Investigation of Modified Screen-Printing Al Pastes for Local Back Surface Field Formation

This paper reports on a low-cost screen-printing process to form a self-aligned local back surface field (LBSF) through dielectric rear surface passivation. The process involved formation of local openings through a dielectric (SiN_x or stacked SiO₂/SiN_x) prior to full area Al screen-printing and a rapid firing. Conventional Al paste with glass frit degraded the SiN_x surface passivation quality because of glass frit induced pinholes and etching of SiN_x layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiN_x, but did not provide an acceptably thick and uniform LBSF. Al pastes containing appropriate additives gave better LBSF because of the formation of a thicker and more uniform Al-BSF region. However, they exhibited somewhat lower internal back surface reflectance (<90%) compared to conventional Al paste on SiN_x. More insight on these competing effects is provided by fabrication and analysis of complete solar cells