Patent Publication Number: US-2013230665-A1

Title: Method of preparing organic light-emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2012-0022518, filed on Mar. 5, 2012 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Aspects of embodiments of the present invention relate to a method of preparing an organic light-emitting device. 
     2. Description of the Related Art 
     Organic light-emitting devices, which are self-emitting devises, have advantages such as a wide viewing angle, excellent contrast, quick response, high brightness, excellent driving voltage characteristics, and can provide multicolored images. 
     The organic light-emitting device includes an organic emission unit including a first electrode, an organic layer, and a second electrode. Since the organic emission unit is vulnerable to an external environment, such as oxygen and moisture, a sealing structure that seals the organic emission unit from the external environment is required. 
     Meanwhile, there is a need to develop a thin organic light-emitting device and/or a flexible organic light-emitting device. 
     SUMMARY 
     According to an aspect of embodiments of the present invention, a method of preparing an organic light-emitting device is provided in which the organic light-emitting device includes a thin film encapsulation layer having excellent sealing characteristics against an external environment and flexibility. 
     According to an embodiment of the present invention, a method of preparing an organic light-emitting device includes: forming an organic emission unit on a substrate; and forming a thin film encapsulation layer including at least one inorganic layer including a low temperature viscosity transition (LVT) inorganic material, the forming the thin film encapsulation layer including forming the at least one inorganic layer, and the forming the at least one inorganic layer includes: forming a pre-inorganic layer including the LVT inorganic material on the organic emission unit by heating a source including the LVT inorganic material using plasma in a vacuum and depositing the LVT inorganic material or ions of the LVT inorganic material separated from the source on the organic emission unit; and applying a healing process to the pre-inorganic layer at a temperature greater than a viscosity transition temperature of the LVT inorganic material. 
     The forming the pre-inorganic layer may include applying a negative potential to the substrate. 
     The viscosity transition temperature of the LVT inorganic material may be a minimum temperature capable of providing fluidity to the LVT inorganic material. 
     The viscosity transition temperature of the LVT inorganic material may be less than a lowest temperature of denaturation temperatures of materials contained in the organic emission unit. 
     The LVT inorganic material may include a tin oxide, 
     The LVT inorganic material may further include at least one selected from the group consisting of phosphorus oxide, boron phosphate, tin fluoride, niobium oxide, and tungsten oxide. 
     The LVT inorganic material may include SnO; SnO and P 2 O 5 ; SnO and BPO 4 ; SnO, SnF 2 , and P 2 O 5 ; SnO, SnF 2 , P 2 O 5 , and NbO; or SnO, SnF 2 , P 2 O 5 , and WO 3 . 
     The applying the healing process may be performed by heat-treating the pre-inorganic layer at a temperature in the range of the viscosity transition temperature of the LVT inorganic material to a lowest temperature of denaturation temperatures of materials contained in the organic emission unit. 
     The applying the healing process may be performed by heat-treating the pre-inorganic layer at a temperature of 80° C. to 132° C. for 1 to 3 hours. 
     The applying the healing process may be performed in a vacuum or in an inert gas atmosphere. 
     The applying the healing process may include scanning the pre-inorganic layer while irradiating a laser beam to the pre-inorganic layer. 
     The forming the at least one inorganic layer may further include applying another healing process by using chemical treatment, plasma treatment, hot chamber treatment including oxygen, or hot chamber treatment including oxygen and moisture. 
     The forming the thin film encapsulation layer may further include forming at least one organic layer of the thin film encapsulation layer, the at least one organic layer including a polymer. 
     The forming the at least one organic layer may include providing a curable precursor, and curing the curable precursor. The providing the curable precursor may be performed by using a flash evaporator. The curing the curable precursor may be performed by using at least one of UV rays, infrared rays, or laser beams. 
     The thin film encapsulation layer may include one organic layer and one inorganic layer, and the organic layer and the inorganic layer may be sequentially stacked on the organic emission unit. 
     The thin film encapsulation layer may include one organic layer and one inorganic layer, and the inorganic layer and the organic layer may be sequentially stacked on the organic emission unit. 
     The thin film encapsulation layer may include two organic layers including first and second organic layers, and one inorganic layer, and the first organic layer, the inorganic layer, and the second organic layer may be sequentially stacked on the organic emission unit. 
     The thin film encapsulation layer may include one organic layer, and two inorganic layers including first and second inorganic layers, and the first inorganic layer, the organic layer, and the second inorganic layer may be sequentially stacked on the organic emission unit. 
