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Si thin films are key material for electronic/energy devices such as photovoltaic cells and battery electrodes, as well as various MEMS devices. For their fabrication process, electrochemical approaches attract attention featuring high productivity and precise controllability to form micro/nano structures to wide surfaces. Since Si has very negative redox potential, non-aqueous electrolytes with molten salts, ionic liquids (ILs), or organic solvents are required. We have applied the ILs such as trimethylhexyl ammonium (TMHA+) bis-trifluorosulfonyl imide (TFSI-) and investigated their deposition process focusing upon the initial stage. The analysis using electrochemical quartz crystal microbalance (EQCM) [1] suggested that the reaction of the source of Si in the electrolyte, SiCl4, proceeded via Si (III) and Si(II) intermediate species. From the results of X-ray reflection (XRR) analysis and density functional theory calculation [2], it was indicated that the SiCl4 species adsorped onto the surface initially formed multimer, and this “precursor” was subsequently transformed into Si layers. The authors would like to thank Dr. J. Komadina, Mr. M. Nishida, and Mr. T. Fujimura for their experimental help and valuable discussion. This study was financially supported in part by the Japan Science and Technology Agency (JST) CREST program. [1] Y. Tsuyuki, A. Pham, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta, 183, 49 (2015). [2] Y. Tsuyuki, T. Fujimura, M. Kunimoto, Y. Fukunaka, P. Pianetta, T. Homma, J. Electrochem. Soc., 164, D994 (2017).

The reaction mechanism of Si electrodeposition in the ionic liquid trimethyl- n -hexylammonium bis(trifluoromethylsulfonyl) imide (TMHATFSI) with SiCl 4 was investigated using an electrochemical quartz crystal microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). The deposited films were further characterized by Raman spectroscopy. The EQCM method measured the frequency change in situ during electrodeposition, whereas XPS provided the Si mass concentration in the electrodeposited films. By combining these results, the mass change due to SiCl 4 reduction was estimated. Then, the current efficiency for Si electrodeposition at various potentials was evaluated. The calculated current efficiency was almost 100% when it was assumed that SiCl 4 reduction progressed by four-electron electroreduction. The XPS and Raman results showed that the deposited films were composed primarily of amorphous Si with Si–Si bonds. Crystalline Si thin films were obtained after annealing in an Ar gas stream.

Si thin films and micro/nano structures have been widely used for various applications such as semiconductor and MEMS devices, solar cells, battery electrodes, etc. While dry processes such as chemical vapor deposition have been conventionally applied for their fabrication, electrochemical approaches attract attention for their capability to form uniform and precise structures to wide and non-flat surfaces, with high productivity. In this paper we describe the results of our investigation on Si electrodeposition, mainly focusing upon the process development as well as mechanistic understanding of the deposition process through experimental and theoretical approaches. Since the redox potential of Si species is very negative, nonaqueous systems such as organic solvents, molten salts or ionic liquids have been applied for the electrodeposition of Si [1-3]. We have mainly used low temperature (up to 40°C) ionic liquid (IL), trimethylhexyl ammonium (TMHA+) bis-trifluorosulfonyl imide (TFSI-), as electrolyte [4]. The TMHA-TFSI is hydrophobic IL in which SiCl4, the precursor of Si, could be sufficiently dissolved. The electrodeposition was carried out potentiostatically using a three electrode sell. Au/n-Si or FTO substrate, Pt wire, and Ag/Ag+ were used as working, counter, and reference electrodes, respectively. By optimizing the deposition parameters, uniform Si films were obtained. The results of structural and compositional analyses indicated that the as-deposited films were amorphous and contained impurities such as C, while their amount could be reduced by annealing. It was also confirmed that the electric properties of the deposited films could be controlled by adding dopant species such as AlCl3 in the bath for the electrodeposition [5]. In order to investigate the mechanism of the deposition process, we applied electrochemical quartz crystal microbalance (EQCM) and the results indicated that the reduction of SiCl4 to Si proceeded via intermediate species such as Si (III) and Si (II) [4, 6]. In situ X-ray reflection measurements were also employed using synchrotron radiation lightsource in combination with density functional theory (DFT) calculation for nanoscopic analysis of the structure of IL-electrode interface during the deposition [7]. It was indicated that layered structure consisting of Si multimer such as Si2Cl6 dimer was formed at the surface of electrode prior to the deposition of elemental Si, suggesting formation of the intermediate states during the initial deposition stage of Si. The authors would like to thank Dr. J. Komadina, Dr. A. Mehta, Mr. M. Nishida, and Mr. H. Takai for their experimental help and valuable discussion. This study was financially supported in part by the Japan Science and Technology Agency (JST) CREST program. [1] T. Munisamy, A. J. Bard, Electrochim. Acta, 55, 3797 (2010). [2] M. Bechelany, J. Elias, P. Brodard, J. Michler, L. Philippe, Thin Solid Films, 520, 1895 (2012). [3] F. Endres, A.P. Abbott, D.R. MacFarlane, Electrodeposition from Ionic Liquids, Wiley-VCH, 2008. [4] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim. Acta, 100, 236 (2013). [5] Y. Tsuyuki, H. Takai, Y. Fukunaka, T. Homma, Jpn. J. Appl. Phys., in press. [6] Y. Tsuyuki, A. Pham, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta, 183, 49 (2015). [7] Y. Tsuyuki, T. Fujimura, M. Kunimoto, Y. Fukunaka, P. Pianetta, T. Homma, J. Electrochem. Soc., 164, D994 (2017).

