Empa research team sets new flexible CIGS efficiency record

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  Flexible thin film CIGS solar cell on polymer substrate developed at Empa. Image: Empa
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  Improvement in energy conversion efficiency of flexible CIGS solar cells on polymer film.

A team of scientists at Empa, the Swiss Federal Laboratories for Materials Science and Technology, led by Ayodhya Tiwari, has broken their own conversion efficiency record for flexible CIGS solar cells with an 18.7% efficiency measurement. The team previously set the 17.6% record in June 2010 and has had its new record independently certified by the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany.

"The new record value for flexible CIGS solar cells of 18.7% nearly closes the ‘efficiency gap’ to solar cells based on polycrystalline silicon (Si) wafers or CIGS thin film cells on glass", says Tiwari. He continued by expressing his belief that, "flexible and lightweight CIGS solar cells with efficiencies comparable to the ‘best-in-class’ will have excellent potential to bring about a paradigm shift and to enable low-cost solar electricity in the near future."

The Empa team worked with scientists at Flisom, to advance the low-temperature growth of CIGS layers to produce flexible CIGS cells that improved efficiency to 18.7% from 2005’s 14.1%. Empa researchers maintain that this is the highest score for any type of flexible solar cell grown on polymer or metal foil.

Empa scientists explained that they achieved the record efficiency by decreasing the recombination losses through the structural properties of the CIGS layer and used its proprietary low-temperature deposition process to grow the layers. Additionally, they used in situ doping with Na during the final stage, which they state led to the polymer films to achieve such a high efficiency. "Our results clearly show the advantages of the low-temperature CIGS deposition process for achieving highest efficiency flexible solar cells on polymer as well as metal foils", says Tiwari.

Empa noted “record efficiencies of up to 17.5% on steel foils covered with impurity diffusion barriers were so far achieved with CIGS growth processes at temperature exceeding 550°C.” However, when the team used the low-temperature CIGS deposition process it developed with Flisom on steel foil without a diffusion barrier, the performance was in line with that attained through the high-temperature procedure and was followed by a 17.7% efficiency. This has led the scientists to conclude, “that commonly used barrier coatings for detrimental impurities on metal foils would not be required.”

Empa Director Gian-Luca Bona acknowledge the achievement by stating, "What we see here is the result of an in-depth understanding of the material properties of layers and interfaces combined with an innovative process development in a systematic manner. Next, we need to transfer these innovations to industry for large-scale production of low cost solar modules to take off." Empa and Flisom are continuing their partnership to enhance the manufacturing processes and increase production.