     An environmental element may be covered with the thin film encapsulation layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and aspects of the present invention will become more apparent by describing in further detail some exemplary embodiments thereof with reference to the attached drawings in which: 
         FIGS. 1A to 1D  are schematic diagrams illustrating a method of preparing an organic light-emitting device according to an embodiment of the present invention; 
         FIG. 2  is a schematic view of a device used to form a pre-inorganic layer of an organic light-emitting device according to an embodiment of the present invention; 
         FIGS. 3A to 3D  are schematic diagrams illustrating a method of preparing an organic light-emitting device according to another embodiment of the present invention; and 
         FIGS. 4 to 6  are schematic views of organic light-emitting devices prepared according to methods of preparing an organic light-emitting device according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which some exemplary embodiments of the invention are shown. However, embodiments of the present invention may be embodied in different forms and should not be construed as limited to the exemplary embodiments illustrated and set forth herein. Rather, these exemplary embodiments are provided by way of example for understanding of the invention and to convey the scope of the invention to those skilled in the art. As those skilled in the art would realize, the described embodiments may be modified in various ways, all without departing from the spirit or scope of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     As shown in  FIG. 1A , an organic emission unit  13  is formed on a substrate  11 . The organic emission unit  13  may have a structure including a first electrode  13   a , an organic layer  13   b , and a second electrode  13   c  which are sequentially stacked. 
     The substrate  11  may be any suitable substrate to be used in an organic light-emitting device, and, in one embodiment, may be a glass substrate or a transparent plastic substrate with excellent mechanical strength, thermal stability, transparency, surface smoothness, ease of handling, and water resistance. The substrate  11  may be a flexible substrate that may be bent. In one embodiment, a bending radius of the substrate  11  may be 10 cm or less. 
     The first electrode  13   a  may be formed by depositing or sputtering a material used to form the first electrode  13   a  on the substrate  11 . When the first electrode  13   a  constitutes an anode, the material used to form the first electrode  13   a  may be a high work function material so as to facilitate hole injection. The first electrode  13   a  may be a reflective, semi-transmissive, or transmissive electrode. Transparent and conductive materials, such as indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO 2 ), and zinc oxide (ZnO), may be used to form the first electrode  13   a.  The first electrode  13   a  may also be formed as a reflective electrode using magnesium (Mg), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), or the like. 
     The first electrode  13   a  may have a single-layered or a multi-layered structure. For example, the first electrode  13   a  may have a triple-layered structure of ITO/Ag/ITO, but is not limited thereto. 
     The organic layer  13   b  is formed on the first electrode  13   a.    
     The organic layer  13   b  may include at least one layer selected from the group consisting of a hole injection layer, a hole transport layer, a functional layer having both hole injecting and hole transporting capabilities, a buffer layer, an electron blocking layer, an emission layer, a hole blocking layer, an electron transport layer, and an electron injection layer. 
     In one embodiment, for example, the organic layer  13   b  may include at least one of compounds  301 ,  311 , and  321  below. 
     
       
         
         
             
             
         
       
     
     The second electrode  13   c  is disposed on the organic layer  13   b . The second electrode  13   c  may be a cathode, which is an electron injecting electrode. A metal used to form the second electrode  13   c  may be a metal, an alloy, or an electrically conductive compound, which has a low work function, or a mixture thereof. In one embodiment, for example, the second electrode  13   c  may be a reflective, semi-transmissive, or transmissive electrode by forming a thin film using lithium (Li), magnesium (Mg), aluminum (Al), aluminum-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), or the like. In one embodiment, in order to manufacture a top-emission type organic light-emitting device, a transmissive electrode formed of ITO or IZO may be used, and various modifications may be applied thereto. 
     Although not shown in  FIG. 1A , the organic emission unit  13  includes one pixel circuit per each pixel, and the pixel circuit may include at least one thin film transistor (TFT) (not shown) and a capacitor (not shown). The first electrode  13   a  may be electrically connected to the TFT. 
     In one embodiment, the first electrode  13   a  may be patterned in each pixel independent from one another, and the second electrode  13   c  may be a common electrode covering all of the pixels. 
     In a bottom-emission type organic light-emitting display device in which an image is formed toward the substrate  11 , emission efficiency toward the substrate  11  may be increased by forming the second electrode  13   c  relatively thick. 
     In a top-emission type organic light-emitting display device in which an image is formed toward the organic layer  13   b , the second electrode  13   c  may be a semi-reflective layer by forming the second electrode  13   c  relatively thin, or the second electrode  13   c  may be formed of a transparent conductive material, and the first electrode  13   a  may further include a reflective layer. 
     Although not shown in  FIG. 1A , a protective layer may be formed on the second electrode  13   c.  The protective layer may be formed of LiF, lithium quinolate, Alq3, or the like. 
