Highly passivating and blister-free hole selective poly-silicon based contact for large area crystalline silicon solar cells

Abstract

Passivating the contacts of crystalline silicon (c-Si) solar cells with a poly-crystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) is currently sparking interest for reducing recombination at the interface between the metal electrode and the c-Si substrate. Hole-selective poly-Si/SiOx structures could be particularly relevant to passivate the rear side of mass-produced p-type c-Si solar cells (i.e. PERC solar cells). In this study, we elaborate on the optimization of a hole-selective passivating structure consisting of a boron-doped poly-Si layer on top of a chemically grown thin SiOx. The poly-Si layer is prepared by Plasma Enhanced Chemical Vapor Deposition, which enables single-side deposition. However, if not optimized, this deposition technique leads to degradation of the poly-Si layer through a “blistering” phenomenon due to high hydrogen incorporation in the layer. To tackle this, a study of the interplay between process parameters and blistering is undertaken in order to obtain highly passivating and blister-free poly-Si/SiOx structures. By addition of a hydrogenation step, the implied open circuit voltage (iVoc) provided by the structure is further improved, leading to a maximum value of 734 mV demonstrated on symmetrical samples made from large area wafers. We also conduct a Conductive-Atomic Force Microscopy (C-AFM) study with the aim of investigating the pinholes formation in the SiOx interfacial layer that could explain the transport of free charge carriers within the poly-Si/SiOx structure. We show that the current levels detected by C-AFM are affected by an oxide layer that grows at the poly-Si top surface. We also demonstrate that conductive spots detected by C-AFM are not likely to mirror conductive pinholes within the SiOx layer but are rather linked to the poly-Si layer.