BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
FIGS. 1A and 1B are schematic cross-sectional views showing a conventional bottom emission OLED and a conventional top emission OLED, respectively.
FIGS. 2A to 2C are schematic, cross-sectional diagrams illustrating the process flow for fabricating a double side emitting OLED according to a preferred embodiment of the present invention.
FIG. 3 is a schematic view showing a plasma diffusion deposition system.
FIG. 4 is a diagram illustrating a relationship between the thickness of the second film, the driving voltage and the transmission of the transparent electrode measured according to the devices having the stacked structure of CuPc/NPB/Alq3/Alq3 with the different transparent electrode (the second film of the transparent electrode has different thickness) of the present invention.
DESCRIPTION OF THE EMBODIMENTS
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
FIGS. 2A to 2C are schematic, cross-sectional diagrams illustrating the process flow for fabricating a double side emitting OLED according to a preferred embodiment of the present invention. First, referring to FIG. 2A, a transparent substrate 210 is provided. In one embodiment of the present invention, the transparent substrate 210 may be a glass substrate or a substrate made of other suitable transparent material. Next, referring to FIG. 2B, a first transparent electrode 220 and an OEL layer 230 are sequentially formed on the transparent substrate 210. Generally speaking, a material of the first transparent electrode 220 is selected from the group consisting of indium tin oxide, indium zinc oxide, aluminium zinc oxide, antimony tin oxide, zinc oxide, indium oxide and tin oxide. Besides, in one embodiment of the present invention, the method of fabricating the OEL layer 230 comprises the following steps. First, a hole injection layer 231 is formed on the first transparent electrode 220. Next, a hole transport layer 232 is formed on the hole injection layer 231. Then, a light emitting layer 233 is formed on the hole transport layer 232. Next, an electron transport layer 234 is formed on the light emitting layer 233. Finally, an electron injection layer 235 is formed on the electron transport layer 234. The fabrication of the OEL layer 230 is completed by the above processes. It should be noted that the OEL layer 230 may only comprise any one, any two, any three or any four of the layers except the five-layer structure. The type and the number of the films which the OEL layer 230 comprises are not limited in the present invention.
Next, referring to FIG. 2C, a second transparent electrode 240 is formed on the OEL layer 230. The second transparent electrode 240 of the present invention is composed of a first film 241, a second film 242 and a third film 243. First, a first film 241 is formed on the OEL layer 230 and is made of a lithium compound, such as lithium fluoride, lithium oxide and the like. The thickness of the first film 241 is between 5 angstroms and 100 angstroms. In this embodiment, the first film 241 is made of lithium fluoride. Next, the second film 242 is formed on the first film 241. The second film 242 is a thin metallic film, and the thin metallic film may be made of Al, Au, Ag, Ca, Mg, Mg/Al alloy, Mg/Ag alloy and the like. The transmission of the second transparent electrode 240 depends on the thickness of the second film 242, and therefore the transmission of the second transparent electrode 240 may be adjusted according to the thickness of the second film 242. In one embodiment of the present invention, a preferred thickness of the second film 242 is between 20 angstroms and 50 angstroms. Besides, the first film 241 and the second film 242 may be formed by sputtering, thermal evaporation, physical vapour deposition or other process.
Finally, a plasma diffusion deposition process or an ionic thermal evaporation process is performed to form a third film 243 on the second film 242, wherein the third film 243 is made of a transparent conductive material, such as indium tin oxide, indium zinc oxide and the like. The thickness of the third film is between 500 angstroms and 3000 angstroms. Thus far, the double side emitting OLED 200 is formed according to the above processes.
FIG. 3 is a schematic view showing a plasma diffusion deposition system (PDDS). Referring to FIG. 3, the plasma diffusion deposition system 300 mainly comprises a plasma gun 310, a target 320 and a plasma beam controller 330. The substrate 210 to be deposited is disposed right above the target 320. Besides, in the embodiment, the target 320 is made of indium tin oxide. The plasma 312 generated from the plasma gun 310 is controlled by the plasma beam controller 330, such that the plasma 312 is turned toward the target 320 for heating the target 320. The target 320 may evaporate after heated and then be deposited on the substrate 210 to form the indium tin oxide layer.
In one embodiment of the present invention, the plasma gas used in the plasma diffusion deposition comprises Ar, O2, N2 or water vapor. Besides, the plasma power of the plasma diffusion deposition process is between 5000W and 15000W, and the operation temperature of the plasma diffusion deposition process is between 20° C. and 300° C. Since the plasma diffusion deposition process may reduce the effect of ion bombardment, the double side emitting OLED 200 formed according to the above processes may have a lower driving voltage.
To prove that the double side emitting OLED 200 formed according to the above processes has a higher electrode transmission and a lower driving voltage, the present invention fabricates devices having a stacked structure of CuPc (200 angstroms)/NPB (500 angstroms)/Alq3 (350 angstroms)/Alq3 (150 angstroms) with the different transparent electrode (the second film of the transparent electrode has a different thickness) and measures the driving voltage of the devices and the transmission of the transparent electrode to obtain a relationship between the thickness of the second film, the driving voltage of the devices and the transmission of the transparent electrode. It is clear that from FIG. 4, when the thickness of the second film is between 20 angstroms and 50 angstroms, the transmission of the transparent electrode is larger than 50% and the driving voltage of the device is lower than or equal to 5 volt. Accordingly, the requirements of a higher transmission of the transparent electrode and a lower driving voltage are achieved.
In summary, the transparent electrode of the double side emitting OLED of the present invention comprises three films—the first film made of the lithium compound (such as lithium fluoride);the second film made of the metallic material (such as aluminium); the third film made of the transparent conductive material (such as indium tin oxide). The present invention features that the third film disposed on the second film is formed by the plasma diffusion deposition process or ionic thermal evaporation process. Since the effect of ion bombardment generated during the plasma diffusion deposition process is lower, therefore, the OLED formed according to the processes has a lower driving voltage. From the experiment data shown in FIG. 4, as the thickness of the second film is between 20 angstroms and 50 angstroms, the transmission rate of the transparent electrode is larger than 50% and the driving voltage of the device is lower than or equal to 5 volt. Accordingly, the double side emitting OLED of the present invention has a higher transmission rate of the transparent electrode and a lower driving voltage.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
Patent Application
20080018242
