Portland, Ore. -- Cornell University professor George Malliaras, a materials science engineer, is looking to capitalize on the ability of organic semiconductors to host mobile ions. By bonding mobile ions to the surfaces of these semiconductors in custom-designed ways, circuitry made from organic polymers can be optimized in ways that are impossible for silicon formulations, opening the door to new functionality, Malliaras said.
"Everybody [else] is making doped organic semiconductors," with junctions "exactly analogous to silicon junctions," said Malliaras. "But what we are doing is using mobile ions instead." Traditional doping is essentially a bulk-material operation, but designer ions could endow circuitry with electronic characteristics custom-tailored to their function.
Another advantage, said Malliaras, is that ionic junctions work at the slower time constant associated with diffusing ions. For instance, ionic semiconductor junctions need a bias voltage to put them into the proper operating region. With a change in bias voltage, a junction behaves differently. "We believe that ionic junctions can be programmable--that ionic and electronic mobilities can be used as a part of device architecture," he said. "Ionic junctions will enable circuitry that is reconfigurable by the prolonged application of a different bias voltage."
In tests, Malliaras reports that a forward bias on his organic p-n junction causes recombination of the injected holes and electrons, resulting in electroluminescence--photons generated to release the extra energy of hole-electron recombination. Even in the unoptimized prototype material used at Cornell, the electroluminescence was very bright--about 500 cd/m2 at 5 volts.
In a typical silicon p-n junction, a p-type semiconductor (doped with holes that are free to move) and n-type semiconductor (where electrons are free to move) are butted together. The charge carriers on each side diffuse across the junction, creating a depletion region. In ionic semiconductors, the same diffusion process happens--only with mobile ions instead of electrons or holes.
The main advantage is that instead of using high temperature and pressure to dope semiconductors, Malliaras just coats the surfaces of each polymer with different ions at room temperature and pressure.