     After the organic emission unit  13  is formed, an environmental element  15  is on the organic emission unit  13 . The environmental element  15  is an impurity particle that exists or is generated when the organic emission unit  13  is formed, and may be, for example, a microparticle from an external environment (e.g., dust existing in the external environment) or a microparticle of a material used to form the organic emission unit  13  and remaining on the organic emission unit  13  (e.g., a microparticle formed of a material used to form the second electrode  13   c  and remaining after the second electrode  13   c  is formed). The environmental element  15  may include various organic materials, inorganic materials, organic/inorganic complexes, and the like. The environmental element  15  cannot be removed by using a known method, such as a wet process (e.g., washing) after the organic emission unit  13  is formed. 
     For example, the environmental element  15  may be a particle having an average particle diameter of 5 μm or less, such as an average particle size of 1 μm to 5 μm, but is not limited thereto. 
     In  FIG. 1A , the environmental element  15  is depicted as one spherical particle. 
     A low temperature viscosity transition (LVT) inorganic material is provided onto the organic emission unit  13  on which the environmental element  15  is formed to form a pre-inorganic layer  23  including the LVT inorganic material, as shown in  FIG. 1B . 
     The LVT inorganic material is an inorganic material having a low viscosity transition temperature. 
     As used herein, the “viscosity transition temperature” is not a temperature where the phase of the LVT inorganic material is completely changed from solid to liquid, but is a minimum temperature where the LVT inorganic material has fluidity. 
     The viscosity transition temperature of the LVT inorganic material may be less than a denaturation temperature of a material contained in the organic emission unit  13 . 
     For example, the viscosity transition temperature of the LVT inorganic material may be less than a lowest temperature of the denaturation temperatures of the materials contained in the organic emission unit  13 . 
     The denaturation temperature of the material contained in the organic emission unit  13  refers to a temperature capable of causing chemical and/or physical denaturation in the material contained in the organic emission unit  13 , and the organic emission unit  13  may have a plurality of denaturation temperatures according to the type and number of materials contained therein. 
     The “viscosity transition temperature of the LVT inorganic material” and the denaturation temperature of the material contained in the organic emission unit  13  may indicate a glass transition temperature Tg of the LVT inorganic material and the organic material contained in the organic layer  13   b  of the organic emission unit  13 . The Tg may be measured by performing thermogravimetric analysis (TGA) on the LVT inorganic material, and on the organic material contained in the organic layer  13   b  of the organic emission unit  13 . 
     For example, the Tg may be obtained from thermal analysis of the material contained in the organic emission unit  13  by using TGA and differential scanning calorimetry (DSC) in an N 2  atmosphere at a temperature ranging from room temperature to 600° C. (10° C./min) for TGA, at a temperature ranging from room temperature to 400° C. for DSC (Pan Type: Pt Pan in disposable Al Pan (TGA), disposable Al pan (DSC)), the conditions for which would be understood by one of ordinary skill in the art. 
     The denaturation temperature of the material contained in the organic emission unit  13  may be, but is not limited to, higher than 130° C., and may be efficiently measured via TGA analysis of the material contained in the organic emission unit  13  as described above. 
     The lowest temperature of the denaturation temperatures of the materials contained in the organic emission unit  13  may be 130° C. to 140° C. For example, the lowest temperature of the denaturation temperatures of the materials contained in the organic emission unit  13  may be, but is not limited to, 132° C., and may be measured by measuring Tg of the material contained in the organic emission unit  13  via TGA analysis of the material as described above, and choosing the lowest Tg. 
     In one embodiment, the viscosity transition temperature of the LVT inorganic material may be 80° C. or greater, such as 80° C. to 132° C., but is not limited thereto. For example, the viscosity transition temperature of the LVT inorganic material may be 80° C. to 120° C., or 100° C. to 120° C., but is not limited thereto. For example, the viscosity transition temperature of the LVT inorganic material may be 110° C. 
     The LVT inorganic material may be a single compound or a mixture of at least two compounds. 
     The LVT inorganic material may include a tin oxide, such as SnO or SnO 2 . 
     In one embodiment, the LVT inorganic material includes SnO, and the content of SnO may be 20% by weight to 100% by weight. 
     In one embodiment, the LVT inorganic material may further include at least one selected from the group consisting of phosphorus oxide (e.g., P 2 O 5 ), boron phosphate (BPO 4 ), tin fluoride (e.g., SnF 2 ), niobium oxide (e.g., NbO), and tungsten oxide (e.g., WO 3 ), but is not limited thereto. 
     In one embodiment, the LVT inorganic material may include: SnO; SnO and P 2 O 5 ; SnO and BPO 4 ; SnO, SnF 2 , and P 2 O 5 ; SnO, SnF 2 , P 2 O 5 , and NbO; or SnO, SnF 2 , P 2 O 5 , and WO 3 , but is not limited thereto. 
