Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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As expected the addition of Na also improves significantly the Voc as well as the FF, leading to high
conversion efficiencies.
Figure 3: SIMS Na-profile in the CIGS layer on TiN back contact. Due to the selenization process, for
the "MoSe/TiN" samples, Na diffuses through pinholes into the CIGS layer and the cells reach similar
values than for Na-PDT CIGS cells.
Comparison with literature shows that the best achieved efficiencies, 13.9% on ZrN and 13.8% on TiN
back contacts, are very near to the efficiencies reached on Mo back contacts at a processing
temperatures of 450°C (Table IV).
14.1 % with NaF precursor F. Kessler et al., 2001, MRS Symp. Proc., 668
14.4 % with Na-PDT D. Rudmann , 2004, Ph.D. Thesis
14.8 % Diffusion from SLG
Table IV: CIGS solar cells grown at 450°C on Mo BC.
T. Wada et al., 2000, Proc. 16th European
PVSEC, 1
Investigation of Na Incorporation Methods
Prior to the start of this project, research and development work on Na-supply was conducted in the
EU projects PROCIS and METAFLEX. There is some overlap of the objectives and research work of
those projects and LARCIS to maintain continuity of development.
The influence of Na supply by different methods on the microstructure and the photovoltaic
parameters has been investigated for CIGS solar cells grown by the "3-stage" process. The
investigated Na-supply methods were: Na from precursor layer, Na co-evaporation during CIGS
deposition, diffusion from soda lime glass (SLG) and the Na post-deposition treatment (Na-PDT). All
samples have been grown on SLG with an Al2O3 barrier to prevent the diffusion of Na, excepted for
the "diffusion from SLG" samples. The Na-PDT is the only one where the Na is incorporated
afterwards and therefore has no influence on the growth of the CIGS. The Na-PDT consists of
evaporation of 30nm NaF onto as-grown CIGS layer, followed by annealing at 400°C for 20min in
vacuum.
We observed for all supply methods where the Na is present during growth a reduction of the grain
size, especially near the back contact. We attribute this behavior to a reduced elemental interdiffusion
of In and Ga if Na is present. This results in a composition grading through the CIGS layer and
therefore also in a bandgap grading. Secondary ion mass spectroscopy (SIMS) measurements
confirm the composition change and the grading. On the microstructural aspects as well as for the
photovoltaic parameters we did not observed any significant differences for the Na-supply methods
before or during growth. Therefore we used for the following analyses only samples with "Na diffusion
from SLG" as representative for "Na-precursor" and "Na coevaporation" samples.
In the case of Na-PDT samples we did not see any change in the microstructure, as expected.
Anyhow a significant increase of the efficiency was observed, especially for low processing
LARCIS, D. Brémaud, ETH Zurich
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