The SiO2 nanoparticles (black) have been imprinted directly on the Molybdenum substrate (purple)
which corresponds to the back contact of the solar cell. On top of this structured substrate the
ultrathin CIGSe layer (red) was grown at HZB, and subsequently all the other layers and
contacts needed for the solar cell. Since all layers are extremely thin, even the top layer
is showing deformations according to the pattern of the nanoparticles. Credit: G.Yin / HZB
(October 16, 2015) CIGSe solar cells are made of a thin chalcopyrite layer consisting of Copper, Indium, Gallium and Selenium and can reach high efficiencies. Since Indium is becoming scarce and expensive, it is interesting to reduce the active CIGSe layer, which however decreases the efficiency quite strongly. Now, scientists at Helmholtz-Zentrum Berlin have produced high quality ultrathin CIGSe layers and increased their efficiency by an array of tiny nanoparticles between the back contact and the active layer.
Nanoparticles with sizes the order of a wavelength interact with light in specific ways. A young investigator group at Helmholtz-Zentrum Berlin, led by Professor Martina Schmid, is inquiring how to use arrangements of such nanoparticles to improve solar cells and other opto-electronic devices. Now the scientists report in the Journal of the American Chemical Society ACS Nano a considerable success with ultrathin CIGSe solar cells.
Problems add up below 1 micrometer
CIGSe solar cells have proven high efficiencies and are established thin film devices with active layers of a few micrometers thickness. But since Indium is a rare element, the active layer should be as thin as possible. This reduces the efficiency, since less light is absorbed. And if the active layer is thinner than one micrometer, an additional problem arises: more and more charge carriers meet and recombine at the back contact, getting “lost”.
Ultrathin CIGSe cell with efficiencies of 11.1%
“It took me more than one year to be able to produce ultrathin layers of only 0.46 micrometer or 460 nanometers which still reach reasonable efficiencies up to 11.1 %,” Guanchao Yin says about his PhD project. He then started to enquire how to implement nanoparticles between different layers of the solar cell. His supervisor Martina Schmid discussed this with Prof. Albert Polman, one of the pioneers in the field of nanophotonics, at the Center for Nanooptics, Amsterdam, with whom she was in contact for a while already. They proposed to produce arrays of dielectric nanoparticles by nanoimprinting technologies.