Formation of Crystalline Si Film Using Liquid Zn Electrode in Molten KF–KCl–K2SiF6
Authors: Moteki Wataru , Norikawa Yutaro , Nohira Toshiyuki 
Published: December 2024

Introduction

Recently, crystalline Si solar cells have been the mainstream in the solar cell market, and the demand for crystalline Si solar cells is expected to increase in the future. However, it is difficult to further improve productivity with the current manufacturing method. It is a multi-step process and includes processes with high energy consumption due to the inclusion of the Siemens method and a process with low yield due to kerf losses. As a new production method for crystalline Si solar cells, our research group has proposed direct electrodeposition of Si films in molten KF–KCl using SiCl4 as a Si source [1]. We have already reported electrodeposition of Si film with a dense and smooth surface in molten KF–KCl–K2SiF6, which is the same bath state as after SiCl4 introduction [2]. Although this method produced dense and smooth crystalline Si films, the grain size of electrodeposited Si was as small as 20 µm even at 1073 K. To fabricate Si solar cells with high efficiency, it is necessary to reduce the number of grain boundaries where electrons and holes generated by light recombine, which requires increasing the crystal size.
Referring to a previous report of Si deposition using a liquid Ga electrode at 373 K [3], we focused on using a liquid metal electrode in high-temperature molten salt to increase the grain size. We have already reported the electrodeposition of Si using a liquid Zn electrode held in a BN crucible in molten KF–KCl–K2SiF6 at 923 K [4]. The maximum grain size of electrodeposited Si was approximately 2 mm. However, the obtained Si was not in film form.
To obtain crystalline Si film with large grain size, we propose, in the present study, a direct film formation method using thin liquid Zn. Fig. 1 shows a schematic diagram of the proposed mechanism. In the initial stages, Si(IV) ions are electrochemically reduced to form Si–Zn liquid alloy on the liquid Zn electrode held on the substrate. As the electrodeposition progresses, Si forms a film on the substrate and crystals grow. Finally, Zn is removed to obtain a crystalline Si film with a large grain size on the substrate. The goal for this crystalline Si film is a grain size of at least 1 mm and a thickness of 50–100 µm.

Experimental

All electrodepositions were performed under an Ar atmosphere in a glove box. The electrodeposition process was carried out in two steps as shown in Fig. 2. Electrodeposition to prepare thin liquid Zn was performed in molten NaCl–KCl–ZnCl2 at 723 K using a Si substrate. After electrodeposition of Si in molten KF–KCl–K2SiF6, the samples were washed with water for salt removal, treated with HCl for Zn removal, and analyzed by SEM/EDX.

Result and discussion

Fig. 3(a) shows an SEM image of the sample after Zn deposition in molten NaCl–KCl–ZnCl2 at a current density of 50 mA cm–2 and charge density of 21 C cm–2. EDX analysis showed that the gray areas were Zn and the black areas were Si substrate. Si deposition was then performed using the obtained Zn deposit as a working electrode in molten KF–KCl–K2SiF6 at 923 K at a current density of 20 mA cm–2 and charge density of 90 C cm–2. Fig. 3(b) shows an SEM image of the sample after removal of Zn. Crystalline Si was observed over the entire surface, but the distribution was uneven for Si with large grain sizes. This was thought to be caused by the fact that the Zn film was not uniform.
These results suggest that the preparation of uniform thin Zn is necessary to realize the proposed mechanism. The uniform Zn film may be achieved by adding ZnCl2 to molten KF–KCl–K2SiF6 to co-deposit Si and Zn. Since Zn has a more positive deposition potential than Si, Zn is preferentially deposited in co-deposition. Fig. 4 shows an optical image of the sample prepared at 150 mA cm–2 for 90 C cm–2 in molten KF–KCl–K2SiF6–ZnCl2 at 923 K. A film-like deposit with metallic luster was obtained.

R
eferences

[1] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc., 162, D444 (2015).
[2] K. Yasuda, K. Saeki, T. Kato, R. Hagiwara, T. Nohira, J. Electrochem. Soc., 165, D825 (2018).
[3] J. Gu, E. Fahrenkrug, and S. Maldonado, J Am Chem Soc, 135, 1684 (2013).
[4] W. Moteki, Y. Norikawa, and T. Nohira, J. Electrochem. Soc., 170, 062506 (2023).

Figure 1