Patent Publication Number: US-11653563-B2

Title: Organic light-emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2015-0177363 filed on Dec. 11, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     Exemplary embodiments of the present invention relate to a light-emitting device, and more particularly to an organic light-emitting device. 
     DISCUSSION OF RELATED ART 
     Organic light-emitting devices (OLEDs) may be self-emission devices. OLEDs may have relatively wide viewing angles, relatively high contrast ratios, and relatively short response times. OLEDs may produce full-color images. OLEDs may also have an increased brightness, driving voltage, and response speed characteristics. 
     OLEDs may include a first electrode disposed on a substrate. OLEDs may include a hole transport region, an emission layer, an electron transport region, and a second electrode sequentially disposed on the first electrode. Holes provided from the first electrode may move toward the emission layer through the hole transport region. Electrons provided from the second electrode may move toward the emission layer through the electron transport region. Carriers, such as holes and electrons, may recombine in the emission layer to produce excitons. The excitons may transition from an excited state to a ground state, thus generating light. 
     SUMMARY 
     One or more exemplary embodiments of the present invention include an organic light-emitting device having relatively low driving voltage and relatively high efficiency. 
     According to one or more exemplary embodiments of the present invention, an organic light-emitting device includes: 
     a first electrode; 
     a second electrode facing the first electrode; and 
     an organic layer disposed between the first electrode and the second electrode, the organic layer including an emission layer, a first compound, and a second compound. 
     The first compound is represented by Formula 1. The second compound is represented by one of Formulae 2A and 2B. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 1, 1-1, 1-2, 2A, 2B, 10-1, 10-2, and 20: 
     ring A 2 , ring A 3 , and ring A 4  are condensed with each other; and ring A 12 , ring A 13 , and ring A 14  are condensed with each other; 
     ring A 1 , ring A 2 , ring A 4 , ring A 11 , ring A 12 , ring A 14 , and ring A 21  to ring A 24  are each independently selected from a C 5 -C 30  carbocyclic group and a C 2 -C 30  heterocyclic group; 
     ring A 3  is a group represented by Formula 10-1, and ring A 13  is a group represented by Formula 10-2; 
     X 1  is selected from N-(L 5 ) a5 -(Ar 2 ) b2 , oxygen (O), and sulfur (S); 
     X 11  is selected from N-(L 12 ) a12 -(Ar 12 ) b12 , O, and S; 
     L 1  to L 5 , L 11 , L 12 , L 21 , and L 31  are each independently 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; 
     a1 to a5, a11, a12, a21, and a31 are each independently an integer selected from 0 to 3; 
     Ar 1 , Ar 2 , Ar 11 , Ar 12 , Ar 21 , Ar 22 , Ar 31 , and Ar 32  are each independently selected from a group represented by Formula 1-1, 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; 
     b1, b2, b11, and b12 are each independently an integer selected from 1 to 5, 
     Ar 31  and Ar 32  are connected to each other to form a saturated ring or an unsaturated ring; 
     R 1  to R 6 , R 11  to R 13 , and R 21  to R 24  are each independently selected from a group represented by Formula 1-2, a group represented by Formula 20, hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, and —Si(Q 1 )(Q 2 )(Q 3 ); 
     c1 to c6, c11 to c13, and c21 to c24 are each independently an integer selected from 0 to 4; 
     n1 to n3 are each independently an integer selected from 0 to 4; 
     at least one of R 21  and R 22  is be a group represented by Formula 20, and at least one of R 23  and R 24  is a group represented by Formula 20; 
     * indicates a binding site to a neighboring atom; 
     at least one substituent selected from 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 is selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 11 )(Q 12 )(Q 13 ), and —N(Q 14 )(Q 15 ); 
     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 at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —Si(Q 21 )(Q 22 )(Q 23 ), and —N(Q 24 )(Q 25 ); and 
     —Si(Q 31 )(Q 32 )(Q 33 ), and —N(Q 34 )(Q 35 ); and 
     Q 1  to Q 3 , Q 11  to Q 15 , Q 21  to Q 25 , and Q 31  to Q 35  are each independently selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a 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, and a monovalent non-aromatic condensed heteropolycyclic group. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention will become more apparent by describing in detail exemplary embodiments thereof, with reference to the accompanying drawings, in which: 
         FIG.  1    is a schematic cross-sectional diagram illustrating an organic light-emitting device according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. “and/or” includes any exemplary embodiments may have different forms and should not be construed as being limited to the exemplary embodiments of the present invention described herein. 
     Like reference numerals may refer to like elements throughout the specification and drawings. 
     Sizes of elements in the drawings may be exaggerated for clarity of description. 
     It will be understood that when a component, such as a layer, a film, a region, or a plate, is referred to as being “on” another component, the component can be directly on the other component or intervening components may be present. 
     According to an exemplary embodiment of the present invention, an organic light-emitting device may include a first electrode, a second electrode facing the first electrode, and an organic layer. The organic layer may include an emission layer. The organic layer may be disposed between the first electrode and the second electrode. The organic layer may include a first compound and a second compound. 
     The first electrode may be an anode. The second electrode may be a cathode. The first electrode and the second electrode may each independently be as defined herein. 
     In the organic layer, the first compound may be represented by Formulae 1, and the second compound may be represented by one of Formulae 2A and 2B: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 1, 1-1, 1-2, 2A, and 2B, ring A 2 , ring A 3 , and ring A 4  may be chemically bonded to each other, and ring A 12 , ring A 13 , and ring A 14  may be chemically bonded to each other. 
     In Formulae 1, 1-1, 1-2, 2A, and 2B, ring A 1 , ring A 2 , ring A 4 , ring A 11 , ring A 12 , ring A 14 , and ring A 21  to ring A 24  may each independently be selected from a C 5 -C 30  carbocyclic group and a C 2 -C 30  heterocyclic group. 
     According to an exemplary embodiment of the present invention, ring A 1 , ring A 2 , ring A 4 , ring A 11 , ring A 12 , ring A 14 , and ring A 21  to ring A 24  in Formulae 1, 1-1, 1-2, 2A, and 2B may each independently be selected from a benzene group, a naphthalene group, a pyridine group, a pyrimidine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a quinoxaline group, a quinazoline group, and a phenanthroline group. 
     According to an exemplary embodiment of the present invention, in Formulae 1, 1-1, 1-2, 2A, and 2B, ring A 1 , ring A 4 , ring A 11 , ring A 14 , and ring A 21  to ring A 24  may each independently be selected from a benzene group and a naphthalene group, and ring A 2  and ring A 12  may each independently be a naphthalene group; however, exemplary embodiments of the present invention are not limited thereto. 
     In Formulae 1, 1-1, and 1-2, ring A 3  may be a group represented by Formula 10-1, and ring A 13  may be a group represented by Formula 10-2: 
     
       
         
         
             
             
         
       
     
