Main process flow of flexible OLED panel
1. ITO substrate pretreatment
ITO is used as the anode, and the film is usually formed in the process of making the TFT backplane. The flatness and cleanliness of the substrate surface will affect the growth of organic thin-film materials and the performance of the OLED, so the ITO surface must be strictly cleaned.
2. EL film formation
After the pretreatment of the ITO substrate, the OLED materials prepared include small organic molecules, high molecular polymers, metals and alloys. Most organic small molecule films are prepared by vacuum thermal evaporation. Soluble organic small molecules and polymer films can be prepared by simpler, faster and low-cost solution methods. Spin coating, inkjet printing, and laser have been developed successively. Technology such as transfer printing. Metal and alloy films are usually prepared by vacuum thermal evaporation. The following focuses on the two processes of vacuum thermal evaporation and inkjet printing.
(1) Vacuum thermal evaporation: refers to the evaporation of the vaporized material into atoms or molecules by means of electric current heating, electron beam bombardment and laser heating in a vacuum. The surface of the sheet is condensed to form a thin film. The specific location of the vapor deposition film is controlled by Fine Metal Mask (FMM, that is, high-definition metal mask).
The evaporation process has strict requirements on the degree of vacuum, film thickness, FMM alignment, and impurity control during material transportation. At present, the vapor deposition process has the highest equipment and technology maturity among various EL film forming methods, and it can be used to prepare injection layers, transport layers, light-emitting layers and cathode materials. Its disadvantages are slow film formation speed, low utilization of organic materials, and poor uniformity of large-size film formation. Therefore, the evaporation process is mainly used for the preparation of small and medium-sized OLED devices.
(2) Inkjet printing: refers to pre-filling various organic functional layer materials into ink cartridges, using a computer to convert graphic information into digital signals, and controlling the movement of nozzles and the extrusion of ink droplets. Spray to the corresponding position to form the required pattern, so as to achieve precise, quantitative and positioning deposition, and complete the final printed product. Inkjet printing can be used to prepare hole transport layers, light-emitting layers, and cathode materials. Other layers still need to be completed with the help of an evaporation process. The full-layer inkjet printing technology is under development. Compared with evaporation, the inkjet printing process is simple, greatly improving the material utilization rate, and is suitable for preparing large-size OLED devices. In the future, with the maturity and large-scale application of printing technology, the manufacturing cost of OLED can be greatly reduced.
Among the related technologies of inkjet printing, the development of high molecular polymer ink is the most important, because the uniformity of ejected droplets mainly depends on the physical properties of the ink. At present, due to the insufficient stability of the ink formulation and printing process, it is easy to cause poor EL performance, uniformity and reliability. This is a technical problem that inkjet printing needs to solve urgently.
3. Packaging process
In order to prevent water, oxygen and dust from entering the inside of the OLED display device and causing its life and performance to decline, it is necessary to develop an OLED packaging process. At present, common packaging technologies are glass or metal cover packaging, and thin-film packaging.
(1) Traditional cover encapsulation: In an environment full of inert gas, the OLED glass is bonded to the glass cover or metal cover with ultraviolet curing glue, thereby sandwiching the organic layer and the cover between the cover and the substrate. The electrode is sealed to isolate water, oxygen and dust from the outside. The disadvantage of cover packaging is that the metal cover is easy to warp and deform, while the glass cover is fragile and is not suitable for the development of flexible OLED technology.
(2) Thin film packaging: replacing the cover plate packaging by depositing a thin film protective layer of a certain thickness is the current mainstream OLED packaging process. Thin-film packaging includes inorganic thin-film packaging, organic thin-film packaging, and composite packaging with overlapping organic/inorganic layers. Thin film packaging usually relies on PECVD (Ion Enhanced Chemical Vapor Deposition, that is, using microwave or radio frequency to ionize the gas containing the film constituent atoms to form plasma locally. The plasma is chemically active and easy to react. On the substrate The desired thin film is deposited), but with the rise of flexible OLEDs and the requirements for lightness and thinness, the thickness of the encapsulation layer is also required to be thinner and thinner. Achieving good density under a small thickness, so as to obtain excellent water and oxygen barrier properties, which requires high process technology. At present, the industry is reviewing the use of ALD (Atomic Layer Deposition, a method that can deposit substances on the surface of the substrate layer by layer in the form of a monoatomic film, similar to ordinary chemical deposition) to achieve the above effects.
