Patent Publication Number: US-2021193927-A1

Title: Heterocyclic compound, organic light-emitting device including heterocyclic compound, and electronic device including organic light-emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2019-0170043, filed on Dec. 18, 2019, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     One or more aspects of embodiments of the present disclosure relate to a heterocyclic compound, an organic light-emitting device including the heterocyclic compound, and an electronic apparatus including the organic light-emitting device. 
     2. Description of Related Art 
     Organic light-emitting devices (OLEDs) are self-emission devices that, as compared with related devices, have wide viewing angles, high contrast ratios, short response times, and excellent characteristics in terms of brightness, driving voltage, and/or response speed, and can produce full-color images. 
     OLEDs may include a first electrode on a substrate, and a hole transport region, an emission layer, an electron transport region, and a second electrode sequentially stacked on the first electrode. Holes provided from the first electrode may move toward the emission layer through the hole transport region, and electrons provided from the second electrode may move toward the emission layer through the electron transport region. Carriers, such as holes and electrons, recombine in the emission layer to produce excitons. These excitons transition from an excited state to a ground state to thereby generate light. 
     SUMMARY 
     One or more aspects of embodiments of the present disclosure are directed toward a heterocyclic 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 of the disclosure. 
     According to one or more embodiments, a heterocylic compound may be represented by Formula 1: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 1, 
     rings A 11  to A 13 , rings A 21  to A 23 , and rings A 31  to A 33  may each independently be a C 5 -C 30  carbocyclic group or a C 2 -C 30  heterocyclic group, 
     X 11  may be O, S, N(R 11a ), C(R 11a )(R 11b ), or Si(R 11a )(R 11b ), 
     X 21  may be O, S, N(R 21a ), C(R 21a )(R 21b ), or Si(R 21a )(R 21b ), 
     X 31  may be O, S, N(R 31a ), C(R 31a )(R 31b ), or Si(R 31a )(R 31b ), 
     a2 may be 0 or 1, and when a2 is 0, a corresponding boron atom may not be present, 
     b2 may be 0 or 1, and when b2 is 0, X 21  may not be present, 
     c2 may be 0 or 1, and when c2 is 0, ring A 23  may not be present, 
     a3 may be 0 or 1, and when a3 is 0, a corresponding boron atom may not be present, 
     b3 may be 0 or 1, and when b3 is 0, X 31  may not be present, 
     c3 may be 0 or 1, and when c3 is 0, ring A 33  may not be present, 
     R 11a , R 11b , R 21a , R 21b , R 31a , R 31b , R 11  to R 13 , R 21  to R 23 , and R 31  to R 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 hydrazine group, a hydrazone group, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 2 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 2 -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 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 ), 
     d11 to d13, d21 to d23, and d31 to d33 may each independently be an integer from 0 to 10, 
     at least two selected from R 11a , R 11b , R 21a , R 21b , R 31a , R 31b , R 11  to R 13 , R 21  to R 23 , and R 31  to R 33  may optionally be bound to form a C 5 -C 30  carbocyclic group that is unsubstituted or substituted with at least one R 10a , or a C 2 -C 30  heterocyclic group that is unsubstituted or substituted with at least one R 10a , 
     R 10a  may be the same as described in connection with R 11  provided herein, and 
     at least one substituent of the substituted C 1 -C 60  alkyl group, the substituted C 2 -C 60  alkenyl group, the substituted C 2 -C 60  alkynyl group, the substituted C 1 -C 60  alkoxy group, the substituted C 3 -C 10  cycloalkyl group, the substituted C 1 -C 10  heterocycloalkyl group, the substituted C 3 -C 10  cycloalkenyl group, the substituted C 1 -C 10  heterocycloalkenyl group, the substituted C 6 -C 60  aryl group, the substituted C 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 (-D), —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone 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 hydrazine group, a hydrazone 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, each independently unsubstituted or 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 hydrazine group, a hydrazone 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 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 21 )(Q 22 )(Q 23 ), —N(Q 21 )(Q 22 ), —B(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O) 2 (Q 21 ), and —P(═O)(Q 21 )(Q 22 ); and 
     —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein Q 1  to Q 3 , Q 11  to Q 13 , Q 21  to Q 23 , and Q 31  to Q 33  may each independently be selected from: hydrogen; deuterium; —F; —Cl; —Br; —I; a hydroxyl group; a cyano group; a nitro group; an amidino group; a hydrazine group; a hydrazone 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 C 1 -C 60  alkyl group substituted with at least one selected from deuterium, —F, and a cyano group; a C 6 -C 60  aryl group substituted with at least one selected from deuterium, —F, and a cyano group; a biphenyl group; and a terphenyl group. 
     According to one or more embodiments, an organic light-emitting device may include: 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 heterocyclic compound of the present embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the disclosure will become more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of an embodiment of an organic light-emitting device; 
         FIG. 2  is a schematic cross-sectional view of an embodiment of an organic light-emitting device; 
         FIG. 3  is a schematic cross-sectional view of an embodiment of an organic light-emitting device; and 
         FIG. 4  is a schematic cross-sectional view of an embodiment of an organic light-emitting device. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in more detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Throughout the disclosure, the expression “at least one of a, b or c” indicates only a, only b, only c, both a and b, both a and c, both b and c, all of a, b, and c, or variations thereof. Expressions such as “at least one of,” “one of,” and “selected from,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. Further, the use of “may” when describing embodiments of the present disclosure refers to “one or more embodiments of the present disclosure.” 
     A heterocyclic compound may be represented by Formula 1: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 1, rings A 11  to A 13 , rings A 21  to A 23 , and rings A 31  to A 33  may each independently be a C 5 -C 30  carbocyclic group or a C 2 -C 30  heterocyclic group. 
     In some embodiments, rings A 11  to A 13 , A 21  to A 23 , and A 31  to A 33  may each independently be a benzene group, a naphthalene group, an anthracene group, a phenanthrene group, a triphenylene group, a pyrene group, a chrysene group, a cyclopentadiene group, a 1,2,3,4-tetrahydronaphthalene group, a thiophene group, a furan group, an indole group, a benzoborole group, a benzophosphole group, an indene group, a benzosilole group, a benzogermole group, a benzothiophene group, a benzoselenophene group, a benzofuran group, a carbazole group, a dibenzoborole group, a dibenzophosphole group, a fluorene group, a dibenzosilole group, a dibenzogermole group, a dibenzothiophene group, a dibenzoselenophenegroup, a dibenzofuran group, a dibenzothiophene 5-oxide group, a 9H-fluorene-9-one group, a dibenzothiophene 5,5-dioxide group, an azaindole group, an azabenzoborole group, an azabenzophosphole group, an azaindene group, an azabenzosilole group, an azabenzogermole group, an azabenzothiophene group, an azabenzoselenophene group, an azabenzofuran group, an azacarbazole group, an azadibenzoborole group, an azadibenzophosphole group, an azafluorene group, an azadibenzosilole group, an azadibenzogermole group, an azadibenzothiophene group, an azadibenzoselenophene group, an azadibenzofuran group, an azadibenzothiophene 5-oxide group, an aza-9H-fluoren-9-one group, an azadibenzothiophene 5,5-dioxide group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a quinoline group, an isoquinoline group, a quinoxaline group, a quinazoline group, a phenanthroline group, a pyrrole group, a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isooxazole group, a thiazole group, an isothiazole group, an oxadiazole group, a thiadiazole group, a benzopyrazole group, a benzimidazole group, a benzoxazole group, a benzothiazole group, a benzoxadiazole group, a benzothiadiazole group, a 5,6,7,8-tetrahydroisoquinoline group, or a 5,6,7,8-tetrahydroquinoline group. 
     In one or more embodiments, ring A 13  may be a benzene group, a naphthalene group, a carbazole group, a fluorene group, a dibenzothiophene group, or a dibenzofuran group. 
     In one or more embodiments, rings A 13 , A 23 , and A 33  may each independently be a benzene group, a naphthalene group, a carbazole group, a fluorene group, a dibenzothiophene group, or a dibenzofuran group. 
     In some embodiments, rings A 11  to A 13 , A 21  to A 23 , and A 31  to A 33  may each independently be a benzene group, a naphthalene group, a carbazole group, a fluorene group, a dibenzothiophene group, or a dibenzofuran group. 
     In some embodiments, rings A 11 , A 12 , A 21 , A 22 , A 31 , and A 32  may each be a benzene group, and rings A 13 , A 23 , and A 33  may each independently be a benzene group, a naphthalene group, a carbazole group, a fluorene group, a dibenzothiophene group, or a dibenzofuran group. 
     In Formula 1, X 11  may be O, S, N(R 11a ), C(R 11a )(R 11b ), or Si(R 11a )(R 11b )X 21  may be O, S, N(R 21a ), C(R 21a )(R 21b ), or Si(R 21a )(R 21b ), and X 31  may be O, S, N(R 31a ), C(R 31a )(R 31b ), or Si(R 31a )(R 31b ). 
     In Formula 1, 
     a2 may be 0 or 1, and when a2 is 0, a corresponding boron atom may not be present, 
     b2 may be 0 or 1, and when b2 is 0, X 21  may not be present, 
     c2 may be 0 or 1, and when c2 is 0, ring A 23  may not be present, 
     a3 may be 0 or 1, and when a3 is 0, a corresponding boron atom may not be present, 
     b3 may be 0 or 1, and when b3 is 0, X 31  may not be present, and 
     c3 may be 0 or 1, and when c3 is 0, ring A 33  may not be present. 
     In some embodiments, a2, b2, and c2 may be identical to one another, and a3, b3, and c3 may be identical to one another. 
     In some embodiments, a2, b2, c2, a3, b3, and c3 may each be 0, or 
     a2, b2, and c2 may each be 1, and a3, b3, and c3 may each be 0, or 
     a2, b2, and c2 may each be 0, and a3, b3, and c3 may each be 1, or 
     a2, b2, c2, a3, b3, and c3 may each be 1. 
     In an embodiment, in Formula 1, b2 may be 1, and X 11  may be identical to X 21 . In some embodiments, in Formula 1, 1) b2 may be 1; and 2) i) X 11  and X 21  may both be O or S, or ii) X 11  may be N(R 11a ), and X 21  may be N(R 21a ). 
     In one or more embodiments, in Formula 1, b2 may be 1, and X 11  and X 21  may be different from each other. 
     In some embodiments, in Formula 1, 1) b2 may be 1; and 2) X 11  may be O, and X 21  may be S. 
     In one or more embodiments, in Formula 1, b2 and b3 may each be 1, and X 11 , X 21 , and X 31  may be identical to one another. 
     In some embodiments, in Formula 1, 1) b2 and b3 may each be 1; and 2) i) X 21 , and X 31  may all be O or S, or ii) X 11  may be N(R 11a ), X 21  may be N(R 21a ), and X 31  may be N(R 31a ). 
     In one or more embodiments, in Formula 1, b2 and b3 may each be 1, and i) X 11  may be different from X 21 , ii) X 21  may be different from X 31 , and/or iii) X 31  may be different from X 11 . 
     In some embodiments, in Formula 1, 1) b2 and b3 may each be 1; and 2) may be O, X 21  may be C(R 21a )(R 21b ), and X 31  may be S. 
     In an embodiment, in Formula 1, a group represented by 
     
