(Invited) Electrodeposition of Si in CsF–CsCl Eutectic Melt
Authors: Norikawa Yutaro , Kondo Airi , Yasuda Kouji , Nohira Toshiyuki 
Published: July 2021

1.Introduction
In recent years, the demand for solar cells as a clean energy source has been increasing. Among the various types of solar cells, crystalline Si solar cells dominate the solar cell market due to their high efficiency, excellent stability, and abundant natural resources. However, the major disadvantages of the conventional Si substrate manufacturing method are the low productivity of the Siemens process and the considerable kerf loss in the Si slicing process. As the demand for crystalline Si solar cells continues to grow, there is a strong need to develop alternative manufacturing methods for Si substrates.
As a promising method, the direct formation of Si films on inexpensive substrates has been investigated [1]. We have proposed the electrodeposition of Si films from molten KF–KCl which has relatively high fluoride-ion concentration at eutectic composition (KF:KCl = 45:55 mol%, m.p.: 878 K) and high solubility in water [2]. We have already studied the optimum conditions for obtaining adherent, compact, and smooth Si films in molten KF–KCl–K2SiF6 at 923 K [3]. Furthermore, good Si films was obtained from KF–KCl after bubbling SiCl4 as a Si-ion source [4].
In this study, we focused on CsF–CsCl eutectic melt (CsF:CsCl = 50:50 mol%, m.p.: 713 K [5]) as a new electrolyte for Si electrodeposition. Molten CsF–CsCl has lower operable temperature and higher fluoride-ion concentration than molten KF–KCl at the eutectic composition. Moreover, CsF and CsCl have extremely high solubility in water (CsF; 573, CsCl; 191 g per 100 g H2O at 293 K [6]), indicating that the solidified salts can be easily removed. In this study, Si was electrodeposited on Ag plate substrates at 923 K at various Si-ion concentrations and cathodic current densities. The obtained Si films were analyzed by XRD and SEM/EDX to determine the optimum electrodeposition conditions.
2. Experimental
The experiments were conducted in CsF–CsCl eutectic melt in an Ar glovebox. The experimental temperature was 923 K and Cs2SiF6was used as the Si ions source. Ag plate and Ag flag electrodes were used as the working electrodes. The counter and reference electrodes were Si rods. The potential of the reference electrode was calibrated by Cl2/Cl− potential measured at a glass-like carbon electrode. Samples prepared by galvanostatic electrolysis of Ag plates substrates were analyzed by SEM/EDX after washing with distilled water to remove adhered salts.
3. Results and Discussion
The electrodeposition was carried out by changing the added amounts of Cs2SiF6 from 0.50 to 3.5 mol%. The cathodic current density was also varied in the range of 25–800 mA cm−2. As typical deposits, Fig. 1 shows SEM images of Si prepared at (a) 600, (b) 100, and (c) 50 mA cm−2 in CsF–CsCl–Cs2SiF6 (2.0 mol%). At a higher current density, Si was nodule-shaped because the concentration gradient of Si(IV) ions increased near the electrode, and Si deposition proceeded preferentially on the convex part close to the bulk molten salt. At medium current densities (e.g., 100 mA cm−2), the electrodeposited Si was film-like and the surface was smooth. Fig. 2 shows a cross-sectional SEM image of the film, indicating the smooth and compact deposit. The thickness is about 20 μm (the theoretical film thickness: 25 μm), indicating a relatively high current efficiency. Furthermore, it was found that the upper current limit to obtain Si with smooth surface is higher at the higher Cs2SiF6 concentration; Si films with smooth surface were obtained even at 600 mA cm−2 in CsF–CsCl–Cs2SiF6 (3.5 mol%). At low current density, Si was film-like, but many humps were observed due to insufficient overvoltage and difficulty in nucleation.
Acknowledgement
The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ.
References
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Figure 1