Abstract:
The present invention discloses a method of fabricating organic light emitting diode array, which adopts a directional spin coating technology to grow different organic light-emitting materials on the same plane so as to control the color of the emitted light and accomplish monochrome or full color organic light emitting diodes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of fabricating organic light emitting diode array, particularly to the one that adopts a directional spin coating technology. 
     2. Description of the Related Art 
     The characteristic of an organic light emitting diode (OLED) is that the material of its electro-luminescent (EL) layer is a small discrete organic molecule such as aluminum tris(8-hydroxyquinoline) (Alq3) or an organic polymer such as polyfluorene (PF). Refer to  FIG. 1  the schematic diagram of a conventional OLED  1 . As shown in  FIG. 1 , the conventional OLED  1  includes the following elements in top-down sequence: an encapsulating layer  12 , which isolates the OLED  1  from the environment; a cathode  14 ; an organic light emitter  16 ; a transparent anode  18 ; and a transparent substrate  20 , whose material may be a glass or a transparent plastic. In a general OLED, only the anode is transparent; however, there is also OLED having a transparent cathode or having transparent anode and cathode.  FIG. 1(   a ) is the schematic diagram of the detailed structure of the organic light emitter  16 . As shown in  FIG. 1(   a ), the organic light emitter  16  includes the following elements in top-down sequence: an electron injection layer (EIL)  161 , an electron transport layer (ETL)  162 , an electro-luminescent (EL) layer  163 , a hole transport layer (HTL)  164 , and a hole injection layer (HIL)  165 . Except the EL layer  163 , other layers of the organic light emitter  16  are optional, depending on designer&#39;s choice. 
     The structures of full color OLED display devices can be divided into a stack one and a parallel one. Referring to  FIG. 2(   a ) the schematic diagram of a stack structured full color OLED display device  3 , herein, three OLEDs  32 ,  34 , and  36 , which emit red (R), green (G), and blue (B) lights respectively, are stacked on a substrate  38  to form a unitary full color pixel. There are three types of parallel structured full color OLED display devices. Referring to  FIG. 2(   b ) the schematic diagram of the first type parallel structured full color OLED display device  4 , herein, three OLEDs  42 ,  44 ,  46 , which emit R, G, and B lights respectively, are disposed on a substrate  48  to form a unitary full color pixel. Referring to  FIG. 2(   c ) the schematic diagram of the second type parallel structured full color OLED display device  5 , herein, a white-light light source  52  in cooperation with three color filters  54 ,  56 , and  58  creates R, G, and B lights. Referring to  FIG. 2(   d ) the schematic diagram of the third type parallel structured full color OLED display device  6 , herein, three color conversion elements  64 ,  66 , and  68  convert the light emitted by a light source  62  and of a specific frequency into R, G, and B lights. 
     A few methods exist for fabricating OLEDs. Thermal evaporation is the de facto choice for fabrication of small molecular OLEDs. For fabrication of polymeric OLEDs, two approaches are commonly used. For monochrome OLEDs, the simple spin coating method is universally adopted. When addressing full color OLEDs, the inkjet printing method is the first choice coming to designer&#39;s mind. The conventional spin coating approach is a simple and inexpensive fabrication method; however, it cannot be utilized to fabricate full color OLEDs, as it can coat only one thin film on the substrate and lacks the ability to coat polymers into arbitrarily geometrical patterns. Feasibility of the inkjet printing method for fabrication of full color polymeric OLEDs was first demonstrated in [CBY98], which reported dual-color polymeric OLED pixels involving a spin-coated EL layer with blue emission topped by inkjet printed EL dots with red-orange emission. Custom-design and careful selection of the EL materials are necessary for the success of the inkjet printed full color OLEDs. 
     Limitation of the thermal evaporation method to small-size OLED displays, inability of the spin coating approach for full color OLED displays, and the fact that the inkjet printing technique is still at laboratory prototyping stage prompt many activities on alternative methods. Proposals directly addressing patterning of the EL layer for fabrication of full color or multi-color OLED displays include methods of thermal transfer [WBF03, HS02, CSS01 and references therein], electrochemical polymerization [ZWW03], photolithography using UV curable EL polymers [MFR03], screen printing [BBH01], and photolithography based on a new photoresist of a photoacid generating material and heat labile monomers [She01]. In the following, a brief review of these alternative methods is prepared by calling upon each method to selectively deposit the R, G, and B EL layers as shown in  FIG. 3(   a ) which depicts a half complete, parallel structured full color OLED consisting of a substrate  102 , an anode layer  104 , an optional HIL  122 , an optional HTL  124 , and three discretely deposited EL patterns  126 , emitting R, B, and G lights respectively. 
