There are two different types of gettering that can occur within a silicon solar cell or wafer, which are classified as either internal gettering and external gettering.
Internal gettering is a process where the impurities form alternate species such as precipitates, which are less recombination active. For instance, precipitated oxygen is known to be detrimental to solar cell performance. However, these precipitates can act as the gettering sites where metallic impurities will diffuse to, due to the higher diffusivity within that region . Nickel (Ni) and copper (Cu) are known to be effectively gettered by oxygen precipitates. Cobalt (Co) is less effectively gettered and furthermore, along with Pb, Co hinders the gettering effect by the oxygen precipitates. Phosphorus gettering is more effective to getter iron than gettering by oxygen precipitates . Low-temperature annealing (LTA) can also be a way to precipitate metallic impurities. For example, Fe has lower recombination activity when it is in the precipitated form instead of in the interstitial form .
External gettering removes the impurities from the bulk region of the silicon wafer. Three prominent examples are: phosphorus diffusion gettering, aluminium gettering and boron diffusion gettering. Phosphorus diffusion gettering occurs during phosphorus diffusion within the phosphorous diffused region. During the diffusion, metallic impurities, such as Cr, Cu, Fe and titanium (Ti), which have high diffusivity, getter and accumulate in the diffused region . There are limitations with this process. Due to the inhomogeneity of mc-Si, gettering is enhanced in some regions relative to others [5-7]. Furthermore, for large dislocation densities (> 106 cm-3), the improvement from phosphorous getting is limited due to the high diffusivity within the dislocations [8-10]. This is because metallic impurities are collected to the dislocations rather than the diffused region, and so they remain detrimental to solar cell performance. In some instances, a heavy phosphorous diffusion will be performed on the wafer, followed by an alkaline etch to remove the gettered and diffused layer. This effectively yields a raw wafer with less metallic impurities and higher lifetime potential.
The gettering efficacy strongly depends on the diffusivity of the impurities towards the heavily diffused region. Higher process temperatures can improve the diffusivity of the impurities towards the surface, but higher temperature can also dissolve metallic impurities, where they become more recombination active compared to the precipitated form. Furthermore, carbon or oxygen can hinder the gettering process. Al-Si alloy, which is used to form a rear p+ region (commonly called a back surface field or “BSF”), also causes aluminium gettering that can act as a sink to collect impurities.
The animation below shows the gettering process taking place during phosporous diffusion.
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