Patent Publication Number: US-2023165022-A1

Title: Light-emitting device and electronic apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2021-0163727, filed on Nov. 24, 2021, in the Korean Intellectual Property Office, the entire content of which is hereby incorporated by reference. 
     BACKGROUND 
     1. Field 
     One or more embodiments of the present disclosure relate to a light-emitting device and an electronic apparatus including the same. 
     2. Description of the Related Art 
     Self-emissive devices among light-emitting devices have wide viewing angles, high contrast ratios, short response times, and excellent characteristics in terms of luminance, driving voltage, and response speed. 
     In a light-emitting device, a first electrode is on a substrate, and a hole transport region, an emission layer, an electron transport region, and a second electrode are sequentially on the first electrode. Holes provided from the first electrode may move toward the emission layer through the hole transport region, and electrons provided from the second electrode may move toward the emission layer through the electron transport region. Carriers, such as holes and electrons, recombine in such an emission layer region to produce excitons. These excitons transition from an excited state to a ground state to thereby generate light. 
     SUMMARY 
     Provided are a light-emitting device and an electronic apparatus including the light-emitting device. 
     Additional aspects of embodiments will be set forth in part in the description, which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments of the disclosure. 
     According to one or more embodiments, 
     provided is a light-emitting device including a first electrode, 
     a second electrode facing the first electrode, and 
     an interlayer between the first electrode and the second electrode and including an emission layer, 
     wherein the emission layer includes a first emission layer and a second emission layer, 
     the first emission layer includes a first host, 
     the second emission layer includes a second host and a third host, and 
     a hole mobility of the first host (μH 1 ), a hole mobility of the second host (μH 2 ), and a hole mobility of the third host (μH 3 ) satisfy Expressions (1) and (2) below. 
       μ H   1   &gt;μH   2   (1)
 
       μ H   1   &gt;μH   3   (2)
 
     According to one or more embodiments, 
     provided is an electronic apparatus including the light-emitting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects and features of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a schematic view of a structure of a light-emitting device according to an embodiment; 
         FIG.  2    is a cross-sectional view of an electronic apparatus according to an embodiment of the present disclosure; and 
         FIG.  3    is a cross-sectional view of an electronic apparatus according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in more detail to embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, embodiments are merely described below, by referring to the figures, to explain aspects of embodiments of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Throughout the disclosure, the expression “at least one of a, b and c” indicates only a, only b, only c, both a and b, both a and c, both b and c, all of a, b, and c, or variations thereof. 
     A fluorescent blue emission layer of a light-emitting device of the related art includes a single host and a single dopant, and such a host has stronger electron transporting characteristics than hole transporting characteristics. For this reason, holes and electrons recombine in an interface between an electron blocking layer and an emission layer, thereby causing triplet-triplet fusion (TTF). As a result, the electron blocking layer is deteriorated and the lifespan of the light-emitting device is decreased. 
     According to one or more embodiments, a light-emitting includes: 
     a first electrode; 
     a second electrode facing the first electrode; and 
     an interlayer between the first electrode and the second electrode and including an emission layer, 
     wherein the emission layer includes a first emission layer and a second emission layer, 
     the first emission layer includes a first host, 
     the second emission layer includes a second host and a third host, and 
     a hole mobility of the first host (μH 1 ), a hole mobility of the second host (μH 2 ), and a hole mobility of the third host (μH 3 ) satisfy Expressions (1) and (2) below: 
       μ H   1   &gt;μH   2   (1)
 
       μ H   1   &gt;μH   3   (2).
 
     In an embodiment, the hole mobility of the second host (μH 2 ) and the hole mobility of the third host (μH 3 ) may satisfy Expression (3) below. 
       μ H   2   μH   3   (3)
 
     In an embodiment, a method of measuring the hole mobility is not limited, but, for example, a time of flight method may be used. In the time of flight method, from an electrode/organic layer/electrode structure, time characteristics of transient current (transient characteristic time) generated by irradiating light of a wavelength corresponding to an absorption wavelength region of the organic layer may be measured, and the hole mobility may be calculated from the following Measurement Equation. In an embodiment, the hole mobility may be measured via a JV curve after a device including an electrode/interlayer/electrode structure is manufactured. 
     An electron mobility may also be measured by a method similar to the method utilized for measuring the hole mobility. 
     Measurement Equation 
       Hole mobility=(thickness of interlayer) 2 /(transient characteristic time applied voltage) 
     In an embodiment, a triplet energy level of the first host (T 1_H1 ), a triplet energy level of the second host (T 1_H2 ), and a triplet energy level of the third host (T 1_H3 ) may satisfy Expressions (4) and (5) below: 
         T   1_H1   &gt;T   1_H2   (4)
 
         T   1_H1   &gt;T   1_H3   (5)
 
     In an embodiment, the triplet energy level of the second host (T 1_H2 ) and the triplet energy level of the third host (T 1_H3 ) may satisfy Expression (6) below. 
         T   1_H2   &gt;T   1_H3   (6)
 
     In an embodiment, a lowest unoccupied molecular orbital (LUMO) energy level of the first host (E LUMO_H1 ), a LUMO energy level of the second host (E LUMO_H2 ), and a LUMO energy level of the third host (E LUMO_H3 ) may satisfy Expressions (7) and (8) below. 
         E   LUMO_H1   &gt;E   LUMO_H2   (7)
 
         E   LUMO_H1   &gt;E   LUMO_H3   (8)
 
     In an embodiment, the LUMO energy level of the second host (E LUMO_H2 ) may be different from the LUMO energy level of the third host (E LUMO_H3 ). 
     In an embodiment, the LUMO energy level of the second host (E LUMO_H2 ) may be greater than the LUMO energy level of the third host (E LUMO_H3 ). 
     In an embodiment, the LUMO energy level of the second host (E LUMO_H2 ) may be less than the LUMO energy level of the third host (E LUMO_H3 ). 
     In an embodiment, a highest occupied molecular orbital (HOMO) energy level of the first host (E HOMO_H1 ), a HOMO energy level of the second host (E HOMO_H2 ), and a HOMO energy level of the third host (E HOMO_H3 ) may satisfy Expressions (9) and (10) below. 
         E   HOMO_H1   &gt;E   HOMO_H2   (9)
 
         E   HOMO_H1   &gt;E   HOMO_H3   (10)
 
     In an embodiment, the HOMO energy level of the second host (E HOMO_H2 ) may be different from the HOMO energy level of the third host (E HOMO_H3 ). 
     In an embodiment, the HOMO energy level of the second host (E HOMO_H2 ) may be greater than the HOMO energy level of the third host (E HOMO_H3 ). 
     In an embodiment, the HOMO energy level of the second host (E HOMO_H2 ) may be less than the HOMO energy level of the third host (E HOMO_H3 ). 
     In an embodiment, an electron mobility of the first host (μE 1 ), an electron mobility of the second host (μE 2 ), and an electron mobility of the third host (μE 3 ) may satisfy Expressions (11) and (12) below. 
       μ E   2   &gt;μE   1   (11)
 
       μ E   3   &gt;μE   1   (12)
 
     In an embodiment, the electron mobility of the second host (μE 2 ) may be different from the electron mobility of the third host (μE 3 ). 
     In an embodiment, the electron mobility of the second host (μE 2 ) may be greater than the electron mobility of the third host (μE 3 ). 
     In an embodiment, the electron mobility of the second host (μE 2 ) may be less than the electron mobility of the third host (μE 3 ). 
     In an embodiment, the first emission layer and the second emission layer may each include a dopant, and the dopant in the first emission layer and the dopant in the second emission layer may be identical to each other. 
     In an embodiment, the first emission layer and the second emission layer may each include a dopant, and the dopant in the first emission layer and the dopant in the second emission layer may be different from each other. 
     In an embodiment, the first emission layer and the second emission layer of the light-emitting device may be in contact with each other. In an embodiment, the first emission layer and the second emission layer may be physically in contact with each other. In an embodiment, the first emission layer and the second emission layer may be physically in direct contact with each other (e.g., direct physical contact with no intervening elements therebetween). 
     In an embodiment, the first emission layer may be between the first electrode and the second emission layer, and the second emission layer may be between the first emission layer and the second electrode. 
     In an embodiment, in the light-emitting device, the first electrode may be an anode, the second electrode may be a cathode, the interlayer may further include a hole transport region between the first electrode and the emission layer and an electron transport region between the emission layer and the second electrode, 
     the hole transport region may include a hole injection layer, a hole transport layer, a emission auxiliary layer, an electron blocking layer, or any combination thereof, and the electron transport region may include a hole blocking layer, an electron transport layer, an electron injection layer, or any combination thereof. 
     In an embodiment, the interlayer of the light-emitting device may further include a hole transport layer and an electron blocking layer, which are between the first electrode and the emission layer, and the first emission layer may be in contact with the electron blocking layer (e.g., direct physical contact with no intervening elements between the first emission layer and the electron blocking layer). In an embodiment, the interlayer may further include a hole injection layer, and the hole injection layer may be in contact with the first electrode (e.g., direct physical contact with no intervening elements between the hole injection layer and the first electrode). In an embodiment, the hole injection layer may include a charge generation material. In an embodiment, the hole injection layer may include a p-dopant compound. 
     In an embodiment, the interlayer of the light-emitting device may further include an electron transport layer and a hole blocking layer, which are between the second electrode and the emission layer, and the second emission layer may be in contact with the hole blocking layer (e.g., direct physical contact with no intervening elements between the second emission layer and the hole blocking layer). 
     In an embodiment, the electron transport layer may include a metal-containing material. The metal-containing material is described further below. 
     In an embodiment, in the light-emitting device, the first electrode may be an anode, the second electrode may be a cathode, the first emission layer and the second emission layer may be in contact with each other (e.g., direct physical contact with no intervening elements between the first emission layer and the second emission layer). 
     and holes injected from the first electrode and electrons injected from the second electrode may recombine at an interface between the first emission layer and the second emission layer. In an embodiment, the first emission layer may be positioned in a first electrode direction. In an embodiment, the first emission layer may be positioned between the first electrode and the second emission layer. 
     In the light-emitting device according to an embodiment, a hole-electron recombination zone may be moved to an interface between the first emission layer and the second emission layer, thereby preventing or reducing deterioration of an electron blocking layer due to generated excitons. 
     In an embodiment, the emission layer of the light-emitting device may emit blue light. 
     In an embodiment, the emission layer of the light-emitting device may be a fluorescent emission layer. 
     In an embodiment, a ratio of a thickness of the first emission layer to a thickness of the second emission layer may be in a range of about 3:7 to about 7:3. In an embodiment, a ratio of a thickness of the first emission layer to a thickness of the second emission layer may be in a range of about 4:6 to about 6:4. In an embodiment, a ratio of a thickness of the first emission layer to a thickness of the second emission layer may be about 5:5. 
     In an embodiment, a weight ratio of the second host to the third host may be in a range of about 1:9 to about 9:1. In an embodiment, a weight ratio of the second host to the third host may be in a range of about 2:8 to about 8:2. In an embodiment, a weight ratio of the second host to the third host may be in a range of about 3:7 to about 7:3. 
     In an embodiment, the first host may be represented by Formula 1. 
     
       
         
         
             
             
         
       
     
     In Formula 1, 
     ring CY 1  and ring CY 2  may each independently be a C 3 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group. 
     In an embodiment, in Formula 1, ring CY 1  and ring CY 2  may each independently be a benzene group, a naphthalene group, anthracenyl group, a carbazole group, a dibenzofuran group, a fluorene group, a dibenzothiophene group, or a dibenzosilole group. 
     In an embodiment, in Formula 1, ring CY 1  and ring CY 2  may be the same group. 
     In an embodiment, in Formula 1, ring CY 1  and ring CY 2  may each be a benzene group. 
     In Formula 1, R 1  to R 4  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 60  alkyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkenyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkynyl group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  alkoxy group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  aryloxy group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  arylthio group unsubstituted or substituted with at least one R 10a , a C 7 -C 60  aryl alkyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  heteroaryl alkyl group unsubstituted or substituted with at least one R 10a , —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), 
     R 10a  may be: 
     deuterium (-D), —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, or a nitro group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, or a C 1 -C 60  alkoxy group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 7 -C 60  aryl alkyl group, a C 2 -C 60  heteroaryl alkyl group, —Si(Q 11 )(Q 12 )(Q 13 ), —N(Q 11 )(Q 12 ), —B(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), or any combination thereof; 
     a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 7 -C 60  aryl alkyl group, or a C 2 -C 60  heteroaryl alkyl group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, a C 7 -C 60  aryl alkyl group, a C 2 -C 60  heteroaryl alkyl group, —Si(Q 21 )(Q 22 )(Q 23 ), —N(Q 21 )(Q 22 ), —B(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O) 2 (Q 21 ), —P(═O)(Q 21 )(Q 22 ), or any combination thereof; or 
     —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), or —P(═O)(Q 31 )(Q 32 ), and 
     Q 1  to Q 3 , Q 11  to Q 13 , Q 21  to Q 23 , and Q 31  to Q 33  may each independently be: hydrogen; deuterium; —F; —C 1 ; —Br; —I; a hydroxyl group; a cyano group; a nitro group; or 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 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 7 -C 60  aryl alkyl group, or a C 2 -C 60  heteroaryl alkyl group, unsubstituted or substituted with deuterium, —F, a cyano group, a C 1 -C 60  alkyl group, a C 1 -C 60  alkoxy group, a phenyl group, a biphenyl group, a C 1 -C 60  heterocyclic group or any combination thereof. 
     a1 and a2 may each independently be an integer from 0 to 10, and a3 and a4 may each independently be an integer from 0 to 2. 
     In an embodiment, in Formula 1, R 1  to R 4  may each independently be: hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, or any combination thereof; a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indenyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a benzoisothiazolyl 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 dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofurano carbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, or azadibenzosilolyl group, each unsubstituted or substituted with deuterium. —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indenyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a benzoisothiazolyl 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 dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofurano carbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), —P(═O)(Q 31 )(Q 32 ), or any combination thereof; or —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), and 
     Q 1  to Q 3  and Q 31  to Q 33  may each independently be: —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, or —CD 2 CDH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a phenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, or a triazinyl group, each unsubstituted or substituted with deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, a triazinyl group, or any combination thereof. 
     In an embodiment, in Formula 1, R 1  to R 4  may each independently be: hydrogen, deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofurano carbazolyl group, a benzothienocarbazolyl group, or a benzosilolocarbazolyl group, each unsubstituted or substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofurano carbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, —Si(Q 31 )(Q 32 )(Q 33 ), or any combination thereof; or —Si(Q 1 )(Q 2 )(Q 3 ), and 
     Q 1  to Q 3  and Q 31  to Q 33  may each independently be: —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, or —CD 2 CDH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a phenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, or a triazinyl group, each unsubstituted or substituted with deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, a triazinyl group, or any combination thereof. 
     In an embodiment, in Formula 1, R 1  to R 4  may each independently be: hydrogen, deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofurano carbazolyl group, a benzothienocarbazolyl group, or a benzosilolocarbazolyl group, each unsubstituted or substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofurano carbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, —Si(Q 31 )(Q 32 )(Q 33 ), or any combination thereof; or —Si(Q 1 )(Q 2 )(Q 3 ), and 
     Q 1  to Q 3  and Q 31  to Q 33  may each independently be: —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, or —CD 2 CDH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a phenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, or a triazinyl group, each unsubstituted or substituted with deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, a triazinyl group, or any combination thereof. 
     In an embodiment, in Formula 1, R 1  to R 4  may each independently be: hydrogen, deuterium, —F, or a cyano group; 
     a cyclohexyl group, an adamantanyl group, a norbornanyl group, a phenyl group, a naphthyl group, a fluorenyl group, an anthracenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, or a dibenzosilolyl group, each unsubstituted or substituted with deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclohexyl group, an adamantanyl group, a norbornanyl group, a phenyl group, a naphthyl group, a fluorenyl group, an anthracenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, —Si(Q 31 )(Q 32 )(Q 33 ) or any combination thereof; or —Si(Q 1 )(Q 2 )(Q 3 ), and 
     Q 1  to Q 3  and Q 31  to Q 33  may each independently be: —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, or —CD 2 CDH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a phenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, or a triazinyl group, each unsubstituted or substituted with deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, a triazinyl group, or any combination thereof. 
     In an embodiment, in Formula 1, a1 and a2 may each independently be an integer from 0 to 3. 
     In an embodiment, Formula 1 may be represented by Formula 1-1. 
     
