9 July 2009

Solar cell costs coming down

Solar power’s heavy cost continues to be the downside to any kind of growth as a potential energy source.

By thinking differently, that may soon change. Today’s solar industry mainly produces solar panels made from crystalline silicon, which is relatively expensive.

However, new players in the solar industry have instead been looking at panels that can harvest energy with copper-indium-gallium-selenide (CIGS) or CIGS-related materials. CIGS panels have a high-efficiency potential, may be cheaper to produce, and would use less raw materials than silicon solar panels. That is the upside. The downside is manufacturing of CIGS panels on a commercial scale has thus far proven to be difficult.

There is now a low-cost solution processing method in development for CIGS-based solar cells that could provide an answer to the manufacturing issue, said Yang Yang, a professor at the UCLA Henry Samueli School of Engineering and Applied Science.

“This CIGS-based material can demonstrate very high efficiency,” said William Hou, a graduate student on Yang’s team and first author of a study on the subject. “People have already demonstrated efficiency levels of up to 20%, but the current processing method is costly. Ultimately, the cost of fabricating the product makes it difficult to be competitive with current grid prices. However, with the solution process that we recently developed, we can inherently reach the same efficiency levels and bring the cost of manufacturing down quite significantly.”

The copper-indium-diselenide thin-film solar cell developed by Yang’s team achieved 7.5% efficiency in the published study but has in a short amount of time already improved to 9.13% in the lab.

“We started this process 16 months ago from ground zero. We spent three to four months getting the material to reach 1%, and today it’s around 9%. That is about an average increase of 1% every two months,” said Yang.

Currently, researchers manufacture most CIGS solar cells using vacuum evaporation techniques called co-evaporation, which can be costly and time-consuming. The active elements, copper, indium, gallium, and selenide, heat up and go onto a surface in a vacuum. Using vacuum processing to create CIGS films with uniform composition on a large scale has also been challenging.

The copper-indium-diselenide material created by Yang’s team does not need to go through the vacuum evaporation process. Their material simply dissolves into a liquid, then they apply it and bake it. To prepare the solution, Yang’s team used hydrazine as the solvent to dissolve copper sulfide and indium selenide in order to form the constituents for the copper-indium-diselenide material. In solar cells, the “absorber layer” (either copper-indium-diselenide or CIGS) is the most critical to performance and the most difficult to control. Researchers can take the copper-indium-diselenide layer, which is in solution form, and paint or coat it evenly onto a surface and bake it.

“In our method, material utilization is one advantage. Another advantage is our solution technology has the potential to be fabricated in a continuous roll-to-roll process. Both are important breakthroughs in terms of cost,” Hou said.

The team’s goal is to reach an efficiency level of 15 to 20%. Yang predicts three to four years before commercialization.

“As we continue to work on enhancing the performance and efficiency of the solar cells, we also look forward to opportunities to collaborate with industry in order to develop this technology further,” Yang said. “We hope this technology will lead to a new green energy company in the U.S., especially here in California so that it may also bring job opportunities to many who need it.”

For related information, go to www.isa.org/manufacturing_automation.