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.
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,
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.