Robust Molecule Powers Organic Electronic Devices

Robust Molecule Powers Organic Electronic Devices

RIKEN chemists have developed a molecule that improves the performance of organic electronic devices and is more stable than previous alternatives, increasing its chances of being used in industrial manufacturing

Traditional electronic devices are made of hard semiconductors such as silicon, but organic semiconductor molecules are increasingly appearing in devices that use organic light-emitting diodes (OLEDs), such as television and mobile phone screens.

“Organic electronic devices are strong candidates for thin, light, and flexible devices that cannot be easily realized using inorganic materials,” explains Kazuo Takimiya of the RIKEN Center for New Materials 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 electron-donating organic dopants are often unstable, making them difficult to design, synthesize and process, Takimiya says.

His team had previously studied derivatives of a molecule called tetraphenyl dipyranylidene, which can easily donate electrons to organic semiconductor materials. Now, they have made further modifications to the molecule to improve its stability at high temperatures.

The most promising modification added nitrogen-based amine groups, which push electrons into the central region of the molecule. Theoretical calculations showed that the resulting molecule, called DP7, has electrons at a high enough energy level. Experiments showed that it is also very stable and can be added to devices via vacuum deposition, one of the most widely used processes in semiconductor manufacturing.

The team incorporated DP7 into a variety of organic electronic devices, including an organic field-effect transistor (OFET), which consists of a thin film of buckminsterfullerene, or ‘buckyball’, 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 a much lower electrical resistance than previous variants of the dopant—in fact, it had one of the lowest resistances of any electron-doped OFET reported to date. This would increase the flow of electrons into the buckminsterfullerene.

Additionally, the device remained stable without showing any degradation even after being stored in an inert atmosphere for two weeks.

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

The researchers are now investigating other stable dopants that have even more electron-donating abilities.