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

Goals of the project
The goals of the project are three-fold; the most important task is the development of a dielectric back
reflector for better electrical and optical matching between the metallic back contact and the silicon
absorber layer. A second task addresses a texturing process for the substrate which yields enhanced
absorption due to diffuse scattering of light at textured interfaces. Finally, a rather technological objective
addresses the development of tandem processes using two stacked amorphous absorber layers
which yields better stability against light induced degradation of the cells.
Work performed in the reporting period 2008
DIELECTRIC REFLECTOR
The idea of the dielectric back reflector is an enhanced reflection of the metallic back contact. FIG. 1
compares the theoretical reflection for typical back contact materials like silver and aluminium in contact
to air, to ZnO, or directly to silicon. In case of silver there are only small differences between the
three cases, but it turns out that the every day experience of good reflection from aluminium is only
true for the case of reflection in air. When aluminium is in contact to ZnO or silicon, the reflection is
drastically reduced. The effect is particularly severe for the wavelength region around 700 nm where a
high reflection of the back reflector is essential for light trapping.
IMT and Flexcell thus investigate the introduction of advanced back reflectors consisting of a double
layer of metal and a dielectric material like ZnO. The beneficial effect of a dielectric reflector could be
well documented at IMT. Also Flexcell have successfully manufactured cells with improved current
density in their R&D line. Currently both partners investigate the compatibility with all other process
steps of module fabrication.
Reflection
1.0
0.8
0.6
0.4
0.2
0.0
400 600 800 1000
lambda [nm]
R (air)
R (ZnO)
R (Si)
Reflection
1.0
0.8
0.6
0.4
0.2
0.0
400 600 800 1000
lambda [nm]
R (air)
R (ZnO)
R (Si)
Fig. 1: Simulated reflection of silver (left) and aluminium (right) in contact to different materials
(air, ZnO, silicon).
SUBSTRATE STRUCTURING
Substrate structuring is an essential for high efficiency solar cells based on thin film silicon; light scattering
at structured interfaces prolongs the light path in the device which permits using thinner absorber
layers. This is important in terms of production throughput, but also because the thin devices
are more stable against light induced degradation of the amorphous silicon absorber layer.
IMT tested the suitability of different substrate structures, and together with VHF and several external
suppliers the feasibility of roll-to-roll production was assessed. Based on these results, the industrial
partner VHF was able to decide which of the available options they will incorporate into their production.
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Flexible photovoltaics: next generation high efficiency and low cost thin film silicon modules, F.-J. Haug, Institute of Microtechnology IMT
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