Though the solar industry today predominately produces solar panels made from crystalline silicon, the panels remain relatively expensive to make. New players in the solar industry instead have been looking at panels that can harvest energy with CIGS (copper-indium-gallium-selenide) or CIGS-related materials. CIGS panels have a high efficiency potential, may be cheaper to produce and would use fewer raw materials than silicon solar panels. However, manufacturing of CIGS panels on a commercial scale thus far has proven to be difficult.

Researchers at the UCLA Henry Samueli School of Engineering and Applied Science have developed a low-cost solution processing method for CIGS-based solar cells that could provide an answer to the manufacturing issue. In a new study to be published in the journal Thin Solid Films today (July 7), Yang Yang, a professor in the school's Department of Materials Science and Engineering, and his research team show how they have developed a low-cost solution processing method for their copper-indium-diselenide solar cells which has the potential to allow production on a large scale.

"This CIGS-based material can demonstrate very high efficiency," said William Hou, a graduate student on Yang's team and first author of the study. "People have already demonstrated efficiency levels of up to 20 percent, 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 percent efficiency in the published study but has in a short amount of time already improved to 9.13 percent 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 percent and today it's around 9 percent. That is about an average increase of 1 percent every two months," said Yang, also a member of the California NanoSystems Institute, where some of the work is being done.

Currently, most CIGS solar cells are produced using a vacuum evaporation technique called co-evaporation, which can be costly and time-consuming. The active elements -- copper, indium, gallium and selenide -- are heated and deposited onto a surface in a vacuum. Using vacuum processing for large-scale production of CIGS films with uniform composition also has been challenging.

The copper-indium-diselenide material created by Yang's team does not require the vacuum evaporation process. Their material simply is dissolved into a liquid, applied and baked. 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. The copper-indium-diselenide layer, which is in solution form, can easily be painted or coated evenly onto a surface and baked.

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

The team's goal is to reach an efficiency level of 15 to 20 percent. 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. 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," said Yang.

The study was funded in part by the NSF Integrative Graduate Education and Research Traineeship-Materials Creation Training Program.

The Department of Materials Science and Engineering is part of the UCLA Henry Samueli School of Engineering and Applied Science. Established in 1945, the school offers 28 academic and professional degree programs, including an interdepartmental graduate degree program in biomedical engineering. Ranked among the top 10 engineering schools at public universities nationwide, the school is home to five multimillion-dollar interdisciplinary research centers in wireless sensor systems, nanotechnology, nanomanufacturing and nanoelectronics, all funded by federal and private agencies.

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