Wüest M. 51 Wykes M. 82 Yamaguchi M. 17 Ybarra G. 129 Yubero F ...

JUNE 26 MONDAY MORNING
RIVA-TF-MoM-OR.8 STUDY OF THE DC-SPUTTERED Mo ON POLYMER SUB-
STRATE FOR FLEXIBLE CIGS SOLAR CELLS M. Andrés and M.T. Gutiérrez. Dep. of Energy,
CIEMAT. Avda. Complutense, 22. 28040 Madrid SPAIN.
For the last few years interest in flexible thin film solar cells has been growing for space power applications,
building integration and portable electronic. Thin film Copper Indium Gallium Selenide
(CIGS) has been established as a leading contender to these applications. In a recent publication different
approaches to flexible CIGS thin-film solar cells has been reviewed [1]. There are numerous
challenges in developing the technology for manufacturing flexible CIGS, the substitution of the
well-established soda-lime-glass substrate by a flexible alternative with drawbacks and without the
generation of new obstacles is not a minor problem. The substrate requirements for flexible CIGS solar
cells are related with a high thermal stability, a sufficient film adhesion and an adequate surface
morphology.
Molybdenum (Mo) has been used almost exclusively as a back contact material for CIGS-based
photovoltaics. Key requirements of the Mo-coated polyimide films for photovoltaic applications are
a high electrical conductivity, ohmic contact to CIGS, and high temperature stability in the presence
of selenium during CIGS absorber deposition.
The aim of the work has been to prepare Mo-coated glass and polyimide and to evaluate their performance
as flexible back contact for CIGS solar cells. The intrinsic stress, orientation, adhesion,
microstructure, reflectance and electrical resistance of dc-sputtered Mo films on glass and polymer
substrates has been studied as a function of deposition parameters.
Following Thorton and Hoffman [2-4], who comprehensively studies stress in sputtered metallic
coatings, we have investigated the effect of Ar gas pressure on the internal film stress in magnetron
sputtered Mo with particular emphasis on the problems that are encountered when relatively high
melting point Mo coatings, with low thermal expansion coefficient , are deposited on polyimide substrates
having relatively high thermal expansion coefficient. Substrate expansion due to heating during
deposition places a tensile stress on the coating which is sufficiently large to cause cracking. This
difficulty is overcome most effectively by depositing the coating under conditions that build a compressive
intrinsic stress into the coating.
[1] F. Kessler , D. Hermann and M. Powalla, Thin Solid Films 480-481 (2005) 491
[2] J.W. Hoffman, J. Vac. Sci. Technol. A 12(4) (1994), 953
[3] J.A. Thorton and D. W. Hoffman, Thin Solid Films, 171(1989) 5
[4] D. W. Hoffman and J.A. Thorton, J. Vac.Sci. Technol. 20 (1982) 355
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