Measuring thermomechanical displacements of solar cells in laminates using digital image correlation

Solar modules are commercially fabricated for more than 30 years but there is little knowledge about its thermomechanical properties. Solid materials shrink or expand when subjected to temperature changes. The coefficients of thermal expansion (CTE) for silicon and glass are small compared to the CTE of the polymer sheets. This CTE-mismatch leads to mechanical stress and strain which can be critical for the solar cells, the interconnects and the polymers. We demonstrate that the digital image correlation technique (DIC) is capable of measuring displacements in modules that are within a climate chamber by an optical measurement through a transparent back sheet and one layer of encapsulation material. As a first demonstration the change of the gaps between adjacent cells is determined at various temperatures for three test laminates with three different interconnection techniques. We find the gap between two solar cells to deform about 0.4-0.6 ¿m/°C in the temperature range of 0°C to 80°C and slightly less in the temperature region below 0°C. The results are verified by a simplified calculation of the gap deformation.