Photoresponsive Supramolecular Networks Self-Assemble on Graphene

Although challenged by the poor ordering of the thin layers on top of the electrodes, organic photovoltaics have shown great promise for large-scale, cost-effective solar power generation.

Utilizing self-assembly on atomically flat, transparent substrates, ordered monolayers of molecular networks with photovoltaic responses could enable bottom-up fabrication of optoelectronic devices with molecular precision.

A scanning tunneling microscopic image of the topography of melamine-linked terrylene-diimide molecules. On the right, an inserted model of the molecular network (scale bar = 2 nm). Courtesy of C.A. Palma/TUM.

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 biomolecular 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, in France, have modified dye molecules in such a manner that allows them to serve as building blocks of self-assembling molecular networks.

The dye molecules self-assemble on the atomically flat surfaces of a graphene coated diamond substrate 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, say the researchers, the molecular network produces a photocurrent when exposed to light.

Sarah Wieghold (front) and Juan Li at the scanning tunneling microscope where the photoresponsive monolayers were characterized. Courtesy of Andreas Battenberg/TUM.

"For a long time, engineered self-assembled molecular architectures were looked upon as arty," says professor Friedrich Esch. "With this publication we present for the first time a serious, practical implementation of this technology."

The researchers now hope to scale up the device configuration and certify the photovoltaic response under standard conditions. Intercalating self-assembled dyes between stacks of 2D electrodes like graphene could allow easy scale-up to efficient photovoltaic monolayer elements, they said.

Using 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 diimide, the authors of the study determine which architectures can form.

The work, published in Nature Communications (doi: 10.1038/ncomms10700), was funded by the European Research Council (ERC Grants MolArt and Suprafunction, as well as the Graphene Flagship project); the German Research Council via the Clusters of Excellence Nanosystems Initiative Munich and Munich-Centre for Advanced Photonics; the China Scholarship Council; as well as the French Agence Nationale de la Recherche; and the International Center for Frontier Research in Chemistry.