Study of non fire-through metallization processes of boron-doped polysilicon passivated contacts for high efficiency silicon solar cells

Abstract

We report on the investigation of a variety of metallization processes for hole selective passivating contacts, which consist in the combination of an ultra-thin silicon oxide (SiOx) interfacial film (grown by wet treatments) and a 25 nm thick boron (B) doped polysilicon layer (poly-Si). Plasma Enhanced Chemical Vapor Deposition (PECVD) of amorphous silicon (a-Si) followed by crystallization annealing is used for poly-Si elaboration. Optimization of metallization steps is required in order to (1) reach low specific contact resistances (ρc), and (2) preserve passivation quality along with structural integrity. This study focuses on the analysis of two processes: electron-beam evaporation deposition, and Physical Vapor Deposition (PVD) by sputtering. Different metals (silver (Ag) and aluminum (Al)) and annealing conditions are studied. Additionally, we investigated the effect of an additional a-Si capping layer on both (1) and (2) items. PVD of Ag appears as the most promising technique combining relatively low ρc (90 mΩ.cm2) as deposited with limited passivation degradations, and thus seems suitable for device integration as a full area back contact.