective electrode, a material for forming the first electrode  110  may include magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), or any combination thereof. 
     The first electrode  11  may have a single-layered structure or a multi-layered structure including a plurality of layers. 
     Emission Layer  15   
     The thickness of the emission layer  15  may be in a range of about 100 Å to about 1,000 Å, and in some embodiments, about 200 Å to about 600 Å. When the thickness of the emission layer is within any of these ranges, improved luminescence characteristics may be obtained without a substantial increase in driving voltage. 
     Host in Emission Layer  15   
     In the emission layer  15 , the host may include a compound represented by Formula 2. 
     For example, the host may be a compound represented by Formula 2. 
     In an embodiment, the host may be a single host compound. 
     Dopant in Emission Layer  15   
     The dopant in the emission layer  15  may be a phosphorescent or fluorescent dopant. 
     For example, the phosphorescent dopant may be a compound represented by Formula 3 or Formula 5, and the fluorescent dopant may be a compound represented by Formula 4. 
     A content (weight) of the dopant in the emission layer  15  may be in a range of about 0.1 parts to about 20 parts by weight of 100 parts by weight of the emission layer  15 . 
     Hole Transport Region  12   
     In the organic light-emitting device  10 , the hole transport region  12  may be 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 injection layer/first hole transport layer/second hole transport layer/electron blocking 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. 
     The hole transport region  12  may include a compound having hole transport characteristics. 
     For example, the hole transport region  12  may include an amine-based compound. 
     In some embodiments, the hole transport region  12  may include m-MTDATA, TDATA, 2-TNATA, NPB, 8-NPB, TPD, spiro-TPD, spiro-NPB, methylated-NPB, TAPC, HMTPD, 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), polyaniline/dodecylbenzenesulfonic acid (PAN I/D BSA), poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), polyaniline/camphor-sulfonic acid (PANI/CSA), polyaniline/poly(4-styrene sulfonate) (PANI/PSS), a compound represented by one of Formulae 201 to 205, or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     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 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, or a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, and two adjacent groups of R 201  to R 206  may optionally be bound via a single bond, a dimethyl-methylene group, or a diphenyl-methylene group. 
     In some embodiments, L 201  to L 209  may be a benzene group, a heptalene group, an indene group, a naphthalene group, an azulene group, a heptalene 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 pentacene group, a rubicene group, a coronene 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 deuterium, a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy 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 any 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 deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 10 -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 any combination thereof, 
     wherein Q 11  to Q 13  and Q 31  to Q 33  may each independently be a C 1  to C 10  alkyl group, a C 1  to C 10  alkoxy 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-based compound. 
     In one or more embodiments, the hole transport region  12  may include a carbazole-containing amine-based compound and a carbazole-free amine-based compound. 
     The carbazole-containing amine-based compound may include, for example, compounds 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, a benzothienocarbazole group, or a combination thereof. 
     The carbazole-free amine-based compound may include, 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, a benzothienocarbazole group, or a combination thereof. 
     In one or more embodiments, the hole transport region  12  may include a compound represented by Formula 201, a compound represented by Formula 202, or any combination thereof. 
     In some embodiments, the hole transport region  12  may include a compound represented by Formula 201-1, 202-1, or 201-2 or any combination thereof: 
     
       
         
         
             
             
         
       
     
     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 respectively be understood by referring to the descriptions of L 201  to L 203 , L 205 , xa1 to xa3, xa5, R 201 , and R 202  provided herein, and R 211  to R 213  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, 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 one of Compounds HT1 to HT39 or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     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 LUMO energy level of the p-dopant may be about −3.5 eV or less. 
     The p-dopant may include a quinone derivative, a metal oxide, a compound containing a cyano group, or any combination thereof. 
     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 (R 4 -TCNQ), or F6-TCNNQ; 
     a metal oxide such as tungsten oxide or molybdenum oxide; 
     1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN); 
     a compound represented by Formula 221, or 
     any combination thereof: 
     
       
         
         
             
             
         
       
     
     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 1 -C 60  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 substituent of the R 221  to R 223  may be a cyano group; —F; —Cl; —Br; —I; a C 1 -C 20  alkyl group substituted with —F; a C 1 -C 20  alkyl group substituted with —Cl; a C 1 -C 20  alkyl group substituted with —Br; a C 1 -C 20  alkyl group substituted with —I; or any combination thereof. 
     The compound represented by Formula 221 may include, for example, Compound HT-D2: 
     
       
         
         
             
             
         
       
     
     A thickness of the hole transport region  12  may be in a range of about 100 Angstroms (Å) to about 10,000 Å, for example, 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, for example, 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. 
     The hole transport region  12  may further include a buffer layer. 
     The buffer layer may compensate for an optical resonance distance depending on a wavelength of light emitted from the emission layer to improve the efficiency of an organic light-emitting device. 
     The hole transport region  12  may further include an electron blocking layer. The electron blocking layer may include a known material, e.g., mCP or DBFPO: 
     
       
         
         
             
             
         
       
     
     Electron Transport Region  17   
     In the organic light-emitting device  10 , the electron transport region  17  may be 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. 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, a buffer layer, a hole blocking layer, an electron control layer, or an electron transport layer in the electron transport region) may include a metal-free compound including at least one π electron-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 of the π electron-depleted nitrogen-containing C 1 -C 60  cyclic group provided herein. 
     In some embodiments, the electron transport region  17  may include a compound represented by Formula 601: 
       [Ar 601 ] xe11 -[(L 601 ) xe1 -R 601 ] xe21    Formula 601
 
     wherein, in Formula 601, 
     Ar 601  and L 601  may each independently be a C 5 -C 60  carbocyclic group unsubstituted or substituted with at least one R 601a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 601a , 
     xe11 may be 1, 2, or 3, 
     xe1 may be an integer from 0 to 5, 
     R 601a  and R 601  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 heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 601 )(Q 602 )(Q 603 ), ——C(═O)(Q 601 ), —S(═O) 2 (Q 601 ), 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 phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group, and 
     xe21 may be an integer from 1 to 5. 
     In some embodiments, at least one of Ar 601 (s) in the number of xe11 and R 601 (s) in the number of xe21 may include a Tr electron-depleted nitrogen-containing C 1 -C 60  cyclic group. 
     In some embodiments, in Formula 601, ring Ar 601  and L 601  may each independently be a benzene group, a naphthalene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a naphthacene group, a picene group, a perylene group, a pentaphene group, an indenoanthracene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyrimidine group, a pyridazine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline group, a quinazoline group, a cinnoline group, a phenanthridine group, an acridine group, a phenanthroline group, a phenazine group, a benzimidazole group, an isobenzothiazole group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a thiadiazole group, an imidazopyridine group, an imidazopyrimidine group, or an azacarbazole group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —S(═O) 2 (Q 31 ), —P(═O)(Q 31 )(Q 32 ), or any 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 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 via a single bond. 
     In one or more embodiments, Ar 601  in Formula 601 may be an anthracene group. 
     In some embodiments, the 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, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group. 
     In one or more embodiments, in Formulae 601 and 601-1, xe1 and xe611 to xe613 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 deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, or any 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  17  may include one of Compounds ET1 to ET36 or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the electron transport region  17  may include 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), NTAZ, DBFPO, or any combination thereof. In some embodiments, when the electron transport region  17  includes a hole blocking layer, the hole blocking layer may include BCP or Bphen: 
     
       
         
         
             
             
         
       
     