     In one embodiment, the LVT inorganic material may include: SnO (100 wt %); SnO (80 wt %) and P 2 O 5  (20 wt %); SnO (90 wt %) and BPO 4  (10 wt %); SnO (20-50 wt %), SnF 2 (30-60 wt %), and P 2 O 5  (10-30 wt %), where the weight percent of the sum of SnO, SnF 2 , and P 2 O 5  is 100 wt %; SnO (20-50 wt %), SnF 2 (30-60 wt %), P 2 O 5  (10-30 wt %), and NbO (1-5 wt %), where the weight percent of the sum of SnO, SnF 2 , P 2 O 5 , and NbO is 100 wt %; or SnO (20-50 wt %), SnF 2  (30-60 wt %), P 2 O 5  (10-30 wt %), and WO 3  (1-5 wt %), where the weight percent of the sum of SnO, SnF 2 , P 2 O 5 , and WO 3  is 100 wt %, but is not limited thereto. 
     In one embodiment, the LVT inorganic material may include SnO (42.5 wt %), SnF 2  (40 wt %), P 2 O 5  (15 wt %), and WO 3  (2.5 wt %), but is not limited thereto. 
     The providing of the LVT inorganic material to the organic emission unit  13  to form the pre-inorganic layer  23  is described below in further detail with reference to  FIG. 2 . 
       FIG. 2  is a schematic view of a device used to form the pre-inorganic layer  23  by applying the LVT inorganic material to the organic emission unit  13 , according to an embodiment of the present invention. 
     The device includes a chamber  100  that is maintained in a vacuum and includes a first gate  101  and a second gate  102 . The substrate  11  may pass through the chamber  100  via the first gate  101  and the second gate  102 . 
     A plasma gun  110  is disposed at one side of a lower portion of the chamber  100 , and a hearth  120  is disposed at a lower portion of the chamber  100 . An exhaust pipe  140  and an exhaust pump  141  are disposed at the other side of the lower portion of the chamber  100 . A gas injector  130  is disposed to be adjacent to the plasma gun  110 . Oxygen and argon gases are injected into the chamber  100  by the gas injector  130  and generate plasma  112  with the plasma gun  110 . 
     In the chamber  100 , the plasma  112  is accelerated in a direction toward the hearth  120  by a negative voltage applied to a source  121 . A plasma controller  122  is disposed near the hearth  120  to control the migration of the plasma  112 . The hearth  120  includes the source  121 . Accordingly, the plasma  112  is adjusted to be curved by an angle of 45 to 135 degrees between a region in which the plasma gun  110  is located and a region in which the hearth  120  is located. The migration of the plasma  112  and adjustment of the regions are performed by electric field and magnetic field. 
     The source  121  contains the LVT inorganic material. 
     The plasma  112  heats the hearth  120 , and due to this heat, the LVT inorganic material, as the source  121  of the hearth  120 , is melted or sublimated to form an active region  150 , and accordingly, the LVT inorganic material forms a film on the organic emission unit  13 . 
     In this regard, when the hearth  120  is heated and electric charges are accumulated on the upper surface of a tablet, a tablet material is separated therefrom to form a layer on the substrate by a kind of sputtering effect. In general, the amount of electric charges is about several mA and an accelerating voltage is about several kV in sputtering. However, according to the method described above, while the accelerating voltage is about several V, the amount of electric charges is about several A, and thus the speed of a deposition material separated from the source is several hundreds to several thousands times greater than the sputtering. Accordingly, the film-forming speed is several tens to several hundreds times greater than that by general sputtering, and the sputtered particles are deposited on the substrate with an initial constant kinetic energy without any acceleration. Thus, adhesion properties of the pre-inorganic layer  23  to the substrate  11  and the organic emission unit  13  may be improved without damaging the organic emission unit  13 . 
     According to one embodiment, a LVT inorganic material including SnO—SnF 2 —P 2 O 5 —WO 3  may be provided to the organic emission unit  13  by using the method described above. According to the method, the pre-inorganic layer  23  may be quickly formed without damaging the organic emission unit  13 , thereby increasing productivity. 
     As shown in  FIG. 1B , the pre-inorganic layer  23  may include a defect such as a film-forming element  23   a , a pin hole  23   b , and a void  23   d  formed between the environmental element  15  and the organic emission unit  13 . 
     The film-forming element  23   a  is an agglomerated particle of the LVT inorganic material that does not contribute to the formation of the LVT inorganic material layer, and the pin hole  23   b  is a region where the organic emission unit  13  is exposed due to the LVT inorganic material not being applied thereto at the region of the pin hole  23   b.  The formation of the film-forming element  23   a  may contribute to the formation of the pin hole  23   b.  The void  23   d  formed between the environmental element  15  and the organic emission unit  13  is an empty space at which the LVT inorganic material is not applied. 
