CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for ORGANIC LIGHT EMITTING DEVICE earlier filed in the Korean Intellectual Property Office on 20 Jan. 2009 and there duly assigned Serial No. 10-2009-0004722. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an organic light-emitting device, and more particularly, to an organic light-emitting device with low driving voltage and excellent light-emitting efficiency. 
     2. Description of the Related Art 
     Conventional display devices are being replaced with portable thin flat-panel display devices. Electroluminescent light-emitting display devices are self-luminous flat-panel display devices, and have advantages such as wide viewing angles, excellent contrast, and fast response speed. Thus, electroluminescent light-emitting display devices are being highlighted as next-generation display devices. Furthermore, organic light-emitting display devices, which include light-emitting layers made out of organic materials, have higher brightness, lower driving voltage, and quicker response speed as compared to inorganic light-emitting display devices. Also, organic light-emitting display devices can be polychromic. 
     An organic light-emitting display device includes an organic light-emitting device. An organic light-emitting device includes an anode, a cathode, and an intermediate layer arranged between the anode and the cathode. The intermediate layer includes an organic light-emitting layer and another organic material layer. A voltage difference is applied across the organic light-emitting layer via the anode or cathode causing the organic light-emitting layer to emit visible light. 
     Here, the organic material layer in the intermediate layer generally includes a plurality of layers. The plurality of organic material layers provide for easy transportation and implantation of electric charges into the organic light-emitting layer. 
     However, it is not easy to combine a plurality of organic material layers for high light-emitting efficiency, optimized charge balance, and long lifetime. In other words, there are limits to the extent to which the properties of an organic light-emitting device can be optimized by mutually combining a plurality of organic material layers due to the types and characteristics of the organic material layers. The electron transport layer can include only three layers. 
     SUMMARY OF THE INVENTION 
     The present invention provides an organic light-emitting device with reduced driving voltage and improved light-emitting efficiency. 
     According to an aspect of the present invention, there is provided an organic light-emitting device that includes a substrate, a first electrode arranged on the substrate, a second electrode arranged to face the first electrode, an organic light-emitting layer arranged between the first electrode and the second electrode, an electron transport layer arranged between the organic light-emitting layer and the second electrode, wherein the electron transport layer includes a multi-layer structure that includes at least one first layer and at least two second layers, wherein ones of said at least one first layer and ones of said at least two second layers are alternately stacked, wherein ones of the at least two second layers are arranged at both opposite ends of the electron transport layer, each of the at least two second layers having a lower electron mobility than that of each of the at least one first layer. 
     The organic light-emitting device can also include a hole implant layer arranged between the first electrode and the organic light-emitting layer. The organic light-emitting device can also include a hole transport layer arranged between the hole implant layer and the organic light-emitting layer. The organic light-emitting device can also include an electron implant layer arranged between the electron transport layer and the second electrode. The electron transport layer can include only three layers. The electron transport layer can include five layers. The electron transport layer can include thirteen layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicated the same or similar components, wherein: 
         FIG. 1  is a cross-sectional view of an organic light-emitting device according to a first embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the electron transport layer of the organic light-emitting device of  FIG. 1 ; and 
         FIG. 3  is a graph showing comparison of the lifetime of an organic light-emitting device according to the present invention and the lifetime of a conventional organic light-emitting device; 
         FIG. 4  is a graph showing current density according to driving voltage in an organic light-emitting device according to another embodiment of the present invention, and current density according to driving voltage of a conventional organic light-emitting device; 
         FIG. 5  is a cross-sectional view of an organic light-emitting device according to a second embodiment of the present invention; 
         FIG. 6  is a cross-sectional view of an electron transport layer of the organic light-emitting device of  FIG. 5 ; 
         FIG. 7  is a cross-sectional view of an organic light-emitting device according to a third embodiment of the present invention; 
         FIG. 8  is a cross-sectional view of an electron transport layer of the organic light-emitting device of  FIG. 7 ; and  FIG. 11  is another cross-sectional view of an electron transport layer of the organic light-emitting device of  FIG. 7 ; 
         FIG. 9  is a cross-sectional view of an organic light-emitting device according to a fourth embodiment of the present invention; and 
         FIG. 10  is a cross-sectional view of an electron transport layer of the organic light-emitting device of  FIG. 9 ; and 
         FIG. 11  is a cross sectional view of an electron transport layer of the organic light-emitting device of  FIG. 9  according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments can be modified in various different ways, all without departing from the spirit or scope of the principles for the present invention. 
