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/SiO
_x
/SiC
_x
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-SiN
_x
: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 SiC
_x
layers. The kinetics of the hydrogenation process seems to be limited by the available hydrogen supply at the c-Si/SiO
_x
interface, rather than by its diffusion within the bulk of the wafer. Moreover, it is affected by the bulk doping as well as the SiC
_x
layer thickness. Finally, it is shown that hydrogenation is also possible with an a-SiN
_x
:H/D reservoir layer deposited on one side of the wafer only, although resulting in a lower passivation level (
${{\boldsymbol{\tau }}_{\mathbf{eff}}}$
∼700
μ
s compared to
${{\boldsymbol{\tau }}_{\mathbf{eff}}}$
∼1300
μ
s for symmetrical samples), and slower kinetics (
${{\boldsymbol{\tau }}_{\mathbf{reac}}}$
∼5 min comparedto
${{\boldsymbol{\tau }}_{\mathbf{reac}}}$
∼0.8 min).