Patent Publication Number: US-2020295280-A1

Title: Organometallic compound and organic light-emitting device including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2019-0030024, filed on Mar. 15, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     One or more embodiments relate to an organometallic compound and an organic light-emitting device including the same. 
     2. Description of the Related Art 
     Organic light-emitting devices are self-emission devices that produce full-color images, and also have wide viewing angles, high contrast ratios, short response times, and excellent characteristics in terms of brightness, driving voltage, and response speed, compared to devices in the art. 
     In an example, an organic light-emitting device may include a first electrode disposed on a substrate, and a hole transport region, an emission layer, an electron transport region, and a second electrode, which are sequentially disposed on the first electrode. Holes provided from the first electrode may move toward the emission layer through the hole transport region, and electrons provided from the second electrode may move toward the emission layer through the electron transport region. Carriers, such as holes and electrons, recombine in the emission layer to produce excitons. These excitons transit from an excited state to a ground state, thereby generating light. 
     SUMMARY 
     One or more embodiments include a novel organometallic compound and an organic light-emitting device including the same. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
     An aspect of the present disclosure provides an organometallic compound represented by Formula 1: 
     
       
         
         
             
             
         
       
     
     In Formula 1, 
     M 1  may be selected from platinum (Pt), palladium (Pd), copper (Cu), zinc (Zn), silver (Ag), gold (Au), rhodium (Rh), iridium (Ir), ruthenium (Ru), rhenium (Re), osmium (Os), titanium (Ti), zirconium (Zr), hafnium (Hf), europium (Eu), terbium (Tb), and thulium (Tm), 
     ring A 1  to ring A 4  may each independently be selected from a C 5 -C 60  carbocyclic group and a C 1 -C 60  heterocyclic group, 
     L 1  and L 4  may each independently be selected from a single bond, *—O—*′, *—S—*&#39;, *—C(R 5 )(R 6 )—*′, *—C(R 5 )—*′, *—C(R 5 )—*′, *—C(R 5 )═C(R 6 )—*′, *—C(═O)—*′, *—C(═S)—*′, *—C≡C*′, *—B(R 5 )—*′, *—N(R 5 )—*′, *—P(R 5 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, 
     L 2  and L 3  may each independently be selected from *—O—*′, *—S—*′, *—C(R 5 )(R 6 )—*′, *—C(R 5 )═*′, *═C(R 5 )—*′, *—C(R 5 )═C(R 6 )—*′, *—C(═O)—*′, *—C(═S)—*′, *—C≡C*′, *—B(R 5 )—*′, *—N(R 5 )—*′, *—P(R 5 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, 
     at least one selected from L 2  and L 3  may be selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, and at least one pair of R 5  and R 6  selected from R 5  and R 6  in L 2  and R 5  and R 6  in L 3  may be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     a1 and a4 may each independently be selected from 0, 1, 2, and 3, wherein the sum of a1 and a4 may be 1 or more, when a1 is 0, ring A 1  and ring A 2  are not linked with each other, and when a4 is 0, ring A 1  and ring A 4  are not linked with each other, 
     a2 and a3 may each independently be selected from 1, 2, and 3, 
     Y 1  to Y 4  may each independently be selected from a carbon atom (C) and a nitrogen atom (N), 
     B 1  to B 4  may each independently be selected from a chemical bond, *—O—*′, and *—S—*′, 
     R 1  to R 6  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), —N(Q 1 )(Q 2 ), —P(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O)(Q 1 ), —S(═O) 2 (Q 1 ), —P(═O)(Q 1 )(Q 2 ), and —P(═S)(Q 1 )(Q 2 ), 
     two neighboring substituents among R 1  to R 6  may optionally be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     b1 to b4 may each independently be an integer from 1 to 5, 
     * and *′ each indicate a binding site to a neighboring atom, 
     at least one substituent of the substituted C 5 -C 60  carbocyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 6 -C 60  aryloxy group, the substituted C 6 -C 60  arylthio group, the substituted C 1 -C 60  heteroaryl group, the substituted monovalent non-aromatic condensed polycyclic group, and the substituted monovalent non-aromatic condensed heteropolycyclic group may be selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 11 )(Q 12 )(Q 13 ), —N(Q 11 )(Q 12 ), —B(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O) 2 (Q 11 ) and —P(═O)(Q 11 )(Q 12 ); 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 21 )(Q 22 )(Q 23 ), —N(Q 21 )(Q 22 ), —B(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O) 2 (Q 21 ), and —P(═O)(Q 21 )(Q 22 ); and 
     —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), and 
     Q 1  to Q 3 , Q 11  to Q 13 , Q 21  to Q 23 , and Q 31  to Q 33  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, a biphenyl group, and a terphenyl group. 
     Another aspect of the present disclosure provides an organic light-emitting device including: a first electrode; a second electrode facing the first electrode; an organic layer between the first electrode and the second electrode and including an emission layer; and at least one organometallic compound represented by Formula 1. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic view of an organic light-emitting device according to an embodiment; 
         FIG. 2  is a schematic view of an organic light-emitting device according to another embodiment; 
         FIG. 3  is a schematic view of an organic light-emitting device according to another embodiment; and 
         FIG. 4  is a schematic view of an organic light-emitting device according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An organometallic compound according to an embodiment is represented by Formula 1 below: 
     
       
         
         
             
             
         
       
     
     In Formula 1, M 1  may be selected from platinum (Pt), palladium (Pd), copper (Cu), zinc (Zn), silver (Ag), gold (Au), rhodium (Rh), iridium (Ir), ruthenium (Ru), rhenium (Re), osmium (Os), titanium (Ti), zirconium (Zr), hafnium (Hf), europium (Eu), terbium (Tb), and thulium (Tm). 
     In one embodiment, M 1  may be selected from Pt, Pd, Cu, Ag, and Au. For example, M 1  may be Pt. 
     In Formula 1, ring A 1  to ring A 4  may each independently be selected from a C 5 -C 60  carbocyclic group and a C 1 -C 60  heterocyclic group. 
     In one embodiment, ring A 1  to ring A 4  in Formula 1 may each independently be selected from a benzene group, a naphthalene group, an anthracene group, a phenanthrene group, an azulene group, a triphenylene group, a pyrene group, a chrysene group, a cyclopentadiene group, a 1,2,3,4-tetrahydronaphthalene group, a furan group, a thiophene group, a silole group, an indene group, a fluorene group, an indole group, a carbazole group, a benzofuran group, a dibenzofuran group, a benzothiophene group, a dibenzothiophene group, a benzosilole group, a dibenzosilole group, an indenopyridine group, an indolopyridine group, a benzofuropyridine group, a benzothienopyridine group, a benzosilolopyridine group, an indenopyrimidine group, an indolopyrimidine group, a benzofuropyrimidine group, a benzothienopyrimidine group, a benzosilolopyrimidine group, a dihydropyridine 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 pyrrole group, a pyrazole group, an imidazole group, a 2,3-dihydroimidazole group, a triazole group, a 2,3-dihydrotriazole group, an oxazole group, an isoxazole group, a thiazole group, an isothiazole group, an oxadiazole group, a thiadiazole group, a benzopyrazole group, a pyrazolopyridine group, a furopyrazole group, a thienopyrazole group, a benzimidazole group, a 2,3-dihydrobenzimidazole group, an imidazopyridine group, a 2,3-dihydroimidazopyridine group, a furoimidazole group, a thienoimidazole group, an imidazopyrimidine group, a 2,3-dihydroimidazopyrimidine group, an imidazopyrazine group, a 2,3-dihydroimidazopyrazine group, a benzoxazole group, a benzothiazole group, a benzoxadiazole group, a benzothiadiazole group, a 5,6,7,8-tetrahydroisoquinoline group, and a 5,6,7,8-tetrahydroquinoline group. 
     In one embodiment, in Formula 1, ring A 3  may be a 6-membered ring including one or more N atoms, and one selected from ring A 1 , ring A 2 , and ring A 4  may be a 5-membered ring including two or more N atoms. 
     In one or more embodiments, in Formula 1, ring A 3  may be a 6-membered ring including one or more N atoms, and one selected from ring A 1  and ring A 2  may be a 5-membered ring including two or more N atoms. 
     In one or more embodiments, in Formula 1, ring A 3  may be a 6-membered ring including one or more N atoms, and Y 3  may be N. For example, ring A 3  may be a pyridine group. 
     In one embodiment, in Formula 1, ring A 3  may be a group which is represented by Formula 2-2(1) and in which Y 15  is N, 
     ring A 1 , ring A 2 , and ring A 4  may each independently be selected from groups represented by Formulae 2-1(1) to 2-1(35) and 2-2(1) to 2-2(25): 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 2-1(1) to 2-1(35) and 2-2(1) to 2-2(25), 
     Y 15  may be a carbon atom (C) or a nitrogen atom (N), 
     X 21  may be N or C(R 21 ), X 22  may be N or C(R 22 ), X 23  may be N or C(R 23 ), X 24  may be N or C(R 24 ), X 25  may be N or C(R 25 ), X 26  may be N or C(R 26 ), X 27  may be N or C(R 27 ), and X 28  may be N or C(R 28 ), 
     X 29  may be C(R 29a )(R 29b ), Si(R 29a )(R 29b ), N(R 29 ), O, or S, 
     X 30  may be C(R 30a )(R 30b ), Si(R 30a )(R 30b ), N(R 30 ), O, or S, 
     R 21  to R 30  and R 25a  to R 30b  may each independently be the same as defined in connection with R 1  to R 4  in Formula 1, 
     * indicates a binding site to B 1 , B 2 , B 3 , or B 4 , and 
     *′ and *″ each indicate a binding site to a neighboring atom. 
     In one embodiment, in Formulae 2-1(1) to 2-1(35) and 2-2(1) to 2-2(25), R 21  to R 30  and R 25a  to R 30b  may each independently be selected from: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a cyano group, a C 1 -C 20  alkyl group, and a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group and a C 1 -C 20  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a cyano group, a phenyl group, and a biphenyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a cyano group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group. 
     For example, in Formulae 2-1(1) to 2-1(35) and 2-2(1) to 2-2(25), R 21  to R 30  and R 25a  to R 30b  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a cyano group, a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a phenyl group, and a pyridinyl group. 
     In one embodiment, in Formula 1, a4 may be 0, ring A 1  and ring A 4  may each independently be selected from groups represented by Formulae 2-1(1) to 2-1(35), ring A 2  may be selected from groups represented by Formulae 2-2(1) to 2-2(25), ring A 3  may be a group which is represented by Formula 2-2(1) and in which Y 15  is N. 
     For example, in Formula 1, a4 may be 0, ring A 1  may be a group represented by Formula 2-1(32), ring A 2  may be a group represented by Formula 2-2(1), ring A 3  may be a group which is represented by Formula 2-2(1) and in which Y 15  is N, and ring A 4  ring may be a group represented by Formula 2-1(1). 
     In one embodiment, in Formula 1, a4 may be 0, ring A 1  and ring A 4  may each independently be a group represented by Formula 2-1(1), ring A 2  may be a group represented by Formula 2-2(10), 2-2(12), or 2-2(20), and ring A 3  may be a group which is represented by Formula 2-2(1) and in which Y 15  is N. 
     In Formula 1, L 1  and L 4  may each independently be selected from a single bond, *—O—*′, *—S—*′, *—C(R 5 )(R 6 )—*′, *—C(R 5 )═*′, *═C(R 5 )—*′, *—C(R 5 )═C(R 6 )—*′, *—C(═O)—*′, *—C(═S)—*′, *—C≡C—*′, *—B(R 5 )—*′, *—N(R 5 )—*′, *—P(R 5 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, and 
     L 2  and L 3  may each independently be selected from *—O—*′, *—S—*′, *—C(R 5 )(R 6 )—*′, C(R 5 )═*′, *═C(R 5 )—*′, *—C(R 5 )≡C(R 6 )—*′, *—C(═O)—*′, *—C(═S)—*′, *—C≡C—*′, *—B(R 5 )—*′, *—N(R 5 )—*′, *—P(R 5 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, wherein at least one selected from L 2  and L 3  may be selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′. 
     When at least one selected from L 2  and L 3  is selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, at least one pair of R 5  and R 6  selected from R 5  and R 6  in L 2  and R 5  and R 6  in L 3  may be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group. 
     That is, when L 2  is selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′ and L 3  is selected from *—O—*′, *—S—*′, *—C(R 5 )═*′, *═C(R 5 )—*′, *—C(R 5 )═C(R 6 )—*′, *—C(═O)—*′, *—C(═S)—*′, *—C≡C—*′, *—B(R 5 )—*′, *—N(R 5 )—*′, and *—P(R 5 )—*′, R 5  and R 6  in L 2  may be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     when L 3  is selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′ and L 2  is selected from *—O—*′, *—S—*′, *—C(R 5 )═*′, *═C(R 5 )—*′, *—C(R 5 )═C(R 6 )—*′, *—C(═O)—*′, *—C(═S)—*′, *—C≡C—*′, *—B(R 5 )—*′, *—N(R 5 )—*′, and *—P(R 5 )—*′, R 5  and R 6  in L 3  may be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, and 
     when L 2  and L 3  are each independently selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, at least one pair of R 5  and R 6  selected from R 5  and R 6  in L 2  and R 5  and R 6  in L 3  may be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group. 
     In Formula 1, a1 and a4 may each independently be selected from 0, 1, 2, and 3, wherein the sum of a1 and a4 is 1 or more. When a1 is 0, ring A 1  and ring A 2  are not linked with each other, and when a4 is 0, ring A 1  and ring A 4  are not linked with each other. 
     a1 indicates the number of L 1 , wherein, when a1 is two or more, a plurality of L 1 (s) may be identical to or different from each other. When L 1  is a single bond, ring A 1  and ring A 2  may be directly linked with each other. 
     a4 indicates the number of L 4 , wherein, when a4 is two or more, a plurality of L 2 (s) may be identical to or different from each other. When L 4  is a single bond, ring A 1  and ring A 4  may be directly linked with each other. 
     In one embodiment, the sum of a1 and a4 in Formula 1 may be 1. That is, a1 may be 1, and a4 may be 0; or a1 may be 0, and a4 may be 1. 
     For example, a1 may be 1, and a4 may be 0. 
     In Formula 1, a2 and a3 may each independently be selected from 1, 2, and 3. 
     a2 indicates the number of L 2 , wherein, when a2 is 2 or more, a plurality of L 2 (s) may be identical to or different from each other. a3 indicates the number of L 3 , wherein, when a3 is 2 or more, a plurality of L 3 (s) may be identical to or different from each other. 
     In one embodiment, in Formula 1, i) a2 and a3 may each be 1, L 2  may be selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, L 3  may be selected from *—O—*′, *—S—*′, *—C(R 5 )(R 6 )—*′, and *—N(R 5 )—*′, and R 5  and R 6  in L 2  may be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group; 
     ii) a2 and a3 may each be 1, L 2  may be selected from *—O—*′, *—S—*′, *—C(R 5 )(R 6 )—*′, and *—N(R 5 )—*′, L 3  may be selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, and R 5  and R 6  in L 3  may be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group; or 
     iii) a2 and a3 may each be 1, L 2  and L 3  may each independently be selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, and at least one pair of R 5  and R 6  selected from R 5  and R 6  in L 2  and R 5  and R 6  in L 3  may be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group. 
     In one embodiment, in Formula 1, at least one selected from L 2  and L 3  may be selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, and at least one pair of R 5  and R 6  selected from R 5  and R 6  in L 2  and R 5  and R 6  in L 3  may be linked to form a group represented by Formula 3: 
     
