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It has previously been shown that ex situ phosphorus-doped polycrystalline silicon on silicon oxide (poly-Si/SiO x ) passivating contacts can suffer a pronounced surface passivation degradation when subjected to a firing treatment at 800 °C or above. The degradation behavior depends strongly on the processing conditions, such as the dielectric coating layers and the firing temperature. The current work further studies the firing stability of poly-Si contacts and proposes a mechanism for the observed behavior based on the role of hydrogen. Secondary ion mass spectrometry is applied to measure the hydrogen concentration in the poly-Si/SiO x structures after firing at different temperatures and after removing hydrogen by an anneal in nitrogen. While it is known that a certain amount of hydrogen around the interfacial SiO x can be beneficial for passivation, surprisingly, we found that the excess amount of hydrogen can deteriorate the poly-Si passivation and increase the recombination current density parameter J 0. The presence of excess hydrogen is evident in selected poly-Si samples fired with silicon nitride (SiN x ), where the injection of additional hydrogen to the SiO x interlayer leads to further degradation in the J 0, while removing hydrogen fully recovers the surface passivation. In addition, the proposed model explains the dependence of firing stability on the crystallite properties and the doping profile, which determine the effective diffusivity of hydrogen upon firing and hence the amount of hydrogen around the interfacial SiO x after firing.

Defects and impurities in silicon limit carrier lifetimes and the performance of solar cells. This work explores the use of fluorine to passivate defects in silicon for solar cell applications. We present a simple method to incorporate fluorine atoms into the silicon bulk and interfaces by annealing samples coated with thin thermally evaporated fluoride overlayers. It is found that fluorine incorporation does not only improve interfaces but can also passivate bulk defects in silicon. The effect of fluorination is observed to be comparable to hydrogenation, in passivating grain boundaries in multicrystalline silicon, improving the surface passivation quality of phosphorus-doped poly-Si-based passivating contact structures, and recovering boron–oxygen-related light-induced degradation in boron-doped Czochralski-grown silicon. Our results highlight the possibility to passivate defects in silicon without using hydrogen and to combine fluorination and hydrogenation to further improve the overall passivation effect, providing new opportunities to improve solar cell performance.

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