Improving ‘plastic’ semiconductors to achieve flexible electronics

September 26, 2013
flexible semiconductor graphic

The yellow electric charge races through a ”speed lane” in this stylized view of a polymer semiconductor, but pauses before leaping to the next fast path. Stanford engineers are studying why this occurs with an eye toward building flexible electronics. (Credit: Professor Andrew Spakowitz)

Understanding how the molecular structure of polymers influences their electrical characteristics could hasten the advent of flexible electronics.

Flexible electronics could spawn new products: clothing wired to cool or heat, reading tablets that could fold like newspaper, and so on.

However, electronic components such as chips, displays and wires are generally made from metals and inorganic semiconductors — materials with physical properties that make them fairly stiff and brittle.

So in the quest for flexibility, many researchers have been experimenting with semiconductors made from plastics (polymers), which bend and stretch readily.

“But at the molecular level polymers look like a bowl of spaghetti,” says Stanford chemical engineering professor Andrew Spakowitz, adding: “Those non-uniform structures have important implications for the conductive properties of polymeric semiconductors.”

Flexibility (“spaghetti”) vs. conductivity (regular)

Spakowitz and two colleagues, Rodrigo Noriega, a postdoctoral researcher at UC Berkeley, and Alberto Salleo, a Stanford professor of Materials Science and Engineering, have created the first theoretical framework that includes this molecular-level structural inhomogeneity, seeking to understand, predict and improve the conductivity of semiconducting polymers.

Their theory deals with the observed tendency of polymeric semiconductors to conduct electricity at differing rates in different parts of the material — a variability that depends on whether the polymer strands are coiled up like a bowl of spaghetti or run relatively true, even if curved, like lanes on a highway.

In other words, the entangled structure that allows plastics and other polymers to bend also impedes their ability to conduct electricity, whereas the regular structure that makes silicon semiconductors such great electrical switches tends to make it a bad fit for our back pockets.

The Stanford paper in PNAS gives experimental researchers a model that allows them to understand the tradeoff between the flexibility and conductivity of polymeric semiconductors.

In the process of experimenting with polymeric semiconductors, researchers discovered that these flexible materials exhibited “anomalous transport behavior” or, simply put, variability in the speed at which electrons flowed through the system.

One of the fundamental insights of the Stanford paper is that electron flow through polymers is affected by their spaghetti-like structure — a structure that is far less uniform than that of the various forms of silicon and other inorganic semiconductors whose electrical properties are much better understood.

The Stanford theory includes a simple algorithm that begins to suggest how to control the process for making polymers — and devices out of the resulting materials — with an eye toward improving their electronic properties.

“A simple theory that works is a good start,” said Spakowitz, who envisions much work ahead to bring bending smart phones and folding e-readers to reality.