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Schematic of a junction between two organic
semiconductors, an anthracene derivative containing free positive
ions and a ruthenium, complex containing negative ions. When the
two are joined, ions diffuse across the junction creating a
difference in energy levels that facilitates rectification,
electroluminiscence and photovoltaic response. For experimental
purposes the materials were sandwiched between electrodes made of
gold and indium tin oxide. The latter is transparent.
Malliaras lab/Cornell University
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The work is described in the Sept. 7 issue of
the journal Science in a paper by Cornell graduate researchers Daniel
Bernards and Samuel Flores-Torres, Héctor Abruña, the E. M. Chamot
Professor of Chemistry and Chemical Biology at Cornell, and Malliaras.
Semiconductors - organic or otherwise - are
materials that contain either an excess of free electrons (N-type) or
"holes" (P-type). Holes are spaces where an atom ought to have an
electron but doesn't, representing a positive charge. N- and P-type
materials can be joined to form diodes and transistors. The Cornell
researchers went a step further by making a diode out of organic
semiconductors that also contain free ions (molecules with an
electrical charge). They laminated together two organic layers, one
that contained free positive ions and the other negative ions. They
then added thin conducting films on the top and bottom; the top
conductor is transparent to allow light in and out.
Where the two films meet, negative ions migrate
across the junction to the positive side and vice versa, until an
equilibrium is reached. This is analogous, the researchers said, to
what happens in a silicon diode, where electrons and holes migrate
across the junction.
When a voltage is applied across the top and bottom
electrodes, a current flows through the junction in the form of
electrons moving one way while holes move the other way. The migration
of ionic charge across the junction causes a higher potential (voltage
difference) than normal, which affects the way electrons combine with
holes. This raises the energy of the molecules, which quickly release
the energy as photons of light. The junction shows "intense light
emission," the researchers said in their paper.
On the other hand, when a bright light is applied,
photons are absorbed by the molecules, causing them to kick out
electrons. The ionic charges create a "preferential direction" for the
electrons to move, and a current flows.
The collection of charges also allows electrons and
holes to move across the junction easily in one direction but only
weakly in the other, making the device a rectifier. It may be possible,
Malliaras said, to change the configuration of the ionic charge by
applying a voltage to the device, telling it whether to conduct or not,
so organic diodes might be used as components for computer memory.
Since the device was created by laminating together
materials that are flexible, large quantities could be manufactured
very cheaply by feeding two films together from rolls, Malliaras said.
The next step, he added, is to try modifying the metal content of the
semiconductors to make more efficient materials.
"There are tons of materials we can use," he said.
The research was supported by the National Science
Foundation, the Cornell Center for Materials Research, the New York
State Office of Science, Technology and Academic Research, the Office
of Naval Research, the Defense Advanced Research Projects Agency and a
Department of Defense fellowship. |
