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The electrical conductivity of inorganic semiconductors, such as silicon, which prevail in today’s microelectronic devices, can be tuned through the controlled incorporation of “impurity” atoms – a process called doping. Organic electronics, based on organic semiconducting materials, is an emerging alternative technology. IRIS-Member Norbert Koch and his colleagues at Humboldt-Universität and the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH have revealed the mechanism that governs the doping of organic semiconductors with dopants, i.e. “impurity” molecules. In contrast to previous suggestions that rely on direct electron hopping from one molecule to another, inter-molecular charge-transfer complexes between organic semiconductor and dopant are formed. These can generate mobile charge carriers after (thermal or optical) excitation, which in turn increases the electrical conductivity of the material. These new insights pave the way for the development of more potent molecular dopants to improve the efficiency of organic electronic devices.

Fig.: “Organic/dopant complexes“: Cartoon of a typical organic electronic device consisting of organic semiconductor layers and conducting electrodes. The magnification shows complexes formed by organic semiconductor molecules (circled in red) and molecular dopants (green line), which need to be excited to generate mobile charge carriers. (Image © by G. Heimel)


Intermolecular Hybridization Governs Molecular Electrical Doping

I. Salzmann, G. Heimel, S. Duhm, M. Oehzelt, P. Pingel, B. M. George, A. Schnegg, K. Lips, R.-P. Blum, A. Vollmer, N. Koch
Phys. Rev. Lett. 108 (2012) 035502
DOI: 10.1103/PhysRevLett.108.035502

One of the key challenges in current science and technology is to assemble functional nanostructures and exploit them in miniaturized devices and various applications. Covalent molecular nanoarchitectures offer the advantage of inherent stability, functional tunability, and increased charge transport capability. The method of on-surface polymerization, which allows the bottom-up construction of these nanoarchitectures, has been pioneered by the groups of Stefan Hecht (IRIS Adlershof, member of the CRC 951) and Leonhard Grill (FHI Berlin, member of the CRC 951) a few years ago (see Nature Nanotechnol. 2 (2007) 687) and was later used to prepare and characterize conjugated molecular wires (see Science 323 (2009) 1193). However, the degree of sophistication was rather limited thus far due to the involvement of single step processes only.
The same research team has now been able to greatly improve their method. In their most recent article in Nature Chemistry the authors demonstrate that the polymerization process can be carried out in a hierarchical fashion and further be directed by the underlaying substrate. On the one hand, the chemists designed suitable monomers, which carry two types of reactive groups allowing for sequential activation. These monomers with “programmed“ reactivity initially form one-dimensional chains, which subsequently “zip up“ in the second dimension (see Figure). On the other hand, the physicists utilized corrugated surfaces to orient monomers and intermediate chains and therefore direct the growth process. The new method greatly improves the quality of the formed nanostructures with regard to increased size as well as decreased defect density and allows for the construction of more complex structures from more than one type of building block.

Fig.: “Molecular zipper“: Monomers (red) initially polymerize at low temperatures into linear polymer chains, which subsequently interconnect at higher temperatures to a two-dimensional network. The “zipping“ step is facilitated by the pre-orientation of the activated groups (blue). (Image © by L. Lafferentz)













This work was carried out in the framework of the EU-Project AtMol and the CRC 951 and published on: 15 January 2012 in: Nat. Chem. 2012, 4,
DOI: 10.1038/nchem.1242