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Print out: This plastic material is used as the backing for Phicot's amorphous silicon electronics. Credit: Phicot

Now a company called Phicot has adapted a technique for printing amorphous silicon electronics onto plastic that could finally make such displays practical. The manufacturing technique, already used to make cheap solar cells, involves depositing chemicals on long sheets of plastic as they are fed through a series of rollers. Phicot is a subsidiary of PowerFilm of Ames, IA, which already makes amorphous silicon solar cells using this roll-to-roll process.

"The basic technology of roll-to-roll can bring the price down and make plastic an excellent option for the back half of the display," says Frank Jeffrey, cofounder and CEO of PowerFilm.

Most modern displays rely on transistors made of polysilicon--a semiconductor that allows electrons to move fast enough for video. The problem with polysilicon is that it needs to be deposited at high temperatures that melt plastic. So Phicot turned to amorphous silicon, which can be deposited at low temperatures and yet is still fast enough to control the pixels of electrophoretic displays such as E-Ink, and eventually those in an organic light-emitting diode (OLED) display.

At Phicot's facility, layers of amorphous silicon and insulating materials are deposited onto plastic. These rolls of plastic are then sent to a facility at HP Labs, where engineers use a novel kind of lithography, called self-aligned imprint lithography (SAIL), to etch transistors onto the plastic's surface.

Once the transistors have been deposited, the screen itself must be added. HP has tested its transistors using E-Ink and with its own reflective display technology, capable of showing color and video. According to Carl Taussig, director of HP's information surfaces labs, the amorphous silicon transistors could, with slight changes to their chemical makeup, also drive OLEDs.

Phicot isn't the only company trying to make plastic-based displays. Polymer Vision, a spin-off of Philips, and Plastic Logic are both promising products in the near future. However, these devices will rely on transistors made of organic materials, which are easy to deposit on plastic at low temperatures, but operate more slowly than those made of amorphous silicon. While organic transistors are good enough to power electrophoretic displays, they are incompatible with OLEDs. Another company, Kovio, is aiming to print silicon on plastic using technology that resembles an ink-jet printer; the main applications at this point are RFID tags.
 

Another advantage of amorphous silicon over organic materials is that its electronic performance is well known. "Organics are less well developed," says John Rogers, professor of materials science at the University of Illinois, Urbana-Champaign. "[Organic components] have uncertain stability and limited long-term reliability; they also have not been used in large-scale applications."

The transition from thin-film solar to printing display transistors was relatively simple, says PowerFilm's Jeffrey. The major difference is that the layers are deposited in a different order. Some layers also need to be doped--adding impurities to change a layer's electrical properties--in a different way. "We used our solar-cell machines to do the first development work," he says. "It was very straightforward."

Phicot is planning to move to a facility in Silicon Valley by the end of the year to take HP's SAIL technology to a large space.

But the process is far from perfect. Jeffrey says that the engineers are quickly solving many of the defect challenges--the major obstacle to mass production at this point. For instance, the process must be fine-tuned so that the plastic isn't stressed in a way that could, down the line, result in defective circuitry.

Another challenge is making sure that the plastic is as defect-free as possible to start with. "Your plastic has to be smooth," says Alberto Salleo, professor of materials science and engineering at Stanford. "And as it goes through the rolls it shouldn't expand or crack."

Jeffrey says there may be ways to adapt the electronics to flaws in the plastic. "We have a possible option of designing our circuitry to tolerate those" he says.

Copyright Technology Review 2010.

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Two-way display: This image shows a detailed layout of the Fraunhofer display chip, which combines photodetectors with an organic light-emitting diode display. Credit: Fraunhofer Institute for Photonic Microsystems

Researchers at Fraunhofer Institute for Photonic Microsystems (IPMS) have now developed a screen technology that could help make wearable displays more compact and simpler to use. By interlacing photodetector cells--similar to those used to capture light in a camera--with display pixels, the researchers have built a system that can display a moving image while also detecting movement directly in front of it. Tracking a person's eye movements while she looks at the screen could allow for eye-tracking control: instead of using hand controls or another form of input, a user could flip through menu options on a screen by looking at the right part of the screen. The researchers envisage eventually integrating the screen with an augmented-reality system.

"We can present an image and, at the same time, track the movement of the user's eye," says Michael Scholles, business unit manager at Fraunhofer's IPMS. "This is of great interest for all kinds of applications where your hands are needed for something else, like a pilot flying an aircraft or a surgeon wanting to access vital parameters while performing a surgery."

Eye-tracking technology is nothing new, of course. Over the years, researchers have developed a number of systems that follow a person's gaze to allow him or her to interface with a computer. Often, the applications are for physically impaired people, but they can also be designed for a general computer user.

Additionally, researchers have been developing wearable display systems for years, but for the most part, these have been clunky, power hungry, and not entirely practical to use, says Alexander Sawchuck, a professor of electrical engineering at the University of Southern California. "Anything that can be done to make [wearable displays] more compact or lighter weight and low power is important," he says. And integrating a display and a camera on one chip is a step toward this, he says.

The researchers built the system by first designing a light-sensing chip, which features a pattern of evenly spaced photodetectors. This was then fabricated at a commercial semiconductor manufacturing facility. A wafer containing multiple chips was then placed in a deposition chamber, where layers of organic material were deposited in between the photodetectors. These layers make up the organic light-emitting diodes, or OLEDs, that create the display. The mosaic of photodetectors and OLEDs is then encapsulated in a thin polymer film to protect it.

The idea of integrating OLEDs with a photodetector chip is intriguing, says Sawchuck. "There are a lot of challenges in building wearable displays for the applications [intended by the researchers], and any advances in this field are very exciting," he says.
 

The Fraunhofer IPMS researchers will demonstrate their prototype at the Society for Information Display conference in San Antonio this week. The current version touts a simple monochromatic display--about 1.25 centimeters on each side, with a resolution of 320 by 240 pixels. Scholles says that full-color displays are possible but trickier to create because they require adding color filters to white OLEDs, which are difficult to make efficiently and aren't always reliable. However, the team at Fraunhofer IPMS has partnered with Novaled, an OLED company that manufactures high-quality white diodes, and plans to make future color prototypes using the company's diodes.

The camera in the researchers' current prototype is still fairly rudimentary. It has a resolution of only 12 pixels, which means that it can't yet track a user's eye movements. However, Scholles says that the team has developed a 160-by-120-resolution version of the camera chip that has been tested in the lab, but not yet integrated with a display. The researchers expect to have an advanced version of the system, complete with higher-resolution camera and full eye-tracking capability, by early 2011.

Copyright Technology Review 2009.