       
         
         
             
             
         
       
     
     may be represented by one of Formulae 2-1 to 2-10: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Formulae 2-1 to 2-10, 
     ring A 11 , ring A 12 , X 11 , R 11  to R 13 , d11, and d12 may each independently be the same as described herein, 
     X 12  may be O, S, N(R 12a ), C(R 12a )(R 12b ), or Si(R 12a )(R 12b ), wherein R 12a  and R 12b  may each independently be the same as described in connection with R 11a  and R 11b  provided herein, 
     e14 may be an integer from 0 to 4, 
     e16 may be an integer from 0 to 6, and 
     * and *′ may each indicate a binding site to a carbon atom in a benzene group of Formula 1. 
     In one or more embodiments, the heterocyclic compound represented by Formula 1 may be represented by one of Formulae 1-1 to 1-3: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 1-1 to 1-3, rings A 11  to A 13 , rings A 21  to A 23 , rings A 31  to A 33 , X 11 , X 21 , X 31 , R 11  to R 13 , R 21  to R 23 , R 31  to R 33 , d11 to d13, d21 to d23, and d31 to d33 may each independently be the same as described in connection with Formula 1. 
     In Formulae 1, 2-1 to 2-10, and 1-1 to 1-3, R 11a , R 11b , R 21a , R 21b , R 31a , R 31b , R 11  to R 13 , R 21  to R 23 , and R 31  to R 33  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a 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 2 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 2 -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 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 ), wherein Q 1  to Q 3  may respectively be understood by referring to the descriptions of Q 1  to Q 3  provided herein. 
     In some embodiments, R 11a , R 11b , R 21a , R 21b , R 31a , R 31b , R 11  to R 13 , R 21  to R 23 , and R 31  to R 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 hydrazine group, a hydrazone 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, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a C 1 -C 10  alkyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a naphthyl group, a pyridinyl group, and a pyrimidinyl group; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a C 1 -C 10  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an oxadiazolyl group, a triazinyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, an azafluorenyl group, and an azadibenzosilolyl group, each substituted with at least one selected from deuterium, —F, —C, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazine group, a hydrazone group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbomanyl group, a norbomenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a C 1 -C 10  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an oxadiazolyl group, a triazinyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —P(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ); and 
     —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 ), 
     wherein Q 1  to Q 3  and Q 31  to Q 33  may each independently be selected from: 
     —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, and —CD 2 CDH 2 ; and 
     an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a phenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, and a triazinyl group, each independently unsubstituted or substituted with at least one selected from deuterium, a C 1 -C 10  alkyl group, a phenyl group, a biphenyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, and a triazinyl group. 
     In some embodiments, R 11a , R 11b , R 21a , R 21b , R 31a , R 31b , R 11  to R 13 , R 21  to R 23 , and R 31  to R 33  may each independently be selected from: 
     hydrogen, deuterium, 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, —CD 3 , —CD 2 H, —CDH 2 , C 1 -C 10  alkyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, and a naphthyl group; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a C 1 -C 10  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, and a dibenzocarbazolyl group, each independently unsubstituted or substituted with at least one selected from deuterium, —CD 3 , —CD 2 H, —CDH 2 , a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a C 1 -C 10  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, —Si(Q 31 )(Q 32 )(Q 33 ), and —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ); and —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ) and —B(Q 1 )(Q 2 ), 
     wherein Q 1  to Q 3  and Q 31  to Q 33  may each independently be selected from: 
     —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, and —CD 2 CDH 2 ; and 
     an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a phenyl group, and a naphthyl group, each independently unsubstituted or substituted with at least one selected from deuterium, a C 1 -C 10  alkyl group, a phenyl group, and a biphenyl group. 
     In Formula 1, d11 to d13, d21 to d23, and d31 to d33 may each independently indicate the number of R 11 (s) to R 13 (s), R 21 (s) to R 23 (s), and R 31 (s) to R 33 (s). d11 to d13, d21 to d23, and d31 to d33 may each independently be an integer from 0 to 10. 
     When d11 is 2 or greater, at least two R 11 (s) may be identical to or different from each other, when d12 is 2 or greater, at least two R 12 (s) may be identical to or different from each other, and when d13 is 2 or greater, at least two R 13 (s) may be identical to or different from each other, 
     when d21 is 2 or greater, at least two R 21 (s) may be identical to or different from each other, when d22 is 2 or greater, at least two R 22 (s) may be identical to or different from each other, and when d23 is 2 or greater, at least two R 23 (s) may be identical to or different from each other, and 
     when d31 is 2 or greater, at least two R 31 (s) may be identical to or different from each other, when d32 is 2 or greater, at least two R 32 (s) may be identical to or different from each other, and when d33 is 2 or greater, at least two R 33 (s) may be identical to or different from each other. 
     In Formula 1, at least two selected from R 11a , R 11b , R 21a , R 21b , R 31a , R 31b , R 11  to R 13 , R 21  to R 23 , and R 31  to R 33  may optionally be bound to form a C 5 -C 30  carbocyclic group that is unsubstituted or substituted with at least one R 10a , or a C 2 -C 30  heterocyclic group that is unsubstituted or substituted with at least one R 10a , and 
     R 10a  may be the same as described in connection with R 11  provided herein. 
     In an embodiment, the heterocyclic compound may satisfy at least one of Conditions 1 to 3: 
     Condition 1 
     X 11  may be N(R 11a ), and 
     R 11a  may be bound to R 13  to form a C 5 -C 30  carbocyclic group that is unsubstituted or substituted with at least one R 10a , or a C 2 -C 30  heterocyclic group that is unsubstituted or substituted with at least one R 10a ; 
     Condition 2 
     a2, b2, and c2 may each be 1, 
     X 21  may be N(R 21a ), and 
     R 21a  may be bound to R 23  to form a C 5 -C 30  carbocyclic group that is unsubstituted or substituted with at least one R 10a , or a C 2 -C 30  heterocyclic group that is unsubstituted or substituted with at least one R 10a ; and 
     Condition 3 
     a3, b3, and c3 may each be 1, 
     X 31  may be N(R 31a ), and 
     R 31a  may be bound to R 33  to form a C 5 -C 30  carbocyclic group that is unsubstituted or substituted with at least one R 10a , or a C 2 -C 30  heterocyclic group that is unsubstituted or substituted with at least one R 10a . 
     In an embodiment, in Formula 1, a group represented by 
     
       
         
         
             
             
         
       
     
     may be represented by Formula 3-1: 
     
       
         
         
             
             
         
       
     
     In Formula 3-1, ring A 11 , ring A 12 , R 11  to R 13 , and d11 to d13 may each independently be the same as described herein, 
     ring A 14  may be a C 5 -C 30  carbocyclic group or a C 2 -C 30  heterocyclic group, 
     R 14  may be the same as described in connection with R 11  provided herein, 
     d14 may be an integer from 0 to 10, 
     X 13  may be a single bond, O, S, N(R 13a ), C(R 13a )(R 13b ), or Si(R 13a )(R 13b ), wherein R 13a  and R 13b  may each independently be the same as described in connection with R 11a  and R 11b  provided herein, and 
     * and *′ may each indicate a binding site to a carbon atom in a benzene group of Formula 1. 
     In an embodiment, the heterocyclic compound may satisfy at least one of Conditions 1A to 3A: 
     Condition 1A 
     a group represented by 
     
       
         
         
             
             
         
       
     
     in Formula 1 may be represented by Formula 3-1: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 3-1, 
     rings A 11  to A 13 , R 11  to R 13 , and d11 to d13 may each independently be the same as described herein, 
     ring A 14  may be a C 5 -C 30  carbocyclic group or a C 2 -C 30  heterocyclic group, 
     R 14  may be the same as described in connection with R 11  provided herein, 
     d14 may be an integer from 0 to 10, 
     X 13  may be a single bond, O, S, N(R 13a ), C(R 13a )(R 13b ), or Si(R 13a )(R 13b ), wherein R 13a  and R 13b  may each independently be the same as described in connection with R 11a  and R 11b  provided herein, and 
     * and *′ may each indicate a binding site to a carbon atom in a benzene group of Formula 1; 
     Condition 2A 
     a group represented by 
     
       
         
         
             
             
         
       
     
     in Formula 1 may be represented by Formula 3-2: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 3-2, 
     rings A 21  to A 23 , R 21  to R 23 , and d21 to d23 may each independently be the same as described herein, 
     ring A 24  may be a C 5 -C 30  carbocyclic group or a C 2 -C 30  heterocyclic group, 
     R 24  may be the same as described in connection with R 21  provided herein, 
     d24 may be an integer from 0 to 10, 
     X 23  may be a single bond, O, S, N(R 23a ), C(R 23a )(R 23b ), or Si(R 23a )(R 23b ), wherein R 23a  and R 23b  may each independently be the same as described in connection with R 21a  and R 21b  provided herein, and 
     * and *′ may each indicate a binding site to a carbon atom in a benzene group of Formula 1. 
     Condition 3A 
     a group represented by 
     
       
         
         
             
             
         
       
     
     in Formula 1 may be represented by Formula 3-3: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 3-3, 
     rings A 31  to A 33 , R 31  to R 33 , and d31 to d33 may each independently be the same as described herein, 
     ring A 34  may be a C 5 -C 30  carbocyclic group or a C 2 -C 30  heterocyclic group, 
     R 34  may be the same as described in connection with R 31  provided herein, 
     d34 may be an integer from 0 to 10, 
     X 33  may be a single bond, O, S, N(R 33a ), C(R 33a )(R 33b ), or Si(R 33a )(R 33b ), wherein R 33a  and R 33b  may each independently be the same as described in connection with R 31a  and R 31b  provided herein, and 
     * and *′ may each indicate a binding site to a carbon atom in a benzene group of Formula 1. 
     In the heterocyclic compound represented by Formula 1, a difference between a singlet energy level and a triplet energy level may be 0.3 electron volts (eV) or lower, for example, about 0 eV to about 0.3 eV. When the difference between a singlet energy level and a triplet energy level in the heterocyclic compound is within this range, the heterocyclic compound may emit delayed fluorescence, e.g., thermal activated delayed fluorescence (TADF). 
     In an embodiment, the heterocyclic compound may be selected from Compounds 1 to 31, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The heterocyclic compound represented by Formula 1 may have a structure in which at least one boron atom is included in a triindolobenzene-based backbone, as shown in Formula 1′: 
     
       
         
         
             
             
         
       
     
