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

Figure 8: J-V characteristic of with 60 nm thin InxSy buffer layer without PBDVA: (-----) as-grown; (-----)
cell air annealed for 5 min at 200°C and, (-----) CBD-CdS reference cell.
CIGS solar cells on TCO contacts
The development of CIGS solar cells on transparent conducting oxide (TCO) coated substrates is
essential for the development of tandem and bi-facial solar cells. Different TCOs are to be evaluated in
terms of their compatibility with the device structure and processing conditions, i.e. the TCOs have to
be stable, should not degas or degrade during the various deposition steps and should be chemically
inert. The formation of an ohmic contact between the absorber and the back contact is also crucial to
achieve high conversion efficiencies, which requires the introduction of a very thin (few nm) layer of
Mo in between. We started our TCO investigations with ITO, analysed the influence of the Mo
intermediate layer and developed semi-transparent solar cells. Continuing earlier work we have
improved the properties of the layers (including post deposition Na incorporation) and cell processing
and consequently achieved 13.3% efficiency solar cells on ITO coated glass substrates (table IV).
Back Contact Na Voc
[mV]
Jsc
[mA/cm 2 ]
FF
[%]
ITO - 503 28.4 53.9 7.7
��
[%]
Mo (1�m) / ITO - 528 28.4 66.7 10.0
Mo (10 nm) / ITO - 516 29.1 65.2 9.7
MoSe2 (10 nm) / ITO - 548 30.2 67.7 11.2
Mo (10 nm) / ITO PDT 621 30.2 71.0 13.3
MoSe2 (10 nm) / ITO PDT 573 29.6 67.1 11.4
Table IV: Photovoltaic properties of CIGS solar cells grown on ITO back contacts.
Evaluation 2007 and outlook 2008
Microstructural and chemical inhomogeneity in the In2S3 powder for evaporation depends on the
source/supplier. While powder-A was chemically and structurally stable upon heating, powder-B
showed large S-loss and regions of different chemical compositions. Interdiffusion at the CIGS-buffer
might occur due to annealing treatment as well as due to due to difference in chemical potentials. The
in-situ annealing above 200°C is detrimental to cell efficiencies as excessive interdiffusion might
deteriorate the cell properties. Some inconsistent results with air annealing of the completed cells
were obtained. The solar cells with efficiency up to 14.1% have been developed.
Those PVD buffer layer have now to be applied to flexible CIGS cells. On glass the reproducibility has
to be improved and further investigations are needed to understand the role of annealing treatment on
the properties of heterojunctions.
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ATHLET, D. Brémaud, ETH Zurich
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