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

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Work and results
1) Sodium incorporation techniques for CIGS solar cells
Introduction
Highest efficiencies of CIGS solar cells have been achieved by using soda lime glass as substrate
material. It was shown, that sodium, which diffuses from the glass substrate into the CIGS absorber
material during its growth process, is beneficial for the solar cell performance. When sodium is
present, the solar cell efficiency is enhanced significantly, in particular due to an improvement in the
open circuit voltage (Voc) and fill factor (FF) of the CIGS cells.
In order to improve the performance also for cells on the flexible (sodium-free) substrates, different
ways of sodium incorporation into the absorber are investigated, recently. Common methods for the
sodium supply are the deposition of a precursor layer on top or beneath the molybdenum back contact
and the deposition of a sodium layer after the CIGS growth (post deposition process), respectively.
For both methods the sodium diffuses into the CIGS absorber, which is a strongly time- and
temperature dependent process. Since a precise sodium dosage is required for the optimum cell
performance this process has to be controlled accordingly. E.g. it was found, when applying a sodium
precursor layer, that an excessive presence of sodium during the CIGS growth process affects both,
the absorber microstructure and probably the elemental (Cu, In, Ga) inter-diffusion. Also adhesion
problems of the layer stack were observed often. The CIGS absorber growth process is not disturbed
or affected, respectively, by sodium incorporation via post deposition treatment. The mechanisms
behind these improvements, the „optimum“ amount of sodium and in particular the influence of sodium
on the formation of the absorber microstructure, are still contradictorily reported in literature.
Both sodium incorporation methods are additional process steps, which might turn out as limiting
factors in an in-line production process in terms of: (i) the processing speed (required time for indiffusion
of Na), (ii) different temperature requirements during sodium incorporation (Na deposition at
low temperature and annealing process at high temperature), (iii) additional cleaning steps in order to
remove excessive sodium from the absorber surface. Based on the above-introduced issues, the
following topics were investigated:
� What is the optimum sodium dosage in order to achieve best solar cell?
� How is this optimum dosage influenced by other process parameter, as e.g. processing speed
and temperature?
� How does the time of Na application influence the growth process and solar cell performance?
� Is there a preferred method of Na application?
Experimental
Substrate and CIGS processing
The substrates used for all experiments are conventional soda-lime glasses with a size of 5x5 cm 2 .
This material was chosen, in order to keep conditions similar to our standard cell processing. In order
to prevent any diffusion from the substrate (e.g. sodium diffusion) towards the absorber layer, a silicon
nitride (Si3N4) diffusion barrier layer was deposited on top off the soda lime glass. Finally a
conventional molybdenum back contact was applied. The CIGS absorber material was grown by coevaporation
with the common 3-stage process. The substrate temperature TS was kept at 400°C
during the first stage and at 450°C during the second and third stage (low temperature process).
Post deposition treatment
The method of sodium incorporation for the first series of experiments was the post deposition
treatment (PDT). Here sodium fluoride (NaF) is deposited onto the as-grown CIGS absorber. The
sample temperature during deposition is 100 °C. After deposition the temperature is increased to
400°C for 20 minutes in order to ensure the diffusion of sodium into the CIGS absorber. Different layer
thicknesses of NaF were applied to the absorbers by extending the evaporation time. The evaporation
rates of the NaF-source were calibrated with a quartz monitor.
LARCIS, A. N. Tiwari, ETH Zurich
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