Possibility of NCS Group Anchor for Ru Dye Adsorption to Anatase

Article pubs.acs.org/JPCC

Possibility of NCS Group Anchor for Ru Dye Adsorption to Anatase TiO2(101) Surface: A Density Functional Theory Investigation Yusuke Ootani,†,‡ Keitaro Sodeyama,†,§ Liyuan Han,‡,∥ and Yoshitaka Tateyama*,†,‡,§,⊥ †

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 333-0012, Japan § Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan ∥ Photovoltaic Materials Unit, NIMS, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ⊥ PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 333-0012, Japan S Supporting Information *

ABSTRACT: We examined the possibility of the presence of isothiocyanate (NCS) group anchor for Ru dye adsorption to anatase TiO2(101) surface in typical dye-sensitized solar cells (DSCs), motivated by the recent X-ray photoelectron spectroscopy (XPS) experimental observations of the NCS anchors. A variety of adsorption configurations were examined with model molecules of CH3NCS by density functional theory (DFT) calculations. To compare with the experimental XPS spectra, we also calculated core− electron binding energies using (TiO2)38 cluster model. We demonstrated that 0.6−0.7 eV of the observed chemical shift in the S 2p XPS spectra can be assigned to the S interactions with the surface Ti atom and O vacancy, while 8.8 eV shift in the S 1s level is connected to the S−Ti vacancy interaction. On the other hand, it is also confirmed that the adsorption energies of COOH group and acetonitrile solvent molecule are usually larger than that for the NCS group. Therefore, the Ru dye adsorption through the NCS anchor is less probable even on the surface in vacuum and further decreases in the solution environment. With these results, we conclude that the NCS group anchoring is possible in the vacuum environment as shown by the XPS studies, while it will be negligible on the working electrode of the solar cell.

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INTRODUCTION Since the seminal work by O’Regan and Grätzel,1 the dyesensitized solar cell (DSC) has been widely studied as a promising next-generation photovoltaic device. So far, many types of DSCs have been proposed, and the highest solar-toelectric power-conversion efficiency (PCE) of 13% was reported.2 Recently, organic−inorganic perovskite solar cells were found to show higher efficiencies and have attracted considerable attention.3,4 Even under these circumstances, understanding the structural and electronic properties at the electrode−dye−electrolyte interfaces on the atomic scale is still important not only for the further development of solar cells but also for the fundamental science. Conventionally, Ru dyes such as N3, N719, and N749 (black dye, BD)1,5−11 show high PCE due to their broad light absorption capability, long-lived photoexcited state, and so on. One of the present authors achieved the highest efficiency record of 11.4% with the BD.10 Usually, Ru dyes consist of isothiocyanate (NCS) ligands and polypyridyl ligands involving carboxyl (COOH) groups. The highest occupied molecular orbital (HOMO) is localized on the NCS ligands and a central metal, while the lowest unoccupied molecular orbital (LUMO) is mainly distributed over the polypyridyl ligand. It is wellknown that Ru dyes adsorb on the TiO2 surface via the COOH © XXXX American Chemical Society

groups, while the NCS groups are exposed to electrolyte with redox mediators. This orientation is of great importance for a smooth electron transfer process and, thus, high PCE, because the excited electrons mainly in the LUMO can be easily injected to the surface and the mediators can easily reduce the photogenerated holes in the HOMO. The number of anchors, the anchoring modes, and the roles of proton are still under debate,12−22 whereas there is no doubt that the Ru dyes adsorb via the COOH groups. This idea is supported by vibrational spectroscopy studies: the vibrational spectra related to the COOH groups change upon the adsorption, while a strong peak related to the CN stretching vibration remains almost unchanged.12,23,24 On the other hand, there are some studies suggesting adsorption of a certain amount of Ru dyes on the TiO2 surface via the NCS groups. The X-ray photoelectron spectroscopy (XPS) study indicated that the S 2p peaks in the XPS spectra shift by 0.6−0.7 eV25,26 upon adsorption. The authors of these studies attributed this chemical shift to a NCS−TiO 2 interaction. From the intensities of the peaks, Johansson et al. Received: July 27, 2014 Revised: December 5, 2014

