With their warm colors and excellent color contrast, OLED displays are superior compared to LCDs, as many end-users will vouch. But OLED displays are also notoriously difficult to produce on a large scale. Now a Chinese company, Guangdong Juhua, has shown what a little-known but budding technology could do to increase production yield rates while also lowering manufacturing costs in OLED display manufacturing.
At the International Conference on Display Technology (ICDT) held April in Guangzhou, southern China, the manufacturer Guangdong Juhua showed in a demo how inkjet technology could be used in three-color RGB printing of materials for OLED displays. On hand were a 31-inch Full HD AMOLED panel as well as a 5-inch AMOLED panel boasting a noteworthy 400 pixels per inch (PPI) of resolution.
Unlike previously demonstrated RGB-printed panels, the smaller model's sample resolution can be used for small/medium display applications. Moreover, if a smaller nozzle were used, resolution could even reach 500 or 600 ppi—a rate competitive with that of high-resolution AMOLED panels currently produced through Fine Metal Mask (FMM) technology.
The successful demonstration by Guangdong Juhua shows the potential of inkjet technology in the manufacture of RGB AMOLED displays, for use in target applications including industrial, medical, and digital signage.
A product of two Chinese giants
Guangdong Juhua was founded in 2016 by two big Chinese display panel makers, China Star Optoelectronics Technology (CSOT, a TCL company) and the Tianma Group. Juhua serves as a research and development facility to focus on printing AMOLED and Quantum Dot (QD) technology, and to act as a platform for integrating the entire supply chain for printing AMOLED and QD panels in China. More companies have joined the open platform this year, and Guangdong Juhua plans to explore the use of printing technology in the production of OLED displays.
At present, Guangdong Juhua can use RGB printing technology in small, medium, and large displays, according to the AMOLED and LCD Supply Demand & Equipment Tracker from IHS Markit.
Pixel design in RGB printing: A new way of thinking
In his demo at the conference, Dr. Jinyao Song, also the media contact at Guangdong Juhua, combined the liquid-flow characteristics of inkjet printing with an S-strip pixel layout, taking advantage of the liquid flow in the bank, as depicted in the figure below.
The basic approach uses a microstructure that splits ink into different areas inside the bank during the liquid evaporation phase. Each area has its own thin-film transistor (TFT), allowing each area to be lit independently. Since the microstructure is made via a photolithography process, finer patterns with smaller widths can be achieved. This means less spacing is required between subpixels from the same bank, and more room is available to place extra pixels in the mother glass. 
As a printing design, this pixel method delivers several advantages, including the following:
- Because multiple pixels are formed by ink-splitting, the printing time can be reduced, with fewer runs for the deposition of one functional layer. This increases production line efficiency.
- Pixel density in PPI can be doubled—or even tripled—without inkjet systems being changed, which saves on costs.
- By using a smaller print head, it is possible to print panels with a resolution of 500 PPI or higher. Unlike the S‑PenTile structure used in current AMOLED panels, it is possible to have all R, G, and B colors present in one pixel via the printing method, resulting in better color performance.
Printing for small/medium OLED display production
What makes the ICDT demo interesting is the potential for RGB OLED manufacturing, through inkjet printing, for use in producing small- and medium-sized displays, as discussed in the IHS Markit AMOLED & Flexible Display Intelligence Service. Adopting this technology in the Gen 6 line, which targets cellphone and tablet panel use, would provide the following advantages:
- Where TV applications are involved, produces higher yield rates than when printing on Gen 8-and-higher lines
- Allows for more panels to be cut from the mother glass, which increases yield
- Cuts risk in the production of small displays—though not as much when large displays are involved. The current evaporator technique is mature for OLED materials such as electron transport layers (ETL) and electron injection layers (EIL), which means there is no need to purchase large evaporators, like Gen 10.5, for TV applications.
- Lowers the cost of AMOLED if the RGB printing yield rate uses an FMM method. RGB printing can increase the competitive strength of AMOLED at a lower price—FMM is not needed for printed functional layers, fewer chambers are needed for the evaporator, and material usage is higher
Despite the various advantages offered by RGB OLED-inkjet, the technology still carries risks, such as concerns on material performance as well as the so-called Mura Effect—the clouding of the display panel because of defective illumination, showing up as blotches, dots or stains when the panel is examined closely in darkly lit environments.
For these reasons, it is best to reserve RGB OLED inkjet printing for small- and medium-sized displays for the future rather than deploy the technology for mass production too soon, IHS Markit believes.
David Hsieh is Research & Analysis Director within the IHS Technology Group at IHS Markit
Posted 30 May 2016
or 600 ppi