       
         
         
             
             
         
       
     
     In Formula 3, 
     X 31  may be selected from a single bond, *—O—*′, *—S—*′, *—Se—*′, *—C(R 33 )(R 34 )—*, *—C(R 33 )═C(R 34 )—*′, *—Si(R 33 )(R 34 )—*′, and *—Ge(R 33 )(R 34 )—*′, 
     A 31  and A 32  may each independently be selected from a C 6 -C 30  carbocyclic group and a C 1 -C 30  heterocyclic group, 
     R 31  to R 34  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), and —P(═O)(Q 1 )(Q 2 ), 
     Q 1  to Q 3  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, a biphenyl group, and a terphenyl group, 
     b31 and b32 may each independently be selected from 1, 2, 3, 4, 5, and 6, and 
     * indicates a binding site to C, Si, P, or Ge. 
     In one or more embodiments, at least one selected from L 2  and L 3  in Formula 1 may be selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, wherein a pair of R 5  and R 6  selected from R 5 (s) in the number of L 2  and R 6 (s) in the number L 3  may be linked with each other to form a group represented by Formula 3A: 
     
       
         
         
             
             
         
       
     
     In Formula 3A, 
     X 31  and R 31  to R 34  may each independently be the same as defined in connection with Formula 3, 
     b31 and b32 may each independently be selected from 1, 2, 3, and 4, and 
     * indicates a binding site to C, Si, P, or Ge. 
     In one embodiment, in Formulae 3 and 3A, R 31  to R 34  may each independently be selected from: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a cyano group, a C 1 -C 20  alkyl group, and a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group and a C 1 -C 20  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a cyano group, a phenyl group, and a biphenyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a cyano group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group. 
     For example, R 31  to R 34  in Formulae 3 and 3A may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a cyano group, a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a phenyl group, and a pyridinyl group. 
     In Formula 1, Y 1  to Y 4  may each independently be selected from a carbon atom (C) and a nitrogen atom (N). 
     In one embodiment, i) Y 1 , Y 2 , and Y 4  may each be C, and Y 3  may be N; ii) Y 1  and Y 2  may each be C, and Y 3  and Y 4  may each be N; iii) Y 1  and Y 4  may each be C, and Y 2  and Y 3  may each be N; or iv) Y 2  and Y 4  may each be C, and Y 1  and Y 3  may each be N. 
     For example, i) Y 1 , Y 2 , and Y 4  may each be C, and Y 3  may be N; or ii) Y 1  and Y 4  may each be C, and Y 2  and Y 3  may each be N. 
     In one embodiment, each of B 1  to B 4  may be a chemical bond, Y 3  may be N, and a bond between Y 3  and M 1  may be a coordinate bond, 
     i) Y 1 , Y 2 , and Y 4  may each be C, one bond selected from a bond between Y 1  and M 1 , a bond between Y 2  and M 1 , and a bond between Y 4  and M 1  may be a coordinate bond, and the others thereof may be a covalent bond; 
     ii) Y 1  and Y 4  may each be C, Y 2  may be N, a bond between Y 2  and M 1  may be a coordinate bond, and a bond between Y 1  and M 1  and a bond between Y 4  and M 1  may be a covalent bond; or 
     iii) Y 1  may be N, Y 2  and Y 4  may each be C, a bond between Y 1  and M 1  may be a coordinate bond, and a bond between Y 2  and M 1  and a bond between Y 4  and M 1  may be a covalent bond. 
     In Formula 1, B 1  to B 4  may each independently be selected from a chemical bond, *—O—*′, and *—S—*′. 
     When B 1  is a chemical bond, Y 1  and M 1  may be directly linked with each other, when B 2  is a chemical bond, Y 2  and M 1  may be directly linked with each other, when B 3  is a chemical bond, Y 3  and M 1  may be directly linked with each other, and when B 4  is a chemical bond, Y 4  and M 1  may be directly linked with each other. 
     In one embodiment, in Formula 1, i) each of B 1  to B 4  may be a chemical bond; or ii) at least one selected from B 1  to B 4  may be *—O—*′ or *—S—*′, and the others thereof may be a chemical bond. 
     For example, each of B 1  to B 4  may be a chemical bond. In one embodiment, each of B 1  to B 3  may be a chemical bond, and B 4  may be *—O—*′. 
     In Formula 1, R 1  to R 6  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 1 )(Q 2 )(Q 3 ), —B(Q 1 )(Q 2 ), —N(Q 1 )(Q 2 ), —P(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O)(Q 1 ), —S(═O) 2 (Q 1 ), —P(═O)(Q 1 )(Q 2 ), and —P(═S)(Q 1 )(Q 2 ), 
     two neighboring substituents among R 1  to R 6  may optionally be linked to form a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group. 
     In one embodiment, R 1  to R 6  may each independently be selected from: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a cyano group, a C 1 -C 20  alkyl group, and a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group and a C 1 -C 20  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a cyano group, a phenyl group, and a biphenyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a cyano group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyridinyl group, a pyrimidinyl group, a carbazolyl group, and a triazinyl group. 
     In one or more embodiments, R 1  to R 6  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a cyano group, a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a phenyl group, and a pyridinyl group. 
     In Formula 1, b1 to b4 may each independently be an integer from 1 to 5. 
     b1 indicates the number of R 1 , wherein, when b1 is two or more, two or more R 1 (s) may be identical to or different from each other. 
     b2 indicates the number of R 2 , wherein, when b2 is two or more, two or more R 2 (s) may be identical to or different from each other. 
     b3 indicates the number of R 3 , wherein, when b3 is two or more, two or more R 3 (s) may be identical to or different from each other. 
     b4 indicates the number of R 4 , wherein, when b4 is two or more, two or more R 4 (s) may be identical to or different from each other. 
     In the description regarding Formula 1, * and *′ each indicate a binding site to a neighboring atom. 
     In the description regarding Formula 1, at least one substituent of the substituted C 5 -C 60  carbocyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 6 -C 60  aryloxy group, the substituted C 6 -C 60  arylthio group, the substituted C 1 -C 60  heteroaryl group, the substituted monovalent non-aromatic condensed polycyclic group, and the substituted monovalent non-aromatic condensed heteropolycyclic group may be selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 11 )(Q 12 )(Q 13 ), —N(Q 11 )(Q 12 ), —B(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O) 2 (Q 11 ), and —P(═O)(Q 11 )(Q 12 ); 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 21 )(Q 22 )(Q 23 ), —N(Q 21 )(Q 22 ), —B(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O) 2 (Q 21 ), and —P(═O)(Q 21 )(Q 22 ); and 
     —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), and 
     Q 1  to Q 3 , Q 11  to Q 13 , Q 21  to Q 23 , and Q 31  to Q 33  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, a biphenyl group, and a terphenyl group. 
     In one embodiment, the organometallic compound may be represented by one of Formulae 1-1 to 1-3: 
     
       
         
         
             
             
         
       
     
     In Formulae 1-1 to 1-3, 
     L 2  and L 3  may each independently be selected from *—O—*′, *—S—*′, *—C(R 5 )(R 6 )—*′, *—C(R 5 )═*′, *═C(R 5 )—*′, *—C(R 5 )═C(R 6 )—*′, *—C(═O)—*′, *—C(═S)—*′, *—C≡C—*′, *—B(R 5 )—*′, *—N(R 5 )—*′, *—P(R 5 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′, 
     X 1  and X 2  may each independently be selected from C, Si, P, and Ge, 
     X 22  may be N or C(R 22 ), X 23  may be N or C(R 23 ), and X 24  may be N or C(R 24 ), 
     X 31  may be selected from a single bond, *—O—*′, *—S—*′, *—Se—*′, *—C(R 33 )(R 34 )—*, *—C(R 33 )═C(R 34 )—*′, *—Si(R 33 )(R 34 )—*′, and *—Ge(R 33 )(R 34 )—*′, 
     X 32  may be selected from a single bond, *—O—*′, *—S—*′, *—Se—*′, *—C(R 37 )(R 38 )—*, *—C(R 37 )═C(R 38 )—*′, *—Si(R 37 )(R 38 )—*′ and *—Ge(R 37 )(R 38 )—*′, 
     R 22  to R 24  may each independently be the same as defined in connection with R 3  in Formula 1, R 31  to R 34  may each independently be the same as described above, and R 35  to R 38  may each independently be the same as defined in connection with R 31  to R 34  in Formula 3, 
     b31, b32, b35, and b36 may each independently be selected from 1, 2, 3, and 4, 
     M 1 , ring A 1 , ring A 2 , ring A 4 , L 1 , L 4 , a1, a4, Y 1  to Y 4 , B 1  to B 4 , R 1 , R 2 , R 4 , b1, b2, and b4 may each independently be the same as defined in connection with Formula 1. 
     In one embodiment, in Formulae 1-1 to 1-3, 
     i) L 2  and L 3  may each independently be selected from *—O—*′, *—S—*′, *—C(R 5 )(R 6 )—*′, and *—N(R 5 )—*′; 
     ii) L 2  may be *—N(R 5 )—*′, and R 5  may be linked with neighboring R 2  or R 22 ; or 
     iii) L 3  may be *—N(R 5 )—*′, and R 5  may be linked with neighboring R 4  or R 24 . 
     In one embodiment, in Formulae 1-1 to 1-3, X 22  may be C(R 22 ), X 23  may be C(R 23 ), and X 24  may be C(R 24 ). For example, X 22  may be C(R 22 ), X 23  may be C(R 23 ), X 24  may be C(R 24 ), R 22  and R 24  may be hydrogen, and R 23  may be hydrogen or a tert-butyl group. 
     In one or more embodiments, in Formulae 1-1 to 1-3, X 22  may be C(R 22 ), X 23  may be C(R 23 ), X 24  may be C(R 24 ), and at least one selected from R 22  to R 24 , R 31 , R 32 , R 35 , and R 36  may not be hydrogen. 
     In one or more embodiments, the organometallic compound may be represented by one of Formulae 1-1A to 1-3A: 
     