     As Formula 1 has a triindolobenzene-based backbone, due to a large planar structure having a condensed ring, multiple resonance may be more (e.g., better) activated, and delocalization in electrons may be expanded. Due to an increased polarizability, an f value may be further increased, thus allowing the heterocyclic compound to be used as a highly efficient delayed fluorescence emitting material. In addition, the triindolobenzene-based backbone includes a substituent condensed to a heterocyclic ring, thus having a smaller number of freely rotating C—N bonds than a substituent that is not so condensed. Accordingly, a molecule (e.g., a molecule of the compound of Formula 1) may be more rigid in view of a bond dissociation energy (BDE), thereby supplementing chemical instability (that may be a weak point due to properties of a boron atom) with an increased electron density. 
     As Formula 1 includes at least one boron atom, the highest occupied molecular orbital (HOMO)/the lowest unoccupied molecular orbital (LUMO) separation may be increased, thereby enhancing multiple resonance, e.g., increasing short range charge transfer (CT). Accordingly, a high oscillator strength may be achieved, and a low ΔE ST  as well, by separating a singlet and a triplet energy levels of a molecule. Therefore, luminescence efficiency may be improved, because a triplet state exciton is used for emission by facilitating reverse intersystem crossing and delayed fluorescence even at room temperature. In addition, when a molecule is transferred, probability of having a charge transfer characteristic in the transition of a triplet level (such as T 2  or T 3 ) higher than T 1  may be increased, and thus the LEST value may be further reduced. In particular, some of the heterocyclic compounds of the present embodiments and Compound A (DABNA-1) were subjected to discrete Fourier transform (DFT) calculation. As shown in Table 1, low ΔE ST  values and increased oscillator strength (OSC) may be obtained. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Dipole 
                   
                   
               
               
                 No. 
                 HOMO 
                 LUMO 
                 T1 (eV) 
                 S1 (eV) 
                 (Debye) 
                 OSC 
                 ΔE ST   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Compound 
                 −4.73 
                 −1.09 
                 2.62 
                 3.11 
                 2.55 
                 0.20 
                 0.492 
               
               
                 A 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound 
                 −4.941 
                 −1.647 
                 2.3882 
                 2.837 
                 2.947 
                 0.2046 
                 0.449 
               
               
                 1 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound 
                 −4.775 
                 −1.416 
                 2.4175 
                 2.871 
                 2.500 
                 0.3546 
                 0.454 
               
               
                 10 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound 
                 −5.108 
                 −1.814 
                 2.3488 
                 2.606 
                 2.729 
                 0.1613 
                 0.257 
               
               
                 16 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound 
                 −5.228 
                 −1.948 
                 2.3115 
                 2.602 
                 2.602 
                 0.0748 
                 0.290 
               
               
                 26 
               
               
                   
               
               
                 *Dipole: Criterion for molecular polarity. 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
            
           
         
       
     
     Therefore, an electronic device, e.g., an organic light-emitting device, applying the heterocyclic compound represented by Formula 1 may have a low driving voltage, high maximum quantum yield, high efficiency, and long lifespan. 
     Methods of synthesizing the heterocyclic compound represented by Formula 1 should be readily apparent to those of ordinary skill in the art by referring to Examples described herein. 
     At least one heterocyclic compound represented by Formula 1 may be included between a pair of electrodes in an organic light-emitting device. In some embodiments, the heterocyclic compound may be included in an emission layer. In some embodiments, the heterocyclic compound represented by Formula 1 may be used as a material for forming a capping layer, which may be positioned on outer sides of at least one of a pair of electrodes in an organic light-emitting device. 
     According to one or more embodiments, an organic light-emitting device may include: 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 heterocyclic compound represented by Formula 1. 
     As used herein, the expression the “(organic layer) includes at least one heterocyclic compound” may be construed as meaning the “(organic layer) may include one heterocyclic compound of Formula 1 or two different heterocyclic compounds of Formula 1”. 
     For example, the organic layer may include only Compound 1 as the heterocyclic compound. In this embodiment, Compound 1 may be included in the emission layer of the organic light-emitting device. In some embodiments, the organic layer may include Compounds 1 and 2 as the heterocyclic compounds. In this embodiment, Compounds 1 and 2 may be included in the same layer (for example, both Compounds 1 and 2 may be included in an emission layer) or in different layers (for example, Compound 1 may be included in an emission layer, and Compound 2 may be included in an electron transport layer). 
     In some embodiments, the first electrode may be an anode, 
     the second electrode may be a cathode, and 
     the organic layer may include the heterocyclic compound, and 
     the organic layer may further include a hole transport region between the first electrode and the emission layer, and an electron transport region between the emission layer and the second electrode. 
     In some embodiments, the hole transport region may include a hole injection layer, a hole transport layer, an emission auxiliary layer, an electron blocking layer, or a combination thereof, and 
     the electron transport region may include a hole blocking layer, an electron transport layer, an electron injection layer, or a combination thereof. 
     In some embodiments, the emission layer may include the heterocyclic compound. 
     In some embodiments, the emission layer may include a host and a dopant, 
     the host may be different from the dopant, 
     a content (e.g., amount) of the host may be greater than a content (e.g., amount) of the dopant, and 
     the dopant may include the heterocyclic compound. 
     In some embodiments, the emission layer may include a dopant and a host, 
     The host may include at least one heterocyclic compound. 
     In some embodiments, the emission layer may emit blue light or blue-green light. 
     In some embodiments, the heterocyclic compound may emit blue light or blue-green light having a maximum emission wavelength in a range of about 400 nanometers (nm) to about 500 nm. 
     In some embodiments, an electronic apparatus may include the organic light-emitting device. 
     In some embodiments, the electronic apparatus may include a thin-film transistor, the thin-film transistor may include a source electrode and a drain electrode, and the first electrode of the organic light-emitting device may be electrically coupled to the source electrode or the drain electrode. 
     The term “organic layer” as used herein refers to a single layer and/or a plurality of layers between the first electrode and the second electrode in an organic light-emitting device. A material included in the “organic layer” is not limited to an organic material. 
     In some embodiments, the organic light-emitting device may have i) a first electrode/organic layer/second electrode/second capping layer structure, ii) a first capping layer/first electrode/organic layer/second electrode structure, or iii) a first capping layer/first electrode/organic layer/second electrode/second capping layer structure, wherein the layers of each structure are sequentially stacked in each stated order. At least one of the first capping layer or the second capping layer may include the heterocyclic compound. 
     Description of FIG.  1   
       FIG. 1  illustrates a schematic cross-sectional view of an organic light-emitting device  10  according to an embodiment. The organic light-emitting device  10  may include 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 an organic light-emitting device according to an embodiment will be described in connection with  FIG. 1 . 
     First Electrode  110   
     In  FIG. 1 , a substrate may be additionally located under the first electrode  110  or above the second electrode  190 . The substrate may be a glass substrate and/or a plastic substrate, each having excellent mechanical strength, thermal stability, transparency, surface smoothness, ease of handling, and/or water resistance. 
     The first electrode  110  may be formed by depositing or sputtering, onto the substrate, a material for forming the first electrode  110 . 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 that facilitate hole injection. 
     The first electrode  110  may be a reflective electrode, a semi-transmissive electrode, or a transmissive electrode. When the first electrode  110  is a transmissive electrode, a material for forming the first electrode  110  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 are not limited thereto. In some embodiments, when the first electrode  110  is a semi-transmissive electrode or a reflective electrode, as a material for forming the first electrode  110 , at least one 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 may be used, but embodiments are not limited thereto. 
     The first electrode  110  may have a single-layered structure, or a multi-layered structure including two or more layers. In some embodiments, the first electrode  110  may have a triple-layered structure of ITO/Ag/ITO, but embodiments are not limited thereto. 
     Organic Layer  150   
     The organic layer  150  may be 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 consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer including a plurality of different materials, or iii) a multi-layered structure having a plurality of layers including a plurality of different materials. 
     The hole transport region may include at least one 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, e.g., a hole injection layer/hole transport layer structure, a hole injection layer/hole transport layer/emission auxiliary layer structure, a hole injection layer/emission auxiliary layer structure, a hole transport layer/emission auxiliary layer structure, or a hole injection layer/hole transport layer/electron blocking layer structure, wherein the layers of each structure are sequentially stacked on the first electrode  110  in each stated order, but embodiments are not limited thereto. 
     The hole transport region may include at least one selected from m-MTDATA, TDATA, 2-TNATA, NPB (NPD), β-NPB, TPD, a spiro-TPD, a 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), CzSi (9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole), a compound represented by Formula 201, and a compound represented by Formula 202: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein, in Formulae 201 and 202, 
     L 201  to L 204  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     L 205  may be selected from *—O—*′, *—S—*′, *—N(Q 201 )—*′, a substituted or unsubstituted C 1 -C 20  alkylene group, a substituted or unsubstituted C 2 -C 20  alkenylene group, a substituted or unsubstituted C 3 -C 10  cycloalkylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkylene group, a substituted or unsubstituted C 3 -C 10  cycloalkenylene group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenylene group, a substituted or unsubstituted C 6 -C 60  arylene group, a substituted or unsubstituted C 1 -C 60  heteroarylene group, a substituted or unsubstituted divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group, 
     xa1 to xa4 may each independently be an integer from 0 to 3, 
     xa5 may be an integer from 1 to 10, and 
     R 201  to R 204  and Q 201  may each independently be selected from a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group. 
     In some embodiments, in Formula 202, R 201  and R 202  may optionally be bound via a single bond, a dimethyl-methylene group, and/or a diphenyl-methylene group, and R 203  and R 204  may optionally be bound via a single bond, a dimethyl-methylene group, and/or a diphenyl-methylene group. 
     In an embodiment, in Formulae 201 and 202, 
     L 201  to L 205  may each independently be selected from: 
     a phenylene group, a pentalenylene group, an indenylene group, a naphthylene group, an azulenylene group, a heptalenylene group, an indacenylene group, an acenaphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenalenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a naphthacenylene group, a picenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a rubicenylene group, a coronenylene group, an ovalenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, and a pyridinylene group; and 
     a phenylene group, a pentalenylene group, an indenylene group, a naphthylene group, an azulenylene group, a heptalenylene group, an indacenylene group, an acenaphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenalenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a naphthacenylene group, a picenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a rubicenylene group, a coronenylene group, an ovalenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, and a pyridinylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, a biphenyl group, a terphenyl group, a phenyl group substituted with a C 1 -C 10  alkyl group, a phenyl group substituted with —F, a pentalenyl group, an indenyl group, a naphthyl group, an azulenyl group, a heptalenyl group, an indacenyl group, an acenaphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a naphthacenyl group, a picenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, an ovalenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), and —N(Q 31 )(Q 32 ), 
     wherein Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     In one or more 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 ), 
     wherein Q 31  to Q 33  may respectively be understood by referring to the descriptions of Q 31  to Q 33  provided herein. 
     In one or more embodiments, in Formula 201, at least one of R 201  to R 203  may 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 are not limited thereto. 
     In one or more embodiments, in Formula 202, i) R 201  and R 202  may be bound via a single bond, and/or ii) R 203  and R 204  may be bound via a single bond. 
     In one or more embodiments, in Formula 202, at least one of R 201  to R 204  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 are not limited thereto. 
     The compound represented by Formula 201 may be represented by Formula 201-1: 
     