       FIG. 3(   b ) illustrates how discrete deposition of an EL pattern is achieved using the thermal transfer method. The key component of the thermal transfer method is a donor element  400  which, in one of many possible embodiments [WBF03], consists of a donor substrate  401 , a light-to-heat conversion layer  402 , and a transfer layer  403 . For our application, the transfer layer is made of an EL material. With light radiation  406  through a mask  405 , a part  404  of the EL transfer layer is transferred onto the HTL  124  due to the heat converted by the light-to-heat conversion layer. The half complete full color PLED is accomplished by repeating the same process for another two EL patterns. 
       FIG. 3(   c ) describes how the electrochemical polymerization method operates. The substrate  102  with patterned anode  104  is used as the positive electrode. Mononers of the desired EL polymer are dissolved in the electrolyte  412 . When a voltage source  416  is applied to the patterned anode and a negative electrode  414 , the monomers are oxidized, resulting in positively charged polymers selectively deposited on the patterned anode. Neutralization of the positively charged polymers is not necessary but it does give rise to an OLED device with “superior” performance [ZWW03]. Since electrochemical polymerization requires deposition on the electrode, the fabricated OLED device can not contain either HIL or HTL layer. Repeating the same process for another two EL patterns makes the half complete full color OLED. 
       FIG. 3(   d ) shows how, with specially synthesized UV curable EL polymers, traditional photolithography is applied to fabrication of full color OLED devices. The UV curable EL polymers are soluble before UV curing and become insoluble when photochemically crosslinked. For OLED applications, the UV curable EL material of one type is spun coated on top of the HTL layer. UV radiation  426  is then applied through a mask  424 . A discrete EL pattern  126  is created after washing away the uncured non-crosslinked part  422 . Repeated applications of the photolithography process gives rise to the needed R, G, and B patterns. 
       FIG. 3(   e ) shows a schematic of the screen printing approach [BBH01]. A screen  434  made of polyester fabric is placed above the HTL layer at a pre-determined gap, called a snap-off distance  432 . A photoresist layer is coated onto the screen and photolithographically patterned as shown  436 . Deposition of an EL pattern is screen printed by applying a soft rubber squeegee  438  over a solution of EL material  439 . Repeating the screen printing process with properly patterned photoresist layer render the discretely printed R, G, and B patterns. 
       FIG. 3(   f ) to  FIG. 3(   h ) highlight development of a full color OLED device through successive applications of photolithographic process whose success is hinged upon the invention of a new photoresist which includes a photoacid generating material and heat labile monomers [She01]. The photoacid generating material releases acid when exposed to light. After light exposure ( 448 ,  FIG. 3(   f )), the photoresist  442  is heated to a predetermined temperature and the monomers are joined by acid labile links to form a polymer. A special feature of this polymer is its solubility in a solvent not containing water and active hydrogen.  FIG. 3(   g ) shows a photolithographically patterned photoresist  452  over layers of cathode  444  and an EL material  446 . Reactive ion etching is then applied to remove the unprotected portion of the cathode and EL layer, creating a needed EL pattern. The remaining photoresist  452  is finally removed. Note that the need of an etching process like the reactive ion etching prevents the inclusion of the HIL and HTL layers in the OLED devices. After creation of one EL pattern, layers of the EL material of second type  466 , cathode  464 , and photoresist  462  are deposited as shown in  FIG. 3(   h ). The same photolithography plus etching process is repeated to create a second EL pattern. 
     From above review, the thermal transfer method seems most feasible, competitive, and mature. The methods of electrochemical polymerization and photolithography using UV curable electroluminescent polymers require specially synthesized EL polymers, possibly resulting in compromised electroluminescence efficiency. One additional drawback of the electrochemical polymerization method is its exclusion of the use of HIL and HTL layers in device design and optimization. The same limitation preventing usage of HIL and HTL layers exists in the method of photolithography based on a new photoresist of a photoacid generating material and heat labile monomers. In its early development stage, the screen printing method still has rooms for improvement in resolution and in the on/off current ratio of the OLED devices such made. 
     Accordingly, by utilizing a directional spin coating technology, the present invention proposes a method for fabrication of OLED array which overcomes the aforementioned inability of the conventional spin coating for fabricating full color OLEDs, wherein the superiorities of simplicity and low-cost of the spin coating method are still maintained. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a method of fabricating OLED array which can obviously reduce the fabrication cost, simplify the fabrication process and promote the competitiveness of OLED in the market. 
     Another objective of the present invention is to provide a method of fabricating OLED array which enjoys the same benefits of low cost and simplicity as the conventional spin coating method but without its limitation to only monochrome OLED. 