       
         
         
             
             
         
       
     
     In Formulae 1-1A to 1-3A, 
     L 2 , L 3 , X 1 , X 2 , X 22  to X 24 , R 31 , R 32 , R 35 , R 36 , b31, b32, b35, and b36 may each independently be the same as defined in connection with Formulae 1-1 to 1-3, and 
     M 1 , ring A 1 , ring A 2 , ring A 4 , L 1 , L 4 , a1, a4, Y 1  to Y 4 , B 1  to B 4 , R 1 , R 2 , R 4 , b1, b2, and b4 may each independently be the same as defined in connection with Formula 1. 
     In one embodiment, in Formulae 1-1A to 1-3A, 
     i) L 2  and L 3  may each independently be selected from *—O—*′, *—S—*′, *—C(R 5 )(R 6 )—*′, and *—N(R 5 )—*′; 
     ii) L 2  may be *—N(R 5 )—*′, and R 5  may be linked with neighboring R 2  or R 22 ; or 
     iii) L 3  may be *—N(R 5 )—*′, and R 5  may be linked with neighboring R 4  or R 24 . 
     In one embodiment, in Formulae 1-1A to 1-3A, X 22  may be C(R 22 ), X 23  may be C(R 23 ), and X 24  may be C(R 24 ). For example, X 22  may be C(R 22 ), X 23  may be C(R 23 ), X 24  may be C(R 24 ), R 22  and R 24  may be hydrogen, and R 23  may be hydrogen or a tert-butyl group. 
     In one or more embodiments, in Formulae 1-1A to 1-3A, X 22  may be C(R 22 ), X 23  may be C(R 23 ), X 24  may be C(R 24 ), and at least one selected from R 22  to R 24 , R 31 , R 32 , R 35 , and R 36  may not be hydrogen. 
     In one embodiment, the organometallic compound represented by Formula 1 may be one selected from Compounds 1 to 40: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In the organometallic compound represented by Formula 1, ring A 3  is linked to ring A 2  and ring A 4  via linkers L 2  and L 3 , thereby increasing the rigidity of ring A 3 . In addition, in the organometallic compound represented by Formula 1, bond dissociation energy of a metal center M 1  and ring A 3  may be increased, as compared with an organometallic complex including an organic ligand in which one of linkers L 2  and L 3  does not exist (that is, an organic ligand with which ring A 2  and ring A 3  are not linked, or ring A 3  and ring A 4  are not linked). Accordingly, the stability in the ground state of the organometallic compound may be increased, thereby improving the lifespan thereof. Therefore, the durability of the organic light-emitting device to which the organometallic compound is applied may be increased, thereby improving the lifespan thereof. 
     Furthermore, since the organometallic compound represented by Formula 1 has a high energy level of a triplet metal-centered state ( 3 MC state), the stability of the compound in the excited state may be improved. Therefore, since a non-radiative decay phenomenon is reduced, when the organometallic compound is applied to an electronic device, for example, an organic light-emitting device, the device may have a long lifespan. 
     Furthermore, at least one selected from the linkers L 2  and L 3  is selected from *—C(R 5 )(R 6 )—*′, *—Si(R 5 )(R 6 )—*′, *—P(R 5 )(R 6 )—*′, and *—Ge(R 5 )(R 6 )—*′. The at least one selected from the linkers L 2  and L 3  includes a ring structure in which R 5  and R 6  bonded to an atom (C, Si, P, or Ge) in the linkers are linked via a single bond or a linking group. This ring structure causes steric hindrance in the organometallic compound represented by Formula 1. When the organometallic compound is applied to an emission layer dopant of the organic light-emitting device, it may hinder the formation of excimer or exciplex due to an interaction between the dopants or between the dopant and the host. Therefore, it is possible to maintain high efficiency and color purity during the driving of the device. 
     A synthesis method for the organometallic compound represented by Formula 1 would be apparent to those of ordinary skill in the art by referring to the following examples. 
     The organometallic compound represented by Formula 1 may be used between a pair of electrodes of an organic light-emitting device. For example, the organometallic compound may be included in an emission layer. The organometallic compound may act as a dopant in the emission layer. In one or more embodiments, the organometallic compound of Formula 1 may be used as a material for a capping layer located outside a pair of electrodes of an organic light-emitting device. 
     Accordingly, there is provided an organic light-emitting device including: a first electrode; a second electrode facing the first electrode; an organic layer between the first electrode and the second electrode; and at least one organometallic compound represented by Formula 1. 
     In one embodiment, the organic layer of the organic light-emitting device may include the at least one organometallic compounds represented by Formula 1. 
     The expression “(an organic layer) includes at least one organometallic compounds” used herein may include a case in which “(an organic layer) includes identical organometallic compounds represented by Formula 1” and a case in which “(an organic layer) includes two or more different organometallic compounds represented by Formula 1”. 
     For example, the organic layer may include, as the organometallic compound, only Compound 1. In this regard, Compound 1 may exist only in the emission layer of the organic light-emitting device. In one or more embodiments, the organic layer may include, as the organometallic compound, Compound 1 and Compound 2. In this regard, Compound 1 and Compound 2 may exist in an identical layer (for example, Compound 1 and Compound 2 may all exist in an emission layer), or different layers (for example, Compound 1 may exist in an emission layer and Compound 2 may exist in an electron transport region). 
     In one embodiment, the first electrode may be an anode, and the second electrode may be a cathode, and the organic layer may include the organometallic compound, and 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. The hole transport region may include a hole injection layer, a hole transport layer, an emission auxiliary layer, an electron blocking layer, or any combination thereof. The electron transport region includes a buffer layer, a hole blocking layer, an electron control layer, an electron transport layer, an electron injection layer, or any combination thereof. 
     In one or more embodiments, the emission layer of the organic light-emitting device may include the at least one organometallic compound. 
     In one or more embodiments, the emission layer of the organic light-emitting device may include the at least one organometallic compound, the emission layer may further include a host, and an amount of the host included in the emission layer may be larger than an amount of the organometallic compound included in the emission layer. For example, the amount of the organometallic compound may be in a range of about 0.01 parts by weight to about 30 parts by weight based on 100 parts by weight of the emission layer. 
     In one embodiment, the emission layer may include the organometallic compound, and the emission layer may emit blue light or green light. For example, the emission layer may emit blue light or green light having a wavelength of maximum emission in a range of about 400 nm or more and about 600 nm or less, for example, a wavelength of maximum emission in a range of about 460 nm or more and about 570 nm or less. The wavelength of maximum emission of the organometallic compound is a value obtained through quantum simulation using a density functional theory (DFT) calculation method. 
     In one embodiment, the hole transport region may include an electron blocking layer, and the electron blocking layer may include the organometallic compound; or the electron transport region may include a hole blocking layer, and the hole blocking layer may include the organometallic compound. 
     In one embodiment, the hole transport region may include a p-dopant having a lowest unoccupied molecular orbital (LUMO) energy level of −3.5 eV or less. 
     In one embodiment, the electron transport region may include a phosphine oxide-containing compound and/or a silyl-containing compound. For example, the electron transport region may include a hole blocking layer, the hole blocking layer may directly contact the emission layer, and the hole blocking layer may include the phosphine oxide-containing compound and/or a silyl-containing compound. 
     The term “an organic layer” as used herein refers to a single layer and/or a plurality of layers disposed between the first electrode and the second electrode of an organic light-emitting device. A material included in the “organic layer” is not limited to an organic material. 
     [Description of  FIG. 1 ] 
       FIG. 1  is a schematic cross-sectional view of an organic light-emitting device  10  according to an embodiment. The organic light-emitting device  10  includes a first electrode  110 , an organic layer  150 , and a second electrode  190 . 
     Hereinafter, the structure of the organic light-emitting device  10  according to an embodiment and a method of manufacturing the organic light-emitting device  10  will be described in connection with  FIG. 1 . 
     [First Electrode  110 ] 
     In  FIG. 1 , a substrate may be additionally disposed under the first electrode  110  or above the second electrode  190 . The substrate may be a glass substrate or a plastic substrate, each having excellent mechanical strength, thermal stability, transparency, surface smoothness, ease of handling, and water resistance. 
     The first electrode  110  may be formed by depositing or sputtering a material for forming the first electrode  110  on the substrate. When the first electrode  110  is an anode, the material for forming the first electrode  110  may be selected from materials with a high work function to facilitate hole injection. 
     The first electrode  110  may be a reflective electrode, a semi-reflective electrode, or a transmissive electrode. When the first electrode  110  is a transmissive electrode, a material for forming a first electrode may be selected from indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO 2 ), zinc oxide (ZnO), and any combinations thereof, but embodiments of the present disclosure are not limited thereto. In one or more embodiments, when the first electrode  110  is a semi-transmissive electrode or a reflectable electrode, a material for forming a first electrode may be selected from magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), and any combinations thereof, but embodiments of the present disclosure are not limited thereto. 
     The first electrode  110  may have a single-layered structure, or a multi-layered structure including two or more layers. For example, the first electrode  110  may have a three-layered structure of ITO/Ag/ITO, but the structure of the first electrode  110  is not limited thereto. 
     [Organic Layer  150 ] 
     The organic layer  150  may be disposed on the first electrode  110 . The organic layer  150  may include an emission layer. 
     The organic layer  150  may further include a hole transport region between the first electrode  110  and the emission layer and an electron transport region between the emission layer and the second electrode  190 . 
     [Hole Transport Region in Organic Layer  150 ] 
     The hole transport region may have i) a single-layered structure including a single layer including a single material, ii) a single-layered structure including a single layer including a plurality of different materials, or iii) a multi-layered structure having a plurality of layers including a plurality of different materials. 
     The hole transport region may include at least one layer selected from a hole injection layer, a hole transport layer, an emission auxiliary layer, and an electron blocking layer. 
     For example, the hole transport region may have a single-layered structure including a single layer including a plurality of different materials, or a multi-layered structure having a hole injection layer/hole transport layer structure, a hole injection layer/hole transport layer/emission auxiliary layer structure, a hole injection layer/emission auxiliary layer structure, a hole transport layer/emission auxiliary layer structure, or a hole injection layer/hole transport layer/electron blocking layer structure. For each structure, constituting layers are sequentially stacked from the first electrode  110  in this stated order, but the structure of the hole transport region is not limited thereto. 
     The hole transport region may include at least one selected from m-MTDATA, TDATA, 2-TNATA, NPB (NPD), β-NPB, TPD, spiro-TPD, spiro-NPB, methylated-NPB, TAPC, HMTPD, 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), polyaniline/dodecylbenzenesulfonic acid (PANI/DBSA), poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), polyaniline/camphor sulfonic acid (PANI/CSA), polyaniline/poly(4-styrenesulfonate) (PANI/PSS), a compound represented by Formula 201, and a compound represented by Formula 202: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 201 and 202, 
     L 201  to L 204  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     L 205  may be selected from *—O—*′, *—S—*′, *—N(Q 201 )—*′, a substituted or unsubstituted C 1 -C 20  alkylene group, a substituted or unsubstituted C 2 -C 20  alkenylene group, a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     xa1 to xa4 may each independently be an integer from 0 to 3, 
     xa5 may be an integer from 1 to 10, 
     R 201  to R 204  and Q 201  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group. 
     For example, in Formula 202, R 201  and R 202  may optionally be linked via a single bond, a dimethyl-methylene group, or a diphenyl-methylene group, and R 203  and R 204  may optionally be linked via a single bond, a dimethyl-methylene group, or a diphenyl-methylene group. 
     In one embodiment, in Formulae 201 and 202, 
     L 201  to L 205  may each independently be selected from: 
     a phenylene group, a pentalenylene group, an indenylene group, a naphthylene group, an azulenylene group, a heptalenylene group, an indacenylene group, an acenaphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenalenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a naphthacenylene group, a picenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a rubicenylene group, a coronenylene group, an ovalenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, and a pyridinylene group; and 
     a phenylene group, a pentalenylene group, an indenylene group, a naphthylene group, an azulenylene group, a heptalenylene group, an indacenylene group, an acenaphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenalenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a naphthacenylene group, a picenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a rubicenylene group, a coronenylene group, an ovalenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, and a pyridinylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), and —N(Q 31 )(Q 32 ), and 
     Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     In one or more embodiments, xa1 to xa4 may each independently be 0, 1, or 2. 
     In one or more embodiments, xa5 may be 1, 2, 3, or 4. 
     In one or more embodiments, R 201  to R 204  and Q 201  may each independently be selected from: 
     a phenyl group, a biphenyl group, a terphenyl group, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, and a pyridinyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, and a pyridinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), and —N(Q 31 )(Q 32 ), and 
     Q 31  to Q 33  are the same as described above. 
     In one or more embodiments, at least one selected from R 201  to R 203  in Formula 201 may each independently be selected from: 
     a fluorenyl group, a spiro-bifluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; and 
     a fluorenyl group, a spiro-bifluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, 
     but embodiments of the present disclosure are not limited thereto. 
     In one or more embodiments, in Formula 202, i) R 201  and R 202  may be linked via a single bond, and/or ii) R 203  and R 204  may be linked via a single bond. 
     In one or more embodiments, at least one selected from R 201  to R 204  in Formula 202 may be selected from: 
     a carbazolyl group; and 
     a carbazolyl group substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, 
     but embodiments of the present disclosure are not limited thereto. 
     The compound represented by Formula 201 may be represented by Formula 201A: 
     
