Innovative Approaches to p- and n-Type Poly-Si/SiOx Passivating Contacts for Silicon Solar Cells

In the last years, PV manufacturing has seen a shift from Al-BSF (Back Surface Field) to PERC (Passivated Emitter and Rear Contact) solar cells to drive down the recombination current at the rear surface of the cells. Other shifts, e.g. transition from multi-crystalline to monocrystalline material, and from p-type to n-type base doping, are also taking place to minimize recombination currents in the bulk of the device. However, today, the weakest point of crystalline silicon solar cells is still the recombination at the semiconductor/metal contact interface. To maximize the open-circuit voltage of silicon solar cells and approach the thermodynamic efficiency limit, commonly known as Shockley-Queisser limit, a lot of the recent research in the field of crystalline silicon photovoltaics has been addressed to mitigate the recombination losses at that interface. In crystalline silicon solar cells, this problem can be effectively tackled with the implementation of polysilicon-based passivating contacts. A polysilicon-based contact consists of the stack ‘interfacial oxide/n-type or p-type polysilicon thin-film/metal’ at the un-doped or doped semiconductor surface to be contacted. As a passivating contact, this structure must be designed to shield the minority carriers from the recombination sites at the metal contact while enabling a good transport and collection of the majority carriers to that contact. Thus, the ideal passivating contact would feature minimum minority carrier recombination as well as low specific contact resistance. From a technological point of view, this work will focus on the fabrication of a polysilicon-based passivating contact starting from PECVD a-Si:H, which will be recrystallized to form the polycrystalline silicon contact after a high-temperature annealing. The goal of the research will be to investigate the effect of the crystalline structure and the grain boundaries, as well as the doping impurities, on the recombination mechanisms when these layers are used for contact passivation. P-type or n-type doping can be performed in-situ during PECVD a-Si:H deposition or ex-situ after recrystallization. In this in-depth characterization, the impact of the metallization approach and the diffusion of metal species will be also taken into account. Plating, screen-printing, evaporation and/or sputtering are the available metallization technologies to be explored. Additional aspects to be included as part of this research are the influence of the surface morphology (from planar to a textured surface) and the patterning of the passivating contact when required. Experimental tests during this Ph.D. will be conducted on both specialized test structures and complete solar cells. This work will make use of advanced electrical characterization tools (examples: quasi-steady-state photoconductance, spectral response, current-voltage measurements,...) and material analysis techniques (examples: secondary ion mass spectroscopy, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy,...). The research will also use analytical modeling tools to support the experimental observations.