In early 2000, AVI began exploring the use of Light-Emitting Polymer (LEP) materials for low-resolution displays and specialty lighting applications. The aim was to exploit the key benefits of LEP materials, while developing a manufacturing process based on rapid, low cost manufacturing methods common in the graphics printing industry, not the LCD display industry.
The fabrication process for flexible P-OLEDs can be divided into three stages: (1) pre-production, (2) production and (3) final encapsulation. The pre-production stage includes artwork design; preparation of screens, and ink preparation, to name but a few. The production stage includes pretreatment of barrier-coated substrates; ITO patterning; LEP printing; and cathode printing. Final encapsulation involves the removal of oxygen and/or moisture from the printed display, and laminating a barrier-film to the back of the display to protect it from environmental degradation.
Stage 1: Pre-production Processes:
Whether the product is a segmented display, pixilated sign, or some other specialty lighting application, a Product Designer first generates a CAD drawing of the P-OLED component, creating artwork for each component layer of the display. Component layers include the ITO anode, LEP layer, cathode layer, electrical traces as well as any graphic overlay that the final product may require. With printed P-OLED technology, the designer is afforded considerable design freedom as display pixels can be printed using a variety of colored LEP inks, to virtually any shape and size.
AVI’s P-OLED technology offers high versatility in accommodating an array of printing and coating methods. While the current manufacturing platform is largely based on screen-printing, AVI has demonstrated other printing methods in the fabrication of P-OLEDs, including gravure printing and slot coating. Display manufacturers may wish to combine various printing methods with the aim of realizing certain benefits, such as perhaps improved high-resolution capabilities, throughput, or material usage. In spite of the array of printing methods available, screen-printing remains one of the leading methods deployed throughout the printed electronics industry, such as in the fabrication of thick-film EL displays, membrane switch assemblies, flexible circuits and graphic overlays. If using screen-printing, the manufacturer uses its CAD artwork to create screens for the screen-printing press(es), creating a different screen for each component layer of the display.
Display manufacturers also require specialty materials in display manufacture. To this aim, AVI has developed specialty formulated inks, most importantly LEP and cathode inks. Regarding LEP inks, AVI has developed a system for converting LEP powders into LEP inks which it specially formulates with additives to exploit the electrical and optical properties of the LEP materials, while also ensuring the ink can handle the rigors of a high-volume printing environment. For example, inks must demonstrate a certain amount of robustness against air-borne dust from the printing environment, while still enabling high yield deposition of ultra thin, defect-free films on flexible substrates. In addition to LEP inks, AVI has developed a patented ink system for its cathode technology. As a requisite, AVI’s cathode inks must be air-stable and printable, as well as demonstrate strong compatibility with the underlying LEP layer. Cathode technology plays a direct role in display characteristics, including the display’s power efficiency, operating lifetime, and manufacturing yield. AVI is committed to ensuring there is an availability of ‘ready-to-print’ inks for display manufacturers.
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Stage 2: Production Processes:
Most fundamentally, a flexible P-OLED device is a three layer device consisting of an anode, LEP layer and cathode. The display is built onto a transparent barrier-coated, ITO sputtered film or substrate. First, the anode layer is patterned through a subtractive process that applies a print-resist step to the substrate film to remove ITO from those areas of the substrate where anode is not required. Once patterned, the film is cleaned and dried. Following anode patterning, the LEP layer is then printed onto the ITO anode and then dried. Finally, a cathode ink is printed onto the LEP layer, and then dried to form a high-efficiency cathode and interconnect traces. To this point, the display has been entirely printed in air using conventional printing equipment and practices. During fabrication in air, the display can become saturated with moisture. However, unlike conventional RGB OLEDs, a printed P-OLED with its air-stable cathode is not irreversibly harmed by this moisture so long as the display is not powered up in this condition.
Stage 3: Final Encapsulation:
Before powering up the P-OLED, encapsulation is required. To accelerate the implementation of flexible encapsulation technology, AVI is working with a JDA partner to finalize the materials set, processing and related tooling. As an overview, flexible encapsulation begins by first extracting moisture from the display using elevated temperatures in an inert environment. Following rapid moisture extraction, displays are laminated with a barrier film to the back-side of the display using a specialty conformal adhesive technology. Once laminated encapsulation is complete, sheets are transferred back into to ambient where they are die-cut into components, QA tested and shipped.
Future Challenges:
Printed P-OLED technology is an emerging technology that offers enormous upside potential to greatly impact the display and specialty lighting industries. This large market opportunity also presents technological challenges. Today, AVI is closely engaged with other members of the value chain (e.g. customers) to create increasingly sophisticated P-OLED prototype displays, as well as collaborating with early-adopter manufacturers in technology transfer to scale to high volume. AVI is working with display manufacturers and key suppliers to secure availability of key materials, including LEP ‘powders’ and specialty barrier-coated substrates; finalizing a lamination-encapsulation process and related tooling; advancing the performance of multiple P-OLED colors; and pushing the frontier in ink development for novel (semi-)conducting materials to handle large-area, high-volume printing that is anticipated in the near future.
Summarizing Thoughts:
The manufacture of printed P-OLEDs share many commonalities with the manufacture of other printed electronics, including thick-film “EL displays”, interface panels and other membrane switch assemblies. The capital investment to build a single production line for printed P-OLEDs, which AVI defines as a capacity to produce 30,000ft2 of flexible P-OLED displays per month using a single shift, is ~$1 million. However, in many cases, display manufacturers already possess a significant portion of relevant expertise and infrastructure required for printed P-OLEDs, greatly reducing capital investment. Similar to other printable electronic technologies, the manufacturing lead time and setup tooling costs associated with this printed P-OLED technology will be remarkably low. These manufacturing characteristics make printed P-OLED technology highly attractive for adoption.
