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

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Conclusion – sodium incorporation
Sodium incorporation into CIGS absorbers is beneficial for the performance of CIGS solar cells. The
deposition of different layers thicknesses of sodium via post deposition treatment revealed an optimum
of thickness a 20 nm for the low temperature growth process. It can be concluded, that the required
sodium depends on the processing speed and the applied substrate temperature during the CIGS
growth process.
Based on the findings of an optimum Na-dosage, sodium was co-evaporated during the CIGS growth.
With this technique, the low temperature growth process yielded efficiencies of approx. 12,5%, which
are comparable or even higher than those gain with PDT. This result is important, as it opens the
applicability of the sodium incorporation method in a in-line CIGS process.
Investigations of the cell microstructure reveal a reduced grain size near the cell back contact, if
sodium is present during the growth process (co-evaporation). It is assumed, that a Ga-rich structures
forms within this absorber region. Further analyses, like SIMS and Raman spectroscopy are planed in
cooperation with other project partners in order to determine composition gradients along the absorber
thickness.
2) Modified CIGS absorber for buffer-free cells
Introduction
It is commonly accepted, that a buffer between the CIGS absorber and the transparent conducting
front contact (TCO) is needed for the following reasons:
� Improvement of electronic interface properties due to better band alignment between window
front contact and absorber layer
� Protection of CIGS absorber from sputter damage during window layer deposition
� Doping of absorber to improve the homo-junction properties
Thus, in most laboratories, the standard device structure of Cu(In,Ga)Se2 (CIGS)-based solar cells
includes a very thin CdS buffer layer. In view of an industrial production process the non-vacuum
chemical bath deposition process of this buffer involves technological problems. But also for ecological
reasons efforts are undertaken to substitute the CdS buffer layer. Alternative buffer layers are e.g.
indium-sulfide, zinc-sulfide and magnesium-oxide, which can be deposited via vacuum or non-vacuum
techniques. As these buffer layers require additional process steps and equipment, a different
approach could be, to modify the CIGS surface in such a way, that the buffer layer could be omitted.
Even though this way is accompanied by significant cost reduction potential, it is desired to achieve
efficiencies comparable to CIGS cells with other alternative buffer layers.
Experimental
At ETH Zurich state-of-the-art CIGS solar cells are grown on 1 mm thick soda-lime glass. A 1 µm thick
Molybdenum back contact is deposited by DC sputtering. The ~2 µm thick CIGS absorber is grown by
elemental co-evaporation in a high-vacuum chamber using a three-stage process. Our standard buffer
layer is a 50 µm CdS layer deposited by chemical bath deposition. As front contact a bi-layer of 50 nm
i-ZnO and 250 nm ZnO:Al is deposited by RF sputtering. The cells are finished by e-beam evaporated
nickel and aluminium grids and mechanical scribing. Efficiencies generally yield 12% to 17%,
depending on the substrate and processing temperature.
In order to avoid the additional buffer layer deposition the finishing of the CIGS deposition process has
been modified in this work. After a standard three-stage process the samples were cooled down to
200°C and a thin layer of i) InxSey, ii) InxGa1-xSey and iii) GaxSey was evaporated onto the CIGS. A
reference sample was prepared without this additional step.
Results
Figure 2.1 shows IV parameters of solar cells, processed as described above. High open circuit
voltage almost comparable to standard cells with CdS buffer layers can be reached with a 2:1 ratio of
In:Ga for the CIGS finishing (sample 2 in figure 2.1). However, the short-circuit current strongly
decreases with increasing gallium content and reaches almost zero for pure GaxSey. Due to very poor
fill factors of around 50%, efficiencies of buffer free cells did not exceed 5% in these experiments.
LARCIS, A. N. Tiwari, ETH Zurich
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