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dx.doi.org/10.1021/jp5075434 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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natural atomic orbitals with NBO version 3.1.42−44 For the assessment calculations, functional dependence of the geometries, population analysis, and bond-order analysis were investigated with BLYP functional. We employed 6-31G(d) basis sets for the H, C, N, O, and S atoms, LANL2DZ effective core potential (ECP) and corresponding pseudobasis set45 for Ti atom, and Stuttgart−Dresdon (SDD) ECP and corresponding pseudobasis set46 with additional f-type polarized functions for Ru atom. The chemical shifts of the S 1s CEBE were calculated for the adsorption structure of the CH3NCS molecule. The calculation of the CEBE is useful to investigate the molecular structure on the surface.47−49 Patrick and Giustino compared the calculated CEBEs and measured XPS spectra to investigate the detailed adsorption structure of the N3 dye on TiO2.48 Delta selfconsistent field (ΔSCF) method is usually used to calculate the accurate CEBE calculation;47−50 however, since such a calculation is demanding, we approximated the CEBEs with the corresponding Kohn−Sham orbital energies (=−ε).47,50 For the degenerate states, the orbital energies were averaged. The chemical shift was calculated as the difference between the CEBE of the target and the reference systems (=CEBEtarget − CEBEreference). We confirmed that the absolute CEBEs underestimate the experimental values at the present calculation level, while the chemical shifts were well-reproduced with the relative Kohn−Sham orbital energies. The calculated chemical shifts of selected sulfur compounds and corresponding experimental values are listed in the Supporting Information. The CEBE chemical shifts upon the adsorption were calculated with the anatase (TiO2)38 cluster exposing (101) surface.33,51−53 The initial (TiO2)38 configuration was extracted from the experimentally determined crystal structure, which is similar to that used by De Angelis et al.51,52 The geometry optimization of the (TiO2)38 cluster was then carried out without any geometrical constraints. The S0−S1 excitation energy of the (TiO2)38 cluster was calculated by timedependent DFT (TDDFT) method. The adsorption energies including the basis set superposition error (BSSE) correction are calculated with the counterpoise method. All molecular structures were visualized with VMD.54

estimated that 10−30% of the NCS groups interact with the TiO2 surface.25 Syres et al.27 and Karlsson et al.28 also obtained similar XPS spectra and implied the existence of the NCS− TiO2 interaction. Recently, Honda et al. showed that the S 1s peak shifts by 8.8 eV and the peak of noninteracting S atom disappears.29 Considering a short mean free path of photoelectrons, they concluded that, at several nanometers of the outermost surface layer, all the NCS groups interact with the TiO2 surface. A chemical shift of the Ti 2p peak was also observed and assigned to the NCS−TiO2 interaction. Moreover, they showed that the NCS−TiO2 interaction could be removed by adding D131 coadsorbate. Lee et al. ascribed a small shift in the CN stretching vibrational spectra to the NCS−TiO2 interaction.14 There are reports on the adsorption of the NCS group on a metal surface30,31 and a copper-based semiconductor surface32 as well. Such adsorption of the NCS group may lower the PCE. However, to our knowledge, the NCS anchor has not been well-examined for the DSC applications. Although some theoretical studies consider the adsorption structure via the NCS group,26,33,34 the detailed mechanism of the NCS anchor adsorption is still an open question. In this work, we examined whether adsorption via the NCS group anchor can exist for Ru dyes on anatase TiO2(101) surface, by using density functional theory (DFT) calculations. A methyl isothiocyanate (CH3NCS) molecule was used to model the NCS group of Ru dyes. The substitute was justified by comparing with the N3 dye. For the adsorption modes, we selected the molecular adsorptions on the Ti site as well as on a surface oxygen vacancy (VO) and a titanium vacancy (VTi). The adsorption structures together with the electronic properties were investigated by employing the plane-wave basis technique at the DFT/BLYP level. To clarify the difference between the NCS and COOH groups, we also investigated the COOH group anchor by using an isonicotinic acid (PyCOOH) model molecule. To compare with the experimental chemical shifts in the XPS spectra, we calculated the chemical shift of core− electron binding energy (CEBE) at the DFT/B3LYP level with the localized Gaussian basis sets.