Electrochemical processing can potentially be used for the cost-effective and large-scale fabrication of solar cells based on nanostructured Si thin films. Herein, we fabricated Si thin films for solar cell applications by electrodeposition in ionic liquids and performed a systematic investigation of film structure control and doping effects. Notably, relatively smooth and compact films were obtained by the modulation of the rest time during the electrodeposition under light irradiation, and the addition of AlCl3 to the electrolyte allowed the electrodeposition of p-type Si thin films.

Low-cost and simple fabrication methods of Si thin films have been required to meet increasing demands for solar cells, semiconductor materials, and so on. Si electrodeposition is one of the useful techniques, for it is low-cost and suitable for large scale production and nano-structure fabrication. We have proposed the Si electrodeposition process using SiCl4 as Si source and ionic liquids Trimethyl-n-hexylammmonium bis(trifluoromethylsulfonyl)imide (TMHA-TFSI) as solvents [1,2], and have attempted to optimize the deposition process, for which understanding of the molecular level mechanisms of Si electrodeposition reaction is significant. In this study we carried out theoretical analysis on the reaction mechanism of SiCl4 in ionic liquids by density functional theory (DFT), mainly focusing on Si-Si bond formation just after electron transfer from Si electrode surface. Two reaction schemes about the Si-Si bond formation as an initial stage of Si electrodeposition process are hypothesized; (i) SiCl4 forms Si-Si bond with Si surface directly, (ii) SiCl4 forms Si-Si bond with another SiCl4nearby. More favorable scheme was identified by comparing theoretically obtained energy profiles of scheme (i) and (ii). All DFT calculations were performed by Gaussian 09, with B3LYP as an exchange-correlation functional, 6-31+G** as basis sets for C and H, LANL2DZdp ECP as a basis set for Si. To model ionic liquids as solvent surrounding reactants, ONIOM method was applied; main reactants were expressed by quantum mechanics and solvent molecules were expressed by molecular mechanics with universal force field (UFF) parameters. Since ionic liquids generally exhibit complicated behaviors in dielectric constant, polarized continuum model (PCM) that usually provides sufficiently appropriate solvent model in the case of water solvent is not capable to work well in this situation. Energy profiles obtained by the DFT calculation showed that the scheme (ii) was more favorable than the scheme (i), suggesting that just after accepting electrons, SiCl4 forms Si-Si bond with another SiCl4 species, rather than with Si surface. Mulliken charge analyses showed that the electron population of covalent Si-Si bonds in Si surface decreased after the bond formation with SiCl4, which was caused by highly electronegative Cl atoms of SiCl4. This charge distribution destabilized the covalent bond within products of scheme (i), which made scheme (ii) more favorable. Solvation structure of products with ionic liquids showed that products of scheme (ii) interacted with both cation TMHA+ and anion TFSI-, whereas the product of scheme (i) interacted with only TMHA+. This result implies that the ionic liquid molecules significantly influence the stability of the products, promoting Si-Si bond formation among SiCl4species as shown in scheme (ii). From these results, we hypothesized the scheme of Si electrodeposition at initial stage as follows; a SiCl4 molecule near electrode surface receives electrons to react with another SiCl4molecule nearby to form Si-Si bond, generating relatively smaller size intermediates, which are stabilized by the solvation of ionic liquids. These smaller size intermediates continue to receive electrons near electrode surface to organize larger size intermediates or to deposit on the Si surface, forming Si film. This study was financially supported by JST Core Research for Evolutionary Science and Technology (CREST). T. F. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University from MEXT, Japan. References [1] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim, Acta., 100, 236, 2013 [2] Y. Tsuyuki, T. A. Pham Huynh, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta., 183, 49, 2015. Figure 1

