Comparison of cleaning processes for enabling solderability of LFC-PERC silicon solar cells with evaporated Al as rear side metallization

Silicon solar cells with evaporated aluminum rear metallization suffer from incompatibility with common soldering processes. In this paper we present a way of making industrially processed passivated emitter and rear cells (PERC) with evaporated Al rear side and laser fired contacts (LFC) subsequently solderable and module capable. Solderability is enabled by sputtering a solderable stack on the Al metallization, but it becomes apparent that the deposited stack lacks of adhesion due to dust originating from the LFC process. Therefore prior to the deposition of the solderable stack, cleaning of the rear side is essential. Therefore, test samples and PERC solar cells with evaporated Al rear side and LFC are fabricated and four cleaning methods are introduced and applied with varying influence on adhesion and cell efficiency. We show that proper cleaning processes enable a well-adhering solderable rear side metallization which permits cell interconnection and encapsulation into mini-modules.