Optoelectronic Devices Go Organic

Laurel Sheppard

Light-emitting diodes made from organic polymers were first developed at Cavendish Laboratory a decade ago. Now researchers at Cavendish have taken this technology a step further.

The light that is emitted from these devices is typically controlled by sandwiching a polymer layer between two mirrors made from inorganic materials to form a microcavity. Peter K.H. Ho and colleagues at the lab, whose work appears in the July 9 issue of Science, have used layered structures of semiconducting organic polymers to control the emitted light.

This development arose from research geared toward making an electrically driven polymer laser. The most popular laser structure used for electrical pumping is the waveguide laser, which is made of three layers, with the middle one having the highest refractive index. The challenge was to find a match of three polymers that would have both the right refractive index and the desired current transport properties.

The solution was to take a well-known polymer -- poly(p-phenylenevinylene), or PPV -- and tune the refractive index. To accomplish this, the team added SiO₂, a material with a refractive index of 1.45 at a wavelength of 550 nm, to the PPV, which has an index of 2.3 to 2.7. By varying the amount of SiO₂, the researchers could tune the refractive index of the composite. They have produced refractive indices from 1.6 to 2.7 by using levels of up to 50 percent by volume. To minimize scattering, the researchers selected particles of SiO₂ that were 5 nm in diameter and chemically doped them to maintain electrical conductivity.

Other polymer-based optoelectronics rely on inorganic layers to provide the refractive-index contrast. Typically, the polymer is spin-coated on a prepatterned inorganic material for optical contrast. To take full advantage of the polymeric materials, all or most of the components in a device should be polymerlike.

"Our mirrors are also made of conducting materials, while common distributed-Bragg-reflector mirrors used in conjunction with polymers are insulating, which complicates the design," said Nir Tessler, a principal investigator. "Namely, the mirror has to be coated by a conducting layer that degrades its reflectivity."

Although several major technical issues need to be solved before the technology is fully commercialized, the potential is enormous. "Our goal was to make a building block so that any electro-optical element can be made," Tessler said.