Electrodeposition of semiconductor materials such as Si with very negative reduction potentials has been attempted in various non-aqueous solvents [1-3]. In particular, since ionic liquids exhibit relatively large electrochemical windows and are applicable under low temperature condition, they are expected to be one of the promising solvents for Si electrodeposition. Several studies have suggested the capability of ionic liquids to electrodeposit Si [4-6]. We also applied trimethylhexylammonium bis(trifluoromethylsulfonyl) imide (TMHATFSI) ionic liquid for the electrodeposition of Si [7-9]. To control this deposition process more precisely, its reaction mechanism should be elucidated further in detail. In the present study, we analyzed the reaction mechanism of SiCl4 during Si electrodeposition in ionic liquids using three approaches; 1) Electrochemical quartz crystal microbalance (EQCM), 2) X-ray reflectivity (XRR), and 3) Density functional theory (DFT) calculation. Electrodeposition of Si was conducted in TMHATFSI at 40 oC, controlled by thermo block. 3 electrodes cell (W.E. : Au, R.E. : Ag/Ag+, C.E. : Pt) was employed for the Si film electrodeposition. During XRR measurement, 2 electrodes cell (W.E. : Au, C.E. : Au) was used at 30 oC, controlled by thin thermo plate. EQCM measurement was carried out in Ar atmosphere, while the mixture of Ar and N2 was introduced into the electrolytic cell during the XRR measurement. The XRR measurement data were analyzed by GenX software. DFT calculation was performed by Gaussian09 package to support the XRR measurements. Results of the EQCM measurements during the cyclic voltammetry indicated a preliminary stage prior to the electrodeposition, in which no mass increase was detected in spite of substantial amount of charge transfer, suggesting the sequential reduction of SiCl4 through Si (III) and Si (II) species over the multiple stages. These were thought to be intermediate state in Si electrodeposition. Such a state was then investigated by using XRR measurement. The intensity of the reflectivity gradually decreased with the progress of the Si electrodeposition. Analyzed results of the XRR measurement indicated that a layer, which is thought to be polymer-like structure of Si containing dimer such as Si2Cl6, would be generated prior to the formation of metallic Si (0) films. This polymer-like structure was also supported by the results of DFT calculation; energy profiles showed that Si-Si bond formation between two SiCl4 molecules was more favorable than that between SiCl4 and Si surface, after SiCl4 received electrons from the surface. These results suggest that the cathodic reaction of SiCl4 proceeds through the formation of intermediate states containing polymer-like structure of Si. We would like to thank Dr. Apurva Mehta (SLAC National Accelerator Laboratory) and Mr. Trevor Petach (Department of Physics, Stanford University) for their kind advices and helps with the experiment. This study was financially supported in part by the Japan Science and Technology Agency (JST) CREST program. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Y. T. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University, from MEXT, Japan. References [1] Y. Nishimura, Y. Fukunaka, Electrochim. Acta, 53 (2007) 111. [2] T. Munisamy, A. J. Bard, Electrochim. Acta, 55 (2010) 3797. [3] M. Bechelany, J. Elias, P. Brodard, J. Michler, L. Philippe, Thin Solid Films, 520 (2012) 1895. [4] S. Zein El Abedin, N. Borrissenko, F. Endres, Electrochem. Comm., 6 (2004) 510. [5] J. Mallet, M. Molinari, F. Martineau, F. Delavoie, P. Fricoteaux, M. Troyon, Nano Lett., 8 (2010) 3468. [6] S. Ivanov, C. Vlaic, A. Bund, I. Efimov, Electrochim. Acta, 219 (2016) 251. [7] T. Homma, J. Komadina, Y. Nakano, T. Ouchi, T. Akiyoshi, Y. Ishibashi, Y. Nishimura, T. Nishida, and Y. Fukunaka, Electrochem. Soc. Trans., 41 (2012) 9. [8] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim. Acta, 100 (2013) 236. [9] Y. Tsuyuki, A. Pham, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta, 183 (2015) 49.