     As described above, any of the defects of the pre-inorganic layer  23  may be a pathway of external environmental materials, such as moisture and oxygen, while the organic light-emitting device is stored or operates to induce the formation of a progressive dark spot, such that a life span of the organic light-emitting device may be reduced. 
     After the pre-inorganic layer  23  is formed, a healing process that rectifies the defects of the pre-inorganic layer  23  is performed. The healing process, in one embodiment, includes first and second healing processes. 
     The first healing process is performed at a temperature greater than the viscosity transition temperature of the LVT inorganic material. In one embodiment, the first healing process may be performed by heat-treating the pre-inorganic layer  23  at a temperature in the range of the viscosity transition temperature of the LVT inorganic material to the denaturation temperature of the material contained in the organic emission unit  13 . In another embodiment, the first healing process may be performed by heat-treating the pre-inorganic layer  23  at a temperature in the range of the viscosity transition temperature of the LVT inorganic material to a lowest temperature of the denaturation temperatures of the material contained in the organic emission unit  13 . In another embodiment, the first healing process may be performed at the viscosity transition temperature of the LVT inorganic material. 
     The “viscosity transition temperature of the LVT inorganic material” may vary according to the composition of the LVT inorganic material, and the denaturation temperature of the material contained in the organic emission unit  13  and the lowest temperature of the denaturation temperatures of the material contained in the organic emission unit  13  may vary according to the material used in the organic emission unit  13 . However, these temperatures will be understood by one of ordinary skill in the art according to the composition of the LVT inorganic material and the material used in the organic emission unit  13 , such as by using a Tg evaluation obtained from TGA analysis results of the material contained in the organic emission unit  13 . 
     In one embodiment, the first healing process may be performed by heat-treating the pre-inorganic layer  23  at a temperature of 80° C. to 132° C., such as, in the range of 80° C. to 120° C., or 100° C. to 120° C., for 1 to 3 hours, such as at 110° C. for 2 hours, but is not limited thereto. If the temperature of the first healing process is within the range described above, the LVT inorganic material of the pre-inorganic layer  23  may be fluidized, and the denaturation of the organic emission unit  13  may be prevented or substantially prevented. 
     The first healing process may be performed in a vacuum or in an inert gas atmosphere, such as in an N 2  atmosphere or an Ar atmosphere, using an IR oven in order to prevent or substantially prevent the organic emission unit  13  from being exposed to the external environment. 
     The LVT inorganic material contained in the pre-inorganic layer  23  may be fluidized by the first healing process. The fluidized LVT inorganic material may have flowability. Thus, in the first healing process,
         i) the fluidized LVT inorganic material may flow into the void  23   d  formed between the environmental element  15  and the organic emission unit  13 ,   ii) the fluidized LVT inorganic material may flow into the pin hole  23   b  of the pre-inorganic layer  23 , and/or   iii) the film-forming element  23   a  may be fluidized and flow into the pin hole  23   b.          

     As a result, as shown in  FIG. 1C , a pre-inorganic layer  23 ′ rectified by the first healing process is formed, from which the defects of the pre-inorganic layer  23 , i.e. the film-forming element  23   a , the pin hole  23   b , and the void  23   d  formed between the environmental element  15  and the organic emission unit  13 , are removed. 
     The pre-inorganic layer  23 ′ rectified by the first healing process may include a region  23   e  having a weak binding force between the environmental element  15  and the LVT inorganic material or among the LVT inorganic materials. The region  23   e  having a weak binding force between the environmental element  15  and the LVT inorganic material or among the LVT inorganic materials may be used as a pathway of external environmental materials, such as moisture and oxygen, while the organic light-emitting device is stored or operates to induce the formation of a progressive dark spot, such that the life span of the organic light-emitting device may be reduced. 
     Thus, in one embodiment, a second healing process is performed to remove the region  23   e  having a weak binding force between the environmental element  15  and the LVT inorganic material or among the LVT inorganic materials by accelerating vigorous substitution reaction between the environmental element  15  and the LVT inorganic material and among the LVT inorganic materials and improving heat resistance and mechanical strength of the pre-inorganic layer  23 ′ rectified by the first healing process. 
     The second healing process may be performed by using chemical treatment, plasma treatment, hot chamber treatment including oxygen, or hot chamber treatment including oxygen and moisture. 
     In one embodiment, the second healing process may be performed by using a chemical treatment by which the pre-inorganic layer  23 ′ is treated with at least one of an acidic solution, an alkaline solution, and a neutral solution. The alkaline solution may be a nitrate solution (e.g., a potassium nitrate solution). 