       
         
         
             
             
         
       
     
     In some embodiments, the compound represented by Formula 201 may be represented by Formula 201-2, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In some embodiments, the compound represented by Formula 201 may be represented by Formula 201-2(1), but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     The compound represented by Formula 201 may be represented by Formula 201A: 
     
       
         
         
             
             
         
       
     
     In some embodiments, the compound represented by Formula 201 may be represented by Formula 201A(1), but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In some embodiments, the compound represented by Formula 201 may be represented by Formula 201A-1, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     In some embodiments, the compound represented by Formula 202 may be represented by Formula 202-1: 
     
       
         
         
             
             
         
       
     
     In one or more embodiments, the compound represented by Formula 202 may be represented by Formula 202-1(1): 
     
       
         
         
             
             
         
       
     
     In some embodiments, the compound represented by Formula 202 may be represented by Formula 202A: 
     
       
         
         
             
             
         
       
     
     In some embodiments, the compound represented by Formula 202 may be represented by Formula 202A-1: 
     
       
         
         
             
             
         
       
     
     In Formulae 201-1, 201-2, 201-2(1), 201A, 201A(1), 201A-1, 202-1, 202-1(1), 202A, and 202A-1, 
     L 201  to L 203 , xa1 to xa3, xa5, and R 202  to R 204  may respectively be understood by referring to the description of L 201  to L 203 , xa1 to xa3, xa5, and R 202  to R 204  provided herein, 
     L 205  may be selected from a phenylene group and a fluorenylene group, 
     X 211  may be selected from O, S, and N(R 211 ), 
     X 212  may be selected from O, S, and N(R 212 ), 
     R 211  and R 212  may each be understood by referring to the description of R 203  provided herein, 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 HT48, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The thickness of the hole transport region may be in a range of about 100 (Angstroms) Å to about 10,000 Å, and in some embodiments, about 100 Å to about 1,000 Å. When the hole transport region includes at least one selected from a hole injection layer and a hole transport layer, the thickness of the hole injection layer may be in a range of about 100 Å to about 9,000 Å, and in some embodiments, about 100 Å to about 1,000 Å, and the thickness of the hole transport layer may be in a range of about 50 Å to about 2,000 Å, and in some embodiments, 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 each independently within any of these ranges, excellent (or improved) hole transport characteristics may be obtained without a substantial increase in driving voltage. 
     The emission auxiliary layer may increase light emission efficiency by compensating for an optical resonance distance according to the wavelength of light emitted by an emission layer. The electron blocking layer may reduce or eliminate the flow of electrons from an electron transport region. The emission auxiliary layer and the electron blocking layer may each independently include any of the aforementioned materials. 
     p-Dopant 
     The hole transport region may include a charge generating material, as well as any of the aforementioned materials, to improve conductive properties of the hole transport region. The charge generating material may be substantially homogeneously or non-homogeneously dispersed in the hole transport region. 
     The charge generating material may include, for example, a p-dopant. 
     In some embodiments, the lowest unoccupied molecular orbital (LUMO) energy level of the p-dopant may be −3.5 eV or less. 
     The p-dopant may include at least one selected from a quinone derivative, a metal oxide, and a cyano group-containing compound, but embodiments are not limited thereto. 
     In some embodiments, the p-dopant may include at least one selected from: 
     a quinone derivative, such as tetracyanoquinodimethane (TCNQ) and/or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ); 
     a metal oxide, such as tungsten oxide and/or molybdenum oxide; 
     1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN); and 
     a compound represented by Formula 221, 
     but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 221, 
     R 221  to R 223  may each independently be 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 include 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. The stacked structure may include two or more layers selected from a red emission layer, a green emission layer, and a blue emission layer. The two or more layers may be in direct contact with each other. In some embodiments, the two or more layers may be separated from each other. In one or more embodiments, the emission layer may include two or more materials. The two or more materials may include a red light-emitting material, a green light-emitting material, or a blue light-emitting material. The two or more materials may be mixed with each other in a single layer. The two or more materials mixed with each other in the single layer may emit white light. 
     The emission layer may include the heterocyclic compound represented by Formula 1. 
     The emission layer may include a host and a luminescent material. The luminescent material may include at least one of a phosphorescent dopant, a fluorescent dopant, or a quantum dot. 
     The amount of the dopant in the emission layer may be, for example, in a range of about 0.01 parts to about 15 parts by weight based on 100 parts by weight of the host, but embodiments are not limited thereto. 
     In some embodiments, the emission layer may emit blue light or blue-green light. 
     In some embodiments, the heterocyclic compound emits blue light or blue-green light having a maximum emission wavelength in a range of about 400 nm to about 500 nm. 
     The thickness of the emission layer may be in a range of about 100 Å to about 1,000 Å, and in some embodiments, about 200 Å to about 600 Å. When the thickness of the emission layer is within any of these ranges, improved luminescence characteristics may be obtained without a substantial increase in driving voltage. 
     Host in Emission Layer 
     The host may be different from the dopant, and a content (e.g., amount) of the host may be greater than a content (e.g., amount) of the dopant, and the host may include the heterocyclic compound represented by Formula 1. 
     The host may further include a compound represented by Formula 301: Formula 301 
       [Ar 301 ] xb11 -[(L 301 ) xb1 -R 301 ] xb21    
     wherein, in Formula 301, 
     Ar 301  may be selected from a substituted or unsubstituted C 5 -C 60  carbocyclic group and 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 ), and 
     xb21 may be an integer from 1 to 5, 
     wherein 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 are not limited thereto. 
     In some embodiments, in Formula 301, Ar 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 ), 
     wherein Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group, but embodiments are not limited thereto. 
     When xb11 in Formula 301 is 2 or greater, at least two Ar 301 (s) may be bound 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: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 301-1 to 301-2, 
     A 301  to A 304  may each independently be selected from a benzene group, a naphthalene group, a phenanthrene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a pyridine group, a pyrimidine group, an indene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, an indole group, a carbazole group, a benzocarbazole group, a dibenzocarbazole group, a furan group, a benzofuran group, a dibenzofuran group, a naphthofuran group, a benzonaphthofuran group, a dinaphthofuran group, a thiophene group, a benzothiophene group, a dibenzothiophene group, a naphthothiophene group, a benzonapthothiophene group, and a dinaphthothiophene group, 
     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 respectively be understood by referring to the descriptions of L 301 , xb1, R 301 , and Q 31  to Q 33  provided herein, 
     L 302  to L 304  may each be understood by referring to the descriptions of L 301  provided herein, 
     xb2 to xb4 may each be understood by referring to the descriptions of xb1 provided herein, and 
     R 302  to R 304  may each be understood by referring to the descriptions of R 301  provided herein. 
     In some embodiments, in Formulae 301, 301-1, and 301-2, L 301  to L 304  may each independently be selected from: 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group; and 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-bifluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a fluoranthenylene group, a triphenylenylene group, a pyrenylene group, a chrysenylene group, a perylenylene group, a pentaphenylene group, a hexacenylene group, a pentacenylene group, a thiophenylene group, a furanylene group, a carbazolylene group, an indolylene group, an isoindolylene group, a benzofuranylene group, a benzothiophenylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a dibenzosilolylene group, a pyridinylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a thiadiazolylene group, an oxadiazolylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a triazinylene group, a quinolinylene group, an isoquinolinylene group, a benzoquinolinylene group, a phthalazinylene group, a naphthyridinylene group, a quinoxalinylene group, a quinazolinylene group, a cinnolinylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an imidazopyridinylene group, an imidazopyrimidinylene group, and an azacarbazolylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azacarbazolyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein Q 31  to Q 33  may respectively be understood by referring to the descriptions of Q 31  to Q 33  provided herein. 
     In some embodiments, in Formulae 301, 301-1, and 301-2, R 301  to R 304  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 ), 
     wherein Q 31  to Q 33  may respectively be understood by referring to the descriptions of Q 31  to Q 33  provided herein. 
     In some embodiments, the host may include an alkaline earth metal complex. For example, the host may include a beryllium (Be) complex (e.g., Compound H55), a magnesium (Mg) complex, and/or a zinc (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 are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Phosphorescent Dopant Included in Emission Layer of Organic Layer  150   
     The phosphorescent dopant may include an organometallic complex represented by Formula 401: 
     
       
         
         
             
             
         
       
     
     wherein, 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; when xc1 is 2 or greater, at least two L 401 (s) may be identical to or different from each other, 
     L 402  may be an organic ligand, and xc2 may be an integer selected from 0 to 4; when xc2 is 2 or greater, at least two L 402 (s) may be identical to or different from each other, 
     X 401  to X 404  may each independently be a nitrogen or a carbon, 
     X 401  and X 403  may be bound to each other via a single bond or a double bond, X 402  and X 404  may be bound to each other via a single bond or a double bond, 
     A 401  and A 402  may each independently be a C 5 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, 
     X 405  may be a single bond, *—O—*′, *—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, 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 an embodiment, in Formula 402, A 401  and A 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. 
     In an embodiment, in Formula 402, 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 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 adamantyl group, a norbornanyl group, and a norbornenyl group; 
     a cyclopentyl group, a cyclohexyl group, an adamantyl 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 adamantyl 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 adamantyl 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 ), 
     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 phenyl group, a biphenyl group, and a naphthyl group, but embodiments are not limited thereto. 
     In one or more embodiments, when xc1 in Formula 401 is 2 or greater, two A 401 (s) of at least two L 401 (s) may optionally be linked via X 407  as a linking group; and/or two A 402 (s) may optionally be linked via X 408  as a linking group (see e.g., Compounds PD1 to PD4 and PD7). X 407  and X 408  may each independently be selected from a single bond, *—O—*′, *—S—*′, *—C(═O)—*′, *—N(Q 413 )-*′, *—C(Q 413 )(Q 414 )-*′, and *—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 are not limited thereto. 
     L 402  in Formula 401 may be any suitable monovalent, divalent, or trivalent organic ligand. For example, L 402  may be selected from halogen, diketone (e.g., acetylacetonate), a carboxylic acid (e.g., picolinate), —C(═O), isonitrile, —CN, and phosphorus (e.g., phosphine and/or phosphite), but embodiments are not limited thereto. 
     In some embodiments, the phosphorescent dopant may include, for example, at least one selected from Compounds PD1 to PD25, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Fluorescent Dopant in Emission Layer 
     In some embodiments, the fluorescent dopant may include a heterocyclic compound represented by Formula 1: 
     The fluorescent dopant may further include an arylamine compound and/or a styrylamine compound. 
     In some embodiments, the fluorescent dopant may further include a compound represented by Formula 501: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 501, 
     Ar 501  may be selected from a substituted or unsubstituted C 5 -C 60  carbocyclic group and 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 some embodiments, in Formula 501, Ar 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 an embodiment, in Formula 501, L 501  and L 503  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 an embodiment, in Formula 501, R 501  and R 502  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 ), 
     wherein Q 31  to Q 33  may 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 are not limited thereto. 
     In some embodiments, the fluorescent dopant may be selected from Compounds FD1 to FD22: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the fluorescent dopant may be selected from the following compounds, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
     
