Effect of bulk resistivity on the performance of spherical Si solar cells

Effect of bulk resistivity (rho) on the performance of spherical Si solar cells has been investigated. The light absorption part of the spherical Si solar cells is based on a p-type multicrystalline Si (mc-Si) sphere with a diameter of 1 mm. Thus, a longest current path for hole in the cells reaches as high as 1mm, which is considerably greater than 0.2 mm for typical mc-Si wafer-based solar cells. In this study, spherical Si crystals with rho=0.2~5 Omegacm were fabricated by a dropping method and spherical solar cells were fabricated through our baseline process. The current-voltage characteristics revealed that the series resistance (R_s) of the spherical Si solar cells strongly depends on its bulk resistivity; rho=2.0Omegacm, which is a typical value for mc-Si wafer-based solar cells, caused a high R_s of 1.2Omegacm² for spherical Si solar cells, resulting in a poor fill factor of less than 0.70. Also, the results showed that a bulk resistivity of lower than 1.0Omegacm is required to obtain R_s of lower than 0.5 Omegacm². These results demonstrated that the bulk resistivity is one of the key factors to improve the performance of the spherical Si solar cells