Organic electroluminescent device

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device with a buffer layer in the cathode.

2. Description of the Prior Art

In various types of flat panel displays, since an organic electroluminescent display (OLED) has many beneficial characteristics, such as having a spontaneous light source, a wide viewing angle, fast response time, full-color, simpler structure, and power savings, the OLED has been used extensively in small and medium scale portable display fields.

An OLED is composed of many organic electroluminescent devices that comprise organic electroluminescent materials. U.S. Pat. No. 6,548,956 has disclosed an organic electroluminescent device with vertically stacked layers of a dual emission color display. Referring to <figref idref="DRAWINGS">FIG. 1</figref>, <figref idref="DRAWINGS">FIG. 1</figref> is a cross-sectional view of an organic electroluminescent device according to U.S. Pat. No. 6,548,956. The organic electroluminescent device <b>100</b> is grown on a glass substrate <b>102</b> pre-coated with a transparent indium tin oxide (ITO) thin film <b>104</b>. The layer <b>106</b> includes hole conducting compound, and the layer <b>108</b> includes electron conducting and highly electroluminescent materials, wherein the layers <b>106</b>, <b>108</b> are composed of organic materials. The layer <b>110</b> provides an electron injecting contact to the device <b>100</b>, which is made by deposition and composed of metal material, including a thin semi-transparent Mg-Ag alloy electrode. The top layer <b>112</b> is a thick ITO or a thick indium zinc oxide (IZO). Numerals <b>114</b> and <b>116</b> represent electrode contacts. The ITO thin film <b>104</b> serves as an anode while the top layer <b>112</b> and the thin metal layer <b>110</b> serve as a cathode of the organic electroluminescent device <b>100</b>.

For electron injection, the work function of the thin metal layer <b>110</b> has to match the lowest unoccupied molecular orbital (LUMO) energy level of the organic materials in the layer <b>108</b>. On the other hand, since the organic electroluminescent device <b>100</b> is a dual emission color display, the top layer <b>112</b> and the thin metal layer <b>110</b> must be transparent. Accordingly, the thin metal layer <b>110</b> has to be very thin, which insulted in a bad conductivity. Therefore, the top layer with a transparent conductive material, ITO or IZO, is essential to compensate the conductivity of the cathode. However, the transparent top layer <b>112</b> formed with ITO or IZO is sputter-deposited onto the Mg-Ag alloy surface of the thin metal layer <b>110</b>, which easily damages the thin metal layer <b>110</b> and the organic materials in the layers <b>106</b>, <b>108</b> due to the electrons and ions bombardment during sputter process. The damage would result in lower light-emitting efficiency and lifetime of the organic electroluminescent devices. Therefore, one of the disadvantages of the above-mentioned disclose is that the light-emitting efficiency and lifetime of the organic electroluminescent devices are decreased.

Another disclosure of an organic electroluminescent device is disclosed in U.S. Pat. No. 6,420,031, Parthasarathy et al. <figref idref="DRAWINGS">FIG. 2</figref> is a sectional-view of a transparent OLED (TOLED) <b>200</b> shown in the application of Parthasarathy et al. The TOLED includes a non-metallic cathode <b>202</b>, an electron injecting interface layer (EIL) <b>204</b>, an electron transporting layer (ETL) <b>206</b>, a hole transporting layer (HTL) <b>208</b>, an anode layer <b>210</b>, and a substrate <b>212</b>. After depositing the hole transporting layer <b>208</b> and the electron transporting layer <b>206</b>, the electron injecting interface layer <b>204</b> is added by depositing a thin film of copper phthalocyanine (CuPc) which is then capped with a film of sputtered ITO. This ITO layer functions as the cathode <b>202</b> of the TOLED <b>200</b>.

However, the CuPc material absorbs light with wavelength of about 625 nm which resulted in influence of light efficiency. In addition, the utilization of CuPc near the cathode leads to high operating voltages. Furthermore, the evaporation temperature of CuPc is much higher than other organic materials and it is hard to clean CuPc materials so that the evaporation chamber is easily contaminated during forming the CuPc layer. Accordingly, the TOLED <b>200</b> with CuPc material is not suitable for applying to mass production.

Accordingly, to provide an organic electroluminescent device with preferable light-emitting efficiency, easily fabricated in mass production, is still an important issue for manufactures.

