Etching Paste for Innovative Solar-Cell Applications

This paper reports the development of an etching paste for selective etching of a phosphor silicate glass (PSG) layer, which is used as a mask for the processing of solar cells. The etching paste should thoroughly open a thick PSG layer that has high oxygen content in 2 minutes. Moreover, the paste must be completely water based and etch the PSG layer at room temperature without damaging the silicon surface. One of the applications for this kind of etching paste is making solar cells with a selective emitter structure. In this study, a selective structure with a double-diffusion process is considered. The first PSG layer is created by adding oxygen and POCl₃ gases into the diffusion furnace. By using the ETCH-TEC .Ox etching paste, this oxide-rich PSG layer is locally opened, and the second diffusion without oxygen is carried out. Light and heavy phosphorous doping of silicon leads to locally doped areas with sheet resistances of 100 and 50 Ω/square, respectively. The opened structures were characterized microscopically and physically. An experiment with 800 wafers in a solar-cell production line of the company MAGI Solar was carried out. Compared with the wafers with a homogeneous emitter, an average increase of +0.52% in absolute efficiency was measured.