Patent Publication Number: US-2005118362-A1

Title: Thermal transfer element with light-to-heat conversion layer having concentration gradient

Description:
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 THERMAL TRANSFER ELEMENT WITH LTHC HAVING GRADIENT CONCENTRATION earlier filed in the Korean Intellectual Property Office on Nov. 29, 2003 and there duly assigned Serial No. 2003-87791.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a thermal transfer element and, more particularly, to a laser thermal transfer element capable of preventing a characteristic deterioration of an organic thin-film layer.  
      2. Description of the Related Art  
      In general, an organic electroluminescent display device includes an anode electrode which is a lower electrode formed on an insulating substrate, an organic thin-film layer formed on the anode electrode, and a cathode electrode which is an upper electrode formed on the organic thin-film layer. The organic thin-film layer includes at least one of a hole injecting layer, a hole transporting layer, an emission layer, a hole blocking layer, an electron transporting, and an electron injecting layer.  
      A method of forming the organic thin-film layer includes a deposition method and a lithography method. The deposition method is one which forms an organic emission layer by vacuum-depositing an organic light-emitting material using a shadow mask. The deposition method has disadvantages in that it is difficult to form fine patterns of fine pitch due to a transformation of a mask and it is difficult to be applied to a large-size display device. The lithography method is one which forms the organic emission layer by depositing an organic light-emitting material layer and then patterning the deposited organic light-emitting material layer using a photoresist. The lithography method can form the fine patterns of fine pitch but has a disadvantage in that characteristics of the organic emission layer are degraded by a developing solution used to form a photoresist pattern or an etching solution of the organic emission layer.  
      In order to resolve the above problems, an ink jet method has been suggested that patterns directly the organic emission layer. The ink jet method is one which dissolves or disperses a light-emitting material in a solvent and discharges liquid droplets containing the light-emitting material from a head of an ink jet printer to form an organic emission layer. The ink jet method is simple in process but has disadvantages in that a manufacturing yield is low, a film thickness is not uniform and also is difficult to be applied to a large-size display device.  
      Meanwhile, a method of forming an organic emission layer using a thermal transfer method which is a dry-etching method has been suggested. The thermal transfer method is one which converts light from a light source to thermal energy, and transfers an image forming material onto an insulting substrate by using the converted thermal energy to form R, G, and B organic emission layers.  
     SUMMARY OF THE INVENTION  
      The present invention provides a thermal transfer element which has a light-to-heat conversion layer in which a concentration distribution is lower as it is closer to an organic emission layer and thus can prevent characteristic deterioration of an organic emission layer.  
      The present invention also provides a thermal transfer element suitable for an organic electroluminescent display device, which can prevent the characteristic deterioration of an emission layer.  
      In an exemplary embodiment of the present invention, a thermal transfer element includes: a base substrate which is a support substrate; a light-to-heat conversion layer formed on the base substrate, converting incident light to heat energy and containing a radiation absorber; and a transfer layer for image formation, wherein the radiation absorber of the light-to-heat conversion layer has a concentration distribution that a concentration is lower as it is closer to the transfer layer.  
      The radiation absorber of the light-to-heat conversion layer has a concentration distribution that the concentration is gradually lower as it is closer to the transfer layer.  
      The radiation absorber of the light-to-heat conversion layer has a concentration distribution that the concentration is stepwise lower as it is closer to the transfer layer.  
      The radiation absorber of the light-to-heat conversion layer has a concentration distribution that the concentration is stepwise lower as it is farther from the base substrate and as it is closer to the transfer layer.  
      The radiation absorber of the light-to-heat conversion layer has a concentration distribution that the concentration is gradually lower as it is farther from the base substrate and as it is closer to the transfer layer.  
      The radiation absorber of the light-to-heat conversion layer absorbs light generated from one of infrared laser, visible laser and ultraviolet laser.  
      The radiation absorber of the light-to-heat conversion layer contains at least one of carbon black, metal, infrared ray dye and pigment as a material which absorbs infrared rays to generate heat energy. The radiation absorber of the light-to-heat conversion layer contains an organic binder material which is hardened by ultraviolet rays or heat.  
      The thermal transfer element further includes an interlayer formed between the light-to-heat conversion layer and the transfer layer, which serves to protect the light-to-heat conversion layer. The transfer layer contains an image forming material to transfer an organic thin film including at least an emission layer.  