       
         
         
             
             
         
       
     
     In one embodiment, the compound represented by Formula 201 may be represented by Formula 201A(1) below, but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In one embodiment, the compound represented by Formula 201 may be represented by Formula 201A-1 below, but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In one embodiment, the compound represented by Formula 202 may be represented by Formula 202A: 
     
       
         
         
             
             
         
       
     
     In one embodiment, the compound represented by Formula 202 may be represented by Formula 202A-1: 
     
       
         
         
             
             
         
       
     
     In Formulae 201A, 201A(1), 201A-1, 202A, and 202A-1, 
     L 201  to L 203,  xa1 to xa3, xa5, and R 202  to R 204  may be understood by referring to corresponding description presented herein, 
     R 211  and R 212  may each independently be the same as defined in connection with R 203 , and 
     R 213  to R 217  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, and a pyridinyl group. 
     The hole transport region may include at least one compound selected from Compounds HT1 to HT39, but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A thickness of the hole transport region may be from about 100 Å to about 10,000 Å, for example, about 100 Å to about 3,000 Å. When the hole transport region includes at least one selected from a hole injection layer and a hole transport layer, a thickness of the hole injection layer may be in a range of about 100 Å to about 9,000 Å, for example, about 100 Å to about 1,000 Å, and a thickness of the hole transport layer may be in a range of about 50 Å to about 2,000 Å, for example about 100 Å to about 1,500 Å. When the thicknesses of the hole transport region, the hole injection layer, and the hole transport layer are within these ranges, satisfactory hole transporting characteristics may be obtained without a substantial increase in driving voltage. 
     The emission auxiliary layer may increase light-emission efficiency by compensating for an optical resonance distance according to the wavelength of light emitted by an emission layer, and the electron blocking layer may block the flow of electrons from an electron transport region. The emission auxiliary layer and the electron blocking layer may include the materials as described above. 
     [p-Dopant] 
     The hole transport region may further include, in addition to these materials, a charge-generation material for the improvement of conductive properties. The charge-generation material may be homogeneously or non-homogeneously dispersed in the hole transport region. 
     The charge-generation material may be, for example, a p-dopant. 
     In one embodiment, the p-dopant may have a LUMO energy level of −3.5 eV or less. 
     The p-dopant may include at least one selected from a quinone derivative, a metal oxide, and a cyano group-containing compound, but embodiments of the present disclosure are not limited thereto. 
     In one embodiment, the p-dopant may include at least one selected from: 
     a quinone derivative, such as tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ); 
     a metal oxide, such as tungsten oxide or molybdenum oxide; 
     1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN); and 
     a compound represented by Formula 221, 
     but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In Formula 221, 
     R 221  to R 223  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, wherein at least one selected from R 221  to R 223  may have at least one substituent selected from a cyano group, —F, —Cl, —Br, —I, a C 1 -C 20  alkyl group substituted with —F, a C 1 -C 20  alkyl group substituted with —Cl, a C 1 -C 20  alkyl group substituted with —Br, and a C 1 -C 20  alkyl group substituted with —I. 
     [Emission Layer in Organic Layer  150 ] 
     When the organic light-emitting device  10  is a full-color organic light-emitting device, the emission layer may be patterned into a red emission layer, a green emission layer, or a blue emission layer, according to a sub-pixel. In one or more embodiments, the emission layer may have a stacked structure of two or more layers selected from a red emission layer, a green emission layer, and a blue emission layer, in which the two or more layers contact each other or are separated from each other. In one or more embodiments, the emission layer may include two or more materials selected from a red light-emitting material, a green light-emitting material, and a blue light-emitting material, in which the two or more materials are mixed with each other in a single layer to emit white light. 
     The emission layer may include a host and a dopant. The dopant may include at least one selected from a phosphorescent dopant and a fluorescent dopant. 
     In the emission layer, an amount of the dopant may be in a range of about 0.01 parts by weight to about 15 parts by weight based on 100 parts by weight of the host, but embodiments of the present disclosure are not limited thereto. 
     A thickness of the emission layer may be in a range of about 100 Å to about 1,000 Å, for example, about 200 Å to about 600 Å. When the thickness of the emission layer is within this range, excellent light-emission characteristics may be obtained without a substantial increase in driving voltage. 
     [Host in Emission Layer] 
     In one or more embodiments, the host may include a compound represented by Formula 301 below: 
       [Ar 301 ] xb11 -[(L 301 ) xb1 -R 301 ] xb21    &lt;Formula 301&gt;
 
     In Formula 301, 
     Ar 301  may be a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     xb11 may be 1, 2, or 3, 
     L 301  may be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     xb1 may be an integer from 0 to 5, 
     R 301  may be selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 301 )(Q 302 )(Q 303 ), —N(Q 301 )(Q 302 ), —B(Q 301 )(Q 302 ), —C(═O)(Q 301 ), —S(═O) 2 (Q 301 ), and —P(═O)(Q 301 )(Q 302 ), 
     xb21 may be an integer from 1 to 5, 
     Q 301  to Q 303  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group, but embodiments of the present disclosure are not limited thereto. 
     In one embodiment, Ar 301  in Formula 301 may be selected from: 
     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, and a dibenzothiophene group; and 
     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, and a dibenzothiophene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), and 
     Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group, but embodiments of the present disclosure are not limited thereto. 
     When xb11 in Formula 301 is two or more, two or more Ar 301 (s) may be linked via a single bond. 
     In one or more embodiments, the compound represented by Formula 301 may be represented by Formula 301-1 or 301-2: 
     
       
         
         
             
             
         
       
     
     In Formulae 301-1 to 301-2, 
     ring A 301  to ring A 304  may each independently be selected from a benzene ring, a naphthalene ring, a phenanthrene ring, a fluoranthene ring, a triphenylene ring, a pyrene ring, a chrysene ring, a pyridine ring, a pyrimidine ring, an indene ring, a fluorene ring, a spiro-bifluorene ring, a benzofluorene ring, a dibenzofluorene ring, an indole ring, a carbazole ring, a benzocarbazole ring, a dibenzocarbazole ring, a furan ring, a benzofuran ring, a dibenzofuran ring, a naphthofuran ring, a benzonaphthofuran ring, a dinaphthofuran ring, a thiophene ring, a benzothiophene ring, a dibenzothiophene ring, a naphthothiophene ring, a benzonaphthothiophene ring, and a dinaphthothiophene ring, 
     X 301  may be O, S, or N-[(L 304 ) xb4 -R 304 ], 
     R 311  to R 314  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), 
     xb22 and xb23 may each independently be 0, 1, or 2, 
     L 301 , xb1, R 301 , and Q 31  to Q 33  may be understood by referring to corresponding description presented herein, 
     L 302  to L 304  may each independently be the same as defined in connection with L 301 , 
     xb2 to xb4 may each independently be the same as defined in connection with xb1, and 
     R 302  to R 304  may each independently be the same as defined in connection with R 301 . 
     For example, L 301  to L 304  in Formulae 301, 301-1, and 301-2 may each independently be selected from: 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group; and 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl croup, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), and 
     Q 31  to Q 33  are the same as described above. 
     In one embodiment, R 301  to R 304  in Formulae 301, 301-1, and 301-2 may each independently be selected from: 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), and 
     Q 31  to Q 33  are the same as described above. 
     In one embodiment, the host may include an alkaline earth-metal complex. For example, the host may be selected from a Be complex (for example, Compound H55), a Mg complex, and a Zn complex. 
     The host may include at least one selected from 9,10-di(2-naphthyl)anthracene (ADN), 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN), 9,10-di-(2-naphthyl)-2-t-butyl-anthracene (TBADN), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), 1,3-di-9-carbazolylbenzene (mCP), 1,3,5-tri(carbazol-9-yl)benzene (TCP), and Compounds H1 to H55, but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     [Phosphorescent Dopant Included in Emission Layer in Organic Layer  150 ] 
     The phosphorescent dopant may include the organometallic compound represented by Formula 1. 
     The phosphorescent dopant may include an organometallic complex represented by Formula 401 below: 
       M(L 401 ) xc1 (L 402 ) xc2    &lt;Formula 401&gt;
 
     
       
         
         
             
             
         
       