       
         
         
             
             
         
       
     
     In Formula 1-1, 
     ring CY 2 , a2 to a4, and R 2  to R 4  may respectively be the same as those described in the present specification, and R 11  to R 13  may each independently be the same as described in connection with R 1  in the present specification. 
     In an embodiment, the first host may be a pyrene derivative compound. The first host may be a pyrene derivative compound in which pyrene is substituted with Si(Q 1 )(Q 2 )(Q 3 ). In an embodiment, Q 1  to Q 3  may each independently be the same as described in the present specification. 
     In an embodiment, the first host may be Compound 1-1 below: 
     
       
         
         
             
             
         
       
     
     In an embodiment, the second host may be represented by Formula 2. 
     
       
         
         
             
             
         
       
     
     In Formula 2, X 2  may be O, S, Se, N(Ar 1 ), or Si(Ar 1 )(Ar 2 ). 
     In an embodiment, in Formula 2, X 2  may be O, S, or Se. 
     In an embodiment, in Formula 2, X 2  may be O. 
     In Formula 2, ring CY 21  and ring CY 22  may each independently be a C 3 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group. 
     In an embodiment, in Formula 2, ring CY 21  and ring CY 22  may each independently be a benzene group, a naphthalene group, anthracenyl group, a carbazole group, a dibenzofuran group, a fluorene group, a dibenzothiophene group, or a dibenzosilole group. 
     In an embodiment, in Formula 2, ring CY 21  and ring CY 22  may be identical to each other. 
     In an embodiment, in Formula 2, ring CY 21  and ring CY 22  may be different from each other. 
     In an embodiment, in Formula 2, ring CY 21  and ring CY 22  may each be a benzene group or a naphthalene group. 
     In Formula 2, T 21  may be *-(L 21 ) b21 -(Ar 21 ) c21 . * in T 21  may indicate a binding site to a neighboring atom. 
     In Formula 2, L 21  may be a single bond or a C 5 -C 30  carbocyclic group that is unsubstituted or substituted with at least one R 10a , and b21 may be an integer from 0 to 3. 
     In an embodiment, in Formula 2, L 21  may be a single bond, a benzene group unsubstituted or substituted with at least one R 10a , a naphthalene group unsubstituted or substituted with at least one R 10a , or an anthracene group unsubstituted or substituted with at least one R 10a . 
     In an embodiment, in Formula 2, L 21  may be a benzene group, a naphthalene group, or an anthracene group. 
     In an embodiment, in Formula 2, a group represented by 
     
       
         
         
             
             
         
       
     
     may be represented by one selected from rings CY21-1 to CY21-22: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In rings CY21-1 to CY21-22, T 21  may be the same as T 21  in the present specification, R 23  to R 28  may each independently be the same as described in connection with R 21  in the present specification (R 21  is described in more detail herein below), * may indicate a binding site to X 2  in Formula 2, and *′ may indicate a binding site to ring CY 22  in Formula 2. 
     
       
         
         
             
             
         
       
     
     In an embodiment, in Formula 2, a group represented by may be represented by one selected from rings CY22-1 to CY22-4. 
     
       
         
         
             
             
         
       
     
     In rings CY22-1 to CY22-4, R 23  to R 28  may each independently be the same as described in connection with R 22  in the present specification, * may indicate a binding site to X 2  in Formula 2, and * may indicate a binding site to ring CY 21  in Formula 2. 
     In an embodiment, R 21 , R 22 , Ar 1 , Are, and Ar 21  in Formula 2 may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 60  alkyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkenyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkynyl group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  alkoxy group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  aryloxy group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  arylthio group unsubstituted or substituted with at least one R 10a , a C 7 -C 60  aryl alkyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  heteroaryl alkyl group unsubstituted or substituted with at least one R 10a , —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), and a21, a22, and c21 may each independently be an integer from 0 to 10. 
     In an embodiment, in Formula 2, 
     R 21 , R 22 , and Ar 21  may each independently be: hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkyl phenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indenyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl 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 dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphthosilolyl group, an indenocarbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, or an azadibenzosilolyl group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CDH, —CDH, —CF 3 , —CFH, —CFH, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkyl phenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indenyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl 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 dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphthosilolyl group, an indenocarbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), —P(═O)(Q 31 )(Q 32 ), or any combination thereof; or 
     —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(a), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), 
     Q 1  to Q 3  and Q 31  to Q 33  may each independently be: 
     —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, or —CD 2 CDH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a phenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, or a triazinyl group, each unsubstituted or substituted with deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, a triazinyl group, or any combination thereof. 
     In an embodiment, in Formula 2, R 21 , R 22 , and Ar 21  may each independently be: hydrogen, deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, or a benzosilolocarbazolyl group, each unsubstituted or substituted with deuterium, —F, —CD 3 , —CDH, —CDH, —CF 3 , —CF 2 H, —CFH 2 , a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, or any combination thereof. 
     In an embodiment, in Formula 2, R 21 , R 22 , and Ar 21  may each independently be: hydrogen, deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, or a benzosilolocarbazolyl group, each unsubstituted or substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, —Si(Q 31 )(Q 32 )(Q 33 ), or any combination thereof. 
     In an embodiment, in Formula 2, R 21 , R 22 , and Ar 21  may each independently be: hydrogen, deuterium, —F, or a cyano group; 
     a cyclohexyl group, an adamantanyl group, a norbornanyl group, a phenyl group, a naphthyl group, a fluorenyl group, an anthracenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, or a dibenzosilolyl group, each unsubstituted or substituted with deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclohexyl group, an adamantanyl group, a norbornanyl group, a phenyl group, a naphthyl group, a fluorenyl group, an anthracenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, or any combination thereof. 
     In an embodiment, the second host may be a dibenzofuran derivative compound. 
     In an embodiment, the second host may be one selected from Compound 2-1 to 2-20: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In an embodiment, the third host may be represented by Formula 3. 
     
       
         
         
             
             
         
       
     
     In Formula 3, L 31  to L 34  may each independently be a single bond, a C 5 -C 30  carbocyclic group that is unsubstituted or substituted with at least one R 10a , or a C 1 -C 30  heterocyclic group that is unsubstituted or substituted with at least one R 10a . 
     In Formula 3, a31 to a34 may each independently be an integer from 0 to 3. 
     In an embodiment, in Formula 3, L 31  to L 34  may each independently be a single bond, a benzene group unsubstituted or substituted with at least one R 10a , a naphthalene group unsubstituted or substituted with at least one R 10a , a phenanthrene group unsubstituted or substituted with at least one R 10a , an anthracene group unsubstituted or substituted with at least one R 10a , or a pyrene group unsubstituted or substituted with at least one R 10a . 
     In an embodiment, in Formula 3, L 31  to L 34  may each independently be a single bond, a benzene group, a naphthalene group, a phenanthrene group, an anthracene group, or a pyrene group. 
     In Formula 3, R 31  to R 34  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 60  alkyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkenyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkynyl group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  alkoxy group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  aryloxy group unsubstituted or substituted with at least one R 10a , a C 6 -C 60  arylthio group unsubstituted or substituted with at least one R 10a , a C 7 -C 60  aryl alkyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  heteroaryl alkyl group unsubstituted or substituted with at least one R 10a , —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), and b31 to b34 may each independently be an integer from 0 to 10. 
     In an embodiment, in Formula 3, R 31  to R 34  may each independently be: hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthrenyl group, a pyrenyl group, a pyridinyl group, a pyrimidinyl group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indenyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a benzoisothiazolyl 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 dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an azafluorenyl group, an azacarbazolyl group, an azadibenzofuranyl group, an azadibenzothiophenyl group, or azadibenzosilolyl group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a pyrenyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a pyrrolyl group, a thiophenyl group, a furanyl group, an imidazolyl group, a pyrazolyl group, a thiazolyl group, an isothiazolyl group, an oxazolyl group, an isoxazolyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, an indenyl group, an isoindolyl group, an indolyl group, an indazolyl group, a purinyl group, a quinolinyl group, an isoquinolinyl group, a benzoquinolinyl group, a quinoxalinyl group, a quinazolinyl group, a cinnolinyl group, a carbazolyl group, a phenanthrolinyl group, a benzimidazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a benzoisothiazolyl 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 dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2 (Q 31 ), —P(═O)(Q 31 )(Q 32 ), or any combination thereof; or 
     —Si(Q 1 )(Q 2 )(Q 3 ), —N(Q 1 )(Q 2 ), —B(Q 1 )(Q 2 ), —C(═O)(Q 1 ), —S(═O) 2 (Q 1 ), or —P(═O)(Q 1 )(Q 2 ), and 
     Q 1  to Q 3  and Q 31  to Q 33  may each independently be: 
     —CH 3 , —CD 3 , —CD 2 H, —CDH 2 , —CH 2 CH 3 , —CH 2 CD 3 , —CH 2 CD 2 H, —CH 2 CDH 2 , —CHDCH 3 , —CHDCD 2 H, —CHDCDH 2 , —CHDCD 3 , —CD 2 CD 3 , —CD 2 CD 2 H, or —CD 2 CDH 2 ; or 
     an n-propyl group, an iso-propyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a phenyl group, a naphthyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, or a triazinyl group, each unsubstituted or substituted with deuterium, a C 1 -C 20  alkyl group, a phenyl group, a biphenyl group, a pyridinyl group, a pyrimidinyl group, a pyridazinyl group, a pyrazinyl group, a triazinyl group, or any combination thereof. 
     In an embodiment, in Formula 3, R 31  to R 34  may each independently be: hydrogen, deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, or a benzosilolocarbazolyl group, each unsubstituted or substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, or any combination thereof. 
     In an embodiment, in Formula 3, R 31  to R 34  may each independently be: hydrogen, deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, or a C 1 -C 20  alkoxy group; 
     a C 1 -C 20  alkyl group or a C 1 -C 20  alkoxy group, each substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, or any combination thereof; 
     a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, or a benzosilolocarbazolyl group, each unsubstituted or substituted with deuterium, —F, —CD 3 , —CD 2 H, —CDH 2 , —CF 3 , —CF 2 H, —CFH 2 , a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group, a norbornenyl group, a cyclopentenyl group, a cyclohexenyl group, a cycloheptenyl group, a phenyl group, a biphenyl group, a terphenyl group, a C 1 -C 20  alkylphenyl group, a naphthyl group, a fluorenyl group, a phenanthrenyl group, an anthracenyl group, a pyrenyl group, a thiophenyl group, a furanyl group, an indenyl group, an isoindolyl group, an indolyl group, a carbazolyl group, a benzofuranyl group, a benzothiophenyl group, a benzosilolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, a benzofluorenyl group, a benzocarbazolyl group, a naphthobenzofuranyl group, a naphthobenzothiophenyl group, a naphthobenzosilolyl group, a dibenzofluorenyl group, a dibenzocarbazolyl group, a dinaphthofuranyl group, a dinaphthothiophenyl group, a dinaphtho silolyl group, an indeno carbazolyl group, an indolocarbazolyl group, a benzofuranocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, —Si(Q 31 )(Q 32 )(Q 33 ), or any combination thereof. 
     In an embodiment, in Formula 3, R 31  to R 34  may each independently be: hydrogen, deuterium, —F, or a cyano group; 
     a cyclohexyl group, an adamantanyl group, a norbornanyl group, a phenyl group, a naphthyl group, a phenanthrenyl group, a fluorenyl group, an anthracenyl group, a pyrenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, or a dibenzosilolyl group, each unsubstituted or substituted with deuterium, —F, a cyano group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a cyclohexyl group, an adamantanyl group, a norbornanyl group, a phenyl group, a naphthyl group, a phenanthrenyl group, a fluorenyl group, an anthracenyl group, a pyrenyl group, a carbazolyl group, a dibenzofuranyl group, a dibenzothiophenyl group, a dibenzosilolyl group, or any combination thereof. 
     In an embodiment, the third host may be an anthracene derivative compound. In an embodiment, the third host may be an anthracene derivative compound in which an anthracene compound is substituted with at least one selected from an aryl group, a naphthyl group, a phenanthrenyl group, and a pyrenyl group. In an embodiment, the third host may be an asymmetric compound. 
     In an embodiment, the third host may be one selected from Compound 3-1 to 3-18: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The second host and the third host may satisfy Expressions (1) and (2) in relation to the first host. 
     The light-emitting device may include an emission layer including a first emission layer and a second emission layer. The first emission layer includes a first host, the second emission layer includes a second host and a third host, and a hole mobility of the first host (μH 1 ), a hole mobility of the second host (μH 2 ), and a hole mobility of the third host (μH 3 ) satisfy Expressions (1) and (2) below. 
       μ H   1   &gt;μH   2   (1)
 
       μ H   1   &gt;μH   3   (2)
 
     In an embodiment, holes and electrons recombine to generate TTF and form a light-emitting zone emitting light, wherein, when Expressions (1) and (2) are satisfied, a region where the holes and the electrons recombine may move to an interface between the first emission layer and the second emission layer, and thus, deterioration of an electron blocking layer in light-emitting devices of the related art does not occur (or substantially does not occur). Therefore, the lifespan of the light-emitting device may be greatly improved. 
     Also, even when electrons migrate from the second emission layer to the first emission layer, the first host included in the first emission layer may emit light from a singlet state, thereby contributing to light emission. In an embodiment, the second host included in the second emission layer may narrow a light-emitting zone to increase generation of TTF. In an embodiment, the third host may improve driving voltage by adjusting injection of holes. 
     Therefore, the light-emitting device including the first emission layer including the first host and the second emission layer including the second host and the third host may, for example, improve the low voltage characteristics, luminance, luminescence efficiency, and/or lifespan of an electronic apparatus including the light-emitting device, as a result of an increase in TTF and a decrease in driving voltage due to the first host, the second host, and the third host. 
     Synthesis methods of the first host represented by Formula 1, the second host represented by Formula 2, and the third host represented by Formula 3 may be recognizable by one of ordinary skill in the art by referring to Examples provided below. 
     The emission layer may emit red light, green light, blue light, and/or white light. In an embodiment, the emission layer may emit blue light. The blue light may have a maximum emission wavelength of, for example, about 400 nm to about 490 nm. 
     In an embodiment, the first emission layer and the second emission layer may each independently further include a dopant. 
     In an embodiment, the emission layer (e.g., the first emission layer and/or the second emission layer) may further include a phosphorescent dopant, a delayed fluorescence dopant, or any combination thereof. In an embodiment, the emission layer may further include a phosphorescent dopant, in addition to a host and a dopant. 
     In an embodiment, the dopant may include a transition metal and ligand(s) in the number of m, m may be an integer from 1 to 6, the ligand(s) in the number of m may be identical to or different from each other, at least one of the ligand(s) in the number of m may be bound to the transition metal via a carbon-transition metal bond, and the carbon-transition metal bond may be a coordinate bond (e.g., a coordinate covalent bond, which may also be referred to as a dative bond). In some embodiments, at least one of the ligand(s) in the number of m may be a carbene ligand (e.g., as found in Ir(pmp)3 and/or the like). The transition metal may be, for example, iridium, platinum, osmium, palladium, rhodium, or gold. The emission layer and the dopant may be the same as described in the present specification. 
     