     In one embodiment, the second healing process may be performed by using a plasma treatment by which the pre-inorganic layer  23 ° is treated using at least one of O 2  plasma, N 2  plasma, and Ar plasma in a vacuum. 
     In one embodiment, the second healing process may be performed by using a plasma treatment by which the pre-inorganic layer  23 ′ is treated using at least one of O 2  plasma, N 2  plasma, and Ar plasma under atmospheric pressure. 
     In one embodiment, the second healing process may be performed by exposing the pre-inorganic layer  23 ′ to a chamber having an oxygen partial pressure of 2% to 100%, such as an oxygen partial pressure in the atmospheric pressure, and a temperature of 25° C. to 150° C. 
     In one embodiment, the second healing process may be performed by exposing the pre-inorganic layer  23 ′ to a chamber having an oxygen partial pressure of 2% to 100%, such as an oxygen partial pressure in the atmospheric pressure, a relative humidity of 10% to 100%, and a temperature of 25° C. to 150° C. 
     The oxygen partial pressure is a value with respect to 100% of the pressure in the chamber. 
     As a result of the second healing process, an inorganic layer  23 ″ is formed, as shown in  FIG. 1D , from which the region  23   e  having a weak binding force between the environmental element  15  and the LVT inorganic material or among the LVT inorganic materials is removed. In addition, the binding force between the inorganic layer  23 ″ and the organic emission unit  13  may be improved by the second healing process, such that a high-quality organic light-emitting device is prepared. In one embodiment, the surface of the inorganic layer  23 ″ is flat. 
     In one embodiment, the healing process may not include both of the above-described first and second healing processes, and only the first healing process may be performed. 
     After increasing temperature as described above in the healing process, a cooling process may be performed. 
     In one embodiment, the healing process may be performed by irradiating laser beams to the pre-inorganic layer  23 , and then scanning the pre-inorganic layer  23 . That is, the inorganic layer  23 ″ in which various defects are rectified may be formed by increasing a temperature of the pre-inorganic layer  23  by irradiating laser beams to the pre-inorganic layer  23 , providing fluidity to the pre-inorganic layer  23 , and cooling the pre-inorganic layer  23 . 
     A thickness of the inorganic layer  23 ″ may be 1 μm to 30 μm, such as 1 μm to 5 μm. If the thickness of the inorganic layer  23 ″ is 1 μm to 5 μm, a flexible organic light-emitting device having a bending characteristic may be prepared. 
     The organic light-emitting device of  FIG. 1D  includes a thin film encapsulation layer  20  including the inorganic layer  23 ″. By performing the healing process, the inorganic layer  23 ″ is formed having a strong binding force between the environmental element  15  and the LVT inorganic material, among the LVT inorganic materials, and between the LVT inorganic material and the organic emission unit  13 , from which the voids formed between the environmental element  15  and the LVT inorganic material and voids formed between the environmental element  15  and the organic emission unit  13  are removed. Since the inorganic layer  23 ″ may be formed in a thin film as described above, a flexible organic light-emitting device having a bending characteristic may be prepared. Accordingly, the organic light-emitting device may have a long life span and excellent flexibility. 
       FIGS. 3A to 3D  are schematic diagrams illustrating a method of preparing an organic light-emitting device according to another embodiment of the present invention. 
     As shown in  FIG. 3A , the organic emission unit  13  is formed on the substrate  11 , wherein the organic emission unit  13  includes the environmental element  15  as described with reference to  FIG. 1A , and then an organic layer  21  including a polymer is formed to cover the organic emission unit  13 . 
     The formation of the organic layer  21  may include providing a curable precursor contained in the organic layer  21  and curing the curable precursor. 
     The precursor may be a thermosetting or photocurable precursor having a viscosity of 5 to 15 cp at room temperature and a boiling point of 300° C. to 500° C. In one embodiment, the precursor may be an acrylate precursor such as mono-acrylate, dimethacrylate, and triacrylate, but is not limited thereto. The curable precursor may be a single compound or a mixture of at least two different compounds. 
     The providing of the curable precursor onto the organic emission unit  13  may be performed by using a flash evaporating method, but is not limited thereto. 
     Then, the curable precursor provided onto the organic emission unit  13  is cured by using a known method. For example, the precursor may be cured by at least one of UV rays, infrared rays, or laser beams to form the organic layer  21 , but is not limited thereto, 
     The thickness of the organic layer  21 , in one embodiment, is 1 μm to 5 μm. If the thickness of the organic layer  21  is within the range described above, the thin film encapsulation layer  20 ′ (see  FIG. 3D ) may have a bending characteristic since at least a portion of the environmental element  15  formed on the organic emission unit  13  is covered with the organic layer  21 . 