     Quantum Dot 
     The emission layer included in the organic light-emitting device of the present disclosure may include a quantum dot material. 
     The quantum dot is a particle having a crystal structure of several to tens of nanometers in size. The quantum dot may include hundreds to thousands of atoms. 
     Because the quantum dot is very small in size, quantum confinement effect may occur. The quantum confinement is a phenomenon in which a band gap of an object becomes larger when the object becomes smaller, such as when the object becomes smaller than or reaches a nanometer size. Accordingly, when light of a wavelength having an energy larger than a band gap of the quantum dot is incident on the quantum dot, the quantum dot is excited by absorbing the light, emits light of a specific wavelength, and falls to the ground state. In this case, the wavelength of the emitted light may have a value corresponding to the band gap. 
     A core of the quantum dot may include a Group II-VI compound, a Group III-VI compound, a Group III-V compound, a Group IV-VI compound, a Group IV element or compound, a Group compound, or a combination thereof. 
     The Group II-VI compound may be selected from a binary compound selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, MgS, and mixtures thereof; a ternary compound selected from the group consisting of CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, and mixtures thereof; and a quaternary compound selected from the group consisting of CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe and mixtures thereof. 
     The Group III-VI compound may include a binary compound such as In 2 S 3  and/or In 2 Se 3 ; a ternary compound such as InGaS 3  and/or InGaSe 3 ; or any combination thereof. 
     The Group III-V compound may be selected from a binary compound selected from the group consisting of GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb and mixtures thereof; a ternary compound selected from the group consisting of GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InGaP, InAlP, InNP, InNAs, InNSb, InPAs, InPSb, GaAlNP, and mixtures thereof; and a quaternary compound selected from the group consisting of GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb, and mixtures thereof. The Group III-V compound may further include a Group II metal (e.g., e.g., the Group III-V compound may be InZnP). 
     The Group IV-VI compound may be selected from a binary compound selected from the group consisting of SnS, SnSe, SnTe, PbS, PbSe, PbTe, and mixtures thereof; a ternary compound selected from the group consisting of SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, and mixtures thereof; and a quaternary compound selected from the group consisting of SnPbSSe, SnPbSeTe, SnPbSTe, and mixtures thereof. The Group IV element may be selected from the group consisting of Si, Ge, and a mixture thereof. The Group IV compound may be a binary compound selected from the group consisting of SiC, SiGe, and a mixture thereof. 
     In this case, the binary compound, the ternary compound, or the quaternary compound may each independently be present in particles at a uniform concentration or may be present in the same particle by being partially divided into different concentrations. One quantum dot may have a core-shell structure surrounding another quantum dot. An interface between a core and a shell of the quantum dot may have a concentration gradient where a concentration of elements present in the shell decreases toward the center. 
     In some embodiments, the quantum dot may have a core-shell structure including a core including the nano-sized crystals described above and a shell surrounding the core. The shell of the quantum dot may serve as a protective layer for preventing or reducing chemical denaturation of the core to maintain semiconductor characteristics and/or as a charging layer for imparting electrophoretic characteristics to the quantum dot. The shell may be monolayer or multilayer. An interface between a core and a shell may have a concentration gradient where a concentration of elements present in the shell decreases toward the center. Examples of the shell of the quantum dot include metal oxide, nonmetal oxide, a semiconductor compound, and a combination thereof, but embodiments are not limited thereto. 
     In some embodiments, the metal oxide or nonmetal oxide may each independently be a binary compound such as SiO 2 , Al 2 O 3 , TiO 2 , ZnO, MnO, Mn 2 O 3 , Mn 3 O 4 , CuO, FeO, Fe 2 O 3 , Fe 3 O 4 , CoO, Co 3 O 4 , and/or NiO; or a ternary compound such as MgAl 2 O 4 , CoFe 2 O 4 , NiFe 2 O 4 , and/or CoMn 2 O 4 , but embodiments are not limited thereto. 
     In some embodiments, the semiconductor compound may be CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnSeS, ZnTeS, GaAs, GaP, GaSb, HgS, HgSe, HgTe, InAs, InP, InGaP, InSb, AlAs, AlP, and/or AlSb, but embodiments are not limited thereto. 
     The quantum dot may have a full width of half maximum (FWHM) of a spectrum of an emission wavelength of about 45 nm or less, about 40 nm or less, or about 30 nm or less. When the FWHM of the quantum dot is within any of these ranges, color purity and/or color reproducibility may be improved. In addition, because light emitted through the quantum dot is emitted in all directions, an optical viewing angle may be improved. 
     The form (e.g., shape) of the quantum dot may be any suitable form and is not particularly limited. The quantum dot may be a spherical form, a pyramidal form, a multi-armed form, and/or a cubic nanoparticle, a nanotube, a nanowire, a nanofiber, a nano-plate particle, and/or the like. 
     The quantum dot may control color of emitted light according to the particle size. Accordingly, the quantum dot may have various emission colors such as blue, red, or green. 
     Electron Transport Region in Organic Layer  150   
     The electron transport region may have i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer including a plurality of different materials, or iii) a multi-layered structure each having a plurality of layers, each having 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 are not limited thereto. 
     In some embodiments, the electron transport region may have an electron transport layer/electron injection layer structure, a hole blocking layer/electron transport layer/electron injection layer structure, an electron control layer/electron transport layer/electron injection layer structure, or a buffer layer/electron transport layer/electron injection layer structure, wherein the layers of each structure are sequentially stacked on the emission layer in each stated order, but embodiments are not limited thereto. 
     The electron transport region (e.g., the buffer layer, the hole blocking layer, the electron control layer, the electron transport layer, and/or the electron injection layer in the electron transport region) may include the heterocyclic compound represented by Formula 1. 
     In some embodiments, the electron transport region may include the heterocyclic compound represented by Formula 1, and may further include a metal-free compound containing at least one π electron-depleted nitrogen-containing ring. 
     The term “π electron-depleted nitrogen-containing ring” as used herein refers to 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 at least two 5-membered to 7-membered heteromonocyclic groups, each having at least one *—N═*′ moiety, are condensed, or iii) a heteropolycyclic group in which at least one of a 5-membered to 7-membered heteromonocyclic group, 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 may include imidazole, pyrazole, thiazole, isothiazole, oxazole, isoxazole, pyridine, pyrazine, pyrimidine, pyridazine, indazole, purine, quinoline, isoquinoline, benzoquinoline, phthalazine, naphthyridine, quinoxaline, quinazoline, cinnoline, phenanthridine, acridine, phenanthroline, phenazine, benzimidazole, isobenzothiazole, benzoxazole, isobenzoxazole, triazole, tetrazole, oxadiazole, triazine, thiadiazole, imidazopyridine, imidazopyrimidine, and azacarbazole, but embodiments are not limited thereto. 
     In some embodiments, the electron transport region may include a compound represented by Formula 601: Formula 601 
       [Ar 601 ] xe11 -[(L 601 ) xe1 -R 601 ]) xe21 , 
     wherein, in Formula 601, 
     Ar 601  may be selected from a substituted or unsubstituted C 5 -C 60  carbocyclic group and 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 ), 
     wherein Q 601  to Q 603  may each independently be a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, or a naphthyl group, and 
     xe21 may be an integer from 1 to 5. 
     In some embodiments, at least one selected from 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 some embodiments, in Formula 601, Ar 601  may be selected from: 
     a benzene group, a naphthalene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a naphthacene group, a picene group, a perylene group, a pentaphene group, an indenoanthracene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyrimidine group, a pyridazine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline group, a quinazoline group, a cinnoline group, a phenanthridine group, an acridine group, a phenanthroline group, a phenazine group, a benzimidazole group, an isobenzothiazole group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a thiadiazole group, an imidazopyridine group, an imidazopyrimidine group, and an azacarbazole group; and 
     a benzene group, a naphthalene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, a dibenzofluorene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a naphthacene group, a picene group, a perylene group, a pentaphene group, an indenoanthracene group, a dibenzofuran group, a dibenzothiophene group, a carbazole group, an imidazole group, a pyrazole group, a thiazole group, an isothiazole group, an oxazole group, an isoxazole group, a pyridine group, a pyrazine group, a pyrimidine group, a pyridazine group, an indazole group, a purine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a phthalazine group, a naphthyridine group, a quinoxaline group, a quinazoline group, a cinnoline group, a phenanthridine group, an acridine group, a phenanthroline group, a phenazine group, a benzimidazole group, an isobenzothiazole group, a benzoxazole group, an isobenzoxazole group, a triazole group, a tetrazole group, an oxadiazole group, a triazine group, a thiadiazole group, an imidazopyridine group, an imidazopyrimidine group, and an azacarbazole group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group. 
     When xe11 in Formula 601 is 2 or greater, at least two Ar 601 (s) may be bound via a single bond. 
     In one or more embodiments, Ar 601  in Formula 601 may be an anthracene group. 
     In some embodiments, the compound represented by Formula 601 may be represented by Formula 601-1: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 601-1, 
     X 614  may be N or C(R 614 ), X 615  may be N or C(R 615 ), X 616  may be N or C(R 616 ), and at least one selected from X 614  to X 616  may be N, 
     L 611  to L 613  may each independently be understood by referring to the description of L 601  provided herein, 
     xe611 to xe613 may each independently be understood by referring to the description of xe1 provided herein, 
     R 611  to R 613  may each independently be understood by referring to the description of R 601  provided herein, and 
     R 614  to R 616  may each independently be 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 some embodiments, in Formulae 601 and 601-1, L 601  and L 611  to L 613  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 are not limited thereto. 
     In one or more embodiments, in Formulae 601 and 601-1, xe1 and xe611 to xe613 may each independently be 0, 1, or 2. 
     In one or more embodiments, in Formulae 601 and 601-1, R 601  and R 611  to R 613  may each independently be selected from: 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a hexacenyl group, a pentacenyl group, a thiophenyl group, a furanyl group, a carbazolyl group, an indolyl group, an isoindolyl group, a benzofuranyl group, a benzothiophenyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a dibenzosilolyl group, a pyridinyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a thiadiazolyl group, an oxadiazolyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and an azacarbazolyl group; and 
     —S(═O) 2 (Q 601 ) and —P(═O)(Q 601 )(Q 602 ), 
     wherein Q 601  and Q 602  may respectively be understood by referring to the descriptions of Q 601  and Q 602  provided herein. 
     The electron transport region may include at least one compound selected from Compounds ET1 to ET36, but embodiments are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the electron transport region may include at least one compound selected from 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (Bphen), Alq 3 , BAlq, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ), NTAZ, diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (TSPO1), and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi): 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The thicknesses of the buffer layer, the hole blocking layer, and the electron control layer may each independently be in a range of about 20 Å to about 1,000 Å, and in some embodiments, about 30 Å to about 300 Å. When the thicknesses of the buffer layer, the hole blocking layer, or the electron control layer are each independently within any of these ranges, excellent (or improved) hole blocking characteristics and/or excellent (or improved) electron controlling characteristics may be obtained without a substantial increase in driving voltage. 
     The thickness of the electron transport layer may be in a range of about 100 Å to about 1,000 Å, and in some embodiments, about 150 Å to about 500 Å. When the thickness of the electron transport layer is within any of these ranges, excellent (or improved) electron transport characteristics may be obtained without a substantial increase in driving voltage. 
     The electron transport region (for example, the electron transport layer in the electron transport region) may further include, in addition to the materials described above, a metal-containing material. 
     The metal-containing material may include at least one selected from an alkali metal complex and an alkaline earth metal complex. The alkali metal complex may include a metal ion selected from a lithium (Li) ion, a sodium (Na) ion, a potassium (K) ion, a rubidium (Rb) ion, and a cesium (Cs) ion. The alkaline earth metal complex may include a metal ion selected from a beryllium (Be) ion, a magnesium (Mg) ion, a calcium (Ca) ion, a strontium (Sr) ion, and a barium (Ba) ion. A ligand coordinated with the metal ion of the alkali metal complex and the alkaline earth metal complex may each independently be selected from hydroxyquinoline, hydroxyisoquinoline, hydroxybenzoquinoline, hydroxyacridine, hydroxyphenanthridine, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxyphenyloxadiazole, hydroxyphenylthiadiazole, hydroxyphenylpyridine, hydroxyphenylbenzimidazole, hydroxyphenylbenzothiazole, bipyridine, phenanthroline, and cyclopentadiene, but embodiments are not limited thereto. 
     For example, the metal-containing material may include a Li complex. The Li complex may include, e.g., Compound ET-D1 (LiQ) and/or Compound ET-D2: 
     
