New organic transistors can make electronic displays bendable and nearly unbreakable
Wrap-around smartphones and roll-up computer tablets could soon be coming to a store near you. A British electronics firm has created a plastic transistor. That could make possible a host of devices with flexible electronic displays.
To illustrate the possibilities, Plastic Logic showed off a flexible smartwatch with its new transistors at the Society for Information Display meeting last week in San Diego, Calif.
Like other digital displays, a flat plane of transistors sits underneath the light-up layer of the new smartwatch screen. Those transistors tell individual pixels on the plane above when and how much to light up. The arrangement of those pixels creates the pictures and text on a screen.
Both layers in the new smartwatch screen contain organic semiconductors. These are carbon-based materials that sometimes conduct electricity.
Organic light-emitting diodes, or OLEDs, form the display’s light-up layer. The OLEDs allow electricity to pass through organic semiconductors. Along the way, those materials release energy as light.
Engineers had already made some flexible OLEDs. The big advance for Plastic Logic’s display is the flexible plane of transistors.
Digital electronic displays need “at least one transistor behind each pixel to act as a switch to turn that pixel on and off,” explains Paul Cain at Plastic Logic. The physicist is marketing director for the company, which is based in Cambridge, England.
Until now, commercially available transistors have been made on glass sheets. That’s because those earlier transistors were made from materials such as silicon. And making those transistors requires high temperatures.
Polymers are long chain molecules, often a building block of plastics. “What we’ve done is create a completely new kind of transistor that’s made out of plastic — out of polymers in solution,” explains Cain.
The transistors can be printed onto a surface, so that they coat it, using a process that takes place at room temperature. It is “a bit like spreading out cooked spaghetti onto a sheet,” notes Cain. The process makes very uniform layers.
Plastic transistors can make displays “virtually unbreakable,” he says. “You can put them in objects and not worry about dropping them in the way that you do with glass displays.” Goodbye, bulky cases for phones and tablet computers!
Displays also can be as thin as several cellophane sheets stacked atop one another. A whole mobile phone screen could wrap around the wrist like a thin piece of plastic. Other flexible gadgets become possible too.
Plastic Logic’s demo device “is elegant and impressive,” says Qibing Pei. “What is important here is that organic transistors can be made flexible over large areas.”
Pei works as a materials scientist and engineer at the University of California Los Angeles. Last year, he and his colleagues developed a stretchable OLED light that “is more flexible than skin.” They described their new development last fall in the journal Nature Photonics. Pei’s group also works on stretchable transistors.
Plastic Logic’s transistor sheets are able to bend with curves “well under 1 millimeter radius of curvature,” says Cain. For perspective, a sharp pencil point measures about 1 millimeter (or about 4 one-hundredths of an inch) across. What this means, Cain says, is that an OLED display effectively could be folded in half without damage.
Controlling distortion was a challenge in developing the new transistors. Plastic Logic prints its transistors as a series of layers. “We put down one pattern, and then we put down a spacer layer. And then we put down another pattern on top,” Cain explains.
Features of each pattern measure just a few micrometers (hundred-thousands of an inch). All patterns must line up precisely. If not, the pixels won’t display properly.
Flexible OLED devices like Plastic Logic’s sample smartwatch could be in stores within two to three years, says Cain. Other types of roll-up devices might go in your back pocket. A computer tablet could unfold from a small envelope size.
But that will still take a while because some challenges remain. For instance, those flexible displays will need some sort of coating or barrier to protect them from the damaging effects of moisture and air. “Barrier films currently available have limited flexibility,” Pei notes.
Cain agrees. But he also notes that other companies are working on that issue. Solving the barrier problem and other challenges will open up the possibility of many new types of electronic products.
“With plastic transistors, you can really unlock design features that just haven’t been possible” until now, Cain says.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
diode An electronic part that works like a one-way valve for electric current.
electric current A flow of charge, called electricity, along a given path. This usually results from the movement of negatively charged particles, called electrons.
electrode An electric conductor through which current leaves or enters an object, substance or region.
electroluminescence The process by which the passage of an electric current through some semiconductors will discharge some energy as visible light.
electronics Devices that are powered by electricity but whose properties are controlled by the semiconductors or other circuitry that channel or gate the movement of electric charges.
engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.
LED (light-emitting diode) A type of semiconductor device that produces light.
micrometer (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s equivalent to a few one-hundred-thousandths of an inch.
organic light-emitting diode (or OLED) A type of digital lighting technology that uses a semiconductor to produce illumination.
organic In chemistry and materials science, a term that means something contains carbon. (Not to be confused with the more general definition for organic, which refers to living or once-living things. In food, organic also can mean “natural.”)
photonics Technology and research on the properties and transmission of light particles, called photons.
physicist A scientist who studies the nature and properties of matter and energy.
pixel Short for picture element. A tiny area of illumination on a computer screen, or a dot on a printed page, usually placed in an array to form a digital image. Photographs are made of thousands of pixels, each of different brightness and color, and each too small to be seen unless the image is magnified.
polymer Substances whose molecules are made of long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics.
radius A straight line from the center to the circumference of a circle or sphere.
semiconductor A material that sometimes conducts electricity. Semiconductors are important parts of computer chips.
silicon A nonmetal, semiconducting element used in making electronic circuits. Pure silicon exists in a shiny, dark-gray crystalline form and as a shapeless powder.
smartwatch A watch that can perform a host of functions, including search for information on the Internet.
tablets (in computing) A small, hand-held computer that can connect to the Internet and that users can control using a touch screen. An Apple iPad, Samsung Galaxy and Amazon Kindle Fire are all examples of tablets.
transistor A device that can act like a switch for electrical signals.
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Original meeting source: P. Cain. “Flexible displays — portable & wearable.” Society for Information Display. Display Week 2014. June 5, 2014. San Diego, Calif.
Mike Banach.” Flexible AMOLED display driven by organic TFTs on a plastic substrate.” Society for Information Display. Display Week 2014. June 4, 2014. San Diego, Calif.
Original journal source: J. Liang, et al. Elastomeric polymer light-emitting devices and displays. Nature Photonics, Vol. 7, Sept. 22, 2013, p. 817. doi:10.1038/nphoton.2013.242.