Patent Publication Number: US-2011069000-A1

Title: System for displaying images

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Taiwan Patent Application No. 098132280, filed on Sep. 24, 2009, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The invention relates to a system for displaying images, and more particularly to a system for displaying images including a full-color organic electroluminescent device. 
     2. Description of the Related Art 
     Recently, with the development and wide application of electronic products such as mobile phones, personal digital assistants, and notebook computers, there has been an increased demand for flat display devices which consume less power and occupy less space. Organic electroluminescent devices are self-emitting and highly luminous, have a wide viewing angle, fast response time, and simple fabrication process, making them an industry display of choice. 
     In order to fabricate a top-emission organic electroluminescent device, U.S. Pat. No. 5,739,545 discloses an organic electroluminescent device with a transparent cathode, wherein the transparent cathode includes a low work-function metallic layer and a wide energy-gap layer formed on the low work-function metallic layer. The wide energy-gap layer, serving as a protection layer, prevents the organic electroluminescent layers and the low work-function metallic layer from being damaged during fabrication process of the top-emission organic electroluminescent device. However, because properties of the wide energy-gap layer, such as color purity and luminescence intensity, are poor, similar properties for the organic electroluminescent device of the invention made therefrom are also poor. 
     U.S. Pat. No. 6,984,934 discloses a method to prevent light between a class substrate and an anode from dissipating by forming a micro-lens array on the class substrate of the organic electroluminescent device. Although the method can increase extraction of light from the organic electroluminescent device, fabrication of the micro-lens array is complicated and difficult. Further, since the micro-lens array is formed outside of the glass substrate of the organic electroluminescent device, color purity of the organic electroluminescent device cannot be enhanced by the micro-lens array. 
     BRIEF SUMMARY 
     An exemplary embodiment provides a system for displaying images. The system includes a full-color organic electroluminescent device including: a bottom substrate; a reflective layer disposed on the bottom substrate; a first transparent electrode; an organic electroluminescent element; and a second transparent electrode sequentially formed on the reflective layer. A light enhancing layer, disposed between the bottom substrate and the first transparent electrode. 
     In another exemplary embodiment, the top surface of the transparent connection layer, the top surface of the first transparent electrode, and/or the bottom surface of the protection layer comprise a plurality of protrusions. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a cross-section of a full-color organic electroluminescent device according to an embodiment of the invention. 
         FIG. 2  is a cross-section of a full-color organic electroluminescent device according to another embodiment of the invention. 
         FIG. 3  is a cross-section of a full-color organic electroluminescent device according to yet another embodiment of the invention. 
         FIG. 4  is a cross-section of the full-color organic electroluminescent device (1) as disclosed in Comparative Example 1. 
         FIG. 5  shows a graph plotting RGB luminescence intensity of the full-color organic electroluminescent device (1) as disclosed in Comparative Example 1. 
         FIG. 6  shows a graph plotting RGB luminescence intensity of the full-color organic electroluminescent device (2) as disclosed in Example 1. 
         FIG. 7  is a cross-section of the full-color organic electroluminescent device (3) as disclosed in Example 2. 
         FIG. 8  shows a graph plotting RGB luminescence intensity of the full-color organic electroluminescent device (3) as disclosed in Example 2. 
         FIG. 9  is a cross-section of the full-color organic electroluminescent device (4) as disclosed in Example 3. 
         FIG. 10  shows a graph plotting RGB luminescence intensity of the full-color organic electroluminescent device (4) as disclosed in Example 3. 
         FIG. 11  is a cross-section of the full-color organic electroluminescent device (5) as disclosed in Example 4. 
         FIG. 12  shows a graph plotting RGB luminescence intensity of the full-color organic electroluminescent device (5) as disclosed in Example 4. 
         FIG. 13  schematically shows a block diagram of a system for displaying images according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     Referring to  FIG. 1 , a full-color organic electroluminescent device  100  employed by a system for displaying images according to an embodiment of the invention is provided. Herein, the full-color organic electroluminescent device  100  includes a bottom substrate  12 , and a reflective layer  14 , a transparent connection layer  16 , a light enhancing layer  18 , a first transparent electrode  20  (such as an anode), an organic electroluminescent element  22 , and a second transparent electrode  24  (such as a cathode) subsequently formed on the bottom substrate  12 . It should be noted that, since the organic electroluminescent device  100  has a specific sequence structure (reflective layer  14 /transparent connection layer  16 /light enhancing layer  18 /first transparent electrode  20 ), the organic electroluminescent device  100  of the invention can exhibit increased luminescence intensity and improve RGB color purity. 
