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

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A new regime for the microcrystalline solar cell deposition was developed, despite the good results
obtained in 2006 with a regime at 0.7 nm/s. Two reasons which triggered this development were first
the empirical observation of powder formation during deposition and secondly the substantial substrate
heating above 260°C measured during deposition. Thus, a new deposition regime at lower RF
power was developed, reducing powder production and reducing the substrate heating to below
210°C. The deposition rate is 0.55 nm/s. The best µc-Si cell so far has 8.0% efficiency and is characterized
by Voc= 513 mV, FF= 70.1%, Jsc= 22.48 mA/cm 2 for a cell surface of 0.25 cm 2 and 1.4 �m
thickness (cf. Fig. 3).
Currrent density (mA/cm 2 )
25
20
15
10
5
0
-5
-100 0 100 200 300 400 500
Voltage (V)
Fig. 3: I(V) curve of micro-crystalline silicon solar cell in KAI-S system at IMT. The best cell efficiency
for 1.4 �m absorber thickness is 8.0%
The two cells were combined in a micromorph cell. The intermediate reflector was deposited ex situ in
a small area system to increase the current in the top cell. The area of the cell was 1.2 cm 2 . The I(V)
curve of the best cell is displayed in Fig. 4. The cell parameters are as follows: Voc= 1.32 V,
FF= 70.2%, Jsc = 11.4 mA/cm 2 which adds up in an initial efficiency of 10.6%. This latter value satisfies
the first milestones mentioned earlier. Short-term improvement is expected to boost the efficiency
above the 9% efficiency stable target. The next challenge will be to keep that efficiency while rising the
deposition rate to the final target of 10 Å/s.
Finally it should be noted that all results are given for solar cells prepared on ZnO transparent front
contact prepared at IMT.
10
5
0
-1000
V (mV)
-500 0 500 1000 1500
-5
-10
-15
-20
J (mA/cm 2 )
Fig. 4: I(V) curve of micromorph solar cell in KAI-S system at IMT. Best cell efficiency for layer thicknesses
of 250 nm for the amorphous and 1.6 �m for the microcrystalline thickness is 10.6%
ATHLET, N. Wyrsch, IMT
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