Media release
Dübendorf, St. Gall, Thun, March 24, 2011
Novel electrode for flexible thin-film solar cells
Collecting the sun's energy
Conventional silicon-based rigid solar cells generally found on the market are not suitable for
manufacturing moldable thin-film solar cells, in which a transparent, flexible and electrically
conductive electrode collects the light and carries away the current. A woven polymer electrode
developed by Empa has now produced first results which are very promising, indicating that the new
material may be a substitute for indium tin oxide coatings.
The scarcity of raw materials and increasing usage of rare metals is making electronic components and
devices more and more costly. Such rare metals are used, for example, to make the transparent electrodes
found in mobile phone touchscreen displays, liquid-crystal displays, organic LEDs and thin-film solar cells.
The material of choice in these cases is indium tin oxide (ITO), a largely transparent mixed oxide. Because ITO
is relatively expensive, however, it is uneconomic to use in large area applications such as solar cells.
The search for alternatives
Indium-free transparent oxides do exist, but with demand for them increasing they too are tending to
become scarce. In addition, the principal disadvantages such as brittleness remain. The search for alternative
coatings which are both transparent and electrically conductive is therefore intense, with materials such as
conductive polymers, carbon nanotubes or graphenes coming under scrutiny. Carbon-based electrodes,
however, generally show excessive surface resistance values which make them poor electrical conductors. If a
metallic grid is integrated into the organic layer, it reduces not just its resistance but also its mechanical
stability. If a solar cell made out of this material is bent, the electrode layers break and are no longer
conductive. The challenge thus consists of manufacturing flexible yet stable conductive substrates, ideally in a
cost-effective industrial rolling process.
One solution: woven electrodes
One particularly promising possibility is the use of a transparent flexible woven polymer, which Empa has
developed together with the company Sefar AG in a project financially supported by the Swiss Commission
for Technology and Innovation (CTI). Sefar, which specializes in precision fabrics, is able to produce the
Empa Überlandstrasse 129 CH-8600 Dübendorf Telefon +41 44 823 55 11 Fax +41 44 823 40 31
■ www.empa.ch
S e i t e 2/ 3
woven polymer economically and in large quantities using a roll to roll process similar to the way newspapers
are printed. Metal wires woven into the material ensure that it is electrically conductive. In a second process
step the material is embedded in an inert plastic layer which does not, however, completely cover the metal
filaments, thus retaining its conductivity. The electrode which results is transparent, stable and yet flexible.
The Empa researchers then applied a series of coatings to this new substrate to create a novel organic solar
cell whose efficiency is compatible to conventional ITO-based cells. In addition, the woven electrode is
significantly more stable when deformed than commercially available flexible plastic substrates to which a
thin layer of conductive ITO has been applied.
Literature reference
William Kylberg, Fernando Araujo de Castro, Peter Chabrecek, Uriel Sonderegger, Bryan Tsu-Te Chu, Frank
Nüesch and Roland Hany: Woven Electrodes for Flexible Organic Photovoltaic Cells, Adv. Mater. 2011, 23,
1015-1019, doi: 10.1002/adma.201003391
Further Informationen
Dr. Roland Hany, Funktionspolymere, +41 58 765 40 84, [email protected]
Editor
Rémy Nideröst, Kommunikation, Tel. +41 58 765 45 98, [email protected]
Empa Überlandstrasse 129 CH-8600 Dübendorf Telefon +41 44 823 55 11 Fax +41 44 821 40 31
■ www.empa.ch
S e i t e 3/ 3
Flexible precision fabric which, in cooperation
c
w
with the Swisss company Sefar AG, waas developed
d into an
electrode for thin-film
m solar cells.
Circular organic solar cells manufactured in thee laboratory. Red-brown: active organnic layer; silve
er-grey: top
electrode.. Clearly visib
ble in the pho
oto are the w
woven metal wires.
Cross-secction of a thin
n-film solar cell
c with a wo
oven electrod
de.
The text and
d images are ava
ailable in electro
onic form from: [email protected]
Empa Überlaandstrasse 129 CH-8600
C
Dübendorf Telefon +441 44 823 55 11 Fax +41 44 821
1 40 31
■ www.empa.ch