      In another exemplary embodiment of the present invention, a thermal transfer element includes: a base substrate which is a support substrate; a light-to-heat conversion layer formed on the base substrate, converting incident light to heat energy and containing a radiation absorber; and a transfer layer for image formation, wherein the radiation absorber of the light-to-heat conversion layer has a concentration distribution that a concentration is lower as it is closer to the transfer layer, and the transfer layer contains a patterned organic thin-film layer which includes at least an emission layer.  
      In yet another exemplary embodiment of the present invention, an organic EL (emission layer) display device includes: an organic emission layer including at least an emission layer, which is formed using the thermal transfer element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become 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 indicate the same or similar components, wherein:  
       FIG. 1  is a cross-sectional view of a thermal transfer element for forming an organic thin film layer;  
       FIG. 2  is a graph illustrating a concentration distribution of a radiation absorber contained in a light-to-heat conversion layer of the thermal transfer element;  
       FIG. 3  is a cross-sectional view of a thermal transfer element according to a first embodiment of the present invention;  
       FIG. 4  is a cross-sectional view of a thermal transfer element according to a second embodiment of the present invention;  
       FIGS. 5   a  to  5   c  are cross-sectional views illustrating a process of forming an organic thin-film layer of an organic emission layer (EL) display device by using the laser thermal transfer element according to the first embodiment of the present invention; and  
       FIGS. 6   a  and  6   b  are graphs illustrating a concentration distribution of a radiation absorber contained in a light-to-heat conversion layer of a laser thermal transfer element of the present invention. 
    
    
     DETAILED DESCRIPTION  
      The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout the specification.  
       FIG. 1  is a cross-sectional view of a laser thermal transfer element for forming an organic thin film layer.  
      Referring to  FIG. 1 , the laser thermal transfer element is composed of a base substrate  10 , a light-to-heat conversion layer  11 , an interlayer  12 , and a transfer layer  13 .  
      A method of forming an organic emission layer using the thermal transfer element is as follows. In the state that a substrate, on which an organic emission layer is to be formed, is closely contacted with the thermal transfer element, laser is irradiated, so that the light-to-heat conversion layer converts the laser light to heat and discharges the heat. As a result, the transfer layer is transferred to the substrate, thereby forming the organic emission layer.  
       FIG. 2  is a graph illustrating a concentration distribution of a radiation absorber contained in a light-to-heat conversion layer of the thermal transfer element.  
      Referring to  FIG. 2 , the light-to-heat conversion layer  11  has a concentration distribution  11   c  that a radiation absorber is uniformly distributed regardless of a depth “t” thereof. In other words, a concentration of the radiation absorber on a surface  11   a  contacting the base substrate  10  of the light-to-heat conversion layer  11  and a concentration of the radiation absorber on a surface  11   b  contacting the interlayer  12  are uniform. As a result, the light-to-heat conversion layer  11  has a temperature distribution, depending on the depth “t,” illustrated by segmented line  11   d . That is, a temperature is lower as it is closer to the transfer layer  13 .  
      However, as shown in  FIG. 2 , when the organic emission layer is formed using the laser thermal transfer element, since the concentration distribution of the radiation absorber in the light-to-heat conversion layer is uniform, defects or characteristic deterioration may occur in the transferred organic emission layer when a heat which is generated in the light-to-heat conversion layer is excessively high while transferring the transfer layer by irradiating the laser.  
       FIG. 3  is a cross-sectional view of a thermal transfer element used to form an organic emission layer of an organic electroluminescent display device according to a first embodiment of the present invention.  
      Referring to  FIG. 3 , the thermal transfer element of the present invention includes a base substrate  30 , a light-to-heat conversion layer  31 , an interlayer  32 , and a transfer layer  33 . The base substrate  30  serves as a support substrate for supporting the thermal transfer element and is made of a transparent high-molecular material such as Polyethylene Terephthalate (PET). The base substrate can be a glass substrate or film.  
      The light-to-heat conversion layer  31  contains a radiation absorber which absorbs laser light and converts it to heat energy. The radiation absorber of the light-to-heat conversion layer  31  has a concentration distribution depending on a depth “t” as shown in  FIGS. 6   a  and  6   b . As a radiation absorber for absorbing the laser light, the light-to-heat conversion layer  31  contains a radiation absorber which absorbs the light to generate the heat energy and an organic binder material which is curable by ultraviolet rays or heat. The light-to-heat conversion layer  31  contains infrared ray absorber such as carbon black, carbon, infrared dye, and pigment in addition to aluminum, aluminum oxide and sulfide.  