     In Formulae 10-1 and 10-2, X 1  may be selected from N-(L 5 ) a5 -(Ar 2 ) b2 , oxygen (O), and sulfur (S), and X 11  may be selected from N-(L 12 ) a12 -(Ar 12 ) b12 , O, and S. L 5 , L 12 , a5, a12, Ar 2 , Ar 12 , b2, and b12 may each independently be as defined herein. 
     According to an exemplary embodiment of the present invention, X 1  and X 11  may each independently be selected from O and S. According to an exemplary embodiment of the present invention, X 1  and X 11  may each independently be 5, however, exemplary embodiments of the present invention are not limited thereto. 
     In Formulae 1, 1-2, 2A, 2B, 10-1, 10-2, and 20, L 1  to L 5 , L 11 , L 12 , L 21 , and L 31  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. 
     According to an exemplary embodiment of the present invention, L 1  to L 5 , L 11 , L 12 , L 21 , and L 31  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-fluorenylene 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 pyrrolylene group, a thiophenylene group, a furanylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a pyridinylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, an isoindolylene group, an indolylene group, an indazolylene group, a purinylene 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 carbazolylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, a benzofuranylene group, a benzothiophenylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an oxadiazolylene group, a triazinylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a thiadiazolylene group, an imidazopyridinylene group, and an imidazopyrimidinylene 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-fluorenylene 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 pyrrolylene group, a thiophenylene group, a furanylene group, an imidazolylene group, a pyrazolylene group, a thiazolylene group, an isothiazolylene group, an oxazolylene group, an isoxazolylene group, a pyridinylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, an isoindolylene group, an indolylene group, an indazolylene group, a purinylene 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 carbazolylene group, a phenanthridinylene group, an acridinylene group, a phenanthrolinylene group, a phenazinylene group, a benzimidazolylene group, a benzofuranylene group, a benzothiophenylene group, an isobenzothiazolylene group, a benzoxazolylene group, an isobenzoxazolylene group, a triazolylene group, a tetrazolylene group, an oxadiazolylene group, a triazinylene group, a dibenzofuranylene group, a dibenzothiophenylene group, a benzocarbazolylene group, a dibenzocarbazolylene group, a thiadiazolylene group, an imidazopyridinylene group, and an imidazopyrimidinylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 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 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-fluorenyl 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 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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl 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, a thiadiazolyl group, an imidazopyridinyl group, and an imidazopyrimidinyl group. 
     According to an exemplary embodiment of the present invention, L 1  to L 5 , L 11 , L 12 , L 21 , and L 31  may each independently be selected from groups represented by Formulae 3-1 to 3-41: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 3-1 to 3-41: 
     Y 1  may be selected from O, S, C(Z 3 )(Z 4 ), N(Z 5 ), and Si(Z 6 )(Z 7 ); 
     Z 1  to Z 7  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a 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-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a triazinyl group, and —Si(Q 33 )(Q 34 )(Q 35 ); 
     Q 33  to Q 35  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; 
     d2 may be an integer selected from 1 and 2; 
     d3 may be an integer selected from 1 to 3; 
     d4 may be an integer selected from 1 to 4; 
     d5 may be an integer selected from 1 to 5; 
     d6 may be an integer selected from 1 to 6; 
     d8 may be an integer selected from 1 to 8; and 
     * and *′ may each independently indicate a binding site to a neighboring atom. 
     According to an exemplary embodiment of the present invention, L 1  to L 5 , L 11 , L 12 , L 21 , and L 31  may each independently be selected from groups represented by Formulae 4-1 to 4-37: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 4-1 to 4-37, * and *′ may each independently indicate a binding site to a neighboring atom. 
     In Formulae 1, 1-2, 2A, 2B, 10-1, 10-2, and 20, a1 to a5, a11, a12, a21, and a31 may each independently be an integer selected from 0 to 3. a1 may indicate the number of L 1 (s) in Formula 1. When a1 is 2 or greater, at least two L 1 (s) may be the same as or different from each other. When a1 is 0, *-(L 1 ) a1 -*′ may be a single bond. a2 to a5, a11, a12, a21, and a31 may each independently be the same as described herein with reference to with a1 and Formulae 1, 1-2, 2A, 2B, 10-1, 10-2, and 20. 
     According to an exemplary embodiment of the present invention, a1 and a2 may each independently be an integer selected from 0, 1, and 2. a3 to a5, a11, a1, and a31 may each independently be an integer selected from 0 and 1. a21 may be an integer selected from 1 and 2. However, exemplary embodiments of the present invention are not limited thereto. 
     In Formulae 1, 1-2, 2A, 10-1, 10-2, and 20, Ar 1 , Ar 2 , Ar 11 , Ar 12 , Ar 21 , Ar 22 , Ar 31 , and Ar 32  may each independently be selected from a group represented by Formula 1-1, 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. 
     As an example, Ar 1 , Ar 2 , Ar 11 , Ar 12 , Ar 21 , Ar 22 , Ar 31 , and Ar 32  may each independently be selected from a group represented by Formula 1-1, 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-fluorenyl 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 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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzothiazolyl group, an isobenzothiazolyl group, an benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an oxadiazolyl group, a triazinyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a thiadiazolyl group, an imidazopyridinyl group, and an imidazopyrimidinyl 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-fluorenyl 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 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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzothiazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an oxadiazolyl group, a triazinyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a thiadiazolyl group, an imidazopyridinyl group, and an imidazopyrimidinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 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 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-fluorenyl 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 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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzothiazolyl group, an isobenzothiazolyl group, a benzoxazolyl group, an isobenzoxazolyl group, a triazolyl group, a tetrazolyl group, an oxadiazolyl group, a triazinyl group, a benzocarbazolyl group, a dibenzocarbazolyl group, a thiadiazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and —Si(Q 31 )(Q 32 )(Q 33 ). 
     Q 31  to Q 33  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, and a triazinyl group. 
     According to an exemplary embodiment of the present invention, Ar 1 , Ar 2 , Ar 11 , Ar 12 , Ar 21 , Ar 22 , Ar 31 , and Ar 32  may each independently be selected from groups represented by Formulae 1-1 and 5-1 to 5-79: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 5-1 to 5-79; 
     Y 11  may be selected from O, S, C(Z 13 )(Z 14 ), N(Z 15 ), and Si(Z 16 )(Z 17 ); 
     Z 11  to Z 17  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a 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-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a triazinyl group, and —Si(Q 33 )(Q 34 )(Q 35 ); 
     Q 33  to Q 35  may each independently be selected from a C 1 -C 10  alkyl group, a alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group; 
     e2 may be an integer selected from 1 and 2; 
     e3 may be an integer selected from 1 to 3; 
     e4 may be an integer selected from 1 to 4; 
     e5 may be an integer selected from 1 to 5; 
     e6 may be an integer selected from 1 to 6; 
     e8 may be an integer selected from 1 to 8; and 
     * may indicate a binding site to a neighboring atom. 
     According to an exemplary embodiment of the present invention, Ar t , Ar 2 , Ar 11 , Ar 12 , Ar 21 , Ar 22 , Ar 31 , and Ar 32  in Formulae 1, 1-2, 2A, 10-1, 10-2, and 20 may each independently be selected from groups represented by Formulae 1-1 and 6-1 to 6-158: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 6-1 to 6-158, * may indicate a binding site to a neighboring atom. 
     According to an exemplary embodiment of the present invention, Ar 1  may be selected from groups represented by Formulae 1-1 and 5-1 to 5-79, e.g., groups represented by Formulae 6-1 to 6-158 
     Ar 2 , Ar 11 , Ar 12 , Ar 21 , Ar 22 , Ar 31 , and Ar 32  may each independently be selected from groups represented by Formulae 5-1 to 5-79, e.g., groups represented by Formulae 6-1 to 6-158. 
     In Formula 20, Ar 31  and Ar 32  may be connected to each other and may form a saturated or unsaturated ring. 
     In Formulae 1, 1-2, 10-1, and 10-2, b1, b2, b11, and b12 may each independently be an integer selected from 1 to 5. b1 may indicate the number of Ar 1 (s) in Formula 1. When b1 is 2 or greater, at least two Ar 1 (s) may be the same or different from each other. b2, b11, and b12 may each independently be the same as described herein with reference to b1 and Formulae 1-2, 10-1, and 10-2. 
