Methyl Termination and ATR-FTIR Evaluation of nSi ( 111 ) Electrode towards Photoelectrochemical Cell Fabrication

We confirmed methyl termination on n-Si(111) surface by ATR-FTIR measurement, which was fabricated by a photo chloro-reaction and its methylation. The coverage of the methylation was about 63.7%, and the surface was not re-terminated by hydrogen. Photoelectrochemical properties of the n-Si(111) were measured as an electrode for a photoelectrochemical cell, and an onset potential obtaining photocurrent for the methyl terminated n-Si(111) electrode was observed as negative shift at 70 mV comparing with that of the hydrogen terminated n-Si(111) electrode. Therefore, the negative shift would be expected for improving open circuit voltage towards solar cell.


Introduction
Recently, we have remarked photovoltaic power generation, and have produced higher efficiency solar cells by proceeding to the influential investigation.However, those include great problems which take high costs to produce them by semiconductor fabrication processes as p-n junction and use of transparent conductive indium oxide etc.The low costs and energy conversion efficiency are very important to the spread of the solar cell utilization [1,2].Previously, higher open circuit voltage was discovered with n-Si photoelectrochemical cells (PEC) comparing with familiar p-n junction solar cell, and was able to fabricate them with low costs and ease.But, the cell has a point that the n-Si surface is gradually oxidized by exposing to an electrolyte solution, and then SiO 2 is slowly formed as an insulation layer on the surface.
Researches have been carried out to solve the problem by immobilizing the surface with alkyl base etc. [3][4][5][6][7][8].For example, the alkyl termination on the Si surface is investigated toward developments of bio sensors for DNA and protein detection by giving functionalize on the surface [9], furthermore, the termination is known to make the surface stable for the oxidation in the solution [4,10,11].Therefore, we designed the functional n-Si surface toward PEC in this study.
We investigated alkyl terminated n-Si surface by ATR-FTIR, which was known hardly to oxidate the surface in electrolyte solution, and we selected methyl base as the alkyl species simply, and then n-Si was measured about photoelectrochemical properties electrochemically as a working electrode by Pt eletrodeposition onto the surface.Especially, an onset potential (U oc ) for obtaining photocurrent was examined to know the termination effects, and to estimate expected photovoltage properties toward PEC.

Methodology
We carried out the methyl termination with a method reported by Bansal et al., which was immobilization on the surface by two steps alkyl termination via a chlorination in Scheme 1 [5,12].The n-Si(111) surface was exposed by so-called RCA washing process as HF, H 2 O, NH 3 + H 2 O 2 solution, H 2 O, HCl + H 2 O 2 solution and H 2 O 2 and then the surface was terminated hydrogen.After the surface was washed by diethyl ether, the chlorine termination was performed by immersing the Si(111) to sat.PCl 5 /chlorobenzene solution under heating at 100˚C for 1 h irradiating UV light in flowing Ar gas.Next immersing them in CH 3 Li/diethyle ether in Ar, and refluxing at 50˚C for 3 h, and the methyl termination was completed on the surface.Before the surface was measured by ART-FTIR, they are washed by diethyl ether, 2-propanol and pure water.The surface was not re-terminated by hydrogen for parts of non-methyl termination, which was immersed in HF solution for 15 min, and ART-FTIR was measured after washing pure water.
In Figure 1, an electrode was fabricated by using the n-Si(111) as a working electrode to be in contact with In-Ga alloy, and electrochemical measurements was performed with a Pt counter electrode and a Ag/AgCl (sat.KCl) reference electrode under 100 mW/cm 2 irradiation by a solar simulator at AM 1.5.The n-Si(111) working electrode was carried out at Pt electrodeposition to induce electron transfer easily in 5 mM K 2 PtCl 6 + 0.1 M LiClO 4 at 83 mC/cm 2 imposing at −1.0 V, before properties of the electrode were measured.

Results and Discussion
ATR-FTIR spectrum is shown about the hydrogen and methyl terminated n-Si(111) surface in  (symmetry) in Figure 2(B).Furthermore, trial of hydrogen re-termination results in not obtaining the spectra based on hydrogen termination on the n-Si(111) surface.This result would show that the surface was covered sufficiently with the methyl bases by this processes, hence the re-termination by hydrogen will not happen.
The rough coverage of the methyl termination was calculated from the area based on the Figure 2(A).The result gave ca.63.7% by Table 1, and it would suppose that the methyl termination was completed adequately.A coverage has been reported at about 50% by alkene (1-octene, 1-octadecene, 1-octyne and styrene etc.) on the n-Si(111) surface [13], hence this coverage of methyl termination will be enough.
The n-Si(111) was measured as an electrode for photocurrent-potential curves in 1 M KI solution.In Figure 3, onset potential (U oc ) for photocurrent indicated −0.25 V at the hydrogen terminated electrode, and −0.32 V at the methyl terminated electrode.The methyl termination gave the electrode effect of the negative shift of the onset potential about 70 mV, which is concretely not so clear yet, but the methyl termination can raise the electrode  2 (A) and (B), and calculation of the methyl base coverage, which assumed that the hydrogen coverage is at 100% by hydrogen termination on Si(111).* Therefore, the coverage of methyl base was decided by at 63.7 % regarding that is (100 -37.6)%.

Conclusion
By measuring the Si(111) surface by ART-FTIR, we confirmed the methyl termination on the surface by the UV irradiation in PCl 5 /chlorobenzene solution and immersing them in CH 3 Li/diethyle ether in Ar, and refluxing at 50˚C for 3 h.Furthermore, the coverage was estimated at about 63.7%, and the re-termination of hydrogen by NH 4 F solution was not obtained.Photoelectrochemical properties showed the negative shift of U oc in I − / solution, hence improvement of open circuit volt-

Figure 3 .
Figure 3. Photocurrent-voltage curves of hydrogen and methyl termination Si(111) electrode under 100 mW/cm 2 Xe lump irradiation in 1 M KI solution.potential by the negative shift.This shift would be expected for improving open circuit voltage U oc as photoelectrochemical cell (PEC).