     Recognizing that sizes and thicknesses of constituent members shown in the accompanying drawings are arbitrarily given for better understanding and ease of description, the present invention is not limited to the illustrated sizes and thicknesses. 
     In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements can also be present. Alternatively, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     In order to clarify the present invention, elements extrinsic to the description are omitted from the details of this description, and like reference numerals refer to like elements throughout the specification. 
     In several exemplary embodiments, constituent elements having the same configuration are representatively described in a first exemplary embodiment by using the same reference numeral and only constituent elements other than the constituent elements described in the first exemplary embodiment will be described in other embodiments. 
       FIG. 1  is a cross-sectional view of an organic light-emitting device  100  according to a first embodiment of the present invention. Referring to  FIG. 1 , the organic light-emitting device  100  according to the first embodiment of the present invention includes a substrate  101 , a first electrode  110 , an organic light-emitting layer  150 , an electron transport layer  170 , and a second electrode  190 . 
     The substrate  101  can be a transparent glass of which the main ingredient is SiO 2 , however the present invention is not limited thereto, and the substrate  101  can instead be made out of a transparent plastic. If substrate  101  is plastic, the plastic can be an insulating organic material, such as polyethersulphone (PES), polyacrylate (PAR), polyetherimide (PEI), polyethylene napthalate (PEN), polyethyeleneterepthalate (PET), polyphenylene sulfide (PPS), polyallylate, polyimide, polycarbonate (PC), cellulose tree acetate (TAC), or cellulose acetate propionate (CAP), however the present invention is not limited thereto, and the substrate  101  can also be made out of a metal and still be within the scope of the present invention. 
     If the organic light-emitting device  100  is a bottom emission type organic light-emitting device in which images are projected through the substrate  101 , it is necessary for the substrate  101  to be made out of a transparent material. However, if the organic light-emitting device  100  is a top emission type organic light-emitting device in which images are projected in a direction away from the substrate  101 , it is not necessary for the substrate  101  to be transparent. In the latter case, the substrate  101  can be made out of a metal. When made out of a metal, the substrate  101  can contain one or more of carbon, iron, chrome, manganese, nickel, titanium, molybdenum, stainless steel (SUS), Invar alloy, Inconel alloy, and Kovar alloy, however the present invention is not limited to the metals stated above. The substrate  101  can also be made out of a metal foil. 
     A buffer layer (not shown) can be formed on the top surface of the substrate  101  to prevent osmosis of impure elements into the substrate  101  and to provide for a flat surface over the substrate  101 . 
     The first electrode  110  can be formed on the substrate  101 . The first electrode  110  can have a predetermined pattern that is produced via a photolithography technique. The first electrode  110  can be either a transparent electrode or a reflective electrode. If the first electrode  110  is a transparent electrode, the first electrode  110  can be made out of ITO, IZO, ZnO, or In 2 O 3 . If the first electrode  110  is a reflective electrode, the first electrode  110  can include a reflective layer that includes one or more of Ag, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, or a compound thereof and transparent layer that includes one or more of ITO, IZO, ZnO, or In 2 O 3 . The surface of the first electrode  110  can be ultraviolet-ray/ozone treated to reduce contact resistance between the first electrode  110  and the organic light-emitting layer  150  formed thereon. 