       
         
         
             
             
         
       
     
     In an embodiment, the interlayer may include m emitting units and m-1 charge generation unit(s) between adjacent emitting units among the m emitting units, and 
     at least one of the m emitting units may include the first emission layer and the second emission layer. 
     The light-emitting device may include m-1 charge generation unit(s) between adjacent emitting units among the m emitting units. m is an integer from 1 to 6. 
     For example, when m is 2, the first electrode, a first emitting unit, a first charge generation unit, and a second emitting unit may be sequentially arranged. In this state, the first emitting unit may emit a first-color light, the second light emitting unit may emit a second-color light, and the maximum emission wavelength of the first-color light and the maximum emission wavelength of the second-color light may be identical to or different from each other. Here, at least one selected from the first emitting unit and the second emitting unit may include the first emission layer and the second emission layer. 
     As another example, when m is 3, the first electrode, a first emitting unit, a first charge generation unit, a second emitting unit, a second charge generation unit, and a third emitting unit may be sequentially arranged. In this state, the first emitting unit may emit a first-color light, the second emitting unit may emit a second-color light, the third emitting unit may emit a third-color light, and a maximum emission wavelength of the first-color light, a maximum emission wavelength of the second-color light, and a maximum emission wavelength of the third-color light may be identical to or different from each other. Here, at least one selected from the first emitting unit, the second emitting unit, and the third emitting unit may include the first emission layer, and the second emission layer. 
     As another example, when m is 4, the first electrode, a first emitting unit, a first charge generation unit, a second emitting unit, a second charge generation unit, a third emitting unit, a third charge generation unit, and a fourth emitting unit may be sequentially arranged. In this state, the first emitting unit may emit a first-color light, the second emitting unit may emit a second-color light, the third emitting unit may emit a third-color light, the fourth emitting unit may emit a fourth-color light, and a maximum emission wavelength of the first-color light, a maximum emission wavelength of the second-color light, a maximum emission wavelength of the third-color light, and a maximum emission wavelength of the fourth-color light may be identical to or different from each other. Here, at least one selected from the first emitting unit, the second emitting unit, the third emitting unit, and the fourth emitting unit may include the first emission layer, and the second emission layer. 
     An electronic apparatus according to another aspect of embodiments includes the light-emitting device. 
     In an embodiment, the electronic apparatus may further include a thin-film transistor, 
     thin-film transistor includes a source electrode and a drain electrode, and 
     the first electrode of the light-emitting device may be electrically connected to at least one selected from the source electrode and the drain electrode of the thin-film transistor. 
     In an embodiment, the electronic apparatus may further include a color filter, a color conversion layer, a touch screen layer, a polarizing layer, or any combination thereof. 
     In an embodiment, the electronic apparatus may further include quantum dots. For example, the electronic apparatus may include a color conversion layer, and the color conversion layer may include quantum dots. 
     The term “interlayer,” as used herein, refers to a single layer and/or all of a plurality of layers between the first electrode and the second electrode of the light-emitting device. 
     Description of FIG.  1   
       FIG.  1    is a schematic cross-sectional view of a light-emitting device  10  according to an embodiment of the disclosure. The light-emitting device  10  includes a first electrode  110 , an interlayer  130 , and a second electrode  150 . The interlayer  130  includes an emission layer  120 . The emission layer  120  includes a first emission layer  122  and a second emission layer  124 . 
     Hereinafter, a structure of the light-emitting device  10  according to an embodiment and a method of manufacturing the light-emitting device  10  will be described in connection with  FIG.  1   . 
     First Electrode  110   
     In  FIG.  1   , a substrate may be additionally under the first electrode  110  and/or above the second electrode  150 . As the substrate, a glass substrate and/or a plastic substrate may be used. In an embodiment, the substrate may be a flexible substrate, and may include plastics having excellent heat resistance and durability, such as polyimide, polyethylene terephthalate (PET), polycarbonate, polyethylene naphthalate, polyarylate (PAR), polyetherimide, or any combination thereof. 
     The first electrode  110  may be formed by, for example, depositing and/or sputtering a material for forming the first electrode  110  on the substrate. When the first electrode  110  is an anode, a material for forming the first electrode  110  may be a high work function material that facilitates injection of holes. 
     The first electrode  110  may be a reflective electrode, a semi-transmissive electrode, or a transmissive electrode. When the first electrode  110  is a transmissive electrode, a material for forming the first electrode  110  may include indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO 2 ), zinc oxide (ZnO), or any combinations thereof. In one or more embodiments, when the first electrode  110  is a semi-transmissive electrode or a reflective electrode, magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), or any combinations thereof may be used as a material for forming the first electrode  110 . 
     The first electrode  110  may have a single-layered structure consisting of a single layer or a multilayer structure including a plurality of layers. In an embodiment, the first electrode  110  may have a three-layered structure of ITO/Ag/ITO. 
     Interlayer  130   
     The interlayer  130  may be on the first electrode  110 . The interlayer  130  includes the emission layer  120 . 
     The interlayer  130  may further include a hole transport region between the first electrode  110  and the emission layer  120  and an electron transport region between the emission layer  120  and the second electrode  150 . 
     The interlayer  130  may further include metal-containing compounds such as organometallic compounds, inorganic materials such as quantum dots, and/or the like, in addition to various suitable organic materials. 
     In one or more embodiments, the interlayer  130  may include, i) two or more emitting units sequentially stacked between the first electrode  110  and the second electrode  150 , and ii) a charge generation layer between the two emitting units. When the interlayer  130  includes emitting units and a charge generation layer as described above, the light-emitting device  10  may be a tandem light-emitting device. 
     Hole Transport Region in Interlayer  130   
     The hole transport region may have: i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer consisting of a plurality of different materials, or iii) a multi-layered structure including a plurality of layers including different materials. 
     The hole transport region may include a hole injection layer, a hole transport layer, an emission auxiliary layer, an electron blocking layer, or any combination thereof. 
     For example, the hole transport region may have a multi-layered structure including a hole injection layer/hole transport layer structure, a hole injection layer/hole transport layer/emission auxiliary layer structure, a hole injection layer/emission auxiliary layer structure, a hole transport layer/emission auxiliary layer structure, or a hole injection layer/hole transport layer/electron blocking layer structure, the layers of each structure being stacked sequentially from the first electrode  110 . 
     The hole transport region may include a compound represented by Formula 201, a compound represented by Formula 202, or any combination thereof: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 201 and 202, 
     L 201  to L 204  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     L 205  may be *—O—*, *—S—*, *—N(Q 201 )-*, a C 1 -C 20  alkylene group unsubstituted or substituted with at least one R 10a , a C 2 -C 20  alkenylene group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     xa1 to xa4 may each independently be an integer from 0 to 5, 
     xa5 may be an integer from 1 to 10, 
     R 201  to R 204  and Q 201  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     R 201  and R 202  may optionally be linked to each other, via a single bond, a C 1 -C 5  alkylene group unsubstituted or substituted with at least one R 10a , or a C 2 -C 5  alkenylene group unsubstituted or substituted with at least one R 10a , to form a C 8 -C 60  polycyclic group (for example, a carbazole group or the like) unsubstituted or substituted with at least one R 10a  (for example, Compound HT16), 
     R 203  and R 204  may optionally be linked to each other, via a single bond, a C 1 -C 5  alkylene group unsubstituted or substituted with at least one R 10a , or a C 2 -C 5  alkenylene group unsubstituted or substituted with at least one R 10a , to form a C 8 -C 60  polycyclic group unsubstituted or substituted with at least one R 10a , and 
     na1 may be an integer from 1 to 4. 
     In an embodiment, each of Formulae 201 and 202 may include at least one selected from groups represented by Formulae CY201 to CY217: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     R 10b  and R 10c  in Formulae CY201 to CY217 are respectively the same as those described in connection with R 10a , ring CY 201  to ring CY 204  may each independently be a C 3 -C 20  carbocyclic group or a C 1 -C 20  heterocyclic group, and at least one hydrogen in Formulae CY201 to CY217 may be unsubstituted or substituted with R 10a  as described above. 
     In an embodiment, ring CY 201  to ring CY 204  in Formulae CY201 to CY217 may each independently be a benzene group, a naphthalene group, a phenanthrene group, or an anthracene group. 
     In an embodiment, each of Formulae 201 and 202 may include at least one selected from groups represented by Formulae CY201 to CY203. 
     In an embodiment, Formula 201 may include at least one selected from groups represented by Formulae CY201 to CY203 and at least one selected from groups represented by Formulae CY204 to CY217. 
     In an embodiment, xa1 in Formula 201 may be 1, R 201  may be a group represented by one selected from Formulae CY201 to CY203, xa2 may be 0, and R 202  may be a group represented by one selected from Formulae CY204 to CY207. 
     In an embodiment, each of Formulae 201 and 202 may not include groups represented by Formulae CY201 to CY203. 
     In an embodiment, each of Formulae 201 and 202 may not include groups represented by Formulae CY201 to CY203, and may include at least one selected from groups represented by Formulae CY204 to CY217. 
     In an embodiment, each of Formulae 201 and 202 may not include groups represented by Formulae CY201 to CY217. 
     In an embodiment, the hole transport region may include one selected from Compounds HT1 to HT44, m-MTDATA, TDATA, 2-TNATA, NPB(NPD), β-NPB, TPD, Spiro-TPD, Spiro-NPB, methylated NPB, TAPC, HMTPD, 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), polyaniline/dodecylbenzenesulfonic acid (PANI/DBSA), poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), polyaniline/camphor sulfonic acid (PANT/CSA), polyaniline/poly(4-styrenesulfonate) (PANT/PSS), or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A thickness of the hole transport region may be in a range of about 50 Å to about 10,000 Å, for example, about 100 Å to about 4,000 Å. When the hole transport region includes a hole injection layer, a hole transport layer, or any combination thereof, a thickness of the hole injection layer may be in a range of about 100 Å to about 9,000 Å, for example, about 100 Å to about 1,000 Å, and a thickness of the hole transport layer may be in a range of about 50 Å to about 2,000 Å, for example, about 100 Å to about 1,500 Å. When the thicknesses of the hole transport region, the hole injection layer and the hole transport layer are within these ranges, suitable or satisfactory hole-transporting characteristics may be obtained without a substantial increase in driving voltage. 
     The emission auxiliary layer may increase light-emission efficiency by compensating for an optical resonance distance according to the wavelength of light emitted by an emission layer  120 , and the electron blocking layer may block or reduce the flow of electrons from an electron transport region. The emission auxiliary layer and the electron blocking layer may include the materials as described above. 
     p-Dopant 
     The hole transport region may further include, in addition to these materials, a charge-generation material for the improvement of conductive properties (e.g., electrically conductive properties). The charge-generation material may be uniformly or non-uniformly dispersed in the hole transport region (for example, in the form of a single layer consisting of a charge-generation material). 
     The charge-generation material may be, for example, a p-dopant. 
     In an embodiment, a LUMO energy level of the p-dopant may be about −3.5 eV or less. 
     In an embodiment, the p-dopant may include a quinone derivative, a cyano group-containing compound, a compound containing element EL1 and element EL2, or any combination thereof. 
     Examples of the quinone derivative may include TCNQ, F4-TCNQ, and the like. 
     Examples of the cyano group-containing compound may include HAT-CN, a compound represented by Formula 221 below, and the like. 
     