     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 an alkali metal complex, an alkaline earth metal complex, or any combination thereof. A metal ion of the alkali metal complex may be a lithium (Li) ion, a sodium (Na) ion, a potassium (K) ion, a rubidium (Rb) ion, a cesium (Cs) ion, or any combination thereof. A metal ion of the alkaline earth metal complex may be a beryllium (Be) ion, a magnesium (Mg) ion, a calcium (Ca) ion, a strontium (Sr) ion, a barium (Ba) ion, or any combination thereof. 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, cyclopentadiene, or any combination thereof. 
     For example, the metal-containing material may include a Li complex. The Li complex may include, e.g., Compound ET-D1 (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, Cs or any combination thereof. In some embodiments, the alkali metal may be Li, Na, or Cs. In an embodiment, the alkali metal may be Li or Cs. 
     The alkaline earth metal may be Mg, Ca, Sr, Ba, or any combination thereof. 
     The rare earth metal may be Sc, Y, Ce, Tb, Yb, Gd, or any combination thereof. 
     The alkali metal compound, the alkaline earth metal compound, and the rare earth metal compound may respectively be oxides, halides (e.g., fluorides, chlorides, bromides, or iodides), or any combination thereof of each of the alkali metal, the alkaline earth metal, and the rare earth metal. 
     The alkali metal compound may be one of alkali metal oxides such as Li 2 O, Cs 2 O, or K 2 O, one of alkali metal halides such as LiF, NaF, CsF, KF, LiI, Nal, CsI, or KI, or any combination thereof. In some embodiments, the alkali metal compound may include LiF, Li 2 O, NaF, LiI, NaI, CsI, KI, or any combination thereof. 
     The alkaline earth-metal compound may include one of alkaline earth-metal compounds, such as BaO, SrO, CaO, Ba x Sr 1-x O (wherein 0&lt;x&lt;1), or Ba x Ca 1-x O (wherein 0&lt;x&lt;1), or any combination thereof. In some embodiments, the alkaline earth metal compound may include BaO, SrO, CaO, or any combination thereof. 
     The rare earth metal compound may include YbF 3 , ScF 3 , Sc 2 O 3 , Y 2 O 3 , Ce 2 O 3 , GdF 3 , TbF 3 , or any combination thereof. In an embodiment, the rare earth metal compound may include YbF 3 , ScF 3 , TbF 3 , YbI 3 , ScI 3 , TbI 3 , or any combination thereof. 
     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 rare earth metal. Each ligand coordinated with the metal ion of the alkali metal complex, the alkaline earth metal complex, and the rare earth metal complex may independently be hydroxyquinoline, hydroxyisoquinoline, hydroxybenzoquinoline, hydroxyacridine, hydroxyphenanthridine, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxyphenyloxadiazole, hydroxyphenylthiadiazole, hydroxyphenylpyridine, hydroxyphenylbenzimidazole, hydroxyphenylbenzothiazole, bipyridine, phenanthroline, cyclopentadiene, or any combination thereof. 
     The electron injection layer may consist of an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal 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 lithium (Li), silver (Ag), magnesium (Mg), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), ITO, IZO, or any combination thereof. 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. 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, and the term “C 1 -C 60  alkylene group” as used herein refers to a divalent group having the same structure as the C 1 -C 60  alkyl group. 
     Examples of the C 1 -C 60  alkyl group, the C 1 -C 20  alkyl group, and/or the C 1 -C 10  alkyl group alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, an n-hexyl group, an isohexyl group, a sec-hexyl group, a tert-hexyl group, an n-heptyl group, an isoheptyl group, a sec-heptyl group, a tert-heptyl group, an n-octyl group, an isooctyl group, a sec-octyl group, a tert-octyl group, an n-nonyl group, an isononyl group, a sec-nonyl group, a tert-nonyl group, an n-decyl group, an isodecyl group, a sec-decyl group, or a tert-decyl group, each unsubstituted or substituted with a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, an n-hexyl group, an isohexyl group, a sec-hexyl group, a tert-hexyl group, an n-heptyl group, an isoheptyl group, a sec-heptyl group, a tert-heptyl group, an n-octyl group, an isooctyl group, a sec-octyl group, a tert-octyl group, an n-nonyl group, an isononyl group, a sec-nonyl group, a tert-nonyl group, an n-decyl group, an isodecyl group, a sec-decyl group, a tert-decyl group, or any combination thereof. 
     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, a propoxy group, a butoxy group, and a pentoxy 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 60  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 saturated hydrocarbon cyclic group having 3 to 10 carbon atoms. The term “C 3 -C 10  cycloalkylene group” as used herein refers to a divalent group having the same structure as the C 3 -C 10  cycloalkyl group. 
     Examples of the  c3 -C 10  cycloalkyl group as used herein include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl (bicyclo[2.2.1]heptyl) group, a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, and a bicyclo[2.2.2]octyl group. 
     The term “C 1 -C 10  heterocycloalkyl group” as used herein refers to a monovalent cyclic group having 1 to 10 carbon atoms and at least one heteroatom of N, O, P, Si, S, Se, Ge, and B as a ring-forming atom. The term “C 1 -C 10  heterocycloalkylene group” as used herein refers to a divalent group having the same structure as the C 1 -C 10  heterocycloalkyl group. 
     Examples of the C 1 -C 10  heterocycloalkyl group as used herein may include a silolanyl group, a silinanyl group, a tetrahydrofuranyl group, a tetrahydro-2H-pyranyl group, or a tetrahydrothiophenyl group. 
     The term “C 3 -C 10  cycloalkenyl group” as used herein refers to a monovalent cyclic group that has 3 to 10 carbon atoms and at least one carbon-carbon double bond in its ring, and is not aromatic. Examples thereof include a cyclopentenyl group, a cyclohexenyl group, and a cycloheptenyl group. The term “C 3 -C 10  cycloalkenylene group” as used herein refers to a divalent group having 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 cyclic group including at least one heteroatom of N, O, P, Si, S, Se, Ge, and B 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 1 -C 60  heteroaryl group” as used herein refers to a monovalent group having a heterocyclic aromatic system having at least one heteroatom of N, O, P, Si, S, Se, Ge, and B 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 of N, O, P, Si, S, Se, Ge and B 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 6 -C 60  aryloxy group” as used herein indicates —OA 102  (wherein A 102  may be the C 6 -C 60  aryl group), and the term “C 6 -C 60  arylthio group” as used herein indicates —SA 103  (wherein A 103  may be the C 6 -C 60  aryl group), and the term “C 7 -C 60  aryl alkyl group” indicates an alkyl group in which at least one hydrogen of a C 1 -C 54  alkyl group is substituted with a C 6 -C 59  aryl group, e.g., a cumyl group, and the term “C 2 -C 60  heteroaryl alkyl group” as used herein indicates an alkyl group in which at least one hydrogen of a C 1 -C 59  alkyl group is substituted with a C 1 -C 59  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, S, Se, Ge, and B 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 “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═*′ (wherein * and *′ each indicate a binding site to an adjacent atom) as a ring-forming moiety. 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 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 at least one *—N═*′ (wherein * and *′ each indicate a binding site to an adjacent atom) as a ring-forming moiety. 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. 
     The term “C 5 -C 60  cyclic group” as used herein refers to a monocyclic or polycyclic group having 5 to 60 carbon atoms, e.g., a) a third ring or b) a condensed ring in which at least two third rings are condensed. 
     The “C 1 -C 60  heterocyclic group” as used herein refers to a monocyclic or polycyclic group including at least one heteroatom and 1 to 60 carbon atoms, e.g., a) a fourth ring, b) a condensed ring in which at least two fourth rings are condensed, or c) a condensed ring in which at least one third ring is condensed with at least one fourth ring. 
     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 benzene group, a cyclopentadiene group, a pyrrole group, a furan group, a thiophene group, or a silole group. 
     The “third ring” as used herein may be a cyclopentane group, a cyclopentadiene group, an indene group, an adamantane group, a norbornene group, a bicyclo[1.1.1]pentane group, a bicyclo[2.1.1]hexane group, a bicyclo[2.2.1]heptane group (a norbornane group), a bicyclo[2.2.2]octane group, a cyclohexane group, a cyclohexene group, or a benzene group. 
     The “fourth ring” as used herein may be a furan group, a thiophene group, a pyrrole group, a silole group, an oxazole group, an isoxazole group, an oxadiazole group, an isooxadiazole group, oxatriazole group, an isooxatriazole group, a thiazole group, an isothiazole group, a thiadiazole group, an isothiadiazole group, a thiatriazole group, an isotriazole group, a pyrazole group, an imidazole group, a triazole group, a tetrazole group, an azasilole group, a diazasilole group, a trazasilole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, or a triazine group. 
     In some embodiments, the π electron-depleted nitrogen-containing C 1 -C 60  cyclic group may be an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyridazine group, a pyrimidine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a benzoisoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline group, a benzoquinoxaline group, a quinazoline group, a cinnoline group, a phenanthridine group, an acridine group, a phenanthroline group, a phenazine group, a benzimidazole group, an isobenzothiazole group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a thiadiazole group, an imidazopyridine group, an imidazopyrimidine group, an azacarbazole group, an azadibenzofuran group, an azadibenzothiophene group, an azadibenzosilole group, an acridine group, or a pyridopyrazine group: 