     According to one embodiment, a curable precursor mixture including mono-acrylate, dimethacrylate, and triacryrate at a weight ratio of about 2:7:1, which has a viscosity of 5 to 15 cp at room temperature, and a boiling point of 300° C. to 500° C., may be formed on the organic emission unit  13 , as a curable precursor, by using a flash evaporating method (film-forming rate: about 200 Å/s and film-forming time: 3 to 4 min). In one embodiment, the curable precursor mixture is condensed to a liquid phase as soon as the curable precursor mixture is provided onto the organic emission unit  13 , and thus at least a portion of the surface of the environmental element  15  is surrounded by the curable precursor without empty space with the environmental element  15 . Then, the curable precursor mixture provided onto the organic emission unit  13  is cured to form the organic layer  21  by using a UV curing device (wavelength: 390 nm and light quantity: 500 mJ). 
     The LVT inorganic material is provided onto the organic layer  21  to form the pre-inorganic layer  23  including the LVT inorganic material as shown in  FIG. 3B . As shown in  FIG. 3B , the pre-inorganic layer  23  may include a defect such as a film-forming element  23   a , a pin hole  23   b , or a region  23   c  through which the environmental element  15  is exposed. As described above, the defect of the pre-inorganic layer  23  may be a pathway of external environmental materials, such as moisture and oxygen, while the organic light-emitting device is stored or operates to induce the formation of a progressive dark spot, such that the life span of the organic light-emitting device may be reduced. 
     Thus, in one embodiment, after the pre-inorganic layer  23  is formed, a first healing process that rectifies the defect of the pre-inorganic layer  23  is performed. 
     The first healing process according to one embodiment is as described above. 
     By the first healing process, as shown in  FIG. 3C , a pre-inorganic layer  23 ′ is formed from which the defects of the pre-inorganic layer  23 , i.e. the film-forming element  23   a , the pin hole  23   b , and the region  23   c  through which the environmental element  15  is exposed are removed. 
     In one embodiment, the second healing process is performed on the pre-inorganic layer  23 ′. The second healing process according to one embodiment is as described above. 
     As a result, as shown in  FIG. 3D , an inorganic layer  23 ″ is formed having excellent heat resistance and mechanical strength from which the region  23   e  having a weak binding force between the environmental element  15  and the LVT inorganic material or among the LVT inorganic materials is removed. In addition, the binding force between the inorganic layer  23 ″ and the organic emission unit  13  may be improved by the second healing process, such that a high-quality organic light-emitting device is prepared. In one embodiment, the surface of the inorganic layer  23 ″ is flat. The healing process, in one embodiment, may include only the first healing process as described above, or may be performed by laser scanning. 
     The thin film encapsulation layer  20 ′ of  FIG. 3D  includes one organic layer  21  and one inorganic layer  23 ″, and has a structure in which the organic layer  21  and the inorganic layer  23 ″ are sequentially stacked on the organic emission unit  13 . Since the thin film encapsulation layer  20  of  FIG. 3D  may considerably reduce a take time by forming the inorganic layer  23 ″ after forming the organic layer  21  having a thickness (e.g., a predetermined thickness), compared with the thin film encapsulation layer  20  including only the inorganic layer  23 ″ as shown in  FIG. 1  D, such that mass productivity may be improved. In addition, the thin film encapsulation layer  20 ′ of  FIG. 3D  may have an excellent bending characteristic since the organic layer  21  may have a better bending characteristics than the inorganic layer  23 ″. The thin film encapsulation layer  20 ′ of  FIG. 3D  has excellent sealing characteristics due to the inorganic layer  23 ″. 
     A thickness of the thin film encapsulation layer  20 ′ of  FIG. 3D , in one embodiment, is greater than an average particle diameter of the environmental element  15 . Thus, in one embodiment, the environmental element  15  is covered by the thin film encapsulation layer  20 ′, and thus a possibility of generation of a progressive dark spot caused by exposure of the environmental element  15  may be reduced or eliminated. 
     An organic light-emitting device shown in  FIG. 4  has a same structure as the organic light-emitting device of  FIG. 3D , except that the organic light-emitting device of  FIG. 4  includes a thin film encapsulation layer  20 ″ including one inorganic layer  23 ″ and one organic layer  21  which are sequentially stacked on the organic emission unit  13 . 
     The organic light-emitting device of  FIG. 4 , in one embodiment, is prepared in the same manner as in preparation of the organic light-emitting device of  FIG. 3D , except that the inorganic layer  23 ″ is formed on the organic emission unit  13 , and then the organic layer  21  is formed thereon. 
