Electrodeposition of Si Film from Water-Soluble KF-KCl Molten Salt and Feasibility of SiCl4 as a Si Source
Authors: Yasuda Kouji , Saeki Kazumi , Maeda Kazuma , Nohira Toshiyuki , Hagiwara Rika , Homma Takayuki 
Published: February 2020

1. Introduction

Presently, Si substrate used for solar cells are manufactured by a multi-step process of production of polycrystalline Si, pulling up of single crystalline Si ingot or casting into a mold, and slicing by diamond wire saws. However, this process inevitably produces a large amount of kerf loss, which lowers the yield of the product. Thus, a direct formation of high-quality crystalline Si films on desired substrates at a low cost has been desired as a new production process of crystalline Si solar cells. For this purpose, electrodeposition in high temperature molten salts has been studied as a potential method [1,2]. Recently, we proposed a new electrodeposition process of crystalline Si in which molten KF–KCl is used as an electrolyte and SiCl4as a Si source [3].
In the present study, we first explored the optimum condition for electrodepositing compact and smooth Si films in molten KF–KCl, and then investigated the feasibility of SiCl4gas as a Si source.

2. Experimental

The electrodeposition was conducted in a dry Ar atmosphere at 923 K, 1023 K or 1073 K. Reagent-grade KF and KCl were mixed to a eutectic composition (KF:KCl = 45:55 mol%, melting point = 878 K) and loaded in a graphite crucible. The crucible was placed at the bottom of a quartz or stainless steel vessel in an air-tight Kanthal container and dried under vacuum at 673 K for 24 h. A Ag plate was used as the working electrode and a glassy carbon rod as the counter electrode. A Pt wire was employed as the quasi-reference electrode. The potential was calibrated and given with reference to K+/K electrode potential. Galvanostatic electrolysis was conducted with various K2SiF6 concentrations and current densities. The electrolyzed samples were washed in hot distilled water at 333 K for 24 h to remove the adhered salt on the deposit, and dried under vacuum for 12 h. The samples were analyzed by SEM, EDX, EBSD, XRD and Raman spectroscopy. An SiCl4 gas was introduced into KF-KCl molten salt containing no K2SiF6by using argon as a carrier gas. After the introduction, cyclic voltammetry and galvanostatic electrolysis were conducted to confirm the electrodeposition of silicon.

3. Results and discussion

The galvanostatic electrolysis was carried out in various current densities and K2SiF6 concentrations in a eutectic KF-KCl melt. The ionic state after the introduction of SiCl4 gas in the melt is regarded as the same with the addition of K2SiF6to the melt.
SiCl4 + 6F− → SiF6
2− + 4Cl− (1)
From cross-sectional SEM/EDX analysis of the samples, the optimum condition for obtaining compact and smooth Si films has been found to be the K2SiF6 concentration range of 2.0–3.5 mol% and the current density range of 50–200 mA cm−2at 923 K [4]. XRD and Raman spectrocscopy confirmed that the obtained film was crystalline Si. According to the EBSD analysis, the typical crystallite size of the deposited Si was <0.1μm at 923 K, 1–10 μm at 1023 K and 5-30 μm at 1073 K. The dependence of the crystallite size on the deposition temperature could be explained by the crystallization rate of deposited Si.
In order to investigate the feasibility of SiCl4 as a Si source, 2.88 mol% of SiCl4 gas was introduced into a pure KF–KCl molten salt. After the introduction, cyclic voltammetry was performed on a Ag electrode to check the existence of Si(IV) ions. The obtained cyclic voltammogram shown in Fig. 1 indicates a couple of redox peaks which is indicative for Si(IV) ions. The concentration of Si(IV) ions is estimated to be 2.30 mol% from the height of peak current density. The sample prepared by galvanostatic electrolysis on a Ag wire at 155 mA cm-2for 30 minutes was confirmed to be a crystalline Si film.

Acknowledgement

This study was partly supported by the Core Research for Evolutionary Science and Technology (CREST) of the Japan Science and Technology Agency (JST).

Reference

[1] U. Cohen and R.A. Huggins, J. Electrochem. Soc., 123, 381 (1976).
[2] D. Elwell and R.S. Feigelson, Sol. Energ. Mat., 6,　123 (1982).
[3] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara and T. Homma, J. Electrochem. Soc., 162, D444 (2015).
[4] K. Yasuda, K. Maeda, T. Nohira, R. Hagiwara and T. Homma, J. Electrochem. Soc., 163, D95 (2016).

Figure 1