Process Induced Deflection and Stress on Encapsulated Solar Cells

Cell cracking is one of the most common factors that limit the lifetime of PV modules. Until now electroluminescence (EL) has been the tool of choice to inspect cracks in finished modules. However, there are intrinsic limitations to the size of the cracks that this technique can resolve making it complicated to study the origins of crack formation. We also argue that the process of module assembly today is optimized from the point of view of the inactive materials (e.g. encapsulant cross-linking) offering no insights into the solar cell status. To this regard, even the correlation of module degradation to cell cracking in flat modules is only evident when cracks are of detectable size and detrimental to the electrical performance as measured by EL or PL. We have shown that in-house X-ray topography (XRT) is a unique technology that provides a non-destructive way of assessing the mechanical state of encapsulated solar cells, not only the evaluation of cracks and microdefects developed during handling and outdoor operation, but also the analysis of intrinsic deflection and stress induced by materials and processes (e.g. soldering, lamination). In this contribution, we present results on the deflection of the solar cells caused by different processing temperatures, various lamination materials, accelerated testing as well as metal ribbon.