Tough Molecule Strengthens Organic Electronics

RIKEN chemists have developed a molecule that boosts the performance of organic electronic devices while being more stable than previous alternatives, raising the prospects for its use in industrial production processes.1.

Conventional electronic devices are made of hard semiconductors such as silicon, but organic semiconductor molecules are increasingly being used in devices such as televisions and mobile phone displays that use organic light-emitting diodes (OLEDs).

“Organic electronic devices are excellent candidates for thin, lightweight, and flexible devices that cannot be easily realized using inorganic materials,” explains Kazuo Takimiya of the RIKEN Center for Emerging Matter Science, who led the research.

But organic semiconductors need help from other molecules, known as dopants, to increase the flow of charge through them. For example, some dopants contain electrons at high energy levels that can be easily released into the semiconductor. But existing organic electron-donating dopants tend to be unstable, making them difficult to design, synthesize, and handle, Takimiya says.

His team had previously studied derivatives of a molecule called tetraphenyldipyranylidene, which could easily donate electrons to organic semiconductor materials. Now, by making further modifications to the molecule, they improved its stability at high temperatures.

The most promising change added nitrogen-based amine groups, which push electrons into the molecule’s central region. Theoretical calculations suggested that the resulting molecule, called DP7, had electrons at a sufficiently high energy level. Experiments showed that it was also very stable and could be added to devices via vacuum deposition—one of the most widely used processes in semiconductor manufacturing.

The team incorporated DP7 into several organic electronic devices, including an organic field-effect transistor (OFET) that consisted of a thin layer of buckminsterfullerene, or “buckyballs,” on top of a silicon-based substrate. They added ultrathin patches of DP7 to connect the buckminsterfullerene layer to gold electrodes.

The researchers found that the interface between buckminsterfullerene and DP7 had significantly lower electrical resistance than previous dopant variants—in fact, it had one of the lowest resistances of any electron-doped OFET reported to date. This would enhance electron flow into buckminsterfullerene.

Moreover, the device was stable and showed no signs of degradation after two weeks of storage in an inert atmosphere.

DP7 can be easily made from commercially available chemicals using just two chemical reactions, and Takimiya is optimistic that it could find industrial applications. “For commercial devices, it could be used to improve the conductivity of the electron transport layer in OLEDs that are made in vacuum processes.”

Scientists are now looking for other, stable dopants that will have an even greater ability to donate electrons.