     The organic light-emitting layer  150  is formed on the first electrode  110 . The organic light-emitting layer  150  contains either a small molecular organic material or a polymer organic material. For example, the organic light-emitting layer  150  can contain a blue organic material, such as oxadiazole dimer dyes (Bis-DAPDXP), Spiro compounds (Spiro-DPVBi, Spiro-6P), triarylamine compounds, bis(styryl)amine (DVPBi, DSA), 4,4′-bis(9-ethyl-3-carbazovinylene)-1-1′-bisphenyl (BCzVBi), perylene, 2,5,8,11-tetra-tert-butylphenylen (TPBe), 9H-carbazole,3,3′-(1,4-phenylenedi-2,1-ethenediyl)bis[9-ethyl-(9c) (BCzVB), 4,4′-Bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi), 4-(di-p-tolyamino)-4′-[(di-p-tolyamino)styril]stilbene (DPAVB), 4,4′-Bis(4-(diphenylamino)styryl)biphenyl (BDAVBi), Bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FlrPic), etc., a green organic material, such as 3-(2-Benzothiazolyl)-7-(diethyl-amino)coumarin (coumarin 6), 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl) quinolizino-[9,9a,1gh]coumarin (C545T), N,N′-dimethyl-quinacridone (DMQA), tris(2-phenylpyridine)iridium(III) (Ir(ppy) 3 ), or the like, and a red organic material, such as Tetraphenylnaphthacene (Rubrene), tris(1-phenylisoquinoline)iridum(III) (Ir(piq) 3 ), Bis(2-benzo[1,1]thiophen-2-yl-pyridine)(acetylacetonate)iridium(III) (Ir(btp) 2 (acac)), tris(dibenzoylmethan)(phenanthroline)europium(III) (Eu(dbm) 3 (phen)), tris[4,4′-di-tert-butyl-(2,2)-bipyridine]ruthenium(III) complex (Ru(dtb-bpy) 3 ,2(PF 6 )), DCM1, DCM2, Eu(thenoylfluoroacetone)3(Eu(TTA)3, butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), or the like. 
     Furthermore, the organic light-emitting layer  150  can contain an aromatic compound that contains a polymer and nitrogen, wherein examples of the polymer can include phenylene-based polymers, phenylene vinylene-based polymers, thiophene-based polymers, fluorine-based polymers, spiro-fluorene-based polymers, etc, but the present invention is in no way so limited. 
     If required, the organic light-emitting layer  150  can be produced by adding a dopant to a host. The host can be a light-emitting host or a phosphoric host. Examples of the light-emitting host can include tris(9-hydroxy-quinolinato)aluminum (Alq 3 ), 9,10-di(2-naphth-2-yl)anthracene (ADN), 3-tert-buthyl-9,10-di(2-naphth-2-yl)anthracene (TBADN), 4,4′-bis(2,2-diphenyl-ethene-1-yl)-4,4′-dimethylphenyl (DPVBi), (p-DMDPVBi), tert(9,9-diarylfluorene)s (TDAF), 2-(9,9′-spirobifluorene-2-yl)-9,9′spirobifluorene (BSDF), 2,7-bis(9,9′-spirobifluorene-2-yl)-9,9′-spirobifluorene (TSDF), bis(9,9-diarylfluorene)s (BDAF), 4,4′-bis(2,2-diphenyl-ethene-1-yl)-4,4′-di-(tert-buthyl)phenyl (p-TDPVBi), etc. Examples of the phosphoric host can include 1,3-bis(carbazole-9-yl)benzene (mCP), 1,3,5-tris(carbazole-9-yl)benzene (tCP), 4,4′,4″-tris(carbazole-9-yl)triphenylamine (TcTa), 4-4′-bis(carbazole-9-yl)biphenyl (CBP), 4,4′-bis[bis(9-carbazolyl)]-2,2′-dimethyl-biphenyl (CBDP), 4,4′-bis(carbazole-9-yl)-9,9′-dimethyl-fluorene (DMFL-CBP), 4,4′-bis(carbazole-9-yl)-9,9-bis[bis(9-phenyl-9H-carbazole)]fluorene (FL-4CBP), 4,4′-bis(carbazole-9-yl)-9,9-di-tolyl-fluorene (DPFL-CBP), 9,9-bis(9-phenyl-9H-carbazole)fluorene (FL-2CBP), etc. The content of the dopant can vary according to a material constituting the organic light-emitting layer  150 . 
