Physics sheds light on light-emitting semiconductors

A recent study by an interdisciplinary team of scientists at Cornell University in New York has unravelled some of the fundamental physics of light-emitting, flexible semiconductors of the type used in electrochemical cells and displays. And the results are more than academic; they contain an important lesson in how such devices should be designed for optimal performance.

For some years the Cornell team has been interested in the molecular semiconductor ruthenium tris-bipyridine, which has potential for use in flexible light-emitting devices. Sensing, microscopy and flat-panel displays are among the possible applications.

George Malliaras and his colleagues wondered what happens to the material when it encounters an applied electric field, both inside the bulk of a film and also at its boundaries. To investigate this the researchers constructed an electrochemical cell from the ruthenium complex by spin-coating the material onto an insulating substrate with pre-patterned gold electrodes.

“Critical to this work was our ability to pattern the organic semiconductor and achieve the right device geometry,” says Malliaras. “We did this using a patterning technology developed at the Cornell NanoScale Facility.” The CNF is one of the US’ national nanotech research facilities, and is open to all with a feasible project and sufficient financial resources to carry it through.

Illustration of a ruthenium tris-bipyridine light-emitting device created by Cornell researchers

In this case the patterning involves laying down a gold electrode and a polymer called parylene. The ruthenium complex is then deposited on top of the parylene layer, filling in an etched gap between the gold electrodes. Finally, the parylene material is peeled away leaving behind a perfect ruthenium semiconductor device.

Using an electron force microscope to measure the electric field distribution, the researchers found that the field in the material is concentrated at the boundaries. This was not unexpected as it has been predicted by electrodynamics modelling, but there is no consensus within the research community on the electric field distribution.

If electric fields are concentrated at surfaces and edges of these semiconductor devices, this has clear implications for their design. “The ramification of the interfacial fields is that one should be careful in designing materials,” says Malliaras. “For example, if the counter ions are too small, then too many of them might accumulate near the interface and create too high electric fields that will degrade the material.”

The high interfacial fields also explain the ability of the devices to function efficiently when used with high energy barrier cathodes that would normally inhibit the free flow of charged particles. The energy barrier is reduced in width by the interfacial field, and as a result smaller counter ions can bunch up near the electrodes producing higher electric fields that help inject electrons and positively charged holes more efficiently.

“A number of groups have worked on such systems, and the simple expectation is that the electric field in the bulk of the semiconducting region should be close to zero with all the potential drop occurring near the contacts,” says Henning Sirringhaus, a Cambridge University physicist and founder of flexible electronics firm Plastic Logic. “The present work shows very nicely under which conditions this is indeed the case, and identifies patterning of the semiconductor as an important boundary condition in such devices.”

The researchers stress that the results apply to all solid-state ionic devices, and not just light-emitting electrochemical cells. Electron force microscope measurements on planar and patterned devices can help to quantify the effects, uncover the basic physics and improve the performance of mixed-conductor devices.

“This is a very nice application of electrostatic force microscopy to light-emitting electrochemical cells,” says Sirringhaus. “It demonstrates the power of this technique for nanoscale imaging of potential profiles in such devices.”

Further reading: Direct measurement of the electric-field distribution in a light-emitting electrochemical cell, Slinker et al., Nature Materials (2007).

Figure: Illustration of a ruthenium tris-bipyridine light-emitting device created by Cornell researchers. The ruthenium metal complex is represented by red spheres, and counter ions by green spheres. The material is sandwiched between two gold electrodes. Also visible is the probe of an electron force microscope used to measure the electric field distribution in the device. (source: George Malliaras/Cornell University).

Article first published in Nanomaterials News.