       
         
         
             
             
         
       
     
     The electron transport region may include an electron injection layer that facilitates injection of electrons from the second electrode  190 . The electron injection layer may be in direct contact with the second electrode  190 . 
     The electron injection layer may have i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer including a plurality of different materials, or iii) a multi-layered structure having a plurality of layers, each including a plurality of different materials. 
     The electron injection layer may include an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth metal compound, a rare earth metal compound, an alkali metal complex, an alkaline earth metal complex, a rare earth metal complex, or a combination thereof. 
     The alkali metal may be selected from Li, Na, K, Rb, and Cs. In some embodiments, the alkali metal may be Li, Na, or Cs. In one or more embodiments, the alkali metal may be Li or Cs, but embodiments are not limited thereto. 
     The alkaline earth metal may be selected from Mg, Ca, Sr, and Ba. 
     The rare earth metal may be selected from Sc, Y, Ce, Tb, Yb, and Gd. 
     The alkali metal compound, the alkaline earth metal compound, and the rare earth metal compound may each independently be selected from oxides and halides (e.g., fluorides, chlorides, bromides, and/or iodides) of the alkali metal, the alkaline earth metal, and the rare earth metal, respectively. 
     The alkali metal compound may be selected from alkali metal oxides (such as Li 2 O, Cs 2 O, and/or K 2 O), and alkali metal halides (such as LiF, NaF, CsF, KF, LiI, NaI, CsI, and/or KI). In some embodiments, the alkali metal compound may be selected from LiF, Li 2 O, NaF, LiI, NaI, CsI, and KI, but embodiments are not limited thereto. 
     The alkaline earth-metal compound may be selected from alkaline earth-metal compounds, such as BaO, SrO, CaO, Ba x Sr 1-x O (wherein 0&lt;x&lt;1), and/or Ba x Ca 1-x O (wherein 0&lt;x&lt;1). In some embodiments, the alkaline earth metal compound may be selected from BaO, SrO, and CaO, but embodiments 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 some embodiments, the rare earth metal compound may be selected from YbF 3 , ScF 3 , TbF 3 , YbI 3 , ScI 3 , and TbI 3 , but embodiments are not limited thereto. 
     The alkali metal complex, the alkaline earth metal complex, and the rare earth metal complex may each include ions of the above-described alkali metal, alkaline earth metal, and rare earth metal, respectively. The ligand coordinated with the metal ion of the alkali metal complex, the alkaline earth metal complex, and the rare earth metal complex may each independently be selected from hydroxyquinoline, hydroxyisoquinoline, hydroxybenzoquinoline, hydroxyacridine, hydroxyphenanthridine, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxyphenyloxadiazole, hydroxyphenylthiadiazole, hydroxyphenylpyridine, hydroxyphenylbenzimidazole, hydroxyphenylbenzothiazole, bipyridine, phenanthroline, and cyclopentadiene, but embodiments are not limited thereto. 
     The electron injection layer may include (e.g., may consist of) an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth metal compound, a rare earth metal compound, an alkali metal complex, an alkaline earth metal complex, a rare earth metal complex, or a combination thereof, as described above. In some embodiments, the electron injection layer may further include an organic material. When the electron injection layer further includes an organic material, the alkali metal, the alkaline earth metal, the rare earth metal, the alkali metal compound, the alkaline earth metal compound, the rare earth metal compound, the alkali metal complex, the alkaline earth metal complex, the rare earth metal complex, or a combination thereof may be homogeneously or non-homogeneously dispersed in a matrix including the organic material. 
     The thickness of the electron injection layer may be in a range of about 1 Å to about 100 Å, and in some embodiments, about 3 Å to about 90 Å. When the thickness of the electron injection layer is within any of these ranges, excellent (or improved) electron injection characteristics may be obtained without a substantial increase in driving voltage. 
     Second Electrode  190   
     The second electrode  190  may be on the organic layer  150 . In an embodiment, the second electrode  190  may be a cathode, that is an electron injection electrode. In this embodiment, a material for forming the second electrode  190  may be a material having a low work function, for example, a metal, an alloy, an electrically conductive compound, or a combination thereof. 
     The second electrode  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), silver-magnesium (Ag—Mg), ytterbium (Yb), silver-ytterbium (Ag—Yb), ITO, and IZO, but embodiments 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   
     Referring to  FIG. 2 , an organic light-emitting device  20  has a first capping layer  210 , the first electrode  110 , the organic layer  150 , and the second electrode  190  structure, wherein the layers are sequentially stacked in this stated order. Referring to  FIG. 3 , an organic light-emitting device  30  has the first electrode  110 , the organic layer  150 , the second electrode  190 , and a second capping layer  220  structure, wherein the layers are sequentially stacked in this stated order. Referring to  FIG. 4 , an organic light-emitting device  40  has the first capping layer  210 , the first electrode  110 , the organic layer  150 , the second electrode  190 , and the second capping layer  220  structure, wherein the layers are stacked in this stated order. 
     The first electrode  110 , the organic layer  150 , and the second electrode  190  illustrated in  FIGS. 2 to 4  may be substantially the same as those illustrated in and described in connection with  FIG. 1 . 
     In the organic light-emitting devices  20  and  40 , light emitted from the emission layer in the organic layer  150  may pass through the first electrode  110  (which may be a semi-transmissive electrode or a transmissive electrode) and through the first capping layer  210  to the outside. In the organic light-emitting devices  30  and  40 , light emitted from the emission layer in the organic layer  150  may pass through the second electrode  190  (which may be a semi-transmissive electrode or a transmissive electrode) and through the second capping layer  220  to the outside. 
     The first capping layer  210  and the second capping layer  220  may improve the external luminescence efficiency based on the principle of constructive interference. 
     The first capping layer  210  and the second capping layer  220  may each independently include (e.g., be) an organic matter (e.g., an organic material), an inorganic matter (e.g., an inorganic material), or any combination thereof. 
     At least one of the first capping layer  210  and the second capping layer  220  may each independently include at least one selected from a carbocyclic compound, a heterocyclic compound, an amine group-containing compound, a porphine derivative, a phthalocyanine derivative, a naphthalocyanine derivative, an alkali metal complex, an alkaline earth metal complex, a silicon-based inorganic matter (such as SiON, SiNx, and/or SiOx), a silicon-based organic matter, an acrylic compound, and an epoxy compound. The carbocyclic compound, the heterocyclic compound, and the amine group-containing compound may each independently be optionally substituted with a substituent containing at least one element selected from O, N, S, Se, Si, F, Cl, Br, and I. In some embodiments, at least one of the first capping layer  210  and the second capping layer  220  may each independently include an amine-based compound. 
     In one or more embodiments, at least one of the first capping layer  210  and the second capping layer  220  may each independently include a compound represented by Formula 201 or a compound represented by  202 . 
     In one or more embodiments, at least one of 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 are not limited thereto: 
     
       
         
         
             
             
         
       
     
