Specific PVMaT R&D on Siemens Cz Silicon Product Manufacturing: Annual Subcontract Report, June 1998 - June 1999

This report describes work done by Siemens Solar Industries (SSI) from June 1998 to June 1999 during Phase I of a three-phase Photovoltaic Manufacturing Technology (PVMaT 5A2) subcontract from DOE/NREL. The work focuses on improvements in the cost per watt of Cz modules and improved PV module manufacturing technology. The first step toward reducing cost was to reduce wafer thickness. The approachhas been a two-step reduction in thickness, from 385 microns to 250 micron cells, and then from 250 microns to 125 micron cells during Phase II. During Phase I, the handling tools, the back-surface-field process, and the confirmation of the environmental integrity of thinner wafers have all been accomplished. Cells with efficiency over 15.5% have been demonstrated in high-volume production. SSIhas initiated the development and growth of 200-mm ingot to be fabricated into wafers and eventually cells. Cell and module production with 200-mm cells will begin during Phase II. Hazardous waste reduction has been attacked in two ways. The largest consumable item aside from polysilicon is silicon carbide (SiC) used in the wafer-slicing process. This SiC use has been reduced significantlythrough recycling and re-use. This program approach is well under way at SSI, with more than 25% of the SiC used being recycled. The largest hazardous waste volume at SSI is the caustic waste generated in the wafer etching processes. The reduction of this waste will be accomplished using subcontractors with extensive environmental compliance experience such that the solution is driven by bestavailable techniques, lowering operating cost as a secondary motive. These three areas of focus-thinner cells, larger cells and modules, and hazardous waste reduction-have the potential to reduce cost by about 30% per watt. This first phase of large 150-mm-thinner 250-micron cells has demonstrated a potential for 10% cost reduction, with the final yield improvements being implemented in the SSIline now. The follow-on work during Phase II, with thinner cells, larger cells, and continued waste reduction, will allow the 30%, three-year goal to be met.