     Further, the full-color organic electroluminescent device  100  can further include a top substrate (serving as a packaging substrate)  32 , and a color filter film  30  and a protection layer (serving as a passivation layer)  28  subsequently formed on the bottom surface of the top substrate. Next, the top substrate  32  (with the protection layer  28  and the color filter film  30 ) is fixed on the second transparent electrode  24  via a buffer layer  26 . Finally, an adhesive layer  34  is disposed between the top substrate  32  and the bottom substrate  12  to seal the full-color organic electroluminescent device  100 , as shown in  FIG. 1 . According to another embodiment of the invention, the color filter film  30  can also be formed on the top substrate  32 , as shown in  FIG. 2 . 
     The bottom substrate  12  can be a glass substrate, a plastic substrate, or a semiconductor substrate. The substrate  202  can be a substrate including a required element (such as a thin film transistor). The accompanying drawings illustrate the substrate  12  as a plain rectangle in order to simplify the illustration. 
     The reflective layer  14  can be a distributed Bragg reflector (DBR) reflecting scattered lights toward the bottom substrate  12  for total reflection. The reflective layer  14  can include Ag, Al, Au, or combinations thereof. The transparent connection layer  16  exhibits high transmittance and can include indium tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), zinc oxide (ZnO), or combinations thereof. The light enhancing layer  18  includes a material with high refractive index. For example, the refractive index of the light enhancing layer  18  is more than the refractive index of the transparent connection layer  16  or the refractive index of the first transparent electrode  20 . The light enhancing layer  18  can include materials with a refractive index of more than 2.1, such as ZnSe (refractive index 2.6), or ZnS (refractive index 2.4). 
     The first transparent electrode  20  serving as an anode can include transparent metal or metallic oxide, such as tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), or zinc oxide (ZnO). The method for forming the above layers (the reflective layer  14 , the transparent connection layer  16 , the light enhancing layer  18 , and the first transparent electrode  20 ) is unlimited and can be a sputtering, electron beam evaporation, thermal evaporation, or chemical vapor deposition process. 
     The organic electroluminescent element  22  can at least include a light emitting layer, and can further include a hole injection layer, a hole transport layer, an electron transport layer, and/or an electron injection layer. The layers of the organic electroluminescent element  22  can include small molecule organic electroluminescent materials or polymer organic electroluminescent materials. 
     The layer of small molecule organic electroluminescent materials can be formed by a thermal vacuum evaporation process, and the layer of polymer organic electroluminescent materials can be formed by a spin coating, ink-jet printing, or screen printing process. 
     Each emitting layer of the organic electroluminescent element  22  can include one or multiple light-emitting materials and electroluminescent dopants doped into the light-emitting materials and can perform energy transfer or carrier trapping under electron-hole recombination in the emitting layer. The light-emitting material can be fluorescent or phosphorescent. The layers, materials, dopant dose, and the thickness of the organic electroluminescent element  22  are not limited and can be optionally modified by a person of ordinary skill in the field. Suitable materials of the second transparent electrode  24  serving as a cathode can include transparent metal or metallic oxide, such as tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), or zinc oxide (ZnO). The method for forming the second transparent electrode  24  is unlimited and can be a sputtering, electron beam evaporation, thermal evaporation, or chemical vapor deposition process. The buffer layer  26  can be formed on the second transparent electrode  24  to combine the top substrate  32  and the bottom substrate  12 . Since the buffer layer  26  and the protection layer  28  can be dovetailed into each other, there is no residual moisture and air in the full-color organic electroluminescent device  100 . After combining the buffer layer  26  and the protection layer  28 , the adhesive layer  34  is subjected to a curing process. Suitable materials of the buffer layer can be visible-light-induced photocurable material or an ultraviolet-light-induced photocurable material, such as photocurable resin. The method for forming the buffer layer can be a spin coating, ink-jet printing, or screen printing process. Since the protection layer (passivation layer)  28  can prevent oxygen and moisture in the ambient environment from entering into the full-color organic electroluminescent device, the electrodes and the organic layers can be prevented from being damaged and the operating lifespan of the full-color organic electroluminescent device can be extended. 
