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.