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

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Mechanical testing:
The mechanical reliability of the Si thin film is an important issue for the quality and the life time of the
solar cells. During the processing, extended defect configurations and thermal mismatch between
silicon and glass can destabilize the material. Mechanical crack formation in the silicon layer on the
glass substrate can be studied by carrying out various standard tests as scratches, indentation, bending,
and pulling (Fig. 4). The key materials parameters that needed to be measured are the fracture
energy of the coating and the fracture energy for interfacial debonding.
Three different tests have been applied to the Si thin film on glass substrate: Scratch tests, nanoindentations
and pull tests. Scratch test is currently the adhesion test the more widely used by industry.
It could be treated as two operations: normal indentation process and horizontal tip motion. A vertical
increasing load is applied to the tip during scratching until the coating detaches from the substrate.
The minimal critical load at which delamination occurs is used as a measure of the practical work of
adhesion.
Indentation testing is a simple method that consists essentially of touching the materiel of interest in
order to determine the Young’s modulus and the hardness, quantities which will be used for the determination
of fracture toughness and adhesion. Nanoindentation is an indentation test in which the
length scale of penetration is measured in nanometers. In brittle material such as silicon, cracking of
the specimen may occur during the indent especially when using a pyramidal tip as the three–sided
Berkovich. The length of the crack, which often begins at the corner of the indentation impression, can
be used to calculate the fracture toughness of the material. Microindents of higher length scale can
also be processed down to the glass substrate in order to test the fracture resistivity of all materials.
In Pull test the sample is glued between two pistons. The adhesive strength is determined by subjecting
the specimen to some external load and measuring the critical values at which its fails.
Figure 4: Sketch of the three types of tests applied to the Si thin film samples on glass substrates
Other reports and results are confidential
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Large grained, low stress multi-crystalline silicon thin film solar cells on glass by a novel combined diode laser and solid phase crystallization process, X. Maeder, Empa