The Study of Electrodeposition of Si in Tmhatfsi-SiCl4 analyzed by EQCM Method

   An inexpensive fabrication method of Si is required for achieving Ultra-Large-Scale Si solar power generation. Electrodeposition method has attracted recently because it has numbers of advantages such as simplicity of process and precision control of reaction etc. compared with conventional methods such as CVD, sputtering and e-beam evaporation.

   Si electrodeposition has been conducted using nonaqueous solvents (molten solt¹⁾, organic solvent²⁾ and ionic liquid³⁾) because the reduction potential of SiO₂ is very negative. However, the organic solvents have a drawback due to safety concerns, with high flammability. On the other hand, ionic liquids (sometimes referred as room temperature molten salts) exhibit low volatility and in many cases are nonflammable. The primary benefit of both systems is the large electrochemical window, allowing for study of elements such as Si and Li. We have reported the electrodeposition of Si from an ionic liquid.⁴⁾

   However, high purity Si electrodeposition has not been demonstrated yet. Si electrodeposition layers include some impurities stemmed from ionic liquids.⁵⁾ It is caused by the uncertainty on Si electrochemical reaction mechanism.

   Electrochemical quartz microbalance (EQCM) impedance method could provide us to obtain in-situ information about the mass change during electrodeposition. Such information might further support our deduction on electrochemical reaction mechanism.

   Here we present the results of our recent work on the analysis of Si electrodeposition using EQCM method. Si is electrodeposited at 40 °C in an ionic liquid, specifically tri-methylhexyl ammonium bis-(trifluorosulfonyl) imide (TMHATFSI). In particular, we focus on the mass change during cyclicvoltammogram and chronoamperometry. Figure1 demonstrates that Si deposition starts around -2.2 V because a significant mass increase is observed with the increase of the current density. It corresponds to our previous study.⁵⁾ Therefore we could propose Si electrochemical reaction mechanism more clearly than before from this EQCM result.

   This study was financially supported in part by CREST program by Japan Science and Technology Agency (JST).

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