INFLUENCE OF MICRO CRACKS IN MULTI-CRYSTALLINE SILICON SOLAR CELLS ON THE RELIABILITY OF PV MODULES

The mechanical strength of multi- and monocrystalline silicon wafers and cells is strongly dependent on the length and the position of micro cracks in the silicon wafer material. Micro cracks are increasing the breakage risk over the whole value chain from the wafer to the finished module, because the wafer or cell is exposed to tensile stress during handling and processing. The actual need to use larger and thinner crystalline silicon wafers increases the risk of yield losses. Besides the breakage risk in the production, the cells have to withstand the tensile stresses under outdoor operation in the finished modules. Here the tensile stress is induced by temperature changes and mechanical loads from wind and snow. Currently, commercial PV modules have 25 years warranty and their reliability over this time period is simply considered as proven, when a module type was successfully qualified according to IEC 61215. In industrial practice, there is currently no parameter for the mechanical strength of cells and wafers available, which guarantees the reliability of an individual module. Wafers of different thickness were laminated into modules, which were then exposed to mechanical load tests similar to IEC 61215. The micro crack propagation due to the load test was monitored by scattered light on the micro crack planes in the near infrared range. Finally, the power drops resulting from mechanical load tests were compared for standard modules using cells with and without micro cracks.