     The electron transport layer  170  is formed on the organic light-emitting layer  150 .  FIG. 2  is a cross-sectional view of the electron transport layer  170  according to the first embodiment of the present invention. Referring to  FIG. 2 , the electron transport layer  170  according to the first embodiment includes a first layer  171  and two second layers  172 . The second layers  172  are formed to have lower electron mobility than that of the first layer  171 . Referring to  FIG. 2 , one of the second layers  172  is formed on the organic light-emitting layer  150 , the first layer  171  is formed on the second layer  172 , and the other of the second layer  172  is formed on the first layer  171  so that first layer  171  is arranged between the two second layers  172 . 
     The electron transport layer  170  according to the first embodiment of the present invention includes three layers, and thus it is easy to adjust the charge balance in the organic light-emitting layer  150 . In other words, it is easy to adjust the speed of electrons transmitted to the organic light-emitting layer  150  via the electron transport layer  170  by having many layers in the electron transport layer  170 . In particular, ones of the second layers  172  are respectively arranged between the first layer  171  and the organic light-emitting layer  150 , and between the first layer  171  and the second electrode  190 . The second layers  172  can be made out of a material having a lower electron mobility than the material constituting the first layer  171  to compensate for fast electron movement in the first layer  171 . 
     The first layer  171  and the second layers  172  can be made out of any material as along as the material constituting the second layers  172  have lower electron mobility than the material constituting the first layer  171 . Furthermore, each of the second layers  172  can differ from each other. For example, the second layer  172  interposed between the first layer  171  and the organic light-emitting layer  150  can be made out of a material different from a material constituting the second layer  172  interposed between the first layer  171  and the second electrode  190 . 
     Accordingly, electron mobility in the electron transport layer  170  can be freely adjusted, and thus the charge balance in the organic light-emitting layer  150  can be easily adjusted. As a result, the organic light-emitting device  100  according to the first embodiment of the present invention has excellent light-emitting efficiency and long lifetime. 
     The electron transport layer  170  can be produced via a deposition method. In a case where the electron transport layer  170  is produced in a single chamber, a source unit, which includes a material for producing the first layer  171  and a material for producing the second layer  172 , are arranged in the chamber. Then, a second layer  172 , the first layer  171 , and the other second layer  172  can be easily and sequentially deposited by moving the source unit back and forth in the chamber. Alternatively, with the source unit being fixed, the substrate  101  can be put on a carrier, and a second layer  172 , the first layer  171 , and the other second layer  172  can be sequentially deposited by moving the carrier back and forth. However, the present invention is not limited to these techniques. In other words, various other techniques and apparatuses for producing thin-films can be used to form the first layer  171  and the second layers  172  and still be within the scope of the present invention. 
     The second electrode  190  is formed on the electron transport layer  170 . The second electrode  190  can be a transparent electrode or a reflective electrode. If the second electrode  190  is a transparent electrode, the second electrode  190  can be produced by depositing a transparent conductive material, such as ITO, IZO, ZnO or In 2 O 3  on the electron transport layer  170  and forming either an assistant electrode or a bus electrode line using Li, Ca, LiF/Ca, LiF/Al, Al, Mg, or a compound thereof. If the second electrode  190  is a reflective electrode, the second electrode  190  can be produced by depositing Li, Ca, LiF/Ca, LiF/Al, Al, Mg, or a compound thereof on the electron transport layer  170 . 
