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

Fig. 3: Current voltage and external quantum efficiency curves of the highest efficiency micromorph
cell (�=13.3%, Voc=1.36V, FF=70.8%, Jsc=13.8 mA/cm 2 ).
As seen in Fig. 1 and 2, the open circuit voltage is not yet satisfactory when targeting for achieving the
goal of 14% stable efficiency. It has been shown that in the case microcrystalline single junction cells
deposited on LP-CVD ZnO front contact, the value of open circuit voltage critically depends on the
surface texture. This remains also true for micromorph cells, even though the dependence is not so
strong. It has been shown that surface texture can be modified using plasma treatments and that open
circuit voltage clearly improves. However short circuit current tends to decrease upon plasma treatment,
because of reduced light scattering/trapping capability of the TCO and therefore a trade-off has
to be found. Plasma treatment was optimized in order to keep higher open circuit voltages. In a parallel
effort the amorphous cell was optimized on the TCO in order to increase the single junction open
circuit voltage above 930 mV. Deposition on thin glass coated with a broad-band antireflective layer
was systematically used in order to compensate for the current loss induced by the plasma treatment.
In order to tune the currents to values above 13 mA/cm 2 , the layer thicknesses were adjusted for
amorphous, intermediate reflector and microcrystalline to 340 nm, 100 nm and 3.4 µm, respectively.
Figure 4 displays current voltage and external quantum efficiency measurement on the best cell
achieving the deliverable (Voc=1.4V, FF=67.8%, Jsc=13.0 mA/cm 2 ). Further work will be needed in
order to improve the fill factor which at the moment limits the cell efficiency to 12.3%.
Current density (mA/cm 2 )
14
12
10
8
6
4
2
0
-2
-4
-0.4 -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
13.2 mA/cm 2
13.0 mA/cm 2
0.0
400 500 600 700 800 900 1000 1100
Wavelength (nm)
Fig. 4: Left – current voltage characteristic of a micromorph cell having 1.401V open circuit voltage
13.03 mA/cm 2 short circuit current; right: external quantum efficiency measurement of the same
cell. Current values have been rounded to the first digit after the point.
ATLET, N. Wyrsch, Institut de Microtechnique
66/290
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