Is impurity gettering or passivation by hydrogen the improvement key of mc-Si solar cells during processing steps?

Abstract

During conventional processing steps needed to make solar cells from mc-Si wafers the lifetime or diffusion length of minority carriers, are drastically improved. The first step, P diffusion, induces a marked gettering of fast diffusers. The second step is the deposition of an antireflection hydrogen rich layer on the front face and a thick aluminium layer on the back face, followed by a rapid thermal annealing which forms an Al–Si alloy. It was proposed that during this last step, the alloy formation induces an additional gettering and injects Si vacancies which favour the penetration of hydrogen in the bulk. We arrive to the conclusion that: the main improvement of the bulk properties results of gettering effects. P diffusion must be applied first, then Al–Si alloy formation improves drastically the material. Hydrogenation is also efficient provided the material was previously purified.