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COMPUTATIONAL DETAIL We carried out DFT-based geometry optimizations of the adsorption structures of CH3NCS and PyCOOH on the anatase TiO2(101) surface. A supercell approach with periodic boundary condition (PBC) was adopted. The supercell has an orthorhombic shape with the parameters a = 11.353 Å, b = 10.240 Å, and c = 36.0 Å, where a and b were taken from the experimental lattice constants of the TiO2 anatase.35 The TiO2 slab consists of 3 × 1 × 5 primitive unit cells. The thickness of the vacuum layer is ca. 20 Å, which is enough to suppress an artificial dipole moment. An asymmetric slab, where the molecules adsorb only on one surface, was used. Plane-wave basis set with cutoff energy of 70 Ry and Troullier−Martin-type norm-conserving pseudopotential were used.36 For the PBC calculations, the BLYP functional37,38 was used. The geometry optimizations were performed without any geometrical constraint. The analyses of the projected density of state (PDOS) were performed for the optimized structures. All of these PBC calculations were carried out with CPMD package.39 The DFT calculations with the cluster boundary condition were carried out with the Gaussian 09 program package.40 Here we used the B3LYP hybrid functional.41 The population analysis and bond-order analysis were performed based on the

RESULTS AND DISCUSSION In this work, we used the CH3NCS and PyCOOH molecules to model the NCS and COOH group anchors of Ru dyes. We chose the simple molecules to avoid an undesired interaction between the other substituents and the substrate. To confirm the validity of our model, we compared with the N3 dye, a most representative Ru dye. The geometrical parameters, bond order, and atomic charge of the NCS groups of CH3NCS and N3 dye are compared in Table 1. Although we replaced a transition metal by a carbon atom, the S−C and C−N bond lengths are almost the same at all calculation levels. The bond-order and population analysis also exhibit the same trend for the CH3NCS and N3 dye. We confirmed that the characters of the MOs are also similar to those of the N3 dye. Although there are small differences in the atomic charge of the S atom, CH3NCS appropriately mimics the nature of the NCS group in the N3 dye. The geometry parameters of the PyCOOH and N3 dye as well as the atomic charges are listed in Table 2. Both geometries and atomic charges of PyCOOH are in good agreement with those of the N3 dye. Because the COOH groups do not directly bond with the Ru atom, the consistency B



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Table 1. Selected Geometry Parameters (in Å), Bond Orders, and Atomic Charge of the NCS Group of the CH3NCS and N3 Dye B3LYP geometry parameters bond order atomic charge

a

S−C C−N S−C C−N S C N

BLYP a

CH3NCS

N3

1.60 1.19 1.79 2.07 −0.052 0.212 −0.417

1.63 1.18 1.59 2.28 −0.212 0.154 −0.469

CH3NCS

N3a

1.60 1.21 1.83 2.05 −0.013 0.166 −0.362

1.63 1.20 1.63 2.24 −0.138 0.120 −0.430

Averaged value over the two NCS groups.

of the results for the PyCOOH and N3 dye is better than in the case of the NCS group. The calculation results are insensitive to the choice of the functional. Thus, the PyCOOH is an appropriate molecule to model the polypyridyl ligands of the Ru dyes. We carried out the geometry optimization of the adsorption structure of the CH3NCS molecule on the anatase TiO2(101) surface. For the adsorption sites, we selected five-coordinated titanium Ti5c and two-coordinated oxygen O2c on the surface as well as the two types of surface vacancies, VO and VTi. The VO (VTi) site was created by removing one O2c (Ti5c) atom. We observed that CH3NCS on the surface O2c always shows the desorption behavior. On the other hand, CH3NCS adsorbed to the Ti5c and the two types of vacancy sites. The adsorption structures are shown in Figure 1a−c. For the Ti5c site, the calculated S−Ti5c bond length is 3.45 Å, which is consistent with the previous calculations where the N3 dye adsorbs through the NCS anchor with ∼2.92 Å of the S−Ti distance.34 The slight difference in the S−Ti distance can be attributed to the smaller atomic charge of the S atom in the present CH3NCS and the steric effects of the bulky bipyridine ligand of N3 dye. The calculated adsorption energy (=Edesorbed − Eadsorbed) is 1.4 kcal mol−1. For the VO site, the NCS group adsorbs to a neighboring Ti site (Ti4c) with a smaller S−Ti4c distance of 2.60 Å. The adsorption structure at the VTi site is different from those at the Ti5c and VO sites. During the geometry optimizations, the S atom is immediately dissociated from the CH3NCS molecule and then bounded to neighboring three oxygen atoms, while the CH3NC moiety adsorbs to the Ti5c. Since the present DFT functional (BLYP) is usually insufficient to evaluate the vacancy states in TiO2,55−57 we did not calculate the adsorption energies for the vacancy sites. However, we expect that the total energy cost of the vacancy formation and the NCS group bonding will be less favorable

