Color Collective: Polymer self-assembles into light-emitting film

In the past year or so, organic light-emitting diodes have appeared in a handful of products, such as the tiny screens in some cell phones and digital cameras. Manufacturing large and long-lasting flexible displays for computer screens and flat-panel televisions, however, is expected to require new and improved organic materials.

To create displays that are more efficient and more luminescent than existing technologies based on organic substances, researchers are turning to organic molecules that self-assemble into cell membrane–like structures. Samuel Stupp and his colleagues at Northwestern University in Evanston, Ill., have now devised a way to enhance the electronic and optical properties of a polymer that is widely used for making organic light-emitting diodes.

In standard polymer films, molecules are randomly oriented. To avoid that, the researchers engineered the polymer molecules to automatically assume an organized liquid-crystal structure when mixed in water. The group attached different small appendages to the ends of the polymer molecules. They put a water-repelling molecule at one end and a water-attracting molecule on the other.

When mixed with water and poured onto a glass surface, the modified polymer molecules assembled into sheets that resembled cell membranes. Also, the sheets stacked on top of each to create a film hundreds of nanometers thick.

“By controlling the orientation of the molecules within the film, we can control the material’s conductivity and luminescence,” says the team’s James F. Hulvat. For example, because the film’s molecules are packed tightly and aligned, electrical charges in the resulting structure move through the material more efficiently than they do through disordered polymer films. At the same time, the highly ordered assemblies harbor fewer defects that can quench the material’s luminescence.

The researchers also found that varying the length of the water-attracting appendages caused the resulting films to produce different colors. Shorter chains resulted in films that glowed green, while longer chains made the films appear blue.

Scott Watkins of the University of Melbourne in Australia says that this is an important strategy for maximizing the efficiencies and lifetimes of organic light-emitting materials. “Without self-assembly, you’d have to go through multiple processing steps” to get the same result, he says.

The next step, says Hulvat, will be to produce an actual organic light-emitting diode, which glows when stimulated electrically. The Northwestern group is also working to make solar cells out of its new materials. By coating the hydrophilic segments of the film with a semiconductor, the researchers plan to create alternating layers of organic and inorganic materials.

The researchers presented their polymer films on Dec. 2 at the Materials Research Society Meeting in Boston. A detailed report will appear in an upcoming Journal of the American Chemical Society.