Electrodeposition of Crystalline p-Si in Molten KF–KCl for Application to Solar Cells
Authors: Hou Zhengyang , Moteki Wataru , Norikawa Yutaro , Nohira Toshiyuki 
Published: December 2024

Introduction
Electrodeposition of Si film is a promising manufacturing method to solve the problem of high cost of crystalline silicon solar cells produced by conventional methods. Bard et al. made a crystalline-Si solar cell by a two-step electrodeposition from molten CaCl2–CaO–SiO2 at 1123 K [1]. The p-Si film was firstly electrodeposited onto the graphite plate in the molten salt using Al2O3 or B2O3 as a dopant source. Then, n-Si was electrodeposited onto the obtained p-Si in another molten CaCl2–CaO–SiO2 using Sb2O3 or Ca3(PO4)2 as a dopant source to make a solar cell. The power conversion efficiency (PCE) was reported to be 3.1%. However, the current efficiency of electrodeposition was low and the deposition rate was low, making the process time consuming. Furthermore, they did not report the concentration of dopants, meaning that there is a large margin to improve the PCE by optimizing the concentration of dopants.
Our research group has investigated electrodeposition of crystalline Si films in molten KF–KCl using SiCl4 or K2SiF6 as a Si source [2,3]. Smooth Si films have been obtained with high current efficiency and high current density, i.e., high deposition rate. Moreover, we have reported that dense and smooth Si films electrodeposited in KF–KCl–K2SiF6 exhibit n-type semiconductor characteristics, and in KF–KCl–SiCl4 exhibit p-type semiconductor characteristics [4]. Impurities of group 13 and 15 elements are thought to be from the Si source reagents, indicating that the semiconductor properties of the electrodeposited Si films can be controlled if the impurity concentration can be controlled. In order to fabricate Si solar cells with p-n junctions by electrodeposition, it is necessary to find the optimum conditions for electrodeposition of p- and n-Si films. In this study, we investigated the optimal KBF4 concentration to improve the semiconductor properties of p-Si films for solar cells.
Experimental
The experiments were conducted in KF–KCl–K2SiF6 (KF:KCl = 60:40 mol%, K2SiF6 = 3.5 mol%) at 1023 K in an Ar glovebox. KBF4 as a dopant source was added to the molten salt. The concentration of KBF4 was 0.5, 2, and 10 mol ppm. Graphite plate electrodes were used as the working electrodes. The counter electrode was a glassy carbon rod. The reference electrode was Pt wire. Semiconductor properties of the Si samples prepared by galvanostatic electrolysis were evaluated by linear sweep voltammetry in CH3CN–TBAPF6 (0.1 M)–EV(ClO4)2 (0.05 M) at 293 K under 100 mW cm−2 light with a chopper. The samples were also analyzed by SEM and XRD to observe the morphology and crystallinity of the film.
Results and Discussion
Figure 1 shows the optical images and surface SEM images of the samples electrodeposited at 100 mA cm−2 for 180 C cm−2 at 1023 K. The part of graphite plates immersed in molten salt was covered with dark blue electrodeposits. XRD analysis showed that the obtained Si films were well-crystalized Si.
Figure 2 shows linear sweep voltammograms of the Si samples and a commercial p-Si wafer in CH3CN–TBAPF6 (0.1 M)–EV(ClO4)2 (0.05 M) at 293 K under light with a chopper. In the case of Si film obtained with 0.5 mol ppm KBF4 addition, no photocurrent was observed, as indicated by the purple line. This is considered to be a compensated semiconductor as the donor and acceptor (added boron) coexist and the effects of the donor and the acceptor cancel each other out. In the photoelectrochemical measurements of Si films obtained with 2 and 10 mol ppm KBF4 addition and a commercial p-Si wafer, stepwise currents were observed corresponding to the chopping of light, as shown by the red, blue, and black lines. This indicates the presence of photoexcited electrons, confirming that it is p-Si. However, the Si film obtained with 10 mol ppm KBF4 addition had a larger dark current (current when no light is irradiated), indicating that the concentration of KBF4 was too high. Taking these into consideration, it is thought that the optimal concentration of KBF4 addition is around 2 mol ppm.
References
Zou, L. Ji, J. Ge, D. R. Sadoway, E. T. Yuand A. J. Bard, Nat Commun, 10, 5772 (2019).
Maeda, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, J. Electrochem. Soc.,162, D444 (2015).
Yasuda, K. Maeda, R. Hagiwara, T. Homma, and T. Nohira, J. Electrochem. Soc., 164, D67 (2017).
Yasuda, T. Kato, Y. Norikawa, and T. Nohira, J. Electrochem. Soc., 168, 112502 (2021).

Figure 1