     The approach to achieving the aforementioned objectives of the present invention is the utilization of a directional spin coating technology to linearly coat different organic light-emitting materials to accomplish the fabrication of full color OLED. 
     In the method of fabricating OLED array of the present invention, one embodiment thereof comprises: providing a substrate, whereon multiple anodes are arranged in rows; forming multiple parallel insulating banks which are perpendicular to the anodes; utilizing the directional spin coating method to coat a light-emitting layer between two neighboring insulating banks; and forming cathodes on the substrate. 
     Another embodiment of the present invention comprises: providing a substrate; forming pixelized anodes on the substrate; forming multiple parallel insulating banks; utilizing the directional spin coating method to coat a light-emitting layer between two neighboring insulating banks; and forming a patterned cathode on the substrate. 
     To further understand the objectives, technical contents, and accomplishments of the present invention, a detailed description, with the aid of drawings of the embodiments, is stated below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is the schematic diagram of a general conventional OLED. 
         FIG. 1(   a ) is the schematic diagram of a conventional multiple-layer light emitter. 
         FIG. 2(   a ) is the schematic diagram of a conventional stack structured full color OLED display device. 
         FIG. 2(   b ) is the schematic diagram of a conventional parallel structured full color OLED display device. 
         FIG. 2(   c ) is the schematic diagram of another conventional parallel structured full color OLED display device 
         FIG. 2(   d ) is the schematic diagram of yet another conventional parallel structured full color OLED display device. 
         FIG. 3(   a ) is the schematic diagram of a half complete, parallel structured, full color OLED. 
         FIG. 3(   b ) is an illustration of the thermal transfer method for fabrication of full color OLED. 
         FIG. 3(   c ) is an illustration of the electrochemical polymerization method for fabrication of full color OLED. 
         FIG. 3(   d ) is an illustration of the photolithographical method for fabrication of full color OLED using UV curable EL polymers. 
         FIG. 3(   e ) is an illustration of the screen printing method for fabrication of full color OLED. 
         FIG. 3(   f ) to  FIG. 3(   h ) highlight development of a full color OLED through successive applications of photolithographic process using a specially prepared photoresist made of a photoacid generating material and heat labile monomers. 
         FIG. 4  is the flowchart of the fabrication process according to the present invention. 
         FIG. 5(   a ) to  FIG. 5(   d ) are the schematic diagrams of the steps of the fabrication process according to the present invention. 
         FIG. 6(   a ) and  FIG. 6(   b ) are the schematic diagrams of the directional spin coating technology. 
         FIG. 7(   a ) to  FIG. 7(   c ) are the schematic sectional views according to the steps of the fabrication process of the present invention when the light emitter is a multiple-layer one. 
         FIG. 8(   a ) is the schematic diagram of a pixelized anode. 
         FIG. 8(   b ) is the schematic diagram of a stripe-like anode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention pertains to a method which utilizes a directional spin coating technology to fabricate monochrome or full color OLED array. 
     Firstly, the fabrication of passive matrix OLED array is described herein. Refer to  FIG. 4  for the flowchart of fabrication process of the present invention. The fabrication process of the passive matrix OLED array comprises the following steps: providing a substrate  72  (step S 1 ), which may be a glass or a general transparent material such as a transparent plastic; forming multiple anodes  74 , which are arranged in rows, onto the substrate  72  (step S 2 ), wherein the material of the anode  74  is not limited to a metallic material or a metallic compound but can also be an electrically conductive polymeric material, as showing in  FIG. 5(   a ); forming multiple parallel insulating banks  76  on the anodes  74  (step S 3 ), wherein the insulating banks are perpendicular to the rows of the anodes and two neighboring insulating banks define a trench  78 , as shown in  FIG. 5(   b ), and wherein the insulating banks  76  are not only needed in the directional spin coating of the next step and but also provide an electrically insulating function, and wherein any dielectric material, which can be patterned and will not harm the anodes  74  disposed there below in the patterning process, can be adopted as the material of the insulating banks  76 ; utilizing the directional spin coating technology to coat an organic solution of the light-emitting layer  80  on the trenches  78  defined by the insulating banks  76  to form light-emitting layers  80  arranged in columns (step S 4 ), as shown in  FIG. 5(   c ); and fabricating cathodes  82  (step S 5 ), which can be achieved via forming stripe-like cathodes  82  on the light-emitting layers  80  as shown in  FIG. 5(   d   1 ) or via utilizing a non-directional deposition method such as thermal evaporation to form the cathode  82  on the substrate  72  as shown in  FIG. 5(   d   2 ) wherein the concern of electrical short between neighboring columns of cathodes  82  does not exist because of the presence of the insulating banks  76 . 
