Nanocrystalline Silicon Superlattice Solar cells

Abstract

We report on the growth and properties of nanocrystalline Si superlattice solar cells. The solar cells consisted of a stack of alternating layers of amorphous and nanocrystalline Si. The thickness of each of the two layers in the superlattice structure was varied independently. It was found that when the nanocrystalline layer thickness was low, increasing the thickness of the amorphous layer in the superlattice systematically reduced the <220> grain size, while the <111> grain size remained essentially invariant. This fact shows that by interposing an amorphous layer between two nanocrystalline layers forces the nano grains to renucleate and regrow. It was also found that when the amorphous Si layer was too thick, there were significant problems with hole transport through the device. Measurements of defect densities and effective diffusion lengths showed that there was an optimum thickness of the amorphous layer (about 10 nm) for which the defect density was the lowest and the diffusion length was the highest. We also show that the absorption coefficient in nano Si depends upon the grain size and can be increased significantly by increasing the grain size.