The Mechanical Theory behind the Peel Test

The peel test is a very simple and fast method to determine the adhesion of interconnector ribbons to solar cell metallizations. It is part of the solar cell standard DIN EN 50461 and is, due to its ease of use, widely accepted to qualify cell metallizations and the soldering process. In the standard a force of 1 N per mm of joint width is specified but other relevant quantities are missing, for example the peeling angle. We show that this lack of specification enables the manipulation of peel testing results. We therefore apply the mechanical theory of Kinloch [1] where measured peel forces are translated into adhesive fracture energies GA. The fracture energy is a geometry-independent parameter that describes the energy to break the interfacial bondings at the peel front. It incorporates the dimensions of the ribbon and its stress-strain-curve. We perform 86 peel experiments at 90°, 135° and 180° of ribbons on continuous front side busbars of cells from one stringing batch. While the median forces for 90°(3.07 N), 135°(2.35 N) and 180°(3.39 N) differ by up to 30.4% we find the median adhesive fracture energies to deviate by only 17.4%. Using the same adhesive fracture energy (260 J/m2) for a 45° peel test we expect peel forces of 7.45 N which is factor 2.4 (2.2) higher than the 90° (180°) peel forces.