     Although not shown, the organic light-emitting device  100  can also include a sealing member (not shown). The sealing member can be formed on the second electrode  190 . The sealing member is included to protect the organic light-emitting device  100  from external moisture, oxygen, etc. If the organic light-emitting device  100  is a top emission type organic light-emitting device, the sealing member is made out of a transparent material. In this regard, the sealing member can be made out of a glass substrate, a plastic substrate, or a stack structure of organic materials and inorganic materials. 
       FIG. 3  is a graph showing comparison of the lifetime of an organic light-emitting device according to the present invention and the lifetime of a conventional organic light-emitting device. In  FIG. 3 , curve (a) indicates the conventional organic light-emitting device including an electron transport layer having a single-layer structure, and curve (b) indicates an organic light-emitting device according to the present invention. 
     Referring to  FIG. 3 , curve (a) shows that the brightness of the conventional organic light-emitting device is decreased by 10% as compared to the initial brightness of the same in less than one hour. Meanwhile, curve (b) shows that the brightness of the organic light-emitting device according to the present invention is decreased by only 4.5% as compared to the initial brightness of the same after 16 hours. Thus, it is clear that the organic light-emitting device according to the present invention exhibits less brightness decrease as compared to a conventional organic light-emitting device, and thus the organic light-emitting device according to the present invention has a longer lifetime. 
     According to the first embodiment of the present invention, the electron transport layer  170  can be formed by repeatedly stacking the first layer  171  and the second layer  172 . In other words, although  FIG. 2  illustrates a structure in which the first layer  171 , the second layer  172 , and the first layer  171  are sequentially stacked in a triple-layer structure, however the present invention is not limited thereto. The first layer  171  and the second layer  172  can be further stacked in a multi-layer structure to include four or more layers and still be within the scope of the present invention. A result of an experiment related thereto is shown in  FIG. 4 . 
       FIG. 4  is a graph showing current density according to driving voltage in an organic light-emitting device according to another embodiment of the present invention and current density according to driving voltage of a conventional organic light-emitting device. In this regard, the organic light-emitting device according to the present invention includes an electron transport layer  170  having a nonuple-layer (or 9-layered) structure where the first layer  171  and the second layer  172  are stacked in a sequence of second layer  172 , first layer  171 , second layer  172 , first layer  171 , second layer  172 , first layer  171 , second layer  172 , first layer  171  and second layer  172 . The conventional organic light-emitting device used to produce curve (a) in  FIG. 4  includes an electron transport layer having a double-layer structure. In  FIG. 4 , curve (a) indicates the conventional organic light-emitting device electron transport, and curve (b) indicates the organic light-emitting device according to the present embodiment as described above. 
     Referring to  FIG. 4 , curve (b) indicates reduced driving voltage and increased current density as compared to curve (a). This is because the charge balance of the organic light-emitting device according to the present invention is effectively adjusted. Thus, overall light-emitting efficiency and lifetime of the organic light-emitting device according to the present invention are better than those of the conventional organic light-emitting device. 
     Turning now to  FIGS. 5 and 6 ,  FIG. 5  is a cross-sectional view of an organic light-emitting device  200  according to a second embodiment of the present invention, and  FIG. 6  is a cross-sectional view of an electron transport layer  270  of the organic light-emitting device  200  of  FIG. 5 . Referring to  FIG. 5 , the organic light-emitting device  200  according to the second embodiment includes a substrate  201 , a first electrode  210 , a hole implant layer  230 , a hole transport layer  240 , an organic light-emitting layer  250 , the electron transport layer  270 , an electron implant layer  280 , and a second electrode  290 . For convenience of explanation, descriptions below will focus on differences between the second embodiment and the first embodiment illustrated in  FIGS. 1 and 2 . 
     As compared to the organic light-emitting device  100  of  FIGS. 1 and 2 , the organic light-emitting device  200  according to the second embodiment further includes the hole implant layer  230 , the hole transport layer  240 , and the electron implant layer  280 . The hole implant layer  230 , the hole transport layer  240 , and the electron implant layer  280  can either be all included or be selectively included in the organic light-emitting device  200  according to the second embodiment. 
