Flexible thin film CIGS solar cell on polymer substrate developed at Empa (Copyright: Empa) |
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.”
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 concepts.
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 low-temperature 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 low-cost
solar modules to take off.” Empa scientists are currently working
together with FLISOM to further develop manufacturing processes and to
scale up production.
Swiss Federal Laboratories for Materials Science and Technology (EMPA)
