for flexible application
Solar cell designs traditionally are based on crystalline silicon, but not only has that material been in short
supply, the silicon-solar process is relatively expensive. And as the solar industry
strives to decrease the cost to make it a
viable alternative to fossil fuels, many are
turning an eye toward panels with copper
indium gallium selenide (CIGS) as an
Although CIGS cells have proved efficient and have the potential to cost less, a
low-cost production method has eluded the
industry. Researchers at Henry Samueli
School of Engineering and Applied Science at the University of California, Los
Angeles (UCLA), are aiming to change
that by developing a low-cost production
method for solar cells based on CIGS.
The group, led by Yang Yang, a professor in the department of materials science
and engineering, recently published a study
in the journal Thin Solid Films that describes a low-cost method for manufacturing on a large scale. The study reports the
efficiency at 7. 5 percent, but the team has
surpassed that, improving to 9. 3 percent.
Doctoral candidate William Hou works with the
UCLA team that is developing a low-cost production
method for CIGS solar cells.
The dissolution method
Key to the method is the fact that it does
not use a vacuum evaporation process.
Most CIGS solar cells are produced by
heating each of the active elements and
depositing them onto a surface in a vacuum. This “co-evaporation” method can be
costly and time-consuming, Yang said.
Instead, the investigators dissolved the
materials into a liquid, applied it onto a
substrate and baked it. They had been
dissolving organic materials for both LED
and solar cell applications, and it was only
recently that they applied this concept to
inorganic materials such as CIGS.
They used hydrazine to dissolve the
copper sulfide and indium selenide to
form the constituents for the copper
indium sulfur selenide. According to Yang,
they also can dissolve gallium, but they
left if out to simplify the material system
for research purposes. He also said
gallium may be replaced by sulfur because
of cost, adding that gallium costs 500
times more than sulfur.
Not only did they find their method of
liquefying the materials cheaper and easier
than the vacuum method, but the materials
can be applied to various surfaces, including film that can be manufactured in a
Yang said that, even though the material
system is unchanged, “the quality of the
material is very different between the conventional methods and our process.” The
challenges have been in understanding the
type of defects that result from the solu-tion-processing method “and to either
eliminate or passivate the defects.”
As far as efficiency goes, he said they
are seeing an increase of about 1 percent
every two months and expect to reach
15 to 20 percent within a few years.
Currently, the best CIGS method achieves
about 20 percent but is more costly and
challenging to produce and cannot be
applied to the range of surfaces that the
new UCLA method can.
A flexible future
The significance of this work is the
potential of using a flexible insulating
material such as polyimide, which cannot
tolerate the traditional method of processing CIGS. “Most demonstrations of flexible CIGS solar cells are done on metal
foils,” Yang explained, adding that “this
creates various problems.” The low-temperature process offers a way to fabricate CIGS on polyimide without serious
degradation, he said.
Yang expects to see commercial products based on this method in three or