Figure 1. Adsorption structures of CH3NCS and PyCOOH model molecules. The geometry parameters are Å and degree.

than the molecular adsorption on the Ti5c site, referring to the vacancy formation energies calculated by more sophisticated

Table 2. Selected Geometry Parameters (in Å) and Atomic Charge of the COOH Group of the PyCOOH and N3 Dye B3LYP geometry parameters

atomic charge

a

CO C−O O−H O(CO) C O(O−H) H

BLYP a

PyCOOH

N3

1.21 1.36 0.98 −0.590 0.817 −0.707 0.504

1.21 1.35 0.98 −0.584 0.815 −0.703 0.510

PyCOOH

N3a

1.23 1.38 0.99 −0.559 0.768 −0.682 0.495

1.23 1.37 0.99 −0.556 0.763 −0.680 0.500

Averaged value over four COOH groups. C



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methods.56,57 In either case, we can conclude that the three types of S−TiO2 interactions are metastable. To compare with the NCS anchor under the same calculation conditions, geometry optimizations of the PyCOOH adsorption were performed for four possible adsorption modes: monodentate protonated (prot), deprotonated with hydrogen bond (dep-hb), deprotonated (dep), and bidentate bridging (brid). We did not consider the bidentate chelating mode, because the previous theoretical study showed that this mode is unstable.58 Our preliminary calculation also showed that this mode has higher energy than the other adsorption structures. The adsorption structures and the energies of PyCOOH are shown in Figure 1d−g and Table 3, respectively.

occupied states of the whole system correspond to the HOMOs of the molecules, which lie at ∼1 eV. We confirmed that there are no spurious states localized on the opposite side surface. Although all the peaks of CH3NCS shift by −0.4 eV, there is no significant difference between them. This result indicates that there is no hybridization between the NCS group and the substrate. Thus, the adsorption of the NCS group is mainly attributed to the electrostatic interaction between the negatively charged S atom and the positively charged Ti5c. Regarding the COOH group, the peaks of the hydrogen, carbon, and nitrogen atoms shift by similar values as in the case of CH3NCS, while the significant changes are found in the position of the peaks of the oxygen atoms. This result indicates that there are considerable orbital interactions between the oxygen atoms and the substrate. As mentioned in the introduction, several XPS studies observed the chemical shifts in the S 1s and 2p peaks and ascribed them to the NCS−TiO2 interaction.25,26,29 To understand the origin, we calculated the CEBE chemical shift of the S atom upon the adsorption. The (TiO2)38 cluster was used to model the substrate.33,51−53 The optimized structure was slightly distorted due to the lack of PBC, which is similar to the previous studies.33,53 The VBM and CBM are located at −8.4 and −4.1 eV, respectively, deducing a band gap of 4.3 eV at the DFT/B3LYP level. These are consistent with previous theoretical calculations.20,33,53 The valence band and the conduction band mainly consist of the 2p orbitals of oxygen and the 3d orbitals of titanium, respectively. The S0−S1 excitation energy was calculated to be 3.6 eV, which is in satisfactory agreement with the experimentally observed optical gap of 3.2−3.3 eV.65,66 The optimized geometry of the CH3NCS adsorption on the (TiO2)38 cluster is shown in Figure 3. Although, the adsorption structure is slightly different

