A New Electrodeposition Process of Crystalline Silicon Utilizing Water-Soluble KF–KCl Molten Salt

As a new production process of silicon solar cells, electrodeposition in high temperature molten salts is worth investigating. One of the most studied molten salt systems for electrodeposition of silicon is LiF-KF or LiF-NaF-KF containing K₂SiF₆. Many important works regarding this molten salt were undertaken in 1970s and 1980s [1-4]. Cohen reported that both epitaxial and polycrystalline continuous films of compact, coherent and well-adherent silicon were obtained from LiF-KF containing K₂SiF₆ at 1023 K [2]. One of the problems in the previous works is the removal of the adhered salt on the deposited silicon. This is due to the very low solubility of LiF and NaF to water. Another problem of the previous studies, when aiming at the solar grade purity, is the availability of high purity and low cost silicon ion sources. Conventionally, either K₂SiF₆ or anode dissolution of Si rod was utilized. However, it is difficult to prepare K₂SiF₆and Si rod having solar grade purity with low cost.

From this background, we propose a new electrodeposition process of crystalline silicon, as shown in Fig. 1.  Here, fluoride–chloride molten salt is used as an electrolyte and SiCl₄ as a silicon ion source. Among many kinds of fluoride–chloride molten salts, we especially focus on the eutectic KF–KCl (45:55 mol%) molten salt. Since both KF and KCl are highly soluble to water, the adhered salt can be easily removed by water washing. Moreover, high purity SiCl₄, which is produced with low cost, can be used as an ion source. When gaseous SiCl₄is introduced into the molten KF–KCl, the following reaction proceeds:

SiCl₄ + 6F⁻ → SiF₆²⁻ + 4Cl⁻                                                      (1)

Thus, after the introduction of SiCl₄ gas, the resultant molten salt is same as the molten KF–KCl containing K₂SiF₆. Another advantage of the use of fluoride–chloride molten salt is the capability of Cl₂ gas evolution on the carbon anode, which realizes the fixed composition of molten salt as well as the reuse of Cl₂ for the production of SiCl₄.

Firstly, electrochemical reduction of Si(IV) ion on a silver electrode was investigated by cyclic voltammetry in a molten KF–KCl–K₂SiF₆ system at 923 K. Then, the optimum conditions were explored for electrodepositing adherent, compact and smooth silicon layer. Galvanostatic electrolysis was conducted for silver electrodes with various current densities and K₂SiF₆ concentrations. As typical results, Fig. 2 shows cross-sectional SEM images of the samples electrodeposited at (a) 38.8, (b) 155 and (c) 310 mA cm^-2 in 2.0 mol% K₂SiF₆. A compact and smooth Si layer was obtained at an intermediate current density of 155 mA cm^-2. At a lower current density of 38.8 mA cm^-2, detachment of Si layer from the Ag substrate occurred during the water washing. At a higher current density of 310 mA cm^-2, the morphology changed into nodular.

Acknowledgements

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

References

[1] U. Cohen and R. A. Huggins, J. Electrochem. Soc., 123, 381 (1976).

[2] U. Cohen, J. Electron. Mater., 6, 607 (1977).

[3] D. Elwell and R. S. Feigelson, Sol. Energ. Mat., 6, 123 (1982).

[4] D. Elwell and G. M. Rao, J. Appl. Electrochem., 18, 15 (1988).