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     In Formula 221, 
     R 221  to R 223  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , and 
     at least one selected from R 221  to R 223  may each independently be a C 3 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, each substituted with: a cyano group; —F; —C 1 ; —Br; —I; a C 1 -C 20  alkyl group substituted with a cyano group, —F, —Cl, —Br, —I, or any combination thereof; or any combination thereof. 
     In the compound containing element EL1 and element EL2, element EL1 may be metal, metalloid, or any combination thereof, and element EL2 may be non-metal, metalloid, or any combination thereof. 
     Examples of the metal may include: an alkali metal (for example, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), etc.); an alkaline earth metal (for example, beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc.); a transition metal (for example, titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), technetium (Tc), rhenium (Re), iron (Fe), ruthenium (Ru), osmium (Os), cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt), copper (Cu), silver (Ag), gold (Au), etc.); a post-transition metal (for example, zinc (Zn), indium (In), tin (Sn), etc.); a lanthanide metal (for example, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), etc.); or any combination thereof. 
     Examples of the metalloid may include silicon (Si), antimony (Sb), tellurium (Te), or any combination thereof. 
     Examples of the non-metal may include oxygen (O) halogen (for example, F, Cl, Br, I, etc.), or any combination thereof. 
     In an embodiment, examples of the compound containing element EL1 and element EL2 may include metal oxide, metal halide (for example, metal fluoride, metal chloride, metal bromide, and/or metal iodide), metalloid halide (for example, metalloid fluoride, metalloid chloride, metalloid bromide, and/or metalloid iodide), metal telluride, or any combination thereof. 
     Examples of the metal oxide may include tungsten oxide (for example, WO, W 2 O 3 , WO 2 , WO 3 , W 2 O 5 , etc.), vanadium oxide (for example, VO, V 2 O 3 , VO 2 , V 2 O 5 , etc.), molybdenum oxide (MoO, Mo 2 O 3 , MoO 2 , MoO 3 , Mo 2 O 5 , etc.), rhenium oxide (for example, ReO 3 , etc.), or any combination thereof. 
     Examples of the metal halide may include alkali metal halide, alkaline earth metal halide, transition metal halide, post-transition metal halide, lanthanide metal halide, or any combination thereof. 
     Examples of the alkali metal halide may include LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, CsI or any combination thereof. 
     Examples of the alkaline earth metal halide may include BeF 2 , MgF 2 , CaF 2 , SrF 2 , BaF 2 , BeCl 2 , MgCl 2 , CaCl 2 ), SrCl 2 , BaCl 2 , BeBr 2 , MgBr 2 , CaBr 2 , SrBr 2 , BaBr 2 , Bel 2 , Mgl 2 , Cal 2 , Srl 2 , Bale, or any combination thereof. 
     Examples of the transition metal halide may include titanium halide (for example, TiF 4 , TiCl 4 , TiBr 4 , Til 4 , etc.), zirconium halide (for example, ZrF 4 , ZrCl 4 , ZrBr 4 , ZrI 4 , etc.), hafnium halide (for example, HfF 4 , HfCl 4 , HfBr 4 , HfI 4 , etc.), vanadium halide (for example, VF 3 , VCl 3 , VBr 3 , VI 3 , etc.), niobium halide (for example, NbF 3 , NbCl 3 , NbBr 3 , NbI 3 , etc.), tantalum halide (for example, TaF 3 , TaCl 3 , TaBr 3 , TaI 3 , etc.), chromium halide (for example, CrF 3 , CrCl 3 , CrBr 3 , CrI 3 , etc.), molybdenum halide (for example, MoF 3 , MoCl 3 , MoBr 3 , MoI 3 , etc.), tungsten halide (for example, WF 3 , WCl 3 , WBr 3 , WI 3 , etc.), manganese halide (for example, MnF 2 , MnCl 2 , MnBr 2 , MnI 2 , etc.), technetium halide (for example, TcF 2 , TcCl 2 , TcBr 2 , TcI 2 , etc.), rhenium halide (for example, ReF 2 , ReCl 2 , ReBr 2 , ReI 2 , etc.), iron halide (for example, FeF 2 , FeCl 2 , FeBr 2 , Felt, etc.), ruthenium halide (for example, RuF 2 , RuCl 2 , RuBr 2 , Rule, etc.), osmium halide (for example, OsF 2 , OsCl 2 , OsBr 2 , OsI 2 , etc.), cobalt halide (for example, CoF 2 , CoCl 2 , CoBr 2 , Cole, etc.), rhodium halide (for example, RhF 2 , RhCl 2 , RhBr 2 , RhI 2 , etc.), iridium halide (for example, IrF 2 , IrCl 2 , IrBr 2 , IrI 2 , etc.), nickel halide (for example, NiF 2 , NiCl 2 , NiBr 2 , NiI 2 , etc.), palladium halide (for example, PdF 2 , PdCl 2 , PdBr 2 , PdI 2 , etc.), platinum halide (for example, PtF 2 , PtCl 2 , PtBr 2 , PtI 2 , etc.), copper halide (for example, CuF, CuCl, CuBr, Cul, etc.), silver halide (for example, AgF, AgCI, AgBr, AgI, etc.), gold halide (for example, AuF, AuCl, AuBr, AuI, etc.), or any combination thereof. 
     Examples of the post-transition metal halide may include zinc halide (for example, ZnF 2 , ZnCl 2 , ZnBr 2 , ZnI 2 , etc.), indium halide (for example, InI 3 , etc.), tin halide (for example, SnI 2 , etc.), or any combination thereof. 
     Examples of the lanthanide metal halide may include YbF, YbF 2 , YbF 3 , SmF 3 , YbCl, YbCl 2 , YbCl 3  SmCl 3 , YbBr, YbBr 2 , YbBr 3 , SmBr 3 , YbI, YbI 2 , YbI 3 , SmI 3  or any combination thereof. 
     Examples of the metalloid halide may include antimony halide (for example, SbCl 5 , etc.). 
     Examples of the metal telluride may include alkali metal telluride (for example, Li 2 Te, Na 2 Te, K 2 Te, Rb 2 Te, Cs 2 Te, etc.), alkaline earth metal telluride (for example, BeTe, MgTe, CaTe, SrTe, BaTe, etc.), transition metal telluride (for example, TiTe 2 , ZrTe 2 , HfTe 2 , V 2 Te 3 , Nb 2 Te 3 , Ta 2 Te 3 , Cr 2 Te 3 , Mo 2 Te 3 , W 2 Te 3 , MnTe, TcTe, ReTe, FeTe, RuTe, OsTe, CoTe, RhTe, IrTe, NiTe, PdTe, PtTe, Cu 2 Te, CuTe, Ag 2 Te, AgTe, Au 2 Te, etc.), post-transition metal telluride (for example, ZnTe, etc.), lanthanide metal telluride (for example, LaTe, CeTe, PrTe, NdTe, PmTe, EuTe, GdTe, TbTe, DyTe, HoTe, ErTe, TmTe, YbTe, LuTe, etc.) or any combination thereof. 
     Emission Layer  120  in Interlayer  130   
     When the light-emitting device  10  is a full-color light-emitting device, the emission layer  120  may be patterned into a red emission layer, a green emission layer, and/or a blue emission layer, according to a subpixel. In an embodiment, the emission layer  120  may have a stacked structure of two or more layers of a red emission layer, a green emission layer, and a blue emission layer, in which the two or more layers contact (e.g., physically contact) each other or are separated (e.g., spaced apart) from each other. In one or more embodiments, the emission layer  120  may include two or more materials of a red light-emitting material, a green light-emitting material, and a blue light-emitting material, in which the two or more materials are mixed together with each other in a single layer to emit white light. 
     The emission layer  120  may include the first emission layer  122  and the second emission layer  124 . In an embodiment, the first emission layer  122  may include the first host, and the second emission layer  124  may include the second host and the third host. In an embodiment, the first emission layer  122  and the second emission layer  124  may each independently further include a dopant. The dopant may include a phosphorescent dopant, a fluorescent dopant, or any combination thereof. 
     An amount of the dopant in each of the first emission layer  122  and the second emission layer  124  may be in a range of about 0.01 parts by weight to about 15 parts by weight, based on 100 parts by weight of a host. 
     In an embodiment, the first emission layer  122  and the second emission layer  124  may each independently further include a delayed fluorescence material. The delayed fluorescence material may act as a host or a dopant in the first emission layer  122  and the second emission layer  124 . 
     A thickness of the emission layer  120  may be in a range of about 100 Å to about 1,000 Å, for example, about 200 Å to about 600 Å. When the thickness of the emission layer  120  is within the range, excellent light-emission characteristics may be obtained without a substantial increase in driving voltage. 
     Host 
     The first host, the second host, and the third host may each further include, for example, a carbazole-containing compound, an anthracene-containing compound, or any combination thereof as a host. 
     In an embodiment, the first host, the second host, and the third host may each further include a compound represented by Formula 301 as a host: 
       [Ar 301 ] xb11 -[(L 301 ) xb1 -R 301 ] xb21   Formula 301
 
     wherein, in Formula 301, 
     Ar 301  and L 301  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     xb11 may be 1, 2, or 3, 
     xb1 may be an integer from 0 to 5, 
     R 301  may be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 60  alkyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkenyl group unsubstituted or substituted with at least one R 10a , a C 2 -C 60  alkynyl group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  alkoxy group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , —Si(Q 301 )(Q 302 )(Q 303 ), —N(Q 301 )(Q 302 ), —B(Q 301 )(Q 302 ), —C(═O)(Q 301 ), —S(═O) 2 (Q 301 ), or —P(═O)(Q 301 )(Q 302 ), 
     xb21 may be an integer from 1 to 5, and 
     Q 301  to Q 303  are respectively the same as those described in connection with Q 1 . 
     In an embodiment, when xb11 in Formula 301 is 2 or more, two or more of Ar 301 (s) may be linked to each other via a single bond. 
     In an embodiment, the host that is further included in each of the first host, the second host, and the third host may include a compound represented by Formula 301-1 below, a compound represented by Formula 301-2 below, or any combination thereof: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 301-1 and 301-2, 
     ring A 301  to ring A 304  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     X 301  may be O, S, N-[(L 304 ) xb4 -R 304 ], C(R 304 )(R 305 ), or Si(R 304 )(R 305 ), 
     xb22 and xb23 may each independently be 0, 1, or 2, 
     L 301 , xb1, and R 301  are respectively the same as those described in the present specification in connection with Formula 301, 
     L 302  to L 304  are each independently the same as described in connection with L 301 , 
     xb2 to xb4 are each independently the same as described in connection with xb1, and 
     R 302  to R 305  and R 311  to R 314  are respectively the same as those described in connection with R 301 . 
     In an embodiment, the host may include an alkali earth metal complex, a post-transition metal complex, or any combination thereof. In an embodiment, the host may include a Be complex (for example, Compound H55), an Mg complex, a Zn complex, or any combination thereof. 
     In an embodiment, the host may include one selected from Compounds H1 to H124, 9,10-di(2-naphthyl)anthracene (ADN), 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN), 9,10-di-(2-naphthyl)-2-t-butyl-anthracene (TBADN), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), 1,3-di-9-carbazolylbenzene (mCP), 1,3,5-tri(carbazol-9-yl)benzene (TCP), or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Phosphorescent Dopant 
     The phosphorescent dopant may include at least one transition metal as a central metal atom. 
     The phosphorescent dopant may include a monodentate ligand, a bidentate ligand, a tridentate ligand, a tetradentate ligand, a pentadentate ligand, a hexadentate ligand, or any combination thereof. 
     The phosphorescent dopant may be electrically neutral. 
     In an embodiment, the phosphorescent dopant may include an organometallic compound represented by Formula 401: 
     
       
         
         
             
             
         
       
     
     wherein, in Formulae 401 and 402, 
     M may be a transition metal (for example, iridium (Ir), platinum (Pt), palladium (Pd), osmium (Os), titanium (Ti), gold (Au), hafnium (Hf), europium (Eu), terbium (Tb), rhodium (Rh), rhenium (Re), or thulium (Tm)), 
     L 401  may be a ligand represented by Formula 402, and xc1 may be 1, 2, or 3, wherein, when xc1 is two or more, two or more of L 401 (s) may be identical to or different from each other, 
     L 402  may be an organic ligand, and xc2 may be 0, 1, 2, 3, or 4, wherein, when xc2 is 2 or more, two or more of L 402 (s) may be identical to or different from each other, 
     X 401  and X 402  may each independently be nitrogen or carbon, 
     ring A 401  and ring A 402  may each independently be a C 3 -C 60  carbocyclic group or a C 1 -C 60  heterocyclic group, 
     T 401  may be a single bond, *—O—*, *—S—*, *—C(═O)—*, *—N(Q 411 )-*, *—C(Q 411 )(Q 412 )-*, *—C(Q 411 )═C(Q 412 )-*, *—C(Q 411 )=*, or *═C═*, 
     X 403  and X 404  may each independently be a chemical bond (for example, a covalent bond or a coordinate bond (e.g., a coordinate covalent bond which may also be referred to as a dative bond)), O, S, N(Q 413 ), B(Q 413 ), P(Q 413 ), C(Q 413 )(Q 414 ), or 
     Q 411  to Q 414  are respectively the same as those described in connection with Q 1 , 
     R 401  and R 402  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group unsubstituted or substituted with at least one R 10a , a C 1 -C 20  alkoxy group unsubstituted or substituted with at least one R 10a , a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , —Bi(Q 401 )(Q 402 )(Q 403 ), —N(Q 401 )(Q 402 ), —B(Q 401 )(Q 402 ), —C(═O)(Q 401 ), —B(═O) 2 (Q 401 ), or —P(═O)(Q 401 )(Q 402 ), 
     Q 401  to Q 403  are respectively the same as described in connection with Q 1 , 
     xc11 and xc12 may each independently be an integer from 0 to 10, and 
     * and * in Formula 402 each indicate a binding site to M in Formula 401. 
     In an embodiment, in Formula 402, i) X 401  may be nitrogen, and X 402  may be carbon, or ii) each of X 401  and X 402  may be nitrogen. 
     In an embodiment, when xc1 in Formula 401 is 2 or more, two ring A 401  in two or more of L 401 (s) may be optionally linked to each other via T 402 , which is a linking group, and two ring A 402  may optionally be linked to each other via T 403 , which is a linking group (see Compounds PD1 to PD4 and PD7). T 402  and T 403  are respectively the same as those described in connection with T 401 . 
     L 402  in Formula 401 may be an organic ligand. In an embodiment, L 402  may include a halogen group, a diketone group (for example, an acetylacetonate group), a carboxylic acid group (for example, a picolinate group), —C(═O) group, an isonitrile group, —CN group, a phosphorus group (for example, a phosphine group, a phosphite group, etc.), or any combination thereof. 
     The phosphorescent dopant may include, for example, one selected from compounds PD1 to PD39, or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Fluorescent Dopant 
     The fluorescent dopant may include an amine group-containing compound, a styryl group-containing compound, or any combination thereof. 
     In an embodiment, the fluorescent dopant may include a compound represented by Formula 501: 
     
       
         
         
             
             
         
       
     
