Recombination via transition metals in solar silicon: The significance of hydrogen–metal reactions and lattice sites of metal atoms

Abstract

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.The move toward lower cost sources of solar silicon has intensified the efforts to investigate the possibilities of passivating or reducing the recombination activity caused by deep states associated with transition metals. This is particularly important for the case of the slow diffusing metals early in the periodic sequence which are not removed by conventional gettering. In this paper, we examine reactions between hydrogen and transition metals and discuss the possibility of such reactions during cell processing. We analyse the case of hydrogenation of iron in p-type Si and show that FeH can form under non-equilibrium conditions. We consider the electrical activity of the slow diffusing metals Ti, V, and Mo, how this is affected in the presence of hydrogen, and the stability of TM-H complexes formed. Finally, we discuss recent experiments which indicate that re-siting of some transition metals from the interstitial to substitutional site is possible in the presence of excess vacancies, leading to a reduction in recombination activity.Periodic ordering of the 3d, 4d, and 5d transition metals with adjacent columns of the periodic table. The metals with hydrogen complexes which have had their electronic properties reported in the literature or in this paper are highlighted in green.

Hydrogenation in multicrystalline silicon: The impact of dielectric film properties and firing conditions

“…This explains our previous observation that changing the cooling rate has a negligible influence on the measured lifetimes, as shown in Figure . However, our result disagrees with some previous works that suggest that the passivation reaction mainly occurs during the cooling down period because the binding between hydrogen and metals are rather weak and unstable at high temperature (higher than 200°C) . Moreover, it is found that the same inactive hydrogenated GB became recombination active and remained active afterwards when subjected to a second annealing without the presence of hydrogen containing dielectric films on the surfaces, possibly due to the out diffusion of hydrogen from the Si bulk to the ambient.…”

“…The underlying mechanism of bulk hydrogenation in mc‐Si is less clear. There is still a lack of consensus in literature on how hydrogen diffuses from the dielectric films into Si bulk, exactly which defects can be passivated, and the fundamental physics of the passivation reaction . In previous work, we found that hydrogenation is effective in passivating GBs but rather ineffective for passivation of dislocation clusters and hence is particularly beneficial for high‐performance (HP) mc‐Si that contains a large density of GBs and a lower density of dislocation clusters …”

Temporary Surface Passivation for Characterisation of Bulk Defects in Silicon: A Review

“…. Secondly, although not directly proven to our knowledge at these temperatures, various authors have provided evidence to suggest that hydrogen diffuses into the bulk from the dielectric layers, and hydrogen has the potential to affect bulk lifetime by interacting with other defects . Thirdly, impurities such as iron have been shown to be highly soluble in dielectrics such as SiN x and can be gettered to the Al 2 O 3 –Si interface and thus sufficiently mobile metallic impurities can be potentially removed from the bulk during the passivation step.…”

First-principles calculations of iron-hydrogen reactions in silicon

“…Hydrogenation of Silicon wafers has been recurrently applied in order to passivate, or at least reduce, the recombination activity from several defects and contaminants, including iron and other transition metals. 2,[5][6][7][8][9][10] In Si photovoltaics this hydrogenation process is usually accomplished by means of depositing and firing a hydrogensoaked SiN x layer on top of Si, which besides the surfacepassivation effect, it also works as an anti-reflection coating for the front surface of the cell. 11,12 Other types of hydrogen introduction for passivation treatments have also been considered, including proton implantation 13 or H-plasma exposure, 14 but none is as convenient as the nitridation process.…”