     According to an exemplary embodiment of the present invention, b1, b2, b11, and b12 may each independently be an integer selected from 1 and 2; however, exemplary embodiments of the present invention are not limited thereto. 
     In Formulae 1, 1-1, 1-2, 2A, and 2B, R 1  to R 6 , R 11  to R 13  and R 21  to R 24  may each independently be selected from a group represented by Formula 1-2, a group represented by Formula 20, hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a substituted or unsubstituted C 1 -C 60  alkyl group, a substituted or unsubstituted C 2 -C 60  alkenyl group, a substituted or unsubstituted C 2 -C 60  alkynyl group, a substituted or unsubstituted C 1 -C 60  alkoxy group, a substituted or unsubstituted C 3 -C 10  cycloalkyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkyl group, a substituted or unsubstituted C 3 -C 10  cycloalkenyl group, a substituted or unsubstituted C 1 -C 10  heterocycloalkenyl group, a substituted or unsubstituted C 6 -C 60  aryl group, a substituted or unsubstituted C 6 -C 60  aryloxy group, a substituted or unsubstituted C 6 -C 60  arylthio group, a substituted or unsubstituted C 1 -C 60  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, and —Si(Q 1 )(Q 2 )(Q 3 ). 
     As an example, R 1  to R 6 , R 11  to R 13 , and R 21  to R 24  in Formulae 1, 1-1, 1-2, 2A, and 2B may each independently be selected from a group represented by Formula 1-2, a group represented by Formula 20, hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a substituted or unsubstituted C 1 -C 20  alkyl group, a substituted or unsubstituted C 1 -C 20  alkoxy group, a substituted or unsubstituted C 6 -C 20  aryl group, a substituted or unsubstituted C 1 -C 20  heteroaryl group, a substituted or unsubstituted monovalent non-aromatic condensed polycyclic group, a substituted or unsubstituted monovalent non-aromatic condensed heteropolycyclic group, and —Si(Q 1 )(Q 2 )(Q 3 ). 
     According to an exemplary embodiment of the present invention, R 1  to R 6 , R 11  to R 13 , and R 21  to R 24  in Formulae 1, 1-1, 1-2, 2A, and 2B may each independently be selected from: 
     a group represented by Formula 1-2, a group represented by Formula 20, hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a 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 amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, or a phosphoric acid group and a salt thereof; 
     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-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group (triphenylenyl), 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 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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl 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 a dibenzosilolyl group, a thiadiazolyl group, an imidazopyridinyl group, or an imidazopyrimidinyl group; 
     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-fluorenyl 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 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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl 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 a dibenzosilolyl group, a thiadiazolyl group, an imidazopyridinyl group, and an imidazopyrimidinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 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 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-fluorenyl 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 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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl 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, a thiadiazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, and —Si(Q 33 )(Q 34 )(Q 35 ); and 
     —Si(Q 3 )(Q 4 )(Q 5 ); and 
     Q 3  to Q 5  and Q 33  to Q 35  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. 
     According to an exemplary embodiment of the present invention, R 1  to R 6  in Formula 1 may each independently be selected from a group represented by Formula 1-2, a group represented by Formula 20, hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 10  alkyl group, and a C 1 -C 10  alkoxy group, 
     R 11  to R 13  in Formulae 1-1 and 1-2 may each independently be selected from: 
     hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 10  alkyl group, and a C 1 -C 10  alkoxy group; 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, and a triazinyl group; 
     a phenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, and a triazinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, a triazinyl group, and —Si(Q 31 )(Q 32 )(Q 33 ); and 
     —Si(Q 1 )(Q 2 )(Q 3 ); 
     Q 1  to Q 3  and Q 31  to Q 33  may each independently be selected from a C 1 -C 10  alkyl group, a C 1 -C 10  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, and a naphthyl group; and 
     R 21  to R 24  in Formulae 2A and 2B may each independently be selected from a group represented by Formula 20, hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 10  alkyl group, and a C 1 -C 10  alkoxy group. 
     According to an exemplary embodiment of the present invention, R 1  in Formula 1 may be selected from hydrogen and groups represented by Formulae 1-2 and 20; 
     R 1  to R 6  and R 11  to R 13  in Formulae 1, 1-1, and 1-2 may each independently be hydrogen; and 
     R 21  to R 24  in Formulae 2A and 2B may each independently be selected from hydrogen and a group represented by Formula 20. At least one of R 21  and R 22  may be a group represented by Formula 20. At least one of R 23  and R 24  may be a group represented by Formula 20. 
     In Formulae 1, 1-1, 1-2, 2A, and 2B, c1 to c6, c11 to c13, and c21 to c24 may each independently be an integer selected from 0 to 4. c1 may indicate the number of R 1 (s) in Formula 1. When c1 is 2 or greater, at least two R 1 (s) may be the same as or different from each other. c2 to c6, c11 to c13, and c21 to c24 may each independently be the same as c1 and as described herein with reference to Formulae 1, 1-1, 1-2, 2A, and 2B. 
     According to an exemplary embodiment of the present invention, c1 to c6, c11 to c13, and c21 to c24 in Formulae 1, 1-1, 1-2, 2A, and 2B may each independently be an integer selected from 0 and 1. 
     In Formula 1, n1 to n3 may each independently be an integer selected from 0 to 4. n1 may indicate the number of *-(L 2 ) a2 -(R 1 ) c1 (s). When n is 2 or greater, at least two *-(L 2 ) a2 -(R 1 ) c1 (s) may be the same as or different from each other. n2 and n3 may each independently be the same as n1 as described herein with reference to Formula 1. According to an exemplary embodiment of the present invention, n1 to n3 may each independently be an integer selected from 0 and 1. 
     According to an exemplary embodiment of the present invention, the first compound may be represented by one selected from Formulae 1A to 1L, and the second compound may be represented by one selected from Formulae 2A-1 and 2B-1: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 1A to 1L, 2A-1, and 2B-1, ring A 1 , ring A 4 , ring A 11 , ring A 14 , X 1 , X 11 , L 1 , L 2 , L 11 , L 21 , a1, a2, a11, a21, Ar 1 , Ar 11 , Ar 21 , Ar 22 , b1, b11, R 1  to R 6 , R 11  to R 13 , R 21  to R 24 , C1 to c6, c11 to c13, and c21 to c24 may each independently be as defined herein. 
     According to an exemplary embodiment of the present invention, the first compound may be represented by one selected from Formulae 1A-1, 1A-2, 1B-1, and 1C-1, and the second compound may be represented by one selected from Formulae 2A-1(1) and 2B-1(1): 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 1A-1, 1A-2, 1B-1, 1C-1, 2A-1(1), and 2B-1(1): 
     X 1 , X 11 , L 1 , L 2 , L 11 , L 21 , L 31 , a1, a2, a11, a31, Ar 1 , Ar 11 , Ar 21 , Ar 22 , Ar 31 , Ar 32 , b1, and b11 may each independently be as defined herein; 
     Ar 31a  and Ar 31b  may each independently be the same as Ar 31  as described herein; and 
     Ar 32a  and Ar 32b  may each independently be the same as Ar 32  as described herein. 
     As an example, X 1  and X 11  may each independently be O or S; 
     L 1 , L 2 , L 11 , L 21 , and L 31  may each independently be selected from groups represented by Formulae 4-1 to 4-37; 
     a1 may be an integer selected from 0, 1, and 2; 
     a2, a11, and a31 may each independently be an integer selected from 0 and 1; and 
     Ar 1 , Ar 11 , Ar 21 , Ar 22 , Ar 31 , Ar 31a , Ar 31b , Ar 32 , Ar 32a , and Ar 32b  may each independently be selected from groups represented by Formulae 6-1 to 6-158. 
     According to an exemplary embodiment of the present invention, the first compound may be one selected from Compounds 1-1 to 1-72: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In various example embodiments, the second compound may be one selected from Compounds 2-1 to 2-51: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to an exemplary embodiment of the present invention, in the organic light-emitting device, the first electrode may be an anode. The second electrode may be a cathode. The organic layer may include a hole transport region and an electron transport region. The hole transport region may be disposed between the first electrode and the emission layer. The hole transport region may include at least one selected from a hole injection layer, a hole transport layer, a buffer layer, and an electron blocking layer. The electron transport region may be disposed between the emission layer and the second electrode. The electron transport region may include at least one selected from a hole blocking layer, an electron transport layer, and an electron injection layer. 
     According to an exemplary embodiment of the present invention, in the organic light-emitting device, the first electrode may be an anode. The second electrode may be a cathode. The emission layer may include the first compound. The hole transport region may include the second compound. 