     In Formula 501, 
     Ar 501 , L 501  to L 503 , R 501 , and R 502  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     xd1 to xd3 may each independently be 0, 1, 2, or 3, and 
     xd4 may be 1, 2, 3, 4, 5, or 6. 
     In an embodiment, Ar 601  in Formula 501 may be a condensed cyclic group (for example, an anthracene group, a chrysene group, or a pyrene group) in which three or more monocyclic groups are condensed together. 
     In an embodiment, xd4 in Formula 501 may be 2. 
     In an embodiment, the fluorescent dopant may include: one selected from Compounds FD1 to FD36; DPVBi; DPAVBi; or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Delayed Fluorescence Material 
     The emission layer  120  may include a delayed fluorescence material. 
     In the present specification, the delayed fluorescence material may be selected from compounds capable of emitting delayed fluorescence based on a delayed fluorescence emission mechanism. 
     The delayed fluorescence material included in the emission layer  120  may act as a host or a dopant depending on the type or kind of other materials included in the emission layer  120 . 
     In an embodiment, the difference between the triplet energy level (eV) of the delayed fluorescence material and the singlet energy level (eV) of the delayed fluorescence material may be greater than or equal to 0 eV and less than or equal to 0.5 eV. When the difference between the triplet energy level (eV) of the delayed fluorescence material and the singlet energy level (eV) of the delayed fluorescence material satisfies the above-described range, up-conversion from the triplet state to the singlet state of the delayed fluorescence materials may effectively occur, and thus, the luminescence efficiency of the light-emitting device  10  may be improved. 
     In an embodiment, the delayed fluorescence material may include i) a material including at least one electron donor (for example, a π electron-rich C 3 -C 60  cyclic group, such as a carbazole group) and at least one electron acceptor (for example, a sulfoxide group, a cyano group, or a π electron-deficient nitrogen-containing C 1 -C 60  cyclic group), and ii) a material including a C 8 -C 60  polycyclic group in which two or more cyclic groups are condensed together while sharing a boron atom (B). 
     Examples of the delayed fluorescence material may include at least one selected from the following Compounds DF1 to DF9: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Quantum Dot 
     In the present specification, a quantum dot refers to a crystal of a semiconductor compound, and may include any suitable material capable of emitting light of various suitable emission wavelengths according to the size of the crystal. 
     A diameter of the quantum dot may be, for example, in a range of about 1 nm to about 10 nm. 
     The quantum dot may be synthesized by a wet chemical process, a metal organic chemical vapor deposition process, a molecular beam epitaxy process, and/or any suitable process similar thereto. 
     According to the wet chemical process, a precursor material is mixed together with an organic solvent to grow a quantum dot particle crystal. When the crystal grows, the organic solvent naturally acts as a dispersant coordinated on the surface of the quantum dot crystal and controls the growth of the crystal so that the growth of quantum dot particles may be controlled through a process which is more easily performed than vapor deposition methods, such as metal organic chemical vapor deposition (MOCVD) process or molecular beam epitaxy (MBE) process, and which has relatively lower costs. 
     The quantum dot may include: a Group II-VI semiconductor compound; a Group III-V semiconductor compound; a Group III-VI semiconductor compound; a Group I-III-VI semiconductor compound; a Group IV-VI semiconductor compound; a Group IV element or compound; or any combination thereof. 
     Examples of the Group II-VI semiconductor compound may include: a binary compound, such as CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, and/or MgS; a ternary compound, such as CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, and/or MgZnS; a quaternary compound, such as CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, and/or HgZnSTe; or any combination thereof. 
     Examples of the group III-V semiconductor compound may include: a binary compound such as GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, and/or InSb; a ternary compound such as GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InGaP, InNP, InAlP, InNAs, InNSb, InPAs, and/or InPSb; a quaternary compound such as GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, and/or InAlPSb; or any combination thereof. In an embodiment, the Group III-V semiconductor compound may further include Group II elements. Examples of the Group III-V semiconductor compound further including Group II elements may include InZnP, InGaZnP, InAlZnP, and the like. 
     Examples of the Group III-VI semiconductor compound may include: a binary compound, such as GaS, GaSe, Ga 2 Se 3 , GaTe, InS, InSe, In 2 S 3 , In 2 Se 3 , and/or InTe; a ternary compound, such as InGaS 3 , and/or InGaSe 3 ; or any combination thereof. 
     Examples of the Group I-III-VI semiconductor compound may include: a ternary compound, such as AgInS, AgInS 2 , CuInS, CuInS 2 , CuGaO 2 , AgGaO 2 , and/or AgAlO 2 ; or any combination thereof. 
     Examples of the Group IV-VI semiconductor compound may include: a binary compound, such as SnS, SnSe, SnTe, PbS, PbSe, PbTe, and/or the like; a ternary compound, such as SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, and/or the like; a quaternary compound, such as SnPbSSe, SnPbSeTe, SnPbSTe, and/or the like; or any combination thereof. 
     The Group IV element or compound may include: a single element compound, such as Si or Ge; a binary compound, such as SiC and/or SiGe; or any combination thereof. 
     Each element included in a multi-element compound such as the binary compound, ternary compound and quaternary compound, may exist in a particle with a uniform concentration or non-uniform concentration. 
     In an embodiment, the quantum dot may have a single structure or a dual core-shell structure. In the case of the quantum dot having a single structure, the concentration of each element included in the corresponding quantum dot is uniform (e.g., substantially uniform). In an embodiment, the material contained in the core and the material contained in the shell may be different from each other. 
     The shell of the quantum dot may act as a protective layer to prevent or reduce chemical degeneration of the core to maintain semiconductor characteristics and/or as a charging layer to impart electrophoretic characteristics to the quantum dot. The shell may be a single layer or a multi-layer. The element presented in the interface between the core and the shell of the quantum dot may have a concentration gradient that decreases along a direction toward the center of the quantum dot. 
     Examples of the shell of the quantum dot may be an oxide of metal, metalloid, or non-metal, a semiconductor compound, or any combination thereof. 
     Examples of the oxide of metal, metalloid, or non-metal may include: a binary compound, such as SiO 2 , Al 2 O 3 , TiO 2 , ZnO, MnO, Mn 2 O 3 , Mn 3 O 4 , CuO, FeO, Fe 2 O 3 , Fe 3 O 4 , CoO, Co 3 O 4 , and/or NiO; a ternary compound, such as MgAl 2 O 4 , CoFe 2 O 4 , NiFe 2 O 4 , and/or CoMn 2 O 4 ; or any combination thereof. Examples of the semiconductor compound may include, as described herein, a Group II-VI semiconductor compound, a Group III-V semiconductor compound, a Group III-VI semiconductor compound, a Group I-III-VI semiconductor compound, a Group IV-VI semiconductor compound, or any combination thereof. In addition, the semiconductor compound may include CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnSeS, ZnTeS, GaAs, GaP, GaSb, HgS, HgSe, HgTe, InAs, InP, InGaP, InSb, AlAs, AlP, AlSb, or any combination thereof. 
     A full width at half maximum (FWHM) of an emission wavelength spectrum of the quantum dot may be about 45 nm or less, for example, about 40 nm or less, for example, about 30 nm or less, and within these ranges, color purity or color reproducibility may be increased. In addition, because the light emitted through the quantum dot is emitted in all (e.g., substantially all) directions, the wide viewing angle can be improved. 
     In addition, the quantum dot may be a spherical particle, a pyramidal particle, a multi-arm particle, a cubic nanoparticle, a nanotube particle, a nanowire particle, a nanofiber particle, and/or a nanoplate particle. 
     Because the energy band gap can be adjusted by controlling the size of the quantum dot, light having various suitable wavelength bands can be obtained from the quantum dot emission layer. Therefore, by using quantum dots of different sizes, a light-emitting device that emits light of various suitable wavelengths may be implemented. In an embodiment, the size of the quantum dot may be selected to emit red, green and/or blue light. In addition, the size of the quantum dot may be configured to emit white light by combining light of various suitable colors. 
     Electron Transport Region in Interlayer  130   
     The electron transport region may have: i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer consisting of a plurality of different materials, or iii) a multi-layered structure including a plurality of layers including different materials. 
     The electron transport region may include a hole blocking layer, an electron transport layer, an electron injection layer, or any combination thereof. 
     In an embodiment, the electron transport region may have an electron transport layer/electron injection layer structure or a hole blocking layer/electron transport layer/electron injection layer structure, wherein, in each structure, constituting layers are sequentially stacked from the emission layer. 
     The electron transport region (for example, the hole blocking layer or the electron transport layer in the electron transport region) may include a metal-free compound including at least one 7 electron-deficient nitrogen-containing C 1 -C 60  cyclic group. 
     In an embodiment, the electron transport region may include a compound represented by Formula 601 below: 
       [Ar 601 ] xe11 -[(L 601 ) xe1 -R 601 ] xe21   Formula 601
 
     wherein, in Formula 601, 
     Ar 601  and L 601  may each independently be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a  or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , 
     xe11 may be 1, 2, or 3, 
     xe1 may be 0, 1, 2, 3, 4, or 5, 
     R 601  may be a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a , —Si(Q 601 )(Q 602 )(Q 603 ), —C(═O)(Q 601 ), —S(═O) 2 (Q 601 ), or —P(═O)(Q 601 )(Q 602 ), 
     Q 601  to Q 603  are respectively the same as those described in connection with Q 1 , 
     xe21 may be 1, 2, 3, 4, or 5, and 
     at least one selected from Ar 601 , L 601 , and R 601  may each independently be a π electron-deficient nitrogen-containing C 1 -C 60  cyclic group unsubstituted or substituted with at least one R 10a . 
     In an embodiment, when xe11 in Formula 601 is 2 or more, two or more of Ar 601  (s) may be linked via a single bond. 
     In an embodiment, Ar 601  in Formula 601 may be a substituted or unsubstituted anthracene group. 
     In an embodiment, the electron transport region may include a compound represented by Formula 601-1: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 601-1, 
     X 614  may be N or C(R 614 ), X 615  may be N or C(R 615 ), X 616  may be N or C(R 616 ), at least one selected from X 614  to X 616  may be N, 
     L 611  to L 613  are respectively the same as those described in connection with L 601 , 
     xe611 to xe613 are respectively the same as those described in connection with xe1, 
     R 611  to R 613  are respectively the same as those described in connection with R 601 , and 
     R 614  to R 616  may each independently be hydrogen, deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 20  alkyl group, a C 1 -C 20  alkoxy group, a C 3 -C 60  carbocyclic group unsubstituted or substituted with at least one R 10a , or a C 1 -C 60  heterocyclic group unsubstituted or substituted with at least one R 10a . 
     In an embodiment, xe1 and xe611 to xe613 in Formulae 601 and 601-1 may each independently be 0, 1, or 2. 
     The electron transport region may include one selected from Compounds ET1 to ET45, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (Bphen), Alq3, BAlq, TAZ, NTAZ, or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A thickness of the electron transport region may be from about 100 Å to about 5,000 Å, for example, about 160 Å to about 4,000 Å. When the electron transport region includes a hole blocking layer, an electron transport layer, or any combination thereof, thicknesses of the hole blocking layer and the electron transport layer may each independently be from about 20 Å to about 1,000 Å, for example, from about 30 Å to about 300 Å, and a thickness of the electron transport layer may be from about 100 Å to about 1,000 Å, for example, from about 150 Å to about 500 Å. When the thicknesses of the hole blocking layer and/or electron transport layer are within the ranges described above, suitable or satisfactory electron-transporting characteristics may be obtained without a substantial increase in driving voltage. 
     The electron transport region (for example, the electron transport layer in the electron transport region) may further include, in addition to the materials described above, a metal-containing material. 
     The metal-containing material may include an alkali metal complex, an alkaline earth metal complex, or any combination thereof. A metal ion of the alkali metal complex may be a Li ion, a Na ion, a K ion, a Rb ion, or a Cs ion, and a metal ion of the alkaline earth metal complex may be a Be ion, a Mg ion, a Ca ion, a Sr ion, or a Ba ion. A ligand coordinated with the metal ion of the alkali metal complex or the alkaline earth-metal complex may include a hydroxyquinoline, a hydroxyisoquinoline, a hydroxybenzoquinoline, a hydroxyacridine, a hydroxyphenanthridine, a hydroxyphenyloxazole, a hydroxyphenylthiazole, a hydroxydiphenyloxadiazole, a hydroxydiphenylthiadiazole, a hydroxyphenylpyridine, a hydroxyphenylbenzimidazole, a hydroxyphenylbenzothiazole, a bipyridine, a phenanthroline, a cyclopentadiene, or any combination thereof. 
     In an embodiment, the metal-containing material may include a Li complex. The Li complex may include, for example, Compound ET-D1 (LiQ) or ET-D2: 
     
       
         
         
             
             
         
       
     
