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Why is it that we need ~99% pure silicon in order to have efficient solar cells? I understand that as silicon purity decreases so does solar cell efficiency but I'd like to understand why.

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  • $\begingroup$ I suppose purity needs to be quite a bit higher than that to create a $pn$-junction. $\endgroup$ – Pieter Aug 30 '20 at 22:16
  • $\begingroup$ I’m device grade silicon, dopants are in the parts per million range. Random crud at higher levels is Bad with a capital B. (Well, carbon and oxygen have their own reasons and uses). $\endgroup$ – Jon Custer Aug 31 '20 at 0:06
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Impurities increase recombination rate and will reduce efficiency of the solar cell.

A high-efficiency silicon solar cell has quite a thick p-doped region,

silicon solar cell

This is required because to achieve decent optical absorption; silicon has quite a low absorption coefficient in the visible and near infrared.

Fortunately, the diffusion length of photo-generated minority carries in high-quality silicon is very long.

The cell is designed to balance these competing processes: a thick cell increases the number of solar photons absorbed (increasing efficiency), but making it too thick, above the diffusion length, will cause an increase in recombination and decrease efficiency.

If the silicon is not highly pure this delicate balance will be disrupted. An electron (minority carrier) diffusing in the p-region will recombine, by interaction with an impurity, before is can diffuse across the junction and be collected.

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for a piece of Si to behave as a semiconductor requires there be almost no other elements in solid solution within it except for the dopants; otherwise the junction structures will not function properly.

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  • $\begingroup$ you are right, will edit. -NN $\endgroup$ – niels nielsen Aug 31 '20 at 16:00

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