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Posted: May 19, 2011
Record efficiency of 18.7% for flexible CIGS solar cells on plastics
(Nanowerk News) Scientists at Empa, the Swiss Federal Laboratories for Materials Science and Technology, have further
boosted the energy conversion efficiency of flexible solar cells made of copper indium gallium
(di)selenide (also known as CIGS) to a new world record of 18.7% – a significant improvement over
the previous record of 17.6% achieved by the same team in June 2010. The measurements have been
independently certified by the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany.
It's all about the money. To make solar electricity affordable on a large scale, scientists and engineers
worldwide have long been trying to develop a low-cost solar cell, which is both highly efficient and easy to
manufacture with high throughput. Now a team at Empa's Laboratory for Thin Film and Photovoltaics, led by
Ayodhya N. Tiwari, has made a major step forward.
"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 is convinced 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."
Improvement in energy conversion efficiency of flexible CIGS solar cells on polymer film.
One major advantage of flexible high-performance CIGS solar cells is the potential to lower manufacturing
costs through roll-to-roll processing while at the same time offering a much higher efficiency than the ones
currently on the market. What's more, such lightweight and flexible solar modules offer additional cost
benefits in terms of transportation, installation, structural frames for the modules etc., i.e. they significantly
reduce the so-called "balance of system" costs. Taken together, the new CIGS polymer cells exhibit numerous
advantages for applications such as facades, solar farms and portable electronics. With high-performance
devices now within reach, the new results suggest that monolithically-interconnected flexible CIGS solar
modules with efficiencies above 16% should be achievable with the recently developed processes and
At the forefront of efficiency improvements
In recent years, thin film photovoltaic technology based on glass substrates has gained sufficient maturity
towards industrial production; flexible CIGS technology is, however, still an emerging field. The recent
improvements in efficiency in research labs and pilot plants – among others by Tiwari's group, first at ETH
Zurich and since a couple of years now at Empa – are contributing to performance improvements and to
overcoming manufacturability barriers.
Working closely with scientists at FLISOM, a start-up company who is scaling up and commercializing the
technology, the Empa team made significant progress in low-temperature growth of CIGS layers yielding
flexible CIGS cells that are ever more efficient, up from a record value of 14.1% in 2005 to the new "high
score" of 18.7% for any type of flexible solar cell grown on polymer or metal foil. The latest improvements in
cell efficiency were made possible through a reduction in recombination losses by improving the structural
properties of the CIGS layer and the proprietary low-temperature deposition process for growing the layers
as well as in situ doping with Na during the final stage. 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.
Record efficiencies of up to 17.5% on steel foils covered with impurity diffusion barriers were so far achieved
with CIGS growth processes at temperatures exceeding 550°C. However, when applied to steel foil without
any diffusion barrier, the proprietary low temperature CIGS deposition process developed by Empa and
FLISOM for polymer films easily matched the performance achieved with high-temperature procedure,
resulting in an efficiency of 17.7%. The results suggest that commonly used barrier coatings for detrimental
impurities on metal foils would not be required. "Our results clearly show the advantages of the lowtemperature
CIGS deposition process for achieving highest efficiency flexible solar cells on polymer as well as
metal foils", says Tiwari. The projects were 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.
Scaling up production of flexible CIGS solar cells
The continuous improvement in energy conversion efficiencies of flexible CIGS solar cells is no small feat,
says Empa Director Gian-Luca Bona. "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 lowcost
solar modules to take off." Empa scientists are currently working together with FLISOM to further
develop manufacturing processes and to scale up production.