Bulk Defects and Hydrogenation Kinetics in Crystalline Silicon Solar Cells With Fired Passivating Contacts

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 °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 °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 (${{\boldsymbol{\tau }}_{\mathbf{eff}}}$ ∼700 μs compared to ${{\boldsymbol{\tau }}_{\mathbf{eff}}}$ ∼1300 μs for symmetrical samples), and slower kinetics (${{\boldsymbol{\tau }}_{\mathbf{reac}}}$ ∼5 min comparedto ${{\boldsymbol{\tau }}_{\mathbf{reac}}}$ ∼0.8 min).