Si thin films are attracted as materials for ultra-large-scale solar cell systems at relatively low cost. Electrodeposition is expected as one fabrication method of Si thin films due to its advantages such as relatively simplicity process, precision control of nanostructure and continuous deposition in large area. Si thin films have been electrodeposited in non-aqueous solvents owing to their large electrochemical windows which allows for the study of Si electrodeposition [1-3]. Among various non-aqueous solvents, organic solvents and ionic liquids, which can be used under low temperature, have been used as electrolytes for the electrodeposition of Si thin films in our laboratory. We have previously succeeded to electrodeposit Si thin films in organic solvents and ionic liquids [4-6]. However, there is a problem for applications to solar cells that some impurities (mainly carbon and oxygen) are incorporated in thin films. We have reported the carbon content can be reduced by annealing treatment [7]. On the other hand, the decrease of oxygen content is still required. Thus, we evaluate the composition of Si thin films electrodeposited in an organic solvent and an ionic liquid without oxygen atoms in order to analysis the effect of bath conditions on the composition of thin films (especially, where the oxygen incorporated in Si thin films comes from). Si thin films were electrodeposited with 3 electrode cell (W.E. : Au, R.E. : Ag/Ag+, C.E. : Pt) in acetonitrile, 0.3 M tetrabutylammonium chloride and 0.5 M SiCl4 was used as an organic solvent, and 1-butyl-3-methylimidazolium hexafluorophosphate and 0.5 M SiCl4 as an ionic liquid. The composition of Si thin films were evaluated by X-ray photoelectron spectroscopy. As a result, the oxygen content in Si thin films were decreased by using the solvents without oxygen atoms in both organic solvent and ionic liquid cases. This results suggest that the reason the oxygen inclusion is derived from the incorporation of solvents themselves in films during electrodeposition, and propose us the way to obtain films with low impurities. In addition, the rotating disk electrode was used in the organic solvent case in order to promote the convection. The usage of this rotating disk electrode showed the decrease of impurities in Si thin films. Therefore, it is indicated that the stable supply of SiCl4 to the electrode surface is important factor to obtain films with low impurities. This study reveals that bath condition significantly affects to the composition of Si thin films, and could help us to fabricate films for applications to solar cells. This study was financially supported in part by the Japan Science and Technology Agency (JST) CREST program, and Y. T. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University, from MEXT, Japan. [1] S. Zein El Abedin, N. Borrissenko, F. Endres, Electrochem. Comm., 6 (2004) 510. [2] T. Munisamy, A. J. Bard, Electrochim. Acta, 55 (2010) 3797. [3] M. Bechelany, J. Elias, P. Brodard, J. Michler, L. Philippe, Thin Solid Films 520 (2012) 1895. [4] Y. Nishimura and Y. Fukunaka, Electrochim. Acta, 53 (2007) 111. [5] T. Homma, J. Komadina, Y. Nakano, T. Ouchi, T. Akiyoshi, Y. Ishibashi, Y. Nishimura, T. Nishida, and Y. Fukunaka, Electrochem. Soc. Trans., 41 (2012) 9. [6] J. Komadina, T. Akiyoshi, Y. Ishibashi, Y. Fukunaka, T. Homma, Electrochim. Acta, 100 (2013) 236. [7] Y. Tsuyuki, A. Pham, J. Komadina, Y. Fukunaka, T. Homma, Electrochim. Acta, 183 (2015) 49.

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