     
     In Formulae 401 and 402, 
     M may be selected from iridium (Ir), platinum (Pt), palladium (Pd), osmium (Os), titanium (Ti), zirconium (Zr), hafnium (Hf), europium (Eu), terbium (Tb), rhodium (Rh), and thulium (Tm), 
     L 401  may be selected from ligands represented by Formula 402, and xc1 may be 1, 2, or 3, wherein, when xc1 is two or more, two or more L 401 (s) may be identical to or different from each other, 
     L 402  may be an organic ligand, and xc2 may be an integer from 0 to 4, wherein, when xc2 is two or more, two or more L 402 (s) may be identical to or different from each other, 
     X 401  to X 404  may each independently be nitrogen or carbon, 
     X 401  and X 403  may be linked via a single bond or a double bond, and X 402  and X 404  may be linked via a single bond or a double bond, 
     A 401  and A 402  may each independently be selected from a C 5 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, 
     X 405  may be a single bond, *—O—*′, *—S—*′, *—C(═O)—*′, *—N(Q 411 )—*′, *—C(Q 411 )(Q 412 )-*′, *—C(Q 411 )═C(Q 412 )-*′, *—C(Q 411 )=*′, or *═C═*′, wherein Q 411  and Q 412  may each independently be hydrogen, deuterium, 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, 
     X 406  may be a single bond, O, or S, 
     R 401  and R 402  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a substituted or unsubstituted C 1 -C 20  alkyl group, a substituted or unsubstituted C 1 -C 20  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, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 401 )(Q 402 )(Q 403 ), —N(Q 401 )(Q 402 ), —B(Q 401 )(Q 402 ), —C(═O)(Q 401 ), —S(═O) 2 (Q 401 ), and —P(═O)(Q 401 )(Q 402 ), wherein Q 401  to Q 403  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a C 6 -C 20  aryl group, and a C 1 -C 20  heteroaryl group, 
     xc11 and xc12 may each independently be an integer from 0 to 10, and 
     * and *′ in Formula 402 each indicate a binding site to M in Formula 401. 
     In one embodiment, A 401  and A 402  in Formula 402 may each independently be selected from a benzene group, a naphthalene group, a fluorene group, a spiro-bifluorene group, an indene group, a pyrrole group, a thiophene group, a furan 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, a quinoline group, an isoquinoline group, a benzoquinoline group, a quinoxaline group, a quinazoline group, a carbazole group, a benzimidazole group, a benzofuran group, a benzothiophene group, an isobenzothiophene group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a dibenzofuran group, and a dibenzothiophene group. 
     In one or more embodiments, in Formula 402, i) X 401  may be nitrogen, and X 402  may be carbon, or ii) X 401  and X 402  may each be nitrogen at the same time. 
     In one or more embodiments, R 402  and R 402  in Formula 402 may each independently be selected from: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, and a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group and a C 1 -C 20  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a phenyl group, a naphthyl group, a cyclopentyl group, a cyclohexyl group, an adamantanyl group, a norbornanyl group, and a norbornenyl group; 
     a cyclopentyl group, a cyclohexyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; 
     a cyclopentyl group, a cyclohexyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; and 
     —Si(Q 401 )(Q 402 )(Q 403 ), —N(Q 401 )(Q 402 ), —B(Q 401 )(Q 402 ), —C(═O)(Q 401 ), —S(═O) 2 (Q 401 ), and —P(═O)(Q 401 )(Q 402 ), and 
     Q 401  to Q 403  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, and a naphthyl group, but embodiments of the present disclosure are not limited thereto. 
     In one or more embodiments, when xc1 in Formula 401 is two or more, two A401(s) in two or more L 401 (s) may optionally be linked via X 407 , which is a linking group, or two A 402 (s) in two or more L 401 (s) may optionally be linked via X 408 , which is a linking group (see Compounds PD1 to PD4 and PD7). X 407  and X 408  may each independently be a single bond, *—O—*′, *—S—*′, *—C(═O)—*′, *—N(Q 413 )-*′, *—C(Q 413 )(Q 414 )-*′, or *—C(Q 413 )=C(Q 414 )-*′ (wherein Q 413  and Q 414  may each independently be hydrogen, deuterium, 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), but embodiments of the present disclosure are not limited thereto. 
     L 402  in Formula 401 may be a monovalent, divalent, or trivalent organic ligand. For example, L 402  may be halogen, diketone (for example, acetylacetonate), carboxylic acid (for example, picolinate), —C(═O), isonitrile, —CN, and phosphorus (for example, phosphine or phosphite), but embodiments of the present disclosure are not limited thereto. 
     In one or more embodiments, the phosphorescent dopant may be selected from, for example, Compounds PD1 to PD25, but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     [Fluorescent Dopant in Emission Layer] 
     The fluorescent dopant may include an arylamine compound or a styrylamine compound. 
     The fluorescent dopant may include a compound represented by Formula 501 below: 
     
       
         
         
             
             
         
       
     
     In Formula 501, 
     Ar 501  may be a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     L 501  to L 503  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     xd1 to xd3 may each independently be an integer from 0 to 3, 
     R 501  and R 502  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, and 
     xd4 may be an integer from 1 to 6. 
     In one embodiment, Ar 501  in Formula 501 may be selected from: 
     a naphthalene group, a heptalene 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, and an indenophenanthrene group; and 
     a naphthalene group, a heptalene 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, and an indenophenanthrene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     In one or more embodiments, L 501  to L 503  in Formula 501 may each independently be selected from: 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, and a pyridinylene group; and 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, and a pyridinylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a 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, and a pyridinyl group. 
     In one or more embodiments, R 501  and R 502  in Formula 501 may each independently be selected from: 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, and a pyridinyl group; and 
     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, and a pyridinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 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, and —Si(Q 31 )(Q 32 )(Q 33 ), and 
     Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     In one or more embodiments, xd4 in Formula 501 may be 2, but embodiments of the present disclosure are not limited thereto. 
     For example, the fluorescent dopant may be selected from Compounds FD1 to FD22: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In one or more embodiments, the fluorescent dopant may be selected from the following compounds, but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
     
     [Electron Transport Region in Organic Layer 150] 
     The electron transport region may have i) a single-layered structure including a single layer including a single material, ii) a single-layered structure including a single layer including a plurality of different materials, or iii) a multi-layered structure having a plurality of layers including a plurality of different materials. 
     The electron transport region may include at least one selected from a buffer layer, a hole blocking layer, an electron control layer, an electron transport layer, and an electron injection layer, but embodiments of the present disclosure are not limited thereto. 
     For example, the electron transport region may have an electron transport layer/electron injection layer structure, a hole blocking layer/electron transport layer/electron injection layer structure, an electron control layer/electron transport layer/electron injection layer structure, or a buffer layer/electron transport layer/electron injection layer structure, wherein for each structure, constituting layers are sequentially stacked from an emission layer. However, embodiments of the structure of the electron transport region are not limited thereto. 
     The electron transport region (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 containing at least one π electron-depleted nitrogen-containing ring. 
     The “π electron-depleted nitrogen-containing ring” indicates a C 1 -C 60  heterocyclic group having at least one *—N═*′ moiety as a ring-forming moiety. 
     For example, the “π electron-depleted nitrogen-containing ring” may be i) a 5-membered to 7-membered heteromonocyclic group having at least one *—N═*′ moiety, ii) a heteropolycyclic group in which two or more 5-membered to 7-membered heteromonocyclic groups each having at least one *—N═*′ moiety are condensed with each other, or iii) a heteropolycyclic group in which at least one of 5-membered to 7-membered heteromonocyclic groups, each having at least one *—N═*′ moiety, is condensed with at least one C 5 -C 60  carbocyclic group. 
     Examples of the π electron-depleted nitrogen-containing ring include an imidazole, a pyrazole, a thiazole, an isothiazole, an oxazole, an isoxazole, a pyridine, a pyrazine, a pyrimidine, a pyridazine, an indazole, a purine, a quinoline, an isoquinoline, a benzoquinoline, a phthalazine, a naphthyridine, a quinoxaline, a quinazoline, a cinnoline, a phenanthridine, an acridine, a phenanthroline, a phenazine, a benzimidazole, an isobenzothiazole, a benzoxazole, an isobenzoxazole, a triazole, a tetrazole, an oxadiazole, a triazine, a thiadiazole, an imidazopyridine, an imidazopyrimidine, and an azacarbazole, but are not limited thereto. 
     For example, the electron transport region may include a compound represented by Formula 601: 
       [Ar 601 ] xe11 -[(L 601 ) xe1 -R 601 ] xe21    &lt;Formula 601&gt;
 
     In Formula 601, 
     Ar 601  may be a substituted or unsubstituted C 5 -C 60  carbocyclic group or a substituted or unsubstituted C 1 -C 60  heterocyclic group, 
     xe11 may be 1, 2, or 3, 
     L 601  may be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     xe1 may be an integer from 0 to 5, 
     R 601  may be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 601 )(Q 602 )(Q 603 ), —C(═O)(Q 601 ), —S(═O) 2 (Q 601 ), and —P(═O)(Q 601 )(Q 602 ), 
     Q 601  to Q 603  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group, and 
     xe21 may be an integer from 1 to 5. 
     In one embodiment, at least one of Ar 601 (s) in the number of xe11 and R 601 (s) in the number of xe21 may include the π electron-depleted nitrogen-containing ring. 
     In one embodiment, ring Ar 601  in Formula 601 may be selected from: 
     a benzene group, a naphthalene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a naphthacene group, a picene group, a perylene group, a pentaphene group, an indenoanthracene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyrimidine group, a pyridazine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline group, a quinazoline group, a cinnoline group, a phenanthridine group, an acridine group, a phenanthroline group, a phenazine group, a benzimidazole group, an isobenzothiazole group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a thiadiazole group, an imidazopyridine group, an imidazopyrimidine group, and an azacarbazole group; and 
     a benzene group, a naphthalene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a naphthacene group, a picene group, a perylene group, a pentaphene group, an indenoanthracene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyrimidine group, a pyridazine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline group, a quinazoline group, a cinnoline group, a phenanthridine group, an acridine group, a phenanthroline group, a phenazine group, a benzimidazole group, an isobenzothiazole group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a thiadiazole group, an imidazopyridine group, an imidazopyrimidine group, and an azacarbazole group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), and 
     Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     When xe11 in Formula 601 is two or more, two or more Ar601(s) may be linked via a single bond. 
     In one or more embodiments, Ar 601  in Formula 601 may be an anthracene group. 
     In one or more embodiments, the compound represented by Formula 601 may be represented by Formula 601-1: 
     
       
         
         
             
             
         
       
     
     In Formula 601-1, 
     X 614  may be N or C(R 614 ), X 615  may be N or C(R 615 ), and X 616  may be N or C(R 616 ), wherein at least one selected from X 614  to X 616  may be N, 
     L 611  to L 613  may each independently be the same as defined in connection with L 601 , 
     xe611 to xe613 may each independently be the same as defined in connection with xe1, 
     R 611  to R 613  may each independently be the same as defined in connection with R 601 , and 
     R 614  to R 616  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     In one embodiment, L 601  and L 611  to L 613  in Formulae 601 and 601-1 may each independently be selected from: 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group; and 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group, 
     but embodiments of the present disclosure are not limited thereto. 
     In one or more embodiments, xe1 and xe611 to xe613 in Formulae 601 and 601-1 may each independently be 0, 1, or 2. 
     In one or more embodiments, R 601  and R 611  to R 613  in Formulae 601 and 601-1 may each independently be selected from: 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group; and 
     —S(═O) 2 (Q 601 ) and —P(═O)(Q 601 )(Q 602 ), and 
     Q 601  and Q 602  are the same as described above. 
     The electron transport region may include at least one compound selected from Compounds ET1 to ET36, but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In one or more embodiments, the electron transport region may include at least one selected from 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-dphenyl-1,10-phenanthroline (Bphen), Alq 3 , BAlq, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ), and NTAZ: 
     
       
         
         
             
             
         
       
     
     A thickness of the buffer layer, the hole blocking layer, or the electron control layer may be in a range of about 20 Å to about 1,000 Å, for example, about 30 Å to about 300 Å. When the thicknesses of the buffer layer, the hole blocking layer, and the electron control layer are within these ranges, the electron transport region may have excellent hole blocking characteristics or electron control characteristics without a substantial increase in driving voltage. 
     A thickness of the electron transport layer may be from about 100 Å to about 1,000 Å, for example, about 150 Å to about 500 Å. When the thickness of the electron transport layer is within the range described above, the electron transport layer may have satisfactory electron transport characteristics without a substantial increase in driving voltage. 
     The electron transport region (for example, the electron transport layer in the electron transport region) may further include, in addition to the materials described above, a metal-containing material. 
     The metal-containing material may include at least one selected from alkali metal complex and alkaline earth-metal complex. The alkali metal complex may include a metal ion selected from a Li ion, a Na ion, a K ion, a Rb ion, and a Cs ion, and the alkaline earth-metal complex may include a metal ion selected from a Be ion, a Mg ion, a Ca ion, a Sr ion, and a Ba ion. A ligand coordinated with the metal ion of the alkali metal complex or the alkaline earth-metal complex may be selected from a hydroxy quinoline, a hydroxy isoquinoline, a hydroxy benzoquinoline, a hydroxy acridine, a hydroxy phenanthridine, a hydroxy phenyl oxazole, a hydroxy phenylthiazole, a hydroxy diphenyl oxadiazole, a hydroxy diphenylthiadiazole, a hydroxy phenylpyridine, a hydroxy phenylbenzimidazole, a hydroxy phenylbenzothiazole, a bipyridine, a phenanthroline, and a cyclopentadiene, but embodiments of the present disclosure are not limited thereto. 
     For example, the metal-containing material may include a Li complex. The Li complex may include, for example, Compound ET-D1 (lithium quinolate, LiQ) or ET-D2: 
     
       
         
         
             
             
         
       
     