     The electron transport region may include an electron injection layer that facilitates the injection of electrons from the second electrode  150 . The electron injection layer may be in direct contact (e.g., physical contact) with the second electrode  150 . 
     The electron injection layer may have: i) a single-layered structure consisting of a single layer consisting of a single material, ii) a single-layered structure consisting of a single layer consisting of a plurality of different materials, or iii) a multi-layered structure including a plurality of layers including different materials. 
     The electron injection layer may include an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal-containing compound, an alkaline earth metal-containing compound, a rare earth metal-containing compound, an alkali metal complex, an alkaline earth metal complex, a rare earth metal complex, or any combination thereof. 
     The alkali metal may include Li, Na, K, Rb, Cs, or any combination thereof. The alkaline earth metal may include Mg, Ca, Sr, Ba, or any combination thereof. The rare earth metal may include Sc, Y, Ce, Tb, Yb, Gd, or any combination thereof. 
     The alkali metal-containing compound, the alkaline earth metal-containing compound, and the rare earth metal-containing compound may include oxides, halides (for example, fluorides, chlorides, bromides, or iodides), or tellurides of the alkali metal, the alkaline earth metal, and the rare earth metal, or any combination thereof. 
     The alkali metal-containing compound may include alkali metal oxides, such as Li 2 O, Cs 2 O, and/or K 2 O, alkali metal halides, such as LiF, NaF, CsF, KF, LiI, NaI, CsI, and/or KI, or any combination thereof. The alkaline earth metal-containing compound may include an alkaline earth metal oxide, such as BaO, SrO, CaO, Ba x Sr 1-x O (x is a real number satisfying the condition of 0&lt;x&lt;1), Ba x Ca 1-x O (x is a real number satisfying the condition of 0&lt;x&lt;1), and/or the like. The rare earth metal-containing compound may include YbF 3 , ScF 3 , Sc 2 O 3 , Y 2 O 3 , Ce 2 O 3 , GdF 3 , TbF 3 , YbI 3 , ScI 3 , TbI 3 , or any combination thereof. In an embodiment, the rare earth metal-containing compound may include lanthanide metal telluride. Examples of the lanthanide metal telluride may include LaTe, CeTe, PrTe, NdTe, PmTe, SmTe, EuTe, GdTe, TbTe, DyTe, HoTe, ErTe, TmTe, YbTe, LuTe, La 2 Te 3 , Ce 2 Te 3 , Pr 2 Te 3 , Nd 2 Te 3 , Pm 2 Te 3 , Sm 2 Te 3 , Eu 2 Te 3 , Gd 2 Te 3 , Tb 2 Te 3 , Dy 2 Te 3 , Ho 2 Te 3 , Er 2 Te 3 , Tm 2 Te 3 , Yb 2 Te 3 , or Lu 2 Te 3 , or any combination thereof. 
     The alkali metal complex, the alkaline earth-metal complex, and the rare earth metal complex may include i) one of metal ions of the alkali metal, the alkaline earth metal, and the rare earth metal and ii), as a ligand bonded to the metal ion, for example, hydroxyquinoline, hydroxyisoquinoline, hydroxybenzoquinoline, hydroxyacridine, hydroxyphenanthridine, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxydiphenyloxadiazole, hydroxydiphenylthiadiazole, hydroxyphenylpyridine, hydroxyphenyl benzimidazole, hydroxyphenylbenzothiazole, bipyridine, phenanthroline, cyclopentadiene, or any combination thereof. 
     The electron injection layer may include (e.g., consist of) an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal-containing compound, an alkaline earth metal-containing compound, a rare earth metal-containing compound, an alkali metal complex, an alkaline earth metal complex, a rare earth metal complex, or any combination thereof, as described above. In an embodiment, the electron injection layer may further include an organic material (for example, a compound represented by Formula 601). 
     In an embodiment, the electron injection layer may include (e.g., consist of) i) an alkali metal-containing compound (for example, an alkali metal halide), ii) a) an alkali metal-containing compound (for example, an alkali metal halide); and b) an alkali metal, an alkaline earth metal, a rare earth metal, or any combination thereof. In an embodiment, the electron injection layer may be a KI:Yb co-deposited layer, an RbI:Yb co-deposited layer, and/or the like. 
     When the electron injection layer further includes an organic material, alkali metal, alkaline earth metal, rare earth metal, an alkali metal-containing compound, an alkaline earth metal-containing compound, a rare earth metal-containing compound, alkali metal complex, alkaline earth-metal complex, rare earth metal complex, or any combination thereof may be homogeneously or non-homogeneously dispersed in a matrix including the organic material. 
     A thickness of the electron injection layer may be in a range of about 1 Å to about 100 Å, and, for example, about 3 Å to about 90 Å. When the thickness of the electron injection layer is within the range described above, suitable or satisfactory electron injection characteristics may be obtained without a substantial increase in driving voltage. 
     Second Electrode  150   
     The second electrode  150  may be on the interlayer  130  having such a structure. The second electrode  150  may be a cathode, which is an electron injection electrode, and as the material for the second electrode  150 , a metal, an alloy, an electrically conductive compound, or any combination thereof, each having a low work function, may be used. 
     In an embodiment, the second electrode  150  may include lithium (Li), silver (Ag), magnesium (Mg), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), ytterbium (Yb), silver-ytterbium (Ag—Yb), ITO, IZO, or any combination thereof. The second electrode  150  may be a transmissive electrode, a semi-transmissive electrode, or a reflective electrode. 
     The second electrode  150  may have a single-layered structure or a multi-layered structure including two or more layers. 
     Capping Layer 
     A first capping layer may be outside the first electrode  110 , and/or a second capping layer may be outside the second electrode  150 . In more detail, the light-emitting device  10  may have a structure in which the first capping layer, the first electrode  110 , the interlayer  130 , and the second electrode  150  are sequentially stacked in this stated order, a structure in which the first electrode  110 , the interlayer  130 , the second electrode  150 , and the second capping layer are sequentially stacked in this stated order, or a structure in which the first capping layer, the first electrode  110 , the interlayer  130 , the second electrode  150 , and the second capping layer are sequentially stacked in this stated order. 
     Light generated in an emission layer  120  of the interlayer  130  of the light-emitting device  10  may be extracted toward the outside through the first electrode  110 , which is a semi-transmissive electrode or a transmissive electrode, and the first capping layer or light generated in an emission layer  120  of the interlayer  130  of the light-emitting device  10  may be extracted toward the outside through the second electrode  150 , which is a semi-transmissive electrode or a transmissive electrode, and the second capping layer. 
     The first capping layer and the second capping layer may increase external emission efficiency according to the principle of constructive interference. Accordingly, the light extraction efficiency of the light-emitting device  10  is increased, so that the luminescence efficiency of the light-emitting device  10  may be improved. 
     Each of the first capping layer and second capping layer may include a material having a refractive index (at a wavelength of 589 nm) of 1.6 or more. 
     The first capping layer and the second capping layer may each independently be an organic capping layer including an organic material, an inorganic capping layer including an inorganic material, or a composite capping layer including an organic material and an inorganic material. 
     At least one selected from the first capping layer and the second capping layer may each independently include carbocyclic compounds, heterocyclic compounds, amine group-containing compounds, porphyrin derivatives, phthalocyanine derivatives, naphthalocyanine derivatives, alkali metal complexes, alkaline earth metal complexes, or any combination thereof. The carbocyclic compound, the heterocyclic compound, and the amine group-containing compound may be optionally substituted with a substituent containing O, N, S, Se, Si, F, Cl, Br, I, or any combination thereof. In an embodiment, at least one selected from the first capping layer and the second capping layer may each independently include an amine group-containing compound. 
     In an embodiment, at least one selected from the first capping layer and the second capping layer may each independently include a compound represented by Formula 201, a compound represented by Formula 202, or any combination thereof. 
     In an embodiment, at least one selected from the first capping layer and the second capping layer may each independently include one selected from Compounds HT28 to HT33, one selected from Compounds CP1 to CP6, p-NPB, or any combination thereof: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Electronic Apparatus 
     The light-emitting device may be included in various suitable electronic apparatuses. In an embodiment, the electronic apparatus including the light-emitting device may be a light-emitting apparatus, an authentication apparatus, and/or the like. 
     The electronic apparatus (for example, light-emitting apparatus) may further include, in addition to the light-emitting device, i) a color filter, ii) a color conversion layer, or iii) a color filter and a color conversion layer. The color filter and/or the color conversion layer may be in at least one traveling direction of light emitted from the light-emitting device. For example, the light emitted from the light-emitting device may be blue light or white light. The light-emitting device may be the same as described above. In an embodiment, the color conversion layer may include quantum dots. The quantum dot may be, for example, a quantum dot as described herein. 
     The electronic apparatus may include a first substrate. The first substrate may include a plurality of subpixel areas, the color filter may include a plurality of color filter areas respectively corresponding to the subpixel areas, and the color conversion layer may include a plurality of color conversion areas respectively corresponding to the subpixel areas. 
     A pixel-defining film may be located among the subpixel areas to define each of the subpixel areas. 
     The color filter may further include a plurality of color filter areas and light-shielding patterns located among the color filter areas, and the color conversion layer may include a plurality of color conversion areas and light-shielding patterns located among the color conversion areas. 
     The color filter areas (or the color conversion areas) may include a first area emitting first color light, a second area emitting second color light, and/or a third area emitting third color light, and the first color light, the second color light, and/or the third color light may have different maximum emission wavelengths from one another. In an embodiment, the first color light may be red light, the second color light may be green light, and the third color light may be blue light. In an embodiment, the color filter areas (or the color conversion areas) may include quantum dots. In more detail, the first area may include a red quantum dot, the second area may include a green quantum dot, and the third area may not include a quantum dot. The quantum dot is the same as described in the present specification. The first area, the second area, and/or the third area may each further include a scatterer (e.g., a light scatterer). 
     In an embodiment, the light-emitting device may emit a first light, the first area may absorb the first light to emit a first first-color light, the second area may absorb the first light to emit a second first-color light, and the third area may absorb the first light to emit a third first-color light. In this regard, the first first-color light, the second first-color light, and the third first-color light may have different maximum emission wavelengths. In more detail, the first light may be blue light, the first first-color light may be red light, the second first-color light may be green light, and the third first-color light may be blue light. 
     The electronic apparatus may further include a thin-film transistor in addition to the light-emitting device as described above. The thin-film transistor may include a source electrode, a drain electrode, and an activation layer, wherein any one selected from the source electrode and the drain electrode may be electrically connected to any one selected from the first electrode and the second electrode of the light-emitting device. 
     The thin-film transistor may further include a gate electrode, a gate insulating film, etc. 
     The activation layer may include crystalline silicon, amorphous silicon, organic semiconductor, oxide semiconductor, or the like. 
     The electronic apparatus may further include a sealing portion for sealing the light-emitting device. The sealing portion and/or the color conversion layer may be between the color filter and the light-emitting device. The sealing portion allows light from the light-emitting device to be extracted to the outside, while concurrently (e.g., simultaneously) preventing or reducing penetration of ambient air and/or moisture into the light-emitting device. The sealing portion may be a sealing substrate including a transparent glass substrate and/or a plastic substrate. The sealing portion may be a thin-film encapsulation layer including at least one layer of an organic layer and/or an inorganic layer. When the sealing portion is a thin film encapsulation layer, the electronic apparatus may be flexible. 
     Various suitable functional layers may be additionally on the sealing portion, in addition to the color filter and/or the color conversion layer, according to the use of the electronic apparatus. The functional layers may include a touch screen layer, a polarizing layer, and/or the like. The touch screen layer may be a pressure-sensitive touch screen layer, a capacitive touch screen layer, and/or an infrared touch screen layer. The authentication apparatus may be, for example, a biometric authentication apparatus that authenticates an individual by using biometric information of a living body (for example, fingertips, pupils, etc.). 
     The authentication apparatus may further include, in addition to the light-emitting device, a biometric information collector. 
     The electronic apparatus may be applied to various suitable displays, light sources, lighting, personal computers (for example, a mobile personal computer), mobile phones, digital cameras, electronic diaries, electronic dictionaries, electronic game machines, medical instruments (for example, electronic thermometers, sphygmomanometers, blood glucose meters, pulse measurement devices, pulse wave measurement devices, electrocardiogram displays, ultrasonic diagnostic devices, and/or endoscope displays), fish finders, various measuring instruments, meters (for example, meters for a vehicle, an aircraft, and/or a vessel), projectors, and/or the like. 
     Description of FIGS.  2  and  3   
       FIG.  2    is a cross-sectional view of an electronic apparatus according to an embodiment. 
     The electronic apparatus of  FIG.  2    includes a substrate  100 , a thin-film transistor (TFT), a light-emitting device, and an encapsulation portion  300  that seals the light-emitting device. 
     The substrate  100  may be a flexible substrate, a glass substrate, and/or a metal substrate. A buffer layer  210  may be on the substrate  100 . The buffer layer  210  may prevent or reduce penetration of impurities through the substrate  100  and may provide a flat surface on the substrate  100 . 
     A TFT may be on the buffer layer  210 . The TFT may include an activation layer  220 , a gate electrode  240 , a source electrode  260 , and a drain electrode  270 . 
     The activation layer  220  may include an inorganic semiconductor such as silicon or polysilicon, an organic semiconductor, or an oxide semiconductor, and may include a source region, a drain region and a channel region. 
     A gate insulating film  230  for insulating the activation layer  220  from the gate electrode  240  may be on the activation layer  220 , and the gate electrode  240  may be on the gate insulating film  230 . 
     An interlayer insulating film  250  is on the gate electrode  240 . The interlayer insulating film  250  may be between the gate electrode  240  and the source electrode  260  to insulate the gate electrode  240  from the source electrode  260  and between the gate electrode  240  and the drain electrode  270  to insulate the gate electrode  240  from the drain electrode  270 . 
     The source electrode  260  and the drain electrode  270  may be on the interlayer insulating film  250 . The interlayer insulating film  250  and the gate insulating film  230  may expose the source region and the drain region of the activation layer  220 , and the source electrode  260  and the drain electrode  270  may be in contact (e.g., physical contact) with the exposed portions of the source region and the drain region of the activation layer  220 . 
     The TFT is electrically connected to a light-emitting device to drive the light-emitting device, and is covered by a passivation layer  280 . The passivation layer  280  may include an inorganic insulating film, an organic insulating film, or any combination thereof. A light-emitting device is provided on the passivation layer  280 . The light-emitting device may include a first electrode  110 , an interlayer  130 , and a second electrode  150 . 
     The first electrode  110  may be on the passivation layer  280 . The passivation layer  280  does not completely cover the drain electrode  270  and exposes a portion of the drain electrode  270 , and the first electrode  110  is connected to the exposed portion of the drain electrode  270 . 
     A pixel-defining layer  290  containing an insulating material may be on the first electrode  110 . The pixel-defining layer  290  exposes a region of the first electrode  110 , and an interlayer  130  may be in the exposed region of the first electrode  110 . The pixel-defining layer  290  may be a polyimide and/or polyacrylic organic film. In some embodiments, at least some layers of the interlayer  130  may extend beyond the upper portion of the pixel-defining layer  290  in the form of a common layer. 
     The second electrode  150  may be on the interlayer  130 , and a capping layer  170  may be additionally on the second electrode  150 . The capping layer  170  may cover the second electrode  150 . 
     The encapsulation portion  300  may be on the capping layer  170 . The encapsulation portion  300  may be on a light-emitting device to protect the light-emitting device from moisture and/or oxygen. The encapsulation portion  300  may include: an inorganic film including silicon nitride (SiNx), silicon oxide (SiOx), indium tin oxide, indium zinc oxide, or any combination thereof; an organic film including polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, polyethylene sulfonate, polyoxymethylene, polyarylate, hexamethyldisiloxane, an acrylic-based resin (for example, polymethyl methacrylate, polyacrylic acid, and/or the like), an epoxy-based resin (for example, aliphatic glycidyl ether (AGE), and/or the like), or a combination thereof; or any combination of the inorganic film and the organic film. 
       FIG.  3    is a cross-sectional view of an electronic apparatus according to another embodiment of the present disclosure. 
     The electronic apparatus of  FIG.  3    is the same as the light-emitting apparatus of  FIG.  2   , except that a light-shielding pattern  500  and a functional region  400  are additionally on the encapsulation portion  300 . The functional region  400  may be a combination of i) a color filter area, ii) a color conversion area, or iii) a combination of the color filter area and the color conversion area. In an embodiment, the light-emitting device included in the light-emitting apparatus of  FIG.  3    may be a tandem light-emitting device. The color conversion area refers to an area including the color conversion layer. 
     Manufacture Method 
     Respective layers included in the hole transport region, the emission layer, and respective layers included in the electron transport region may be in a certain region by using one or more suitable methods selected from vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB) deposition, ink-jet printing, laser-printing, and laser-induced thermal imaging. 
     When layers constituting the hole transport region, an emission layer, and layers constituting the electron transport region are formed by vacuum deposition, the deposition may be performed at a deposition temperature of about 100° C. to about 500° C., a vacuum degree of about 10-8 torr to about 10-3 torr, and a deposition speed of about 0.01 Å/sec to about 100 Å/sec, depending on a material to be included in a layer to be formed and the structure of a layer to be formed. 
     When layers constituting the hole transport region, an emission layer, and layers constituting the electron transport region are formed by spin coating, the spin coating may be performed at a coating speed of about 2,000 rpm to about 5,000 rpm and at a heat treatment temperature of about 80° C. to 200° C. by taking into account a material to be included in a layer to be formed and the structure of a layer to be formed. 
     General Definitions of Substituents 
     The term “C 3 -C 60  carbocyclic group,” as used herein, refers to a cyclic group consisting of carbon only and having three to sixty carbon atoms, and the term “C 1 -C 60  heterocyclic group,” as used herein, refers to a cyclic group that has one to sixty carbon atoms and further has, in addition to carbon, a heteroatom. The C 3 -C 60  carbocyclic group and the C 1 -C 60  heterocyclic group may each be a monocyclic group consisting of one ring or a polycyclic group in which two or more rings are condensed together with each other. For example, the number of ring-forming atoms of the C 1 -C 60  heterocyclic group may be from 3 to 61. 