Table 3. Adsorption Energies (kcal mol−1) of the PyCOOH prot

dep-hb

dep

brid

14.6

10.1

−2.4

1.7

The dep mode has negative adsorption energy, indicating that the adsorption structure is unstable. The prot and dep-hb modes are more stable than the brid and dep modes. The former have a hydrogen bond between the molecule and the substrate. The hydrogen bond strongly stabilizes the adsorption structure of the COOH group, which was also observed in the previous theoretical studies for organic molecules58−62 as well as organic dye63 and BD.19 Comparing between the prot and dep-hb adsorption modes, the prot modes are more stable than dep-hb modes as we previously showed for BD in the vacuum environment.19 The energy gain on the molecular adsorption of CH3NCS (1.4 kcal mol−1) is smaller than that of the prot and dep-hb modes of PyCOOH. Considering that the adsorption energy of CH3NCS is comparable with that of the brid mode, the presence of the hydrogen bond is expected to be a major cause of the difference, which is ∼10 kcal mol−1. In consequence, Ru dyes will mainly adsorb via the COOH (COO−) group as common view, whereas the adsorption through the NCS group seems minor. We also calculated the adsorption geometries with dispersion correction64 and found that the relative stability is almost the same with the above results. To further investigate the adsorption mechanism, we analyzed the PDOSs of the desorbed and representative adsorbed structures as shown in Figure 2. The valence band maximum (VBM) and the conduction band minimum (CBM) of TiO2 are located at ∼0 and ∼2 eV, respectively. The highest

Figure 3. Adsorption structure of the CH3NCS on the (TiO2)38 cluster.

from that obtained by the PBC calculation (Figure 1a) because of the distorted surface structure, the adsorption structure is qualitatively the same. The calculated adsorption energy was 4.4 kcal mol−1, which is larger than that obtained with PBC calculation (1.4 kcal mol−1). The adsorption energy calculated with BLYP functional was 2.0 kcal mol−1, and thus, the difference is mainly attributed to the employed functional. The adsorption structures on the VO and VTi sites are also qualitatively similar to those obtained in the PBC calculations. Table 4 shows the calculated CEBE chemical shifts of the CH3NCS adsorbate and the experimental values for Ru dyes.

Figure 2. Projected density of states (PDOSs) of CH3NCS and PyCOOH on the anatase TiO2(101) surface. D



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comparable with the experimental value of 8.8 eV (Table 4). Thus, we expect that many VTi sites were formed in their TiO2 nanoparticle due to the different preparation procedures. For example, it was reported that the concentration of VTi depends on the crystal size.70 The observed XPS chemical shift in the Ti 2p level may correspond to adsorption of the NC group on the Ti5c atom. Finally, we investigate the solvent effects. Typical DSCs are working in an electrolyte solution environment such as acetonitrile (AN), while the XPS measurements are performed in vacuum. To examine the solvent effect, the adsorption energies of AN molecules were calculated with the same calculation scheme for the PBC calculations. As reported previously, the Ti5c atom was considered as an adsorption site.72 The obtained adsorption energy is 9.2 kcal mol−1, which is smaller than that of the COOH group but larger than that of the NCS group (1.4 kcal mol−1). In the solution environment, the AN adsorption competes with that of the NCS group; therefore, the amount of the NCS adsorption decreases.

Table 4. CEBE Chemical Shifts of the S Atom on the NCS− Surface Interaction (in eV) our results molecule

exp.

CH3NCS

adsorption site

Ti5c

VO

VTi

1s 2p

0.43 0.48a

1.17 1.21a

6.98 6.23a

N3

N719

BD

N/A 0.7b,c,d

8.8e 0.6b,c

N/A 0.6b,c

a

Because spin−orbit interaction is not considered, 1/2 and 3/2 components are degenerate. bOnly the 2p3/2 components are taken. c Experimental results taken from ref 25. dExperimental results taken from ref 26. eExperimental results taken from ref 29.

