By Hyun Jun Jang
During the 2015 OLED Evaluation Seminar (December 4) hosted by UBI Research, Professor Hong Mun-Pyo of Korea University gave a talk titled Flexible AMOLED Gas Barrier Technology Development Status. Through this presentation, he discussed in detail flexible OLED’s outline, technological issues, and encapsulation among other key issues.
Flexible display signifies a display that was produced on top of flexible substrate, and not an existing glass substrate, which can bend, fold, or roll without breaking. Hong emphasized flexible display is the next generation display that can simultaneously satisfy consumers and panel makers, and an area that OLED can be more valuable compared to LCD.
There are 3 essential issues in flexible display, substrate, TFT array, and display processes, as well as ancillary issues such as application and cost. Hong reported key issues regarding substrate and display process.
Flexible display uses plastic substrate, instead of glass, that is strong against shock and can bend. Therefore, handling technology that manages plastic substrate is considered a key technology in flexible display production. Hong revealed that for handling technology, a film lamination method and vanish coating method are mainly used. A film lamination method is where plastic substrate is attached to carrier glass using adhesive before being processed and a vanish coating method is where the PI substrate is coated to the carrier glass before processing. He emphasized that no matter which method is used, the debonding technology used to detach the plastic substrate from the glass plays a crucial role in deciding yield.
Hong followed the substrate discussion with encapsulation technology. Encapsulation technology prevents moisture and oxygen that affect OLED panel’s performance from infiltrating in order to increase the display’s lifetime. As it is a core process that decides OLED panel’s yield, OLED panel production companies are focusing on optimum encapsulation technology development.
Key issues of encapsulation technology that is currently being applied to flexible OLED, barrier coating related issues are considered the most important. Barrier coating is the coating applied to the plastic substrate to overcome the limitations that occur as existing glass substrate is replaced by plastic. For flexible encapsulation, as can type or frit seal technologies that were used for glass encapsulation cannot be used, face seal or TFE technologies that can be applied to flexible are used. Also, as the permeability of oxygen and moisture has to be 10-6g/m2day or less, high performing barrier coating technology is needed.
When barrier coating is used to flexible OLED, generally 3 problems occur. Firstly, physically cracks or particles can develop. Regarding this, Hong explained that this issue can be solved if process is properly maintained. The second problem is micro defects that can arise on the surface of plastic film, which can be solved through optimized process, according to Hong. Lastly, nano-sized pin holes can come up. Hong revealed that multi-layers of barrier coating can solve this problem.
Generally, when OLED panel is produced the thickness of encapsulation layer is not a big issue. However, Hong emphasized that the thickness becomes a core issue when producing flexible OLED panel. He reported that hybrid structure of encapsulation where gas barrier cover plate is attached to passivation layer placed via PECVD can be the solution.
Although the most suitable process technology for hybrid encapsulation production is R2R, as appropriate results are not obtained when CVD is applied to R2R, Hong reported that research is being carried out toward the sputtering using direction. He revealed that if reflection plate is added to the sputtering equipment and neutron beam release is induced, defects that occur during the sputtering process can be reduced as the target thin film stabilizes.