Developments in Flexible Printed LCDs
A killer new product is one that shows consistent rapid growth in sales at a level that is high enough to be profitable for suppliers. The i-Pod™ which sold 300,000 in its first year turned out to be one and the Amazon Kindle™ electronic book looks like being another, with 500,000 sales in its first year. Both are exceptionally easy to use, appealing to the young as trendy and to the old as convenient.
For example, electronic books permit the elderly to access a larger font without difficulty and they store one hundred or more volumes in one thin tablet. The text is even more readable in bright sunshine thanks to the electrophoretic display.
It is generally accepted that the next improvement must be the A4 size that is most familiar to those used to paper. This will permit business files to be accessed without tedious reformatting. Then, or so it is thought, the e-books will be further improved by being flexible like paper – even foldable or at least tightly rollable. Such technology would also lead to pull out screens for laptops and mobile phones that snap back into the device when not needed.
Electrophoretics have been successfully printed on polyester films, with printed transistor backplane drivers by Plastic Logic. Indeed, the simpler passive matrix versions have appeared as small displays for such things as shelf edge labels that are updated by radio (SiPix) and samples of ultrathin wristwatches, intelligent keyfobs (Delphi) and other devices have been demonstrated.
Electrophoretic displays deposited on flexible polymer film are commonly referred to as e-paper, the leading supplier of such front planes being a company called E-Ink. The e-paper name is unfortunate because there is now considerable work and some commercialisation on electronics including very primitive displays on real paper which can be lower in cost and biodegradable. Genuine paper displays are starting to be used in packaging but they are not suitable for the long life flexible display in electronic equipment. By contrast, electrophoretic displays tolerate wetness and repeated distortion, need no backlight, have good brightness, high contrast, good viewing angle and ultra low power consumption.
However, for years it has been hoped that organic light emitting diodes OLEDs, probably using printed polymeric light emitters and printed polymer or metal oxide transistor backplane drivers would be used for this. OLEDs emit light and have superb color and contrast, narrowest angle of view and fastest video response combined with safe, low voltages. Unfortunately, their chemicals remain extremely fragile and they need electricity even when the image is not changing as with the page of a book. Even their profitable use in television and mobile phones, where they can be protected by glass, continues to be difficult to achieve, absorbing billions of dollars of investment, despite slower, non-printed technology being used to get the show on the road.
Although LCD displays in television have become unprofitable for many suppliers, they are a huge success with about $100 billion annual sales and huge improvements in picture quality despite an inelegant brushing process being needed on the inside face of the glass sandwich to lock the twisted molecules on which they rely. This cretes problems of dust, static and reproduceability. Some can be bistable, needing no electricity when the image is fixed and thus preserving power as in today’s electrophoretic e-books but that brushing process is not possible with flexible film so the wind-up display is impossible with LCDs.
The desirable color video performance will not therefore be available with flexible e-books if they are made with LCD technology and we must settle for the black and white electrophoretic e-books expected within one year, followed by the rather muddy, slow responding color versions of electrophoretics currently being developed. Or so it was thought until recently. Now several developers have defied this logic by making experimental samples of flexible LCD displays and, although most have serious limitations as yet, they may provide a way forward. The developers have partnerships with major potenitial users such as Tesco, the UK’s largest supermarket chain and Epson.
One of the most promising advances comes from Professor Vladimir Chigrinov at Hong Kong University of Science and Technology, so we visited him to find out more for Printed Electronics World. We learnt that experimental LCD “electronic paper” as he calls it, is lightweight, flexible, thin, robust, durable and potentially low cost. In Part Two we shall look at how this is achieved and how it compares with other experimental forms of bistable, flexible LCD.