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.