Effects of Coordination Structure of Silicon Ions on Electrodeposition Process of Silicon in Fluoride Melts

Silicon electrodeposition in molten salts is a key technology for fabricating silicon films, as a material for solar cells, by environmentally friendly and inexpensive processes. We have already reported the electrodeposition of silicon films from SiO2 in fluoride melts. However, there are only few studies about the effects of ions in fluoride melts, such as O2- anion, and Li+/Na+/K+ cations, on silicon electrodeposition processes. In particular, it is important to understand the coordination structure of silicon ions for controlling the nucleation and growth processes between solid metal substrates and fluoride melts interface. In this study, we investigated the electrodeposition of silicon from SiO2 in LiF-NaF-KF melts mixed with Li2O as a source of O2- anion at 873 K, and binary LiF-KF, LiF-NaF, and NaF-KF melts. The results of the in-situ Raman spectroscopy and DFT calculations indicated that the O2− anions could cleave the Si-O-Si bonds of SiO2 or silicate ions such as [Si2O5]2−; consequently, the formation of tetrahedron [SiO3F]3− species was promoted. In addition, electrochemical measurements, XRD, SEM, and EDS analysis showed thickness and current efficiency of electrodeposited polycrystalline silicon was affected by the difference in coordination structures of silicon ions in fluoride melts. This study suggested the design of the molten salts bath was significant for controlling interfacial phenomena at electrode/molten salts to fabricate high-quality Si layers with high current efficiency.