Optimization of Phosphoric Acid-Based Emitter Formation on Silicon Wafer

Crystalline silicon (c-Si) wafer-based solar cells have been dominating the current photovoltaic industries. However, prevalent manufacturing practices are based on environmentally-harmful chemicals and expensive methodologies. This paper reports on the development of inexpensive, environmentally-benign phosphoric acid-based emitter formation methods as an alternative to conventional highly toxic and poisonous POCl3 gas source-based chemistry. Two emitter formation approaches at temperatures in 850-925 degrees C range have been investigated. The first approach is referred to as the doctor blade (DB) technique, where the flat Si wafer surface is uniformly coated by phosphoric acid (H3PO4) via a moving blade. A small gap between the blade and wafer is maintained in order to form a thin uniform film on the wafer. The second method is referred to as the extension of the blade method (EDB), where an un-doped wafer is placed proximately to the deposited H3PO4 wafer. During the high temperature drive-in process, phosphorous emitter was formed on the un-doped wafer surface through evaporation and deposition of phosphorus atoms from H3PO4 coated wafer. All diffusion processes were carried out on 180 mu m thick, planar boron-doped Si wafers in a conventional quartz tube furnace. The variation of sheet resistances over a broad range from similar to 20-180 Omega/sq were consistent with temperature dependence. Highest diffusion uniformity was observed for 10% H3PO4 solution. Diffusion process simulations based on DifCad software were in good agreement with experimental data. The work reported here illustrates that an environmentally-benign approach in emitter formation based on H3PO4 is feasible for manufacturing solar cells.