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How current flows in organic semiconductors

Grid computing is helping material scientists to understand how electric charges are transmitted through organic materials.

Foldable Organic Light-Emitting Device (OLED)

A foldable Organic Light-Emitting Diode (OLED).

Image: meharris/wikicommons

 

Organic semiconductors (such as pentacene or naphthalene) hold great promise for innovation as an alternative to silicon, the dominant material in conventional electronics. First, they are easy and cheap to produce. Second, organic semiconductors have interesting mechanical properties (they can be bent without damage, for example), while retaining their electrical characteristics.

The practical applications of organic semiconductors can be found everywhere, in solar cells, transistors or light-emitting diodes (LEDs). But the electronic properties of these devices is still lagging behind conventional materials.

Nenad Vukmirović, a material scientist based at the Institute of Physics Belgrade in Serbia, studies how electrical current flows in organic materials. His goal is to understand the relationship between current and crystalline structure and see how they can be improved.

His recent work, published in the Physical Review Letters, used grid computing to calculate how electrons (carrying the electrical current) interact with phonon waves (formed by oscillations within the material). This interaction determines the properties of the electric current: the stronger the interaction, the weaker the current flow.

Nenad's research demonstrates that naphthalene and other organic materials are efficient semiconductors.

Calculating e-ph interactions on the grid

Nenad studied the problem using the example of naphthalene – the compound that gives mothballs their biting smell. Under the microscope, naphthalene is made up of fused pairs of hydrocarbon benzene rings, stacked together in piles.

Working with Vladimir Stojanović and Christoph Bruder, both based at the University of Basel, Nenad determined how current flows through naphthalene using simulations that take into account the positions of every atom and electron in the crystal lattice.

Like any other wave, phonons are characterised by their wavelength and it is possible to calculate the interaction of electrons with the different wavelengths separately. "This is where the grid infrastructure naturally comes in," explains Nenad. "The calculations are performed on separate processors for each wavelength and then assembled together to get the final result."

The main advantage offered by grid computing is time-saving: "we needed to perform many calculations and it would take too much time if I used just a single computer," he says.

Weak interactions, strong flows

For this work, Nenad submitted about 100 jobs to AEGIS, the Serbian National Grid Initiative and part of EGI.

Nenad and his colleagues concluded that the electron-phonon interaction in naphthalene is not strong enough to form polarons – a conceptual particle used to describe both the electron and the effect it causes in crystalline matter. This is further evidence of how efficient naphthalene can be as semiconductor.

"Our study provides a useful insight into the nature of charge carriers in organic molecular crystals," Nenad concludes. "Our work opens a way to properly model the current flow in these materials, which will in future certainly help us to improve their characteristics."

More information

References

N. Vukmirović et al. (2012) Electron-Phonon Coupling in Crystalline Organic Semiconductors: Microscopic Evidence for Nonpolaronic Charge Carriers. Phys Rev Lett. 109, 126407 (abstract)

Grid computing details

Nenad Vukmirović submitted 100 jobs to EGI, each using 250 CPUs and lasting about 6 hours. User support was provided by AEGIS, the Serbian National Grid Infrastructure.