     In some embodiments, the π electron-rich C 3 -C 60  cyclic group may be a benzene group, a heptalene group, an indene group, a naphthalene group, an azulene group, an indacene group, an acenaphthylene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a naphthacene group, a picene group, a perylene group, a pentacene group, a hexacene group, a pentaphene group, a rubicene group, a coronene group, an ovalene group, a pyrrole group, a furan group, a thiophene group, an isoindole group, an indole group, an indene group, a benzofuran group, a benzothiophene group, a benzosilole group, a naphthopyrrole group, a naphthofuran group, a naphthothiophene group, a naphthosilole group, a benzocarbazole group, a dibenzocarbazole group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, a dibenzosilole group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, a benzothienocarbazole group, a benzosilolocarbazole group, a triindolobenzene group, a pyrrolophenanthrene group, a furanophenanthrene group, a thienophenanthrene group, a benzonaphthofuran group, a benzonapthothiophene group, an (indolo)phenanthrene group, a (benzofurano)phenanthrene group, or a (benzothieno)phenanthrene group. 
     For example, the C 5 -C 60  carbocyclic group may be a cyclopentane group, a cyclohexane group, a cyclohexene group, a benzene group, a naphthalene group, an anthracene group, a phenanthrene group, a triphenylene group, a pyrene group, a chrysene group, a 1 ,2,3,4-tetrahydronaphthalene group, a cyclopentadiene group, an indene group, a fluorene group, a 5,6,7,8-tetrahydroisoquinoline group, a 5,6,7,8-tetrahydroquinoline group, an adamantane group, a norbornane group, or a norbornene group. 
     For example, the C 1 -C 60  heterocyclic group may be a thiophene group, a furan group, a pyrrole group, a cyclopentadiene group, a silole group, a borole group, a phosphole group, a selenophene group, a germole group, a benzothiophene group, a benzofuran group, an indole group, an indene group, a benzosilole group, a benzoborole group, a benzophosphole group, a benzoselenophene group, a benzogermole group, a dibenzothiophene group, a dibenzofuran group, a carbazole group, a dibenzosilole group, a dibenzoborole group, a dibenzophosphole group, a dibenzoselenophene group, a dibenzogermole group, a dibenzothiophene 5-oxide group, a 9H-fluorene-9-one group, a dibenzothiophene 5,5-dioxide group, an azabenzothiophene group, an azabenzofuran group, an azaindole group, an azaindene group, an azabenzosilole group, an azabenzoborole group, an azabenzophosphole group, an azabenzoselenophene group, an azabenzogermole group, an azadibenzothiophene group, an azadibenzofuran group, an azacarbazole group, an azafluorene group, an azadibenzosilole group, an azadibenzoborole group, an azadibenzophosphole group, an azadibenzoselenophene group, an azadibenzogermole group, an azadibenzothiophene 5-oxide group, an aza-9H-fluoren-9-one group, an azadibenzothiophene 5,5-dioxide group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a quinoline group, an isoquinoline group, a quinoxaline group, a quinazoline group, a phenanthroline group, a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isooxazole group, a thiazole group, an isothiazole group, an oxadiazole group, a thiadiazole group, a benzopyrazole group, a benzimidazole group, a benzoxazole group, a benzothiazole group, a benzoxadiazole group, or a benzothiadiazole group. 
     The π electron-depleted nitrogen-containing C 1 -C 60  cyclic group, a the π electron-rich C 3 -C 60  cyclic group, the C 5 -C 60  cyclic group, and the C 1 -C 60  heterocyclic group may each be a part of a condensed ring or a monovalent, divalent, trivalent, quadrivalent, pentavalent, or hexavalent, group, depending on the structure of the formula. 
     The “fluorinated C 1 -C 60  alkyl group (or fluorinated C 1 -C 20  alkyl group or the like)”, “fluorinated C 3 -C 10  cycloalkyl group”, “fluorinated C 1 -C 10  heterocycloalkyl group”, and the “fluorinated phenyl group” as used herein may respectively be a C 1 -C 60  alkyl group (or C 1 -C 20  alkyl group or the like), a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, and a phenyl group, each substituted with at least one fluoro group (—F). For example, the term “fluorinated C 1  alkyl group (i.e., a fluorinated methyl group)” as used herein includes —CF 3 , —CF 2 H, and —CFH 2 . The term “fluorinated C 1 -C 60  alkyl group (or a fluorinated C 1 -C 20  alkyl group or the like)”, “fluorinated C 3 -C 10  cycloalkyl group”, “fluorinated C 1 -C 10  heterocycloalkyl group”, or “fluorinated phenyl group” as used herein respectively indicates i) a fully fluorinated C 1 -C 60  alkyl group (or a fully fluorinated C 1 -C 20  alkyl group or the like), a fully fluorinated C 3 -C 10  cycloalkyl group, a fully fluorinated C 1 -C 10  heterocycloalkyl group, or a fully fluorinated phenyl group, each of which all hydrogens included in each group are substituted with fluoro groups, or ii) a partially fluorinated C 1 -C 60  alkyl group (or a partially fluorinated C 1 -C 20  alkyl group or the like), a partially fluorinated C 3 -C 10  cycloalkyl group, a partially fluorinated C 1 -C 10  heterocycloalkyl group, or a partially fluorinated phenyl group, each of which all hydrogens included in each group are not substituted with fluoro groups. 
     The “deuterated C 1 -C 60  alkyl group (or deuterated C 1 -C 20  alkyl group or the like)”, “deuterated C 3 -C 10  cycloalkyl group”, “deuterated heterocycloalkyl group”, and the “deuterated phenyl group” as used herein may respectively be a C 1 -C 60  alkyl group (or a C 1 -C 20  alkyl group or the like), a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, and a phenyl group, each substituted with at least one deuterated. Examples of the “deuterated C 1  alkyl group (i.e., a deuterated methyl group)” may include —CD 3 , —CD 2 H, and —CDH 2 . Examples of the “deuterated C 3 -C 10  cycloalkyl group” may include Formula 10-501. The term “deuterated C 1 -C 60  alkyl group (or a deuterated C 1 -C 20  alkyl group or the like)”, “deuterated C 3 -C 10  cycloalkyl group”, “deuterated heterocycloalkyl group”, or “deuterated phenyl group” as used herein respectively indicates i) a fully deuterated C 1 -C 60  alkyl group (or a fully deuterated C 1 -C 20  alkyl group or the like), a fully deuterated C 3 -C 10  cycloalkyl group, a fully deuterated heterocycloalkyl group, or a fully deuterated phenyl group, each of which all hydrogens included in each group are substituted with deuteriums, or ii) a partially deuterated C 1 -C 60  alkyl group (or a partially deuterated C 1 -C 20  alkyl group or the like), a partially deuterated C 3 -C 10  cycloalkyl group, a partially deuterated heterocycloalkyl group, or a partially deuterated phenyl group, each of which all hydrogens included in each group are not substituted with deuteriums. 
     The term “(C 1 -C 20  alkyl) ‘X’ group” as used herein refers to an ‘X’ group substituted with at least one C 1 -C 20  alkyl group. For example, the term “(C 1 -C 20  alkyl)C 3 -C 10  cycloalkyl group” as used herein refers to a C 3 -C 10  cycloalkyl group substituted with at least one C 1 -C 20  alkyl group. For example, the term “(C 1 -C 20  alkyl)phenyl group” as used herein refers to a phenyl group substituted with at least one C 1 -C 20  alkyl group. Examples of the (C 1  alkyl)phenyl group may include a toluyl group. 
     In the present specification, the number of carbons in each group that is substituted (e.g., C 1 -C 60 ) excludes the number of carbons in the substituent. For example, a C 1 -C 60  alkyl group can be substituted with a C 1 -C 60  alkyl group. The total number of carbons included in the C 1 -C 60  alkyl group substituted with the C 1 -C 60  alkyl group is not limited to 60 carbons. In addition, more than one C 1 -C 60  alkyl substituent may be present on the C 1 -C 60  alkyl group. This definition is not limited to the C 1 -C 60  alkyl group and applies to all substituted groups that recite a carbon range. 
     In the present specification, “an azaindole group, an azabenzoborole group, an azabenzophosphole group, an azaindene group, an azabenzosilole group, an azabenzogermole group, an azabenzothiophene group, an azabenzoselenophene group, an azabenzofuran group, an azacarbazole group, an azadibenzoborole group, an azadibenzophosphole group, an azafluorene group, an azadibenzosilole group, an azadibenzogermole group, an azadibenzothiophene group, an azadibenzoselenophene group, an azadibenzofuran group, an azadibenzothiophene 5-oxide group, an aza-9H-fluoren-9-one group, and an azadibenzothiophene 5,5-dioxide group” each refer to a hetero ring in which at least one ring-forming carbon atom is substituted with nitrogen atom and respectively having an identical backbone as “an indole group, a benzoborole group, a benzophosphole group, an indene group, a benzosilole group, a benzogermole group, a benzothiophene group, a benzoselenophene group, a benzofuran group, a carbazole group, a dibenzoborole group, a dibenzophosphole group, a fluorene group, a dibenzosilole group, a dibenzogermole group, a dibenzothiophene group, a dibenzoselenophene group, a dibenzofuran group, a dibenzothiophene 5-oxide group, a 9H-fluorene-9-one group, and a dibenzothiophene 5,5-dioxide group”. 