     Hereinbefore, the organic light-emitting device has been described with reference to  FIGS. 1 to 4 , but embodiments are not limited thereto. 
     The layers constituting the hole transport region, the emission layer, and the layers constituting the electron transport region may be formed in a specific region by using one or more suitable methods such as vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB) deposition, ink-jet printing, laser printing, and laser-induced thermal imaging. 
     When any of the layers constituting the hole transport region, the emission layer, and the layers constituting the electron transport region are each independently formed by vacuum deposition, the vacuum deposition may be performed at a deposition temperature in a range of about 100° C. to about 500° C. at a vacuum degree in a range of about 10 −8  torr to about 10 −3  torr, and at a deposition rate in a range of about 0.01 Angstroms per second (A/sec) to about 100 Å/sec, depending on the material to be included in each layer and the structure of each layer to be formed. 
     When any of the layers constituting the hole transport region, the emission layer, and the layers constituting the electron transport region are each independently formed by spin coating, the spin coating may be performed at a coating rate of about 2,000 revolutions per minute (rpm) to about 5,000 rpm and at a heat treatment temperature of about 80° C. to about 200° C., depending on the material to be included in each layer and the structure of each layer to be formed. 
     General Definitions of Substituents 
     The term “C 1 -C 60  alkyl group” as used herein refers to a linear or branched aliphatic hydrocarbon monovalent group having 1 to 60 carbon atoms. Non-limiting examples thereof include a methyl group, an ethyl group, a propyl group, an iso-butyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an iso-amyl group, and a hexyl group. The term “C 1 -C 60  alkylene group” as used herein refers to a divalent group having 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 and/or at either terminus of the C 2 -C 60  alkyl group. Non-limiting 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 and/or at either terminus of the C 2 -C 60  alkyl group. Non-limiting 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). Non-limiting examples thereof include a methoxy group, an ethoxy group, and an isopropyloxy group. 
     The term “C 3 -C 10  cycloalkyl group” as used herein refers to a monovalent saturated hydrocarbon monocyclic group including 3 to 10 carbon atoms as ring-forming atoms. Non-limiting 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 including at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, and 1 to 10 carbon atoms as the remaining ring-forming atoms. Non-limiting 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 double bond in its ring, and is not aromatic. Non-limiting 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 including at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, 1 to 10 carbon atoms as the remaining ring-forming atoms, and at least one 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  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 6 -C 60  aryl group” as used herein refers to a monovalent group having a carbocyclic aromatic system having 6 to 6 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. The term “C 6 -C 60  arylene group” as used herein refers to a divalent group having the same structure as the C 6 -C 60  aryl group. When the C 6 -C 60  aryl group and the C 6 -C 60  arylene group each independently include two or more rings, the respective rings may be fused. 
     The term “C 1 -C 60  heteroaryl group” as used herein refers to a monovalent group having a heterocyclic aromatic system having at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, and 1 to 60 carbon atoms as the remaining ring-forming 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. The term “C 1 -C 60  heteroarylene group” as used herein refers to a divalent group having the same structure as the C 1 -C 60  heteroaryl group. When the C 1 -C 60  heteroaryl group and the C 1 -C 60  heteroarylene group each independently include two or more rings, the respective rings may be fused. 
     The term “C 6 -C 60  aryloxy group” as used herein refers to a monovalent group represented by -OA 102  (wherein A 102  is the C 6 -C 60  aryl group). The term “C 6 -C 60  arylthio group” as used herein refers to a monovalent group represented by -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 that has two or more rings condensed and only carbon atoms as ring-forming atoms (e.g., 8 to 60 carbon atoms), wherein the entire molecular structure is non-aromatic. Non-limiting examples of the monovalent non-aromatic condensed polycyclic group may include a fluorenyl group. The term “divalent non-aromatic condensed polycyclic group” as used herein refers to a divalent group having substantially the same structure as the monovalent non-aromatic condensed polycyclic group. 
     The term “monovalent non-aromatic condensed heteropolycyclic group” as used herein refers to a monovalent group that has two or more condensed rings and at least one heteroatom selected from N, O, Si, P, and S, in addition to carbon atoms (e.g., 1 to 60 carbon atoms), as a ring-forming atom, wherein the entire molecular structure is non-aromatic. Non-limiting examples of the monovalent non-aromatic condensed heteropolycyclic group may include a 9H-xanthenyl group. The term “divalent non-aromatic condensed heteropolycyclic group” as used herein refers to a divalent group having substantially the same structure as the monovalent non-aromatic condensed heteropolycyclic group. 
     The term “C 5 -C 60  carbocyclic group” as used herein refers to a monocyclic or polycyclic group having 5 to 60 carbon atoms only as ring-forming atoms. The C 5 -C 60  carbocyclic group may be an aromatic carbocyclic group or a non-aromatic carbocyclic group. The term “C 5 -C 60  carbocyclic group” as used herein refers to a ring (e.g., a benzene group), a monovalent group (e.g., a phenyl group), or a divalent group (e.g., a phenylene group). Also, 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 substantially the same structure as the C 5 -C 60  carbocyclic group, except that at least one heteroatom selected from N, O, Si, P, and S is used as a ring-forming atom, in addition to carbon atoms (e.g., 1 to 60 carbon atoms). 
     In the present specification, at least one substituent of the substituted C 5 -C 60  carbocyclic group, the substituted C 1 -C 60  heterocyclic group, the substituted C 3 -C 10  cycloalkylene group, the substituted C 1 -C 10  heterocycloalkylene group, the substituted C 3 -C 10  cycloalkenylene group, the substituted C 1 -C 10  heterocycloalkenylene group, the substituted C 6 -C 60  arylene group, the substituted C 1 -C 60  heteroarylene group, the substituted divalent non-aromatic 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 (-D), —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 11 )(Q 12 )(Q 13 ), —N(Q 11 )(Q 12 ), —B(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O) 2 (Q 11 ), and —P(═O)(Q 11 )(Q 12 ); 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amidino group, a hydrazino group, a hydrazono group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 21 )(Q 22 )(Q 23 ), —N(Q 21 )(Q 22 ), —B(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O) 2 (Q 21 ), and —P(═O)(Q 21 )(Q 22 ); and 
     —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), and —P(═O)(Q 31 )(Q 32 ), 
     wherein Q 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 hydrazine group; a hydrazone 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 C 1 -C 60  alkyl group substituted with at least one selected from deuterium, —F, and a cyano group; a C 6 -C 60  aryl group substituted with at least one selected from deuterium, —F, and a cyano group; a biphenyl group; and a terphenyl group. 
     “Ph” used herein represents a phenyl group, “Me” used herein represents a methyl group, “Et” used herein represents an ethyl group, “ter-Bu” or “But” used herein represents a tert-butyl group, and “OMe” used herein represents a methoxy group. 
     The term “biphenyl group” as used herein refers to a phenyl group substituted with at least one phenyl group. The “biphenyl group” may be “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 at least one phenyl group. The “terphenyl group” may be “a substituted phenyl group” having a “C 6 -C 60  aryl group substituted with a C 6 -C 60  aryl group” as a substituent. 
     The symbols * and *′ as used herein, unless defined otherwise, refer to a binding site to an adjacent atom in a corresponding formula. 
     Hereinafter, compounds and an organic light-emitting device according to one or more embodiments will be described in more detail with reference to Synthesis Examples and Examples. The wording “B was used instead of A” used in describing Synthesis Examples means that an amount of B used was identical to an amount of A used in terms of molar equivalents. 
     EXAMPLES 
     Synthesis Example 1: Synthesis of Compound 1 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Synthesis of Intermediate 1-1 
     10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole)(1 eq.), bromobenzene (2 eq.), tris(dibenzylideneacetone)dipalladium (0) (0.05 eq.), BINAP (0.1 eq.), and sodium t-butoxide (3 eq.) were dissolved in toluene under a nitrogen atmosphere, followed by stirring at a temperature of 100° C. for 12 hours. Once the mixture was cooled and washed three times using ethyl acetate and water, the resulting organic layer was dried using anhydrous magnesium sulfate under reduced pressure. Subsequently, the residue was separated and purified through column chromatography to thereby obtain Intermediate 1-1 (yield: 65%). 
     Synthesis of Intermediate 1-2 
     Intermediate 1-1 (1 eq.), 1-bromo-3-fluorobenzene (1.5 eq.), tris(dibenzylideneacetone)dipalladium (0) (0.05 eq.), tri-t-butylphosphine (0.1 eq.), and sodium t-butoxide (3 eq.) were dissolved in toluene under a nitrogen atmosphere, followed by stirring at a temperature of 100° C. for 12 hours. Once the mixture was cooled and washed three times using ethyl acetate and water, the resulting organic layer was dried using anhydrous magnesium sulfate under reduced pressure. Subsequently, the residue was separated and purified through column chromatography to thereby obtain Intermediate 1-2 (yield: 85%). 
     Synthesis of Intermediate 1-3 
     Intermediate 1-2 (1 eq.), 2-bromophenol (1.5 eq.), and potassium (III) phosphate (2 eq.) were dissolved in dimethylformamide (DMF), followed by stirring at a temperature of 160° C. for 12 hours. Once the mixture was cooled, the solvent was removed therefrom under reduced pressure, and the resultant was washed three times using dichloromethane and water. The resulting organic layer was dried using anhydrous magnesium sulfate under reduced pressure. Subsequently, the residue was separated and purified through column chromatography to thereby obtain Intermediate 1-3 (yield: 55%). 
     Synthesis of Compound 1 
     Intermediate 1-3 was dissolved in o-xylene, and the mixture was cooled to a temperature of −20° C. and stirred. Then, n-butyl lithium (1.2 eq.) was added thereto, followed by raising the temperature to 70° C. and stirring for 2 hours. The reaction glass was cooled to a temperature of −30° C., and boron tribromide (1.3 eq.) was slowly added thereto, followed by stirring at room temperature for 1 hour. The reaction glass was cooled to a temperature of 0° C., triethylamine (1.5 eq.) was added thereto, followed by raising the temperature to 120° C. and stirring for 5 hours. Once the mixture was cooled, a sodium acetate aqueous solution was added thereto to complete the reaction. Then, the reaction solution was removed by drying under reduced pressure in a rotary evaporator. The resultant was washed using diethyl ether and acetone, and filtered to thereby synthesize Compound 1 (yield: 21%). 
     Synthesis Example 2: Synthesis of Compound 10 
     
       
         
         
             
             
         
       
     
     Synthesis of Intermediate 10-1 
     Intermediate 10-1 was synthesized in substantially the same manner as in Synthesis of Intermediate 1-2, except that 1-bromo-3-chlorobenzene was used instead of 1-bromo-3-fluorobenzene (yield: 80%). 
     Synthesis of Intermediate 10-2 
     Intermediate 10-2 was synthesized in substantially the same manner as in Synthesis of Intermediate 10-1, except that the Intermediate 10-1 was used instead of the Intermediate 1-1, and 2-bromo-N-phenylaniline was used instead of 1-bromo-3-chlorobenzene (yield: 74%). 
     Synthesis of Compound 10 
     Compound 10 was synthesized in substantially the same manner as in Synthesis of Compound 1, except that Intermediate 10-2 was used instead of Intermediate 1-3 (yield: 19%). 
     Synthesis Example 3: Synthesis of Compound 13 
     
       
         
         
             
             
         
       
     
     Synthesis of Intermediate 13-1 
     1,3-dibromo-5-fluorobenzene (1 eq.), diphenylamine (1 eq.), tris(dibenzylideneacetone)dipalladium (0) (0.05 eq.), tri-t-butylphosphine (0.1 eq.), and sodium t-butoxide (3 eq.) were dissolved in toluene under a nitrogen atmosphere, followed by stirring at a temperature of 80° C. for 12 hours. Once the mixture was cooled and washed three times using ethyl acetate and water, the resulting organic layer was dried using anhydrous magnesium sulfate under reduced pressure. Subsequently, the residue was separated and purified through column chromatography to thereby obtain Intermediate 13-1 (yield: 65%). 
     Synthesis of Intermediate 13-2 
     Intermediate 13-2 was synthesized in substantially the same manner as in Synthesis of Intermediate 1-2, except that Intermediate 13-1 was used instead of 1-bromo-3-fluorobenzene (yield: 60%). 
     Synthesis of Intermediate 13-3 
     Intermediate 13-3 was synthesized in substantially the same manner as in Synthesis of Intermediate 1-3, except that Intermediate 13-2 was used instead of Intermediate 1-2 (yield: 61%). 
     Synthesis of Compound 13 
     Compound 13 was synthesized in substantially the same manner as in Synthesis of Compound 1, except that Intermediate 13-3 was used instead of Intermediate 1-3 (yield: 15%). 
     Synthesis Example 4: Synthesis of Compound 16 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Synthesis of Intermediate 16-1 
     Intermediate 16-1 was synthesized in substantially the same manner as in Synthesis of Intermediate 1-1, except that 1-bromo-3-fluorobenzene was used instead of bromobenzene (yield: 60%). 
     Synthesis of Intermediate 16-2 
     Intermediate 16-2 was synthesized in substantially the same manner as in Synthesis of Intermediate 1-2, except that bromobenzene was used instead of 1-bromo-3-fluorobenzene (yield: 82%). 
     Synthesis of Intermediate 16-3 
     Intermediate 16-3 was synthesized in substantially the same manner as in Synthesis of Intermediate 1-3, except that 2-bromophenol was used in an amount of 2.2 eq. instead of 1.5 eq. (yield: 71%). 
     Synthesis of Compound 16 
     Intermediate 16-3 was dissolved in o-xylene, and the mixture was cooled to a temperature of −20° C. and stirred. Then, n-butyl lithium (2.2 eq.) was added thereto, followed by raising the temperature to 70° C. and stirring for 2 hours. The reaction glass was cooled to a temperature of −30° C., and boron tribromide (3.0 eq.) was slowly added thereto, followed by stirring at room temperature for 3 hour. The reaction glass was cooled to a temperature of 0° C., triethylamine (3.0 eq.) was added thereto, followed by raising the temperature to 120° C. and stirring for 5 hours. Once the mixture was cooled, a sodium acetate aqueous solution was added thereto to complete the reaction. Then, the reaction solution was removed by drying under reduced pressure in a rotary evaporator. The resultant was washed using diethyl ether and acetone, and filtered to thereby synthesize Compound 16 (yield: 15%). 
     Synthesis Example 5: Synthesis of Compound 26 
     