SUMMARY OF THE INVENTION

An electronic device comprising an organic electroluminescent device for displaying images is provided. An embodiment of such the organic electroluminescent device comprises an anode, an organic electroluminescent layer, and a multi-layer transparent cathode on a substrate in sequence. The transparent cathode comprises a thin metal layer in the bottom of the transparent cathode, a doped buffer layer on the thin metal layer, and a transparent electrode on the doped buffer layer.

The doped buffer layer provides a function of protecting the thin metal layer and the underlying and maintains the electron injection efficiency even when the materials of the transparent electrode have a high work function. Therefore, an embodiment of the present invention provides a top-emission or a dual emission OLED having the organic electroluminescent devices, which has preferable light-emitting efficiency and a long lifetime.

These and other aspects of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a cross-sectional view of an organic electroluminescent device according to the prior art.

<figref idref="DRAWINGS">FIG. 2</figref> is a sectional-view of a TOLED according to the prior art.

<figref idref="DRAWINGS">FIG. 3</figref> is a top view of an electronic device for displaying images according to an embodiment of the present invention.

<figref idref="DRAWINGS">FIG. 4</figref> is a sectional-view of the organic electroluminescent device shown in <figref idref="DRAWINGS">FIG. 3</figref>.

DETAILED DESCRIPTION

<figref idref="DRAWINGS">FIG. 3</figref> is a top view of an electronic device for displaying images according to an embodiment of the present invention. As shown in <figref idref="DRAWINGS">FIG. 3</figref>, the electronic device <b>1</b> that comprises an input device <b>15</b> and an organic electroluminescent display (OLED) <b>10</b>. The electronic device <b>1</b> may be a portable device such as a PDA, notebook computer, tablet computer, cellular phone, or a display monitor device, etc. Input device <b>15</b> can be coupled to the OLED <b>10</b>. The input device <b>15</b> can include a processor or the like to provide image data to a control circuit <b>14</b> to render images. The OLED <b>10</b> comprises a display area <b>12</b> including a matrix composed of a plurality of data lines <b>22</b> (such as D<b>1</b>, D<b>2</b>, and D<b>3</b>) and scan lines <b>24</b> (such as S<b>1</b>, S<b>2</b>, and S<b>3</b>). The display area <b>12</b> also comprises a plurality of sub-pixel circuits <b>26</b>, wherein each sub-pixel circuit <b>26</b> has at least one thin film transistor (TFT) and an organic electroluminescent device <b>20</b> at each intersection of a data line <b>22</b> and a scan line <b>24</b>. Each sub-pixel circuit <b>26</b> is electrically connected to a corresponding data line <b>22</b> and a corresponding scan line <b>24</b> for driving the organic electroluminescent device <b>20</b> in the corresponding sub-pixel. The data lines D<b>1</b>, D<b>2</b>, and D<b>3</b> connect to a data line driver <b>16</b> for receiving an image data signal, and the scan lines S<b>1</b>, S<b>2</b>, and S<b>3</b> connect to a scan line driver <b>18</b> for receiving a switch/address signal. Both the scan line driver <b>18</b> and the data line driver <b>16</b> are controlled by a control circuit <b>14</b>. The OLED <b>10</b> can be a top-emission display. However, the present invention can also be applied to a dual emission display.

<figref idref="DRAWINGS">FIG. 4</figref> is a sectional-view of the organic electroluminescent device <b>20</b> shown in <figref idref="DRAWINGS">FIG. 3</figref>. As shown in <figref idref="DRAWINGS">FIG. 4</figref>, the organic electroluminescent device <b>20</b> comprises a substrate <b>22</b> and an anode <b>24</b>, a hole injection layer <b>26</b>, a hole transport layer <b>28</b>, an emitting layer <b>30</b>, an electron transport layer <b>32</b>, an electron injection layer <b>34</b>, and a multi-layer transparent cathode <b>42</b> positioned on the substrate <b>22</b> in sequence. The OLED <b>10</b> can be a top emission display, wherein the substrate <b>22</b> and the anode electrode <b>24</b> can be both transparent. In this embodiment, the substrate <b>22</b> can be a glass substrate. According to various embodiments, the substrate can be plastic foil or metal foil. The anode <b>24</b> can be composed of ITO or IZO. However, in other embodiments, the anode <b>24</b> can be formed with aurum (Au), silver (Ag), aluminum (Al) or platinum (Pt) when it is not required to be transparent.

