21 December 2011—Materials scientists have found a way to double or even quadruple the speed with which charge moves through organic semiconductors, potentially opening a path toward cheap, plastic 3-D TVs.
Organic semiconductors have been intensely studied because they can be printed onto flexible plastic to produce large areas of cheap, durable circuits. But these circuits have been limited because charge moves through them at a snail’s pace compared to the way it speeds through silicon.
The new technique, invented in the laboratory of Zhenan Bao at Stanford University, could lead to organic circuits that operate at frequencies up to four times as high as the best of today’s organic devices. That’s still barely one-hundredth the speed through crystalline silicon, but it would mean cheap printed organics could more easily substitute for amorphous silicon in displays and other gadgets.
Bao, an associate professor of chemical engineering, and her colleagues actually borrowed a trick called strain that is widely used in advanced silicon chips. Essentially, it involves stretching the semiconductor’s crystal lattice in one direction and pinning it in place.
Bao’s team worked with a widely researched organic semiconductor called TIPS-pentacene. You can think of it as a being shaped like a broad bar (the pentacene) with a ball on each side (the TIPS). Researchers like it because it can dissolve in solution, be printed like an ink, and then, when it dries, form crystals with relatively high charge-carrier mobility.
Pentacene is a p-type semiconductor, meaning that the charge is carried by holes, or electron vacancies. These holes hop from molecule to molecule when current is flowing, explains Bao. By putting the molecules under strain, Bao hoped to make them pack closer together and make it easier for the holes to hop. However, earlier attempts at adding strain could not get the molecules to hold their positions, because unlike silicon crystals, pentacene crystals are held together only by relatively weak forces.
Bao’s team induced strain in the material using a setup they’d developed years earlier. First they spread the semiconductor, dissolved in a solvent, over a temperature-controlled substrate. Then they used an angled plate, held at a fixed distance from the substrate, to smear the solution at a precise pace. As the knifelike shear plate moved along the surface, it pulled the molecules and induced the solvent to evaporate. The result was pentacene crystals strained in the direction of the moving plate.
”We hypothesized that shearing the solution would change the molecular packing,” says Bao. ”But we didn’t know which way it would change.” X-ray analysis of the material by Stefan Mannsfeld, a staff scientist at the Stanford Synchrotron Radiation Lightsource, showed that the TIPS-pentacene molecules were a little bit elongated and packed much more tightly in a crucial way.