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

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The system consists in two chambers separated by a membrane. Fig.9 illustrates the quality of the
replication that can be achieved with this system, taking the exemple of a “standard LPCVD-ZnO”
layer.
At the moment
the loss in RMS roughness is typically around 17% between masters and replica. Some
of these substrates obtained by UV-NIL replication at the IMT were tested in cells and Jsc currents
up to 15mA/cm2 could be achieved in n-i-p cell configuration. Nevertheless, the FF, Voc and yield
must still be improved.
2.3.
Other processes
A special effort was done
to improve laser scribing based on an old IMT set-up. Here the milestone
was to achieve single cell isolation (single and multiple junctions) with laser scribe with less than 1%
fill factor loss compared to current lift-off/etch process. This milestone was achieved for single-junction
aSi:H cells with the current (old) set-up at PVLab. The loss in FF is indeed lower than 1% when laser
scribing is used as compared to standard lift-off/etch procedure. The value of FF at low illumination is
given as well, as an indicator of shunts: we observed that the laser scribing procedure, although optimized
on the old set-up, leads to shunts more easily than lift-off and thus needs careful development.
It was decided to acquire a better set-up with new laser sources. (see section 5.).
Most other process developments are included in the sctions 3 or 1.
3. DEVICES
3.1 High efficiency
micromorph devices (p-i-n) on glass [Dom08, Dom08b, Bue08]
The best SiOx layers prepared in the work package 1 were used to fabricate state-of- the art micromorph
devices. One chamber is dedicated to the deposition of doped layers whereas the intrinsic silicon
layers are deposited in the other chamber. In order to minimize reflections at the air/glass interface,
AF45 glass plates were used with one side covered with a wide band anti-reflection (AR) coating
from Schott. In agreement with the reflectance measurement of the AR coated glass a relative increase
of 3% was measured for the sum of the Jsc values of the individual cells for the device deposited
on the glass with this AR coating. The back contacts of the cells consist of a LPCVD ZnO layer
covered with a dielectric back reflector.
Optimization of the micromorph device was obtained by choosing the best plasma post-treatment for
the front ZnO in terms of high open circuit voltages and short-circuit current values and adapting the
thicknesses of the intermediate reflector layer. To reach initial conversion efficiencies above 13% top
and bottom cell thickness had to be increased to 375 nm and 3 µm, respectively, while the intermediate
reflector had a thickness of 75 nm. The current are perfectly matched, and the added current densities
of the cells reached 26.6 mA/cm2, yielding an initial conversion efficiency of 13.1% (Voc=1.36 V,
FF=72.2%), as shown in Fig.10. Further improvement made in the frame of the Athlet project with a
focus on higher current cells leads to 13.3% initial efficiency. Next steps will be to achieve a minimization
of the degradation effects (including linked to micro-cracks in the cells, as reported in a previous
SFOE report 2005-2007).
Current density (mA/cm 2 )
14
12
10
8
6
4
2
0
-2
-4
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Voltage (V)
EQE
1.0
0.8
0.6
0.4
0.2
13.3 mA/cm 2
13.3 mA/cm 2
0.0
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Fig.10: Left: current-voltage curve of a 13.1% efficient micromorph cell measured in its initial state.
Right: external quantum efficiency curve of the same cell.
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New processes and device structures for the fabrication of high efficiency thin film silicon photovoltaic modules, C. Ballif, University of Neuchâtel