OPVs are polymer-based devices that harness the Sun's energy. Although they are typically less efficient than conventional silicon photovoltaics at converting sunlight into useful photocurrent, the materials are more versatile and can be applied to flexible substrates. OPVs consist of active light-absorbing layers made up of interconnected networks of nanoscale semiconductor aggregates. However, controlling the composition of these structures is no easy task because of the numerous kinetic processes that occur during fabrication.

“Balancing these processes has been more of an art than a science until now,” explains team leader Dhandapani Venkataraman, “but our method provides the first rational and consistent way to tailor and organize each active layer component over multiple length scales (from the micron to the nanoscale).”

An added bonus is that the researchers have managed to replace the haloarene solvents traditionally used to fabricate OPVs with water, which makes things much more environmentally friendly – and, of course, cheaper.

Tuning sphere assembly

Venkataraman and colleagues say that they exploited the fundamental concept of sphere packing to fabricate multiscale structures, and assembled spherical nanoparticles of either poly(3-hexylthiophene) (P3HT) or 6,6-phenyl-C61-butyric acid methyl ester (PCBM). The spheres can aggregate into either ordered or randomly packed assemblies, and we tuned assembly by simply changing the number of spheres or their sizes, explains Venkataraman.

The technique allows the researchers to independently pre-assemble the semiconductor domains within each active light-absorbing layer and obtain stable, continuous structures in one single step. “We can also systematically alter how the semiconductor packs in the structure through changes in the radii of the nanoparticles or via interparticle interactions, or both,” Venkataraman tells nanotechweb.org. “And that is not all: we can employ multiple hole conductors in the active layer to broaden the range of light wavelengths over which the finished device absorbs.”

General method

The basic principles of geometric packing might be a general way to make fully functioning assemblies of nanoparticles and to fabricate materials with active layers containing more than one component, he adds. So the implications of our work go far beyond just making OPVs.

The team, which also includes scientists from Thomas Russell and Paul Lahti’s groups at Amherst, now plans to adapt its technique for low-bandgap polymers. Such polymers are reported to have higher light-to-power conversion efficiencies.

The present work is detailed in Nano Letters.