Solution-based processes such as spin or dip coating and inkjet printing are cost-effective ways to make large-area solar cells – for example, in inexpensive rooftop panels. These techniques require semiconductors in some soluble form that can be used as "ink". Semiconductor nanocrystals, which are tiny pieces of semiconductor material, are perfect candidates for such ink.

The problem, however, is that individual nanocrystals in printed arrays made using these techniques communicate poorly with each other. This is because surface ligands, consisting of bulky, insulating organic molecules, block electric charge transfer from one nanocrystal to another. The drawback has prevented nanocrystal materials from being more widely employed in solar cells and other devices.

Now, Dmitri Talapin of the University of Chicago and colleagues at the Lawrence Berkeley National Laboratory have overcome this challenge by developing a new chemistry that allows coupling of individual nanocrystals into arrays of strongly communicating building blocks. "Our approach provides a versatile platform for materials design and could impact electronics, photovoltaics and thermoelectrics," Talapin told "The possibility of all-solution device fabrication also makes it appealing for roll-to-roll applications such as manufacture of thin-film solar cells."

The researchers "glued" colloidal nanocrystals together using a class of compounds called molecular metal chalcogenide complexes. The ligands are more stable and robust than the previously employed organic ligands and do not alter the chemistry of the nanocrystals. They also allow efficient charge transfer between the nanocrystals. Indeed, Talapin and colleagues have observed dramatic increases in electrical conductivity in the systems that they have explored so far.

The team is now looking at how to use the coupled nanocrystals in real applications and investigating materials other than metal chalcogenides. The University of Chicago has also licensed the underlying technology for thermoelectric applications to Evident Technologies.

The work was published in Science.