     The first electrode  210  is formed on the substrate  201 , and the hole implant layer  230  is formed on the first electrode  210 . The hole implant layer  230  can be made out of various organic materials, wherein examples of the organic materials can include phthalocyanine compounds such as copper phthalocyanine, or starburst amine derivatives such as TCTA, m-MTDATA, m-MTDAPB, etc. The hole implant layer  230  can be produced using a technique such as vacuum thermal deposition, spin coating, or the like. 
     The hole transport layer  240  is formed on the hole implant layer  230 . Although the hole transport layer  240  can be produced using various techniques such as vacuum deposition, spin coating, casting, Langmuir Blodgett (LB) deposition, etc., it is preferable to use vacuum deposition. By using vacuum deposition, it is easy to obtain a uniform film and pin holes are unlikely to be formed. If a hole transport layer is formed using vacuum deposition, deposition conditions vary according to compounds used, but are generally selected within a range of conditions used to form the hole implant layer  230 . The hole transport layer  240  can be made of various materials, such as N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4′-diamine (TPD), N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (a-NPD), etc. 
     The organic light-emitting layer  250  is formed on the hole transport layer  240 . Since the configuration of the organic light-emitting layer  250  according to the second embodiment is identical to that of the organic light-emitting layer  150  of  FIG. 1 , a detailed description thereof will not be repeated here. 
     The electron transport layer  270  is formed on the organic light-emitting layer  250 . As in the first embodiment, the electron transport layer  270  according to the second embodiment includes a first layer  271  and two second layers  272 . The second layers  272  are have lower electron mobility than that of the first layer  271 . Referring to  FIG. 6 , a second layer  272  is formed on the organic light-emitting layer  250 , the first layer  271  is formed on the second layer  272 , and another second layer  272  is formed on the first layer  271  so that the first layer  271  is sandwiched between the two second layers  272 . 
     As with the electron transport layer  170  illustrated in  FIG. 2 , the electron transport layer  270  includes a plurality of layers, so that it is easy to adjust the charge balance in the organic light-emitting layer  250 . The first layer  271  and the second layers  272  can be made out of any materials as along as the material constituting the second layers  272  have lower electron mobility than the material constituting the first layer  271 . Furthermore, each of the second layers  272  can differ from each other. For example, the second layer  272  interposed between the first layer  271  and the organic light-emitting layer  250  can be made out of a material different from the material constituting the second layer  272  interposed between the first layer  271  and the electron implant layer  280 . 
     The electron implant layer  280  is formed on the electron transport layer  270 . The electron implant layer  280  can be made out of various materials, such as LiF, NaCl, CsF, Li 2 O, BaO, etc. Conditions for forming the electron implant layer  280  vary according to compounds used, but are generally selected within the range of conditions used to form the hole implant layer  230 . The second electrode  290  is formed on the electron implant layer  280 . Although not shown, a sealing member can be arranged on the second electrode  290 . 
     Turning now to  FIGS. 7 and 8 ,  FIG. 7  is a cross-sectional view of an organic light-emitting device  300  according to a third embodiment of the present invention and  FIG. 8  is a cross-sectional view of an electron transport layer  370  of the organic light-emitting device  300 . Referring to  FIG. 7 , the organic light-emitting device  300  according to the third embodiment includes a substrate  301 , a first electrode  310 , an organic light-emitting layer  350 , an electron transport layer  370 , and a second electrode  390 . For convenience of explanation, descriptions below will focus on differences between the current third embodiment and the previous embodiments. As compared to the organic light-emitting  100  illustrated in  FIG. 1 , the electron transport layer  370  of the organic light-emitting device  300  according to the third embodiment has a different configuration. 
     The first electrode  310  is formed on the substrate  301 , and the organic light-emitting layer  350  is formed on the first electrode  310 . The configuration of the organic light-emitting layer  350  is same as in the previous embodiments, and thus a detailed description thereof will not be repeated here. 