     The first compound may include at least two five-membered heterorings. Due to the inclusion of the two or more five-membered heterorings, the first compound may provide a relatively high glass transition temperature (Tg) or a relatively high melting point. Thus, the first compound may contribute to a relatively high heat resistance to Joule heating, which may occur during electroluminescent emission. The first compound may contribute to a relatively high reliability under high-temperature environments. Thus, the organic light-emitting device including the first compound may exhibit a relatively high durability during high-temperature storage and driving. 
     Use of the first compound and the second compound in combination with each other may generate synergistic effects. The synergistic effects may lead to an increased balance between electrons and holes. Thus, the organic light-emitting device including the first compound and the second compound may achieve a relatively low driving voltage and a relatively high efficiency. Additionally, the inclusion of the first compound in the organic light-emitting device may lead to increased emission efficiency. The inclusion of the second compound in the organic light-emitting device may lead to a decrease or prevention of exciton leakage. Thus, the organic light-emitting device including the first compound and the second compound may have a relatively reduced power consumption, which may lead to an increased emission efficiency. 
       FIG.  1    is a schematic cross-sectional diagram of an organic light-emitting device according to an exemplary embodiment of the present invention. 
     Referring to  FIG.  1   , an organic light-emitting device  10  may include a first electrode  110 , a hole transport region  130 , an emission layer  150 , an electron transport region  170 , and a second electrode  190 . The first electrode  110 , the hole transport region  130 , the emission layer  150 , the electron transport region  170 , and the second electrode may be sequentially stacked. 
     The substrate may be disposed under the first electrode  110 . Alternatively, the substrate may be disposed above the second electrode  190 . The substrate may include a glass substrate or a plastic substrate. The glass substrate and the plastic substrate may each have a relatively high mechanical strength, thermal stability, transparency, surface smoothness, ease of handling, and water resistance. 
     The first electrode  110  may be formed by, for example, depositing or sputtering a material for forming the first electrode  110  on the substrate. When the first electrode  110  is an anode, the material for forming the first electrode  110  may include materials with a relatively high work function, which may 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, the material included in the first electrode  110  may include indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO 2 ), zinc oxide (ZnO), or any combinations thereof; however, exemplary embodiments of the present invention are not limited thereto. When the first electrode  110  is a semi-transmissive electrode or a reflective electrode, the material for forming the first electrode  110  may include magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), and any combinations thereof; however, exemplary embodiments of the present invention are not limited thereto. 
     The first electrode  110  may have a single-layered structure. The first electrode  110  may have a multi-layered structure including two or more layers. For example, the first electrode  110  may have a three-layered structure of ITO/Ag/ITO; however, the structure of the first electrode  110  is not limited thereto. 
     Holes provided from the first electrode  110  may move toward the emission layer  150  through the hole transport region  130 . 
     The hole transport region  130  may have a single-layered structure including a single material. The hole transport region  130  may have a single-layered structure including a plurality of different materials. The hole transport region  130  may have a multi-layered structure having a plurality of layers each including a single material or a plurality of different materials. 
     The hole transport region  130  may have a single-layered structure formed of a hole injection layer. The hole transport region  130  may have a single-layered structure formed of a hole transport layer. The hole transport region  130  may have a multi-layered structure having a hole injection layer/hole transport layer structure, a hole injection layer/hole transport layer/buffer layer structure, a hole injection layer/buffer layer structure, or a hole transport layer/buffer layer structure. Layers of each structure may be sequentially stacked from the first electrode  110 ; however, the structure of the hole transport region  130  is not limited thereto. 
     When the hole transport region  130  includes a hole injection layer, the hole injection layer may be formed on the first electrode  110  by using one or more suitable methods including vacuum deposition, spin coating, casting a Langmuir-Blodgett (LB) method, ink-jet printing, laser-printing, and laser induced thermal imaging (LITI). 
     When a hole injection layer is formed by vacuum deposition, for example, the vacuum deposition may be performed at a deposition temperature of from about 100° C. to about 500° C., at a vacuum degree of from about 10 −8  torr to about 10 −3  torr, and at a deposition rate of from about 0.01 Å/sec to about 100 Å/sec, by taking into account the compound for the hole injection layer to be deposited, and the structure of the hole injection layer to be formed. 
     When a hole injection layer is formed by spin coating, the spin coating may be performed at a coating rate of from about 2,000 rpm to about 5,000 rpm and at a temperature of from about 80° C. to about 200° C., by taking into account the compound for the hole injection layer to be deposited, and the structure of the hole injection layer to be formed. 
     When the hole transport region  130  includes a hole transport layer, the hole transport layer may be formed on the first electrode  110  or the hole injection layer by using one or more suitable methods selected from vacuum deposition, spin coating, casting, an LB method, ink-jet printing, laser-printing, and LITI. When the hole transport layer is formed by vacuum deposition or spin coating, deposition and coating conditions for the hole transport layer may be determined by referring to the deposition and coating conditions for the hole injection layer. 
     The hole transport region  130  may include the second compound described above. According to an exemplary embodiment of the present invention, the hole transport region  130  may include a hole transport layer and hole injection layer. The hole transport layer may be disposed between the first electrode and the emission layer. The hole injection layer may be disposed between the first electrode and the hole transport layer. The hole transport layer may include the second compound. The second compound may be as defined herein. 
     The hole transport region may further include, in addition to the second compound, for example, at least one selected from m-MTDATA, TDATA, 2-TNATA, NPB, β-NPB, TPD, spiro-TPD, spiro-NPB, methylated-NPB, TAPC, HMTPD, 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), polyaniline/dodecylbenzene sulfonic acid (PANI/DBSA), poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), polyaniline/camphor sulfonic acid (PANI/CSA), polyaniline/poly(4-styrenesulfonate) (PANI/PSS), a compound represented by Formula 201, and a compound represented by Formula 202: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 201 and 202: 
     X 201  may be selected from nitrogen (N), boron (B), and phosphorus (P); 
     L 201  to L 205  may each independently be the same as L 1  as described herein; 
     xa1 to xa4 may each independently be an integer selected from 0 to 3; 
     xa5 may be an integer selected from 1 to 5; and 
     R 201  to R 204  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. 
     As an example, in Formulae 201 and 202: 
     L 201  to L 205  may each independently be selected from: 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-fluorenylene group, a benzofluorene group, a dibenzofluorene group, a phenanthrenylene group, an anthracenylene group, a pyrenylene group, a chrysenylene group, a pyridinylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a quinolinylene group, an isoquinolinylene group, a quinoxalinylene group, a quinazolinylene group, a carbazolylene group, and a triazinylene group; and 
     a phenylene group, a naphthylene group, a fluorenylene group, a spiro-fluorenylene group, a benzofluorenylene group, a dibenzofluorenylene group, a phenanthrenylene group, an anthracenylene group, a pyrenylene group, a chrysenylene group, a pyridinylene group, a pyrazinylene group, a pyrimidinylene group, a pyridazinylene group, a quinolinylene group, an isoquinolinylene group, a quinoxalinylene group, a quinazolinylene group, a carbazolylene group, and a triazinylene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 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-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an isoindolyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, and a triazinyl group; 
     xa1 to xa4 may each independently be an integer selected from 0 to 2; 
     xa5 may be an integer selected from 1 to 3; and 
     R 201  to R 204  may each independently be selected from: 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, and a triazinyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, and a triazinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, an azulenyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, and a triazinyl group; however, exemplary embodiments of the present invention are not limited thereto. 
     According to an exemplary embodiment of the present invention, the compound represented by Formula 201 may be represented by Formula 201A: 
     
