It’s possible to print large, flexible arrays of cheap, plastic transistors to drive displays. But the performance of these organic electronics is still not consistent enough for commercial devices. A new method for printing a wide variety of semiconducting organic compounds such as polymers is much more reliable–and on top of that, it improves the performance of a wide variety of these materials by a few orders of magnitude.
Smooth going: This thin film, made of a polymer consisting of silane and a hydrocarbon tail, is very smooth, which makes it a good substrate for growing high-performance organic electronics. Very few bumps can be seen in this atomic-force microscope image.
Organic electronics are cheaper than silicon-based electronics, but they tend to have lower performance. So they are being developed for applications where large area is important but performance doesn’t need to be as high as it does inside a computer processor. What has kept these devices out of commercial products has been the difficulty of manufacturing them consistently.
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In particular, the quality of the layered thin films used to make organic transistors often varies at the molecular level. The smoother and more regular the films are, the better they are at conducting electrons. So, Zhenan Bao, associate professor of chemistry at Stanford University, has developed a set of techniques for making consistently smooth, high-performance organic films.
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“This work is very important in terms of getting us to market with high-performance devices,” says Vitaly Podzorov, professor of chemistry at Rutgers University.
Bao’s group makes better organic transistors by focusing on the layer that lies directly underneath the electron-carrying semiconductor. In these devices, an electric current flows at the interface between the semiconductor and this underlying layer. Though the underlying layer is insulating, the properties of the interface between it and the semiconductor determine how fast electrons can move through the device. If it’s rough, electrons can get trapped, cutting into device performance.
Bao makes this layer by spin-coating a film of a polymer made up of the insulator silane trailed by a long hydrophobic carbon tail. “It self-assembles, and after cleaning and treating, you end up with a single layer that’s highly ordered,” says Bao. Her group then deposited several organic semiconductors on top of these surfaces and found that they also grew in regular, smooth layers. “On a disordered surface, they grow like islands instead of planes,” leaving holes that impede electron flow, she says. What makes her method work so well, she explains, is the structure of the molecules in the insulating layer. “We think that by having very densely packed hydrophobic tails on the surface, we lowered the barrier to semiconductor assembly.”
The Stanford researchers then tested the performance of these devices. When pentacene, one of the most commonly used organic semiconductors, is deposited on the new surface instead of on a conventional one, its ability to carry an electrical charge jumps up by two orders of magnitude. Other semiconductors that Bao’s group tested showed similar performance gains.”People would kill for a twofold improvement in performance, let alone tenfold,” says Hagen Klauk, head of the organic electronics group at the Max Planck Institute for Solid State Research in Stuttgart, Germany. More importantly, he points out, Bao “can make a really good self-assembled layer every time.”
Such consistency is vital, agrees Do Hwan Kim, a researcher in the display-device group at the Samsung Advanced Institute of Technology in South Korea. “For the application of organic semiconductors into the commercial display market, it is crucial to obtain reproducibility and reliability,” he says.
Bao says her technique is simple and should be scalable to large areas and applicable to other stubstrates, although the Stanford researchers haven’t yet made the devices on flexible backings. Now Bao’s method must be proven on a large scale.