     The electron transport region may include an electron injection layer that facilitates electron injection from the second electrode  190 . The electron injection layer may directly contact the second electrode  190 . 
     The electron injection layer may have i) a single-layered structure including a single layer including a single material, ii) a single-layered structure including a single layer including a plurality of different materials, or iii) a multi-layered structure having a plurality of layers including a plurality of different materials. 
     The electron injection layer may include an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth-metal compound, a rare earth metal compound, an alkali metal complex, an alkaline earth-metal complex, a rare earth metal complex, or any combinations thereof. 
     The alkali metal may be selected from Li, Na, K, Rb, and Cs. In one embodiment, the alkali metal may be Li, Na, or Cs. In one or more embodiments, the alkali metal may be Li or Cs, but embodiments of the present disclosure are not limited thereto. 
     The alkaline earth metal may be selected from Mg, Ca, Sr, and Ba. 
     The rare earth metal may be selected from Sc, Y, Ce, Tb, Yb, and Gd. 
     The alkali metal compound, the alkaline earth-metal compound, and the rare earth metal compound may be selected from oxides and halides (for example, fluorides, chlorides, bromides, or iodides) of the alkali metal, the alkaline earth-metal, and the rare earth metal. 
     The alkali metal compound may be selected from alkali metal oxides, such as Li 2 O, Cs 2 O, or K 2 O, and alkali metal halides, such as LiF, NaF, CsF, KF, LiI, NaI, CsI, or KI. In one embodiment, the alkali metal compound may be selected from LiF, Li 2 O, NaF, LiI, NaI, CsI, and KI, but embodiments of the present disclosure are not limited thereto. 
     The alkaline earth-metal compound may be selected from alkaline earth-metal oxides, such as BaO, SrO, CaO, Ba x Sr 1-x O (0&lt;x&lt;1), or Ba x Ca 1-x O (0&lt;x&lt;1). In one embodiment, the alkaline earth-metal compound may be selected from BaO, SrO, and CaO, but embodiments of the present disclosure are not limited thereto. 
     The rare earth metal compound may be selected from YbF 3 , ScF 3 , Sc 2 O 3 , Y 2 O 3 , Ce 2 O 3 , GdF 3 , and TbF 3 . In one embodiment, the rare earth metal compound may be selected from YbF 3 , ScF 3 , TbF 3 , YbI 3 , ScI 3 , and TbI 3 , but embodiments of the present disclosure are not limited thereto. 
     The alkali metal complex, the alkaline earth-metal complex, and the rare earth metal complex may include an ion of alkali metal, alkaline earth-metal, and rare earth metal as described above, and a ligand coordinated with a metal ion of the alkali metal complex, the alkaline earth-metal complex, or the rare earth metal complex may be selected from hydroxy quinoline, hydroxy isoquinoline, hydroxy benzoquinoline, hydroxy acridine, hydroxy phenanthridine, hydroxy phenyl oxazole, hydroxy phenylthiazole, hydroxy diphenyloxadiazole, hydroxy diphenylthiadiazole, hydroxy phenylpyridine, hydroxy phenylbenzimidazole, hydroxy phenylbenzothiazole, bipyridine, phenanthroline, and cyclopentadiene, but embodiments of the present disclosure are not limited thereto. 
     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 any combinations thereof, as described above. In one or more embodiments, the electron injection layer may further include an organic material. When the electron injection layer further includes an organic material, an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth-metal compound, a rare earth metal compound, an alkali metal complex, an alkaline earth-metal complex, a rare earth metal complex, or any combinations thereof may be homogeneously or non-homogeneously dispersed in a matrix including the organic material. 
     A thickness of the electron injection layer may be in a range of about 1 Å to about 100 Å, for example, about 3 Å to about 90 Å. When a thickness of the electron injection layer is within these ranges, satisfactory electron injection characteristics may be obtained without substantial increase in driving voltage. 
     [Second Electrode  190 ] 
     The second electrode  190  may be disposed on the organic layer  150  having such a structure. The second electrode  190  may be a cathode which is an electron injection electrode, and in this regard, a material for forming the second electrode  190  may be selected from metal, an alloy, an electrically conductive compound, and a combination thereof, which have a relatively low work function. 
     The second electrode  190  may include at least one selected from lithium (Li), silver (Ag), magnesium (Mg), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), ITO, and IZO, but embodiments of the present disclosure are not limited thereto. The second electrode  190  may be a transmissive electrode, a semi-transmissive electrode, or a reflective electrode. 
     The second electrode  190  may have a single-layered structure, or a multi-layered structure including two or more layers. 
     [Description of  FIGS. 2 to 4 ] 
       FIG. 2  is a schematic view of an organic light-emitting device  20  according to an embodiment. The organic light-emitting device  20  includes a first capping layer  210 , the first electrode  110 , the organic layer  150 , and the second electrode  190 , which are sequentially stacked in this stated order.  FIG. 3  is a schematic view of an organic light-emitting device  30  according to an embodiment. The organic light-emitting device  30  includes the first electrode  110 , the organic layer  150 , the second electrode  190 , and a second capping layer  220 , which are sequentially stacked in this stated order.  FIG. 4  is a schematic view of an organic light-emitting device  40  according to an embodiment. The organic light-emitting device  40  includes the first capping layer  210 , the first electrode  110 , the organic layer  150 , the second electrode  190 , and the second capping layer  220 , which are sequentially stacked in this stated order. 
     Regarding  FIGS. 2 to 4 , the first electrode  110 , the organic layer  150 , and the second electrode  190  may be understood by referring to the description presented in connection with  FIG. 1 . 
     In the organic layer  150  of each of the organic light-emitting devices  20  and  40 , light generated in an emission layer may pass through the first electrode  110  and the first capping layer  210  toward the outside, wherein the first electrode  110  may be a semi-transmissive electrode or a transmissive electrode. In the organic layer  150  of each of the organic light-emitting devices  30  and  40 , light generated in an emission layer may pass through the second electrode  190  and the second capping layer  220  toward the outside, wherein the second electrode  190  may be a semi-transmissive electrode or a transmissive electrode. 
     The first capping layer  210  and the second capping layer  220  may increase external luminescence efficiency according to the principle of constructive interference. 
     The first capping layer  210  and the second capping layer  220  may each independently be an organic capping layer including an organic material, an inorganic capping layer including an inorganic material, or a composite capping layer including an organic material and an inorganic material. 
     At least one selected from the first capping layer  210  and the second capping layer  220  may each independently include at least one material selected from carbocyclic compounds, heterocyclic compounds, amine-based compounds, porphyrine derivatives, phthalocyanine derivatives, naphthalocyanine derivatives, alkali metal complexes, and alkaline earth-based complexes. The carbocyclic compound, the heterocyclic compound, and the amine-based compound may be optionally substituted with a substituent containing at least one element selected from O, N, S, Se, Si, F, Cl, Br, and I. In one embodiment, at least one selected from the first capping layer  210  and the second capping layer  220  may each independently include an amine-based compound. 
     In one embodiment, at least one selected from the first capping layer  210  and the second capping layer  220  may each independently include the compound represented by Formula 201 or the compound represented by Formula 202. 
     In one or more embodiments, at least one selected from the first capping layer  210  and the second capping layer  220  may each independently include a compound selected from Compounds HT28 to HT33 and Compounds CP1 to CP5, but embodiments of the present disclosure are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Hereinbefore, the organic light-emitting device according to an embodiment has been described in connection with  FIGS. 1 to 4 , but embodiments of the present disclosure are not limited thereto. 
     Layers constituting the hole transport region, an emission layer, and layers constituting the electron transport region may be formed in a certain region by using one or more suitable methods selected from vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB) deposition, ink-jet printing, laser-printing, and laser-induced thermal imaging. 
     When layers constituting the hole transport region, an emission layer, and layers constituting the electron transport region are formed by vacuum deposition, the deposition may be performed at a deposition temperature of about 100° C. to about 500° C., a vacuum degree of about 10 −8  torr to about 10 −3  torr, and a deposition speed of about 0.01 Å/sec to about 100 Å/sec by taking into account a material to be included in a layer to be formed, and the structure of a layer to be formed. 
     When layers constituting the hole transport region, an emission layer, and layers constituting the electron transport region are formed by spin coating, the spin coating may be performed at a coating speed of about 2,000 rpm to about 5,000 rpm and at a heat treatment temperature of about 80° C. to 200° C. by taking into account a material to be included in a layer to be formed, and the structure of a layer to be formed. 
     [General Definition of Substituents] 
     The term “C 1 -C 60  alkyl group” as used herein refers to a linear or branched aliphatic saturated hydrocarbon monovalent group having 1 to 60 carbon atoms, and examples thereof include a methyl group, an ethyl group, a propyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an isoamyl group, and a hexyl group. The term “C 1 -C 60  alkylene group” as used herein refers to a divalent group having the same structure as the C 1 -C 60  alkyl group. 
     The term “C 2 -C 60  alkenyl group” as used herein refers to a hydrocarbon group having at least one carbon-carbon double bond in the middle or at the terminus of the C 2 -C 60  alkyl group, and examples thereof include an ethenyl group, a propenyl group, and a butenyl group. The term “C 2 -C 60  alkenylene group” as used herein refers to a divalent group having 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 hydrocarbon group having at least one carbon-carbon triple bond in the middle or at the terminus of the C 2 -C 60  alkyl group, and examples thereof include an ethynyl group, and a propynyl group. The term “C 2 -C 60  alkynylene group” as used herein refers to a divalent group having the same structure as the C 2 -C 60  alkynyl group. 
     The term “C 1 -C 60  alkoxy group” as used herein refers to a monovalent group represented by —OA 101  (wherein A 101  is the C 1 -C 60  alkyl group), and examples thereof include a methoxy group, an ethoxy group, and an isopropyloxy group. 
     The term “C 3 -C 10  cycloalkyl group” as used herein refers to a monovalent saturated hydrocarbon monocyclic group having 3 to 10 carbon atoms, and examples thereof include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and a cycloheptyl group. The term “C 3 -C 10  cycloalkylene group” as used herein refers to a divalent group having the same structure as the C 3 -C 10  cycloalkyl group. 
     The term “C 1 -C 10  heterocycloalkyl group” as used herein refers to a monovalent monocyclic group having at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom and 1 to 10 carbon atoms, and examples thereof include a 1,2,3,4-oxatriazolidinyl group, a tetrahydrofuranyl group, and a tetrahydrothiophenyl group. The term “C 1 -C 10  heterocycloalkylene group” as used herein refers to a divalent group having the same structure as the C 1 -C 10  heterocycloalkyl group. 
     The term “C 3 -C 10  cycloalkenyl group” as used herein refers to a monovalent monocyclic group that has 3 to 10 carbon atoms and at least one carbon-carbon double bond in the ring thereof and no aromaticity, and examples thereof 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 the same structure as the C 3 -C 10  cycloalkenyl group. 
     The term “C 1 -C 10  heterocycloalkenyl group” as used herein refers to a monovalent monocyclic group that has at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, 1 to 10 carbon atoms, and at least one carbon-carbon double bond in its ring. Non-limiting examples of the C 1 -C 10  heterocycloalkenyl group include a 4,5-dihydro-1,2,3,4-oxatriazolyl group, a 2,3-dihydrofuranyl group, and a 2,3-dihydrothiophenyl group. The term “C 1 -C 10  heterocycloalkenylene group” as used herein refers to a divalent group having 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, and a C 6 -C 60  arylene group used herein refers to a divalent group having a carbocyclic aromatic system having 6 to 60 carbon atoms. Non-limiting examples of the C 6 -C 60  aryl group include a phenyl group, a naphthyl group, an anthracenyl group, a phenanthrenyl group, a pyrenyl group, and a chrysenyl group. When the C 6 -C 60  aryl group and the C 6 -C 60  arylene group each include two or more rings, the rings may be fused to each other. 
     The term “C 1 -C 60  heteroaryl group” as used herein refers to a monovalent group having a carbocyclic aromatic system that has at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, in addition to 1 to 60 carbon atoms. The term “C 1 -C 60  heteroarylene group” as used herein refers to a divalent group having a carbocyclic aromatic system that has at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, in addition to 1 to 60 carbon atoms. Non-limiting examples of the C 1 -C 60  heteroaryl group include a pyridinyl group, a pyrimidinyl group, a pyrazinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, and an isoquinolinyl group. When the C 1 -C 60  heteroaryl group and the C 1 -C 60  heteroarylene group each include two or more rings, the rings may be condensed with each other. 
     The term “C 6 -C 60  aryloxy group” as used herein refers to —OA 102  (wherein A 102  is the C 6 -C 60  aryl group), and a C 6 -C 60  arylthio group used herein indicates —SA 103  (wherein A 103  is the C 6 -C 60  aryl group). 
     The term “monovalent non-aromatic condensed polycyclic group” as used herein refers to a monovalent group (for example, having 8 to 60 carbon atoms) having two or more rings condensed with each other, only carbon atoms as ring-forming atoms, and no aromaticity in its entire molecular structure. A detailed example of the monovalent non-aromatic condensed polycyclic group is a fluorenyl group. The term “divalent non-aromatic condensed polycyclic group” as used herein refers to a divalent group having the same structure as the monovalent non-aromatic condensed polycyclic group. 
