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Making gold act more like platinum improves organic devices

Organic semiconductors are a class of materials made by organic chemistry techniques that have electronic properties similar in some ways to technologically important materials like silicon. Semiconducting polymers are particularly interesting technologically, because they hold out the promise of inexpensive "plastic electronics", circuitry that can be patterned by ink-jet printing onto flexible backing. Much remains poorly understood, however, about the basic physics of charge motion in these organic materials.

Making good electrical contact between metals and organic semiconductors is challenging, since the physics models developed for junctions between metals and inorganic semiconductors do not apply. In recent research (Hamadani et al., Phys. Rev. B 72, 235302 (2005)), Prof. Natelson's group and collaborators at the University of Rochester showed that platinum electrodes make excellent contacts for hole-type conduction in polythiophene, a common polymer semiconductor. Under elevated temperatures, platinum contacts remained highly effective long after gold contacts became practically insulating. This is believed to be due to the strong binding of electrons to platinum: it requires significantly more energy to remove an electron from platinum than from gold.

Now, collaborating with the Tour group in the Rice Department of Chemistry, Hamadani et al. have shown that simple surface chemistry may be used to increase the binding energy of electrons in gold, making the resulting surface electronically resemble platinum. Contacts between the modified gold electrodes and polythiophene now act like the platinum contacts described above, and greatly outperform untreated gold. The basic idea of manipulating metal properties with surface chemistry has long been known, but this work is the first systematic demonstration and characterization of this process in organic transistors.

Graphs of current vs. voltage for a polythiophene transistor at a fixed gate potential, using gold electrodes modified by self-assembly of the pictured molecule. Curves from top to bottom are the results of successive heat treatments. While the currents do drop with subsequent heating, the curves remain linear (slope of 1 on log-log plot, blue solid line), in dramatic contrast to devices made with untreated electrodes.