Our results show that the CEBE of the S atom in the NCS group shifts by 0.4−0.5 eV due to the adsorption on the Ti5c. The obtained chemical shift for the 2p level is consistent with the experiments. Thus, the chemical shift of 0.7−0.6 eV can be ascribed to the S−Ti5c interaction. However, the calculated adsorption energy of the NCS group (1.4 kcal mol−1) is too small to explain the reported amount of interacting the NCS groups.25 Another possibility is the adsorption to the VO site, where the neighboring Ti4c may be strongly bound to the S atom due to the dangling bond, leading to the similar chemical shift. As listed in Table 4, the chemical shift for the S−VO interaction is 1.21 eV, which is also comparable to the experimental values. Since the existence of the VO sites on the TiO2 surface is possible, we conclude that the observed S 2p chemical shift also can be assigned to the S−VO interaction. Considering the small adsorption energy of the NCS group on the Ti5c, the S−VO interaction may be dominant in the observed chemical shift. For the 1s level, the calculated chemical shifts for the S−Ti5c and S−VO interactions are much smaller than the experimental value obtained by Honda et al.,29 while the S−VTi interaction gives the closer chemical shift. To investigate this discrepancy, we calculated the 1s CEBE chemical shift of several sulfur compounds, which have different oxidation numbers. As shown in Table 5, the S 1s CEBE increases as the oxidation number increases, while the CH3NCS and N3 dye negatively shift with respect to that of the H2S reference. Comparing between the chemical shift of the N3 and the sulfur compounds, to obtain the 8.8 eV of chemical shifts, the formal oxidation number should change to 4−6. Therefore, the origin of the 1s chemical shift of 8.8 eV is different from that of the 2p chemical shift, indicating that a different surface structure is formed in their experiment. Considering that the chemical shifts of 2p and 1s level are comparable (see Supporting Information, Tables S1 and S2), this explanation seems reasonable. One plausible explanation for the large 1s chemical shift is adsorption on the Ti vacancy site VTi. The properties of the VTi are discussed elsewhere.68−71 Because the VTi is surrounded by the oxygen atoms, adsorption on the VTi may lead to a sort of sulfur oxide state, which can lead to a large chemical shift. The calculated 1s CEBE chemical shift is 6.98 eV, which is

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CONCLUSION In this work, we examined whether the NCS groups of Ru dyes adsorb on the anatase TiO2(101) surface by using the model molecules CH3NCS and PyCOOH. The adsorption energies and the electronic states of both molecules were calculated by employing plane-wave DFT/BLYP method. We found three types of possible S−TiO2 interactions: S− Ti5c, S−VO, and S−VTi interactions. To assign the experimental chemical shift in the XPS spectra upon adsorption, we calculated the CEBE chemical shift of the sulfur atom by using the DFT/B3LYP method with localized Gaussian basis set. The (TiO2)38 cluster model was used to model the substrate. The calculated 2p CEBE chemical shifts for the S− Ti5c interaction are in good agreement with the experimental values, and that for the S−VO interaction is comparable. Considering the small adsorption energy of the NCS group on the Ti5c site, we assign the observed chemical shift mainly to the S−VO interaction with smaller contribution of the S−Ti5c. On the other hand, we propose that the NCS group adsorbed in the surface Ti vacancy site is the possible origin of the observed 1s level chemical shift. Comparing between the adsorption energies of the NCS and COOH groups, Ru dye adsorbs on TiO2 surface via the COOH groups even on the vacuum surface, while the NCS adsorption is minor as the common view. Because the adsorption energy of CH3NCS is comparable with that of the brid structure of PyCOOH, the presence of hydrogen bond is expected to be a major cause of the difference in the stability, which is ∼10 kcal mol−1. It is also found that the adsorption energy of AN is larger than that of the NCS group. Thus, the NCS adsorption possibility further decreases in the presence of AN solvent, leading to the negligible population of the NCS adsorption in the working electrode of the solar cell as the conventional scenario. Therefore, the adsorption via the NCS anchor, which

Table 5. Calculated S 1s CEBE Chemical Shifts of the Various Sulfur Compounds with Respect to That of the Reference H2S; The Numbers in Parentheses Are Experimental Resultsa formal oxidation number 1s CEBE chemical shift a

H2S

S8

SO

SO2

H2SO4

CH3NCS

N3

−2 0.00 (0.00)

0 1.68

2 2.54

4 5.40 (5.20)

6 8.22

N/A −0.35

N/A −1.89

Experimental results taken from ref 67. E



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will lower the PCE of Ru dye based DSCs, should not be a serious issue in the typical setup of the Ru dye DSCs.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated core−electron binding energies (CEBEs) of various sulfur compounds and corresponding experimental results, calculated CEBE chemical shift of various sulfur compounds, and full descriptions of refs 8 and 40. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-29859-2626. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.T. and K.S. were partly supported by KAKENHI (no. 23340089). This work was also supported by the Strategic Programs for Innovative Research (SPIRE), MEXT, and the Computational Materials Science Initiative (CMSI), Japan. The calculations in this work were carried out on the supercomputers in NIMS, the Information Technology Center, The University of Tokyo, Institute for Solid State Physics, Kyushu University, as well as the K computer at the RIKEN AICS through the HPCI Systems Research Projects.

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