Optimization of laser processes for local rear contacting of passivated silicon solar cells

Abstract

Laser Firing Contact (LFC) and Laser Doping (LD) have become potential alternatives to the Al BSF thermal processing conventionally used in p-type c-Si solar cell rear contacts. Optimized LFC and LD processes allow, not only the generation of efficient micro-contacts, but also the diffusion of p-type doping impurities reducing the surface recombination velocity due to the formation of a local back surface field (BSF). In this work, three different laser strategies to create ohmic micro-contacts are studied: 1) evaporated Aluminum LFC, 2) Aluminum foil LFC and 3) Aluminum oxide (Al2O3) LD. The laser source used was a pulsed Nd-YAG 1064 nm laser working in the nanosecond regime. Laser parameters were explored to optimize the electrical behavior of the contacts and their carrier recombination rate. Optimized laser parameters lead to specific contact resistance in the 1.0 - 1.3 mΩ·cm2 range for all three strategies. From the point of view of carrier recombination, better results were obtained for Al2O3 LD, probably related to the lower energy pulse needed to create the contact. Next, the three proposed laser approaches were applied to the back surface of heterojunction silicon solar cells. Contact quality was not limiting any cell performance indicating that the contact quality is good enough to be applied in high-efficiency c-Si cell concepts. On the other hand, surface recombination velocity at the rear surface on the final devices also points out to Al2O3 LD as the best alternative.