     A substituent of the substituted π electron-deficient nitrogen-containing C 1 -C 60  cyclic group, the substituted π electron-rich C 3 -C 60  cyclic group, the substituted C 5 -C 60  cyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 1 -C 60  alkylene group, the substituted C 2 -C 60  alkenylene group, the substituted C 2 -C 60  alkynylene group, the substituted C 3 -C 10  cycloalkylene group, the substituted C 1 -C 10  heterocycloalkylene group, the substituted C 3 -C 10  cycloalkenylene group, the substituted C 1 -C 10  heterocycloalkenylene group, the substituted C 6 -C 60  arylene group, the substituted C 1 -C 60  heteroarylene group, the substituted divalent non-aromatic condensed polycyclic group, the substituted divalent non-aromatic condensed heteropolycyclic group, the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 6 -C 60  aryloxy group, the substituted C 6 -C 60  arylthio group, the substituted C 1 -C 60  heteroaryl group, the substituted monovalent non-aromatic condensed polycyclic group, and the substituted monovalent non-aromatic condensed heteropolycyclic group may be: 
     deuterium, —F, —Cl, —Br, —I, —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 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, or a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, or a C 1 -C 60  alkoxy group, each substituted with deuterium, —F, —Cl, —Br, —I, —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 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  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 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 ), —Ge(Q 13 )(Q 14 )(Q 15 ), —B(C 216 )(Q 17 ), —P(═O)(Q 18 )(Q 19 ), —P(Q 18 )(Q 19 ), or any 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  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 monovalent non-aromatic condensed polycyclic group, or a monovalent non-aromatic condensed heteropolycyclic group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, —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 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 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  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 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 ), —Ge(Q 23 )(Q 24 )(Q 25 ), —B(Q 26 )(Q 27 ), —P(═O)(Q 28 )(Q 29 ), —P(Q 28 )(Q 29 ), or any combination thereof; 
     —N(Q 31 )(Q 32 ), —Si(Q 33 )(Q 34 )(Q 35 ), —Ge(Q 33 )(Q 34 )(Q 35 ), —B(Q 36 )(Q 37 ), —P(═O)(Q 38 )(Q 39 ), or —P(Q 38 )(Q 39 ); or 
     any combination thereof. 
     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; 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 unsubstituted or substituted with deuterium, a C 1 -C 60  alkyl group, a C 6 -C 60  aryl group, or any combination thereof; a C 2 -C 60  alkenyl group; a C 2 -C 60  alkynyl group; a C 1 -C 60  alkoxy group; a C 3 -C 10  cycloalkyl group; a C 1 -C 10  heterocycloalkyl group; a C 3 -C 10  cycloalkenyl group; a C 1 -C 10  heterocycloalkenyl group; a C 6 -C 60  aryl group unsubstituted or substituted with deuterium, a C 1 -C 60  alkyl group, a C 6 -C 60  aryl group, or any combination thereof; a C 6 -C 60  aryloxy group; a C 6 -C 60  arylthio group; a C 1 -C 60  heteroaryl group; a monovalent non-aromatic condensed polycyclic group; or a monovalent non-aromatic condensed heteropolycyclic group. 
     For example, Q 1  to Q 9 , Q 11  to Q 19 , Q 21  to Q 29 , and Q 31  to Q 39  in this specification may each independently be: 
     —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, or —CD 2 CDH 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 deuterium, a C 1 -C 20  alkyl group, a phenyl group, or any 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 via a single bond, respectively. 
     Hereinafter, a compound and an organic light-emitting device according to an embodiment will be described in detail with reference to Synthesis Examples and Embodiments, 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 1 (Compound E1) 
     Compound E1 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     9.56 grams (g) (21.45 millimole (mmol)) of 9,9′-(6-chloro-1,3,5-triazine-2,4-diyl)bis(9H-carbazole), 10.91 g (23.59 mmol) of triphenyl(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)silane, 2.48 g (2.14 mmol) of Pd(PPh 3 ) 4 , and 5.93 g (42.90 mmol) of K 2 CO 3  were dissolved in tetrahydrofuran/distilled water (54 milliliters (mL)/22 mL). The temperature of the mixture was raised, followed by stirring under reflux for 12 hours. Once the reaction was complete, the temperature of the reaction mixture was cooled to room temperature, and methanol (1,000 mL) was added thereto. The resulting solid was filtered and purified using silica gel column chromatography to thereby obtain 8.50 g of Compound E1 (yield: 53%). 
     LC-Mass (calculated value: 745.27 g/mol, found value: M+1=746 g/mol) 
     Synthesis Example 2 (Compound P4) 
     Compound P4 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Intermediate (P4-IM2) 
     5.46 g (20 mmol) of 1-(3-bromophenyl)-1H-benzo[d]imidazole, 7.59 g (24 mmol) of 9-(4-(tert-butyl)pyridin-2-yl)-9H-carbazol-2-ol, 0.76 g (4 mmol) of copper (I) iodide, 0.98 g (8 mmol) of picolinic acid, and 12.7 g (60 mmol) of potassium phosphate tribasic were mixed with 133 mL of dimethyl sulfoxide (DMSO), followed by stirring at a temperature of 120° C. for 12 hours. Once the reaction was complete, the mixture was cooled to room temperature, and then, saturated NH 4 Cl was added, and the resulting mixture was extracted with ethyl acetate (EA). The organic layer was dried with anhydrous MgSO 4  and subjected to filtration, followed by concentration under reduced pressure. The resulting product was subjected to silica gel column chromatography to thereby obtain 6.61 g (13 mmol) of Intermediate P4-IM2 (yield: 65%). 
     LC-Mass (calculated value: 508.23 g/mol, found value: M+1=509 g/mol) 
     (2) Synthesis of Intermediate (P4-IM1) 
     7.42 g (13 mmol) of Intermediate P4-IM2, 10.30 g (19.5 mmol) of (3-(tert-butyl)phenyl)(mesityl)iodonium triflate, and 0.24 g (1.3 mmol) of copper (II) acetate were mixed with 130 mL of DMF, followed by stirring at a temperature of 100° C. for 3 hours. Once the reaction was complete, the mixture was cooled to room temperature, and then, saturated NH 4 Cl was added. The resulting mixture was extracted with EA. The organic layer was dried with anhydrous MgSO 4  and subjected to filtration, followed by concentration under reduced pressure. The resulting product was subjected to silica gel column chromatography to thereby obtain 8.74 g (11.05 mmol) of Intermediate P4-IM1 (yield: 85%). 
     LC-Mass (calculated value: 641.33 g/mol, measured value: M +1 =641 g/mol) 
     (3) Synthesis of Compound (P4) 
     4.35 g (5.50 mmol) of Intermediate P4-IM1, 2.26 g (6.05 mmol) of Pt(COD)Cl 2 , and 1.35 g (16.50 mmol) of sodium acetate were mixed with 275 mL of benzonitrile, followed by stirring at a temperature of 180° C. for 18 hours. Once the reaction was complete, the mixture was cooled to room temperature and concentrated under reduced pressure. The resulting product was subjected to silica gel column chromatography to thereby obtain 2.53 g (3.03 mmol) of Compound P4 (yield: 55%). 
     LC-Mass (calculated value: 833.27 g/mol, found value: M+1=834 g/mol) 
     Synthesis Example 3 (Compound P8) 
     Compound P8 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Intermediate (P8-IM2) 
     5.46 g (20 mmol) of 1-(3-bromophenyl)-1H-benzo[d]imidazole, 9.42 g (24 mmol) of 9-(4-(tert-butyl)pyridin-2-yl)-6-phenyl-9H-carbazol-2-ol, 0.76 g (4 mmol) of copper (I) iodide, 0.98 g (8 mmol) of picolinic acid, and 12.7 g (60 mmol) of potassium phosphate tribasic were mixed with 133 mL of DMSO, followed by stirring at a temperature of 120° C. for 12 hours. Once the reaction was complete, the mixture was cooled to room temperature, saturated NH 4 Cl was added, and the mixture was extracted with EA. The organic layer was dried with anhydrous MgSO 4  and subjected to filtration, followed by concentration under reduced pressure. The resulting product was subjected to silica gel column chromatography to thereby obtain 7.84 g (13.4 mmol) of Intermediate P8-IM2 (yield: 67%). 
     LC-Mass (calculated value: 584.26 g/mol, found value: M+1=585 g/mol) 
     (2) Synthesis of Intermediate (P8-IM1) 
     7.84 g (13.4 mmol) of Intermediate P8-IM2, 11.75 g (20.1 mmol) of (3,5-di-tert-butylphenyl)(mesityl)iodonium triflate, and 0.25 g (1.34 mmol) of copper (II) acetate were mixed with 130 mL of DMF, followed by stirring at a temperature of 100° C. for 3 hours. Once the reaction was complete, the mixture was cooled to room temperature, saturated NH 4 Cl was added, and the mixture was extracted with EA. The organic layer was dried with anhydrous MgSO 4  and subjected to filtration, followed by concentration under reduced pressure. The resulting product was subjected to silica gel column chromatography to thereby obtain 10.27 g (11.12 mmol) of Intermediate P8-IM1 (yield: 83%). 
     LC-Mass (calculated value: 773.42 g/mol, measured value: M +1 =773 g/mol) 
     (3) Synthesis of Compound (P8) 
     5.08 g (5.50 mmol) of Intermediate P8-M1, 2.26 g (6.05 mmol) of Pt(COD)Cl 2 , and 1.35 g (16.50 mmol) of sodium acetate were mixed with 275 mL of benzonitrile, followed by stirring at a temperature of 180° C. for 18 hours. Once the reaction was complete, the mixture was cooled to room temperature and concentrated under reduced pressure. The resulting product was subjected to silica gel column chromatography to thereby obtain 2.66 g (2.75 mmol) of Compound P8 (yield: 50%). 
     LC-Mass (calculated value: 965.36 g/mol, found value: M+1=966 g/mol) 
     Synthesis Example 4 (Compound P9) 
     Compound P9 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P9) 
     Intermediate P9-IM2 was synthesized in the same manner as in Synthesis of Intermediate P4-IM2, except that Compound 9-(1) was used instead of 5.46 g (20 mmol) of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then, Intermediate P9-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P9-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P9, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P9-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 945.40 g/mol, found value: M+1=946 g/mol) 
     Synthesis Example 5 (Compound P13) 