     Suitable materials of the protection layer can be silicon oxide, aluminium oxide, silicon nitride, silicon oxynitride, or combinations thereof. The materials of the color filter film are unlimited and can include a green color filter, red color filter, blue color filter, white color filter, or permutations and combinations thereof for achieving full-color display. The material of the packaging substrate  32  is unlimited and the packaging substrate  32  can be a transparent substrate, such as a glass substrate, or a plastic substrate. 
     According to an embodiment of the invention, the process for forming the full-color organic electroluminescent device  100  can be divided into a process for forming the first part and a process for forming a second part. The process for forming the first part includes subsequently forming the reflective layer  14 , the transparent connection layer  16 , the light enhancing layer  18 , the first transparent electrode  20 , the organic electroluminescent element  22 , the first transparent electrode  24 , and the buffer layer  26  on the bottom substrate  12 . Meanwhile, the process for forming the second part includes subsequently forming the color filter film  30  and the protection layer  28  on the top substrate  32 . Next, the first part and the second part are combined, forcing the buffer layer  26  and the protection layer  28  to be dovetailed into each other. After, the buffer layer  26  is subjected to a curing process (such as UV curing). Finally, the top substrate and the bottom substrate are enveloped by an adhesive layer  34 . 
     According to another embodiment of the invention, at least one of the top surface  15  of the transparent connection layer  56 , the top surface  17  of the first transparent electrode  60  and the bottom surface  19  of the protection layer  68  can have a plurality of protrusions disposed thereon, as shown in  FIG. 3 . As a result, the transparent connection layer  56 , the light enhancing layer  58 , the first transparent electrode  60 , and/or the protection layer  68  may have an alternate thickness at the interface thereof contacting to the top surface  15 , the top surface  17 , and/or the bottom surface  19 . Therefore, lights emitted by the organic electroluminescent device  100  are apt to emit to the outside of the device (i.e. the light extraction efficiency of the organic electroluminescent device  100  is increased). Meanwhile, the RGB luminescence intensity and the color purity of the organic electroluminescent device  100  are simultaneously improved. The shapes of the protrusion are unlimited and can be circular, elliptical, polygonal or combinations thereof. The method for forming the protrusions (or depressions) can be a photolithography process or phase shift mask process having a halftone mask. 
     In an embodiment of the invention, the ratio between the height X of the protrusion and the thickness Y of the layer can be from 1:1 to 1:3. For example, the height X of the protrusion can be 70 nm and the thickness Y of the layer can be 130 nm. 
     The following examples are intended to illustrate the invention more fully without limiting their scope, since numerous modifications and variations will be apparent to those skilled in this art. 
     Comparative Example 1 
     A glass substrate  120  was provided and then washed by a cleaning agent, acetone, and isopropanol with ultrasonic agitation. After drying at 120□, the glass substrate  120  was subjected to a uv/ozone treatment. Next, as shown in  FIG. 4 , a reflective layer  140 , a first transparent electrode  200 , an organic electroluminescent element  220 , a second transparent electrode  240 , a buffer layer  260 , a protection layer  280 , a color filter film  300  and a packaging substrate  320  were subsequently disposed on the glass substrate  120 . Finally, the adhesive layer  340  was disposed between the packaging substrate  320  and the glass substrate  120  to bond the packaging substrate  320  with the glass substrate  120 , obtaining the organic electroluminescent device (1). For purposes of clarity, the materials of the above layers are described in the following. 