       FIGS. 6   a  and  6   b  are graphs illustrating a concentration distribution of a radiation absorber contained in a light-to-heat conversion layer of a laser thermal transfer element of the present invention.  
      Referring to  FIG. 6   a , the light-to-heat conversion layer  31  has a concentration gradient that a radiation absorber is decreased linearly according to a depth “t” thereof as shown by segmented line  31   c . That is, a concentration of the radiation absorber on a surface  31   a  of the light-to-heat conversion layer  31  contacting the base substrate  30  are relatively higher than that of the radiation absorber on a surface  31   b  of the light-to-heat conversion layer  31  contacting the interlayer  32 , and the radiation absorber has a concentration distribution that a concentration is gradually decreased according to a depth “t” thereof.  
      Thus, the light-to-heat conversion layer  31  has a temperature distribution that is lower as it is closer to the transfer layer  33  according to a depth “t” like illustrated by segmented line  31   d . That is, as it is closer to the surface  31   b  contacting the transfer layer  33  from the surface  31   a  contacting the base substrate  30 , a concentration of the radiation absorber is gradually decreased.  
      Therefore, the thermal transfer element of  FIG. 1 , has a temperature value  11   e  ( FIG. 2 ) on the surface  11   b  that the light-to-heat conversion layer  11  contacts the interlayer  12 , whereas the thermal transfer element of the present invention ( FIG. 3 ) has a temperature value  31   e  on the surface that the light-to-heat conversion layer  31  contacts the interlayer  32 . As a result, the thermal transfer element of the present invention has a lower temperature distribution at a portion close to the transfer layer  33  compared to the thermal transfer element of  FIG. 1  having the uniform concentration distribution of the radiation absorber.  
      The radiation absorber of the light-to-heat conversion layer  31  of  FIG. 6   a  has a concentration distribution which a concentration is gradually decreased as shown by segmented line  31   c , whereas that of  FIG. 6   b  has a concentration distribution which a concentration is stepwise decreased, as shown by segmented line  31   f , as it is farther from the base substrate  30  and is closer to the transfer layer  33 . Since the radiation absorber of the light-to-heat conversion layer has a concentration distribution which is stepwise decreased as it is closer to the transfer layer  33 , a temperature on an interface  31   b  between the light-to-heat conversion layer  31  and the interlayer  32  is lower than that of the thermal transfer element like that of  FIG. 6   a.    
      Referring back to  FIG. 3 , the interlayer  32  acts to protect the light-to-heat conversion layer and thus has a high heat resistance, and is made of an organic material or an inorganic material. The transfer layer  33  is formed of an image forming material corresponding to a thin film to be formed on the substrate. When an organic thin-film layer is formed using the thermal transfer element according to the first embodiment of the present invention, the transfer layer  33  includes at least one of a hole injecting layer, a hole transporting layer, an emission layer, a hole blocking layer, an electron transporting layer, and an electron injecting layer. The organic thin-film layer is a thin film selected from a high-molecular organic thin-film layer and a low-molecular organic thin-film layer.  
       FIG. 4  is a cross-sectional view of a thermal transfer element according to a second embodiment of the present invention.  
      Referring to  FIG. 4 , the laser thermal transfer element according to the second embodiment of the present invention includes a base substrate  40 , a light-to-heat conversion layer  41 , an interlayer, and a transfer layer  43 . The base substrate  40  and the interlayer  42  are the same as those of  FIG. 3 . Like that of  FIG. 3 , the light-to-heat conversion layer  41  has a radiation absorber that a concentration is gradually or stepwise decreased as it is father from the base substrate  40  or closer to the transfer layer  43  as shown in  FIGS. 6   a  and  6   b.    
      The transfer layer  33  of  FIG. 3  is formed over the entire surface of the base substrate  30 , whereas the patterned transfer layer  43  is formed as shown in  FIG. 4 . When an organic thin-film layer is formed using the thermal transfer element according to the second embodiment of the present invention, the patterned transfer layer  43  includes at least one of a hole injecting layer, a hole transporting layer, an emission layer, a hole blocking layer, an electron transporting layer, and an electron injecting layer. The organic thin-film layer is a thin film selected from a high-molecular organic thin-film layer and a low-molecular organic thin-film layer. Thus, the patterned organic thin-film layer is formed on the substrate.  