The hole injection layer <b>26</b>, hole transporting layer <b>28</b>, emitting layer <b>30</b>, electron transporting layer <b>32</b>, and electron injection layer <b>34</b> compose an organic electroluminescent material layer, and can be doped with materials of the emitting layer <b>30</b>, wherein the concentration of the dopant is about 0.01%-10% by weight. The main materials of the hole injection layer <b>26</b> is LGC101®, produced by LG Chem. The material of the hole transporting layer <b>28</b> comprises 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB). The emitting layer <b>30</b> comprises tris (8-quinolinato-N,08)-aluminum (Alq3) doped by 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-<b>1-1</b>-H,5H,11H-[1]BENZOPYRANO[6,7,8-ij]quionlizin-11-one (C545T). The electron transporting <b>32</b> comprises Alq3 while the electron injection layer <b>34</b> comprises lithium fluoride (LiF). The above-mentioned organic electroluminescent materials in each layer may be formed on the anode <b>24</b> by evaporation, spin coating or ink jet printing individually. According to various embodiments, the layers comprising the organic electroluminescent materials are formed by vacuum evaporation, evaporation on molecular beam epitaxy (MBE), dipping, spin coating, casting, bar code, and roll coating processes.

The multi-layer transparent cathode <b>42</b> is composed of a thin metal layer <b>36</b>, a doped buffer layer <b>38</b>, and a transparent electrode <b>40</b> from bottom to top. The thin metal layer <b>36</b> can be fabricated by an evaporation process, and selectively comprises aluminum (Al), silver (Ag), barium (Ba), calcium (Ca), magnesium (Mg)/Ag alloy, Al/Li alloy, Al/Ba alloy, or alloy of the above metal materials. For transmitting light, the thickness h of the thin metal layer <b>36</b> as shown in <figref idref="DRAWINGS">FIG. 4</figref> can be less than or equal to 20 nm. In an embodiment, the thin metal layer can have a range of about 1 nm to 20 nm. The doped buffer layer <b>38</b> comprise electron transporting materials and doped with a low work function dopant, wherein the electron transporting materials can beAlq3 or bis (10-hydroxyben-zo[h]quinolinato) beryllium (Bebq2). The doped buffer layer <b>38</b> can be formed by a co-evaporation process. The dopant of the doped buffer layer <b>38</b> comprises metal materials with a low work function, wherein the low work function can be less than or equal to 4.2 electron volts (eV). In an embodiment, the metal materials of the dopant comprise alkali metals, alkali earth metals, transition metals, or rare earth metals. According to various embodiments, the metal materials of the dopant can selected from lithium (Li), cesium (Cs), strontium (Sr), or samarium (Sm). The dopant concentration of the metal materials in the doped buffer layer <b>38</b> can be about 0.1-99% by weight. In an embodiment, the dopant concentration can be 0.1-30% by weight. The thickness of the doped buffer layer <b>38</b> can be about 1 nm to 50 nm. After forming the doped buffer layer <b>38</b>, a transparent electrode <b>40</b> can be formed by a sputter process, wherein the transparent electrode <b>40</b> comprises ITO or IZO and has a thickness of about 10 nm to 400 nm. The doped buffer layer <b>38</b> can prevent the thin metal layer <b>36</b> and the organic electroluminescent materials below the thin metal layer <b>36</b> from damages during the sputtering process for forming the transparent electrode <b>40</b>. In addition, the electron transporting materials doped with low-work-function metals in the doped buffer layer <b>38</b> have high electron injection and transporting efficiency, thus the organic electroluminescent device <b>20</b> still has a high electron injection efficiency even though the materials of the transparent electrode <b>40</b> has a high work function.

According to various embodiments, the present invention provides an organic electroluminescent device with a doped buffer layer in its multi-layer transparent cathode. The organic electroluminescent device is capable of applying to an OLED or any electronic devices. With specific materials disclosed above, the doped buffer layer protects the thin metal layer and underlying organic materials without losing electron injection and transporting efficiencies, and the thin metal layer can be kept in the cathode layer for matching the LUMO energy level of the underlying organic materials so that the device has a preferable emitting efficiency. Therefore, a top-emission or a dual emission organic electroluminescent device or OLED with a long lifetime and preferable performance are provided according to the present invention.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.