     In the thin film encapsulation layer  20 ″ of  FIG. 4 , the organic layer  21  formed on the external surface of the inorganic layer  23 ″ protects the inorganic layer  23 ″ from external impact and compensates for bending characteristics of the inorganic layer  23 ″, and the inorganic layer  23 ″ may provide excellent sealing characteristics for a long period of time as described above. Furthermore, by forming the organic layer  21 , the environmental element  15 , at least a portion of which is exposed outside of the inorganic layer  23 ″, may be completely covered by the thin film encapsulation layer  20 ″. 
     Thus, a possibility of generation of a progressive dark spot caused by exposure of the environmental element  15  may be reduced or eliminated. A thickness of the thin film encapsulation layer  20 ″ of  FIG. 4 , in one embodiment, is greater than an average particle diameter of the environmental element  15 . 
     The organic light-emitting device according to embodiments of the present invention may include a thin film encapsulation layer including a plurality of inorganic layers and/or a plurality of organic layers. If the thin film encapsulation layer includes a plurality of inorganic layers and/or a plurality of organic layers, the plurality of inorganic layer and/or the plurality of organic layers may be alternately stacked on each other. 
     In one embodiment, an organic light-emitting device shown in  FIG. 5  has a same structure as the organic light-emitting device shown in  FIG. 3D , except that the organic light-emitting device of  FIG. 5  includes a thin film encapsulation layer  200  that includes two organic layers, i.e. a first organic layer  21  and a second organic layer  25 , and one inorganic layer  23 ″, wherein the first organic layer  21 , the inorganic layer  23 ″, and the second organic layer  25  are sequentially stacked on the organic emission unit  13 . 
     In one embodiment, the first organic layer  21  and the second organic layer  25  and a method of preparing the same are as described above with reference to  FIG. 3A . The second organic layer  25  may be formed by using screen printing, inkjet printing, spin coating, or the like, and various modifications may be made. The organic materials used to form the first organic layer  21  and the second organic layer  25  may be the same or different. In one embodiment, the inorganic layer  23 ″ and a method of preparing the same are as described above with reference to  FIGS. 1B to 2 . 
     The first organic layer  21  and the second organic layer  25  of the thin film encapsulation layer  200  of  FIG. 5  compensate for a poor bending characteristic of the inorganic layer  23 ″, and the second organic layer  25  formed on an external surface of the inorganic layer  23 ″ may protect the inorganic layer  23 ″ from external impact. 
     A thickness of the thin film encapsulation layer  200  of  FIG. 5 , in one embodiment, is greater than an average particle diameter of the environmental element  15 . 
     In one embodiment, an organic light-emitting device shown in  FIG. 6  has a same structure as the organic light-emitting device shown in  FIG. 3D , except that the organic light-emitting device of  FIG. 6  includes a thin film encapsulation layer  200 ′ that includes one organic layer  21  and two inorganic layers, i.e. a first inorganic layer  23 ″ and a second inorganic layer  27 ″, wherein the first inorganic layer  23 ″, the organic layer  21 , and the second inorganic layer  27 ″ are sequentially stacked on the organic emission unit  13 . In one embodiment, the first inorganic layer  23 ″ and the second inorganic layer  27 ″ of the organic light-emitting device of  FIG. 6  and a method of preparing the same are as described above with reference to  FIGS. 1B to 2 . The second inorganic layer  27 ″ may also be formed of a different inorganic material from the first inorganic layer  23 ″ such as silicon-based oxide, silicon-based nitride, aluminum-based oxide, and/or aluminum-based nitride, and various modifications may be made. In one embodiment, the organic layer  21  and a method of preparing the same are as described above with reference to  FIG. 3A . 
     The organic light-emitting devices including a thin film encapsulation layer including a plurality of inorganic layers and/or a plurality of organic layers, according to embodiments of the present invention, are described above with reference to  FIGS. 5 and 6 . However, various modifications can be made referring to  FIGS. 5 and 6 . For example, the thin film encapsulation layer may include a first inorganic layer, a first organic layer, a second inorganic layer, and a second organic layer which are sequentially stacked. 
     In the organic light-emitting devices shown in  FIGS. 1A to 6 , the thin film encapsulation layer is formed to cover the organic emission unit  13 , but the location of the thin film encapsulation layer is not limited thereto. 
     In one embodiment, the thin film encapsulation layer shown in  FIGS. 1D ,  3 D,  4 ,  5 , and  6  may be formed under the substrate  11 . If the substrate  11  is a TFT substrate, the TFT is sealed from the external environment by the thin film encapsulation layer, such that the organic light-emitting device may have a long life span and flexibility. 
     In one embodiment, the thin film encapsulation layer shown in  FIGS. 1D ,  3 D,  4 ,  5 , and  6  may be formed between the organic emission unit  13  and the substrate  11 . Accordingly, a pixel circuit including a TFT and/or a capacitor may be formed on the thin film encapsulation layer. 
     While the present invention has been particularly shown and described with reference to some exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.