     The reflective layer  140 , with a thickness of 150 nm, comprised Al. The first transparent electrode  200 , with a thickness of 30 nm, comprised ITO (Indium tin oxide). The organic electroluminescent element  220 , formed from a bottom side to a top side, included: a hole injection layer with a thickness of 5 nm comprising 4,4′,4″-tris[N,(3-methylphenyl)-N-phenyl-amino]riphenylamine (m-TDATA); a hole transport layer with a thickness of 10 nm comprising 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD); a first emitting layer (emitting red light) with a thickness of 40 nm comprising 8-hydroxyquinoline aluminum (Alq 3 ) as a host, and a red dopant (with a trade No. RD3, sold and manufactured by Kodak), wherein the weight ratio between Alq 3  and RD3 was 100:1; a second emitting layer (emitting green light) with a thickness of 40 nm comprising 8-hydroxyquinoline aluminum (Alq 3 ) and C545T (10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)benzopyropyrano(6, 7-8-l,j)quinolizin-11-one) as a dopant, wherein the weight ratio between Alq 3  and C545T was 10:1; a third emitting layer (emitting blue light) with a thickness of 40 nm comprising 9,10-bis(2-naphthyl)anthracene (AND) as a host and bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi) as a dopant, wherein the weight ratio between Alq 3  and C545T was 100:7.5; an electron transport layer with a thickness of 40 nm comprising bis(10-hydroxybenzo[h]quinolinato)beryllium (BeBq 2 ); and an electron injection layer with a thickness of 1 nm, comprising LiF. The second transparent electrode  240 , with a thickness of 80 nm, comprised IZO (indium zinc oxide). The buffer layer  260 , with a thickness of 6 μm, comprised acrylic resin. The protection layer  280 , with a thickness of 400 nm, comprised silicon nitride (SiNx). The color filter film  300  had a thickness of 1 μm. 
     The optical properties of the electroluminescent device (1), as described in the Comparative Example 1, were measured by a PR650 (purchased from Photo Research Inc.) and Minolta LS110. Referring to  FIG. 5 , the RGB luminescence intensity of the electroluminescent device (1) were 0.013, 0.015, and 0.022 respectively. 
     Example 1 
     A glass substrate  12  was provided and then washed by a cleaning agent, acetone, and isopropanol with ultrasonic agitation. After drying at 120□, the glass substrate  12  was subjected to a uv/ozone treatment. Next, as shown in  FIG. 1 , a reflective layer  14 , transparent connection layer  16 , light enhancing layer  18 , first transparent electrode  20 , organic electroluminescent element  22 , second transparent electrode  24 , buffer layer  26 , protection layer  28 , color filter film  30 , and a packaging substrate  32  were subsequently disposed on the glass substrate  12 . Finally, the adhesive layer  34  was disposed between the packaging substrate  32  and the glass substrate  12  to bond the packaging substrate  32  with the glass substrate  12 , obtaining the organic electroluminescent device (2). For purposes of clarity, the materials of the above layers are described in the following. 
     The reflective layer  14 , with a thickness of 150 nm, comprised Al. The transparent connection layer  16 , with a thickness of 30 nm, comprised ITO (Indium tin oxide). The light enhancing layer  18 , with a thickness of 5 nm, comprised zinc selenide (ZnSe). The first transparent electrode  20 , with a thickness of 30 nm, comprised ITO (Indium tin oxide). The organic electroluminescent element  22  had the same layers and sequence of layers as the organic electroluminescent element  220  disclosed in Comparative Example 1. The second transparent electrode  24 , with a thickness of 80 nm, comprised IZO (indium zinc oxide). The buffer layer  26 , with a thickness of 6 μm, comprised acrylic resin. The protection layer  28 , with a thickness of 400 nm, comprised silicon nitride (SiNx). 
     The optical properties of the electroluminescent device (2), as described in Example 1, were measured by a PR650 (purchased from Photo Research Inc.) and Minolta LS110. Referring to  FIG. 6 , the RGB luminescence intensity of the electroluminescent device (2) were 0.018, 0.016, and 0.023 respectively. 
     Due to the structure (reflective layer/transparent connection layer/light enhancing layer/first transparent electrode) of the electroluminescent device (2), the electroluminescent device (2) showed increased RGB luminescence intensity in comparison with the electroluminescent device (1). 
     Example 2 
     A glass substrate  12  was provided and then washed by a cleaning agent, acetone, and isopropanol with ultrasonic agitation. After drying at 120□, the glass substrate  12  was subjected to a uv/ozone treatment. Next, as shown in  FIG. 7 , a reflective layer  14 , transparent connection layer  56 , light enhancing layer  58 , first transparent electrode  20 , organic electroluminescent element  22 , second transparent electrode  24 , buffer layer  26 , protection layer  28 , color filter film  30 , and a packaging substrate  32  were subsequently disposed on the glass substrate  12 . Particularly, the transparent connection layer  56  had a plurality of protrusions disposed on the top surface  15  of the transparent connection layer, wherein the height X 1  of the protrusions was 70 nm, and the thickness Y 1  of the transparent connection layer  56  was 130 nm. Finally, the adhesive layer  34  was disposed between the packaging substrate  32  and the glass substrate  12  to bond the packaging substrate  32  with the glass substrate  12 , obtaining the organic electroluminescent device (3). For purposes of clarity, the materials of the above layers are described in the following. 
