Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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Fig. 7: J-V curve and comparison with world record cell (left) and external quantum efficiency (right)
of the best CIGS solar cell on flexible polyimide substrates with TiN/Ti back contacts.
A conversion efficiency of 13.1% (VOC = 613mV, JSC = 29.6mA/cm 2 , FF = 73.7%) could be achieved
for a small cell area of 0.16 cm 2 , by using a Ti/TiN bilayer stack and a Mo interface layer. It should be
mentioned that only a few solar cells could be processed in this preliminary work, and most of the
solar cells of typical area of 0.5 cm 2 were in the range of 8 to 10% efficiency. In Fig. 7 the comparison
with the best reported conversion efficiency of 14.1% (VOC = 649mV, JSC = 31.5mA/cm 2 , FF = 69.1%)
on polymer foil [1] is shown along with the J-V curve of 13.1% efficiency cell on alternative contact.
Even if the best cell efficiency achieved with a TiN/Ti back contact is for the moment slightly lower than
the world record cell, this result shows the potential of this back contact to reach as high or even
higher efficiencies than on Mo back contacts. TiN offers additionally the advantage of higher stability
against oxidation and corrosion, a higher optical reflectivity and impurity diffusion barrier properties
National and international collaboration
Some aspects of this project benefit from the collaboration with FLISOM AG, an ETHZ spin-off company
and other national and European projects in the thin film physics group. Support of Blösch AG on
alternative back contact work is gratefully acknowledged.
Evaluation 2008 and outlook 2009
Improvements in the design of the evaporators was achieved and the simulation of evaportaion fluxes
provided in-depth knowledge on the evaporation behaviour and uniformity profile for large area deposition
of layers. The chemical bath deposition process was improved for defect reduction and waste
mininimisation. Successful development of high efficiency flexible solar cells with alternative electrical
back contacts based on TCO and TiN/Ti is of particularly interest for further advancement in the field.
Industrial involvement to support this research idea was specially encouraging. Although still in a preliminary
development stage flexible CIGS solar cell of 13.1% efficiency achieved with alternative back
contact is an important milestone for further innovation. These alternative stacked back contacts offer
several advantages for manufacturing as well as for long term performance stability of flexible solar
cells. However, a large amount of R&D is necessary to understand the scientific and technological
issues of this unconventional heterostructure. There is a significant industrial and academic interest to
pursue this work. FLISOM AG and Blösch AG have provided valuable support and might be interested
in further collaboration on industrial aspects of the technology.
References
[1] D. Brémaud, D. Rudmann, G. Bilger, H. Zogg, A. N. Tiwari: Towards the development of flexible CIGS solar cells on
polymer films with efficiency exceeding 15%, 31 st IEEE Photovoltaics Specialists Conference, Orlando, 2005, p. 223-
226.
Large Area Flexible CIGS, D. Brémaud, ETH Zurich
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