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

Abstract

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> or stacked SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) prior to full area Al screen-printing and a rapid firing. Conventional Al paste with glass frit degraded the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> surface passivation quality because of glass frit induced pinholes and etching of SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> . More insight on these competing effects is provided by fabrication and analysis of complete solar cells