     The reflective layer  14 , with a thickness of 150 nm, comprised Al. The transparent connection layer  56  comprised ITO (Indium tin oxide), wherein the thickness Y 1  of the transparent connection layer  56  was 130 nm, and the height X 1  of the protrusions was 70 nm. The light enhancing layer  58 , with a thickness of 5 nm, comprised zinc selenide (ZnSe). The first transparent electrode  20 , with a thickness of 30 nm, comprised ITO (Indium tin oxide). The organic electroluminescent element  22  had the same layers and sequence of layers as the organic electroluminescent element  220  disclosed in Comparative Example 1. The second transparent electrode  24 , with a thickness of 80 nm, comprised IZO (indium zinc oxide). The buffer layer  26 , with a thickness of 6 μm, comprised acrylic resin. The protection layer  28 , with a thickness of 400 nm, comprised silicon nitride (SiNx). 
     The optical properties of the electroluminescent device (3), as described in Example 2, were measured by a PR650 (purchased from Photo Research Inc.) and Minolta LS110. Referring to  FIG. 8 , the RGB luminescence intensity of the electroluminescent device (3) were 0.0242, 0.018, and 0.0293 respectively. 
     Due to the structure (reflective layer  14 /transparent connection layer  56 /light enhancing layer  58 /first transparent electrode  20 ) and the plurality of protrusions of the transparent connection layer  56  of the electroluminescent device (3), the electroluminescent device (3) showed increased RGB luminescence intensity in comparison with the electroluminescent device (1). 
     Example 3 
     A glass substrate  12  was provided and then washed by a cleaning agent, acetone, and isopropanol with ultrasonic agitation. After drying at 120□, the glass substrate  12  was subjected to a uv/ozone treatment. Next, as shown in  FIG. 9 , a reflective layer  14 , transparent connection layer  16 , light enhancing layer  18 , first transparent electrode  60 , organic electroluminescent element  62 , second transparent electrode  24 , buffer layer  26 , protection layer  28 , color filter film  30 , and a packaging substrate  32  were subsequently disposed on the glass substrate  12 . Particularly, the first transparent electrode  60  had a plurality of protrusions disposed on the top surface  17  of the first transparent electrode  60 , wherein the height X 2  of the protrusions was 70 nm, and the thickness Y 2  of the first transparent electrode  60  was 130 nm. Finally, the adhesive layer  34  was disposed between the packaging substrate  32  and the glass substrate  12  to bond the packaging substrate  32  with the glass substrate  12 , obtaining the organic electroluminescent device (4). For purposes of clarity, the materials of the above layers are described in the following. 
     The reflective layer  14 , with a thickness of 150 nm, comprised Al. The transparent connection layer  16  with a thickness of 30 nm, comprised ITO (Indium tin oxide). The light enhancing layer  18 , with a thickness of 5 nm, comprised zinc selenide (ZnSe). The first transparent electrode  60  comprised ITO (Indium tin oxide), wherein the thickness Y 2  of the first transparent electrode  60  was 130 nm, and the height X 2  of the protrusions was 70 nm. The organic electroluminescent element  22  had the same layers and sequence of layers as the organic electroluminescent element  220  disclosed in Comparative Example 1. The second transparent electrode  24 , with a thickness of 80 nm, comprised IZO (indium zinc oxide). The buffer layer  26 , with a thickness of 6 μm, comprised acrylic resin. The protection layer  28 , with a thickness of 400 μm, comprised silicon nitride (SiNx). 
     The optical properties of the electroluminescent device (4), as described in Example 3, were measured by a PR650 (purchased from Photo Research Inc.) and Minolta LS110. Referring to  FIG. 10 , the RGB luminescence intensity of the electroluminescent device (4) were 0.0234, 0.021, and 0.0265 respectively. 
     Due to the structure (reflective layer  14 /transparent connection layer  16 /light enhancing layer  18 /first transparent electrode  60 ) and the plurality of protrusions of the first transparent electrode  60  of the electroluminescent device (4), the electroluminescent device (4) showed increased RGB luminescence intensity in comparison with the electroluminescent device (1). 