       
         
         
             
             
         
       
     
     Synthesis of Intermediate 26-1 
     10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (1 eq.), 1-bromo-3-fluorobenzene (3.3 eq.), tris(dibenzylideneacetone)dipalladium (0) (0.05 eq.), tri-t-butylphosphine (0.1 eq.), and sodium t-butoxide (3 eq.) were dissolved in toluene under a nitrogen atmosphere, followed by stirring at a temperature of 100° C. for 12 hours. Once the mixture was cooled and washed three times using ethyl acetate and water, the resulting organic layer was dried using anhydrous magnesium sulfate under reduced pressure. Subsequently, the residue was separated and purified through column chromatography to thereby obtain Intermediate 26-1 (yield: 80%). 
     Synthesis of Intermediate 26-2 
     Intermediate 26-2 was synthesized in substantially the same manner as in Synthesis of Intermediate 1-3, except that 2-bromophenol was used in an amount of 3.3 eq. instead of 1.5 eq. (yield: 75%). 
     Synthesis of Compound 26 
     Compound 26 was synthesized in substantially the same manner as in Synthesis of Compound 16, except that n-butyl lithium was used in an amount of 3.3 eq. instead of 2.2 eq., and boron tribromide and triethyl amine were used in an amount of 3.5 eq. each instead of 3.0 eq. each (yield: 12%). 
     Synthesis Example 6: Synthesis of Compound 30 
     
       
         
         
             
             
         
       
     
     Synthesis of Intermediate 30-1 
     Intermediate 30-1 was synthesized in substantially the same manner as in Synthesis of Intermediate 1-2, except that 8-bromo-1,2,3,4-tetrahydroquinoline was used instead of 1-bromo-3-fluorobenzene (yield: 75%). 
     Synthesis of Compound 30 
     Compound 30 was synthesized in substantially the same manner as in Synthesis of Compound 1, except that Intermediate 30-1 was used instead of Intermediate 1-3 (yield: 22%). 
     The  1 H NMR and MS/FAB results of the synthesized compounds are shown in Table 2. Methods of synthesizing compounds other than compounds shown in Table 2 should be easily understood to those skilled in the art by referring to the synthesis pathways and raw materials described above. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Com- 
                   
                 MS/FAB 
               
            
           
           
               
               
               
               
            
               
                 pound 
                 H NMR (δ) 
                 Calc. 
                 Found 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 8.8-8.72(1H, d), 8.52-8.48(1H,d), 
                 673.56 
                 673.54 
               
               
                   
                 7.67-7.60(10H, m), 
                   
                   
               
               
                   
                 7.50-7.25(8H, m), 7.20-7.10(4H,m), 
                   
                   
               
               
                   
                 6.78-6.74(2H, m), 
                   
                   
               
               
                   
                 6.13-6.08(2H, m) 
                   
                   
               
               
                 10 
                 8.96-8.90(1H, d), 8.86-8.80(1H, d), 
                 748.68 
                 748.66 
               
               
                   
                 7.80-7.56(15H, m), 
                   
                   
               
               
                   
                 7.48-7.20(8H, m), 7.20-7.02(6H, m), 
                   
                   
               
               
                   
                 6.86-6.80(2H, m) 
                   
                   
               
               
                 13 
                 8.8-8.72(1H, d), 8.52-8.48(1H, d), 
                 840.77 
                 840.75 
               
               
                   
                 7.80-7.61(20H, m), 
                   
                   
               
               
                   
                 7.58-7.45(7H, m), 7.41-7.26(4H, m), 
                   
                   
               
               
                   
                 7.15-7.08(4H, m) 
                   
                   
               
               
                 16 
                 9.12-9.06(2H, d), 8.98-8.92(2H, d), 
                 773.45 
                 773.44 
               
               
                   
                 8.12-7.79(12H, m), 
                   
                   
               
               
                   
                 7.68-7.33(8H, m), 7.28-7.11(5H, m) 
                   
                   
               
               
                 26 
                 8.98-8.90(3H, d), 8.80-8.72(3H, d), 
                 873.33 
                 873.31 
               
               
                   
                 8.01-7.72(9H, m), 
                   
                   
               
               
                   
                 7.65-7.51(9H, m), 7.47-7.38(6H, m) 
                   
                   
               
               
                 30 
                 9.12-9.08(1H, d), 9.03-8.48(1H, d), 
                 712.64 
                 712.62 
               
               
                   
                 7.66-7.59(10H, m), 
                   
                   
               
               
                   
                 7.55-7.49(4H, m), 7.43-7.38(4H, m), 
                   
                   
               
               
                   
                 7.33-7.28(5H, m), 
                   
                   
               
               
                   
                 7.21-7.12(2H, m), 3.10-3.04(2H, m), 
                   
                   
               
               
                   
                 2.84-2.78(2H, m), 
                   
                   
               
               
                   
                 1.93-1.88(2H, m) 
               
               
                   
               
            
           
         
       
     
     Example 1 
     A Corning, Inc. 15 Ohms per square centimeter (Ω/cm 2 ) (1,200 Å) ITO glass substrate was cut to a size of 50 millimeters (mm)×50 mm×0.7 mm, sonicated in isopropyl alcohol and pure water for 5 minutes in each solvent, and cleaned by exposure to ultraviolet rays with ozone to use the glass substrate as an anode. Then, the glass substrate was mounted to a vacuum-deposition apparatus. 
     N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (NPD) was vacuum-deposited on the ITO anode formed on the glass substrate to form a hole injection layer having a thickness of 300 Å. TCTA was then vacuum-deposited on the hole injection layer to form a first hole transport layer having a thickness of 200 Å. 
     Compound CzSi as a hole transporting compound was vacuum-deposited on the first hole transport layer to form a second hole transport layer having a thickness of 100 Å. 
     mCP (as a host) and Compound 1 (as a dopant) were co-deposited on the second hole transport layer at a weight ratio of 99:1 to form an emission layer having a thickness of 200 Å. 
     Subsequently, TSPO1 was deposited on the emission layer to form a buffer layer having a thickness of 200 Å, and TPBi as an electron transporting compound was deposited on the buffer layer to form an electron transport layer having a thickness of 300 Å. 
     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 on the electron injection layer to form a LiF/Al electrode having a thickness of 3,000 Å, thereby completing the manufacture of an organic light-emitting device. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Examples 2 to 4 and Comparative Examples A to C 
     Organic light-emitting devices were manufactured in substantially the same manner as in Example 1, except that compounds shown in Table 3 were used instead of Compound 1. 
     Evaluation Example 1 
     To evaluate characteristics of the organic light-emitting devices manufactured in Examples 1 to 4 and Comparative Examples A to C, the driving voltage, luminescence efficiency, and maximum external quantum yield (EQE) of the organic light-emitting devices at a current density of 10 milliamperes per square centimeter (mA/cm 2 ) were measured. The driving voltage of the organic light-emitting devices were measured using a source meter (Keithley Instruments, 2400 series). The maximum external quantum yield of the organic light-emitting devices were measured using Hamamastu Absolute PL Quantum Yield Measurement System C9920-2-12. In evaluation of the maximum external quantum yield, luminance/current density was measured using a luminance meter with calibration of wavelength sensitivity, and the maximum external quantum yield was calculated by the angular luminance distribution on the assumption of the Lambertian surface. The evaluation results of the organic light-emitting devices are shown in Table 3. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                   
                 Maximum 
                   
               
               
                   
                 Dopant in 
                 Driving 
                 Luminescence 
                 external 
                   
               
               
                   
                 emission 
                 voltage 
                 efficiency 
                 quantum 
                 Emission 
               
               
                   
                 layer 
                 (V) 
                 (Cd/A) 
                 yield (%) 
                 color 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 Compound  
                 4.7 
                 16.1 
                 20.3 
                 blue 
               
               
                   
                 1 
                   
                   
                   
                   
               
               
                 Example 2 
                 Compound 
                 4.64 
                 19.5 
                 20.6 
                 Blue- 
               
               
                   
                 10 
                   
                   
                   
                 green 
               
               
                 Example 3 
                 Compound 
                 4.74 
                 23.2 
                 20.5 
                 Blue 
               
               
                   
                 16 
                   
                   
                   
                   
               
               
                 Example 4 
                 Compound 
                 4.8 
                 24.1 
                 20.1 
                 Blue 
               
               
                   
                 26 
                   
                   
                   
                   
               
               
                 Comparative 
                 Compound 
                 5.2 
                 12.1 
                 14.3 
                 Blue 
               
               
                 Example A 
                 A 
                   
                   
                   
                   
               
               
                 Comparative 
                 Compound 
                 5.7 
                 13.8 
                 13.7 
                 Blue- 
               
               
                 Example B 
                 B 
                   
                   
                   
                 green 
               
               
                 Comparative 
                 Compound 
                 5.4 
                 11.7 
                 5.1 
                 Dark blue 
               
               
                 Example C 
                 C 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                   
                     
                     
                         
                         
                     
                   
                 
               
            
           
         
       
     
     Referring to the results of Table 3, the organic light-emitting devices of Examples 1 to 4 were found to have excellent driving voltage, luminescence efficiency, and external quantum yield, as compared with the organic light-emitting devices of Comparative Examples A, B, and C. 
     As used herein, the terms “use,” “using,” and “used” may be considered synonymous with the terms “utilize,” “utilizing,” and “utilized,” respectively. 
     In addition, the terms “substantially,” “about,” and similar terms are used as terms of approximation and not as terms of degree, and are intended to account for the inherent deviations in measured or calculated values that would be recognized by those of ordinary skill in the art. 
     Also, any numerical range recited herein is intended to include all subranges of the same numerical precision subsumed within the recited range. For example, a range of “1.0 to 10.0” is intended to include all subranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein. Accordingly, Applicant reserves the right to amend this specification, including the claims, to expressly recite any sub-range subsumed within the ranges expressly recited herein. 
     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 of the present disclosure as defined by the following claims and their equivalents.