To prepare their modified organic photovoltaic materials, the team created a colloidal solution of silver nanoparticles. Organic capping groups stabilize the nanoparticles and prevent them from sticking together, Transmission electron microscopy reveals that the particles are highly uniform in size, with an average diameter of about 4 nanometres, and align into a regular mosaic with controlled spacing between them, which could be key to light absorption.

The team then applied this colloidal preparation to their organic bulk heterojunction photovoltaic devices, which are constructed from a polythiophene-fullerene material on an indium-tin oxide/glass substrate.

Paul Berger of Ohio State University and his team report [Berger et al., Solar Energy Mater Solar Cells, (2010) in press] that they get 70 amps per square metre with the silver nanoparticles but only 62 amps without the additive.

Their initial tests revealed that the nanolayer boosts optical absorption and photocurrent for the photovoltaic devices. This occurs because an increased electric field is induced in the photoactive layer by the excited localized surface plasmons of the silver nanoparticles. In other words, the nanotechnology essentially amplifies the signal by absorbing solar energy from a wider range of wavelengths.

Previous researchers have attempted to use capped silver nanoparticles to boost solar energy conversion efficiencies. However, those experiments were inconsistent, producing a broad range of nanoparticles sizes and so did not achieve the efficiency boost that Berger and his team have now observed.

Berger explains that implications of this development: “The light absorption of polymer solar cells is inadequate today,” he says, “The top-performing materials have an overall efficiency of about 5 percent. Even with the relatively low production cost of polymers compared to other solar cell materials, you'd still have to boost that efficiency to at least 10 percent to turn a profit.”

“By changing the organic coating, we could change the spacing of the particles and alter the size of each particle,” adds Berger, “By fine-tuning the mosaic pattern, we could move the enhanced absorption to different wavelengths, and thus get even more of an improvement. I think we can get several percent more.”