Photonics Spectra - September 2009

Solar cell designs traditionally are based on crystalline silicon, but not only has that material been in shortsupply, the silicon-solar process is relatively expensive. And as the solar industrystrives to decrease the cost to make it aviable alternative to fossil fuels, many areturning an eye toward panels with copperindium gallium selenide (CIGS) as analternative.

Although CIGS cells have proved efficient and have the potential to cost less, alow-cost production method has eluded theindustry. Researchers at Henry SamueliSchool of Engineering and Applied Science at the University of California, LosAngeles (UCLA), are aiming to changethat by developing a low-cost productionmethod for solar cells based on CIGS.

The group, led by Yang Yang, a professor in the department of materials scienceand engineering, recently published a studyin the journal Thin Solid Films that describes a low-cost method for manufacturing on a large scale. The study reports theefficiency at 7. 5 percent, but the team hassurpassed that, improving to 9. 3 percent.

The dissolution method

Key to the method is the fact that it doesnot use a vacuum evaporation process.Most CIGS solar cells are produced byheating each of the active elements anddepositing them onto a surface in a vacuum. This “co-evaporation” method can becostly and time-consuming, Yang said.Instead, the investigators dissolved thematerials into a liquid, applied it onto asubstrate and baked it. They had beendissolving organic materials for both LEDand solar cell applications, and it was onlyrecently that they applied this concept toinorganic materials such as CIGS.

They used hydrazine to dissolve thecopper sulfide and indium selenide toform the constituents for the copperindium sulfur selenide. According to Yang,they also can dissolve gallium, but theyleft if out to simplify the material systemfor research purposes. He also saidgallium may be replaced by sulfur becauseof cost, adding that gallium costs 500times 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, includ-
ing film that can be manufactured in a
roll-to-roll process.

Yang said that, even though the material
system is unchanged, “the quality of the
material is very different between the con-
ventional 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 thepotential of using a flexible insulatingmaterial such as polyimide, which cannottolerate the traditional method of processing CIGS. “Most demonstrations of flexible CIGS solar cells are done on metalfoils,” Yang explained, adding that “thiscreates various problems.” The low-temperature process offers a way to fabricate CIGS on polyimide without seriousdegradation, he said.