Researchers seeking to improve solar absorption for the next generation of photovoltaic technology are exploring whether surface plasmon resonances of gold nanoparticles can augment the conversion of photons into usable energy.Although similar experiments have been carried out before, researcher Carl Hägglund of Chalmers University of Technology in Göteborg, Sweden, said that not one had been able to control the input of the plasmon excitations explicitly. He and his colleagues achieved control by designing nanoparticles with strong polarization dependence. Researchers used gold nanoparticle plasmons on TiO2 co-sensitized by dye molecules to harvest solar energy. Courtesy of Carl Hägglund.Their model used arrays of identical elliptical gold nanodisks fabricated with a JEOL electron beam lithography system. The disks had a height of 20 nm and minor and major axes of 40 and 120 nm, respectively. The array was deposited on top of a thin TiO2 semiconductor film that was sensitized further with dye molecules to mimic the Grätzel solar cell.Hägglund said that the identical size of the nanoparticles resulted in the well-defined plasmonic response, which could be controlled by the polarization of light because the plasmon resonances differed in the two in-plane directions. The excitations occurred on a femtosecond timescale and initiated an electron injection into the TiO2 conduction band. This stabilized the excited state and prevented the back transfer of energy to the nanoparticle. The researchers found that the technique resulted in a 60 percent enhancement of the charge carrier generation. They concluded that the plasmons were responsible for this enhancement because the system had no other sources of polarization dependence. One issue with the technique was that the dye gradually decomposed because of the lack of a hole conductor, which was problematic in combination with time-consuming measurements. Hägglund said that the team is working to improve the system by studying variations in its geometry.Applied Physics Letters, Jan. 7, 2008, 013113.