     The electron transport layer  370  is formed on the organic light-emitting layer  350 . The electron transport layer  370  includes two first layers  371  and three second layers  372 , but the present invention is in no way so limited. The second layers  372  have lower electron mobility than that of the first layers  371 . The first layers  371  and the second layers  372  are formed on the organic light-emitting layer  350  in a sequence of the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , and the second layer  372 . 
     The electron transport layer  370  according to the third embodiment includes five layers, and thus it is easy to adjust the charge balance in the organic light-emitting layer  350 . In other words, it is easy to adjust the speed of electrons transmitted to the organic light-emitting layer  350  via the electron transport layer  370 . 
     Furthermore, as compared to the first embodiment illustrated in  FIG. 1 , the electron transport layer  370  according to the third embodiment includes two more layers, and thus more interfaces between ones of the first layers  371  and the ones of the second layers  372  are formed in the electron transport layer  370 . The speed of moving electrons changes at the interfaces between ones of the first layers  371  and corresponding ones of the second layers  372 , each of which has different electron mobility. The moving speed of electrons can be adjusted at the interfaces, and movement of electrons can be controlled more precisely as the number of interfaces increases. Thus, the charge balance can be easily adjusted. 
     The first layers  371  and the second layers  372  can be made out of any materials as long as the material constituting the second layers  372  have lower electron mobility than that of a material constituting the first layers  371 . Furthermore, each of the second layers  372  can differ from each other. In other words, each of the three second layers  372  can be made out of different organic materials. Similarly, each of the two first layers  371  can also be made out of different organic materials. Movement of electrons can be precisely controlled by using various materials as described above, and thus the charge balance in the organic light-emitting layer  350  can be easily adjusted. Furthermore, the organic light-emitting device  300  illustrated in  FIG. 7  has excellent light-emitting efficiency and long lifetime due to optimized charge balance in the organic light-emitting layer  350 . 
     After forming the electron transport layer  370 , the second electrode  390  is then formed on the electron transport layer  370 . Although not shown, a sealing member can be arranged on the second electrode  390  to complete the organic light-emitting device according to the third embodiment of the present invention. 
     Although  FIG. 8  illustrates the electron transport layer  370  as having a quintuple-layer structure, in which the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , and the second layer  372  are sequentially stacked, the present invention is in no way so limited. In other words, as described above with reference to  FIG. 4 , the electron transport layer  370  can instead include 9 layers, that is, the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , and the second layer  372 , or more e.g. 11 layers, 13 layers, or the like and still be within the scope of the present invention.  FIG. 11  illustrates the electron transport layer  370  as having a 13-layer structure, in which the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , the second layer  372 , the first layer  371 , and the second layer  372  are sequentially stacked. 
     Turning now to  FIGS. 9 and 10 ,  FIG. 9  is a cross-sectional view of an organic light-emitting device  400  according to a fourth embodiment of the present invention and  FIG. 10  is a cross-sectional view of an electron transport layer  470  of the organic light-emitting device  400 . The organic light-emitting device  400  according to the fourth embodiment includes a substrate  401 , a first electrode  410 , a hole implant layer  430 , a hole transport layer  440 , an organic light-emitting layer  450 , the electron transport layer  470 , an electron implant layer  480 , and a second electrode  490 . For convenience of explanation, descriptions below will focus on differences between the current fourth embodiment and the previous embodiments. Like the organic light-emitting device  200  of  FIG. 5 , the organic light-emitting device  400  according to the fourth embodiment includes the hole implant layer  430 , the hole transport layer  440 , and the electron implant layer  480 . 
     The first electrode  410  is formed on the substrate  401 , and the hole implant layer  430  is formed on the first electrode  410 . The hole implant layer  430  can be made out of various organic materials, wherein examples of the organic materials can include phthalocyanine compounds such as copper phthalocyanine, or starburst amine derivatives such as TCTA, m-MTDATA, m-MTDAPB, etc. The hole implant layer  430  can be produced via a technique such as vacuum thermal deposition, spin coating, or the like. 
