Electrodeposition of Crystalline Silicon Films in Molten KF–KCl–K2SiF6 for Photovoltaic Applications
Authors: Takahashi Shunta , Norikawa Yutaro , Yasuda Kouji , Nohira Toshiyuki 
Published: December 2020

1. Introduction

Recently, the demand for solar cells has been increasing to realize low-carbon society. Among various types of solar cells, crystalline Si solar cells have dominated the photovoltaic market due to the high efficiency, excellent stability, and abundant natural resources. However, the low productivity of the Siemens process and the considerable kerf loss in the Si-slicing step are the main drawbacks in the conventional production method of the Si substrates. Since the demand for crystalline Si solar cells will continue to increase, the development of an alternative production method of Si substrate is strongly required.
One of the expected methods is a direct formation of Si films on an inexpensive substrate [1–3]. We have proposed the electrodeposition of Si films from KF–KCl molten salt, in which SiCl4 gas is used as a Si source [4,5]. We have already investigated the optimum conditions for obtaining adherent, compact, and smooth Si films in molten KF–KCl–K2SiF6 at 923 K [6], and the effect of temperature and current density on the Si electrodeposition in molten KF–KCl–K2SiF6 at 923–1073 K [7]. However, the detailed analysis including the evaluation of semiconductor characteristic has not been performed for the Si films.
In the present study, we prepared crystalline Si films on graphite substrates under several conditions in molten KF–KCl–K2SiF6 at 1023 K. The obtained Si films were analyzed by XRD and SEM/EDX. The semiconductor characteristic was also evaluated by a photoelectrochemical measurement which had been used in the report by Bard et al. [2,3].

2. Experimental

2.1 Electrodeposition of Si
The electrodeposition was conducted in molten KF–KCl–K2SiF6 in Ar atmosphere at 1023 K in a glove box. As a Si source, 3.5 mol% of K2SiF6 was added to the bath. A graphite plate was used as the working electrode. The counter and the reference electrodes were Si rods. Si films prepared by galvanostatic electrolysis were analyzed by XRD, SEM/EDX, and a photoelectrochemical measurement.
2.2 Photoelectrochemical measurement
Photoelectrochemical measurement was conducted by linear sweep voltammetry. The electrolyte was prepared by mixing TBAClO4 (0.3 M) and EVBr2 (0.05 M) in CH3CN at room temperature.
2TBAClO4 + EVBr2 → EV(ClO4)2 + 2TBABr↓
After the reaction was completed, the supernatant liquid in which EV(ClO4)2 was dissolved was used as the electrolyte. A Pt plate and an Ag+/Ag electrode were used as the counter and reference electrode, respectively. A Xe lamp (100 mW cm−2) was used as a light source. The light was chopped at a frequency of 1 Hz during the linear sweep voltammetry.

3.Result and discussion

3.1 Electrodeposition of Si
Galvanostatic electrolysis was conducted under the conditions of the added amount of K2SiF6 of 3.5 mol% and the cathodic current density of 100 mA cm−2 for 15 min. As shown in Fig. 1(a), the deposit on a graphite substrate has dark gray color. From XRD analysis, the deposit was confirmed to be crystalline Si. Fig. 1(b) shows a surface SEM image of the Si film, indicating compact and smooth surface with crystal grains of around 10 μm. The current efficiency was calculated to be 90.1 % from the amount of electricity and the weight increase.
3.2 Photoelectrochemical measurement
The Si film on a graphite substrate was covered with an insulating coating to expose only the Si part and to prescribe the electrode area. Fig. 2 shows a photoresponse of the Si film during the linear sweep voltammetry, where the light is chopped at a frequency of 1 Hz. Cathodic currents change depending on the light chopping, indicating that the obtained Si film is p-type semiconductor.

Acknowledgement

The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto University.

R
eferences

[1] E. Juzeliunas and D. J. Fray, Chem. Rev., 120, 1690 (2020).
[2] J. Zhao, H. Yin, T. Lim, H. Xie, H. Hsu, F. Forouzan, A. J. Bard, J. Electrochem. Soc., 163, D506 (2016).
[3] X. Zou, L. Ji, J. Ge, D. R. Sadoway, E. T. Yu, and A. J. Bard, Nat. Commun., 10, 5772 (2019).
[4] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc., 162, D444 (2015).
[5] K. Yasuda, K. Maeda, R. Hagiwara, T. Homma, and T. Nohira, J. Electrochem. Soc.,
164, D67 (2017).
[6] K. Yasuda, K. Maeda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc.,
163, D95 (2016).
[7] K. Yasuda, K. Saeki, T. Kato, R. Hagiwara, and T. Nohira, J. Electrochem. Soc.,
165, D825 (2018).

Figure 1