       
         
         
             
             
         
       
     
     According to an exemplary embodiment of the present invention, the compound represented by Formula 201 may be represented by Formula 201A-1: 
     
       
         
         
             
             
         
       
     
     According to an exemplary embodiment of the present invention, the compound represented by Formula 202 may be represented by Formula 202A: 
     
       
         
         
             
             
         
       
     
     In Formulae 201A, 201A-1, and 202A: 
     X 201 , L 201  to L 203 , xa1 to xa3, xa5, and R 202  to R 204  may each independently be as described herein; 
     R 211  and R 212  may each independently be the same as R 203  as described herein; 
     R 213  to R 216  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a 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, and a monovalent non-aromatic condensed heteropolycyclic group. 
     The compound represented by Formula 201 and the compound represented by Formula 202 may each independently include at least one compound selected from Compounds HT1 to HT20; however, exemplary embodiments of the present invention are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A thickness of the hole transport region may range from about 100 Å to about 10,000 Å, for example, from about 100 Å to about 1,000 Å. When the hole transport region includes both a hole injection layer and a hole transport layer, a thickness of the hole injection layer may range from about 100 Å to about 10,000 Å, for example, from about 100 Å to less than about 1,000 Å, and a thickness of the hole transport layer may range from about 50 Å to about 2,000 Å, for example, from about 100 Å to about 1,500 Å. When the thicknesses of the hole transport region, the hole injection layer, and the hole transport layer are within any of these ranges, satisfactory hole transporting characteristics may be obtained without a substantial increase in driving voltage. 
     The hole transport region may include a charge-generation material. The charge-generation material may increase conductive properties of the hole transport region. The charge-generation material may be substantially homogeneously or non-homogeneously dispersed in the hole transport region. 
     The charge-generation material may be, for example, a p-dopant. The p-dopant may include at least one selected from a quinone derivative, a metal oxide, and a cyano group-containing compound; however, exemplary embodiments of the present invention are not limited thereto. For example, non-limiting examples of the p-dopant may include a quinone derivative, such as tetracyanoquinodimethane (TCNQ) or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ); a metal oxide, such as a tungsten oxide or a molybdenum oxide; or Compound HT-D1; however, exemplary embodiments of the present invention are not limited thereto: 
     
       
         
         
             
             
         
       
     
     The hole transport region may include at least one selected from a buffer layer and an electron blocking layer. Since the buffer layer may compensate for an optical resonance distance according to a wavelength of light emitted from the emission layer, light emission efficiency of a formed organic light-emitting device may be increased. Materials included in the hole transport region may be included in the buffer layer. The electron blocking layer may decrease or prevent injection of electrons from the electron transport region. 
     The emission layer  150  may be formed on the hole transport region  130  by using one or more suitable methods selected from vacuum deposition, spin coating, casting, an LB method, ink-jet printing, laser-printing, and LITI. When the emission layer  150  is formed by vacuum deposition or spin coating, deposition and coating conditions for the emission layer  150  may be determined by referring to the deposition and coating conditions for the hole injection layer. 
     The emission layer  150  may include the first compound as described above. 
     In  FIG.  1   , the emission layer  150  may include a dopant. The first compound included in the emission layer  150  may serve as a host. The first compound included in the emission layer  150  of  FIG.  1    may be as defined herein. 
     When the organic light-emitting device  10  is a full color organic light-emitting device, the emission layer  150  may be patterned into a red emission layer, a green emission layer, or a blue emission layer, according to a sub-pixel. According to one or more exemplary embodiments of the present invention, the emission layer  150  may have a stacked structure. The stacked structure may include a red emission layer, a green emission layer, and a blue emission layer. Alternatively, the stacked structure may include a red-light emission material, a green-light emission material, and a blue-light emission material. The red-light emission material, the green-light emission material, and the blue-light emission material may be mixed with each other in a single layer. The red-light emission material, the green-light emission material, and the blue-light emission material mixed with each other in a single layer may emit white light. 
     The emission layer  150  may include a phosphorescent dopant or a fluorescent dopant. 
     The phosphorescent dopant may include an organometallic complex represented by Formula 401: 
     
       
         
         
             
             
         
       
     