     The hole transport layer  440  is formed on the hole implant layer  430 . Although the hole transport layer  440  can be produced using various techniques such as vacuum deposition, spin coating, casting, LB deposition, etc., it is preferable to use the vacuum deposition technique. By using vacuum deposition, it is easy to obtain a uniform film and pin holes are unlikely to form. If a hole transport layer is produced using vacuum deposition, deposition conditions can vary according to compounds used, but are generally selected within a range of conditions almost the same as those used to form the hole implant layer  430 . 
     The hole transport layer  440  can be made out of various materials, such as N,N-bis(3-methylphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4-diamine (TPD), N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (a-NPD), etc. 
     The organic light-emitting layer  450  is formed on the hole transport layer  440 . Since the configuration of the organic light-emitting layer  450  according to the fourth embodiment is identical to that of the organic light-emitting layers  150 ,  250 , and  350  according to the first through third embodiments, detailed descriptions thereof will not be repeated here. 
     The electron transport layer  470  is formed on the organic light-emitting layer  450 . Like the organic light emitting device  300  of  FIGS. 7 and 8 , the electron transport layer  470  according to the fourth embodiment of the present invention includes a plurality of first layers  471  and a plurality of second layers  472 . The second layers  472  are formed to have lower electron mobility than that of the first layers  471 . The first layers  471  and the second layers  472  are formed on the organic light-emitting layer  450  in a sequence of the second layer  472 , the first layer  471 , second layer  472 , the first layer  471 , and second layer  472 . 
     Although  FIG. 10  illustrates the electron transport layer  470  as having a quintuple-layer structure, in which the second layer  472 , the first layer  471 , the second layer  472 , the first layer  471 , and the second layer  472  are stacked, the present invention is not limited thereto. In other words, the electron transport layer  470  can include 9 layers, that is, the second layer  472 , the first layer  471 , the second layer  472 , the first layer  471 , the second layer  472 , the first layer  471 , the second layer  472 , the first layer  471 , and the second layer  472 , or more; e.g. 11 layers, 13 layers, or the like and still be within the scope of the present invention.  FIG. 11  shows the scenario where the electron transport layer  470  includes 13 layers. 
     After forming the electron transport layer  470 , the electron implant layer  480  is then formed on the electron transport layer  470 . The electron implant layer  480  can be made out of various materials, such as LiF, NaCl, CsF, Li 2 O, BaO, etc. Conditions for forming the electron implant layer  480  vary according to compounds used, but are generally selected within a range of conditions used to form the hole implant layer  430 . The second electrode  490  is then formed on the electron implant layer  480 . Although not shown, a sealing member can be arranged on the second electrode  490  to complete the formation of the organic light-emitting device  400 . 
     As with the third embodiment, the electron transport layer  470  includes five layers so that it is easy to adjust the charge balance in the organic light-emitting layer  450 . As a result, the organic light-emitting device  400  according to the fourth embodiment has excellent light-emitting efficiency and long lifetime. 
     The organic light-emitting devices according to the embodiments of the present invention are characterized by having low driving voltage and excellent light-emitting efficiency. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.
An organic light emitting device with improved light emitting efficiency, the organic light emitting device includes a substrate, a first electrode arranged on the substrate, a second electrode arranged to face the first electrode, an organic light-emitting layer arranged between the first electrode and the second electrode, an electron transport layer arranged between the organic light-emitting layer and the second electrode, wherein the electron transport layer includes a multi-layer structure that includes at least one first layer and at least two second layers, wherein ones of said at least one first layer and ones of said at least two second layers are alternately stacked, wherein ones of the at least two second layers are arranged at both opposite ends of the electron transport layer, each of the at least two second layers having a lower electron mobility than that of each of the at least one first layer.