     The term “monovalent non-aromatic condensed heteropolycyclic group” as used herein refers to a monovalent group (for example, having 1 to 60 carbon atoms) having two or more rings condensed to each other, at least one heteroatom selected from N, O, Si, P, and S, other than carbon atoms, as a ring-forming atom, and no aromaticity in its entire molecular structure. An example of the monovalent non-aromatic condensed heteropolycyclic group is a carbazolyl group. The term “divalent non-aromatic condensed heteropolycyclic group” as used herein refers to a divalent group having the same structure as the monovalent non-aromatic condensed heteropolycyclic group. 
     The term “C 5 -C 60  carbocyclic group” as used herein refers to a monocyclic or polycyclic group having 5 to 60 carbon atoms in which a ring-forming atom is a carbon atom only. The term “C 5 -C 60  carbocyclic group” as used herein refers to an aromatic carbocyclic group or a non-aromatic carbocyclic group. The C 5 -C 60  carbocyclic group may be a ring, such as benzene, a monovalent group, such as a phenyl group, or a divalent group, such as a phenylene group. In one or more embodiments, depending on the number of substituents connected to the C 5 -C 60  carbocyclic group, the C 5 -C 60  carbocyclic group may be a trivalent group or a quadrivalent group. 
     The term “C 1 -C 60  heterocyclic group” as used herein refers to a group having the same structure as the C 5 -C 60  carbocyclic group, except that as a ring-forming atom, at least one heteroatom selected from N, O, Si, P, and S is used in addition to carbon (the number of carbon atoms may be in a range of 1 to 60). 
     In the present specification, at least one substituent of 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 selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an 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, —Si(Q 11 )(Q 12 )(Q 13 ), —N(Q 11 )(Q 12 ), and —B(Q 11 )(Q 12 ); 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an 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, —Si(Q 21 )(Q 22 )(Q 23 ), —N(Q 21 )(Q 22 ), and —B(Q 21 )(Q 22 ); and 
     —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ) and —P(═O)(Q 31 )(Q 32 ), and 
     Q 11  to Q 13 , Q 21  to Q 23 , and Q 31  to Q 33  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, a biphenyl group, and a terphenyl group. 
     The term “Ph” as used herein refers to a phenyl group, the term “Me” as used herein refers to a methyl group, the term “Et” as used herein refers to an ethyl group, the term “ter-Bu” or “Bu t ” as used herein refers to a tert-butyl group, and the term “OMe” as used herein refers to a methoxy group. 
     The term “biphenyl group” as used herein refers to “a phenyl group substituted with a phenyl group”. In other words, the “biphenyl group” is a substituted phenyl group having a C 6 -C 60  aryl group as a substituent. 
     The term “terphenyl group” as used herein refers to “a phenyl group substituted with a biphenyl group”. In other words, the “terphenyl group” is a phenyl group having, as a substituent, a C 6 -C 60  aryl group substituted with a C 6 -C 60  aryl group. 
     *, *′, and *″ as used herein, unless defined otherwise, each refer to a binding site to a neighboring atom in a corresponding formula. 
     Hereinafter, a compound according to embodiments and an organic light-emitting device according to embodiments will be described in detail with reference to Synthesis Examples and Examples. The wording “B was used instead of A” used in describing Synthesis Examples refers to that an identical molar equivalent of B was used in place of A. 
     SYNTHESIS EXAMPLE 
     Synthesis Example 1: Synthesis of Compound 1 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Intermediate [1-A] 
     2,6-dibromo-4-(tert-butyl)pyridine (1.0 eq), carbazole (3.0 eq), Pd 2 (dba) 3  (0.2 eq), Sphos (0.4 eq), and K 3 PO 4  (2.0 eq) were dissolved in toluene (0.5 M) and stirred at a temperature of 120° C. for 12 hours. After the reaction mixture was cooled to room temperature, an organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulphate and concentrated, and column chromatography was used to obtain Intermediate [1-A] (yield: 65%). 
     (2) Synthesis of Intermediate [1-B] 
     Intermediate [1-A] (1.0 eq) was dissolved in tetrahydrofuran (0.5 M), n-buthylithium (1.2 eq) was slowly added thereto while stirring at a temperature of −78° C., and the reaction mixture was stirred for 2 hours. 3-bromobenzaldehyde (1.2 eq) was slowly added thereto while stirring at a temperature of −78° C., and the reaction mixture was stirred at room temperature for 12 hours. An organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulphate and concentrated, and column chromatography was used to Intermediate [1-B] (yield: 42%). 
     (3) Synthesis of Intermediate [1-C] 
     Intermediate [1-B] (1.0 eq) and DDQ (2.0 eq) were dissolved in dichloromethane (0.5 M) and stirred at room temperature for 0.5 hours. An organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulphate and concentrated, and column chromatography was used to obtain Intermediate [1-C] (yield: 90%). 
     (4) Synthesis of Intermediate [1-D] 
     Magnesium turning (5.0 eq) was dissolved in tetrahydrofuran (0.5 M), and I 2  (0.01 eq) was added thereto. 2-bromo biphenyl (1.2 eq) was slowly added thereto while stirring the reaction mixture at room temperature, and was stirred for 3 hours. After the reaction mixture was cooled to a temperature of −78° C., Intermediate [1-C] (0.1 eq) dissolved in tetrahydrofuran (0.5 M) was slowly added thereto while stirring the reaction mixture. The reaction mixture was heated to a temperature of 70° C. and stirred for 12 hours. An organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulfate and concentrated, and column chromatography was used to obtain Intermediate [1-D] (yield: 46%). 
     (5) Synthesis of Intermediate [1-E] 
     Intermediate [1-D] (1.0 eq) was dissolved in acetic acid (0.5 M) and 35% HCl aqueous solution (0.5 M) and stirred at a temperature of 120° C. for 12 hours. After the reaction mixture was cooled to room temperature, and pH was adjusted to 6 to 7 by using 3N NaOH aqueous solution. An organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulfate and concentrated, and column chromatography was used to obtain Intermediate [1-E] (yield: 55%). 
     (6) Synthesis of Intermediate [1-F] 
     Intermediate [1-E] (1.0 eq), imidazole (1.2 eq), CuI (0.01 eq), K 2 CO 3  (2.0 eq), and L-Proline (0.02 eq) were dissolved in dimethylsulfonate (0.1 M) and stirred at a temperature of 160° C. for 48 hours. After the reaction mixture was cooled to room temperature, an organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulfate and concentrated, and column chromatography was used to obtain Intermediate [1-F] (yield: 43%). 
     (7) Synthesis of Intermediate [1-G] 
     Intermediate [1-F] (1.0 eq) and iodomethane (3.0 eq) were dissolved in THF (1.0 M) and stirred at a temperature of 70° C. for 12 hours. After the reaction mixture was cooled to room temperature, an organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulfate and concentrated, and column chromatography was used to obtain Intermediate [1-G] (yield: 83%). 
     (8) Synthesis of Compound 1 
     Intermediate [1-G] (1.0 eq), dichloro(1,2-dicyclooctadiene)platinum (Pt(COD)Cl 2 ) (1.1 eq), and sodium acetate (2.0 eq) were dissolved in dioxane (0.1M) and stirred at a temperature of 120° C. for 72 hours. After the reaction mixture was cooled to room temperature, an organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulfate and concentrated, and column chromatography was used to Compound 1 (yield: 22%). 
     Synthesis Example 2: Synthesis of Compound 2 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Intermediate [2-A] 
     Intermediate [2-A] (yield: 41%) was synthesized in the same manner as in Synthesis of Intermediate [1-B], except that 2,6-dibromo-4-(tert-butyl)pyridine was used instead of Intermediate [1-A], and benzaldehyde was used instead of 3-bromobenzaldehyde. 
     (2) Synthesis of Intermediate [2-B] 
     Intermediate [2-B] (yield: 67%) was synthesized in the same manner as in Synthesis of Intermediate [1-C], except that Intermediate [2-A] was used instead of Intermediate [1-B]. 
     (3) Synthesis of Intermediate [2-C] 
     Intermediate [2-C] (yield: 43%) was synthesized in the same manner as in Synthesis of Intermediate [1-D], except that Intermediate [2-B] was used instead of Intermediate [1-C]. 
     (4) Synthesis of Intermediate [2-D] 
     Intermediate [2-D] (yield: 43%) was synthesized in the same manner as in Synthesis of Intermediate [1-E], except that Intermediate [2-C] was used instead of Intermediate [1-D]. 
     (5) Synthesis of Intermediate [2-E] 
     Intermediate [2-E] (yield: 44%) was synthesized in the same manner as in Synthesis of Intermediate [1-B], except that Intermediate [2-D] was used instead of Intermediate [1-A]. 
     (6) Synthesis of Intermediate [2-F] 
     Intermediate [2-F] (yield: 62%) was synthesized in the same manner as in Synthesis of Intermediate [1-C], except that Intermediate [2-E] was used instead of Intermediate [1-B]. 
     (7) Synthesis of Intermediate [2-G] 
     Intermediate [2-G] (yield: 43%) was synthesized in the same manner as in Synthesis of Intermediate [1-D], except that Intermediate [2-F] was used instead of Intermediate [1-C]. 
     (8) Synthesis of Intermediate [2-H] 
     Intermediate [2-H] (yield: 47%) was synthesized in the same manner as in Synthesis of Intermediate [1-E], except that Intermediate [2-G] was used instead of Intermediate [1-D]. 
     (9) Synthesis of Intermediate [2-I] 
     Intermediate [2-I] (yield: 52%) was synthesized in the same manner as in Synthesis of Intermediate [1-F], except that Intermediate [2-H] was used instead of Intermediate [1-E]. 
     (10) Synthesis of Intermediate [2-J] 
     Intermediate [2-J] (yield: 86%) was synthesized in the same manner as in Synthesis of Intermediate [1-G], except that Intermediate [2-I] was used instead of Intermediate [1-F]. 
     (11) Synthesis of Compound 2 
     Compound 2 (yield: 24%) was synthesized in the same manner as in Synthesis of Compound 1, except that Intermediate [2-J] was used instead of Intermediate [1-G]. 
     Synthesis Example 3: Synthesis of Compound 3 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Intermediate [3-A] 
     Intermediate [3-A] (yield: 27%) was synthesized in the same manner as in Synthesis of Intermediate [1-B], except that Intermediate [2-D] was used instead of Intermediate [1-A], and 1-methyl-2-phenyl-1H-imidazole-4-carbaldehyde was used instead of 3-bromobenzaldehyde. 
     (2) Synthesis of Intermediate [3-B] 
     Intermediate [3-B] (yield: 62%) was synthesized in the same manner as in Synthesis of Intermediate [1-C], except that Intermediate [3-A] was used instead of Intermediate [1-B]. 
     (3) Synthesis of Intermediate [3-C] 
     Intermediate [3-C] (yield: 51%) was synthesized in the same manner as in Synthesis of Intermediate [1-D], except that Intermediate [3-B] was used instead of Intermediate [1-C]. 
     (4) Synthesis of Intermediate [3-D] 
     Intermediate [3-D] (yield: 45%) was synthesized in the same manner as in Synthesis of Intermediate [1-E], except that Intermediate [3-C] was used instead of Intermediate [1-D]. 
     (5) Synthesis of Compound 3 
     Compound 3 (yield: 31%) was synthesized in the same manner as in Synthesis of Compound 1, except that Intermediate [3-D] was used instead of Intermediate [1-G]. 
     Synthesis Example 4: Synthesis of Compound 4 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Intermediate [4-A] 
     2,6-dibromo-4-(tert-butyl)pyridine (1.0 eq), phenol (1.2 eq), CuI (0.01 eq), K 2 CO 3  (2.0 eq), and L-Proline (0.02 eq) were dissolved in dimethylsulfonate (0.1 M) and stirred at a temperature of 160′ C. for 48 hours. After the reaction mixture was cooled to room temperature, an organic layer was extracted therefrom three times by using dichloromethane and water. The extracted organic layer was dried by using magnesium sulphate and concentrated, and column chromatography was used to obtain Intermediate [4-A] (yield: 57%). 
     (2) Synthesis of Intermediate [4-B] 
     Intermediate [4-B] (yield: 29%) was synthesized in the same manner as in Synthesis of Intermediate [1-B], except that Intermediate [4-A] was used instead of Intermediate [1-A]. 
     (3) Synthesis of Intermediate [4-C] 
     Intermediate [4-C] (yield: 62%) was synthesized in the same manner as in Synthesis of Intermediate [1-C], except that Intermediate [4-B] was used instead of Intermediate [1-B]. 
     (4) Synthesis of Intermediate [4-D] 
     Intermediate [4-D] (yield: 49%) was synthesized in the same manner as in Synthesis of Intermediate [1-D], except that Intermediate [4-C] was used instead of Intermediate [1-C]. 
     (5) Synthesis of Intermediate [4-E] 
     Intermediate [4-E] (yield: 47%) was synthesized in the same manner as in Synthesis of Intermediate [1-E], except that Intermediate [4-D] was used instead of Intermediate [1-D]. 
     (6) Synthesis of Intermediate [4-F] 
     Intermediate [4-F] (yield: 54%) was synthesized in the same manner as in Synthesis of Intermediate [1-F], except that Intermediate [4-E] was used instead of Intermediate [1-E]. 
     (7) Synthesis of Intermediate [4-G] 
     Intermediate [4-G] (yield: 81%) was synthesized in the same manner as in Synthesis of Intermediate [1-G], except that Intermediate [4-F] was used instead of Intermediate [1-F]. 
     (8) Synthesis of Compound 4 
     Compound 4 (yield: 27%) was synthesized in the same manner as in Synthesis of Compound 1, except that Intermediate [4-G] was used instead of Intermediate [1-G]. 
     Synthesis Example 5: Synthesis of Compound 5 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Synthesis of Intermediate [5-A] 
     Intermediate [5-A] (yield: 51%) was synthesized in the same manner as in Synthesis of Intermediate [4-A], except that Intermediate [2-D] (1.0 eq) was used instead of 2,6-dibromo-4-(tert-butyl)pyridine, and 3-bromophenol (1.2 eq) was used instead of phenol. 
     (2) Synthesis of Intermediate [5-B] 
     Intermediate [5-B] (yield: 52%) was synthesized in the same manner as in Synthesis of Intermediate [1-F], except that Intermediate [5-A] was used instead of Intermediate [1-E] 
     (3) Synthesis of Intermediate [5-C] 
     Intermediate [5-C] (yield: 77%) was synthesized in the same manner as in Synthesis of Intermediate [1-G], except that Intermediate [5-B] was used instead of Intermediate [1-F]. 
     (4) Synthesis of Compound 5 
     Compound 5 (yield: 24%) was synthesized in the same manner as in Synthesis of Compound 1, except that Intermediate [5-C] was used instead of Intermediate [1-G]. 
       1 H NMR and LC-MS of Compounds synthesized according to Synthesis Examples 1 to 5 are shown in Table 1. 
     Synthesis methods of compounds other than Compounds shown in Table 1 may also be easily recognized by those of ordinary skill in the art by referring to the synthesis mechanisms and source materials described above. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Com- 
                   
