SID: Flexible Displays Are on Their Way
New displays aimed at E-books and other portable products were at center stage at the Society of Information Displays (SID) conference. Electrophoretic displays used for E-book applications are seeking to move to color, while flexible ultrathin AMOLED displays are adopting a multilayer stack approach to improve lifetimes.
Michael P.C. Watts, Impattern Solutions, Austin, Texas, www.impattern.com -- Semiconductor International, 6/15/2009
New flexible displays with long battery life will appear in 2010, spurring new mobile system markets, participants said at the recent Society for Information Display's SID 2009 conference, held in San Antonio, Texas.
The major display manufacturers showed flexible ultrathin active-matrix organic light-emitting diode (AMOLED) displays, as well as new electrophoretic displays aimed largely at E-books such as the Amazon Kindle. Electrophoretic displays appear to be the ideal choice for E-book applications and are positioned for rapid growth. Vendors also were touting large LCDs with low-power LED light sources, displays with fast refresh rates capable of high-resolution motion, and multiple 3-D displays.
Flexibility was viewed as a competitive advantage. Polymer Vision Ltd. (Eindhoven, Netherlands), which makes the Readius pocket reader, discussed the use of a rollable or foldable display to deliver a display that is larger than the parent device. A cell phone, for example, could contain a display that would unroll to twice the size of the phone.
At SID, less-ambitious projects involved creating an E-book that feels like a magazine. Plastic Logic Ltd. (Cambridge, UK) and Polymer Vision both are focused on reducing weight for E-readers by not using the heavy sheets of glass required in LCDs. Also, they employ a less-rigid mechanical support, due to the flexibility of the display.
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| Plastic Logic's flexible backplane is combined with a frontplane material (e.g. electronic paper) to make a flexible display. |
A flexible display needs to be thin so that the strain in the film surfaces are low, and do not require backlighting or liquid layer-gap control. Though flexible versions of conventional LCDs have been made, the slightest touch causes a change of color on the LCD.
At SID, participants described flexible AMOLED displays, which use organic LEDs (OLEDs) as the source of light, and an active matrix to address each pixel and pass current to the individual diodes in each pixel. (An active matrix display uses a transistor at the intersection of each row and column to allow each pixel to be independently addressed.) As an LED emissive display, AMOLED displays support a large color range know as a "gamut," a high contrast typical of emissive displays — the blacks are really black.
In comparison to LCDs, AMOLEDs are thin and consume little power, making them ideal for mobile applications. The barrier to the use of OLEDs has been lifetimes, but it appears that the display manufactures believe they have a solution using a multilayer stack of organic layers, each tens of nanometers thick.
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A printed TFT array and electrophoretic display are being developed by the Palo Alto Research Corp. (Source: PARC) |
Displays that can be read under ambient light are much easier on the eye, and are closer to the classic idea of a book. The most popular electrophoretic displays rely on oppositely charged white and black particles distributed in a fluid capsule. By applying a voltage, the particles can be moved to the surface, changing the display from black to white. This technology has been commercialized by E Ink, which makes the electrophoretic layer, and Prime View International (PVI), which makes the active-matrix layer under license from Phillips. These displays use ambient light and are bistable, thus using very little power. They have been key to the success of the Kindle E-book, which is being viewed by the display community as a breakthrough product for electrophoretic displays.
E Ink is partnering with a number of active matrix developers, including PVI, Plastic Logic and Polymer Vision. Just before SID, PVI announced the purchase of E Ink, which may have an impact on some of E Ink's partnership strategies.
The next challenge is color in ambient displays, the subject of several papers and demonstrations at the conference. E Ink showed a prototype electrophoretic color display that uses additive RGBW pixels to produce a display with relatively low brightness and low color range. The display was just good enough to communicate color information in a map, but not to show images. Ambient light is a challenge for additive color because the white reflectivity is immediately compromised by two of the three separate color pixels.
There was a lot of discussion at SID about how to do full color in an ambient light display. Matching print color with subtractive CYM colors is the complete solution, but has serious technical challenges. Kent Displays showed a trilayer cholesteric LC color changing skin — effectively a large single pixel with excellent color rendition. A team from Motorola discussed progress on an electrowetting display, which moves each single pixel volume of liquid around using a charged electrode. The color depends on dyed inks and has excellent prospects. The trilayer device is a serious fabrication challenge.
Flexible displays are being made by fabricating displays on a carrier glass using standard LCD process equipment. The plastic is either laminated on the carrier glass or coated on the glass and then separated by laser. The glass support allows equipment with conventional material handling, and allows overlay to be maintained on the stretchable film. Most of the demonstration devices seemed to be fabricated on Gen 2 or Gen 3 lines. There was discussion of the possibility of printable roll-to-roll manufacturing, but it appears to be several years way.
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