     Compound P13 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P13) 
     Intermediate P13-IM2 was synthesized in the same manner as in Synthesis of Intermediate P4-IM2, except that Compound 13-(1) was used instead of 9-(4-(tert-butyl)pyridin-2-yl)-9H-carbazol-2-ol, and then Intermediate P13-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P13-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P13, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P13-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 999.43 g/mol, found value: M+1=1000 g/mol) 
     Synthesis Example 6 (Compound P14) 
     Compound P14 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P14) 
     Intermediate P14-IM2 was synthesized in the same manner as in Synthesis of Intermediate P4-IM2, except that Compound 14-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then, Intermediate P14-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P14-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P14, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P14-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 895.38 g/mol, found value: M+1=896 g/mol) 
     Synthesis Example 7 (Compound P16) 
     Compound P16 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P16) 
     Intermediate P16-IM2 was synthesized in the same manner as in Synthesis of Intermediate P4-IM2, except that Compound 16-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then, Intermediate P16-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P16-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P16, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P16-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 923.41 g/mol, found value: M+1=924 g/mol) 
     Synthesis Example 8 (Compound P18) 
     Compound P18 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P18) 
     Intermediate P18-IM1 was synthesized in the same manner as in Synthesis of Intermediate P4-IM1, except that Intermediate P8-IM2 was used instead of Intermediate P4-IM2. 
     Next, the desired compound, Compound P18, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P18-IM1 was used. 
     LC-Mass (calculated value: 909.30 g/mol, found value: M+1=910 g/mol) Synthesis Example 9 (Compound P19) 
     Compound P19 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P19) 
     P19-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Compound 13-(1) was used instead of 3-(tert-butyl)phenyl)(mesityl)iodonium triflate. 
     The desired compound, Compound P19, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P19-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 985.33 g/mol, found value: M+1=986 g/mol) Synthesis Example 10 (Compound P26) 
     Compound P26 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P26) 
     P26-IM2 was synthesized in the same manner as in Synthesis of Intermediate P8-IM2, except that Compound 26-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then P26-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Compound 26-(2) was used instead of (3,5-di-tert-butylphenyl)(mesityl)iodonium triflate. 
     The desired compound, Compound P26, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P26-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 909.30 g/mol, found value: M+1=910 g/mol) 
     Synthesis Example 11 (Compound P27) 
     Compound P27 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P27) 
     Intermediate P27-IM2 was synthesized in the same manner as in Synthesis of Intermediate P8-IM2, except that Compound 27-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then, Intermediate P27-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P27-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P27, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P19-IM1 was used instead of Intermediate P27-IM1. 
     LC-Mass (calculated value: 1041.40 g/mol, found value: M+1=1042 g/mol) 
     Synthesis Example 12 (Compound P30) 
     Compound P30 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P30) 
     P30-1M1 was synthesized in the same manner as in Synthesis of Intermediate P4-IM1, except that Compound 30-(1) was used instead of 10.30 g (19.5 mmol) of 3-(tert-butyl)phenyl)(mesityl)iodonium triflate. 
     The desired compound, Compound P30, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P30-1M1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 718.21 g/mol, found value: M+1=719 g/mol) 
     Synthesis Example 13 (Compound P44) 
     Compound P44 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P44) 
     Intermediate P44-IM2 was synthesized in the same manner as in Synthesis of Intermediate P8-IM2, except that Compound 44-(1) was used instead of 9-(4-(tert-butyl)pyridin-2-yl)-6-phenyl-9H-carbazol-2-ol, and then Intermediate P44-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P44-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P44, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P44-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 1007.41 g/mol, found value: M+1=1008 g/mol) 
     Synthesis Example 14 (Compound P45) 
     Compound P45 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P45) 
     Intermediate P45-IM1 was synthesized in the same manner as in Synthesis of Intermediate P4-IM2, except that Compound 45-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole. 
     The desired compound, Compound P45, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P45-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 763.29 g/mol, found value: M+1=764 g/mol) 
     Synthesis Example 15 (Compound P46) 
     Compound P46 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P46) 
     Intermediate P46-IM2 was synthesized in the same manner as in Synthesis of Intermediate P8-IM2, except that Compound 46-(1) was used instead of 9-(4-(tert-butyl)pyridin-2-yl)-6-phenyl-9H-carbazol-2-ol, and then Intermediate P46-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P46-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P46, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P46-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 945.40 g/mol, found value: M+1=946 g/mol) 
     Synthesis Example 16 (Compound P47) 
     Compound P47 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P47) 
     Intermediate P47-IM2 was synthesized in the same manner as in Synthesis of Intermediate P4-IM2, except that Compound 47-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then, Intermediate P47-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P47-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P47, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P47-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 897.39 g/mol, found value: M+1=898 g/mol) 
     Synthesis Example 17 (Compound P48) 
     Compound P48 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P48) 
     Intermediate P48-IM2 was synthesized in the same manner as in Synthesis of Intermediate P4-IM2, except that Compound 48-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then, Intermediate P48-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P48-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P48, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P48-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 971.41 g/mol, found value: M+1=972 g/mol) 
     Synthesis Example 18 (Compound P49) 
     Compound P49 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P49) 
     Intermediate P49-IM2 was synthesized in the same manner as in Synthesis of Intermediate P8-IM2, except that Compound 49-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and Compound 49-(2) was used instead of 9-(4-(tert-butyl)pyridin-2-yl)-6-phenyl-9H-carbazol-2-ol, and Intermediate P49-IM1 was synthesized in the same manner as in Synthesis of P8-IM1, except that Intermediate P49-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P47, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P47-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 1081.51 g/mol, found value: M+1=1082 g/mol) 
     Synthesis Example 19 (Compound P50) 
     Compound P50 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P50) 
     Intermediate P50-1M2 was synthesized in the same manner as in Synthesis of Intermediate P8-IM2, except that Compound 50-(1) was used instead of 9-(4-(tert-butyl)pyridin-2-yl)-6-phenyl-9H-carbazol-2-ol, and then Intermediate P50-1M1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P50-1M2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P50, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P50-1M1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 876.31 g/mol, found value: M+1=877 g/mol) 
     Synthesis Example 20 (Compound P51) 
     Compound P51 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P51) 
     Intermediate P51-IM2 was synthesized in the same manner as in Synthesis of Intermediate P8-IM2, except that Compound 51-(1) was used instead of 9-(4-(tert-butyl)pyridin-2-yl)-6-phenyl-9H-carbazol-2-ol, and then Intermediate P51-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P51-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P51, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P51-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 907.32 g/mol, found value: M+1=908 g/mol) 
     Synthesis Example 21 (Compound P53) 
     Compound P53 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P53) 
     Intermediate P53-IM2 was synthesized in the same manner as in Synthesis of Intermediate P4-IM2, except that Compound 53-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then, Intermediate 53-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P53-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P53, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P53-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 945.40 g/mol, found value: M+1=946 g/mol) 
     Synthesis Example 22 (Compound P54) 
     Compound P54 was synthesized based on the following Reaction Scheme: 
     
       
         
         
             
             
         
       
     
     (1) Synthesis of Compound (P54) 
     Intermediate P54-IM2 was synthesized in the same manner as in Synthesis of Intermediate P8-IM2, except that Compound 54-(1) was used instead of 1-(3-bromophenyl)-1H-benzo[d]imidazole, and then, Intermediate 54-IM1 was synthesized in the same manner as in Synthesis of Intermediate P8-IM1, except that Intermediate P54-IM2 was used instead of Intermediate P8-IM2. 
     The desired compound, Compound P54, was synthesized in the same manner as in Synthesis of Compound P4, except that Intermediate P54-IM1 was used instead of Intermediate P4-IM1. 
     LC-Mass (calculated value: 1021.43 g/mol, found value: M+1=1022 g/mol) 
     Synthesis Example 23 (Compound D3) 
     Compound D3 was synthesized in the same manner as in W02021-014001. 
     Synthesis Example 24 (Compound D19) 
     Compound D19 was synthesized in the same manner as in Nature Photonics 2019, 13(1), 678-682. 
     Evaluation Example 1: PL Spectrum Analysis (1) 
     Poly(methyl methacrylate) (PMMA) or E1 as a host compound and a phosphorescent compound shown in the following table as a dopant were co-deposited on a quartz substrate at 13 wt % to form a thin film having a thickness of 40 nm. 
     The prepared sample thin films were subjected to HITACHI F7000 fluorescence spectrometer to thereby obtain a photoluminescence (PL) curve and a transient photoluminescence decay (trPL) curve. 
     Based on the obtained PL spectrum and the trPL curve, a ratio of a 2 nd  peak to a 1 st  peak, a full width at half maximum (FWHM), a transition time to be taken for excitons transitioning from an excited state to a ground state (tau (d)), and a reduced amount of the initial prompt PL intensity were measured. The relative ratios for thin films including the same dopant were calculated. The results thereof are shown in Tables 1 and 2. First, to investigate intrinsic characteristics of dopants, a mixed film with PMMA was formed to thereby calculate optical characteristics of decay time (tau(d)) using photoluminescence (PL, HITACHI, F7000, Fluorescence Spectrophotometer) and transient photoluminescence (trPL, PICOQUANT, FluoTime 300, Fluorescence LifeTime Spectrometer). From PL of the thin films, a ratio of the 2 nd  peak to the 1 st  peak and a FWHM were evaluated, and a decay time (tau(d)) was measured by using trPL. Then, a mixed film of the host and the phosphorescent dopant according to the present disclosure was formed to measure PL, FWHM, and tau(d). Finally, through the ratio of these two optical characteristics, as compared with the intrinsic properties of the phosphorescent dopant, the optical properties changed due to the mixed film of the host(E1) and the phosphorescent dopant were quantified. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 2nd peak 
                 FWHM 
                 tau(d) 
               
               
                   
                 2 nd  Peak 
                   
                 tau(D) 
                 2 nd  Peak 
                   
                 tau(D) 
                 ratio 
                 ratio 
                 ratio 
               
               
                 Dopant 
                 ratio 
                 FWHM 
                 (PMMA:P) 
                 ratio 
                 FWHM 
                 (E1:P) 
                 (E1:P/ 
                 (E1:P/ 
                 (E1:P/ 
               
               
                 (P) 
                 (PMMA:P) 
                 (PMMA:P) 
                 (μS) 
                 (E1:P) 
                 (E1:P) 
                 (μS) 
                 PMMA:P) 
                 PMMA:P) 
                 PMMA:P) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 P4 
                 0.5325 
                 21.1 
                 2.965 
                 0.577 
                 44.40 
                 1.771 
                 108% 
                 210% 
                 60% 
               
               
                 P8 
                 0.4413 
                 17.36 
                 4.199 
                 0.452 
                 23.61 
                 2.382 
                 102% 
                 136% 
                 57% 
               
               
                 P9 
                 0.4839 
                 18.1 
                 3.022 
                 0.529 
                 40.76 
                 1.931 
                 109% 
                 225% 
                 64% 
               
               
                 P13 
                 0.479 
                 22.65 
                 2.7900 
                 0.503 
                 36.54 
                 1.939 
                 105% 
                 161% 
                 70% 
               
               
                 P14 
                 0.4673 
                 17.53 
                 4.605 
                 0.758 
                 69.99 
                 2.168 
                 162% 
                 399% 
                 47% 
               
               
                 P16 
                 0.4829 
                 18.41 
                 4.266 
                 0.871 
                 92.28 
                 2.102 
                 180% 
                 501% 
                 49% 
               
               
                 P18 
                 0.4486 
                 18.11 
                 4.139 
                 0.496 
                 27.83 
                 2.082 
                 111% 
                 154% 
                 50% 
               
               
                 P19 
                 0.4418 
                 18.25 
                 4.143 
                 0.482 
                 26.58 
                 2.117 
                 109% 
                 146% 
                 51% 
               
               
                 P26 
                 0.4572 
                 18.22 
                 3.140 
                 0.558 
                 42.65 
                 1.918 
                 122% 
                 234% 
                 61% 
               
               
                 P27 
                 0.4112 
                 17.16 
                 4.551 
                 0.453 
                 23.29 
                 2.445 
                 110% 
                 136% 
                 54% 
               
               
                 P30 
                 0.46 
                 19.79 
                 3.401 
                 0.744 
                 62.31 
                 1.871 
                 162% 
                 315% 
                 55% 
               
