Quality control using luminescence imaging in production of mcsilicon solar cells from umg feedstock

We use photoluminescence imaging (PL) for quality control of the crystallization process at Fraunhofer ISE in order to find defects which will later limit the efficiencies of solar cells. Bricks of multicrystalline feedstock from electronic grade (EG) and upgraded metallurgical (UMG) silicon are subsequently wafered and solar cells are manufactured using a standard industrial solar cell process. PL is employed on bricks, as-cut wafers and finished cells and supplemented by additional measurement techniques such as microwave photo conductance decay (MW-PCD) or I-V curve measurements. In UMG material, the major problem is the presence of background dopants, which lead to compensation effects such as a pn-type changeover. As the cell efficiency of wafers within or beyond the type changeover drops significantly in the standard process, these wafers have to be detected reliably in the incoming test to be separated and introduced to an adapted solar cell process or discarded. To do such a separation, the position of the wafer from the brick needs to be known.