Last week IDTechEx attended S.I.D. 2009 in San Antonio, Texas - North America's largest event on displays. It increasingly covers printed, flexible and organic displays. In this final article for Printed Electronics World covering the event, we give highlights from some of the technical sessions attended covering progress with OLEDs.
Seiko Epson is working on inkjet printed OLED frontplanes. It has fabricated 14" inkjet printed OLED displays on glass. Seiko Epson believes that inkjet is the best option for scalability to Gen 8 substrates. It weighs up the different pros and cons of different printing techniques as shown below.
Comparison of different printing techniques for OLED frontplanes, as evaluated by Seiko Epson
Source: Seiko Epson
Seiko Epson has produced a green OLED device working at 1000cd/m2 using inkjet printing which demonstrated over 45,000 hrs lifetime. They described that the biggest issue of a printed OLED, the device performance, has been significantly improved for both polymer and phosphorescent small molecule systems by engineering of device architecture and inkjet printing processing.
In polymer devices, they demonstrated that the decrease of electron current by inkjet printing was offset by using a higher electron injection cathode and device lifetime and efficiency was improved. In the case of phosphorescent small molecule devices, uniform emission with high luminous efficiency was achieved by Seiko Epson by selecting a suitable solvent, which does not reduce the quantum efficiency in the ink solution and resultant display. It is also very important for device lifetime to prevent the presence of oxygen in the manufacture environment.
Princeton University discussed a transfer printing method for patterning of thin polymer layers for OLED displays. A hard stamp with a raised feature is brought into contact with a spin coated organic film under elevated pressure and temperature to break the films. The patterned film is then transfer printed onto devices. This method was used to print red/green/blue sub-pixel arrays with pattern size as small as 12 μm at a resolution of 530 ppi. The speaker claimed that this has a low cost potential because it can be processed in parallel, with reuseable stamps.
The process steps are shown in the figure below:
Source: Princeton University
Novaled reported to IDTechEx that they are seeing huge interest in OLED lighting, and their booth had a desk lamp with OLED lighting strips.
Marie O'Regan, Technology Director at DuPont Displays, covered recent advances in solution processable OLEDs at the company. Currently, she says that the relative cost of different manufacturing techniques, if LCD is 1, is that evaporated OLED is 2.2 and solution processed OLED is 0.5. She described the challenges around developing high performance solution OLED materials for high performance displays. Materials need to address issues such as:
- How to coat the blanket layers, containing the printing inks in the active subpixel area
- Keeping successive layers in the device from mixing
- Cleaning the coated materials before encapsulation and bonding
- Printing at high speed
- Preventing atmospheric conditions from damaging the device
The process Dupont uses is shown below:
Source: DuPont Displays
O'Regan described that they have had to make some trade offs between cost and performance. On building up the devices, rather than individually optimising for each colour with different thickness layers, they use common layers based on the weakest colour, which is blue.
This is shown in the figure below:
Source: DuPont Displays
DuPont Displays has formed an alliance with Japanese based Dai Nippon Screen to develop the printing processes and materials. The first Gen 4 nozzle printer has been built and installed in Dupont's lab in Santa Barbara, CA, USA. (720mm x 920 mm size glass).
Lifetime of the devices is shown below:
