Molecular Architectures See the Light

Scanning tunneling microscopic topography of melamine linked terrylene-diimide molecules with inserted model of the molecular network (Scale bar: 2nm, source: C. A. Palma, TUM)

Scanning tunneling microscopic topography of melamine linked terrylene-diimide molecules (Scale bar: 2nm, source: C. A. Palma, TUM)

Organic photo­voltaics bear great potential for large-scale, cost-effective solar power generation. One challenge to be surmounted is the poor ordering of the thin layers on top of the electrodes. Utilizing self-assembly on atomically flat, transparent substrates, a team of scientists at the Technical University of Munich TUM has engineered ordered monolayers of molecular networks with photo­voltaic responses. The findings open up intriguing possi­bilities for the bottom-up fabrication of opto­electronic devices with molecular precision.

Nature is unrivaled when it comes to the self-assembly of complex, high-­performance molecular machinery for light absorption, exciton or charge separation and electron transfer. Molecular nanotechnologists have long dreamt of mimicking such extraordinary bimolecular architectures and rewiring them to produce inex­pensive elec­tricity. Now researchers from the Departments of Physics and Chemistry at the Technical University of Munich TUM, from the Max Planck Institute for Polymer Research in Mainz, Germany, and the Université de Stras­bourg, France, have modified dye molecules in such a manner that allows them to serve as building blocks of self-­assembling molecular networks.

On the atomically flat surfaces of a graphene coated diamond substrate the molecules self-assemble into the target archi­tecture in a manner akin to proteins and DNA nano­technology. The sole driving force stems from the engineered supra­molecular interactions via hydrogen bonds. As expected, the molecular network produces a photo­current when exposed to light.

“For a long time engineered self-assembled molecular architectures were looked upon as arty,” says Friedrich Esch, a lead author of the study. “With this publication we present for the first time a serious practical imple­mentation of this technology.” “In conventional organic photo­voltaics the improvement of molecular order is still a challenge. In contrast, the nano­technology toolbox provides us with the possibility of an atomically precise layout of the constituting components a priori,” says Carlos-Andres Palma, who co-supervised the study. “The possibility of full physicochemical control of the components gives us additional set-screws for functional optimization.”

The scientists now hope to scale up the device confi­guration and certify the photo­voltaic response under standard conditions. “Intercalating self-assembled dyes between stacks of two-dimensional electrodes like graphene, opens up the possibility of easy scale-up to efficient photo­voltaic monolayer elements,” claims Palma “This will put our work on the solar cell technology map.”

The scientists used terrylene-diimide molecules as photo­active dyes. The network is formed when the elongated terrylene molecules link up with trivalent melamine. By choosing adequate side groups for the terrylene divide the authors of the study determine which archi­tectures can form. “This work is an excellent example of the inter­disciplinary cooperation we seek to initiate with the institution of the Catalysis Research Center: a perfect match of chemistry and physics,” says Ulrich Heiz, director of the TUM Catalysis Research Center. (Source: TUM)

Reference: S. Wieghold et al.: Photoresponse of supramolecular self-assembled networks on graphene-diamond interfaces, Nat. Comm., online 25 February 2016, DOI: 10.1038/ncomms10700

Links: Cluster Catalysis and Advanced Spectroscopy (U.Heiz), Physical Chemistry, Technical University of Munich • Synthetic Chemistry, Max Planck Institute for Polymer Research, Mainz, Germany

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