Study of photovoltaic properties and light absorption in poly-Si thin film solar cells grown by the metal-induced growth method

Abstract

In our study, the polycrystalline Si thin films were grown for solar cell applications by using the metal-induced growth (MIG) process. In this process, the poly-Si heteroepitaxally grows from the NiSi₂ or CoSi₂ layers which were formed by the reaction of Ni or Co with Si. Usually, due to the low absorption of light in the crystalline Si, light confinement is an important issue in thin film poly-Si solar cells. The MIG poly-Si thin films have some intrinsic features which assist the light absorption and light trapping. First, the top surfaces of the poly-Si films are relatively rough and have grain facets which reduce the light reflectance. In comparison, the Ni-induced Si film has hemisphere-shaped grain tops while Co-induced Si films have pyramid-shaped grain tops. The Ni-induced Si film has a rougher top-surface, so it appears to be darker under the optical examination compared to the Co-induced sample. This implies that more light may be absorbed in the Ni-induced Si film than in the Co-induced one. Second, in the MIG process, a thin metallic layer under the Si film was formed as a seed-layer for Si growth. This metallic layer could serve as a back contact and a back surface reflector. The above top and back surface structures are naturally formed and will allow MIG poly-Si to absorb the light more efficiently than other techniques. So far, the Schottky solar cell which was fabricated for the intent of MIG poly-Si film property studies has shown a J_sc of 12 mA/cm². By considering the Si film thickness of 5 μm and the photon absorption of 60% at this thickness, these data are reasonable.