     In Formula 401: 
     M may be selected from iridium (Ir), platinum (Pt), osmium (Os), titanium (Ti), zirconium (Zr), hafnium (Hf), europium (Eu), terbium (Tb), and thulium (TM); 
     X 401  to X 404  may each independently be selected from nitrogen (N) and carbon (C); 
     ring A 401  and ring A 402  may each independently be selected from a substituted or unsubstituted benzene, a substituted or unsubstituted naphthalene, a substituted or unsubstituted fluorene, a substituted or unsubstituted spiro-fluorene, a substituted or unsubstituted indene, a substituted or unsubstituted pyrrole, a substituted or unsubstituted thiophene, a substituted or unsubstituted furan, a substituted or unsubstituted imidazole, a substituted or unsubstituted pyrazole, a substituted or unsubstituted thiazole, a substituted or unsubstituted isothiazole, a substituted or unsubstituted oxazole, a substituted or unsubstituted isoxazole, a substituted or unsubstituted pyridine, a substituted or unsubstituted pyrazine, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted pyridazine, a substituted or unsubstituted quinoline, a substituted or unsubstituted isoquinoline, a substituted or unsubstituted benzoquinoline, a substituted or unsubstituted quinoxaline, a substituted or unsubstituted quinazoline, a substituted or unsubstituted carbazole, a substituted or unsubstituted benzimidazole, a substituted or unsubstituted benzofuran, a substituted or unsubstituted benzothiophene, a substituted or unsubstituted isobenzothiophene, a substituted or unsubstituted benzoxazole, a substituted or unsubstituted isobenzoxazole, a substituted or unsubstituted triazole, a substituted or unsubstituted oxadiazole, a substituted or unsubstituted triazine, a substituted or unsubstituted dibenzofuran, and a substituted or unsubstituted dibenzothiophene; 
     at least one substituent selected from the substituted benzene, the substituted naphthalene, the substituted fluorene, the substituted spiro-fluorene, the substituted indene, the substituted pyrrole, the substituted thiophene, the substituted furan, the substituted imidazole, the substituted pyrazole, the substituted thiazole, the substituted isothiazole, the substituted oxazole, the substituted isoxazole, the substituted pyridine, the substituted pyrazine, the substituted pyrimidine, the substituted pyridazine, the substituted quinoline, the substituted isoquinoline, the substituted benzoquinoline, the substituted quinoxaline, the substituted quinazoline, the substituted carbazole, the substituted benzimidazole, the substituted benzofuran, the substituted benzothiophene, the substituted isobenzothiophene, the substituted benzoxazole, the substituted isobenzoxazole, the substituted triazole, the substituted oxadiazole, the substituted triazine, the substituted dibenzofuran, and the substituted dibenzothiophene may be selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group (non-aromatic condensed polycyclic group), a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 401 )(Q 402 ), —Si(Q 403 )(Q 404 )(Q 405 ), and —B(Q 406 )(Q 407 ); 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60 aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 411 )(Q 412 ), —Si(Q 413 )(Q 414 )(Q 415 ), and —B(Q 416 )(Q 417 ). 
     —N(Q 421 )(Q 422 ), —Si(Q 423 )(Q 424 )(Q 425 ), and —B(Q 426 )(Q 427 ). 
     In Formula 401, L 401  may be an organic ligand; 
     xc1 may be an integer selected from 1 to 3; and 
     xc2 may be an integer selected from 0 to 3. 
     In Formula 401, L 401  may be a monovalent, divalent, or trivalent organic ligand. For example, L 401  may be selected from a halogen ligand (e.g., Cl or F), a diketone ligand (e.g., acetylacetonate, 1,3-diphenyl-1,3-propandionate, 2,2,6,6-tetramethyl-3,5-heptanedionate, or hexafluoroacetonate), a carboxylic acid ligand (e.g., picolinate, dimethyl-3-pyrazolecarboxylate, or benzoate), a carbon monoxide ligand, an isonitrile ligand, a cyano group ligand, and a phosphorus ligand (e.g., phosphine or phosphite); however, exemplary embodiments of the present invention are not limited thereto. 
     According to an exemplary embodiment of the present invention, Q 401  to Q 407 , Q 411  to Q 417 , and Q 421  to Q 427  may each independently be selected from hydrogen, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 6 -C 60  aryl group, and a C 2 -C 60  heteroaryl group. 
     In Formula 401, when A 401  has two or more substituents, the substituents of A 401  may be linked to each other to form a saturated or unsaturated ring. 
     In Formula 401, when A 402  has two or more substituents, the substituents of A 402  may be linked to each other to form a saturated or unsaturated ring. 
     When xc1 in Formula 401 is 2 or greater, a plurality of ligands in Formula 401 may be the same as or different from each other. When xc1 in Formula 401 is 2 or greater, A 401  and A 402  may be respectively directly connected to A 401  and A 402  of other neighboring ligands with or without a linking group (e.g., a C 1 -C 5  alkylene group, —N(R′)—, in which R′ is a C 1 -C 10  alkyl group or a C 6 -C 20  aryl group, or —C(═O)—). 
     According to an exemplary embodiment of the present invention, the phosphorescent dopant may be, for example, selected from Compounds PD1 to PD75; however, exemplary embodiments of the present invention are not limited thereto: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to an exemplary embodiment of the present invention, the fluorescent dopant may include a compound represented by Formula 501: 
     
       
         
         
             
             
         
       
     
     In Formula 501: 
     Ar 501  may be selected from: 
     a naphthalene group, a heptalene group, a fluorene group, a spiro-fluorene 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, and an indenoanthracene group; and 
     a naphthalene group, a heptalene group, a fluorene group, a spiro-fluorene 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, and an indenoanthracene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, and —Si(Q 501 )(Q 502 )(Q 503 ) (wherein Q 501  to Q 503  may each independently be selected from hydrogen, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 6 -C 60  aryl group, and a C 1 -C 60  heteroaryl group); 
     L 501  to L 503  may each independently be the same as L 1  as described herein; 
     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-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazole group, a triazinyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a triazinyl group, a dibenzofuranyl group, 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 amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, a triazinyl group, a dibenzofuranyl group, and a dibenzothiophenyl group; 
     xd1 to xd3 may each independently be an integer selected from 0 to 3; and 
     xd4 may be an integer selected from 1 to 4. 
     According to an exemplary embodiment of the present invention, the fluorescent dopant may include at least one selected from Compounds FD1 to FD9: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to an exemplary embodiment of the present invention, the fluorescent dopant may be selected from the following compounds; however, exemplary embodiments of the present invention are not limited thereto: 
     
       
         
         
             
             
         
       
     
     An amount of the dopant in the emission layer  150  may range from about 0.01 parts to about 15 parts by weight based on 100 parts by weight of the host; however, exemplary embodiments of the present invention are limited thereto. 
     A thickness of the emission layer  150  may range from about 100 Å to about 1,000 Å, for example, from about 200 Å to about 600 Å. When the thickness of the emission layer  150  is within any of these ranges, relatively high light emission characteristics may be obtained without a substantial increase in driving voltage. 
     The electron transport region  170  may be disposed on the emission layer  150 . 
     The electron transport region  170  may include at least one selected from a hole blocking layer, an electron transport layer, and an electron injection layer; however, exemplary embodiments of the present invention are not limited thereto. 
     For example, the electron transport region  170  may have an electron transport layer/electron injection layer structure or a hole blocking layer/electron transport layer/electron injection layer structure. Layers included in each structure may be sequentially stacked from the emission layer  150 ; however, the structure of the electron transport region  170  is not limited thereto. 
     When the electron transport region  170  includes a hole blocking layer, the hole blocking layer may be formed on the emission layer  150  by using one or more suitable methods selected from vacuum deposition, spin coating, casting, an LB method, ink-jet printing, laser-printing, and LITI. When the hole blocking layer is formed by vacuum deposition or spin coating, deposition and coating conditions for the hole blocking layer may be determined by referring to the deposition and coating conditions for the hole injection layer. 
     The hole blocking layer may include, for example, at least one selected from BCP and Bphen; however, exemplary embodiments of the present invention are not limited thereto: 
     
       
         
         
             
             
         
       
     
     A thickness of the hole blocking layer may range from about 20 Å to about 1,000 Å, for example, from about 30 Å to about 300 Å. When the thickness of the hole blocking layer is within any of these ranges, a relatively high hole blocking ability may be obtained without a substantial increase in driving voltage. 
     The electron transport region  170  may include an electron transport layer. The electron transport layer may be formed on the emission layer  150  or the hole blocking layer by one or more suitable methods selected from vacuum deposition, spin coating, casting, an LB method, ink-jet printing, laser-printing, and LITI. When the electron transport layer is formed by vacuum deposition or spin coating, deposition and coating conditions for the electron transport layer may be determined by referring to the deposition and coating conditions for the hole injection layer. 
     According to an exemplary embodiment of the present invention, the electron transport layer may include at least one of a compound represented by Formula 601 or a compound represented by Formula 602:
 
Ar 601 -[(L 601 ) xe1 -E 601 ] xe2 ,  &lt;Formula 601&gt;
 
     In Formula 601: 
     Ar 601  may be selected from: 
     a naphthalene group, a heptalene group, a fluorene group, a spiro-fluorene 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, and an indenoanthracene group; and 
     a naphthalene group, a heptalene group, a fluorene group, a spiro-fluorene 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, and an indenoanthracene group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, and —Si(Q 301 )(Q 302 )(Q 303 ); 
     Q 301  to Q 303  may each independently be selected from hydrogen, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 6 -C 60  aryl group, and a C 1 -C 60  heteroaryl group; 
     L 601  may be the same as to L 201  as described herein; 
     E 601  may be selected from: 
     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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl 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, and an imidazopyrimidinyl group; and 
     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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl 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, and an imidazopyrimidinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, 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-fluorenyl 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 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 phthalazinyl group, a naphthyridinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthridinyl group, an acridinyl group, a phenanthrolinyl group, a phenazinyl 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, and an imidazopyrimidinyl group; 
     xe1 may be an integer selected from 0 to 3; and 
     xe2 may be an integer selected from 1 to 4. 
     