                   
               
               
                 pound 
                   
                 LC-MS 
               
            
           
           
               
               
               
               
            
               
                 No. 
                   1 H NMR (CDCl 3 , 400 MHz) 
                 found 
                 calc. 
               
               
                   
               
               
                 1 
                 8.65 (d, 1H), 8.55 (d, 1H), 8.15 (d, 1H), 7.90-  
                 813.22 
                 813.24 
               
               
                   
                 6.54 (m, 20H), 3.67 (s, 3H), 1.32 (s, 9H) 
                   
                   
               
               
                 2 
                 8.61 (d, 1H), 7.87-6.60 (m, 25H), 6.49 (d,  
                 888.30 
                 888.28 
               
               
                   
                 1H), 3.62 (s, 3H), 1.30 (s, 9H) 
                   
                   
               
               
                 3 
                 7.92-7.18 (m, 27H), 7.06 (s, 1H), 3.73 (s,  
                 888.21 
                 888.28 
               
               
                   
                 3H), 1.29 (s, 9H) 
                   
                   
               
               
                 4 
                 8.66 (d, 1H), 8.12-6.55 (m, 18H), 3.58 (s,  
                 740.24 
                 740.21 
               
               
                   
                 3H), 1.34 (s, 9H) 
                   
                   
               
               
                 5 
                 8.58 (d, 1H), 8.03-6.27 (m, 18H), 5.67 (s,  
                 740.19 
                 740.21 
               
               
                   
                 1H), 3.69 (s, 3H), 1.31 (s, 9H) 
               
               
                   
               
            
           
         
       
     
     EVALUATION EXAMPLE 
     Quantum simulation was performed on i) A percentage of a triplet metal-to-ligand charge transfer state ( 3 MLCT), ii) a simulation value (λ max   sim ) and an experiment value (λ max   exp ) of a wavelength of maximum emission, iii) an energy level of a triplet metal-centered state ( 3 MC), and iv) bond dissociation energy of an N atom of pyridine (Py) in a ligand and a metal center Pt of Comparative Examples 1 to 5, and results thereof are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 ΔE HOMO-LUMO   
                   
                   
                   
                 Pt—Py bond 
               
               
                   
                 (eV) 
                 λ max   sim  (nm) 
                 λ max   exp  (nm) 
                   3 MLCT (%) 
                 dissociation energy (eV) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Comparative 
                 2.66 
                 465.8094 
                 453 
                 11.0 
                 2.96 
               
               
                 Compound 1 
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 1.73 
                 714.005 
                 — 
                 — 
                 — 
               
               
                 Compound 2 
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 1.63 
                 756.637 
                 — 
                 — 
                 — 
               
               
                 Compound 3 
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 2.59 
                 478.1594 
                 480 
                 14.8 
                 3.18 
               
               
                 Compound 4 
                   
                   
                   
                   
                   
               
               
                 Comparatve 
                 2.34 
                 529.0396 
                 515 
                 15.1 
                 3.01 
               
               
                 Compound 5 
                   
                   
                   
                   
                   
               
               
                 Compound 1 
                 2.23 
                 555.13 
                 543 
                 16.2 
                 3.21 
               
               
                 Compound 2 
                 2.22 
                 557.64 
                 546 
                 17.6 
                 3.27 
               
               
                 Compound 3 
                 2.64 
                 468.5062 
                 458 
                 17.1 
                 3.10 
               
               
                 Compound 4 
                 2.51 
                 493.5235 
                 479 
                 15.7 
                 3.07 
               
               
                 Compound 5 
                 2.93 
                 422.5108 
                 431 
                 15.4 
                 3.04 
               
               
                   
               
               
                 &lt;Comparative Compound 1&gt; 
               
               
                                   
&lt;Comparative Compound 2&gt; 
               
               
                                   
&lt;Comparative Compound 3&gt; 
               
               
                                   
&lt;Comparative Compound 4&gt; 
               
               
                                   
&lt;Comparative Compound 5&gt; 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
            
           
         
       
     
     EXAMPLE 
     Example 1 
     As an anode, a Corning 15 Ω/cm 2  (1,200 Å) ITO glass substrate was cut to a size of 50 mm×50 mm×0.7 mm, sonicated with isopropyl alcohol and pure water each for 5 minutes, and then cleaned by exposure to ultraviolet rays and ozone for 30 minutes. Then, the ITO glass substrate was provided to a vacuum deposition apparatus. 2-TNATA was vacuum-deposited on the ITO glass substrate to form a hole injection layer having a thickness of 600 Å, and 4,4′-bis[N-(1-naphthyl)-N-phenylaminobiphenyl (NPB) was vacuum-deposited on the hole injection layer to form a hole transport layer having a thickness of 300 Å. Compound 1 (dopant) (weight ratio of 10%) and 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) (host) were co-deposited on the hole transport layer to form an emission layer having a thickness of 300 Å. Then, diphenyl(4-(triphenylsilyl)phenyl)-phosphine oxide (TSPO1) was vacuum-deposited on the emission layer to form a hole blocking layer having a thickness of 50 Å. Then, Alq 3  was deposited on the hole blocking layer to form an electron transport layer having thickness of 300 Å, an alkali metal halide LiF was deposited on the electron transport layer to form an electron injection layer having a thickness of 10 Å, and Al was vacuum-deposited to a thickness 3,000 Å (cathode electrode) to form an LiF/Al electrode, thereby completing the manufacture of an organic light-emitting device. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Examples 2 to 5 
     Organic light-emitting devices were manufactured in the same manner as in Example 1, except that Compounds 2 to 5 were each used instead of Compound 1 as a dopant in forming an emission layer. 
     Comparative Examples 1 to 3 
     Organic light-emitting devices were manufactured in the same manner as in Example 1, except that Comparative Compounds 1, 4, and 5 were each used instead of Compound 1 as a dopant in forming an emission layer. 
     The driving voltage, luminance, luminescence efficiency, and maximum emission efficiency of the organic light-emitting devices manufactured according to Examples 1 to 5 and Comparative Examples 1 to 3 were measured at a current density of 50 mA/cm 2  by using Keithley SMU 236 and a luminance meter PR650, and an amount of time (lifespan) that lapsed when luminance was 50% of initial luminance (100%) was measured. Results thereof are shown in Table 3. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Wavelength 
                   
               
               
                   
                   
                   
                   
                   
                   
                 of 
                   
               
               
                   
                   
                 Driving 
                 Current 
                   
                   
                 maximum 
                   
               
               
                   
                   
                 voltage 
                 density 
                 Luminance 
                 Efficiency 
                 emission 
                 Emission 
               
               
                   
                 Emission layer 
                 (V) 
                 (mA/cm 2 ) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (nm) 
                 color 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 
                 Compound 1 
                 5.16 
                 50 
                 5351 
                 10.7 
                 543 
                 Green 
               
               
                 1 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Example 
                 Compound 2 
                 5.07 
                 50 
                 5603 
                 11.2 
                 546 
                 Green 
               
               
                 2 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Example 
                 Compound 3 
                 5.48 
                 50 
                 4033 
                 8.07 
                 458 
                 Blue 
               
               
                 3 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Example 
                 Compound 4 
                 5.24 
                 50 
                 4272 
                 8.54 
                 479 
                 Blue 
               
               
                 4 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Example 
                 Compound 5 
                 5.62 
                 50 
                 3995 
                 7.99 
                 431 
                 Blue 
               
               
                 5 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 Comparative 
                 5.45 
                 50 
                 3910 
                 7.82 
                 453 
                 Blue 
               
               
                 Example 
                 Compound 1 
                   
                   
                   
                   
                   
                   
               
               
                 1 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 Comparative 
                 6.85 
                 50 
                 3860 
                 7.72 
                 480 
                 Blue 
               
               
                 Example 
                 Compound 4 
                   
                   
                   
                   
                   
                   
               
               
                 2 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 Comparative 
                 5.22 
                 50 
                 4556 
                 9.11 
                 515 
                 Green 
               
               
                 Example 
                 Compound 5 
                   
                   
                   
                   
                   
                   
               
               
                 3 
               
               
                   
               
            
           
         
       
     
     From Table 3, it is confirmed that the green organic light-emitting devices of Examples 1 and 2, in which the compounds according to one or more embodiments are used in an emission layer as a dopant, have an improved driving voltage and increased luminescence efficiency, as compared with the green organic light-emitting device of Comparative Example 3. 
     It is confirmed that the blue organic light-emitting devices of Examples 3 to 5, in which the compounds according to one or more embodiments are used in an emission layer as a dopant, have improved luminescence efficiency, as compared with the blue organic light-emitting device of Comparative Example 1, and have a low driving voltage and high luminescence efficiency, as compared with the blue organic light-emitting device of Comparative Example 2. 
     That is, when the compounds according to one or more embodiments are used in the organic light-emitting device, high color purity may be implemented, and a suitable effect may be exhibited in terms of driving voltage and efficiency. 
     The organic light-emitting device including the organometallic compound may have a low driving voltage, high efficiency, a long lifespan, and high color purity. 
     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.