Promise of Nanowires in Optoelectronics Realized By Getting Them Connected

Researchers fabricate a photovoltaic device with hundreds of organic nanowires connected to nanoelectrodes

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Organic nanowires bound for supramolecular electronics
Photo: University of Strasbourg/Nature Nanotechnology

Supramolecular electronics has been solidifying as the bridge between molecular electronics—in which molecules become the basic building blocks of electronics—and the use polymers for the fabrication of nanoscale circuitry. A supramolecule is actually a number of different molecules that are fused together to act as a single molecule and carry out a particular programmed function. These supramolecules are used, for instance, in block copolymer-based supramolecular solutions that direct the self-assembly of nanoparticles.

Now researchers at the University of Strasbourg and the Le Centre National de la Recherche Scientifique (CRNS) in France, along with collaborators from the University of Nova Gorica in Slovenia, have buoyed the prospects of supramolecular electronics by addressing one of the chief problems of supramolecular organic nanowires for optoelectronics: getting them connected.

In research described in the journal Nature Nanotechnology, the international team of researchers fabricated a photovoltaic device in which they managed to connect and integrate hundreds of organic nanowires.

This is a significant achievement, because supramolecular organic nanowires have long tantalized researchers in the field of optoelectronics. Their highly efficient generation of excitons—essentially energized electrons that are formed when light hits a semiconductor—make devices like solar cells very sensitive to light and boost their light absorption coefficient.

While this is indeed tempting, the rub has been that you couldn’t harvest that photocurrent from the supramolecular nanowires unless they were connected to nanoelectrodes (anodes and cathodes) that carry out different functions. And the best anyone had previously been able to do was connect just a few. Thus, these nanowires were limited to use in very rudimentary devices.

To the rescue is this new approach: a nanomesh scaffold that supports and connects the supramolecular nanowires between two nanoelectrodes with different work functions.

The researchers were able to fabricate this nanomesh scaffold using a technique known as nanosphere lithography, in which nanoscale spheres are used as a mask to fabricate nanoparticle arrays. The result is a nanomesh comprising millions of hole-shaped nanoelectrodes patterned into a hexagonal array with channel lengths less than 100 nanometers.

By using a commercially available n-type organic semiconductor that self-assembles into supramolecular nanowires in combination with the nanomesh scaffold so that the nanowires become connected to the nanoelectrodes, the researchers have been able to fabricate a photovoltaic device with very promising characteristics.

One of the attractive properties of the device is that the polymer/nanowire p-n junction provides a fast photoresponse because the anode and cathode are quite close together. Another feature of this device is that it is possible to chemically modify the anode and cathode separately. This enables tailoring of interfaces that, in turn, makes it possible to replace calcium and aluminum cathodes. It also makes the use of transparent electrodes such as indium tin oxide unnecessary.

In future research, the team intends to optimize the device in a number of different areas, such as using polymer thin films to impart flexibility . The researchers are also considering the use of thinner dielectric layers to increase the photocurrent.

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