Improvement of blue response of black Si solar cells due to graded band structure

We have investigated characteristics of energy band structure of a nanocrystalline Si (nc-Si) layer produced by the surface structure chemical transfer (SSCT) method, especially its effects on quantum efficiency of crystalline Si solar cells. In the nc-Si layer, the size of Si nanocrystals decreases toward the surface because Si dissolves nonuniformly from the surface during the SSCT reaction. The valence band maximum of the outermost surface of the nc-Si layer is located at 0.1-0.3 eV lower than that of Si bulk, and the conduction band minimum increases by 0.17-0.25 eV toward the surface. These results indicate that the nc-Si layer has a graded band structure in which the band-gap energy gradually increases toward the surface. By optimizing the SSCT reaction conditions, thinner thickness (similar to 100 nm) of the nc-Si layer and smaller size of Si nanocrystals can be obtained, resulting in a steep graded band structure. Because of reduction of carrier recombination rate in the surface region by the steep graded band structure, a high internal quantum efficiency over 80% in the 300-400 nm wavelength region is obtained.

Automated summary

The energy band structure of a nc-Si layer produced by the SSCT method was investigated, and it was found to have a graded band structure with increasing band-gap energy towards the surface. By optimizing the SSCT reaction conditions, a thinner nc-Si layer and smaller Si nanocrystals can be obtained, resulting in improved quantum efficiency.