1. Field of Invention
The invention relates to a method for fabricating an organic light emitting diode, and more specifically, to a method for fabricating an organic light emitting diode with a hazy heterojunction.
2. Description of Related Art
An organic light emitting diode converts electric energy into light energy, and a semiconductor device has the maximum conversion efficiency. An organic light emitting diode often is applied in an indicator, a screen display and an emitting device of an optical pick-up head, etc. The organic light emitting diode device has some characteristics, such as non-visual angle, simple production, lower cost, high response speed, a broad range in temperature and full-colour, etc. It fits with the requirements for a multimedia monitor and has become the hottest research topic.
A basic structure of the organic light emitting diode includes a glass substrate, a metal electrode, an indium-tin-oxide (ITO) electrode and an organic emitting layer. In the emitting principle of the organic light emitting diode, the metal electrode is a cathode and the indium-tin-oxide is an anode. When the forward bias is charging on the two electrodes, an electron and a hole are respectively injected from the metal electrode and the ITO electrode into the emitting layer. When two carriers meet in the emitting layer and produce a photon by a radioactive recombination and a light emitting phenomena is achieved. In addition, the conduction speed of the electron is faster than the conduction speed of the hole. In order to achieve a conductive balance parameter of 1 between the electron and the hole, a hole injection layer (HIL) and a hole transport layer (HTL) are formed between the indium-tin-oxide electrode and the emitting layer. An electron injection layer (EIL) and an electron transport layer (ETL) are formed between the emitting layer and the metal electrode. The carriers thus can achieve a balance of the electron and the hole injection/transportation between the difference materials.
FIGS. 1A through 1B are cross-sectional views of a conventional organic light emitting diode and illustrate the steps for fabricating a conventional organic light emitting diode. Referring to FIG. 1A, the substrate 100 is a glass material, and an anode 101 of indium tin oxide material is formed. Then the organic emitting layer 106 builds on the top of the anode 101, and the metal material of cathode 110 is formed over the organic emitting layer 106. When a forward bias charges on the two electrodes, the electron and the hole are injected respectively from anode 101 and cathode 110 into the organic emitting layer 106. The two carriers collide in the emitting layer 106 and emit a photon by radioactive recombination. Further referring to FIG. 1B, the conduction speed of the electron is faster than the conduction speed of the hole. Therefore, there are many methods for fabricating an organic light emitting diode, such as adding an electron injection layer 108, a hole injection layer 102 and a hole transport layer 104, etc. The heterojunction is utilized to get a conduction balance between the electron and the hole.
Accordingly, the invention provides a method for fabricating an organic light emitting diode to haze the heterojunction and increase the efficiency of the radioactive recombination of the electron and the hole.
As embodied and broadly described herein, the invention provides a substrate, such as a glass or plastic substrate. An anode, an organic emitting layer and a cathode are formed sequentially on the substrate by an evaporation or a sputtering deposition method. The material of the anode is indium-tin-oxide, and the material of the organic emitting layer is Alq3. The material of the cathode is aluminum, silver or any metal with good conductivity. When forming the interface between the cathode and the organic emitting layer, the deposition rate of the anode material on the substrate gradually decreases, and the deposition rate of the organic emitting material on the substrate gradually increases. When forming the interface between the organic emitting layer and the cathode, the deposition rate of the organic emitting material on the substrate gradually decreases, and the deposition rate of the cathode material on the substrate gradually increases. Therefore, the gradient concentration is distributed between each layer of the anode, the organic emitting layer and the cathode. The heterojunction between each layer becomes blurred, thus creating an organic light emitting diode with a hazy heterojunction.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.