Enlarged firing window and efficiency boosting of PERC solar cells by ‘laser enhanced contact optimization’ (LECO)

In this contribution PERC solar cells are produced with a large variation of firing temperatures during the contact firing process. In addition the cells are exposed to the novel process 'laser enhanced contact optimization' (LECO). The front metallization is realized with standard silver pastes and LECO specific pastes on ultra-low doped (ULD) emitters with sheet resistances around 150 Ω/sq. The experiment shows that LECO significantly enlarges the firing window for the standard paste allowing fill factors of a minimum of 80 % for peak firing temperatures as low as 740°C. Hence LECO enables the choice of ideal firing conditions for standard pastes on ULD emitters, leading to an efficiency gain of 0.37 %abs. comparing the groups with and without LECO treatment. Moreover the use LECO specific pastes shows a very surprising and strong increase of short circuit current density jsc when going to lower firing temperatures and firing temperature profiles that differ from default. The best group metallized with LECO specific pastes was fired at a pre-peak temperature of 700°C and a peak temperature of 770°C resulting in an average efficiency of 22.54 % which is partially enabled by the high gain in jsc of 0.37 mA/cm2 compared to the best jsc obtained without LECO treatment. We believe that the ideal firing conditions for LECO specific pastes are not fully exploited and that there is even further potential by adjusting the firing profile to a LECO paste specific optimum.