       FIGS. 5   a  to  5   c  are cross-sectional views illustrating a process of forming an organic thin-film layer of an organic emission layer (EL) display device using a laser thermal transfer element according to a first embodiment of the present invention.  
      Referring to  FIG. 5   a , a thermal transfer element having a light-to-heat conversion layer  51 , an interlayer  52  and a transfer layer  53  formed on a base substrate  50  is prepared. An insulting substrate  60  having a lower layer  61  formed thereon is prepared. The thermal transfer element is to form the organic thin-film layer of the organic emission layer (EL) display device, and thus the lower layer is a lower electrode, e.g., anode electrode. Then, the insulating substrate  60  is closely contacted with the thermal transfer element.  
      Referring to  FIG. 5   b , infrared laser is irradiated to the thermal transfer element closely contacted with the insulating substrate  60 , and so the infrared ray absorber contained in the light-to-heat conversion layer  51  absorbs the infrared rays and converts it to heat energy. As a result, the heat energy enables a portion of the transfer layer  53  to be transferred to the insulating substrate  60 , whereby an organic thin-film layer  62  is formed on the lower layer  61  as shown in  FIG. 5   c.    
      Here, since the radiation absorber of the light-to-heat conversion layer  51  has the concentration distribution of  FIGS. 6   a  and  6   b , a temperature on a surface adjacent to the transfer layer  53  used to form the emission layer is relatively lowered, whereby characteristics such as life span and luminous efficiency of the formed emission layer are not degraded, that is, characteristic deterioration of the emission layer does not occur.  
      Here, what the temperature of the light-to-heat conversion layer is relatively lowered means is that a temperature  31   e  on a surface  31   b  of the inventive light-to-heat conversion layer  51  adjacent to the transfer layer  53  is lower than a temperature  11   e  ( FIG. 2 ) on the surface  11   b  of the light-to-heat conversion layer  11  adjacent to the transfer layer  13  if it is assumed that the surface  11   a  of the light-to-heat conversion layer  11  adjacent to the base substrate  10  has the same temperature as the surface  31   a  of the inventive light-to-heat conversion layer  51  adjacent to the base substrate  50 .  
      A method of forming the organic thin-film layer using the laser thermal transfer element according to the second embodiment of the present invention is the same as that of the first embodiment of the present invention. However, the transfer of the transfer layer and the patterning of the organic thin-film layer are simultaneously performed when the laser thermal transfer element according to the first embodiment of the present invention is used, whereas the transfer of the transfer layer and the patterning of the organic thin-film layer are separately performed when the laser thermal transfer element according to the second embodiment of the present invention is used. Thus, the thermal transfer element according to the second embodiment of the present invention is profitable to form fine pitch and large-size display device.  
      The embodiments of the present invention are explained such that the light-to-heat conversion layer contains the radiation absorber which absorbs infrared laser light. However, the light-to-heat conversion layer can contain the radiation absorber which absorbs ultraviolet rays and visible rays as well as an infrared ray, and ultraviolet laser and visible laser can be used as a laser source.  
      The embodiments of the present invention are explained such that a concentration distribution of the light-to-heat conversion layer is lower as it is closer to that transfer layer. This can be applied to the light-to-heat conversion layer of the typical thermal transfer element. Also, the laser thermal transfer element of the present invention is explained to be used to form the organic thin-film layer, but the laser thermal transfer element of the present invention can be used to form other thin-film layers.  
      Furthermore, the embodiments of the present invention are explained such that a concentration distribution of the light-to-heat conversion layer is varied in the thermal transfer element having the light-to-heat conversion layer, the interlayer and the transfer layer stacked on the base substrate. However, a concentration of the light-to-heat conversion layer can be varied in the thermal transfer element having the light-to-heat conversion layer in the same way as the embodiments of the present invention.  
      As described above, according to the embodiments of the present invention, characteristics such as life span and luminous efficiency of the transferred organic emission layer may be improved by varying a concentration distribution of the light-to-heat conversion layer of the laser thermal transfer element.  
      Although the present invention has been described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that a variety of modifications and variations may be made to the present invention without departing from the spirit or scope of the present invention defined in the appended claims, and their equivalents.