Empa’s flexible CIGS approaches Si, CIGS/glass mark

A new 18.7% efficiency mark nearly approaches that of both Si- and CIGS/glass-based solar cells, the group says, thanks to an improved low-temperature process and Na doping.

May 19, 2011 – Scientists at the Swiss Federal Laboratories for Materials Science and Technology (Empa) have hiked the conversion efficiency of their flexible CIGS solar cells to a new record (and Fraunhofer-verified) 18.7%, besting their 17.6% benchmark from last summer.

The group, working with startup Flisom, has developed a low-temperature (<450°C) growth process for CIGS layers. In their latest work, they managed to reduce recombination losses by improving the structural properties of the CIGS layer and the low-temp process, plus in-situ doping with Na during the final growth stage. Efficiencies of 17.5% had been achieved on steel foils covered with impurity diffusion barriers using higher-temperature (>550°C) processes; the new low-temp process on the same steel foil without a diffusion barrier “easily matched the performance” of the higher-temp one, resulting in an efficiency of 17.7%, Empa says — indicating that commonly used barrier coatings could be eliminated.

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Improvement in energy conversion efficiency of flexible CIGS solar cells on polymer film. (Source: Empa)

“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,” said Ayodhya N. Tiwari, research team leader in Empa’s Laboratory for Thin Film and Photovoltaics, in a statement.

“With these results, polymer films have for the first time proven to be superior to metal foils as a carrier substrate for achieving highest efficiency,” the researchers say, noting that >16% efficiency now “should be achievable” in monolithically connected flexible CIGS modules.

The work is supported by the Swiss National Science Foundation (SNSF), the Commission for Technology and Innovation (CTI), the Swiss Federal Office of Energy (SFOE), EU Framework Programmes as well as by Swiss companies W.Blösch AG, and Flisom.

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