Audrey Marie Isabelle Morisset

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In this article, the effect of the various processing steps during the fabrication of c-Si/SiOx/SiCx fired passivating contacts on the silicon bulk lifetime is studied, and the kinetics of defect deactivation by hydrogenation is investigated. It is found that the firing step at 800 degrees C induces shallow bulk defects in float-zone silicon wafers, which can subsequently be passivated with hydrogen provided by an a-SiNx:H/D reservoir layer upon annealing at 450 degrees C. Experimental results and numerical data treatment indicate a rapid passivation of the surface within less than 1 min, followed by a slower passivation of the shallow bulk defects. In situ lifetime measurements are consistent with a slow bulk lifetime improvement by showing similar lifetime evolutions for both p-type and n-type SiCx layers. The kinetics of the hydrogenation process seems to be limited by the available hydrogen supply at the c-Si/SiOx interface, rather than by its diffusion within the bulk of the wafer. Moreover, it is affected by the bulk doping as well as the SiCx layer thickness. Finally, it is shown that hydrogenation is also possible with an a-SiNx:H/D reservoir layer deposited on one side of the wafer only, although resulting in a lower passivation level (s similar to 700 mu s compared to similar to 1300 mu s for symmetrical samples), and slower kinetics (similar to 5 min compared to similar to 0.8 min).

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

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Impact of rapid thermal processing on bulk and surface recombination mechanisms in FZ silicon with fired passivating contacts

Christophe Ballif, Franz-Josef Haug, Andrea Ingenito, Mario Joe Lehmann, Sofia Libraro, Audrey Marie Isabelle Morisset

We investigate bulk and interface recombination in crystalline silicon with p-type fired passivating contacts. These consist of a chemical oxide and an in-situ doped layer of SiCx with low carbon content. Following a rapid thermal annealing to activate the dopants in the SiCx layer, we observe the formation of bulk defect states close to one of the band edges at temperatures between 450 and 700 degrees C. At temperatures of 800 degrees C and above, these bulk defects are increasingly cured. At the same time, the surface passivation provided by the interfacial oxide is increasingly deteriorated at temperatures above 800 degrees C, eventually permitting the injection of a recombination current into the SiCx layer. Consequently, there is a trade-off for the effective minority carrier lifetime with an optimum between 800 and 830 degrees C. We develop a formalism that distinguishes recombination through shallow bulk defects from the recombination current at the surface in a plot of the effective lifetime tau(eff) over the ratio n/p.

ELSEVIER2022

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In Situ Reflectometry and Diffraction Investigation of the Multiscale Structure of p-Type Polysilicon Passivating Contacts for c-Si Solar Cells

Christophe Ballif, Franz-Josef Haug, Andrea Ingenito, Audrey Marie Isabelle Morisset

The integration of passivating contacts based on a highly doped polycrystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) layer has been identified as the next step to further increase the conversion efficiency of current mainstream crystalline silicon (c-Si) solar cells. However, the interrelation between the final properties of poly-Si/SiOx contacts and their fabrication process has not yet been fully unraveled, which is mostly due to the challenge of characterizing thin-film stacks with features in the nanometric range. Here, we apply in situ X-ray reflectometry and diffraction to investigate the multiscale (1 angstrom-100 nm) structural evolution of poly-Si contacts during annealing up to 900 degrees C. This allows us to quantify the densification and thinning of the poly-Si layer during annealing as well as to monitor the disruption of the thin SiOx layer at high temperature >800 degrees C. Moreover, results obtained on a broader range of thermal profiles, including firing with dwell times of a few seconds, emphasize the impact of high thermal budgets on poly-Si contacts' final properties and thus the importance of ensuring a good control of such high-temperature processes when fabricating c-Si solar cells integrating such passivating contacts. Overall, this study demonstrates the robustness of combining different X-ray elastic scattering techniques (here XRR and GIXRD), which present the unique advantage of being rapid, nondestructive, and applicable on a large sample area, to unravel the multiscale structural evolution of poly-Si contacts in situ during high-temperature processes.

AMER CHEMICAL SOC2022

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