P⁺ Doping Analysis of Laser Fired Contacts by Raman Spectroscopy

Abstract

Laser firing of contacts is a simple method to establish local rear contacts of silicon PERC (passivated emitter and rear contact) solar cells. The silicon bulk is contacted point-wise by Al driven by the laser through the rear dielectric layer. The Al is deposited on the full area by physical vapor deposition or screen-printing. Al and B, if the Al paste contains B additives, can thereby establish a p+-doped Si region below the Laser Fired Contact (LFC), which helps to lower carrier recombination because of the local back surface field and also to reduce contact resistance. In this work Raman spectroscopy is used to detect and investigate the p+-layer established by laser firing through screen-printed Al. Scanning Raman measurements allow spatially resolved determination of the free hole concentration in the contact area. In a line scan through a LFC, the step in doping concentration between the lowly doped bulk Si and the highly doped LFC area is clearly seen by a high local hole concentrations in the range of 1019 cm-3 in the LFC region. This shows that scanning Raman spectroscopy is a useful method for the microscopic understanding of LFCs and optimization of the process parameters.