4 May 2010—A transistor that emits light and is made from organic materials could lead to cheaper digital displays and fast-switching light sources on computer chips, according to the researchers who built it. Small displays made from diodes of the same type of materials (organic light-emitting diodes, or OLEDs) are already in commercial production, but the transistor design could improve on those and lead to applications where OLEDs can’t go. The new organic light-emitting transistor (OLET) is much more efficient than previous designs. It has an external quantum efficiency—a key measure of how much light comes out per charge carrier pumped in—of 5 percent. An OLED based on the same material has a quantum efficiency of only 2 percent. Previous OLET designs had an efficiency of only 0.6 percent.
A transistor-based light source would switch much faster than a diode, and because of its planar design it could be more easily integrated onto computer chips, providing faster data transmission across chips than copper wire, says Michele Muccini, who heads a research unit at the Institute for Nanostructured Materials, part of the National Research Council in Bologna, Italy. His team, along with researchers from flexible electronics maker Polyera Corp., of Skokie, Ill., reported their findings this week in the online edition of Nature Materials.
The key to higher efficiency is a three-layer structure, with thin films stacked on top of one another. Current flows horizontally through the top and bottom layers—one carrying electrons and the other holes—while carriers that wander into the central layer recombine and emit photons. Because they’re segregated into their own layer of material, the recombined carriers, known as singlets, don’t run into other carriers, and their energy states change to the point where they won’t emit photons. Such quenching is one of the major limitations of OLED efficiency.
”It is fundamentally different from an OLED because you have the current flowing horizontally, so it doesn’t cross all the layers,” says Antonio Facchetti, chief technology officer at Polyera and a chemistry professor at Northwestern University, in Evanston, Ill., who designed the material.
To build their transistor, the team started with a glass substrate, on top of which they placed a layer of indium tin oxide as the transistor’s gate, then a layer of poly(methyl methacrylate), a common dielectric material. They then used vapor deposition to build their three-layer organic structure, which consisted of a 7-nanometer-thick film of an electron-transporting material, a 40-nm film of emissive material, and a 15-nm hole-transporting material. Finally, they added gold contacts on top as a source and a drain.
”Your emissive layer has to have an energy that is right in between the hole-transporting layer and the electron-transporting layer,” Facchetti says. For the emissive layer, he used a fluorescent polymer semiconductor commonly used in OLEDs, then searched for polymers with the right energy characteristics to match charge-transporting layers. The emitted light peaked at about 600 nm in the red. Facchetti says that if researchers can develop charge-transport layers with a wider band gap, they’ll be able to choose from a selection of wavelengths.
The light in the OLET is emitted as a stripe along the emissive layer, rather than up through the contacts as in an OLED. When light travels through the contacts, there is usually a loss of efficiency of 20 to 30 percent, Muccini says, a loss the transistor does not suffer. The shape of the emitted light would also make it easier to couple it into waveguides and other structures.
One area of weakness for the OLET is brightness, which Muccini would like to increase by an order of magnitude. Facchetti, who hopes to build a commercial device within three years, thinks brightness can be improved by altering the structure of the thin films so the current flows better.
”The light-emitting transistor is a remarkably versatile device architecture,” says Alan Heeger, a physics professor at the University of California, Santa Barbara. Heeger’s lab developed an OLET inverter circuit earlier, but its quantum efficiency was much lower. He called the 5 percent external quantum efficiency ”remarkable,” because it suggests that nearly 100 percent of the carriers in the emissive layer are emitting photons. ”If that is indeed true,” Heeger says, ”they have made an important step forward.”