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NIM nanosystems initiative munich

Wednesday, 24 April, 2013

Make way for electrons!

Transport through organic transistors

Electrons and holes should not encounter any obstacles in their pathways through solar cells and other semiconductor based devices. NIM – scientists have thus been investigating their transport through ultrathin organic structures during device fabrication. 

What do an espresso machine and a semiconductor have in common? Within both, tiny particles navigate through a complex maze. Steam has to penetrate the coffee powder at exactly the right pressure, while in semiconductors charge carriers have to find their way from electrode to electrode as easy as possible. Physicists and mathematicians call the existence of such a connected pathway from a source to its drain percolation. The group of NIM-physicist PD Dr. Bert Nickel (LMU) is engaged in the study of such percolation phenomena in organic transistors.

Delayed transport

Their current publication in the journal Advanced Materials presents charge transport of electrons and holes through ultrathin organic layers during their deposition processes. In the scope of their experiment the scientists begin with the fabrication of a hole conducting layer which consists of small aromatic molecules (pentacene). After finishing this first layer, a second molecule is deposited on top. These molecules are football shaped, consisting of 60 carbon atoms (C60-fullerene or „Buckyballs“), and form an electron conducting film (cf. image).  Simon Noever, the lead author of this work, and his colleagues expected percolation for electron flow after depositing only a single monolayer of C60. However, it took six layers of C60 molecules, until a corresponding electrical current could be observed. Taking a look at the surface with an atomic force microscope revealed that C60 forms small three dimensional droplets on the pentacene layer preventing the formation of connected paths from source to drain.

Floating Islands

The respective charging of the hole and the electron conducting layer upon contact can be tracked via gate voltage modulation. It was a surprise that this charging could only be observed after percolation. This leads to the assumption that the fullerene islands are potential-free before percolation.

Ambipolar components, i.e. electronic structures capable of conducting electrons and holes are key elements of organic solar cells. Such devices will benefit from the insights of the presented NIM research on percolation.

The objects of research, which in reality are nanometer scaled, are depicted in a demonstrative 3D-graphic. It was designed by the NIM - media designer and has lead to publication on the front cover of the printed edition (cf. image).   


Dual Channel Operation Upon n-Channel Percolation in a Pentacene-C60 Ambipolar Organic Thin Film Transistor. Simon J. Noever, Stefan Fischer, Bert Nickel. Advanced Materials, Volume 25, Issue 15, pages 2147–2151, April 18, 2013.


PD Dr. Bert Nickel
Lehrstuhl Prof. Dr. Joachim Rädler
Fakultät Physik der LMU München
Geschwister-Scholl-Platz 1
80539 München

Tel: +49 89 2180 1460
Email: nickel(at)lmu.de
Web: www.softmatter.physik.uni-muenchen.de/nickel_group


About NIM:

Dr. Peter Sonntag
General Manager

Phone: +49 (89) 2180 6794

Opens window for sending emailpeter.sonntag(at)lmu.de 


About science:

Isabella Almstätter

Public Outreach Manager

Phone: +49 (89) 2180 5091

Opens window for sending emailisabella.almstaetter(at)physik.uni-muenchen.de



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