     The term “cyclic group,” as used herein, may include the C 3 -C 60  carbocyclic group and the C 1 -C 60  heterocyclic group. 
     The term “π electron-rich C 3 -C 60  cyclic group,” as used herein, refers to a cyclic group that has three to sixty carbon atoms and does not include *—N═* as a ring-forming moiety, and the term “π electron-deficient nitrogen-containing C 1 -C 60  cyclic group,” as used herein, refers to a heterocyclic group that has one to sixty carbon atoms and includes *—N═* as a ring-forming moiety. 
     In an embodiment, 
     the C 3 -C 60  carbocyclic group may be i) group T1 or ii) a condensed cyclic group in which two or more groups T1 are condensed together with each other (for example, a cyclopentadiene group, an adamantane group, a norbornane group, a benzene group, a pentalene group, a naphthalene group, an azulene group, an indacene group, an acenaphthylene group, a phenalene group, a phenanthrene group, an anthracene group, a fluoranthene group, a triphenylene group, a pyrene group, a chrysene group, a perylene group, a pentaphene group, a heptalene group, a naphthacene group, a picene group, a hexacene group, a pentacene group, a rubicene group, a coronene group, an ovalene group, an indene group, a fluorene group, a spiro-bifluorene group, a benzofluorene group, an indenophenanthrene group, or an indenoanthracene group), 
     the C 1 -C 60  heterocyclic group may be i) group T2, ii) a condensed cyclic group in which two or more groups T2 are condensed together with each other, or iii) a condensed cyclic group in which at least one group T2 and at least one group T1 are condensed together with each other (for example, a pyrrole group, a thiophene group, a furan group, an indole group, a benzoindole group, a naphthoindole group, an isoindole group, a benzoisoindole group, a naphthoisoindole group, a benzosilole group, a benzothiophene group, a benzofuran group, a carbazole group, a dibenzosilole group, a dibenzothiophene group, a dibenzofuran group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, a benzothienocarbazole group, a benzosilolocarbazole group, a benzoindolocarbazole group, a benzocarbazole group, a benzonaphthofuran group, a benzonaphthothiophene group, a benzonaphthosilole group, a benzofurodibenzofuran group, a benzofurodibenzothiophene group, a benzothienodibenzothiophene group, a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isoxazole group, an oxadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, a benzopyrazole group, a benzimidazole group, a benzoxazole group, a benzoisoxazole group, a benzothiazole group, a benzoisothiazole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a benzoisoquinoline group, a quinoxaline group, a benzoquinoxaline group, a quinazoline group, a benzoquinazoline group, a phenanthroline group, a cinnoline group, a phthalazine group, a naphthyridine group, an imidazopyridine group, an imidazopyrimidine group, an imidazotriazine group, an imidazopyrazine group, an imidazopyridazine group, an azacarbazole group, an azafluorene group, an azadibenzosilole group, an azadibenzothiophene group, an azadibenzofuran group, etc.), 
     the π electron-rich C 3 -C 60  cyclic group may be i) group T1, ii) a condensed cyclic group in which two or more groups T1 are condensed together with each other, iii) group T3, iv) a condensed cyclic group in which two or more groups T3 are condensed together with each other, or v) a condensed cyclic group in which at least one group T3 and at least one group T1 are condensed together with each other (for example, the C 3 -C 60  carbocyclic group, a pyrrole group, a thiophene group, a furan group, an indole group, a benzoindole group, a naphthoindole group, an isoindole group, a benzoisoindole group, a naphthoisoindole group, a benzosilole group, a benzothiophene group, a benzofuran group, a carbazole group, a dibenzosilole group, a dibenzothiophene group, a dibenzofuran group, an indenocarbazole group, an indolocarbazole group, a benzofurocarbazole group, a benzothienocarbazole group, a benzosilolocarbazole group, a benzoindolocarbazole group, a benzocarbazole group, a benzonaphthofuran group, a benzonaphthothiophene group, a benzonaphthosilole group, a benzofurodibenzofuran group, a benzofurodibenzothiophene group, a benzothienodibenzothiophene group, etc.), 
     the π electron-deficient nitrogen-containing C 1 -C 60  cyclic group may be i) group T4, ii) a condensed cyclic group in which two or more group T4 are condensed together with each other, iii) a condensed cyclic group in which at least one group T4 and at least one group T1 are condensed together with each other, iv) a condensed cyclic group in which at least one group T4 and at least one group T3 are condensed together with each other, or v) a condensed cyclic group in which at least one group T4, at least one group T1, and at least one group T3 are condensed together with one another (for example, a pyrazole group, an imidazole group, a triazole group, an oxazole group, an isoxazole group, an oxadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, a benzopyrazole group, a benzimidazole group, a benzoxazole group, a benzoisoxazole group, a benzothiazole group, a benzoisothiazole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, a quinoline group, an isoquinoline group, a benzoquinoline group, a benzoisoquinoline group, a quinoxaline group, a benzoquinoxaline group, a quinazoline group, a benzoquinazoline group, a phenanthroline group, a cinnoline group, a phthalazine group, a naphthyridine group, an imidazopyridine group, an imidazopyrimidine group, an imidazotriazine group, an imidazopyrazine group, an imidazopyridazine group, an azacarbazole group, an azafluorene group, an azadibenzosilole group, an azadibenzothiophene group, an azadibenzofuran group, etc.), 
     group T1 may be a cyclopropane group, a cyclobutane group, a cyclopentane group, a cyclohexane group, a cycloheptane group, a cyclooctane group, a cyclobutene group, a cyclopentene group, a cyclopentadiene group, a cyclohexene group, a cyclohexadiene group, a cycloheptene group, an adamantane group, a norbornane (or a bicyclo[2.2.1]heptane) group, a norbornene group, a bicyclo[1.1.1]pentane group, a bicyclo[2.1.1]hexane group, a bicyclo[2.2.2]octane group, or a benzene group, 
     group T2 may be a furan group, a thiophene group, a 1H-pyrrole group, a silole group, a borole group, a 2H-pyrrole group, a 3H-pyrrole group, an imidazole group, a pyrazole group, a triazole group, a tetrazole group, an oxazole group, an isoxazole group, an oxadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, an azasilole group, an azaborole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, or a tetrazine group, 
     group T3 may be a furan group, a thiophene group, a 1H-pyrrole group, a silole group, or a borole group, and 
     group T4 may be a 2H-pyrrole group, a 3H-pyrrole group, an imidazole group, a pyrazole group, a triazole group, a tetrazole group, an oxazole group, an isoxazole group, an oxadiazole group, a thiazole group, an isothiazole group, a thiadiazole group, an azasilole group, an azaborole group, a pyridine group, a pyrimidine group, a pyrazine group, a pyridazine group, a triazine group, or a tetrazine group. 
     The term “cyclic group”, “C 3 -C 60  carbocyclic group”, “C 1 -C 60  heterocyclic group”, “π electron-rich C 3 -C 60  cyclic group”, or “π electron-deficient nitrogen-containing C 1 -C 60  cyclic group,” as used herein, refers to a group condensed to any cyclic group or a polyvalent group (for example, a divalent group, a trivalent group, a tetravalent group, etc.), depending on the structure of a formula in connection with which the terms are used. In an embodiment, “a benzene group” may be a benzo group, a phenyl group, a phenylene group, or the like, which may be easily understood by one of ordinary skill in the art according to the structure of a formula including the “benzene group.” 
     Examples of the monovalent C 3 -C 60  carbocyclic group and the monovalent C 1 -C 60  heterocyclic group may include 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, and examples of the divalent C 3 -C 60  carbocyclic group and the monovalent C 1 -C 60  heterocyclic group may include a C 3 -C 10  cycloalkylene group, a C 1 -C 10  heterocycloalkylene group, a C 3 -C 10  cycloalkenylene group, a C 1 -C 10  heterocycloalkenylene group, a C 6 -C 60  arylene group, a C 1 -C 60  heteroarylene group, a divalent non-aromatic condensed polycyclic group, and a substituted or unsubstituted divalent non-aromatic condensed heteropolycyclic group. 
     The term “C 1 -C 60  alkyl group,” as used herein, refers to a linear or branched aliphatic hydrocarbon monovalent group that has one to sixty carbon atoms, and examples thereof include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, a tert-pentyl group, a neopentyl group, an isopentyl group, a sec-pentyl group, a 3-pentyl group, a sec-isopentyl group, an n-hexyl group, an isohexyl group, a sec-hexyl group, a tert-hexyl group, an n-heptyl group, an isoheptyl group, a sec-heptyl group, a tert-heptyl group, an n-octyl group, an isooctyl group, a sec-octyl group, a tert-octyl group, an n-nonyl group, an isononyl group, a sec-nonyl group, a tert-nonyl group, an n-decyl group, an isodecyl group, a sec-decyl group, and a tert-decyl 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 2 -C 60  alkenyl group,” as used herein, refers to a monovalent hydrocarbon group having at least one carbon-carbon double bond in a main chain (e.g., in the middle) or at a terminal end (e.g., the terminus) of the C 2 -C 60  alkyl group, and examples thereof include an ethenyl group, a propenyl group, and a butenyl group. The term “C 2 -C 60  alkenylene group,” as used herein, refers to a divalent group having 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 monovalent hydrocarbon group having at least one carbon-carbon triple bond in a main chain (e.g., in the middle) or at a terminal end (e.g., the terminus) of the C 2 -C 60  alkyl group, and examples thereof include an ethynyl group and a propynyl group. The term “C 2 -C 60  alkynylene group,” as used herein, refers to a divalent group having substantially the same structure as the C 2 -C 60  alkynyl group. 
     The term “C 1 -C 60  alkoxy group,” as used herein, refers to a monovalent group represented by —OA 101  (wherein A 101  is the C 1 -C 60  alkyl group), and examples thereof include a methoxy group, an ethoxy group, and an isopropyloxy group. 
     The term “C 3 -C 10  cycloalkyl group,” as used herein, refers to a monovalent saturated hydrocarbon cyclic group having 3 to 10 carbon atoms, and examples thereof include a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, an adamantanyl group, a norbornanyl group (or a bicyclo[2.2.1]heptyl group), a bicyclo[1.1.1]pentyl group, a bicyclo[2.1.1]hexyl group, and a bicyclo[2.2.2]octyl group. The term “C 3 -C 10  cycloalkylene group,” as used herein, refers to a divalent group having substantially the same structure as the C 3 -C 10  cycloalkyl group. 
     The term “C 1 -C 10  heterocycloalkyl group,” as used herein, refers to a monovalent cyclic group that further includes, in addition to a carbon atom, at least one heteroatom as a ring-forming atom and has 1 to 10 carbon atoms, and examples thereof include a 1,2,3,4-oxatriazolidinyl group, a tetrahydrofuranyl group, and a tetrahydrothiophenyl group. The term “C 1 -C 10  heterocycloalkylene group,” as used herein, refers to a divalent group having 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 cyclic group that has three to ten carbon atoms and at least one carbon-carbon double bond in the ring thereof and no aromaticity (e.g., is not aromatic), and examples thereof include a cyclopentenyl group, a cyclohexenyl group, and a cycloheptenyl group. The term “C 3 -C 10  cycloalkenylene group,” as used herein, refers to a divalent group having substantially the same structure as the C 3 -C 10  cycloalkenyl group. 
     The term “C 1 -C 10  heterocycloalkenyl group,” as used herein, refers to a monovalent cyclic group that has, in addition to a carbon atom, at least one heteroatom as a ring-forming atom, 1 to 10 carbon atoms, and at least one carbon-carbon double bond in the cyclic structure thereof. Examples of the C 1 -C 10  heterocycloalkenyl group include a 4,5-dihydro-1,2,3,4-oxatriazolyl group, a 2,3-dihydrofuranyl group, and a 2,3-dihydrothiophenyl group. The term “C 1 -C 10  heterocycloalkenylene group,” as used herein, refers to a divalent group having 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 six to sixty carbon atoms, and the term “C 6 -C 60  arylene group,” as used herein, refers to a divalent group having a carbocyclic aromatic system having six to sixty carbon atoms. Examples of the C 6 -C 60  aryl group include a phenyl group, a pentalenyl group, a naphthyl group, an azulenyl group, an indacenyl group, an acenaphthyl group, a phenalenyl group, a phenanthrenyl group, an anthracenyl group, a fluoranthenyl group, a triphenylenyl group, a pyrenyl group, a chrysenyl group, a perylenyl group, a pentaphenyl group, a heptalenyl group, a naphthacenyl group, a picenyl group, a hexacenyl group, a pentacenyl group, a rubicenyl group, a coronenyl group, and an ovalenyl 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 condensed together with each other. 
     The term “C 1 -C 60  heteroaryl group,” as used herein, refers to a monovalent group having a heterocyclic aromatic system that has, in addition to a carbon atom, at least one heteroatom as a ring-forming atom, and 1 to 60 carbon atoms. The term “C 1 -C 60  heteroarylene group,” as used herein, refers to a divalent group having a heterocyclic aromatic system that has, in addition to a carbon atom, at least one heteroatom as a ring-forming atom, and 1 to 60 carbon atoms. Examples of the C 1 -C 60  heteroaryl group include a pyridinyl group, a pyrimidinyl group, a pyrazinyl group, a pyridazinyl group, a triazinyl group, a quinolinyl group, a benzoquinolinyl group, an isoquinolinyl group, a benzoisoquinolinyl group, a quinoxalinyl group, a benzoquinoxalinyl group, a quinazolinyl group, a benzoquinazolinyl group, a cinnolinyl group, a phenanthrolinyl group, a phthalazinyl group, and a naphthyridinyl group. When the C 1 -C 60  heteroaryl group and the C 1 -C 60  heteroarylene group each include two or more rings, the rings may be condensed together with each other. 
     The term “monovalent non-aromatic condensed polycyclic group,” as used herein, refers to a monovalent group having two or more rings condensed to each other, only carbon atoms (for example, having 8 to 60 carbon atoms) as ring-forming atoms, and non-aromaticity in its molecular structure when considered as a whole (e.g., is not aromatic when considered as a whole). Examples of the monovalent non-aromatic condensed polycyclic group include an indenyl group, a fluorenyl group, a spiro-bifluorenyl group, a benzofluorenyl group, an indenophenanthrenyl group, and an indeno anthracenyl group. The term “divalent non-aromatic condensed polycyclic group,” as used herein, refers to a divalent group having substantially the same structure as a monovalent non-aromatic condensed polycyclic group. 
     The term “monovalent non-aromatic condensed heteropolycyclic group,” as used herein, refers to a monovalent group having two or more rings condensed to each other, at least one heteroatom other than carbon atoms (for example, having 1 to 60 carbon atoms), as a ring-forming atom, and non-aromaticity in its molecular structure when considered as a whole (e.g., is not aromatic when considered as a whole). Examples of the monovalent non-aromatic condensed heteropolycyclic group include a pyrrolyl group, a thiophenyl group, a furanyl group, an indolyl group, a benzoindolyl group, a naphtho indolyl group, an isoindolyl group, a benzoisoindolyl group, a naphthoisoindolyl group, a benzosilolyl group, a benzothiophenyl group, a benzofuranyl group, a carbazolyl group, a dibenzosilolyl group, a dibenzothiophenyl group, a dibenzofuranyl group, an azacarbazolyl group, an azafluorenyl group, an azadibenzosilolyl group, an azadibenzothiophenyl group, an azadibenzofuranyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, a tetrazolyl group, an oxazolyl group, an isoxazolyl group, a thiazolyl group, an isothiazolyl group, an oxadiazolyl group, a thiadiazolyl group, a benzopyrazolyl group, a benzimidazolyl group, a benzoxazolyl group, a benzothiazolyl group, a benzoxadiazolyl group, a benzothiadiazolyl group, an imidazopyridinyl group, an imidazopyrimidinyl group, an imidazotriazinyl group, an imidazopyrazinyl group, an imidazopyridazinyl group, an indenocarbazolyl group, an indolocarbazolyl group, a benzofurocarbazolyl group, a benzothienocarbazolyl group, a benzosilolocarbazolyl group, a benzoindolocarbazolyl group, a benzocarbazolyl group, a benzonaphthofuranyl group, a benzonaphthothiophenyl group, a benzonaphthosilolyl group, a benzofurodibenzofuranyl group, a benzofurodibenzothiophenyl group, and a benzothienodibenzothiophenyl group. The term “divalent non-aromatic condensed heteropolycyclic group,” as used herein, refers to a divalent group having substantially the same structure as a monovalent non-aromatic condensed heteropolycyclic group. 
     The term “C 6 -C 60  aryloxy group,” as used herein, indicates —OA 102  (wherein A 102  is the C 6 -C 60  aryl group), and the term “C 6 -C 60  arylthio group,” as used herein, indicates —SA 103  (wherein A 103  is the C 6 -C 60  aryl group). 
     R 10a  may be: 
     deuterium (-D), —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, or a nitro group; 
     a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, or a C 1 -C 60  alkoxy group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, —Si(Q 11 )(Q 12 )(Q 13 ), —N(Q 11 )(Q 12 ), —B(Q 11 )(Q 12 ), —C(═O)(Q 11 ), —S(═O) 2 (Q 11 ), —P(═O)(Q 11 )(Q 12 ), or any combination thereof; 
     a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, or a C 6 -C 60  arylthio group, each unsubstituted or substituted with deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyano group, a nitro group, a C 1 -C 60  alkyl group, a C 2 -C 60  alkenyl group, a C 2 -C 60  alkynyl group, a C 1 -C 60  alkoxy group, a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 6 -C 60  aryloxy group, a C 6 -C 60  arylthio group, —Si(Q 21 )(Q 22 )(Q 23 ), —N(Q 21 )(Q 22 ), —B(Q 21 )(Q 22 ), —C(═O)(Q 21 ), —S(═O) 2  (Q 21 ), —P(═O)(Q 21 )(Q 22 ), or any combination thereof; or 
     —Si(Q 31 )(Q 32 )(Q 33 ), —N(Q 31 )(Q 32 ), —B(Q 31 )(Q 32 ), —C(═O)(Q 31 ), —S(═O) 2  (Q 31 ), or —P(═O)(Q 31 )(Q 32 ). 
     Q 1  to Q 3 , Q 11  to Q 13 , Q 21  to Q 23  and Q 31  to Q 33  used herein may each independently be: hydrogen; deuterium; —F; —C 1 ; —Br; —I; a hydroxyl group; a cyano group; a nitro group; or C 1 -C 60  alkyl group, C 2 -C 60  alkenyl group, C 2 -C 60  alkynyl group, C 1 -C 60  alkoxy group, or a C 3 -C 60  carbocyclic group, a C 1 -C 60  heterocyclic group, a C 7 -C 60  aryl alkyl group, or a C 2 -C 60  heteroaryl alkyl group, each unsubstituted or substituted with deuterium, —F, a cyano group, a C 1 -C 60  alkyl group, a C 1 -C 60  alkoxy group, a phenyl group, a biphenyl group, a C 1 -C 60  heterocyclic group, or any combination thereof. 
     The term “hetero atom,” as used herein, refers to any atom other than a carbon atom. Examples of the heteroatom include O, S, N, P, Si, B, Ge, Se, or any combination thereof. 
     The term “Ph,” as used herein, refers to a phenyl group, the term “Me,” as used herein, refers to a methyl group, the term “Et,” as used herein, refers to an ethyl group, the term “ter-Bu” or “Bu t ,” as used herein, refers to a tert-butyl group, and the term “OMe,” as used herein, refers to a methoxy group. 
     The term “biphenyl group,” as used herein, refers to “a phenyl group substituted with a phenyl group.” In other words, the “biphenyl group” is a substituted phenyl group having a C 6 -C 60  aryl group as a substituent. 
     The term “terphenyl group,” as used herein, refers to “a phenyl group substituted with a biphenyl group”. The “terphenyl group” is a substituted phenyl group having, as a substituent, a C 6 -C 60  aryl group substituted with a C 6 -C 60  aryl group. 
     * and *, as used herein, unless defined otherwise, each refer to a binding site to a neighboring atom in a corresponding formula. 
     Hereinafter, compounds according to embodiments and light-emitting devices according to embodiments will be described in more detail with reference to the following synthesis examples and examples. The wording “B was used instead of A” used in describing Synthesis Examples means that an identical molar equivalent of B was used in place of A. 
     Evaluation Example 1 
     With respect to each of Compounds 1-1, 2-2, 2-19, 3-1, and 3-9 below, a HOMO energy level, a LUMO energy level, a triplet energy level, a hole mobility, and an electron mobility were measured by the following methods and are shown in Table 1. 
     HOMO Energy Level and LUMO Energy Level 
     With respect to each of Compounds 1-1, 2-2, 2-19, 3-1, and 3-9 below, a HOMO energy level and a LUMO energy level were measured via a differential pulse voltammetry under a DMF solvent. 
     Triplet Energy Level 
     Each of Compounds 1-1, 2-2, 2-19, 3-1, and 3-9 below was diluted to a concentration of 5M in a toluene solvent, photoluminescence (PL) was measured at −78° C., and then a triplet energy level was measured from a Max PL value. 
     Hole Mobility 
     Devices having a structure of ITO (120 Å)/host (300 Å)/HATCN (50 Å)/Ag (50 Å)/AgMg (100 Å), which respectively include Compounds 1-1, 2-2, 2-19, 3-1, and 3-9 below as a host, were manufactured, and then hole mobility of an space-charge limited current (SCLC) regime of each of Compounds 1-1, 2-2, 2-19, 3-1, and 3-9 below was measured via a JV curve. 
     Electron Mobility 
     Devices having a structure of AgMg (100 Å)/Yb (10 Å)/host (300 Å)/Yb (10 Å)/AgMg (100 Å), which respectively include Compounds 1-1, 2-2, 2-19, 3-1, and 3-9 below as a host, were manufactured, and then electron mobility of an SCLC regime of each of Compounds 1-1, 2-2, 2-19, 3-1, and 3-9 below was measured via a JV curve. 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 HOMO 
                 LUMO 
                 Triplet 
                 Hole 
                 Electron 
               
