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MATERIALS SCIENCE
Bright cell phone and laptop displays that burden neither power supplies nor pocketbooks may soon become available. At Northwestern University, researchers have come up with a simple procedure for preparing films of conducting polymers with a high degree of order--a property that boosts the performance of the materials when used in electronic applications [Angew. Chem. Int. Ed., 42, 778 (2003)].
Now, graduate student James F. Hulvat and Samuel I. Stupp, professor of materials science, chemistry, and medicine, have shown that ordered films of conducting polymers can be prepared via an aqueous low-temperature technique using liquid crystals as templates to direct the orientation of the growing molecules. The team demonstrated the new method by electropolymerizing 3,4-ethyldioxythiophene in a liquid-crystal medium composed of surfactant molecules. The polymeric product, PEDOT, is commonly used to conduct positive charge in the hole-injection layer in organic light-emitting diodes (LEDs). "You always want to be able to control order in a material," Stupp says. For some properties, greater order is beneficial; for others, less order is needed, he notes. "But control is the key." Using optical microscopy methods, the team showed that the liquid-crystal template controls the polymer's structure so profoundly that, even after the template is washed away, the product looks just like a liquid crystal in terms of its domain structure and birefringence, an optical property characteristic of liquid-crystalline materials. To gauge the electronic properties of templated specimens, the Northwestern University scientists compared current flow through ordered PEDOT films with the flow through disordered PEDOT films, including commercially available materials. Films prepared via the new method deliver significantly more current at a given voltage than other PEDOT films, the team reports. Using standard methods, the group incorporated the new films into LEDs that emit intense blue light. Hulvat explains that the enhanced current flow corresponds to an increase in the luminosity of LEDs made from the ordered materials. With bright and efficient LEDs, he says, people will be able to see their cell-phone displays in the midday sun without draining their batteries. |
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