     Example 4 
     A glass substrate  12  was provided and then washed by a cleaning agent, acetone, and isopropanol with ultrasonic agitation. After drying at 120□, the glass substrate  12  was subjected to a uv/ozone treatment. Next, as shown in  FIG. 11 , a reflective layer  14 , transparent connection layer  56 , light enhancing layer  58 , first transparent electrode  60 , organic electroluminescent element  62 , second transparent electrode  24 , buffer layer  66 , protection layer  68 , color filter film  30 , and a packaging substrate  32  were subsequently disposed on the glass substrate  12 . 
     Particularly, the transparent connection layer  56  has a plurality of protrusions disposed on the top surface  15  of the transparent connection layer  56 , the first transparent electrode  60  has a plurality of protrusions disposed on the top surface  17  of the first transparent electrode  60 , and the protection layer  68  has a plurality of protrusions disposed on the bottom surface  19  of the protection layer  68 . Finally, the adhesive layer  34  was disposed between the packaging substrate  32  and the glass substrate  12  to bond the packaging substrate  32  with the glass substrate  12 , obtaining the organic electroluminescent device (5). For purposes of clarity, the materials of the above layers are described in the following. 
     The reflective layer  14 , with a thickness of 150 nm, comprised Al. The transparent connection layer  56  comprised ITO (Indium tin oxide), wherein the thickness Y 1  of the transparent connection layer  56  was 130 nm, and the height X 1  of the protrusions was 70 nm. The light enhancing layer  58 , with a thickness of 5 nm, comprised zinc selenide (ZnSe). The first transparent electrode  60  comprised ITO (Indium tin oxide), wherein the thickness Y 2  of the first transparent electrode  60  was 130 nm, and the height X 2  of the protrusions was 70 nm. The organic electroluminescent element  62  had the same layers and sequence of layers as the organic electroluminescent element  220  disclosed in Comparative Example 1. The second transparent electrode  24 , with a thickness of 80 nm, comprised IZO (indium zinc oxide). The buffer layer  66 , with a thickness of 6 μm, comprised acrylic resin. The protection layer  68  comprised silicon nitride (SiNx), wherein the thickness Y 3  of the protection layer  68  was 100 nm, and the height X 3  of the protrusions was 70 nm. 
     The optical properties of the electroluminescent device (5), as described in Example 4, were measured by a PR650 (purchased from Photo Research Inc.) and Minolta LS110. Referring to  FIG. 12 , the RGB luminescence intensity of the electroluminescent device (5) were 0.024, 0.021, and 0.029 respectively. 
     Due to the structure (reflective layer  14 /transparent connection layer  56 /light enhancing layer  58 /first transparent electrode  60 ) and the plurality of protrusions of the transparent connection layer, the first transparent electrode, and the protection layer of the electroluminescent device (5), the electroluminescent device (5) showed increased RGB luminescence intensity in comparison with the electroluminescent device (1). Further, since the electroluminescent device (5) had narrower FWHM (full width at half maximum) for RGB lights in comparison with the electroluminescent device (1), the electroluminescent device (5) of the invention had improved color purity, as shown in  FIG. 12 . 
     In comparison with Comparative Example 1, luminescence intensity of the full-color organic electroluminescent devices as disclosed in the Examples were higher due to the reflective layer/transparent connection layer/light enhancing layer/first transparent electrode structure of the Examples. Further, since the transparent connection layer, the first transparent electrode, and/or the protection layer can include a plurality of protrusions on a surface thereof, RGB luminescence intensity and RGB color purity of full-color organic electroluminescent devices utilizing the structure can be simultaneously improved. 
       FIG. 13  schematically shows another embodiment of a system for displaying images which, in this case, is implemented as a display device  300  or an electronic device  400 , such as a notebook computer, mobile phone, digital camera, PDA (personal data assistant), desktop computer, television, car display, or portable DVD player. The display device  300  (such as a full-color display device) includes the full-color organic electroluminescent device  100 . In some embodiments, the display panel  300  can form a portion of a variety of electronic devices (in this case, electronic device  400 ). As shown in  FIG. 13 , the electronic device  400  can include the display device  300  and an input unit  350 . Further, the input unit  350  can be operatively coupled to the display panel  300  and provide input signals (e.g., an image signal) to the display panel  400  to generate images. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.