Photoluminescense for Solar Cell Crack Detection

In the manufacturing process for photovoltaic (PV) cells and modules, a significant percentage of crystalline silicon wafers and solar cells contain microcracks that are difficult or impossible to detect with the human eye or currently available machine vision systems. These microcracks can propagate through the cells, resulting in power loss and cell breakage, due to mechanical and thermal stresses during cell fabrication, module assembly, and outdoor exposure. Photoluminescent imaging is being investigated as a means of crack detection in highly automated production lines, due to its potential for high-speed non-contact measurements. Image processing techniques will be applied to discriminate cracks from other material defects, enabling automated crack detection. Microcracks were detected by photoluminescence in both mono- and multi-crystalline silicon solar cells, using three different excitation wavelengths and two types of near-infrared cameras, a cooled silicon charge-coupled device and a cooled InGaAs focal plane array. In year 1, illumination sources and cameras will be tested to optimize image quality, while image processing software will be developed to identify microcracks in crystalline silicon cells. In year 2, a prototype automated high-speed photoluminescent solar cell inspection system capable of inspecting and handling 1200 cells per hour will be designed, built, and demonstrated. Commercial Applications and Other Benefits: Identifying and removing microcracked silicon wafers from the production line has clear benefits to solar cell and module manufacturers. Automating the inspection and reject part segregation processes reduces the cost of inspection and rework labor in cell and module production lines while increasing product yield, thereby reducing the cost of finished modules. The elimination of microcracked wafers and cells from modules will increase the lifetime (mean time to failure) of installed modules. This will increase the total energy production over the effective lifetime of PV systems, thereby reducing still further the energy generation cost. In order to advance these goals, Spire plans to integrate automated photoluminescent crack detection systems into its commercial cell testing equipment and module assembly lines.