     Refer to  FIG. 6  for a description of the directional spin coating technology. As shown in  FIG. 6(   a ) and  FIG. 6(   b ), the directional spin coating technology utilizes the trenches  86 , which are previously formed on a substrate  84 , to define the directions and the shapes of coated solution, and in the present invention, the patterned insulating banks  76  are utilized to define the positions and shapes of the trenches  78 . Next, via the angular velocity of the spin disc  88 , the coating solution is efficiently coated on the substrate  84 , which is fixed vertically on the spin disc  88 , to achieve the patterning effect.  FIG. 6(   a ) or  FIG. 6(   b ) represents the relationship between ω and θ when the side of the substrate  84  with the patterned trenches  86  points outward or inward, wherein ω is the angular velocity of the rotating spin disc  88  and θ is the angle defined from the radial vector of the spin disc  88  to the normal vector of the side of the substrate  84  with the patterned trenches  86 . 
     The above description relates to the fabrication of OLED with only single light-emitting layer; however, not all OLEDs have only single light-emitting layer, and thus, the multiple-layer light emitter will be further discussed herein. When the light emitter has multiple layers, it is not necessary to adopt the directional spin coating in deposition of all layers, but a mix of the directional spin coating and the non-directional conventional spin coating can also be adopted. In one embodiment thereof, firstly, a substrate  72  with multiple anodes  74  arranged in rows is provided, as shown in  FIG. 7(   a ); then, a HIL  90 , which covers the anodes  74  and the substrate  72 , and a HTL  92 , which is above the HIL  90 , are sequentially formed via the conventional spin coating technology; then, insulating banks  76 , which are parallel to each other and perpendicular to the anodes  74 , are formed on the HTL  92 , as shown in  FIG. 7(   b ); then, the directional spin coating is used to perform coating on the trenches  78 , which are defined by the insulating banks  76 , to form the EL layers  80 , which are arranged in columns, as shown in  FIG. 7(   c ); then, another deposition technology such as the conventional spin coating is used to optionally deposit an ETL  94  on the EL layer  80  and the insulating banks  76 , and to optionally deposit an EIL  96  on the ETL  94 ; finally, multiple stripe-like cathodes are formed on the EIL  96 , or a cathode is formed to overlay the whole substrate  72 , as shown in  FIG. 7(   c ), and owing to the existence of the insulating banks  76 , electrical shorts between the columns are avoided. 
     The active matrix OLED can be fabricated via a slight modification of the aforementioned fabrication process of passive matrix OLED. For example, the anode  74  can be pixelized as that shown in  FIG. 8(   a ), or can be formed into parallel rows as that shown in  FIG. 8(   b ). In  FIG. 8(   a ), the insulating banks need only being parallel, and are not limited to being arranged in rows or columns; next, the approach of fabricating the light emitter is unchanged and is to coat the coating solution on the trenches defined by the insulating banks via the directional spin coating technology. If the light emitter comprises multiple layers, it is to be noted that the modification of the fabrication process is the same as that mentioned above. The cathodes may be pixel-like or stripe-like. When the cathodes are stripe-like, it is necessary for the cathodes to be perpendicular to the stripe-like insulating banks lest supposedly “off” OLED elements should become switched-on when a neighboring OLED element is switched-on. In contrast, when the anodes are formed into rows as shown in  FIG. 8(   b ), the cathodes should be deposited into pixels, and the stripe-like insulating banks must be perpendicular to the rowed anodes lest supposedly “off” OLED elements should become switched-on when a neighboring OLED element is switched-on. There are many available existing technology for fabricating pixelized anodes or cathodes, and they will not be stated herein. 
     In full color OLED, the light emitters must be able to emit R, G, and B lights separately, and thus, the organic solutions of R, G, and B, which are to be filled into the trenches defined by the insulating banks via the directional spin coating, are to be disposed repeatedly according to the sequence of R-G-B or R-B-G in order to accomplish the parallel structured full color OLED display device shown in  FIG. 2(   b ). 
     In summary, the present invention provides a method of fabricating OLED array, wherein the directional spin coating technology is adopted to overcome the problem that the conventional spin coating technology cannot fabricate full color OLED, and wherein a low-cost monochrome/full color OLED is fabricated with a simpler process in order to obviously promote OLED&#39;s competitiveness in the market. 
     Those described above are only the preferred embodiments of the present invention, and any equivalent modification and variation in the shapes, structures, characteristics, and spirit stated inside the claims of the present invention are to be included within the scope of the claims of the present invention.