               
                   
                 energy 
                 energy 
                 energy 
                 mobility 
                 mobility 
               
               
                 Name of 
                 level 
                 level 
                 level 
                 μH 
                 μE 
               
               
                 material 
                 (eV) 
                 (eV) 
                 (eV) 
                 (cm 2 /s · V) 
                 (cm 2 /s · V) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1-1 
                 −5.35 
                 −2.3 
                 2.0 
                 8.0E−06 
                 5.5E−06 
               
               
                 2-1 
                 −5.55 
                 −2.55 
                 1.6 
                 8.0E−07 
                 1.5E−05 
               
               
                 2-19 
                 −5.54 
                 −2.54 
                 1.6 
                 6.5E−07 
                 3.0E−05 
               
               
                 3-1 
                 −5.56 
                 −2.5 
                 1.6 
                 1.7E−07 
                 1.2E−05 
               
               
                 3-9 
                 −5.56 
                 −2.56 
                 1.6 
                 5.0E−07 
                 4.6E−05 
               
               
                   
               
            
           
         
       
     
     Manufacture of Light-Emitting Device 
     Example 1 
     ITO 300 Å/Ag 50 Å/ITO 300 Å (anode) was cut to a size of 50 mm×50 mm×0.7 mm, sonicated with isopropyl alcohol and pure water each for 15 minutes, and then cleaned by irradiation of ultraviolet rays and exposure of ozone thereto for 30 minutes, and the glass substrate was loaded into a vacuum deposition apparatus. 
     HATCN was vacuum-deposited on the substrate to form a hole injection layer having a thickness of 50 Å. Next, NPB as a hole transport compound was vacuum-deposited thereon to form a hole transport layer having a thickness of 1,200 Å. Next, Compound TCTA was vacuum-deposited on the hole transport layer to form an electron blocking layer having a thickness of 50 Å. 
     Compound 1-1 as a first host and a blue dopant as a dopant were co-deposited on the electron blocking layer at a weight ratio of 99:1 to form a first emission layer having a thickness of 100 Å, and a host including Compound 2-2 as a second host and Compound 3-1 as a third host at a weight ratio of 30:70 and a blue dopant as a dopant were co-deposited on the first emission layer at a weight ratio of 99:1 to form a second emission layer having a thickness of 100 Å. 
     Next, T2T was deposited thereon to form a hole blocking layer having a thickness of 50 Å, and then TPM-TAZ and Liq were deposited thereon at a weight ratio of 5:5 to form an electron transport layer having a thickness of 300 Å. 
     Yb was vacuum-deposited on the electron transport layer to a thickness of 10 Å, and consecutively, Al was vacuum-deposited thereon to a thickness of 800 Å, thereby forming a cathode, and CPL was deposited thereon to form a capping layer having a thickness of 600 Å, thereby completing the manufacture of a light-emitting device. 
     
       
         
         
             
             
         
       
     
     Examples 2 to 6 and Comparative Examples 1 to 10 
     Light-emitting devices were manufactured in substantially the same manner as in Example 1, except that, for use as the first host, the second host, the third host, and the dopant, corresponding compounds shown in Table 2 were used. 
     However, in the case of Comparative Examples 1 to 5, an emission layer having a single layer structure rather than a multilayer structure was formed. 
     Evaluation Example 2 
     Driving voltage at 1,000 cd/m2, luminescence efficiency (Cd/A), lifespan (hr, T97@1,000 nit), charge balance (max), and a TTF ratio (%) of the light-emitting devices manufactured in Examples 1 to 6 and Comparative Examples 1 to 10 were measured using a Keithley MU 236, a luminance meter PR650, and a transient EL, and results thereof are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
                 Driving 
                   
                   
                 Charge 
                 TTF 
               
               
                   
                 Emission layer 
                 voltage 
                 Efficiency 
                 Lifespan (hr, 
                 Balance 
                 ratio 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Classification 
                 First emission layer 
                 Second emission layer 
                 (V) 
                 (Cd/A) 
                 T97@1,000 nit) 
                 (max)(%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 Compound 1-1 + 
                 Compound 2-2 (30%) + Compound 3-1 (70%) + 
                 4.0 
                 9.0 
                 155 
                 92 
                 37 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Example 2 
                 Compound 1-1 + 
                 Compound 2-2(50%) + Compound 3-1 (50%) + 
                 3.9 
                 8.9 
                 150 
                 93 
                 36 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Example 3 
                 Compound 1-1 + 
                 Compound 2-2 (70%) + Compound 3-1 (30%) + 
                 3.8 
                 8.8 
                 148 
                 94 
                 35 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Example 4 
                 Compound 1-1 + 
                 Compound 2-19 (30%) + Compound 3-9 (70%) + 
                 4.1 
                 8.9 
                 153 
                 91 
                 36 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Example 5 
                 Compound 1-1 + 
                 Compound 2-19 (50%) + Compound 3-9 (50%) + 
                 3.9 
                 8.8 
                 150 
                 92 
                 35 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Example 6 
                 Compound 1-1 + 
                 Compound 2-19 (70%) + Compound 3-9 (30%) + 
                 3.7 
                 8.6 
                 145 
                 93 
                 34 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 1-1 + 
                 — 
                 4.8 
                 6.5 
                 25 
                 72 
                 5 
               
               
                 Example 1 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 2-2 + 
                 — 
                 3.8 
                 7.4 
                 100 
                 85 
                 26 
               
               
                 Example 2 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 3-1 + 
                 — 
                 4.4 
                 8.2 
                 105 
                 78 
                 33 
               
               
                 Example 3 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 2-19 + 
                 — 
                 3.8 
                 7.6 
                 110 
                 87 
                 26 
               
               
                 Example 4 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 3-9 + 
                 — 
                 4.3 
                 8.1 
                 115 
                 72 
                 34 
               
               
                 Example 5 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 1-1 + 
                 Compound 2-2 + Blue Dopant 
                 3.8 
                 7.8 
                 115 
                 88 
                 28 
               
               
                 Example 6 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 1-1 + 
                 Compound 3-1 + Blue Dopant 
                 4.2 
                 8.6 
                 138 
                 82 
                 35 
               
               
                 Example 7 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 1-1 + 
                 Compound 2-19 + Blue Dopant 
                 3.8 
                 8.0 
                 120 
                 89 
                 29 
               
               
                 Example 8 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 1-1 + 
                 Compound 3-9 + Blue Dopant 
                 4.2 
                 8.4 
                 140 
                 89 
                 36 
               
               
                 Example 9 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 2-2 + 
                 Compound 3-1 + Blue Dopant 
                 4.2 
                 7.8 
                 115 
                 82 
                 31 
               
               
                 Example 10 
                 Blue Dopant 
               
               
                   
               
            
           
         
       
     
     Manufacture of Tandem Light-Emitting Device 
     Example 7 
     A 15 Ω/cm 2  (800 Å) ITO/Ag/ITO glass substrate (a product of Corning Inc.) was cut to a size of 50 mm×50 mm×0.7 mm, sonicated with isopropyl alcohol and pure water each for 5 minutes, cleaned by irradiation of ultraviolet rays and exposure of ozone thereto for 15 minutes, and then loaded onto a vacuum deposition apparatus. 
     HAT-CN was deposited on the ITO/Ag/ITO anode of the glass substrate to form a hole injection layer having a thickness of 50 Å, NPB was deposited on the hole injection layer to form a hole transport layer having a thickness of 250 Å, and Compound TCTA was vacuum-deposited on the hole transport layer to form an electron blocking layer having a thickness of 50 Å. 
     Compound 1-1 as a first host and a blue dopant as a dopant were co-deposited on the electron blocking layer at a weight ratio of 99:1 to form a first emission layer having a thickness of 100 Å, and a host including Compound 2-2 as a second host and Compound 3-1 as a third host at a weight ratio of 30:70 and a blue dopant as a dopant were co-deposited on the first emission layer at a weight ratio of 99:1 to form a second emission layer having a thickness of 100 Å. 
     Next, T2T was deposited on the second emission layer to form a hole blocking layer having a thickness of 50 Å, and then TPM-TAZ and Liq were deposited thereon at a weight ratio of 5:5 to form an electron transport layer having a thickness of 250 Å. 
     Subsequently, BPhen and Li were co-deposited thereon at a weight ratio of 99:1 to form an n-type charge generation layer having a thickness of 50 Å, and HAT-CN was deposited on the n-type charge generation layer to form a p-type charge generation layer having a thickness of 50 Å. 
     NPB was deposited on the p-type charge generation layer to form a hole transport layer having a thickness of 500 Å, and Compound TCTA was vacuum-deposited on the hole transport layer to form an electron blocking layer having a thickness of 50 Å. 
     Compound 1-1 as a first host and a blue dopant as a dopant were co-deposited on the electron blocking layer at a weight ratio of 99:1 to form a first emission layer having a thickness of 100 Å, and a host including Compound 2-2 as a second host and Compound 3-1 as a third host at a weight ratio of 30:70 and a blue dopant as a dopant were co-deposited on the first emission layer at a weight ratio of 99:1 to form a second emission layer having a thickness of 100 Å. 
     Next, TPM-TAZ and LiQ were co-deposited on the second emission layer at a weight ratio of 1:1 to form an electron transport layer having a thickness of 350 Å. 
     Subsequently, Yb was deposited to a thickness of 10 Å, and Ag and Mg were co-deposited thereon to a thickness of 100 Å at a weight ratio of 9:1, thereby forming a cathode, and CPL was deposited thereon to form a capping layer having a thickness of 600 Å, thereby completing the manufacture of a tandem light-emitting device. 
     
       
         
         
             
             
         
       
     
     Examples 8 and 9 and Comparative Examples 11 to 15 
     Light-emitting devices were manufactured in substantially the same manner as in Example 1, except that, for use as the first host, the second host, the third host, and the dopant, corresponding compounds shown in Table 3 were used. However, in the case of Comparative Examples 11 to 13, an emission layer having a single layer structure rather than a multilayer structure was formed. 
     Evaluation Example 3 
     Driving voltage at 1,000 cd/m2, luminescence efficiency (Cd/A), lifespan (hr, T97@1,000 nit), and TTF ratio (%) of the light-emitting devices manufactured in Examples 7 to 9 and Comparative Examples 11 to 15 were measured using a Keithley MU 236, a luminance meter PR650, and a transient EL, and the results thereof are shown in Table 3. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                   
                 Driving 
                   
                   
                 TTF 
               
               
                   
                 Emission layer 
                 voltage 
                 Efficiency 
                 Lifespan (hr, 
                 ratio 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Classification 
                 First emission layer 
                 Second emission layer 
                 (V) 
                 (Cd/A) 
                 T97@1,000 nit) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 7 
                 Compound 1-1 + 
                 Compound 2-2 (30%) + Compound 3-1 (70%) + 
                 7.6 
                 17.5 
                 260 
                 37 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Example 8 
                 Compound 1-1 + 
                 Compound 2-2 (50%) + Compound 3-1 (50%) + 
                 7.5 
                 16.2 
                 230 
                 35 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Example 9 
                 Compound 1-1 + 
                 Compound 2-2 (70%) + Compound 3-1 (30%) + 
                 7.1 
                 16 
                 215 
                 36 
               
               
                   
                 Blue Dopant 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 1-1 + 
                 — 
                 8.8 
                 11.5 
                 30 
                 4 
               
               
                 Example 11 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 2-2 + 
                 — 
                 7.2 
                 13.5 
                 150 
                 25 
               
               
                 Example 12 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 3-1 + 
                 — 
                 9 
                 14.2 
                 180 
                 32 
               
               
                 Example 13 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 1-1 + 
                 Compound 2-2 + Blue Dopant 
                 7.2 
                 14 
                 165 
                 27 
               
               
                 Example 14 
                 Blue Dopant 
               
               
                 Comparative 
                 Compound 1-1 + 
                 Compound 3-1 + Blue Dopant 
                 7.9 
                 14.5 
                 178 
                 34 
               
               
                 Example 15 
                 Blue Dopant 
               
               
                   
               
            
           
         
       
     
     From Tables 2 and 3, it can be seen that all of the light-emitting devices of Examples 1 to 9 exhibited excellent results in terms of efficiency and lifespan, as compared to the light-emitting devices of Comparative Examples 1 to 15. 
     The light-emitting device includes the first emission layer and the second emission layer, the first emission layer includes the first host, the second emission layer includes the second host and the third host, and a hole mobility of the first host, a hole mobility of the second host, a hole mobility of the third host satisfy a specific relationship, and thus the light-emitting device may have excellent luminescence efficiency and excellent luminescence lifespan, and a high-quality electronic apparatus may be manufactured using the light-emitting device. 
     It should be understood that embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. While one or more embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims, and equivalents thereof.