Silicon grain boundary passivation for photovoltaics: A novel approach with small polar molecules

Grain boundaries (GBs) play a major role in determining the device performance of in particular polycrystalline thin film solar cells including Si, CdTe and CIGS. Hydrogen passivation has been traditionally applied to passivate the defects at GBs. However, hydrogenated films such as amorphous silicon (a-Si:H) and microcrystalline silicon (c-Si:H) are subject to light-induced degradation effects. In this study on multicrystalline (mc)-Si wafers, we found an excellent correlation between the grain misorientation and the corresponding electrical resistivity across grain boundaries. In particular, the charge transport across GBs was greatly enhanced after the wafers were properly treated in our polar molecule solutions. The results were explained to be due to the more effective charge neutralization and passivation of polar molecules on localized charges at GBs. These findings may help us achieve high-quality materials at low cost for high-efficiency solar cells by improving the carrier transport and minimizing the carrier recombination. We also believe that this study will help us with a deeper understanding on GBs and their behaviors for the applications not only in photovoltaics, but also in other solid-state devices such as thin-film transistors.