               
                 P44 
                 0.473 
                 19.43 
                 2.994 
                 0.492 
                 28.93 
                 2.058 
                 104% 
                 149% 
                 69% 
               
               
                 P45 
                 0.533 
                 39.9 
                 6.721 
                 1.359 
                 111.46 
                 4.854 
                 255% 
                 279% 
                 72% 
               
               
                 P46 
                 0.5057 
                 20.73 
                 3.145 
                 0.537 
                 41.42 
                 1.912 
                 106% 
                 200% 
                 61% 
               
               
                 P47 
                 0.4321 
                 17.33 
                 4.462 
                 0.595 
                 45.82 
                 2.269 
                 138% 
                 264% 
                 51% 
               
               
                 P48 
                 0.4784 
                 16.91 
                 4.570 
                 0.729 
                 61.93 
                 2.380 
                 152% 
                 366% 
                 52% 
               
               
                 P49 
                 0.5408 
                 39.1 
                 3.544 
                 0.716 
                 50.84 
                 2.321 
                 132% 
                 130% 
                 65% 
               
               
                 P50 
                 0.483 
                 17.01 
                 2.610 
                 0.781 
                 64.06 
                 1.662 
                 162% 
                 377% 
                 64% 
               
               
                 P51 
                 0.4168 
                 15.27 
                 4.147 
                 0.438 
                 22.26 
                 2.623 
                 105% 
                 146% 
                 63% 
               
               
                 P53 
                 0.4749 
                 20.44 
                 3.261 
                 0.510 
                 27.22 
                 1.348 
                 107% 
                 133% 
                 41% 
               
               
                 P54 
                 0.4254 
                 16.77 
                 4.148 
                 0.462 
                 24.12 
                 2.149 
                 109% 
                 144% 
                 52% 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Dopant (P) 
                 PMMA:P 
                 E1:P 
                 Δprompt ratio 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 P4 
                 7723 
                 2860 
                 63% 
               
               
                   
                 P8 
                 10052 
                 3902 
                 61% 
               
               
                   
                 P9 
                 10694 
                 3754 
                 65% 
               
               
                   
                 P13 
                 13494 
                 4589 
                 66% 
               
               
                   
                 P14 
                 4075 
                 1835 
                 55% 
               
               
                   
                 P16 
                 5387 
                 1539 
                 71% 
               
               
                   
                 P18 
                 14421 
                 100 
                 99% 
               
               
                   
                 P19 
                 12727 
                 3739 
                 71% 
               
               
                   
                 P26 
                 10635 
                 2833 
                 73% 
               
               
                   
                 P27 
                 11284 
                 3489 
                 69% 
               
               
                   
                 P30 
                 2924 
                 1886 
                 35% 
               
               
                   
                 P44 
                 8252 
                 4620 
                 38% 
               
               
                   
                 P45 
                 6122 
                 586 
                 44% 
               
               
                   
                 P46 
                 9913 
                 3251 
                 90% 
               
               
                   
                 P47 
                 6886 
                 2463 
                 67% 
               
               
                   
                 P48 
                 11427 
                 2367 
                 64% 
               
               
                   
                 P49 
                 8125 
                 3286 
                 79% 
               
               
                   
                 P50 
                 9397 
                 1247 
                 60% 
               
               
                   
                 P51 
                 15247 
                 3626 
                 87% 
               
               
                   
                 P53 
                 8854 
                 2919 
                 67% 
               
               
                   
                 P54 
                 13417 
                 243 
                 98% 
               
               
                   
                   
               
            
           
         
       
     
     Referring to the results of Tables 1 and 2, it was found that the PL intensity of the 2 nd  peak of the PL curve (that is, the PL curves of PMMA and phosphorescent dopant) of the dopant was higher than that of the 2 nd  peak of the PL curve of the emission layer of the combination of the host and the dopant according to one or more embodiments, and the FWHM of the 2 nd  peak of the PL curve of the dopant was higher than that of the 2 nd  peak of the PL curve of the emission layer, and the tau value (unit: ps) of the 2 nd  peak of the PL curve of the dopant was shorter than that of the 2 nd  peak of the PL curve of the emission layer. 
     In addition, a reduction ratio of the initial prompt PL intensity identified from the trPL curve of the emission layer of the combination of the host and the dopant according to one or more embodiments was 30% or greater. Thus, improvement in lifespan characteristics may be expected. 
     The exciplex formed at around 500 nm in the PL spectrum of the mixed film of the host and the phosphorescent dopant was subject to convolution with the PL spectrum of the dopant. Thus, the 2 nd  peak was increased, as compared with the 1 st  peak, and the FWHM became wider. In addition, in general, a decay time of the exciplex generated between the host and the phosphorescent dopant of several hundreds of nanoseconds (ns) is combined with the decay time of the phosphorescent dopant of microseconds (ps), thereby causing a change in the decay time to be shorter. In particular, an amount of change in the initial prompt signals in the trPL curve reflected a decay time of several hundreds of nanoseconds of the exciplex generated by the host and the phosphorescent dopant. The interaction between the host and the phosphorescent dopant may be inferred according to the prompt change amount (Δprompt). That is, when inferred from the data, it may be seen that the combination of the host and phosphorescent dopant according to one or more embodiments formed an exciplex. 
     Evaluation Example 2: PL Spectrum Analysis (2) 
     PMMA, Z1, or E1 as a host compound and P8, P14, or P30 as a phosphorescent compound were co-deposited on a quartz substrate at 13 wt % to form a thin film having a thickness of 40 nm. 
     
       
         
         
             
             
         
       
     
     The prepared sample thin films were subjected to HITACHI F7000 fluorescence spectrometer to thereby obtain a photoluminescence (PL) curve and a transient photoluminescence decay (trPL) curve. The results thereof are shown in  FIGS.  2 A,  2 B , and  2 C. 
     As shown in  FIGS.  2 A to  2 C , the 2 nd  peak of the PL curve for the emission layer (E1:P8, E1:P14, or E1:P30) according to one or more embodiments was found to have a high PL intensity, a wide FWHM, and a rapid reduction in the initial PL intensity in the trPL curve, as compared with the 2 nd  peak of the PL curve for the control emission layer (PMMA:P8, PMMA:P14, or PMMA:P30) and the comparative emission layer (Z1:P8, Z1:P14, or Z1:P30). Accordingly, it was found that the host and the dopant included in the emission layer according to one or more embodiments formed an exciplex. As described above, at about 500 nm, an increase in the 2 nd  peak and an increase in the peak FWHM may be confirmed in the PL spectrum due to a low energy of the exciplex. In addition, as compared with a long decay time (ps) of the phosphorescent dopant in the trPL decay curve, rapidly decaying components (prompt components) having a relatively short decay time of several hundreds of nanoseconds were added thereto, thereby reducing the decay time. In particular, when a reduction ratio of the initial prompt signal intensity identified from the trPL curve of the emission layer of the combination of the host and the dopant according to one or more embodiments is 30% or greater, lifespan characteristics may be improved. 
     Evaluation Example 3: Lifespan Evaluation of Light-Emitting Devices 
     Examples 1 to 23 and Comparative Examples 1 to 23 
     A glass substrate having an indium tin oxide (ITO) electrode (a first electrode, an anode) deposited thereon at a thickness of 1,500 Å was washed with distilled water in the presence of ultrasound waves. Once the washing with distilled water was complete, ultrasound wave washing was performed on the substrate using solvents, such as isopropyl alcohol, acetone, and methanol. Subsequently, the substrate was dried, transferred to a plasma washer, washed for 5 minutes using oxygen plasma, and mounted in a vacuum depositor. 
     Compound HT3 and Compound HT-D2 were co-deposited on the ITO electrode of the glass substrate to form a hole injection layer having a thickness of 100 Å. 
     Subsequently, Compound HT3 was deposited on the hole injection layer to form a hole transport layer having a thickness of 1,300 Å. mCP was next deposited on the hole transport layer to form an electron blocking layer having a thickness of 100 Å, thereby forming a hole transport region. 
     The host compound and the phosphorescent dopant shown in Table 3 or the host compound, the phosphorescent dopant, and the fluorescent dopant shown in Table 4 were co-deposited on the hole transport region to thereby form an emission layer having a thickness of 400 Å. 
     BCP was vacuum-deposited on the emission layer to form a hole blocking layer having a thickness of 100 Å. Compound ET3 and Liq were then co-deposited on the hole blocking layer to form an electron transport layer having a thickness of 300 Å. Next, Liq was vacuum-deposited on the electron transport layer to form an electron injection layer having a thickness of 10 Å, and then, Al second electrode (a cathode) having a thickness of 1,200 Å was formed on the electron injection layer, thereby completing the manufacture of an organic light-emitting device. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     T 95  lifespan that indicates time (hour) for the luminance of each organic light-emitting device to decline to 95% of its initial luminance of each of the organic light-emitting devices manufactured in Examples 1 to 23 and Comparative Examples 1 to 3 was measured. The T 95  lifespan characteristics of the organic light-emitting devices of Examples 1 to 21 were respectively calculated relative to the T 95  lifespan characteristics of the organic light-emitting devices of Comparative Examples 1 to 21. The results thereof are shown in Table 3. The T 95  lifespan characteristics of the organic light-emitting devices of Examples 22 and 23 were respectively calculated relative to the T 95  lifespan characteristics of the organic light-emitting devices of Comparative Examples 22 and 23. The results thereof are shown in Table 4. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 Phosphorescent 
                   
