Analysis of Cathodic Reaction Process of SiCl₄during Si Electrodeposition in Ionic Liquids

Abstract

Elementary steps in the electrochemical reduction process of SiCl 4 in trimethyl-n-hexylammonium bis(trifluoromethylsulfonyl) imide (TMHATFSI) was investigated, focusing on molecular level behavior of the reactants at solid-liquid interface. Electrochemical measurements using an electrochemical quartz crystal microbalance (EQCM) identified a reduction peak corresponding to Si electrodeposition and several elementary steps with stable intermediates forming prior to the deposition. For detailed analysis, X-ray reflectivity (XRR) measurements with synchrotron radiation were applied in situ. The change in reflectivity of the electrode surface during the deposition was found to be due to the formation of a polymer-like Si such as Si 2 Cl 6 , which is an intermediate layer during the deposition process. These results were theoretically supported by density functional theory (DFT) calculations: after an electron transfers from the electrode, the Si in SiCl 4 forms the bond with another SiCl 4 , rather than the Si of the substrate, resulting in the formation of the intermediate structure. These data suggest an elementary step in the SiCl 4 reduction process which can be described as follows; when SiCl 4 is reduced, a polymer-like Si form such as Si 2 Cl 6 is generated. This intermediate species further reacts with other Si reactants after receiving additional electrons, which then finally deposits as Si on the substrate.

Formation of Si thin films by electrodeposition in ionic liquids for solar cell applications

“…Focusing on such characteristics of ionic liquids, we have been working on developing the Si electrodeposition process in ionic liquids at low temperatures. [34][35][36][37] Thus, we have realized the electrodeposition of Si nanodot arrays on a template substrate formed by UVnanoimprint lithography using N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI) as an ionic liquid and SiCl 4 as a Si source, 34) additionally confirming that annealing results in the crystallization of electrodeposited films and decreases their impurity content. 36) Further development of Si electrodeposition in ionic liquids for the fabrication of solar cells based on Si thin films requires the establishment of methods for controlling their growth and doping.…”

“…More details on the electrochemical behavior can be found in our previous reports. [34][35][36][37] Under the assumption of dense and compact film formation and the operation of a four-electron transfer mechanism, the Si thin films electrodeposited at 1500 mC cm −2 were calculated to be ∼500 nm thick. The large roughness and porosity of electrodeposited films pose two concerns, namely, the observation of diffusion-limited conditions during constant-potential electrodeposition and=or the resistivity of the electrodeposited semiconducting Si films.…”

Challenges in the Electrochemical Synthesis of Si2Cl6 Starting from Tetrachlorosilane and Trichlorosilane

“…This approach will indeed elongate the silicon backbone with Si 2 Cl 6 as potential intermediate as Homma and co-workers reported. [26] However, Si 2 Cl 6 could not be isolated. Depending on constant current or constant potential conditions, an orange polysilane (SiCl 0.7 ) n , [25] or deposits of elemental silicon at the surface of various cathodic materials are obtained.…”

“…Depending on constant current or constant potential conditions, an orange polysilane (SiCl 0.7 ) n , [25] or deposits of elemental silicon at the surface of various cathodic materials are obtained. [26,27] This phenomenon is rationalized by a shift of the reduction potential towards less cathodic values, visible in cyclic voltammetry (CV) measurements at platinum electrodes with the elongation of the silicon backbone (Scheme 2).…”

“…While selective pathways for the dimerization of mono-functionalized substrates have been reported, difunctionalized halosilanes were reductively coupled to linear oligo-and polysilanes with varying chain length dependent on reaction conditions and substrate composition. [23] For higher halo substituted silanes there are only few reports in the literature with silicon deposition for appropriate reaction conditions and network polysilanes as sole products, [24][25][26][27] allowing the reduction of SiCl 4 to silicon nanoparticles. [28] Besides the reductive pathway, an oxidative route to SiÀ Si bond formation of alkyl and allyl H-silanes is known in literature, [29] and a successful dimerization of Me 2 PhSiH via cleavage of SiÀ H bond and cathodic proton reduction to H 2 has been described by Kunai and co-workers (Scheme 1).…”