       
         
         
             
             
         
       
     
     In Formula 602: 
     X 611  may be N or C-(L 611 ) xe611 -R 611 , X 612  may be N or C-(L 612 ) xe612 -R 612 , and X 613  may be N or C-(L 613 ) xe613 -R 613 , in which at least one of X 611  to X 613  may be N; 
     L 611  to L 616  may each independently be the same as L 1  as described herein; 
     R 611  to R 616  may each independently be selected from: 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, and a triazinyl group; and 
     a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, and a triazinyl group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, an azulenyl group, a fluorenyl group, a spiro-fluorenyl group, a benzofluorenyl group, a dibenzofluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a chrysenyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a quinolinyl group, an isoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a carbazolyl group, and a triazinyl group; and 
     xe611 to xe616 may each independently be an integer selected from 0 to 3. 
     According to an exemplary embodiment of the present invention, the compound represented by Formula 601 and the compound represented by Formula 602 may each independently be selected from Compounds ET1 to ET15: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to an exemplary embodiment of the present invention, the electron transport layer may include at least one selected from BCP, Bphen, Alq 3 , BAlq, TAZ, and NTAZ: 
     
       
         
         
             
             
         
       
     
     A thickness of the electron transport layer may range from about 100 Å to about 1,000 Å, for example, from about 150 Å to about 500 Å. When the thickness of the electron transport layer is within any of these ranges, satisfactory electron transport characteristics may be obtained without a substantial increase in driving voltage. 
     The electron transport layer may include a material including metal. 
     The material including metal may include a lithium (Li) complex. The lithium (Li) complex may include, for example, Compound ET-D1 (lithium quinolate, LiQ) or ET-D2: 
     
       
         
         
             
             
         
       
     
     The electron transport region  170  may include an electron injection layer. The electron injection layer may facilitate injection of electrons from the second electrode  190 . 
     The electron injection layer may be formed on the electron transport layer by using one or more suitable methods selected from vacuum deposition, spin coating, casting, an LB method, ink-jet printing, laser-printing, and LITI. When the electron injection layer is formed by vacuum deposition or spin coating, deposition and coating conditions for the electron injection layer may be determined by referring to the deposition and coating conditions for the hole injection layer. 
     The electron injection layer may include at least one selected from LiF, NaCl, CsF, Li 2 O, BaO, and LiQ. 
     A thickness of the electron injection layer may range from about 1 Å to about 100 Å, for example, from about 3 Å to about 90 Å. When the thickness of the electron injection layer is within any of these ranges, satisfactory electron injection characteristics may be obtained without a substantial increase in driving voltage. 
     The second electrode  190  may be disposed on the electron transport region  170 . The second electrode  190  may be a cathode. The cathode may be an electron injection electrode. Thus, a material for forming the second electrode  190  may be a material having a relatively low work function, such as a metal, an alloy, an electrically conductive compound, or a mixture thereof. Examples of the material for forming the second electrode  190  may include Li, Mg, Al, Al—Li, Ca, Mg—In, or Mg—Ag. For example, the material for forming the second electrode  190  may include ITO or IZO. The second electrode  190  may be a semi-transmissive electrode or a transmissive electrode. 
     The term “C 1 -C 60  alkyl group” as used herein refers to a linear or branched aliphatic saturated hydrocarbon monovalent group having 1 to 60 carbon atoms, and non-limiting examples thereof may include a methyl group, an ethyl group, a propyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an iso-amyl group, or a hexyl group. The term “C 1 -C 60  alkylene group” as used herein refers to a divalent group having substantially the same structure as the C 1 -C 60  alkyl group. 
     The term “C 1 -C 60  alkoxy group” as used herein refers to a monovalent group represented by —OA 101 , in which A 101  is the C 1 -C 60  alkyl group, and non-limiting examples thereof may include a methoxy group, an ethoxy group, or an isopropyloxy group. 
     The term “C 2 -C 60  alkenyl group” as used herein refers to a hydrocarbon group formed by substituting at least one carbon-carbon double bond in the middle or at the terminus of the C 2 -C 60  alkyl group, and non-limiting examples thereof may include an ethenyl group, a propenyl group, or a butenyl group. The term “C 2 -C 60  alkenylene group” as used herein refers to a divalent group having substantially the same structure as the C 2 -C 60  alkenyl group. 
     The term “C 2 -C 60  alkynyl group” as used herein refers to a hydrocarbon group formed by substituting at least one carbon-carbon triple bond in the middle or at the terminus of the C 2 -C 60  alkyl group, and non-limiting examples thereof may include an ethynyl group, or a propynyl group. The term “C 2 -C 60  alkynylene group” as used herein refers to a divalent group having substantially the same structure as the C 2 -C 60  alkynyl group. 
     The term “C 3 -C 10  cycloalkyl group” as used herein refers to a monovalent saturated hydrocarbon monocyclic group having 3 to 10 carbon atoms, and non-limiting examples thereof may include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, or 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 saturated monocyclic group having at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom and 1 to 10 carbon atoms, and non-limiting examples thereof may include a tetrahydrofuranyl group or a tetrahydrothiophenyl group. The term “C 1 -C 10  heterocycloalkylene group” as used herein refers to a divalent group having substantially the same structure as the C 1 -C 10  heterocycloalkyl group. 
     The term “C 3 -C 10  cycloalkenyl group” as used herein refers to a monovalent monocyclic group that has 3 to 10 carbon atoms, at least one carbon-carbon double bond in the ring thereof, and does not have aromaticity, and non-limiting examples thereof may include a cyclopentenyl group, a cyclohexenyl group, or a cycloheptenyl group. The term “C 3 -C 10  cycloalkenylene group” as used herein refers to a divalent group having substantially the same structure as the C 3 -C 10  cycloalkenyl group. 
     The term “C 1 -C 10  heterocycloalkenyl group” as used herein refers to a monovalent monocyclic group that has at least one heteroatom selected from N, O, Si, P, and S as a ring-forming atom, 1 to 10 carbon atoms, and at least one carbon-carbon double bond in its ring. Non-limiting examples of the C 1 -C 10  heterocycloalkenyl group may include a 2,3-dihydrofuranyl group or a 2,3-dihydrothiophenyl group. The term “C 1 -C 10  heterocycloalkenylene group” as used herein refers to a divalent group having substantially the same structure as the C 1 -C 10  heterocycloalkenyl group. 
     The term “C 6 -C 60  aryl group” as used herein refers to a monovalent group having a carbocyclic aromatic system having 6 to 60 carbon atoms. The term “C 6 -C 60  arylene group” as used herein refers to a divalent group having a carbocyclic aromatic system having 6 to 60 carbon atoms. Non-limiting examples of the C 6 -C 60  aryl group may include a phenyl group, a naphthyl group, an anthracenyl group, a phenanthrenyl group, a pyrenyl group, or a chrysenyl group. When the C 6 -C 60  aryl group and the C 6 -C 60  arylene group each include two or more rings, the rings may be chemically bonded to each other. 
     The term “C 1 -C 60  heteroaryl group” as used herein refers to a monovalent group having an aromatic system that has at least one heteroatom selected from N, O, P, and S as a ring-forming atom, and 1 to 60 carbon atoms. The term “C 1 -C 60  heteroarylene group” as used herein refers to a divalent group having an aromatic system that has at least one heteroatom selected from N, O, P, and S as a ring-forming atom, and 1 to 60 carbon atoms. Non-limiting examples of the C 1 -C 60  heteroaryl group may include a pyridinyl group, a pyrimidinyl group, a pyrazinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, or an isoquinolinyl group. When the C 1 -C 60  heteroaryl group and the C 1 -C 60  heteroarylene group each include two or more rings, the rings may be chemically bonded to each other. 
     The term “C 6 -C 60  aryloxy group” as used herein refers to a group represented by —OA 102 , in which A 102  is the C 6 -C 60  aryl group. The term “C 6 -C 60  arylthio group” as used herein refers to a group represented by —SA 103 , in which 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 (e.g., having 8 to 60 carbon atoms) that has two or more rings condensed to each other, only carbon atoms as a ring-forming atom, and non-aromaticity in the entire molecular structure. A non-limiting example 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 (e.g., having 1 to 60 carbon atoms) that has two or more rings condensed to each other, has a heteroatom selected from N, O, Si, P, and S, other than carbon atoms as a ring-forming atom, and has non-aromaticity in the entire molecular structure. A non-limiting example of the monovalent non-aromatic condensed heteropolycyclic group may include a carbazolyl. The term “divalent non-aromatic condensed heteropolycyclic group” as used herein refers to a divalent group having the same structure as the monovalent non-aromatic condensed heteropolycyclic group. 
     At least one substituent of substituents of the substituted condensed polycyclic 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, or the substituted monovalent non-aromatic condensed heteropolycyclic group may be selected from: 
     deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, and a C 1 -C 60  alkoxy group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 11 )(Q 12 ), —Si(Q 13 )(Q 14 )(Q 15 ), and —B(Q 16 )(Q 17 ); 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group; 
     a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, and a monovalent non-aromatic condensed heteropolycyclic group, each substituted with at least one selected from deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 10  cycloalkyl group, a C 1 -C 10  heterocycloalkyl group, a C 3 -C 10  cycloalkenyl group, a C 1 -C 10  heterocycloalkenyl group, a C 6 -C 60  aryl group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 1 -C 60  heteroaryl group, a monovalent non-aromatic condensed polycyclic group, a monovalent non-aromatic condensed heteropolycyclic group, —N(Q 21 )(Q 22 ), —Si(Q 23 )(Q 24 )(Q 25 ), and —B(Q 26 )(Q 27 ); and 
     —N(Q 31 )(Q 32 ), —Si(Q 33 )(Q 34 )(Q 35 ), and —B(Q 36 )(Q 37 ); and 
     Q 1  to Q 7 , Q 11  to Q 17 , Q 21  to Q 27 , and Q 37  to Q 37  may each independently be selected from hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, an amino group, an amidino group, a hydrazine group, a hydrazone group, a carboxylic acid group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid group or a salt thereof, a 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, and a monovalent non-aromatic condensed heteropolycyclic group. 
     The term “Ph” as used herein may refer to a phenyl group. The term “Me” as used herein may refer to a methyl group. The term “Et” as used herein may refer to an ethyl group. The terms “ter-Bu” or “Bu t ” as used herein may refer to a tert-butyl group. 
     An organic light-emitting device according to an exemplary embodiment of the present invention will be described in more detail below with reference to Synthesis Examples and Examples. The wording “B was used instead of A” used in describing Synthesis Examples refers to an example in which an identical molar equivalent of B was used in place of A. 
     EXAMPLES 
     Example 1 
     As a substrate and an anode, an ITO glass substrate having a thickness of about 15 Ω/cm 2  (about 1,200 Å) was cut to a size of 50 mm×50 mm×0.7 mm, sonicated by using isopropyl alcohol and pure water for about 5 minutes each, cleaned by exposure to ultraviolet rays for about 30 minutes, and then exposed to ozone, and the resulting ITO glass substrate was mounted on a vacuum deposition apparatus. 
     Compound HT13 was vacuum deposited on the ITO anode to form a hole injection layer having a thickness of about 700 Å, and then, Compound HT-1 was vacuum deposited on the hole injection layer to form a hole transport layer having a thickness of about 1,100 Å. 
     Compound H1-1 and PD76 (as a dopant) were co-deposited on the hole transport layer at a weight ratio of 98:2 to form an emission layer having a thickness of about 400 Å. 
     Compound ET1 and LiQ were co-deposited on the emission layer at a weight ratio of 5:5 to form an electron transport layer having a thickness of about 300 Å. LiQ was deposited on the electron transport layer to form an electron injection layer having a thickness of about 10 Å. Thus, an electron transport region was formed. 
     Al was vacuum deposited on the electron transport region to form a cathode having a thickness of about 1,000 Å. Thus, an organic light-emitting device was formed. 
     
