Enabling the measurement of thermomechanical stress in solar cells and PV modules by confocal micro-Raman spectroscopy

Understanding the origin of thermomechanical stress in solar cells is a key factor to extend the lifetime of photovoltaic modules. However, the methods to determine the stress are very limited. With the confocal micro Raman spectroscopy, we present a contactless method, which is able to measure through the front glass and is well-known in the field of microelectronics. One major challenge for the measurement on crystalline silicon solar cells and modules is the surface texturization of the mono crystalline solar cell, which changes the topology from a plain (100) surface to pyramids with (111) flanks and (100) valleys. We develop a procedure to cover the challenges arising from this topology, namely the inhomogeneous stress distribution on the pyramid flanks and the different crystal planes of the phonon vibrations and the photon back scattering. By studying the procedure on a reference system, we determine a factor for the conversion of a micro-Raman peak shift to stress of Sigma = -(833 +/- 49) MPa/rel. cm(-1). The presented measurements show that the factor holds for uniaxial stress, biaxial stress as well as the stress states occurring from the PV module production processes. We then apply the procedure to measure the stress from soldering 156 x 156 mm(2) solar cells and the lamination. We obtain (-21 +/- 2) MPa for the stress in the unsoldered solar cells, which arise from the cell production steps, like metallization. After soldering, we measure (-26 +/- 3) MPa and after lamination (-53 +/- 6) MPa. Additionally we perform a line scan along the cell diagonal and area scans of the quarter cell as well as the end of one busbar. All results match well with a simulation of stress induced by the soldering process and lamination using the finite element method.