Electrodeposition of Crystalline Si Using a Liquid Zn Electrode in Molten KF–KCl–K2SiF6

Abstract

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, the high energy costs of the Siemens process and the poor yields process due to kerf loss are challenges for conventional production methods. As one of the alternative methods, we have proposed a direct formation of crystalline Si films from KF–KCl molten salt using SiCl4 or K2SiF6 as a Si source [1,2]. In this method, although dense and smooth Si films can be electrodeposited, crystal size of obtained Si films was only 20 µm even at 1073 K [3]. Si solar cells with higher efficiency need to reduce grain boundaries, where electrons and holes generated by light can recombine, meaning that larger crystal size is needed.While high temperatures are advantageous for crystal growth, methods to promote crystal growth without requiring high temperatures are also being investigated. For example, there have been some reports of crystalline Si deposition using a liquid Ga electrode in organic solvents and in ionic liquids at much lower temperatures (< 373 K) [4,5]. Considering that only amorphous Si was obtained at solid electrodes, the use of liquid metal electrodes promoted the Si crystal growth. Therefore, we proposed Si electrodeposition using a liquid metal electrode in a high-temperature molten salt in order to deposit larger crystalline Si size [6].In this study, as an initial investigation of direct formation of crystalline Si film using liquid electrodes in high-temperature molten salt, electrodeposition mechanism of Si using a Zn electrode was studied in molten KF–KCl–K2SiF6 at 923 K by comparing electrodeposited samples at different current densities. Experimental The experiments were conducted in molten KF–KCl–K2SiF6 in Ar atmosphere at 923 K in a glove box. As a Si source, K2SiF6 was added to the bath. Liquid Zn held in a small BN crucible was used as the working electrode. The counter and the reference electrodes were Si rods. After electrodeposition, the Zn electrodes and Si extracted from them by hydrochloric acid treatment were analyzed by SEM/EDX. Result and discussion Galvanostatic electrolysis was conducted under the conditions of the added amount of K2SiF6 of 2.0 mol%, the cathodic current density of 20–30 mA cm−2, and charge of 1252 C. As shown in Fig. 1, the potential during electrolysis was more positive at 20 mA cm−2 than at 30 mA cm−2. This indicates that the concentration of Si near the surface in the liquid Zn electrode at 20 mA cm−2 is smaller than that at 30 mA cm−2. Fig. 2(a) and (b) show the optical and SEM images of the sample at 20 mA cm−2, respectively. In the optical image, black deposits with metallic luster were observed on the Zn surface. Since no clearly discernible grain boundaries were observed in the SEM image, the deposits were expected to be large crystalline Si. On the other hand, brown deposits were observed on the Zn surface after electrolysis at 30 mA cm−2, as shown in Fig. 2(c). The SEM image in Fig. 2(d) confirmed that these deposits were wire-like Si.Based on these results, the mechanism of electrodeposition is considered as follows. When the current density is small, the supply of Si to the liquid Zn electrode surface is slower than the diffusion of alloyed Si into the Zn bulk. In this case, Si concentration inside the Zn increases uniformly and Si is deposited only inside the Zn. On the other hand, when current density is large, the supply of Si to the liquid Zn electrode surface is faster than the diffusion of alloyed Si into the Zn bulk. In this case, since Si concentration becomes large near the surface of the Zn, Si is preferentially deposited near the surface and direct electrodeposition of wire-like Si occurs on the deposited Si. R eferences [1] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc., 162, D444 (2015).[2] K. Yasuda, K. Maeda, R. Hagiwara, T. Homma, and T. Nohira, J. Electrochem. Soc., 164, D67 (2017).[3] K. Yasuda, K. Saeki, T. Kato, R. Hagiwara, T. Nohira, J. Electrochem. Soc., 165, D825 (2018).[4] J. Gu, E. Fahrenkrug, and S. Maldonado, J Am Chem Soc, 135, 1684 (2013).[5] J. Zhang, S. Chen, H. Zhang, S. Zhang, X. Yao, and Z. Shi, RSC Advances, 6, 12061 (2016).[6] G.M. Haarberg, T. Kato, Y. Norikawa, and T. Nohira, ECS Trans., 89, 29 (2019). Figure 1