Investigation of the Mechanism Resulting in low Resistance Ag Thick-Film Contact to Si Solar Cells in the Context of Emitter Doping Density and Contact Firing for Current-Generation Ag Paste

Abstract

Screen-printed thick-film Ag metallization has become highly successful in crystalline Si (c-Si) photovoltaics. However, a complete understanding of the mechanism resulting in low resistance contact is still lacking. In order to shed light on this mechanism for current-generation Ag paste, Si solar cells were fabricated using a range of emitter doping densities and contact firing conditions. Low resistance contact was found to vary as a function of emitter surface P concentration ( [P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">surface</sub> ]) and peak firing temperature. Scanning electron microscope (SEM) analysis revealed thin interfacial glass films (IGF) under the bulk Ag gridline. SEM analysis also showed increasing Ag crystallite density as both emitter [P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">surface</sub> ] and peak firing temperature increased. Two mechanisms are proposed in forming low resistance contact to highly doped emitters: 1) formation of ultrathin IGF and/or nano-Ag colloids at low firing temperature, and 2) formation of Ag crystallites at high firing temperature. However, on lightly doped emitters, low resistance contact was achieved only at higher firing temperatures, concomitant with increasing Ag crystallite density, and suggests that thin IGF decorated with nano-Ag colloids may not be sufficient for low resistance contact to lightly doped emitters.