               
               
                   
                   
                 Host 
                 dopant 
                 rel. T95 
               
               
                   
                   
               
             
            
               
                   
                 Comparative 
                 Z1 
                 P4  
                 100% 
               
               
                   
                 Example 1 
                   
                   
                   
               
               
                   
                 Example 1 
                 E1 
                 P4  
                 1435%  
               
               
                   
                 Comparative 
                 Z1 
                 P8  
                 100% 
               
               
                   
                 Example 2 
                   
                   
                   
               
               
                   
                 Example 2 
                 E1 
                 P8  
                 797% 
               
               
                   
                 Comparative 
                 Z1 
                 P9  
                 100% 
               
               
                   
                 Example 3 
                   
                   
                   
               
               
                   
                 Example 3 
                 E1 
                 P9  
                 954% 
               
               
                   
                 Comparative 
                 Z1 
                 P13 
                 100% 
               
               
                   
                 Example 4 
                   
                   
                   
               
               
                   
                 Example 4 
                 E1 
                 P13 
                 391% 
               
               
                   
                 Comparative 
                 Z1 
                 P14 
                 100% 
               
               
                   
                 Example 5 
                   
                   
                   
               
               
                   
                 Example 5 
                 E1 
                 P14 
                 1771%  
               
               
                   
                 Comparative 
                 Z1 
                 P16 
                 100% 
               
               
                   
                 Example 6 
                   
                   
                   
               
               
                   
                 Example 6 
                 E1 
                 P16 
                 780% 
               
               
                   
                 Comparative 
                 Z1 
                 P18 
                 100% 
               
               
                   
                 Example 7 
                   
                   
                   
               
               
                   
                 Example 7 
                 E1 
                 P18 
                 839% 
               
               
                   
                 Comparative 
                 Z1 
                 P19 
                 100% 
               
               
                   
                 Example 8 
                   
                   
                   
               
               
                   
                 Example 8 
                 E1 
                 P19 
                 641% 
               
               
                   
                 Comparative 
                 Z1 
                 P26 
                 100% 
               
               
                   
                 Example 9 
                   
                   
                   
               
               
                   
                 Example 9 
                 E1 
                 P26 
                 986% 
               
               
                   
                 Comparative 
                 Z1 
                 P27 
                 100% 
               
               
                   
                 Example 10 
                   
                   
                   
               
               
                   
                 Example 10 
                 E1 
                 P27 
                 811% 
               
               
                   
                 Comparative 
                 Z1 
                 P30 
                 100% 
               
               
                   
                 Example 11 
                   
                   
                   
               
               
                   
                 Example 11 
                 E1 
                 P30 
                 2285%  
               
               
                   
                 Comparative 
                 Z1 
                 P44 
                 100% 
               
               
                   
                 Example 12 
                   
                   
                   
               
               
                   
                 Example 12 
                 E1 
                 P44 
                 296% 
               
               
                   
                 Comparative 
                 Z1 
                 P$5 
                 100% 
               
               
                   
                 Example 13 
                   
                   
                   
               
               
                   
                 Example 13 
                 E1 
                 P45 
                 432% 
               
               
                   
                 Comparative 
                 Z1 
                 P46 
                 100% 
               
               
                   
                 Example 14 
                   
                   
                   
               
               
                   
                 Example 14 
                 E1 
                 P46 
                 778% 
               
               
                   
                 Comparative 
                 Z1 
                 P47 
                 100% 
               
               
                   
                 Example 15 
                   
                   
                   
               
               
                   
                 Example 15 
                 E1 
                 P47 
                 849% 
               
               
                   
                 Comparative 
                 Z1 
                 P48 
                 100% 
               
               
                   
                 Example 16 
                   
                   
                   
               
               
                   
                 Example 16 
                 E1 
                 P48 
                 1462%  
               
               
                   
                 Comparative 
                 Z1 
                 P49 
                 100% 
               
               
                   
                 Example 17 
                   
                   
                   
               
               
                   
                 Example 17 
                 E1 
                 P49 
                 318% 
               
               
                   
                 Comparative 
                 Z1 
                 P50 
                 100% 
               
               
                   
                 Example 18 
                   
                   
                   
               
               
                   
                 Example 18 
                 E1 
                 P50 
                 760% 
               
               
                   
                 Comparative 
                 Z1 
                 P51 
                 100% 
               
               
                   
                 Example 19 
                   
                   
                   
               
               
                   
                 Example 19 
                 E1 
                 P51 
                 657% 
               
               
                   
                 Comparative 
                 Z1 
                 P%3 
                 100% 
               
               
                   
                 Example 20 
                   
                   
                   
               
               
                   
                 Example 20 
                 E1 
                 P53 
                 644% 
               
               
                   
                 Comparative 
                 Z1 
                 P54 
                 100% 
               
               
                   
                 Example 21 
                   
                   
                   
               
               
                   
                 Example 21 
                 E1 
                 P54 
                 553% 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Phosphorescent 
                 Fluorescent 
                   
               
               
                   
                 Host 
                 dopant 
                 dopant 
                 rel. T95 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparative 
                 Z1 
                 P7 
                 D19 
                 100% 
               
               
                 Example 22 
               
               
                 Example 22 
                 E1 
                 P7 
                 D19 
                 279% 
               
               
                 Comparative 
                 Z1 
                 P7 
                 D3 
                 100% 
               
               
                 Example 23 
               
               
                 Example 23 
                 E1 
                 P7 
                 D3 
                 325% 
               
               
                   
               
            
           
         
       
     
     As shown in Tables 3 and 4, the organic light-emitting devices of Examples 1 to 21 were found to have improvement in lifespan to a maximum of 2285%, as compared with the organic light-emitting devices of Comparative Examples 1 to 21, and the organic light-emitting devices of Examples 22 and 23 were found to have improvement in lifespan to a maximum of 325%, as compared with the organic light-emitting devices of Comparative Examples 22 and 23. 
     The devices in Table 3 are examples of a phosphorescence device consisting of a host and a phosphorescent dopant. The devices in Table 4 are examples of a hyper device of a multi-resonance (MR) type of a host, a phosphorescent dopant, and a fluorescent dopant. The devices were found to have significant improvement in lifespan, as compared with the devices of the Comparative Examples consisting of Z1 and a phosphorescent dopant or a multi-resonance (MR) type of Z1, a phosphorescent dopant, and a fluorescent dopant, in which formation of exciplex between a host and a phosphorescent dopant is not occurred. This result is due to energy stabilization of the emission layer caused by formation of exciplex of a host and a phosphorescent dopant, which has low energy state identified in optical characteristics. 
     As apparent from the foregoing description, the organic light-emitting devices according to an aspect may have improvement in lifespan characteristics due to energy stabilization caused by interaction between the host and the phosphorescent dopant. 
     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.