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

EQE
1.0
0.8
0.6
0.4
0.2
12.4
11.7
11.9
11.5
0.0
400 500 600 700 800 900 1000 1100
Wavelength [nm]
Jsc [mA/cm 2 ]
15
10
5
0
-5
-10
Voc FF Jtop Jbottom �
�V] [%] [mA/cm 2 ] [%]
initial 1.32 66 12.4 11.9 10.3
stable 1.35 65 11.7 11.5 10.1
-15
-0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 1.5
Fig.16: initial and stabilized performance of a tandem cell in n-i-p structure on glass covered with a hot
silver back contact. The cell processes can also be applied to plastic foil.
4. RELIABILITY
The goal of this first period was to establish a base process for packaging of IMT development cells in
order to achieve a protection against uncontrolled degradation. The chosen strategy was to (symbolically
shown in Fig.17) split an encapsulated thin-film (TF) Si-based solar cell into basic layers, so to
say the building blocks of the cell, and, after embedding with different polymers, identify the main
causes of degradation of either the layer itself and/or of the interface. The major achievements are
summarised here below.
Fig.17: Schematics of the interfaces and layers of an embedded IMT’s lab cells.
4.1 Optimizing adhesion of polymers on glass. (Fig.17(a)),
As far as adhesion is concerned, the main point is the quality of the interface, which has been tested
by measuring the peeling force needed to detach the encapsulant from the glass after lamination. It
has been found that a good control and homogeneity of the intrinsic parameters of the lamination
process (temperature, pressure and time) are crucial for obtaining a good adhesion. Nevertheless,
some extrinsic parameters like the glass cleanliness, the storage conditions and the preparation of the
polymers are equally important in order to reach peeling forces of the order of 5 N/mm or more.
4.2 Encapsulation as a barrier against diffusion of water vapor and other gases. (Fig.17(b))
Because they are made of long and weakly interacting molecular chains, polymers are naturally permeable
to small molecules like those found in air. In addition to oxygen, water vapor is known to diffuse
in the polymer itself and, thus, being able to reach the polymer/something interfaces. Since the
LP-CVD ZnO of the IMT can be made, controllably, more or less sensitive to moisture, glass / TCO /
polymer (as well as glass / polymer, see 4.3) laminates have been made and submitted to "dampheat",
i.e. to a harsh environment of 85 Celsius and 85 % of relative humidity. Depending on the encapsulant,
the increase in the sheet resistance of the zinc oxide ranged from 10% (a factor of 1.1) up
to 1000% (a factor of 10) after a couple of hundreds of hours in damp-heat.
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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
U [V]
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