Non-destructive assessment of solution on based ZnO doped layers for photovoltaic
applications: Raman scattering methodologies
M. Guc
1
, P. Fuchs
2
, F. Tsin
3,4
, J. Rousset
3,4
, Y.E. Romanyuk
2
, V. Izquierdo-Roca
1
,
A. Pérez-Rodríguez
1,5,*
1
Catalonia Institute for Energy Research (IREC), 08930 Sant Adrià de Besòs, Barcelona, Spain
2
Empa – Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf,
Switzerland
3
EDF R&D, 6 quai Watier, 78400 Chatou Cedex, France
4
IRDEP, Institute of Research and Development for Photovoltaic Energy, UMR 7174, CNRS-EDF-
Chimie ParisTech, 6 quai Watier, 78401 Chatou Cedex, France
,
5
IN
2
UB, Departament d’Electrònica, Universitat de Barcelona, 08028 Barcelona, Spain
Recently, solution based transparent conductive oxide (TCO) layers have been developed for
advanced chalcogenide thin film solar cells. These include Al-doped ZnO layers deposited by
chemical bath deposition (CBD) processes and Cl-doped ZnO layers grown by
electrodeposition, and their integration into high efficiency solar cell devices has already
been demonstrated. Solution-based processes appear as an interesting alternative to sputtering
techniques that are typically used in the industrial production of the TCO windows layers in
chalcogenide photovoltaic modules, because of their high potential for reduction of
manufacturing costs. However, implementation of these layers in the mass production scale
requires non-destructive methodologies for monitoring of the quality of the grown layers. In
this sense, Raman scattering is a non-destructive optical technique that has a high potential
for the non-destructive structural and electrical assessment of the processed ZnO layers.
Raman scattering has already been employed to assesssputtered based Al-doped ZnO.
Nevertheless, the full exploitation of the capabilities of Raman scattering based
methodologies for the non-destructive assessment of these processes requires a deeper
knowledge of their vibrational properties, and their dependence on the different process
parameters.
This work reports a detailed Raman scattering study of the solution based doped ZnO layers
that has been performed under resonant (UV) and non-resonant excitation conditions.
Namely, the differences in the Raman spectra have been analyzed in the ZnO:Al made by
CBD and ZnO:Cl made by electrodeposition, and the experimental spectra have been
compared to those measured from reference sputtered ZnO:Al layers with similar
conductivity. The fingerprint of the Raman scattering spectra obtained under different
excitation conditions and their evolution under different post-deposition treatments of the
layers are presented and will be discussed. Comparison with the spectra measured in the
reference AZO layers has revealed the existence of strong structural differences that are
related to the different texture of the solution processed layers. Analysis of the spectra
measured under non-resonant conditions reveals also the existence of different spectral
regions that are strongly sensitive to differences in the density of carriers and to the presence
of structural defects and strain. In this sense, the correlation of the Raman measurements with
XRD analysis of the layers corroborates the existence of a relevant correlation between the
layer strain and the relative intensity of the A1(LO) vibrational peak. These features will be
discussed and analyzed, evaluating the potential of Raman scattering based methodologies for
the non-destructive electrical and structural assessment of TCOs.
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