       
         
         
             
             
         
       
     
     Examples 2 to 5 and Comparative Examples 1 and 2 
     Organic light-emitting devices were each manufactured in substantially the same manner as in Example 1, except that in forming a hole transport layer and an emission layer, compounds shown in Table 1 were used. 
     Evaluation Example 1 
     Driving voltage, current density, and efficiency of the organic light-emitting devices of Examples 1 to 5 and Comparative Examples 1 and 2 were measured by using a Kethley SMU 236 and a PR650 luminance meter, and results thereof are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                   
                 Driving 
                 Current 
                   
               
               
                   
                 Hole transport 
                 Emission layer 
                 voltage 
                 density 
                 Efficiency 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 layer 
                 Host 
                 Dopant 
                 (V) 
                 (mA/cm 2 ) 
                 (cd/A) 
               
               
                   
               
               
                 Example 1 
                 Compound 2-2   
                 Compound 1-22 
                 Compound 
                 4.5 
                 10 
                 38.5 
               
               
                   
                   
                   
                 PD76 
                   
                   
                   
               
               
                 Example 2 
                 Compound 2-7  
                 Compound 1-24 
                 Compound 
                 4.3 
                 10 
                 36.2 
               
               
                   
                   
                   
                 PD76 
                   
                   
                   
               
               
                 Example 3 
                 Compound 2-51 
                 Compound 1-24 
                 Compound 
                 4.2 
                 10 
                 39.3 
               
               
                   
                   
                   
                 PD76 
                   
                   
                   
               
               
                 Example 4 
                 Compound 2-29 
                 Compound 1-25 
                 Compound 
                 4.3 
                 10 
                 37.7 
               
               
                   
                   
                   
                 PD76 
                   
                   
                   
               
               
                 Example 5 
                 Compound 2-51 
                 Compound 1-23 
                 Compound 
                 4.4 
                 10 
                 38.1 
               
               
                   
                   
                   
                 PD76 
                   
                   
                   
               
               
                 Comparative 
                 Compound A1      
                 Compound 1-23 
                 Compound 
                 4.8 
                 10 
                 31.7 
               
               
                 Example 1 
                   
                   
                 PD76 
                   
                   
                   
               
               
                 Comparative 
                 Compound 2-2  
                 Alq 3   
                 Compound 
                 6.2 
                 10 
                 17.6 
               
               
                 Example 2 
                   
                   
                 PD76 
               
               
                   
               
               
                                   
1-22 
               
               
                                   
1-23 
               
               
                                   
1-24 
               
               
                                   
1-25 
               
               
                                   
2-2 
               
               
                                   
2-7 
               
               
                                   
2-29 
               
               
                                   
2-51 
               
               
                                   
Compound A1 
               
            
           
         
       
     
     Referring to Table 1, the organic light-emitting devices of Examples 1 to 5 exhibited a relatively high efficiency and a relatively long lifespan as compared with those of the organic light-emitting devices of Comparative Examples 1 and 2. 
     An organic light-emitting device according to an exemplary embodiment of the present invention may exhibit relatively low driving voltage and relatively high efficiency. 
     It should be understood that exemplary embodiments of the present invention described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each exemplary embodiment should typically be considered as available for other similar features or aspects in other exemplary embodiments of the present invention. 
     While one or more exemplary embodiments of the present invention 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 invention.