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

11/16
Efficiency [%]
FF [%]
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
80
75
70
65
60
55
50
Initial
Stable
400 300 nm 200 nm 140 nm
initial
stable
400 nm 300 nm 200 nm 140 nm
30
25
20
15
10
20
18
16
14
12
10
8
6
Degradation [%]
degradation [%]
V oc [mV]
J sc [mA/cm 2 ]
910
900
890
880
870
860
850
840
830
820
810
800
15
14
13
12
11
10
initial
stable
400 nm 300 nm 200 nm 140 nm
400 nm 300 nm 200 nm 140 nm
Fig.15: Dependency of a-Si n-i-p solar cell parameters on absorber layer thickness; initial and stable
values are denoted by squares and circles, respectively. Triangles refer to the relative degradation
on the scale to the right. The cells are deposited on glass coated with LP-CVD ZnO.
3.3.3 Micromorph tandem cells with textured intermediate reflector [Sod09]
A particularity of the n-i-p configuration is that the deposition is started with the thick bottom cell.
Fig.14 shows that the collision of growth fronts can result in pinched regions and eventually in the
formation of defective material which is highlighted by the arrow the centre of the film shown in the left
panel. Apart from this observation, the microcrystalline material normally tends to smooth out any existing
structure which poses a challenge to the light trapping strategy for n-i-p tandem cells. For light
trapping in the bottom cell the texture of the back reflector should be relatively large (in the order of 1
to 1.5 µm). As such it will be already too large for light trapping in the amorphous top cell which works
best with structures in the order of 300 nm, and additionally it is smoothened out by the growth of the
bottom cell.
In order to solve this dilemma, an asymmetrical intermediate reflector of LPCVD-ZnO was introduced.
This material is known to develop its own intrinsic surface structure, almost independently of the substrate
it grows on. This configuration allows a separation of the two different light trapping requirements;
the bottom cell is grown on a well adapted substrate for microcrystalline cells like the 2D grating
or a hot silver substrate. Then, the textured ZnO reflector is grown which delivers an ideal texture
for the amorphous top cell which is identical to the case of the cells presented in Fig.15.
Fig.16 shows the EQE and J-V characteristics of a tandem device on glass with initial efficiency of
10.3% and a stabilized efficiency of 10.1%. Because all processes are kept compatible with processing
on plastic substrates, it was also possible to apply the recipes to the flexible plastic substrate with
the 2D grating structure. A corresponding device on plastic is shown in the separate report on the EU-
Project Flexcellence and reaches 9.8% stabilised efficiency, with further space for improvement.
initial
stable
41/290
New processes and device structures for the fabrication of high efficiency thin film silicon photovoltaic modules, C. Ballif, University of Neuchâtel
14
12
10
8
6
4
2
4
2
0
degradation [%]
Degradation [%]